Wireless Waffle - A whole spectrum of radio related rubbish
OFDM: How does that work then?signal strength
Monday 27 April, 2009, 16:50 - Spectrum Management
Posted by Administrator
Today, Wireless Waffle's continuing series attempting to explain and simplify the many complex radio technologies, techniques and applications tackles perhaps one of the most complicated spectrum sharing schemes that exists. OFDM or 'Orthogonal Frequency Division Multiplex' to give it its full name is a clever method for sending data across the ether in such a way as to circumvent some specific, commonly occuring, problems. Though many people refer to OFDM as a modulation scheme, it is not! It is more accurately described as a multiplexing or sharing scheme and it can be used as an access scheme to allow the sharing of the spectrum between different users (in which case it becomes known as OFDMA - the 'A' being for 'access').

Before looking at what OFDM is, let's first consider the problems it aims to address. Chief amongst these are the problem of reflections and one of the upshots of reflections, frequency selective fading. The path between any two points on the radio landscape will rarely be straightforward. The signal may be received directly (i.e. the orange path in the picture below) as well as via reflections from various nearby and distant objects (the purple paths). Reflections from distant objects can commonly be seen on (analogue) television pictures where the main signal is followed by several 'ghosts', each representing the same signal arriving slightly delayed due to the path of the reflected signal being longer than that of the direct one. Where reflections are from nearby objects, the effect is somewhat different and manifests as 'holes' being punched into the received radio spectrum causing some frequencies to be severely attenuated whilst others remain largely unaffected.

radio reflections

Into this environment, we now introduce the requirement to transfer large amounts of data. For the sake of argument, let's choose 1 Mbps. If we modulate this data signal onto a radio carrier using basic BPSK (binary phase shift keying - the most basic of digital modulation schemes) the resulting signal has a bandwidth of around 1 MHz and a symbol period (ie the time representing each bit of data) of 1 microSecond. In order to successfully receive this signal, one key factor must hold true: reflections from any delays need to be significantly shorter than 1 microSecond. This is because:

* If a reflected signal arrives at the receiver 1 microSecond later than an undelayed signal, the receiver has finished receiving the bit concerned and has moved onto the next one. Thus the reflection is pure 'interference'. This is equally the case for delays of half a microSecond wherein the delayed signal has equal potential to interfere with the bit we are trying to receive and the one following it.

* A delay of 1 microSecond produces frequency selective fading notches every 1 MHz. As such, if the delay is longer than 1 microSecond, there is every chance that the notch in the frequency spectrum produced by the delay will punch a hole right in the middle of our wanted signal making it unreceivable.

A delay of 1 microSecond represents a reflected path that is 300 metres longer than the unreflected path (the speed of light times 1 microSecond). For a short distance link, this may not be difficult to achieve, but as the length of the link starts to exceed 300 metres, the potential for reflections causing problems increases. With a radio paths over 3 km long, for example, a reflective object which is more than 15 degrees away from the centre line of the path between the two ends will cause such a reflection - clearly a strong likelihood.

One solution to this problem is to minimise the potential for such reflections being caused by focussing the signal carefully between the two ends of the path using highly directional antennas. In this situation, reflections which are 'off-beam' will be heavily attenuated both at the transmit and receive ends of the link. In broadcast situations, however, whilst receiver antennas might be able to be directions, the transmit antenna is, virtually by definition, aiming to send out a signal over as wide an area as possible and in these circumstances reflections are inevitable.

Another solution is OFDM! In OFDM, we take the 1 Mbps of data and break it up into a number of smaller, slower, data streams. For our example, let's break the stream into 100 smaller streams, each which carries only 10 kbps of data. If we modulate one of these streams onto a radio carrier using the same BPSK technique, it now occupies a bandwidth of just 10 kHz and has a symbol period of 100 microSeconds. As such, it can now tolerate delays which are 100 times larger than that the original 1 Mbps conterpart. The problem is that there is only one of them and we need to transmit 100. Normally, when transmitting a 10 kHz wide signal, we would need to leave some space either side of the signal to separate it from its neighbours. A factor of 50% is not unusual meaning that for each 10 kHz signal we might require 15 kHz of spectrum. For our 100 signals, we would therefore require 1.5 MHz of spectrum, making this significantly less efficient in spectrum terms than the single carrier solution. The diagram below shows the spectrum of a single data carrier.

ofdm sinc function

If, however, we modulate each of the adjacent signals intelligently and 'orthogonally' the requirement for space is negated and we can transmit the 100 carriers just 10 kHz apart, putting them back in the 1 MHz of spectrum that the original single carrier solution occupied. Orthogonal implies 'at right angles' and in essence, each adjacent carrier is modulated so that it is 'at spectral right angles' to its neighbour. The diagram below shows the spectrum of multiple orthogonal OFDM carriers. Note that at the centre of each carrier, the signals from all of the adjacent carriers are at a null of zero size.

