Wireless Networking in the Developing World/Physics
1. RADIO PHYSICSEdit
Wireless communications make use of electromagnetic waves to send signals across long distances. From a user’s perspective, wireless connections are not particularly different from any other network connection: your web browser, email, and other applications all work as you would expect. But radio waves have some unexpected properties compared to Ethernet cable. For example, it’s very easy to see the path that an Ethernet cable takes: locate the plug sticking out of your computer, follow the cable to the other end, and you’ve found it! You can also be confident that running many Ethernet cables alongside each other won’t cause problems, since the cables effectively keep their signals contained within the wire itself.
But how do you know where the waves emanating from your wireless device are going? What happens when these waves bounce off objects in the room or other buildings in an outdoor link? How can several wireless cards be used in the same area without interfering with each other?
In order to build stable high-speed wireless links, it is important to understand how radio waves behave in the real world.
What is a wave?Edit
We are all familiar with vibrations or oscillations in various forms: a pendulum, a tree swaying in the wind, the string of a guitar - these are all examples of oscillations.
What they have in common is that something, some medium or object, is swinging in a periodic manner, with a certain number of cycles per unit of time. This kind of wave is sometimes called a mechanical wave, since it is defined by the motion of an object or its propagating medium.
When such oscillations travel (that is, when the swinging does not stay bound to one place) then we speak of waves propagating in space. For example, a singer singing creates periodic oscillations in his or her vocal cords. These oscillations periodically compress and decompress the air, and this periodic change of air pressure then leaves the singers mouth and travels, at the speed of sound.
A stone plunging into a lake causes a disturbance, which then travels across the lake as a wave.
A wave has a certain speed, frequency, and wavelength.
These are connected by a simple relation:
Speed = Frequency * Wavelength
The wavelength (sometimes referred to as lambda, λ) is the distance measured from a point on one wave to the equivalent part of the next (or, in a more technical way, to the next point that is in the same phase), for example from the top of one peak to the next.
The frequency is the number of whole waves that pass a fixed point in a period of time. Speed is measured in metres/second, frequency is measured in cycles per second (or Hertz, represented by the symbol Hz), and wavelength is measured in metres. For example, if a wave on water travels at one metre per second, and it oscillates five times per second, then each wave will be twenty centimetres long:
1 metre/second = 5 cycles/second *W
W = 1 / 5 metres
W = 0.2 metres = 20 cm
Waves also have a property called amplitude. This is the distance from the centre of the wave to the extreme of one of its peaks, and can be thought of as the “height” of a water wave. Frequency, wavelength, and amplitude are shown in Figure RP 1.
Figure RP 1: Wavelength, amplitude, and frequency. For this wave, the frequency is 2 cycles per second, or 2 Hz, while the speed is 1 m/s.
Waves in water are easy to visualize.
Simply drop a stone into the lake and you can see the waves as they move across the water over time. In the case of electromagnetic waves, the part that might be hardest to understand is: “What is it that is oscillating?”
In order to understand that, you need to understand electromagnetic forces.
Electromagnetic forces are the forces between electrical charges and currents. Our most direct access to those is when our hand touches a door handle after walking on synthetic carpet, or brushing up against an electrical fence.
A more powerful example of electromagnetic forces is the lightning we see during thunderstorms.
The electrical force is the force between electrical charges.
The magnetic force is the force between electrical currents.
Electrons are particles that carry a negative electrical charge. There are other charged particles too, but it is the electrons that are responsible for most of what we need to know about how radio behaves.
Let us look at what is happening in a piece of straight vertical wire, in which we push the electrons from one end to the other and back, periodically. At one moment, the top of the wire is negatively charged - all the negative electrons are gathered there. This creates an electric field from the positively charged end to the negatively charged one along the wire.
The next moment, the electrons have all been driven to the other side, and the electric field points the other way. As this happens again and again, the electric field vectors (represented by arrows from plus to minus) are leaving the wire, so to speak, and are radiated out into the space around the wire.
What we have just described is known as a dipole (because of the two differently charged poles, plus and minus, that are created in the straight vertical wire), or more commonly a dipole antenna.
This is the simplest form of an omnidirectional antenna. The moving electric field is commonly referred to as an electromagnetic wave because there is also an associated magnetic field. A moving electric field, such as a wave, always comes together with a magnetic field - you will not find one with out the other. Why is this the case?
An electric field is caused by electrically charged objects.
A moving electric field is produced by moving electrically charged objects, such as we have just described above in a dipole antenna.
Wherever electrical charges are moving, they induce a magnetic field. Mathematically, this is formulated in Maxwell’s equations: https://en.wikipedia.org/wiki/Electromagnetic_field#Mathematical_description
Since the electrical and magnetic components are tied together in this way, we speak of an electromagnetic field.
In practical wireless networking, we focus in the electrical component but there be always a magnetic component as well.
Let us come back to the relation:
Speed = Frequency * Wavelength
In the case of electromagnetic waves, the speed is c, the speed of light.
c = 300,000 km/s = 300,000,000 m/s = 3*108 m/s
c = f * λ
Electromagnetic waves differ from mechanical waves in that they require no medium in which to propagate. Electromagnetic waves will even propagate through perfect vacuum.
The light from the stars is a good example: it reaches us through the vacuum of space.
Symbols of the international system of unitsEdit
In physics, maths, and engineering, we often express numbers by powers of ten.
We will meet these terms again, and the symbols used to represent them, e.g. gigahertz (GHz), centimetres (cm), microseconds (µs), and so on.
These symbols are part of the international system of measurement SI (http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf), they are not abbreviations and should not be changed.
The case is significant and should not be altered.
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Knowing the speed of light, we can calculate the wavelength for a given frequency. Let us take the example of the frequency of 802.11b wireless networking, which is:
f = 2.4 GHz = 2,400,000,000 cycles / second
wavelength (λ) = c / f = 3*108 / 2.4*109 = 1.25*10-1 m = 12.5 cm
Frequency and therefore wavelength determine most of an electromagnetic wave’s behaviour. It governs the dimensions of the antennas that we build as well as the effect of the interactions with objects that are in the propagation path, including the biological effects in living beings.
Wireless standards of course are distinguished by more than just the frequency they are working at - for example, 802.11b, 802.11g, 802.11n and 802.16 can all work at 2.4 GHz -, yet they are very different from one another.
The chapter called Telecommunications Basics will discuss modulation techniques, media access techniques, and other relevant features of wireless communications standards. However, the basic capabilities of electromagnetic waves to penetrate objects, to go long distances, and so forth - these are determined by physics alone. The electromagnetic wave “does not know or care” what modulation or standard or technique you put on top of it. So, while different standards may implement advanced techniques to deal with NLOS (Non Line of Sight), multipath and so forth - they still cannot make a wave go through a wall, if that wall is absorbing the respective frequency. Therefore, an understanding of the basic ideas of frequency and wavelength helps a lot in practical wireless work.
Later in this chapter, we will talk about concepts like interference, multipath and Fresnel zones. In order to understand these, we will need to know about the phase of a wave, or rather, phase differences between waves. Look at the sine wave shown in Fig RP 1 - now imagine we have two such waves moving. These can be in exactly the same position: Where the one has its peak, the other one also has a peak. Then, we would say, they are in phase, or, their phase difference is zero. But one wave could also be displaced from the other, for example it could have its peak where the other wave is at zero. In this case, we have a phase difference. This phase difference can be expressed in fractions of the wavelength, e.g. λ//4, or in degrees, e.g. 90 degrees - with one full cycle of the wave being 360 degrees. A phase difference of 360 degrees is the same as that of 0 degrees: no phase difference.
Another important quality of electromagnetic waves is polarization. Polarization describes the direction of the electrical field vector.
If you imagine a vertically aligned dipole antenna (the straight piece of wire), electrons can only move up and down, not sideways (because there is no room to move) and thus electrical fields only ever point up or down, vertically. The field leaving the wire and travelling as a wave has a strict linear (and in this case, vertical) polarization. If we put the antenna flat on the ground, we would find horizontal linear polarization.
Figure RP 3: Vertically polarized electromagnetic wave
Linear polarization is just one special case, and is never quite so perfect: in general, we will always have some component of the field pointing in other directions too. If we combine two equal dipoles fed with the same signal, we can generate a circularly polarized wave, in which the electric field vector keeps rotating perpendicularly to the wave’s trajectory.
The most general case is elliptical polarization, in which the electric field vector maximum value is not the same in the vertical and horizontal direction. As one can imagine, polarization becomes important when aligning antennas. If you ignore polarization, you might have very little signal even though you have the best antennas. We call this polarization mismatch.
Much in the same way, polarization may also be used in a smart way, to keep two wireless links independent and without interference, even though they might use the same end points (or even share a common reflector) and therefore the same trajectory: if one link is polarized vertically and the other horizontally, they will not “see” each other. This is a convenient way to double data rates over one link using a single frequency.
The antennas used in this kind of application must be carefully built in order to reject the “unwanted” polarization, i.e. an antenna meant for vertical polarization must not receive or transmit any horizontally polarized signal, and vice versa. We say they must have a high “cross polarization” rejection.
The electromagnetic spectrumEdit
Electromagnetic waves span a wide range of frequencies (and, accordingly, wavelengths). This range of frequencies or wavelengths is called the electromagnetic spectrum. The part of the spectrum most familiar to humans is probably light, the visible portion of the electromagnetic spectrum. Light lies roughly between the frequencies of 7.5*1014 Hz and 3.8*1014 Hz, corresponding to wavelengths from circa 400 nm (violet/blue) to 800 nm (red).
We are also regularly exposed to other regions of the electromagnetic spectrum, including Alternating Current (AC) or grid electricity at 50/60 Hz, AM and FM radio, Ultraviolet (at frequencies higher than those of visible light), Infrared (at frequencies lower than those of visible light),
X-Ray radiation, and many others.
Radio is the term used for the portion of the electromagnetic spectrum in which waves can be transmitted by applying alternating current to an antenna. This is true for the range from 30 kHz to 300 GHz, but in the more narrow sense of the term, the upper frequency limit would be about 1 GHz, above which we talk of microwaves and millimetric waves.
When talking about radio, many people think of FM radio, which uses a frequency around 100 MHz. In between radio and infrared we find the region of microwaves - with frequencies from about 1 GHz to 300 GHz, and wavelengths from 30 cm to 1 mm.
The most popular use of microwaves might be the microwave oven, which in fact works in exactly the same region as the wireless standards we are dealing with. These regions lie within the bands that are being kept open for general unlicensed use. This region is called the ISM band, which stands for Industrial, Scientific, and Medical.
Most other parts of the electromagnetic spectrum are tightly controlled by licensing legislation, with license values being a huge economic factor. In many countries the right to use portions of the spectrum have been sold to communications companies for millions of dollars. In most countries, the ISM bands have been reserved for unlicensed use and therefore do not have to be paid for when used.
Figure RP 4: The electromagnetic spectrum.
The frequencies most interesting to us are 2.400 - 2.495 GHz, which is used by the 802.11b and 802.11g standards (corresponding to wavelengths of about 12.5 cm), and 5.150 - 5.850 GHz (corresponding to wavelengths of about 5 to 6 cm), used by 802.11a. The 802.11n standard can work in either of these bands.
See the Chapter called WiFi Family for an overview of standards and frequencies. In addition you can find out more about the Radio portion of the electromagnetic spectrum in the Chapter called Radio Spectrum.
A term you will meet often in radio physics is bandwidth. Bandwidth is simply a measure of frequency range. If a range of 2.40 GHz to 2.48 GHz is used by a device, then the bandwidth would be 0.08 GHz (or more commonly stated as 80 MHz).
It is easy to see that the bandwidth we define here is closely related to the amount of data you can transmit within it - the more room in frequency space, the more data you can fit in at a given moment. The term bandwidth is often used for something we should rather call data rate, as in “my Internet connection has 1 Mbps of bandwidth”, meaning it can transmit data at 1 megabit per second. How much exactly you can fit into a physical signal will depend on the modulation, encoding and other techniques. For example, 802.11g uses the same bandwidth as 802.11b, however it fits more data into those same frequency ranges transmitting up to 5 times more bits per second.
Another example we have mentioned: you may double your data rate by adding a second link at perpendicular polarization to an existing
radio link. Here, frequency and bandwidth have not changed, however the data rate is doubled.
Frequencies and channelsEdit
Let us look a bit closer at how the 2.4 GHz band is used in 802.11b. The spectrum is divided into evenly sized pieces distributed over the band as individual channels. Note that channels are 22 MHz wide, but are only separated by 5 MHz.
Figure RP 5: Channels and centre frequencies for 802.11b.
Note that channels 1, 6, and 11 do not overlap.
Behaviour of radio wavesEdit
There are a few simple rules of thumb that can prove extremely useful when making first plans for a wireless network:
- the longer the wavelength, the further it goes;
- the longer the wavelength, the better it travels through and around things;
- the shorter the wavelength, the more data it can transport.
All of these rules, simplified as they may be, are rather easy to understand by example.
Longer waves travel further
Waves with longer wavelengths tend to travel further than waves with shorter wavelengths. As an example, AM radio stations have a much greater range than FM stations, which use a frequency 100 times higher. Lower frequency transmitters tend to reach much greater distances than high frequency transmitters at the same power.