ofdm orthogonal carriers

wheelbarrow of bricksThe upshot of this clever technique is that we can now transmit the data in the same amount of spectrum but in a way in which reflections and delays of much larger extents can be tolerated without effect, using 100 smaller, slower carriers rather that 1 large, fast one. The best non-technical analogy might be the need to transfer 100 bricks across an area of rough land. If we put all 100 bricks in a single wheelbarrow and push it along, it will get bumped and knocked and bricks will fall out. If there is a big enough obstruction the wheelbarrow will get stuck and nothing will make it to the other side of the land. Alternatively, if we put 1 brick in 100 separate wheelbarrows and push these over the land, whilst some may lose their bricks or be blocked, there is a much higher chance that a goodly proportion will make it to the other side.

An additional advantage of OFDM is that if there is interference on some of the spectrum within our 1 MHz channel, the single carrier solution fails, whereas for the OFDM solution only those carriers where the interference is present fail. Thus it is possible to maintain a connection in the presence of certain types of interference with OFDM. Being even cleverer, if we know which of the frequencies are affected we could change the error correction or modulation of the carriers on those frequencies to compensate for the problem, or even just not use them. Whilst all this would reduce the amount of data we could transmit, at least the connection would remain intact.

Transmitting and receiving OFDM is not straightforward and this is one of the reasons why it has not been used for mobile phones. Transmitters have a high peak-to-average power ratio such that an OFDM transmitter with an average output power of 1 Watt, may produce a peak output of 50 Watts or more, which is not efficient nor would batteries in handsets last long. Decoding the complex OFDM waveform is processor intensive and until recently, the processor power required would also drain batteries pretty pronto. Nonetheless, OFDM offers a number of advantages and many of the proposed fourth generation (4G) mobile standards will adopt it.

OFDM is used in many technologies including the DVB set of digital terrestrial broadcasting standards; for DAB and DRM radio; in some WiFi and WiMAX systems; and in various military and defence links. In these systems the number of carriers differs as does the modulation scheme which each carrier uses (which varies from BPSK to 64QAM) to adapt to the circumstances which are likely to be encountered.

OFDM is not an easy concept to grasp but we, at Wireless Waffle are always keen to try and debunk and demystify difficult radio ideas - we hope we have succeeded.
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Pennies from Heaven?signal strength
Wednesday 9 April, 2008, 15:40 - Spectrum Management
Posted by Administrator
It seems it's not just Wireless Waffle that takes an active interest in matters pertaining to spectrum management. No less than the fourth highest authority in the land (after God, the Queen and Chris Moyles), the Rt. Hon. Alastair Darling MP, chancellor of the Exchequer has obviously been taking note of what we've been saying. Section 5.16 of the his Budget 2008 entitled, innocuously enough 'Spectrum Management', says:
As broadcast media and mobile technologies continue to grow in importance and diversity, efficient use of the electromagnetic spectrum to deliver the services that society demands remains an important issue for both the public and private sectors. In line with the Government's Forward Look on public sector spectrum, the Ministry of Defence will by May 2008 publish an implementation plan setting out its plans for the release of spectrum to the market. Other departments are adopting similar processes. To ensure best use of spectrum by the private sector, Ofcom has also confirmed that in contrast to some previous spectrum releases which were available for specific uses only (notably mobile telephone services) the spectrum released by digital switchover will be available for all technologies. The Government fully supports this decision.

spectrum pricingDoes this tell us anything? Are the Government about to tax spectrum? Will spectrum be blamed for the economic downturn that we now seem to be staring into the barrels of? Will spectrum be the reason for the 2 point increase in income tax at the next budget? Or much worse, will poor spectrum management cause an extra 1 pence on the price of a pint of beer (except 'lager' which isn't really beer in the true British definition of the term)? Probably not, other than the fact that the estimated bill that Ofcom is going to charge the MoD to 'recognise its access' to the spectrum is 300 million, and that decisions on MoD ('and other departments' meaning the Civil Aviation Authority mainly) spectrum use are taken at cabinet level and so are visible on the radar.

One might go so far as to suggest that the chancellor at the time of the 22 billion 3G licence windfall, none other than the now Prime Minister, Gordon Brown, is keeping an eye on matters to make sure that the Government doesn't lose out should the MoD find a way to profit from the sudden interest in one of their previously lesser valued assets. One might do that. One might not be far wrong!
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Farnborough Airport Frequenciessignal strength
Tuesday 11 March, 2008, 09:08 - Spectrum Management
Posted by Administrator
pilotgirlTo add to the previous list of frequencies for London Heathrow, Gatwick and City that went before, here is a set of active frequencies for Farnborough (ICAO code: EGLF) which might come in handy for the 2008 airshow. All frequencies are in MHz and use AM. Those shown in italics are unconfirmed but are listed widely enough to indicate that they are highly likely to remain current.