Longer waves pass around obstacles
A wave on water which is 5 metres long will not be affected by a 5 mm piece of wood floating on the water. If instead the piece of wood were 50 metres big (e.g. a ship), it would modify the behavior of the wave.
The distance a wave can travel depends on the relationship between the wavelength of the wave and the size of obstacles in its path of propagation. It is harder to visualize waves moving “through” solid objects, but this is the case with electromagnetic waves. Longer wavelength (and therefore lower frequency) waves tend to penetrate objects better than shorter wavelength (and therefore higher frequency) waves.
For example, FM radio (88-108 MHz) can travel through buildings and other obstacles easily, while shorter waves (such as GSM phones operating at 900 MHz or 1800 MHz) have a harder time penetrating buildings.
This effect is partly due to the difference in power levels used for FM radio and GSM, but is also partly due to the shorter wavelength of GSM signals. At much higher frequencies, visible light does not go through a wall or even 1 mm of wood - as we all know, from practical experience.
But metal will stop any kind of electromagnetic wave.
Shorter waves can carry more data
The faster the wave swings or beats, the more information it can carry - every beat or cycle could for example be used to transport a digital bit, a ‘0’ or a ‘1’, a ‘yes’ or a ‘no’.
So the data rate scales with bandwidth, and can be further enhanced by advanced modulation and media access techniques such as OFDM, and MIMO (Multiple Input, Multiple Output).
The Huygens PrincipleEdit
There is another principle that can be applied to all kinds of waves, and which is extremely useful for understanding radio wave propagation.
This principle is known as the Huygens Principle, named after Christiaan Huygens, Dutch mathematician, physicist and astronomer, 1629 - 1695.
Imagine you are taking a little stick and dipping it vertically into a still lake’s surface, causing the water to swing and dance. Waves will leave the centre of the stick - the place where you dip in - in circles. Now, wherever water particles are swinging and dancing, they will cause their neighbor particles to do the same: from every point of disturbance, a new circular wave will start. This is, in simple form, the Huygens principle. In the words of wikipedia.org:
“The Huygens’ principle is a method of analysis applied to problems of wave propagation in the far field limit. It recognizes that each point of an advancing wave front is in fact the centre of a fresh disturbance and the source of a new train of waves; and that the advancing wave as a whole may be regarded as the sum of all the secondary waves arising from points in the medium already traversed”.
This view of wave propagation helps better understand a variety of wave phenomena, such as diffraction.” This principle holds true for radio waves as well as waves on water, for sound as well as light, but for light the wavelength is far too short for human beings to actually see the effects directly.
This principle will help us to understand diffraction as well as Fresnel zones, and the fact that sometimes we seem to be able to transmit around corners, with no line of sight.
Let us now look into what happens to electromagnetic waves as they travel.
When electromagnetic waves go through ‘something’ (some material), they generally get weakened or dampened.
How much they lose in power will depend on their frequency and of course the material.
Clear window glass is obviously transparent for light, while the glass used in sunglasses filters out quite a share of the light intensity and most of the ultraviolet radiation.
Often, an absorption coefficient is used to describe a material’s impact on radiation.
For microwaves, the two main absorbent materials are:
Metal. Electrons can move freely in metals, and are readily able to swing and thus absorb the energy of a passing wave.
Water. Microwaves cause water molecules to jostle around, thus taking away some of the wave’s energy.
For the purpose of practical wireless networking, we may well consider metal and water perfect absorbers: we will not be able to go through them (although thin layers of water will let some power pass). They are to microwave what a brick wall is to light.
When talking about water, we have to remember that it comes in different forms: rain, fog and mist, low clouds and so forth, all will be in the way of radio links. They have a strong influence, and in many circumstances a change in weather can bring a radio link down.
When talking about metal, keep in mind that it may be found in unexpected places: it may be hidden in walls (for example, as metal grids in concrete) or be a thin coat on modern types of glass (tinted glass, colored glass).
However thin the layer of metal, it might be enough to significantly absorb a radio wave.
There are other materials that have a more complex effect on radio absorption. For trees and wood, the amount of absorption depends on how much water they contain.
Old dead dry wood is more or less transparent, wet fresh wood will absorb a lot. Plastics and similar materials generally do not absorb a lot of radio energy, but this varies depending on the frequency and type of material.
Lastly, let us talk about ourselves: humans (as well as other animals) are largely made out of water.
As far as radio networking is concerned, we may well be described as big bags of water, with the same strong absorption.
Orienting an office access point in such a way that its signal must pass through many people is a key mistake when building office networks.
The same goes for hotspots, cafe installations, libraries, and outdoor installations.
Just like visible light, radio waves are reflected when they come in contact with materials that are suited for that: for radio waves, the main sources of reflection are metal and water surfaces.
The rules for reflection are quite simple: the angle at which a wave hits a surface is the same angle at which it gets deflected.
Note that in the eyes of a radio wave, a dense grid of bars acts just the same as a solid surface, as long as the distance between bars is small compared to the wavelength.
At 2.4 GHz, a one cm metal grid will act much the same as a metal plate.
Although the rules of reflection are quite simple, things can become very complicated when you imagine an office interior with many many small metal objects of various complicated shapes.
The same goes for urban situations: look around you in city environment and try to spot all of the metal objects.
This explains why multipath effects (i.e. signal reaching their target along different paths, and therefore at different times) play such an important role in wireless networking.
Water surfaces, with waves and ripples changing all the time, effectively make for a very complicated reflection object which is more or less impossible to calculate and predict precisely.
Figure RP 6: Reflection of radio waves. The angle of incidence is always equal to the angle of reflection. A metal parabolic surface uses this effect to concentrate radio waves spread out over it in a common direction.
We should also add that polarization has an impact: waves of different polarization in general will be reflected differently.
We use reflection to our advantage in antenna building: e.g. we put huge parabolas behind our radio transmitter/receiver to collect and bundle the radio signal into a single point, the focal point.
Diffraction is the apparent bending of waves when hitting an object.
It is the effect of “waves going around corners”. Imagine a wave on water traveling in a straight wave front, just like a wave that we see rolling onto an ocean beach.
Now we put a solid barrier, say a wooden solid fence, in its way to block it. We cut a narrow slit opening into that wall, like a small door.
From this opening, a circular wave will start, and it will of course reach points that are not in a direct line behind this opening, but also on either side of it. If you look at this wavefront - and it might just as well be an electromagnetic wave - as a beam (a straight line), it would be hard to explain how it can reach points that should be hidden by a barrier.
When modelled as a wavefront, the phenomenon makes sense.
Figure RP 7: Diffraction through a narrow slit.
The Huygens Principle provides one model for understanding this behavior. Imagine that at any given instant, every point on a wavefront can be considered the starting point for a spherical “wavelet”.
This idea was later extended by Fresnel, and whether it adequately describes the phenomenon is still a matter of debate. But for our purposes, the Huygens model describes the effect quite well.
Figure RP 8: The Huygens Principle.
Through means of the effect of diffraction, waves will “bend” around corners or spread through an opening in a barrier.
The wavelengths of visible light are far too small for humans to observe this effect directly.
Microwaves, with a wavelength of several centimeters, will show the effects of diffraction when waves hit walls, mountain peaks, and other obstacles. It seems as if the obstruction causes the wave to change its direction and go around corners.
Figure RP 9: Diffraction over a mountain top.
Note that diffraction comes at the cost of power: the energy of the diffracted wave is significantly less than that of the wavefront that caused it. But in some very specific applications, you can take advantage of the diffraction effect to circumvent obstacles.
Interference is one of the most misunderstood terms and phenonema in wireless networking.
Interference often gets the blame when we are too lazy to find the real problem, or when a regulator wants to shut down someone else’s network for business reasons. So, why all the misunderstandings?
It is mostly because different people mean different things though they are using the same word.
A physicist and a telecommunications engineer will use the word “Interference” in very different ways. The physicists’ view will be concerned with the “behaviour of waves”. The telecommunications engineer will talk about “… any noise that gets in the way”.
Both views are relevant in wireless, and it is important to be able to know them both and know the difference. Let us start with the physicists’ view:
When working with waves, one plus one does not necessarily equal two. It can also result in zero.
Figure RP 10: Constructive and destructive interference.
This is easy to understand when you draw two sine waves and add up the amplitudes. When the phase difference is zero, peak hits peak, and you will have maximum results (1 + 1 = 2).
This is called constructive interference.
When the phase difference is 180 degrees, or λ/2, peak hits valley, and you will have complete annihilation ((1 + (-)1 = 0) - destructive interference.
You can actually try this with waves on water and two little sticks to create circular waves - you will see that where two waves cross, there will be areas of higher wave peaks and others that remain almost flat and calm. In order for whole trains of waves to add up or cancel each other out perfectly, they have to have the exact same wavelength and a fixed phase relation.
You can see obvious examples of interference in action when you look at the way that antennas are arranged in what are called beamforming arrays, in order to give maximum constructive interference in the directions where you want the signal, and destructive interference (no signal) where you want no signal.
Technically, this is achieved by a combination of physical dimensioning and control of phase shifts.
Simplified, imagine that you have three antennas - and you don’t want antenna 3 to pick up signal from antenna 1 and 2. You would then place antenna 3 at a position where the signals from antennas 1 and 2 cancel each other out.
Now let us have a look at the way the word interference is typically used: in a wider sense, for any disturbance through other RF sources, any noise that might get in our way, e.g. from neighboring channels or competing providers. So, when wireless networkers talk about interference they typically talk about all these kinds of disturbance by other networks, and any other sources of microwave, whether it has exactly the same frequency and a fixed phase relation or not. Interference of this kind is one of the main sources of difficulty in building wireless links, especially in urban environments or closed spaces (such as a conference space) where many networks may compete for use of the spectrum.
But, interference of this kind is also often overrated: for example, imagine you had to build a point to point link that has to cross a crowded inner city area, before reaching its target on the other side of the city. Such a highly directional beam will cross the “electric smog” of the urban centre without any problem. You may imagine this like a green and a red light beam crossing each other in a 90 degrees angle: while both beams will overlap in a certain area, the one will not have any impact on the other at all.
Generally, managing spectrum and coexistence has become a main issue especially in dense indoor environments and urban areas.
Line of sightEdit
The term line of sight, often abbreviated as LOS, is quite easy to understand when talking about visible light: if we can see a point B from point A where we are, we have line of sight. Simply draw a line from A to B, and if nothing is in the way, we have line of sight.
Things get a bit more complicated when we are dealing with microwaves. Remember that most propagation characteristics of electromagnetic waves scale with their wavelength.
This is also the case for the widening of waves as they travel.
Light has a wavelength of about 0.5 micrometres, microwaves as used in wireless networking have a wavelength of a few centimetres.
Consequently, their beams are a lot wider - they need more space, so to speak.
Note that visible light beams widen just the same, and if you let them travel long enough, you can see the results despite their short wavelength. When pointing a well focussed laser at the moon, its beam will widen to well over 100 metres in radius by the time it reaches the surface. You can see this effect for yourself using an inexpensive laser pointer and a pair of binoculars on a clear night. Rather than pointing at the moon, point at a distant mountain or unoccupied structure (such as a water tower). The radius of your beam will increase as the distance increases. This is due to the diffraction.
The line of sight that we need in order to have an optimal wireless connection from A to B is more than just a thin line - its shape is more like that of a cigar, an ellipsoid. Its width can be described by the concept of Fresnel zones - see next section for an explanation. You will also find the abbreviation NLOS, for “non line of sight”, which is mostly used to describe and advertise technologies that allow for dealing with waves that reach the receiver through multiple trajectories (multipath) or diffraction. It does not indicate that the single electromagnetic beam goes “around corners” (other than through diffraction) or “through obstacles” any better than that of other technologies. For example, you might call White Space technology NLOS, as its lower frequencies (longer wavelengths) allow it to permeate objects and utilize diffraction much better than comparable 2.4 GHz or 5 GHz transmissions.
Understanding the Fresnel zoneEdit
The exact theory of Fresnel (pronounced “Fray-nell”) zones is quite complicated. However, the concept is quite easy to understand: we know from the Huygens principle that at each point of a wavefront new circular waves start, we know that microwave beams widen as they leave the antenna, we know that waves of one frequency can interfere with each other. Fresnel zone theory simply looks at a line from A to B, and then at the space around that line that contributes to what is arriving at point B. Some waves travel directly from A to B, while others travel on paths off axis and reach the receiver by reflection.
Consequently, their path is longer, introducing a phase shift between the direct and indirect beam.
Whenever the phase shift is one half wavelength, you get destructive interference: the signals cancel.
Taking this approach you find that when the reflected path is less than half a wavelength longer than the direct path, the reflections will add to the received signal. Conversely, when the reflected path length exceeds the direct path by more than one half wavelength, its contribution will decrease the received power.
Figure RP 11: The Fresnel zone is partially blocked on this link, although the visual line of sight appears clear.
Note that there are many possible Fresnel zones, but we are chiefly concerned with the first zone, because the contributions from the second zone are negative. The contributions from the third zone are positive again, but there is no practical way to take advantage of those without the penalty incurred in going through the second Fresnel Zone.
If the first Fresnel zone is partially blocked by an obstruction, e.g. a tree or a building, the signal arriving at the far end would be diminished. When building wireless links, we therefore need to be sure that the first zone is kept free of obstructions. In practice, it is not strictly necessary that the whole of this zone is clear, in wireless networking we aim to clear about 60 percent of the radius of the first Fresnel zone.
Here is one formula for calculating the radius of the first Fresnel zone:
…where r is the radius of the zone in metres, d1 and d2 are distances from the obstacle to the link end points in metres, d is the total link distance in metres, and f is the frequency in MHz.
The first Fresnel zone radius can also be calculated directly from the wavelength as:
with all the variables in metres
It is apparent that the maximum value of the first Fresnel zone happens exactly in the middle of the trajectory and its value can be found setting d1=d2=d/2 in the preceding formulas. Note that the formulae give you the radius of the zone, not the height above ground.
To calculate the height above ground, you need to subtract the result from a line drawn directly between the tops of the two towers.
For example, let’s calculate the size of the first Fresnel zone in the middle of a 2 km link, transmitting at 2.437 GHz (802.11b channel 6):
r = 7.84 metres
Assuming both of our towers were ten metres tall, the first Fresnel zone would pass just 2.16 metres above ground level in the middle of the link.
But how tall could a structure at that point be to block no more than 60% of the first zone?
r = 0.6 * 7.84 metres
r = 4.70 metres
Subtracting the result from 10 metres, we can see that a structure 5.3 metres tall at the centre of the link would block up to 40% of the first Fresnel zone.
This is normally acceptable, but to improve the situation we would need to position our antennas higher up, or change the direction of the link to avoid the obstacle.
Any electromagnetic wave carries energy - we can feel that when we enjoy (or suffer from) the warmth of the sun.
The amount of energy divided by the time during which we measure it is called power. The power P is measured in W (watts) and is of key importance for a wireless links to work: you need a certain minimum power in order for a receiver to make sense of the signal.
We will come back to details of transmission power, losses, gains and radio sensitivity in the chapter called Antennas/Transmission Lines.
Here we will briefly discuss how the power P is defined and measured.
The electric field is measured in V/m (potential difference per metre), the power contained within it is proportional to the square of the electric field:
P ~ E2
Practically, we measure the power in watts by means of some form of receiver, e.g. an antenna and a voltmetre, power metre, oscilloscope, spectrum analyser or even a radio card and laptop.
Looking at the signal’s power directly means looking at the square of the signal in volts and dividing by the electrical resistance.
Calculating with dB
By far the most important technique when calculating power is calculating with decibels (dB). There is no new physics hidden in this - it is just a convenient method which makes calculations a lot simpler.
The decibel is a dimensionless unit, that is, it defines a relationship between two measurements of power. It is defined by:
dB = 10 * Log (P1 / P0)
where P1 and P0 can be whatever two values you want to compare. Typically, in our case, this will be some amount of power.
Why are decibels so handy to use? Many phenomena in nature happen to behave in a way we call exponential.
For example, the human ear senses a sound to be twice as loud as another one if it has ten times the physical signal power.
Another example, quite close to our field of interest, is absorption. Suppose a wall is in the path of our wireless link, and each metre of wall takes away half of the available signal. The result would be:
0 metres = 1 (full signal)
1 metre = 1/2
2 metres = 1/4
3 metres = 1/8
4 metres = 1/16
n metres = 1/2n = 2-n
This is exponential behaviour.
But once we have used the trick of applying the logarithm (log), things become a lot easier: instead of taking a value to the n-th power, we just multiply by n. Instead of multiplying values, we just add.
Here are some commonly used values that are important to remember:
+3 dB = double power
-3 dB = half the power
+10 dB = order of magnitude (10 times power)
-10 dB = one tenth power
In addition to dimensionless dB, there are a number of definitions that are based on a certain base value P0. The most relevant ones for us are:
dBm relative to P0 = 1 mW
dBi relative to an ideal isotropic antenna
An isotropic antenna is a hypothetical antenna that evenly distributes power in all directions.
It is approximated by a dipole, but a perfect isotropic antenna cannot be built in reality. The isotropic model is useful for describing the relative power gain of a real world antenna.
Another common (although less convenient) convention for expressing power is in milliwatts. Here are equivalent power levels expressed in milliwatts and dBm:
1 mW = 0 dBm
2 mW = 3 dBm
100 mW = 20 dBm
1 W = 30 dBm
For more details on dB refer to the dB math lecture of the Wireless Training kit: http://wtkit.org/sandbox/groups/wtkit/wiki/820cb/attachments/ebdac/02 -dB_Math-v1.12_with-notes.pdf
Physics in the real worldEdit
Don’t worry if the concepts in this chapter seem challenging. Understanding how radio waves propagate and interact with the environment is a complex field of study in itself.
Most people find it difficult to understand phenomenon that they can’t even see with their own eyes.
By now you should understand that radio waves don’t travel only in a straight, predictable path.
To make reliable communication networks, you will need to be able to calculate how much power is needed to cross a given distance, and predict how the waves will travel along the way.
2. TELECOMMUNICATIONS BASICSEdit
The purpose of any telecommunications system is to transfer information from the sender to the receiver by a means of a communication channel.
The information is carried by a signal, which is certain physical quantity that changes with time.
The signal can be a voltage proportional to the amplitude of the voice, like in a simple telephone, a sequence of pulses of light in an optical fibre, or a radio-electric wave irradiated by an antenna.
For analog signals, these variations are directly proportional to some physical variable like sound, light, temperature, wind speed, etc. The information can also be transmitted by digital binary signals, that will have only two values, a digital one and a digital zero. Any analog signal can be converted into a digital signal by appropriately sampling and then coding it. The sampling frequency must be at least twice the maximum frequency present in the signal in order to carry all the information contained therein. Random signals are the ones that are unpredictable and can be described only by statistical means.
Noise is a typical random signal, described by its mean power and frequency distribution. A signal can be characterised by its behaviour over time or by its frequency components, which constitute its spectrum. Some examples of signals are shown in Figure TB 1.
Figure TB 1: Examples of signals
Any periodic signal is composed of many sinusoidal components, all of them multiples of the fundamental frequency, which is the inverse of the period of the signal. So a signal can be characterised either by a graph of its amplitude over time, called a waveform, or a graph of of the amplitudes of its frequency components, called a spectrum.
Figure TB 2: Waveforms, Spectrum and filters
Figure TB 2 shows how the same signal can be seen from two different perspectives. The waveform can be displayed by an instrument called an oscilloscope, while the spectrum can be displayed by what is called a Spectrum Analyzer. The spectrum distribution relays very important information about the signal and allows for the intuitive understanding of the concept of filtering of electrical signals. In the example shown, the signal is formed by the superposition of three sinusoidal components of frequency f1, f2 and f3. If we pass this signal through a device that will remove f2 and f3, the output is a pure sinusoidal at frequency f1.
We call this operation “Low Pass filtering” because it removes the higher frequencies. Conversely, we can apply the signal to a “High Pass Filter”, a device that will remove f1 and f2 leaving only a sinusoidal signal at the f3 frequency.
Other combinations are possible, giving rise to a variety of filters. No physical device can transmit all the infinite frequencies of the radio-electric spectrum, so every device will always perform some extent of filtering to the signal that goes through it.
The bandwidth of a signal is the difference between the highest and the lowest frequency that it contains and is expressed in Hz (number of cycles per second).
While travelling through the communication channel, the signal is subject to interference caused by other signals and is also affected by the electrical noise always present in any electrical or optical component. Intra-channel interference originates in the same channel as our signal. Co-channel interference is due to the imperfection of the filters that will let in signals from adjacent channels.
Consequently, the received signal will always be a distorted replica of the transmitted signal, from which the original information must be retrieved by appropriate means to combat the effect of interference and noise. Furthermore, the received signal will be subject to attenuation and delay that increase with the distance between the transmitter and the receiver.
Figure TB 3: Attenuation and delay
Although it is relatively simple to restore the amplitude of signal by means of an electrical amplifier, the components of the amplifier will add additional noise to the signal, so at very long distances where the received signal is feeble, the amplifier will produce a signal so garbled with noise that the information originally transmitted will no longer be retrievable.
One way to address this problem consists in converting the continuous quantity carrying the information into a sequence of very simple symbols which can be easier to recognise even at great distance. For instance, the flag of a ship is a convenient way to distinguish the nationality of the ship even at distances at which the letters on the hull cannot be read.
This technique has been extended to carry generalised messages by assigning different position of flags to every letter of the alphabet, in an early form of long distance telecommunications by means of digital or numeric signals.
The limitation of this method is obvious; to be able to distinguish among, say, 26 symbols corresponding to each letters of the alphabet, one must be quite close to the communicating ship.
On the other hand, if we code each letter of the alphabet in a sequence of only two symbols, these symbols can be distinguished at much longer distance, for example the dot and dashes of the telegraph system.
The process of transforming a continuous analog signal into a discontinuous digital one is called Analog to Digital Conversion (ADC), and conversely we must have a Digital to Analog Converter (DAC) at the receiving end to retrieve the original information.
This is the reason why most modern telecommunication systems use digital binary signals to convey all sorts of information in a more robust way. The receiver must only distinguish between two possible symbols, or in other words between two possible values of the received bit (binary digit). For instance, the CD has replaced the vinyl record, and analogue television is being replaced by digital television. Digital signals can use less bandwidth, as exemplified by the “digital dividend” currently being harnessed in many countries which consists in bandwidth that has become available thanks to the transition from analog to digital transmission in TV broadcasting.
Although in the process of converting from an analog to a digital information system there is always some loss of information, we can engineer the system so as to make this loss negligible.
Figure TB 4: Undersampled Image
For example, in a digital camera we can choose the number of bits used to record the image.
The greater the number of bits (proportional to the amount of megapixels), the better the rendering, but more memory will be used and longer time to transmit the image will be needed.
So most modern communication systems deal with digital signals, although the original variable that we want to transmit might be analog, like the voice. It can be shown that any analog signal can be reconstructed from discrete samples if the sampling rate is at least twice as high as the highest frequency content of the signal.
Figure TB 5: detection of a noisy signal
Then each sample is coded in as many bits as necessary to achieve the desired amount of precision.
These bits can now be efficiently stored or transmitted, since for the recovery of the information one needs to distinguish among only two states, and not among the infinite nuances of an analog signal.
This is shown in Figure TB 5, where the original data consists of the 0 1 0 1 1 1 0 sequence. The 0’s are represented as zero volts and the 1’s as 1 V. As the signal moves towards the receiver, its amplitude will diminish. This effect is called “attenuation” and is shown in the figure. Likewise, there will also be a delay as the signal moves from the transmitter to the receiver, the variability in the delay of the received signal is called jitter. Attenuation, noise or jitter (or their combination) if severe enough, can cause a detection error. An amplifier can be used to overcome the attenuation, but the electrical noise always present in the system will add to the received signal.
The noisy received signal is therefore quite different from the original signal, but in a digital system we can still recover the information contained by sampling the received signal at the correct time and comparing the value at the sampling time with a suitable threshold voltage. In this example the noise received signal has a peak of 1.8 V, so we might choose e threshold voltage of 1.1 V. If the received signal is above the threshold, the detector will output a digital 1, otherwise, it will output a 0. In this case we can see that because of the effect of the noise the fifth bit was erroneously detected as a zero.
Transmission errors can also occur if the sampling signal period is different from that of the original data (difference in the clock rates), or if the receiver clock is not stable enough (jitter).
Any physical system will have an upper limit in the frequencies that will transmit faithfully (the bandwidth of the system), higher frequencies will be blocked, so the abrupt rise and fall of the voltage will be smoothed out as the signal goes through the channel.
Therefore, we must make sure that each of the elements of the system has enough bandwidth to handle the signal. On the other hand, the greater the bandwidth of the receiver system, the greater the amount of the noise that will affect the received signal.
The robustness of the digital signal is also exemplified by the fact that it was chosen for the first trials of radio transmission. Marconi showed the feasibility of long distance transmission, but pretty soon realised that there was a need to share the medium among different users.
This was achieved by assigning different carrier frequencies which were modulated by each user’s message. Modulation is a scheme to modify the amplitude, frequency or phase of the carrier according with the information one wants to transmit. The original information is retrieved at destination by the corresponding demodulation of the received signal.
Figure TB 6: Sinusoidal Carrier Signal
Figure TB 6 shows a carrier signal with Amplitude A, phase θ, and frequency fo which is the reciprocal of the period T.
The combination of different modulation schemes has resulted in a plethora of modulation techniques depending on which aspect one wants to optimise: robustness against noise, amount of information transmitted per second (capacity of the link in bits/second) or spectral efficiency (number of bits/s per Hertz).
For instance, BPSK -Binary Phase Shift Keying- is a very robust modulation technique but transmits only one bit per symbol, while 256 QAM -Quaternary Amplitude Modulation- will carry 8 bits per symbol, thus multiplying by a factor of eight the amount of information transmitted per second, but to correctly distinguish amongst the 256 symbols transmitted, the received signal must be very strong as compared with the noise (a very high S/N -Signal/Noise ratio- is required).
The ultimate measure of quality in digital transmission is the BER -Bit Error Rate- which corresponds to the fraction of erroneously decoded bits. Typical values of BER range between 10-3 and 10-9.
The modulation also allows us to choose which range of frequency we want to use for a given transmission. All frequencies are not created equal and the choice of the carrier frequency is determined by legal, commercial and technical constraints.
Multiplexing and duplexingEdit
In general, the sharing of a channel among different users is called multiplexing.
This is shown in Figure TB 7.
Figure TB 7: Multiplexing
Assigning different carrier frequencies to different users is called FDMA -Frequency Division Multiple Access-.
An alternative technique consists in assigning different time slots to different users, in what is known as TDMA -Time Division Multiple Access-, or even different codes in CDMA -Code Division Multiple Access- where the different users are recognised at the receiver by the particular mathematical code assigned to them. See Figure TB 8.
By using two or more antennas simultaneously, one can take advantage of the different amount of fading introduced in the different paths to the receiver establishing a difference among users in what is known as SDMA - Space Division Multiple Access-, a technique employed in the MIMO -Multiple Input,Multiple Output- systems that have gained popularity recently.
Figure TB 8: Medium Sharing techniques
Most communication systems transmit information in both directions, for instance from the Base Station to the subscriber in what is called the downlink, and from the subscriber to the base station in the uplink.
To accomplish this, the channel must be shared between the two directions giving rise respectively to FDD -Frequency Division Duplexing- and TDD -Time Division Duplexing-.
The communication system must overcome the noise and interference to deliver a suitable replica of the signal to the receiver.
The capacity of the communication channel in bits/second is proportional to the bandwidth in Hz and to the logarithm of the S/N ratio.
Modulation is used to adapt the signal to the channel and to allow several signals to share the same channel. Higher order modulation schemes allow for a higher transmission rate, but require higher S/N ratio.
The channel can be shared by several users that occupy different frequencies, different time slots, different codes or by taking advantage of different propagation characteristics in what is called spatial multiplexing.
For more information and slides covering this topic please visit http://wtkit.org/groups/wtkit/wiki/820cb/download_page.html
3. LICENSING AND REGULATIONEdit
There are a number of areas where national and international laws and regulations can influence your ability to set up wireless networks.
Since these rules vary from country to country it is impossible to give an overview of which regulations may apply in your region.
It is also worth noting that there may be a huge difference in which laws exist, and how they are regulated in practice. In other words there may be countries where using the 2.4 GHz / 5 GHz spectrum for outdoor wireless is technically illegal, but where everyone does it anyway.
As a rule of thumb, if other people are building similar networks to what you intend, contact them and find out what legal issues they may have run into. If such networks are very widely deployed in your country, then you probably don’t need to worry too much. On the other hand, it is always advisable to seek local advice, from hardware vendors, wireless experts or others who have come before you, before committing time and resources to building a wireless network. Whatever you do, it is important you take local laws and regulations into consideration.
Examples of relevant types of regulationEdit
Each country may have different rules, and each scenario may come across different types of regulations. The areas where regulations may be relevant include licenses for using specific radio frequencies, rules regarding the right to install towers for antennas, the maximum power allowed and telecom licensing rules limiting your ability to provide Internet access to others.
The types of legal issues that may (or may not) be worth considering when planning a wireless network include:
- Spectrum Licensing
- ISP/Telecommunications Licenses
- Tower permits for antennas
- Transmission power and antenna gain limits
- Certification of equipment
Most countries consider RF spectrum as an exclusive property of the state. The RF spectrum is a national resource, much like water, land, gas and minerals. Unlike these, however, RF is reusable. The purpose of spectrum management is to mitigate radio spectrum pollution and maximize the benefit of usable radio spectrum.
The first sentence of the International Telecommunications Union (ITU) constitution fully recognises “the sovereign right of each State to regulate its telecommunication”. Effective spectrum management requires regulation at national, regional and global levels.
Licensing is an orderly way to manage who, when, where and how spectrum resource is used. The unlicensed wireless spectrum was set around the 2.4 GHz band.
In June 2003, the ITU made available the 5 GHz band for license-exempt technology deployment. The 900 MHz band, unlicensed in the United States, is presently used in Western Europe and in many developing countries for GSM phones. Each country has the sovereign right to regulate its telecommunication and to interpret the international Radio Regulations. Governments define the rules and conditions of the frequency use.
(From: Wikipedia “Spectrum Management”)
The technologies described in this book (mostly) use a license-exempt slice of the spectrum referred to as the ISM (Industrial, Scientific and Medical radio bands). Radio frequencies in the ISM bands have been used for communication purposes, although such devices may experience interference from non-communication sources.
The ISM bands are defined by the ITU-R (ITU’s Radiocommunication Sector) at 2.4 and 5 GHz. Individual countries’ use of the bands designated in these sections may differ due to variations in national radio regulations. Because communication devices using the ISM bands must tolerate any interference from ISM equipment, unlicensed operations are typically permitted to use these bands, since unlicensed operation typically needs to be tolerant of interference from other devices anyway.
In the US, the FCC (Federal Communications Commission) first made unlicensed spread spectrum available in the ISM bands in rules adopted on May 9, 1985. Many other countries later adopted these FCC regulations, enabling use of this technology in many countries.
(From: Wikipedia “ISM Band”)
In some countries an ISP license would be required before deploying any network infrastructure for sharing networks over public spaces. In other countries this would only be required to run commercial networks.
Tower permits for antennas
When deploying long-range outdoor networks, it is often necessary to build a tower for the antenna. Many countries have regulations regarding the building of such antenna-towers if they are more than 5 or 10 metres above the roof or ground.
Transmission Power limits
When setting transmission power limits, regulatory agencies generally use the Equivalent Isotropically Radiated Power (EIRP), as this is the power actually radiated by the antenna element. Power limits can be imposed on output power of devices as well.
As an example, the FCC enforces certain rules regarding the power radiated by the antenna element, depending on whether the implementation is point-to-multipoint (PtMp) or point-to-point (PtP). It also enforces certain rules regarding the maximum power transmitted by the radio.
When an omnidirectional antenna is used, the FCC automatically considers the link a PtMP link. In the setup of a 2.4 GHz PtMP link, the FCC limits the EIRP to 4 Watts and the power limit set for the intentional radiator is 1 Watt.
Things are more complicated in the 5 Ghz band. The Unlicensed National Information Infrastructure (U-NII) radio band is part of the radio frequency spectrum used by IEEE-802.11a devices and by many wireless ISPs. It operates over three ranges:
U-NII Low (U-NII-1): 5.15-5.25 GHz. Regulations require use of an integrated antenna. Power limited to 50 mW.
U-NII Mid (U-NII-2): 5.25-5.35 GHz. Regulations allow for a user-installable antenna, subject to Dynamic Frequency Selection (DFS, or radar avoidance). Power limited to 250 mW.
U-NII Worldwide: 5.47-5.725 GHz. Both outdoor and indoor use, subject to Dynamic Frequency Selection (DFS, or radar avoidance). Power limited to 250 mW.
This spectrum was added by the FCC in 2003 to “align the frequency bands used by U-NII devices in the United States with bands in other parts of the world”.
The FCC currently has an interim limitation on operations on channels which overlap the 5600 - 5650 MHz band.
U-NII Upper (U-NII-3): 5.725 to 5.825 GHz.
Sometimes referred to as U-NII / ISM due to overlap with the ISM band. Regulations allow for a user-installable antenna. Power limited to 1W.
Wireless ISPs generally use 5.725-5.825 GHz.
(From: Wikipedia “U-NII”)
For PtP in the 5 GHz band the maximum EIRP allowed is considerable higher, since a high gain antenna produces a very narrow beam and therefore the interference caused to other users is considerably less than in PtMPt topology.
Certification of equipment
Governments may require a formal certification that a given radio equipment comply with specific technical standards and local regulations.
This is often referred to as homologation, and the process must be done by an independent laboratory authorised by the government of the country.
Certified equipment is allowed to operate without an individual license. It is worth noting that certification may only apply to the original factory state for radio equipment.
For example, changing the antenna on a wireless access point in the United States invalidates the FCC Certification.
There may also be commercial grade connections that do not have these limitations.
It is important to note that this is NOT a legal issue, but a clause of the contract with the ISP, and the repercussions for breaching these is usually a disconnection of the Internet connection.
4. RADIO SPECTRUMEdit
What is the electromagnetic spectrum?Edit
There is not a simple definition of the spectrum. From the technical viewpoint the spectrum is simply the range of electromagnetic waves that can be used to transmit information, but from the practical viewpoint the economic and political aspects, as well as the technology actually used to convey the information by means of these waves, play pivotal roles.
As an example, when Marconi in 1902 first spanned the Atlantic with his “wireless telegraph message”, he used the whole spectrum available at the time to send a few bits/s over an area of thousands of square kilometres.
With the spark transmitter used for this achievement that occupied all the frequencies that the existing receivers were able to understand, nobody else could use radio for communications on a radius of some 3500 km from the transmitting station in England. So, if other users wanted to send messages in the same area, they would need to coordinate their transmissions in different “time slots” in order to share the medium. This technique is called “TDMA”, Time Division Multiple Access.
Users located at distances much greater than 3500 km from Marconi’s transmitter could use the spectrum again, since the power of the radio waves decreases as we move farther away from the transmitter. Reusing the spectrum in different geographical areas is called “SDMA”, Space Division Multiple Access. Marconi was later able to build a transmitter that could restrict emissions to just a range of frequencies, and a receiver that could be “tuned” to a particular frequency range. Now, many users could transmit simultaneously in the same area (space) and at the same time. “FDMA”, Frequency Division Multiple Access was borne. Radio then became a practical means of communications, and the only one that was available to reach a ship in the open seas. The coordination of the frequencies allocated to different users was done by national agencies created to this effect, but since radio waves are not stopped by national borders, international agreements were needed. The international organization that had been created to regulate the transmission of telegrams among different countries was commissioned to allocate the use of the electromagnetic spectrum.
Nowadays, ITU, International Telecommunications Union, is the oldest International Organization, tasked with issuing recommendations about which frequencies to use for which services to its 193 nation members.
The use of the spectrum for military applications raised a new issue; “jamming”, the intentional interference introduced by the enemy to impede communication. To avoid jamming, a new technique was developed in which the information to be transmitted was combined with a special mathematical code; only receivers with the knowledge of that particular code could interpret the information. The coded signal was transmitted at low power but using a very wide interval of frequencies to make jamming more difficult.
This technique was later adapted to civilian applications in what is called “CDMA”, Code Division Multiple Access, one of the flavours of spread spectrum communication, extensively used in modern communications systems. In summary, the spectrum can be shared among many users by assigning different time slots, different frequency intervals, different regions of space, or different codes. A combination of these methods is used in the latest cellular systems. Besides issues of sovereignty and its defence, very strong economic and political interests play a determinant role in the management of the spectrum, which also needs to be constantly updated to take advantage of the advances in the communications technology.
Telecommunications engineers keep finding more efficient ways to transmit information using time, frequency and space diversity by means of ever advancing modulation and coding techniques. The goal is to increase the “spectrum efficiency”, defined as the amount of bits per second (bit/s) that can be transmitted in each Hz of bandwidth per square kilometre of area. For example, the first attempts to provide mobile telephone services were done by using a powerful transmitter, conveniently located to give coverage to a whole city.
This transmitter (called a Base Station in this context), divided the allocated frequency band into say, 30 channels. So only 30 conversations could be held simultaneously in the whole city.
As a consequence, the service was very expensive and only the extremely wealthy could afford it. This situation prevailed for many years, until the advances in electronic technology allowed the implementation of a scheme to take advantage of “Space Diversity”.
Instead of using a single powerful transmitter to cover the whole city, the area to be serviced was divided into many “cells”, each one served by a low power transmitter. Now cells that are sufficiently apart can utilize the same channels without interference, in what is known as “frequency reuse”. With the cellular scheme, the first 10 channels are to use frequency band 1, the second 10 channels frequency band 2 and the remaining 10 channels frequency band 3. This is shown in Figure RS 1, in which the colours correspond to different frequency bands. Notice that the colours repeat only at distances far enough to avoid interference. If we divide the city in say, 50 cells, we can now have 10X50 = 500 simultaneous users in the same city instead of 30. Therefore, by adding cells of smaller dimensions (specified by lower transmission power) we can increase the number of available channels until we reach a limit imposed by the interference.
Figure RS 1: Cellular sharing of spectrum
This example shows that a clever use of existing resources can dramatically increase its usefulness. Although the main use of the spectrum is for communication purposes, there are also other uses, like cooking food in microwave ovens, medical applications, garage door openers and so on.
So some frequency bands are allocated for these purposes in what is known as the ISM (Industrial, Scientific and Medical) bands.
This spectrum usage is normally for short distance applications.
A breakthrough occurred in 1985 when the FCC (Federal Commission of Communications), the agency that oversees the spectrum in the U.S., allowed the use of this spectrum for communications applications as well, provided that the transmission power was kept to a very low level to minimize interference.
People could freely use these “Unlicensed” bands without previously applying for a permit, provided that the equipment used had been certified by an authorized laboratory that ensured compliance with interference mitigation measures.
Several manufacturers began taking advantage of this opportunity by offering equipment that could be used to communicate between computers without the need for cables, and some wireless data networks covering significant geographic areas were built with them, but the turning point happened after the 1997 approval of the IEEE (Institute of Electrical and Electronics Engineers) 802.11 Standard, the basis of what is known as WiFi.
The existence of a standard that guaranteed the interoperability of equipment produced by different manufacturers fuelled an impressive growth of the market, which in turn drove the competition that fostered a dramatic decrease in the cost of the devices.
In particular, the portion of the ISM band between 2400 and 2483 MHz is nowadays available in most of the world without the need for previously applying for a license and is widely used by laptops, tablets, smart phones and even photographic cameras.
It is important to stress the role of the unlicensed spectrum in the enormous success of WiFi high speed Internet access.
Many airports, hotels and cafes all over the world offer free WiFi Internet access on their premises, and low cost wireless community networks have been built both in rural area and in cities covering considerable geographic areas, thanks to the availability of free spectrum.
Mobile phone operators, who have to pay dearly for frequency licenses to use the spectrum, were quite hostile to this apparently unfair competition.
But when they started offering smart phones, which make very intensive use of the Internet, they pretty soon realized that off-loading the traffic to WiFi was in their best interest, because it relieved the traffic in their distribution network (known as the backhaul).
So now they encourage their customers to use WiFi wherever it is available and use the more expensive cellular service only when out of range of any WiFi Access Point.
This is a remarkable example of the usefulness of the unlicensed spectrum even to traditional telecommunications operators who often have lobbied against it.
How is the spectrum adjudicated?Edit
Currently the main methods to gain access to a given spectrum band are auctions and the so called “beauty contest”.
The auction method is straightforward; interested parties bid for a given spectrum chunk; whoever commits the higher sum gets the right to use the frequencies.
In theory this method guarantees that the adjudication will be transparent, in practice this has often been circumvented and there have been instances of powerful commercial interests that acquire frequencies only to avoid their use by the competition, with the result of highly valuable spectrum not being used.
Also there is the temptation on the part of governments to use this method as a means to generate revenues and not necessarily in the best public interest.
As an example, in the year 2000 there were auctions in several countries of Europe to adjudicate spectrum for mobile phones, which resulted in a total income of 100 billion (100 000 000 000) euros to the government coffers.
The “beauty contest” method is for the interested parties to submit proposals about how they intend to use the spectrum.
A committee of the spectrum regulating agency then decides which of the proposals better serves the public goals.
This method relies on the objectivity, technical proficiency and honesty of the members of the deciding committee, which is not always guaranteed.
In many countries there are rules for spectrum adjudication that call for the relinquishing of spectrum bands that have been acquired but are not being used; however their enforcement is often lacking due to the strong economic interests affected.
Figure RS 2 shows a photograph of a spectrum monitoring vehicle in Montevideo, Uruguay and Figure RS 3 that of the same kind of equipment being used in Jakarta, Indonesia.
Figure RS 3: The “spectrum Police” at work in Jakarta
Note that the open spectrum used in the unlicensed bands cannot prevent interference issues, especially in very crowded areas, but nevertheless it has proved a fantastic success for short distance applications in cities and also for long distance applications in rural areas.
It is therefore advisable to investigate new forms of spectrum allocation, taking into consideration the needs of many stakeholders and striking a balance among them.
A dynamic spectrum allocation mechanism seems to be the best choice given the advances in technology that make this viable nowadays.
As an example, the current method of spectrum allocation is similar to the railway system, the railroads can be idle a considerable amount of time, whereas the dynamic spectrum allocation is akin to the freeway system that can be used at all times by different users.
The importance of the spectrum as a communications enabler cannot be overstated. Television and radio broadcasting have a strong influence in shaping public perception of any issue, and have been used overtly for political propaganda (It has been said that the election of Kennedy as president of the U.S. was due mainly to his television campaign).
During the cold war, The Voice of America, Moscow Radio and Radio Havana Cuba were very effective ways to sway a global audience.
More recent examples include the influence of CNN and Al Jazeera in the public interpretation of the Arab Spring. Spectrum used for two way communications has also been subject to government interventions, especially in cases of political unrest. On the other hand, economic interests also play a vital role in broadcasting; the consumer society relies heavily on radio and television to create artificial needs or to veer the consumer towards a particular brand. We can conclude that the electromagnetic spectrum is a natural resource whose usefulness is heavily conditioned by technological, economic and political factors.
Explosion in spectrum demandEdit
As the number of tablets and smart phones grows, telecom operators are trying to get access to new frequency bands, but the traditional way of adjudicating the spectrum is facing a dead end.
Keep in mind that the spectrum is used for radio and television broadcasts, for satellite communications, for airplane traffic control, for geolocalisation (Global Postioning Systems-GPS), as well as for military, police and other governmental purposes. Traditionally, the demand for additional spectrum has been met thanks to the advances of electronics that have permitted the use of higher frequencies at an affordable cost. Higher frequencies are well suited for high speed transmissions, but they have a limited range and are highly attenuated by walls and other obstacles as well as by rain.
This is exemplified by comparing the coverage of an AM radio broadcasting station to that of an FM one: the greatest range of the AM station is due to its use of lower frequencies. On the other hand, FM stations can make use of higher bandwidths and as consequence can offer greater audio quality at the expense of a more limited range.
Current cellular operators use even higher frequencies, usually above 800 MHz. Accordingly, the TV broadcasting frequencies are coveted by the cellular telephone providers, because by using lower frequencies they will need less base stations, with huge savings in deployment, operation and maintenance costs. This is why these frequencies are commonly referred to as “beach front property”.
Techniques for more efficient spectrum usage by means of advanced modulation and coding methods have had the greatest impact in allowing more bits/s per Hz of bandwidth availability. This, in turn, was made possible by the great strides in electronics (fabrication of ever advanced integrated circuits) that now make it economically feasible to implement the required sophisticated modulation and coding techniques.
According to the calculations performed in 1948 by Claude Shannon - the father of modern telecommunications, a typical telephone line could carry up to 30 kbit/s. But this was only achieved in the 90’s when integrated circuits implementing the required techniques were actually built. In particular, the transition to digital terrestrial television broadcasting, which is more efficient in spectrum usage compared with analogue transmission, has freed some spectrum in the so called “White Spaces”, the frequencies that had to be left fallow in between analogue television channels to prevent interference.
In traditional analogue TV broadcasting, adjacent channels cannot be used at the same time, because the signal from one channel will “spill” over to the two adjacent channels and would cause interference. This is similar to the central reservation used in freeways to separate the two directions of traffic in order to prevent collisions. So a “White Space” must be left between two contiguous analogue TV channels to prevent interference. Digital TV broadcasting is much more efficient in spectrum utilization, and several digital TV channels can be accommodated in the same frequency band formerly used by a single analogue channel without “spillover” into adjacent channels. So, in places where Analog TV is replaced by Digital TV a “digital dividend” is being harvested.
In conclusion, the concept of white spaces can be applied to three different frequency chunks:
- The spectrum that has been assigned to TV broadcasting but it is not currently being used. This applies particularly to developing countries, in which there has been no economic incentive for broadcaster to use every available TV channel.
- The spectrum that must be left free in between two analogous TV channels to prevent interference.
- The spectrum that has been reclaimed as a consequence of the transition to digital terrestrial TV, which is more spectrum efficient. This currently applies to developed countries, but will soon apply to developing countries as well.
In the last 20 years there has been a tremendous growth in the demand for more spectrum for mobile communication services, in which data
services are consuming much more bandwidth than voice and the growing use of video is presenting an additional challenge.
Not surprisingly, telecom operators everywhere are trying to get a portion of these “White Spaces” allocated to them to fulfill their needs. Broadcasters, on the other hand, are very reluctant to concede any spectrum at all to what are now their direct competitors.
Spectrum scarcity or spectrum hoarding?Edit
Although the available spectrum is currently totally adjudicated in developed countries, many independent studies have found that the actual simultaneous usage of the spectrum is a tiny fraction of the total. This is caused by the way spectrum was originally adjudicated and also because often spectrum is used intermittently; for instance some TV broadcasting stations do not transmit 24 hours a day.
As a consequence, a radically new way to use the spectrum has been suggested; instead of leasing spectrum to a given organization in an exclusive basis, the new dynamic spectrum management paradigm proposes to use whatever spectrum is available in a certain place at a certain time and switch to another frequency whenever interference is detected in a given band.
An analogy can be made to explain this concept: the current way to allocate the spectrum is similar to a railroad system; the railroads are never used 100% of the time, a more efficient use of the same amount of terrain can be done with a highway in which many different users can share the same path according to their current needs.
Of course to implement dynamic spectrum access requires new technologies and new legislation; many vested interests are fighting it alleging the possibility of interference. The key issue is how to determine when a particular chunk of spectrum is really being used in a particular place and how to move quickly to a new frequency band when an existing user with higher priority is detected. The technology to accomplish this feat has already being demonstrated and implemented in the new IEEE 802.22 standard recently approved, as well as in others currently being considered.
Stimulated by the impressive success of WiFi (due mostly to the use of unlicensed, open spectrum), the IEEE created a working group to address the requirements of a Wireless Regional Area Network. The challenge was to develop a technology suitable for long distance transmission that could be deployed in different countries (with quite different spectrum allocations), so they focused on the spectrum currently allocated to TV broadcasting which spans approximately from 50 to 800 MHz. Nowhere is this spectrum being used in its entirety all the time, so there are “White Spaces”, fallow regions that could be “re-farmed” and put to use for bidirectional communications. In rural areas all over the world, but specially in developing countries there are large portions of spectrum currently under utilised. It is expected that IEEE 802.22 will enable dynamic spectrum access in a similar way to IEEE 802.11 (WiFi), allowing access to open spectrum. Of course not all the spectrum can be liberated at once, a gradual process is required as the many technical, legal, economic and political hurdles are solved, but there is no doubt that this is the trend and that IEE 802.22 paves the way to the future of spectrum allocation. In order to assess the availability of a given frequency channel at a given time two methods are being considered: channel sensing and a database of primary users in a given geographic location at a given time.
Channel sensing means that prior to an attempt to use a channel, the base stations will listen to the channel; if it is being used it will try another one, repeating the procedure until a free channel is found. This procedure is repeated at regular intervals to account for the possibility of stations coming alive at any time. This method should suffice, nevertheless current spectrum holders have successfully lobbied the regulators to enforce the implementation of the second method, which is much more complicated and imposes additional complexity and costs in the consumer equipment.
The second method consists in the building of a database of all the existing incumbent transmission stations, with their position and respective coverage area in order to establish an “off limit” zone in a given channel.
A new station wishing to transmit must first determine its exact position (so it must have a GPS receiver or other means to determine its geographic location) and then interrogate the database to ascertain that its present location is not in the forbidden zone of the channel it is attempting to use.
To interrogate the database, it must have Internet access by some other means (ADSL-Asymmetrical Digital Subscriber Loop, Cable, Satellite, or Cellular), besides the 802.22 radio (which cannot be used until the channel is confirmed as usable), so this adds a considerable additional burden to the station hardware which translates into additional cost, beside the cost of building and maintaining the database.
In the US the FCC (Federal Communications Commission, the spectrum regulatory agency) has been promoting the building of the database of registered users and have authorised 10 different private enterprises to build, operate and maintain such repositories.
Furthermore, field trials of the standard are been conducted. In the UK, OFCOM (the spectrum regulator) is also conducting IEE 802.22 trials and concentrating on the database method having ruled out the spectrum sensing method for interference mitigation. Although IEEE 802.22 is the formally approved standard that has received the most publicity, there are several competing candidates that are being explored to leverage the TV White Spaces to provide two-way communication services, among them:
This amendment takes advantage of the enormous success of IEEE 802.11 by adapting the same technology to work in the frequency bands allocated to TV transmission, thus relieving the spectrum crowding of the 2.4 GHz band and offering greater range due to the use of lower transmission frequencies. Its details are still being discussed by the corresponding IEEE 802.11 working group.
This amendment of the 802.16 standard was ratified in 2010 and describes the mechanism for implementing the protocol in uncoordinated operation, licensed or license exempt applications. Although most deployments have been in the 5 GHz band, it can also be applied to the TV band frequencies and can profit from the significant deployments of WiMAX (Wireless Microwave Access) systems in many countries.
Developing countries advantageEdit
It is noteworthy that in developing countries the spectrum allocated to broadcast television is only partially used. This presents a magnificent opportunity to introduce wireless data networking services in the channels that are not currently allocated, and to start reaping the benefits of 802.22 in a more benign environment, where the spectrum sensing and agile frequency changing required to share the crowded spectrum in developed countries can be dispensed with. The usefulness of the lower frequencies for two-way data communications has been proved by the the successful deployment of CDMA (Code Division Modulation Access) cellular systems in the 450 MHz band, right in the middle of the TV allocated frequencies, in rural areas like the Argentinian Patagonian, currently served by “Cooperativa Telefonica de Calafate-COTECAL”. COTECAL offer voice and data services to customers at distances up to 50 km from the Base Station, in the beautiful area shown in the figure below:
Figure RS 5: Region served with voice and data services by COTECAL, in Calafate and El Chalten, Argentina.
So there is an opportunity for stakeholders to lobby for the introduction of TV Band Device based solutions at an early stage, while the issues of the digital transition are being considered. This will help ensure that commercial interests of a few do not prevail over the interests of society at large. Activists/lobbyists should emphasize the need for transparency in the frequency allocation process and for accountability of the administration of spectrum in their country or region.
Also it is important that those who wish to deploy networks gain an understanding of the real spectrum usage by spectrum holders in each region of their country. The monitoring of the spectrum requires expensive instruments with a steep learning curve to use them, but recently an affordable and easy to use device has become available that permits analysis of the frequency band between 240 MHz and 960 MHz, which encompasses the higher part of the TV band. Details of this open hardware based RF Explorer Spectrum Analyzer for the upper TV band are at:
Figure RS 6 shows the RF Explorer for the 2.4 GHz band being used to test an antenna built by participants of the 2012 ICTP Wireless training workshop in Trieste, Italy.
Figure RS 6: Participants from Albania, Nepal, Malawi and Italy testing an antenna with the RF Explorer Spectrum Analyzer in Trieste, February 2012.
This low cost instrument paves the way for a wide involvement of people in the measurement of the real spectrum usage on their own country which hopefully can lead to a better spectrum management.
For additional information see:
5. ANTENNAS / TRANSMISSION LINESEdit
The transmitter that generates the RF power to drive the antenna is usually located at some distance from the antenna terminals. The connecting link between the two is the RF transmission line. Its purpose is to carry RF power from one place to another, and to do this as efficiently as possible. From the receiver side, the antenna is responsible for picking up any radio signals in the air and passing them to the receiver with the minimum amount of distortion and maximum efficiency, so that the radio has its best chance to decode the signal. For these reasons, the RF cable has a very important role in radio systems: it must maintain the integrity of the signals in both directions.
Figure ATL 1: Radio, transmission line and antenna
The simplest transmission line one can envisage is the bifilar or twin lead, consisting of two conductors separated by a dielectric or insulator. The dielectric can be air or a plastic like the one used for flat transmission lines used in TV antennas. A bifilar transmission line open at one end will not radiate because the current in each wire has the same value but opposite direction, so that the fields created on a given point at some distance from the line cancel.
Figure ATL 2: Bifilar transmission line
If we bend the open ends of the transmission line in opposite directions, the currents will now generate electric fields that are in phase and reinforce each other and will therefore radiate and propagate at a distance. We now have an antenna at the end of the transmission line.
Figure ATL 3: Antenna from transmission line
The length of the bent portion of the transmission line will determine the antenna feature. If this length corresponds to a quarter of a wavelength we will have a half wave dipole antenna with a gain of 2.15 dBi.
The functioning of the bifilar transmission line just described is strongly affected by any metal in its proximity, so a better solution is to confine the electrical fields by means of an external conductor that shields the internal one. This constitutes a coaxial cable. Alternatively, a hollow metallic pipe of the proper dimensions will also effectively carry RF energy in what is known as a waveguide.
For frequencies higher than HF the coaxial cables (or coax for short, derived from the words “of common axis”) are used almost exclusively. Coax cables have a core conductor wire surrounded by a non-conductive material called dielectric, or simply insulation.
The dielectric is then surrounded by an encompassing shielding which is often made of braided wires. The dielectric prevents an electrical connection between the core and the shielding. Finally, the coax is protected by an outer casing which is generally made from a PVC material.
The inner conductor carries the RF signal, and the outer shield prevents the RF signal from radiating to the atmosphere, and also prevents outside signals from interfering with the signal carried by the core. Another interesting fact is that high frequency electrical signal travels only along the outer layer of a conductor, the inside material does not contribute to the conduction, hence the larger the central conductor, the better the signal will flow. This is called the “skin effect”.
Figure ATL 4: Coaxial cable with jacket, shield, dielectric, and core conductor.
Even though the coaxial construction is good at transporting the signal, there is always resistance to the electrical flow: as the signal travels along, it will fade away.
This fading is known as attenuation, and for transmission lines it is measured in decibels per metre (dB/m).
The rate of attenuation is a function of the signal frequency and the physical construction of the cable itself. As the signal frequency increases, so does its attenuation.
Obviously, we need to minimise the cable attenuation as much as possible by keeping the cable very short and using high quality cables.
Here are some points to consider when choosing a cable for use with microwave devices:
- ## 1.“The shorter the better!” The first rule when you install a piece of cable is to try to keep it as short as possible. The power loss is not linear, so doubling the cable length means that you are going to lose much more than twice the power. In the same way, reducing the cable length by half gives you more than twice the power at the antenna. The best solution is to place the transmitter as close as possible to the antenna, even when this means placing it on a tower.
- 2.“The cheaper the worse!” The second golden rule is that any money you invest in buying a good quality cable is a bargain. Cheap cables can be used at low frequencies, such as VHF. Microwaves require the highest quality cables available.
- 3.Avoid RG-58. It is intended for thin Ethernet networking, CB or VHF radio, not for microwave.
- 4.Avoid RG-213 or RG-8. They are intended for CB and HF radio. In this case even if the diameter is large the attenuation is significant due to the cheap insulator used.
- 5.Whenever possible, use the best rated LMR cable or equivalent you can find. LMR is a brand of coax cable available in various diameters that works well at microwave frequencies. The most commonly used are LMR-400 and LMR-600. Heliax cables are also very good, but expensive and difficult to use.
- 6.Whenever possible, use cables that are pre-crimped and tested in a proper lab. Installing connectors to cable is a tricky business, and is difficult to do properly even with the specific tools. Never step over a cable, bend it too much, or try to unplug a connector by pulling the cable directly. All of these behaviours may change the mechanical characteristic of the cable and therefore its impedance, short the inner conductor to the shield, or even break the line.
- 7.Those problems are difficult to track and recognise and can lead to unpredictable behaviour on the radio link.
- 8.For very short distances, a thin cable of good quality maybe adequate since it will not introduce too much attenuation.
Above 2 GHz, the wavelength is short enough to allow practical, efficient energy transfer by different means. A waveguide is a conducting tube through which energy is transmitted in the form of electromagnetic waves. The tube acts as a boundary that confines the waves in the enclosed space. The Faraday cage phenomenon prevents electromagnetic effects from being evident outside the guide. The electromagnetic fields are propagated through the waveguide by means of reflections against its inner walls, which are considered perfect conductors. The intensity of the fields is greatest at the center along the X dimension, and must diminish to zero at the end walls because the existence of any field parallel to the walls at the surface would cause an infinite current to flow in a perfect conductor.
The X, Y and Z axis of a rectangular waveguide can be seen in the following figure:
Figure ATL 5: The X, Y, and Z axis of a rectangular waveguide.
There are an infinite number of ways in which the electric and magnetic fields can arrange themselves in a waveguide for frequencies above the low cutoff. Each of these field configurations is called a mode. The modes may be separated into two general groups. One group, designated TM (Transverse Magnetic), has the magnetic field entirely transverse to the direction of propagation, but has a component of the electric field in the direction of propagation. The other type, designated TE (Transverse Electric) has the electric field entirely transverse, but has a component of magnetic field in the direction of propagation.
The mode of propagation is identified by the group letters followed by two subscript numerals. For example, TE 10, TM 11, etc.
The number of possible modes increases with the frequency for a given size of guide, and there is only one possible mode, called the dominant mode, for the lowest frequency that can be transmitted. In a rectangular guide, the critical dimension is X. This dimension must be more than 0.5 λ at the lowest frequency to be transmitted. In practice, the Y dimension is usually about 0.5 X to avoid the possibility of operation in other than the dominant mode. Cross-sectional shapes other than the rectangle can be used, the most important being the circular pipe. Much the same considerations apply as in the rectangular case. Wavelength dimensions for rectangular and circular guides are given in the following table, where X is the width of a rectangular guide and r is the radius of a circular guide. All figures apply to the dominant mode.
Type of guide
Longest wavelength transmitted with little attenuation
Shortest wavelength before next mode becomes possible
Energy may be introduced into or extracted from a waveguide by means of either the electric or magnetic field. The energy transfer typically happens through a coaxial line. Two possible methods for coupling to a coaxial line are using the inner conductor of the coaxial line, or through a loop. A probe which is simply a short extension of the inner conductor of the coaxial line can be oriented so that it is parallel to the electric lines of force. A loop can be arranged so that it encloses some of the magnetic lines of force. The point at which maximum coupling is obtained depends upon the mode of propagation in the guide or cavity. Coupling is maximum when the coupling device is in the most intense field.
If a waveguide is left open at one end, it will radiate energy (that is, it can be used as an antenna rather than a transmission line).
This radiation can be enhanced by flaring the waveguide to form a pyramidal horn antenna.
There are examples of practical waveguide antennas for WiFi shown in Appendix A called Antenna Construction.
Connectors and adaptersEdit
Connectors allow a cable to be connected to another cable or to a component in the RF chain. There are a wide variety of fittings and connectors designed to go with various sizes and types of coaxial lines.
We will describe some of the most popular ones.
BNC connectors were developed in the late 40s. BNC stands for Bayonet Neill Concelman, named after the men who invented it: Paul Neill and Carl Concelman.
The BNC product line is a miniature quick connect/disconnect connector. It features two bayonet lugs on the female connector, and mating is achieved with only a quarter turn of the coupling nut. BNCs are ideally suited for cable termination for miniature to subminiature coaxial cable (RG-58 to RG-179, RG-316, etc.). They are most commonly found on test equipment and 10base2 coaxial Ethernet cables.
TNC connectors were also invented by Neill and Concelman, and are a threaded variation of the BNC. Due to the better interconnect provided by the threaded connector, TNC connectors work well through about 12 GHz. TNC stands for Threaded Neill Concelman.
Type N (again for Neill, although sometimes attributed to “Navy”) connectors were originally developed during the Second World War. They are usable up to 18 GHz, and very commonly used for microwave applications. They are available for almost all types of cable. Both the plug / cable and plug / socket joints are supposedly waterproof, providing an effective cable clamp. Nevertheless for outdoor use they should be wrapped in self agglomerating tape to prevent water from seeping in.
SMA is an acronym for Sub Miniature version A, and was developed in the 60s. SMA connectors are precision, subminiature units that provide excellent electrical performance up to 18 GHz. These threaded high-performance connectors are compact in size and mechanically have outstanding durability.
The SMB name derives from Sub Miniature B, and it is the second subminiature design. The SMB is a smaller version of the SMA with snap-on coupling. It provides broadband capability through 4 GHz with a snap-on connector design.
MCX connectors were introduced in the 80s.
While the MCX uses identical inner contact and insulator dimensions as the SMB, the outer diameter of the plug is 30% smaller than the SMB. This series provides designers with options where weight and physical space are limited. MCX provides broadband capability though 6 GHz with a snap-on connector design. In addition to these standard connectors, most WiFi devices use a variety of proprietary connectors. Often, these are simply standard microwave connectors with the centre conductor parts reversed, or the thread cut in the opposite direction. These parts are often integrated into a microwave system using a short, flexible jumper called a pigtail that converts the non-standard connector into something more robust and commonly available.
Some of these connectors include:
RP-TNC. This is a TNC connector with the genders reversed.
U.FL (also known as MHF). This is possibly the smallest microwave connector currently in wide use. The U.FL/MHF is typically used to connect a mini-PCI radio card to an antenna or larger connector (such as an N or TNC) using a thin cable in waht is known as a pigtail.
The MMCX series, which is also called a MicroMate, is one of the smallest RF connector line and was developed in the 90s. MMCX is a micro-miniature connector series with a lock-snap mechanism allowing for 360 degrees rotation enabling flexibility.
MC-Card connectors are even smaller and more fragile than MMCX. They have a split outer connector that breaks easily after just a few interconnects. Adapters are short, two-sided devices which are used to join two cables or components which cannot be connected directly. For example, an adapter can be used to connect an SMA connector to a BNC.
Adapters may also be used to fit together connectors of the same type, but of different gender.
Figure ATL 6: An N female barrel adapter.
For example a very useful adapter is the one which enables to join two Type N connectors, having socket (female) connectors on both sides.
Choosing the proper connectorEdit
“The gender question.” Most connectors have a well defined gender. Male connectors have an external housing or sleeve (frequently with an inner thread) that is meant to surround the body of the female connector. They normally have a pin that inserts in the corresponding socket of the female connector, which has a housing threaded on the outer surface or two bayonet struds protruding from a cylinder. Beware of reverse polarity connectors, in which the male has an inner socket and the female an inner pin. Usually cables have male connectors on both ends, while RF devices (i.e. transmitters and antennas) have female connectors. Lightning arrestors, directional couplers and line-through measuring devices may have both male and female connectors. Be sure that every male connector in your system mates with a female connector.
“Less is best!” Try to minimise the number of connectors and adapters in the RF chain. Each connector introduces some additional loss (up to a dB for each connection, depending on the connector!)
“Buy, don’t build!” As mentioned earlier, buy cables that are already terminated with the connectors you need whenever possible. Soldering connectors is not an easy task, and to do this job properly is almost impossible for small connectors as U.FL and MMCX. Even terminating “Foam” cables is not an easy task. Don’t use BNC for 2.4 GHz or higher. Use N type connectors (or SMA, SMB, TNC, etc.)
Microwave connectors are precision-made parts, and can be easily damaged by mistreatment. As a general rule, you should rotate the outer sleeve to tighten the connector, leaving the rest of the connector (and cable) stationary. If other parts of the connector are twisted while tightening or loosening, damage can easily occur.
Never step over connectors, or drop connectors on the floor when disconnecting cables (this happens more often than you may imagine, especially when working on a mast over a roof).
Never use tools like pliers to tighten connectors. Always use your hands. When working outside, remember that metals expand at high temperatures and contract at low temperatures: connector too tight in the summer can bind or even break in winter.
Antennas radiation patternsEdit
Antennas are a very important component of communication systems. By definition, an antenna is a device used to transform an RF signal traveling on a transmission line into an electromagnetic wave in free space. Antennas have a property known as reciprocity, which means that an antenna will maintain the same characteristics regardless if whether it is transmitting or receiving. All antennas operate efficiently over a relatively narrow frequency band. An antenna must be tuned to the same frequency band of the radio system to which it is connected, otherwise the reception and the transmission will be impaired. In broadcasting, we can make do with inefficient receiving antennas, because the transmitters are very powerful, but in two-way communications we must have properly sized antennas. When a signal is fed into an antenna, the antenna will emit radiation distributed in space in a certain way. A graphical representation of the relative distribution of the radiated power in space is called a radiation pattern.
Antenna term glossaryEdit
Before we talk about specific antennas, there are a few common terms that must be defined and explained:
For an efficient transfer of energy, the impedance of the radio, antenna, and transmission cable connecting them must be the same. Transceivers and their transmission lines are typically designed for 50 Ω impedance. If the antenna has an impedance different from 50 Ω there will be a mismatch and reflections will occur unless an impedance matching circuit is inserted. When any of these components are mismatched, transmission efficiency suffers.
Return loss is another way of expressing mismatch. It is a logarithmic ratio measured in dB that compares the power reflected by the antenna Pr to the power that is fed into the antenna from the transmission line Pi:
Return Loss (in dB) = 10 log10 Pi/Pr
While some energy will always be reflected back into the system, a high return loss will yield unacceptable antenna performance.
The interaction between the wave travelling from the transmitter to the antenna and the wave reflected by the antenna towards the transmitter creates what is known as a stationary wave, therefore an alternative way to measure the impedance mismatch is by means of the Voltage Standing Wave Ratio (VSWR):
Return Loss (in dB) = 20 log10 (VSWR+1 / VSWR-1)
In a perfectly matched transmission line, VSWR = 1.
In practice, we strive to maintain a VSWR below 2.
The bandwidth of an antenna refers to the range of frequencies FH - FL over which the antenna can operate correctly. The antenna’s bandwidth is
the number of Hz for which the antenna meets certain requirements, like exhibiting a gain within 3 dB of the maximum gain or a VSWR less than 1.5.
The bandwidth can also be described in terms of percentage of the centre frequency of the band.
Bandwidth = 100 (FH – FL )/FC
…where FH is the highest frequency in the band, FL is the lowest frequency in the band, and FC is the centre frequency in the band.
In this way, bandwidth is constant relative to frequency. If bandwidth was expressed in absolute units of frequency, it would be different depending upon the center frequency.
Different types of antennas have different bandwidth limitations.
Directivity and GainEdit
Directivity is the ability of an antenna to focus energy in a particular direction when transmitting, or to receive energy from a particular direction when receiving.
If a wireless link uses fixed locations for both ends, it is possible to use antenna directivity to concentrate the radiation beam in the wanted direction.
In a mobile application where the transceiver is not fixed, it may be impossible to predict where the transceiver will be, and so the antenna should ideally radiate as well as possible in all directions. An omnidirectional antenna is used in these applications. Gain cannot be defined in terms of a physical quantity such as the watt or the ohm, but it is a dimensionless ratio. Gain is given in reference to a standard antenna.
The two most common reference antennas are the isotropic antenna and the half-wave dipole antenna.
Directivity and GainEdit
Directivity is the ability of an antenna to focus energy in a particular direction when transmitting, or to receive energy from a particular direction when receiving. If a wireless link uses fixed locations for both ends, it is possible to use antenna directivity to concentrate the radiation beam in the wanted direction. In a mobile application where the transceiver is not fixed, it may be impossible to predict where the transceiver will be, and so the antenna should ideally radiate as well as possible in all directions. An omnidirectional antenna is used in these applications. Gain cannot be defined in terms of a physical quantity such as the watt or the ohm, but it is a dimensionless ratio. Gain is given in reference to a standard antenna.
The two most common reference antennas are the isotropic antenna and the half-wave dipole antenna.
The isotropic antenna radiates equally well in all directions. Real isotropic antennas do not exist, but they provide useful and simple theoretical antenna patterns with which to compare real antennas. Any real antenna will radiate more energy in some directions than in others. Since antennas cannot create energy, the total power radiated is the same as an isotropic antenna. Any additional energy radiated in the direction it favours is offset by equally less energy radiated in some other direction.The gain of an antenna in a given direction is the amount of energy radiated in that direction compared to the energy an isotropic antenna would radiate in the same direction when driven with the same input power. Usually we are only interested in the maximum gain, which is the gain in the direction in which the antenna is radiating most of the power, the so called boresight. An antenna gain of 3 dB compared to an isotropic antenna would be written as 3 dBi.
The half-wave dipole can be a useful standard for comparing to other antennas at one frequency or over a very narrow band of frequencies.
Unlike the isotropic, is very easy to build and sometimes manufacturers will express the gain with reference to the half-wave dipole instead of the isotropic. An antenna gain of 3 dB compared to a dipole antenna would be written as 3 dBd. Since a half-wave dipole has a gain of 2.15 dBi, we can find the dBi gain of any antenna by adding 2.15 to its dBd gain.
The method of measuring gain by comparing the antenna under test against a known standard antenna, which has a calibrated gain, is technically known as a gain transfer technique.
The radiation pattern or antenna pattern describes the relative strength of the radiated field in various directions from the antenna, at a constant distance. The radiation pattern is a reception pattern as well, since it also describes the receiving properties of the antenna, as a consequence of reciprocity. The radiation pattern is three-dimensional, but usually the published radiation patterns are a two-dimensional slice of the three-dimensional pattern, in the horizontal and vertical planes.
These pattern measurements are presented in either a rectangular or a polar format.
The following figure shows a rectangular plot presentation of a typical ten-element Yagi antenna radiation pattern.
The detail is good but it is difficult to visualize the antenna behaviour in different directions.
Figure ATL 7: A rectangular plot of the radiation pattern of a Yagi antenna.
Polar coordinate systems are used almost universally.
In the polar-coordinate graph, points are located by projection along a rotating axis (radius) to an intersection with one of several concentric circles that represent the correspong gain in dB, referenced to 0 dB at the outer edge of the plot.
This representation makes it easier to grasp the radial distribution of the antenna power.
Figure ATL 8 is a polar plot of the same 10 element Yagi antenna.
Figure ATL 8: The polar radiation pattern plot of the same antenna
The field pattern that exists close to the antenna is different from the one at a distance, which is the one of interest.
The far-field is also called the radiation field.
For radiation pattern measurement it is important to choose a distance sufficiently large.
The minimum permissible distance depends on the dimensions of the antenna in relation to the wavelength.
The accepted formula for this distance is:
rmin = 2d2 /λ
where rmin is the minimum distance from the antenna, d is the largest dimension of the antenna, and λ is the wavelength.
An antenna’s beamwidth is usually understood to mean the half-power beamwidth. The peak radiation intensity is found, and then the points on either side of the peak at which the power has reduced by half are located. The angular distance between the half power points is defined as the beamwidth. Half the power expressed in decibels is -3 dB, so the half power beamwidth is sometimes referred to as the 3 dB beamwidth. Both horizontal and vertical beamwidth are usually considered.
Assuming that most of the radiated power is not divided into sidelobes, the directive and hence the gain is inversely proportional to the beamwidth: as the beamwidth decreases, the gain increases. A very high gain antenna can have a beamwidth of a few degrees and will have to be pointed very carefully in order not to miss the target. The beamwidth is defined by the half power points and in turn determines the coverage area.
Coverage area refers to geographical space “illuminated” by the antenna and it is roughly defined by the intersection of the beamwidth with the earth surface. On a base station, it is normally desired to maximise the coverage area, but sometimes one must resort to “downtilting” the antenna, either mechanically or electrically, in order to provide services to customers very close to the base station and therefore below the beamwidth of a non tilted antenna. This down tilting could be achieved by mechanically inclining the antenna, but often the beam can be steered by changing the phase of the signal applied to the different elements of the antenna in what is known as electrically downtilting.
No antenna is able to radiate all the energy in one preferred direction. Some is inevitably radiated in other directions. These smaller peaks are referred to as sidelobes, commonly specified in dB down from the main lobe. In antenna design, a balance must be struck between gain and sidelobes.
In an antenna radiation pattern, a null is a zone in which the effective radiated power is at a minimum. A null often has a narrow directivity angle compared to that of the main beam. Thus, the null is useful for several purposes, such as suppression of interfering signals in a given direction
Polarization is defined as the orientation of the electric field of an electromagnetic wave. The initial polarization of a radio wave is determined by the antenna. Most antennas are either vertically or horizontally polarized.
Figure ATL 9: The electric field is perpendicular to magnetic field, both of which are perpendicular to the direction of propagation.
The polarization of the transmitting and the receiving antenna must match, or a very big loss will be incurred.
Some modern systems take advantage of polarization by sending two independent signals at the same frequency, separated by the polarization. Polarization is in general described by an ellipse. Two special cases of elliptical polarization are linear polarization and circular polarization.
With linear polarization, the electric field vector stays in the same plane all the time.
The electric field may leave the antenna in a vertical orientation, a horizontal orientation, or at some angle between the two.
Vertically polarized radiation is somewhat less affected by reflections over the transmission path.
Omnidirectional antennas normally have vertical polarization.
Horizontal antennas are less likely to pick up man- made interference, which ordinarily is vertically polarized.
In circular polarization the electric field vector appears to be rotating with circular motion about the direction of propagation, making one full turn for each RF cycle. This rotation may be right-hand or left-hand.
Choice of polarization is one of the design choices available to the RF system designer.
In order to transfer maximum power between a transmit and a receive antenna, both antennas must have the same spatial orientation, and the same polarization sense.
When the antennas are not aligned or do not have the same polarization, there will be a reduction in power transfer between the two antennas. This reduction in power transfer will reduce the overall system efficiency and performance.
When the transmit and receive antennas are both linearly polarized, physical antenna misalignment will result in a polarization mismatch loss, which can be determined using the following formula:
Loss (dB) = 20 log10(cos θ)
…where θ is the difference in the polarization angle between the two antennas.
For 15° the loss is approximately 0.3 dB, for 30° we lose 1.25 dB, for 45° we lose 3 dB and for 90° we have an infinite loss.
In short, the greater the mismatch in polarization between a transmitting and receiving antenna, the greater the loss.
In the real world, a 90° mismatch in polarization is quite large but not infinite. Some antennas, such as Yagis or can antennas, can be simply rotated 90° to match the polarization of the other end of the link.
You can use the polarization effect to your advantage on a point-to-point link.
Use a monitoring tool to observe interference from adjacent networks, and rotate one antenna until you see the lowest received signal. Then bring your link online and orientate the other end to match polarization.
This technique can sometimes be used to build stable links, even in noisy radio environments.
Polarization mismatch can be exploited to send two different signals on the same frequency at the same time, thus doubling the throughput of the link. Special antennas that have dual feeds can be used for this purpose. They have two RF connectors that attach to two independent radios. The real life throughput is somewhat lower than twice the single antenna throughput because of the inevitable cross polarization interference.
It is often useful to compare the front-to-back ratio of directional antennas. This is the ratio of the maximum directivity of an antenna to its directivity in the opposite direction.
For example, when the radiation pattern is plotted on a relative dB scale, the front-to-back ratio is the difference in dB between the level of the maximum radiation in the forward direction and the level of radiation at 180 degrees from it.
This number is meaningless for an omnidirectional antenna, but it is quite relevant when building a system with repeaters, in which the signal sent backward will interfere with the useful signal and must be minimised.
The electrical “aperture” of a receiving antenna is defined as the cross section of a parabolic antenna that would deliver the same power to a matched load.
It is easy to see that a parabolic grid has an aperture very similar to a solid paraboloid.
The aperture of an antenna is proportional to the gain.
By reciprocity, the aperture is the same for a transmitting antenna.
Notice that the concept of aperture is not easily visualised in the case of a wire antenna in which the physical area is negligible. In this case the antenna aperture must be derived from the formula of the gain.
Types of antennasEdit
A classification of antennas can be based on:
Frequency and size.Edit
Antennas used for HF are different from antennas used for VHF, which in turn are different from antennas for microwave. The wavelength is different at different frequencies, so the antennas must be different in size to radiate signals at the correct wavelength.
We are particularly interested in antennas working in the microwave range, especially in the 2.4 GHz and 5 GHz frequencies.
At 2.4 GHz the wavelength is 12.5 cm, while at 5 GHz it is 6 cm.
Antennas can be omnidirectional, sectorial or directive. Omnidirectional antennas radiate roughly the same signal all around the antenna in a complete 360° pattern.
The most popular types of omnidirectional antennas are the dipole and the ground plane. Sectorial antennas radiate primarily in a specific area. The beam can be as wide as 180 degrees, or as narrow as 60 degrees.
Directional or directive antennas are antennas in which the beamwidth is much narrower than in sectorial antennas. They have the highest gain and are therefore used for long distance links.
Types of directive antennas are the Yagi, the biquad, the horn, the helicoidal, the patch antenna, the parabolic dish, and many others.
Figure ATL 10: Antenna types
Antennas can be constructed in many different ways, ranging from simple wires, to parabolic dishes, to coffee cans.
When considering antennas suitable for 2.4 GHz WLAN use, another classification can be used:
Access points tend to make point-to-multipoint networks, while remote links or backbones are point-to-point. Each of these suggest different types of antennas for their purpose. Nodes that are used for multipoint access will likely use omni antennas which radiate equally in all directions, or several sectorial antennas each focusing into a small area. In the point-to-point case, antennas are used to connect two single locations together.
Directive antennas are the primary choice for this application.
A brief list of common type of antennas for the 2.4 GHz frequency is presented now, with a short description and basic information about their characteristics.
1/4 wavelength ground plane.Edit
The 1⁄4 wavelength ground plane antenna is very simple in its construction and is useful for communications when size, cost and ease of construction are important. This antenna is designed to transmit a vertically polarized signal. It consists of a 1⁄4 wavelength element as active element and three or four 1⁄4 wavelength ground elements bent 30 to 45 degrees down. This set of elements, called radials, is known as a ground plane.
Figure ATL 11: Quarter wavelength ground plane antenna.
This is a simple and effective antenna that can capture a signal equally from all directions. The gain of this antenna is in the order of 2 - 4 dBi.
A basic Yagi or more properly Yagi-Uda antenna consists of a certain number of straight elements, each measuring approximately half wavelength. The driven or active element of a Yagi is the equivalent of a centre-fed, half-wave dipole antenna.
Parallel to the driven element, and approximately 0.2 to 0.5 wavelength on either side of it, are straight rods or wires called reflectors and directors, or simply passive elements.
A reflector is placed behind the driven element and is slightly longer than half wavelength; directors are placed in front of the driven element and are slightly shorter than half wavelength. A typical Yagi has one reflector and one or more directors.
The antenna propagates electromagnetic field energy in the direction running from the driven element toward the directors, and is most sensitive to incoming electromagnetic field energy in this same direction. The more directors a Yagi has, the greater the gain.
Following is the photo of a Yagi antenna with 5 directors and one reflector. Yagi antennas are often enclosed in a cylindrical radome to afford protection from the weather.
Figure ATL 12: Yagi-Uda
Yagi antennas are used primarily for Point-to-Point links, have a gain from 10 to 20 dBi and a horizontal beamwidth of 10 to 20 degrees.
The horn antenna derives its name from the characteristic flared appearance.
The flared portion can be square, rectangular, cylindrical or conical.
The direction of maximum radiation corresponds with the axis of the horn.
It is easily fed with a waveguide, but can be fed with a coaxial cable and a proper transition.
While it is cumbersome to make a horn antenna at home, a cylindrical can with proper dimensions will have similar characteristics.
Figure ATL 13: Feed horn made from a food can.
Horn antennas are commonly used as the active element in a dish antenna. The horn is pointed toward the centre of the dish reflector.
The use of a horn, rather than a dipole antenna or any other type of antenna, at the focal point of the dish minimizes loss of energy around the edges of the dish reflector.
At 2.4 GHz, a simple horn antenna made with a tin can has a gain in the order of 10 dBi.
Antennas based on parabolic reflectors are the most common type of directive antennas when a high gain is required.
The main advantage is that they can be made to have gain and directivity as large as required. The main disadvantage is that big dishes are difficult to mount and are likely to have a large wind load.
Randomes can be used to reduce the wind load or windage, as well as for weather protection.
Figure ATL 14: A solid dish antenna.
Dishes up to one metre are usually made from solid material.
Aluminum is frequently used for its weight advantage, its durability and good electrical characteristics.
Windage increases rapidly with dish size and soon becomes a severe problem. Dishes which have a reflecting surface that uses an open mesh are frequently used.
These have a poorer front-to-back ratio, but are safer to use and easier to build.
Copper, aluminum, brass, galvanized steel and steel are suitable mesh materials.
The BiQuad antenna is simple to build and offers good directivity and gain for Point-to-Point communications. It consists of a two squares of the same size of 1⁄4 wavelength as a radiating element and of a metallic plate or grid as reflector. This antenna has a beamwidth of about 70 degrees and a gain in the order of 10-12 dBi. It can be used as stand-alone antenna or as feeder for a Parabolic Dish.
The polarization is such that looking at the antenna from the front, if the squares are placed side by side the polarization is vertical.
Figure ATL 15: The BiQuad.
Log Periodic AntennasEdit
Log periodic antennas have moderate gain over a wide frequency band, They are often used in spectrum analysers for testing purposes and are also popular as TV receiving antennas since they can efficiently cover from channel 2 up to channel 14. These antennas are used in White space devices that require the ability to work in widely different channels.
Figure ATL 16: Log periodic antenna
Many other types of antennas exist and new ones are created following advances in technology.
Sector or Sectorial antennas: they are widely used in cellular telephony infrastructure and are usually built adding a reflective plate to one or more phased dipoles.
Their horizontal beamwidth can be as wide as 180 degrees, or as narrow as 60 degrees, while the vertical is usually much narrower.
Composite antennas can be built with many Sectors to cover a wider horizontal range (multisectorial antenna).
Panel or Patch antennas: they are solid flat panels used for indoor coverage, with a gain up to 23 dBi.
The basic property of a perfect parabolic reflector is that it converts a spherical wave irradiating from a point source placed at the focus into a plane wave. Conversely, all the energy received by the dish from a distant source is reflected to a single point at the focus of the dish. The position of the focus, or focal length, is given by:
f = D2 /16 c
…where D is the dish diameter and c is the depth of the parabola at its centre.
The size of the dish is the most important factor since it determines the maximum gain that can be achieved at the given frequency and the resulting beamwidth. The gain and beamwidth obtained are given by:
Gain = ((3.14 D)2 / λ2) η
Beamwidth = 70 λ / D
…where D is the dish diameter and η is the efficiency. The efficiency is determined mainly by the effectiveness of illumination of the dish by the feed, but also by other factors. Each time the diameter of a dish is doubled, the gain is four times or 6 dB greater. If both stations double the size of their dishes, signal strength can be increased by 12 dB, a very substantial gain. An efficiency of 50% can be assumed when hand- building antennas.
The ratio f / D (focal length/diameter of the dish) is the fundamental factor governing the design of the feed for a dish. The ratio is directly related to the beamwidth of the feed necessary to illuminate the dish effectively. Two dishes of the same diameter but different focal lengths require different design of feed if both are to be illuminated efficiently. The value of 0.25 corresponds to the common focal-plane dish in which the focus is in the same plane as the rim of the dish.
The optimum illumination of a dish is a compromise between maximising the gain and minimising the sidelobes.
As mentioned earlier, antennas do not actually create power. They simply direct all available power into a particular pattern. By using a power amplifier, you can use DC power to augment your available signal. An amplifier connects between the radio transmitter and the antenna, and has an additional cable that connects to a power source.
Amplifiers are available that work at 2.4 GHz, and can add several Watts of power to your transmission. These devices sense when an attached radio is transmitting, and quickly power up and amplify the signal. They then switch off again when transmission ends. When receiving, they also add amplification to the signal before sending it to the radio.
Unfortunately, simply adding amplifiers will not magically solve all of your networking problems.
We do not discuss power amplifiers at length in this book because there are a number of significant drawbacks to using them:
- They are expensive. Amplifiers must work at relatively wide bandwidths at 2.4 GHz, and must switch quickly enough to work for Wi- Fi applications.
- They provide no additional directionality. High gain antennas not only improve the available amount of signal, but tend to reject noise from other directions. Amplifiers blindly amplify both desired and interfering signals, and can make interference problems worse.
- Amplifiers generate noise for other users of the band. By increasing your output power, you are creating a louder source of noise for other users of the unlicensed band. Conversely, adding antenna gain will improve your link and can actually decrease the noise level for your neighbours.
- Using amplifiers is often illegal. Every country imposes power limits on use of unlicensed spectrum. Adding an antenna to a highly amplified signal will likely cause the link to exceed legal limits.
Antennas cost far less than amps, and can improve a link simply by changing the antenna on one end.
Using more sensitive radios and good quality cable also helps significantly on long distance wireless links.
These techniques are unlikely to cause problems for other users of the band, and so we recommend pursuing them before adding amplifiers.
Many manufacturers offer high power versions of their WiFi radios at both 2 and 5 GHz, which have a built in amplifiers.
These are better than external amplifiers, but do not assume that it is always smart to use the high power version, for many application the standard power coupled with a high gain antenna is actually better.
Practical antenna designsEdit
The cost of 2.4 GHz antennas has fallen dramatically with the increased popularity of WiFi. Innovative designs use simpler parts and fewer materials to achieve impressive gain with relatively little machining. Unfortunately, availability of good antennas is still limited in some areas of the world, and importing them can be expensive.
While actually designing an antenna can be a complex and error-prone process, constructing antennas from locally available components is very straightforward, and can be a lot of fun.
In Appendix A called Antenna Construction we present some practical antenna designs that can be built for very little money.
Precise antenna instruments require expensive instruments and installations. It is therefore advisable to obtain the antenna parameters values directly from a reputable manufacturer.
An anechoic chamber is needed to perform accurate antenna measurements, otherwise the reflections will cause false readings.
Ice affects the performance of all antennas to some degree and the problem gets more serious at higher frequencies. The impedance of free space is 377 ohms. If the air immediately surrounding the dipole elements is replaced by ice which has a lower impedance than air, then the impedance match and radiation patterns of the antenna will change.
These changes become progressively worse as the ice loading increases.
Antenna elements are usually encased in a plastic protective housing (radome). This provides an air space between the elements and ice casing so that the lower impedance of the ice layer has only a small effect on the radiators.
Detuning is greatly reduced but radiation pattern distortion may still be encountered (detuning reduces usable antenna bandwidth). For a given ice thickness, deviation from nominal performance values become worse as frequency increases.
In areas where severe icing and wet snow are common, it is prudent to install a full radome over solid parabolic antennas, to use panel antennas instead of corner reflectors, and to stay away from grid parabolics.
Figure ATL 17: Effect of ice on a parabolic grid antenna
1. RADIO PHYSICS 1
1. RADIO PHYSICS
Wireless communications make use of electromagnetic waves to send signals across long distances. From a user's perspective, wireless connections are not particularly different from any other network connection: your web browser, email, and other applications all work as you would expect. But radio waves have some unexpected properties compared to Ethernet cable. For example, it's very easy to see the path that an Ethernet cable takes: locate the plug sticking out of your computer, follow the cable to the other end, and you've found it! You can also be confident that running many Ethernet cables alongside each other won't cause problems, since the cables effectively keep their signals contained within the wire itself.
But how do you know where the waves emanating from your wireless device are going? What happens when these waves bounce off objects in the room or other buildings in an outdoor link? How can several wireless cards be used in the same area without interfering with each other?
In order to build stable high-speed wireless links, it is important to understand how radio waves behave in the real world.
What is a wave?
We are all familiar with vibrations or oscillations in various forms: a pendulum, a tree swaying in the wind, the string of a guitar - these are all examples of oscillations.
What they have in common is that something, some medium or object, is swinging in a periodic manner, with a certain number of cycles per unit of time. This kind of wave is sometimes called a mechanical wave, since it is defined by the motion of an object or its propagating medium.
When such oscillations travel (that is, when the swinging does not stay bound to one place) then we speak of waves propagating in space. For example, a singer singing creates periodic oscillations in his or her vocal cords. These oscillations periodically compress and decompress the air, and this periodic change of air pressure then leaves the singers mouth and travels, at the speed of sound.
A stone plunging into a lake causes a disturbance, which then travels across the lake as a wave. 2
A wave has a certain speed, frequency, and wavelength. These are connected by a simple relation: Speed = Frequency * Wavelength The wavelength (sometimes referred to as lambda, λ) is the distance measured from a point on one wave to the equivalent part of the next (or, in a more technical way, to the next point that is in the same phase), for example from the top of one peak to the next. The frequency is the number of whole waves that pass a fixed point in a period of time. Speed is measured in metres/second, frequency is measured in cycles per second (or Hertz, represented by the symbol Hz), and wavelength is measured in metres. For example, if a wave on water travels at one metre per second, and it oscillates five times per second, then each wave will be twenty centimetres long: 1 metre/second = 5 cycles/second *W W = 1 / 5 metres W = 0.2 metres = 20 cm Waves also have a property called amplitude. This is the distance from the centre of the wave to the extreme of one of its peaks, and can be thought of as the “height” of a water wave. Frequency, wavelength, and amplitude are shown in Figure RP 1.
Figure RP 1: Wavelength, amplitude, and frequency. For this wave, the frequency is 2 cycles per second, or 2 Hz, while the speed is 1 m/s. Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/29 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/30 1. RADIO PHYSICS
SI symbols atto
Knowing the speed of light, we can calculate the wavelength for a given frequency. Let us take the example of the frequency of 802.11b wireless networking, which is: f = 2.4 GHz = 2,400,000,000 cycles / second wavelength (λ) = c / f = 3*108 / 2.4*109 = 1.25*10-1 m = 12.5 cm Frequency and therefore wavelength determine most of an electromagnetic wave’s behaviour. It governs the dimensions of the antennas that we build as well as the effect of the interactions with objects that are in the propagation path, including the biological effects in living beings. Wireless standards of course are distinguished by more than just the frequency they are working at - for example, 802.11b, 802.11g, 802.11n and 802.16 can all work at 2.4 GHz -, yet they are very different from one another. Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/32 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/33 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/34 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/35 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/36 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/37 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/38 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/39 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/40 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/41 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/42 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/43 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/44 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/45 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/46 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/47 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/48 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/49 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/50 Page:Wireless Networking in the Developing World (WNDW) Third Edition.pdf/51