London Farnborough Airport
Radar 125.250
Radar Standby 130.050
Approach 134.350
Tower 122.500
Operations 130.375 (TAG Handling)
ATIS 128.400

One additional frequency of note is 444.3375 MHz (FM not AM), which is a local repeat of the tower and is interestingly in the 443.500 to 445.500 MHz Ministry of Defence (MoD) sub-band. Presumably this is assignment a hang-over from when the airport undertook more military operations than it does now.
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Intermodulation: What's All This Thensignal strength
Friday 29 February, 2008, 08:10 - Spectrum Management
Posted by Administrator
Intermodulation (intermod for short) is a common problem besetting virtually every radio system in existence. This particular problem occurs when two (or more) signals mix together in a non-linear device creating emissions on frequencies which are directly related to the signals being mixed together. It is the same process that is used in the mixers of most superheterodyne (don't worry if this means nothing to you) receivers, where it is a wanted outcome. Intermodulation is therefore mixing which produces unwanted outcomes.

It works a bit like this: Whistle or sing two notes at the same time (OK, this bit is rather difficult, but run with it for now...) In their natural form, each note will be 'pure' or 'clean' and both notes will be distinct from each other. Now whistle or sing the same two notes through a kazoo. If you've ever heard a kazoo played, you will realise that it works by distorting the sound going through it by moving a membrane (often paper) to its extremities, in essence limiting the audio and producing a square wave output from the sine wave input. The effect of any such non-linear distortion on the two notes will be to mix them together and the resulting output will be rich in all sorts of notes and sounds that weren't there in the first place.

The same can happen with radio transmissions. Any two signals passing through a non-linear device produce outputs that were not there to begin with. Though I could run through the maths and prove that such signals actually do exist, it's a little easier just to tell you what the result is.

intermodulation diagramLet's assume that the two frequencies that we are interested in are f and f. The non-linearity will produce harmonics of these frequencies at 2f, 2f, 3f, 3f, 4f, 4f and so on... In addition to this, it will 'mix' these harmonics together with themselves and with the original signals to produce frequencies like f+f, f-f (these are the outputs we would want if we were using the process for mixing). Frequencies of 2f-f and 2f-f are known as the 'third order intermodulation products', third because they are composed of three lots of the input signals (two of one and one of the other) and are usually the most problematic because they are closest in frequency to the original signals. Fifth order intermodulation products 3f-2f and 3f-2f are the next nearest; then seventh (and every odd number thereafter). The problem gets even more complex when there are more than two signals getting mixed together. The even order intermodulation products are usually far removed (in frequency terms) from the original signals and thus cause fewer problems.

If we use real frequencies as an example, let's say we have transmitters on 80 and 85 MHz, the third order products will be at 75 and 90 MHz, the fifth order at 70 and 95 MHz. So we can end up with signals in the FM broadcast band from transmitters that were originally well outside it (and vice versa).

Intermodulation commonly occurs at the receiver (due to distortion in the sensitive amplifiers) but can sometimes occur at a transmitter, though this is more often caused by dodgy connections than by the transmitters themselves. There are stories of 'rusty bolts' on metal structures such as cranes acting as crude diodes (which are highly non-linear) producing intermodulation products if they are in strong radio fields. Because of this problem with receivers, it's not at all uncommon to receive a signal on a frequency where no signal is actually present, a 'ghost signal' as some have called it. Normally, putting an attenuator in line with the antenna will make the ghost signal completely disappear, proving that it is an intermodulation product and not a real signal (for every dB that a real signal decreases, the third order intermodulation products will usually drop by 3dB making them easy to detect).

inter mod ulationThose responsible for choosing frequencies for transmitters in a given area, usually try to avoid putting transmissions on frequencies where the intermods would fall on the frequency of a nearby receiver, especially if the victim receiver is on the same site. Taking the example above, at a site where a receiver operates at 75 MHz, planners would usually avoid a combination of frequencies (e.g. 80 and 85 MHz) that might result in intermodulation causing ghost signals to cause interference. This is normally one of the rules employed when planning the FM broadcast band (though oddly, the frequencies for BBC's national networks are totally counter to this logic and seem to work fine), and within PMR, cellular and microwave bands great care us taken to try and avoid a ghost signal appearing in any particularly scary locations!

With radio transmitters proliferating rapidly, the problem of intermodulation is growing, especially in dense radio environments and on busy sites. Improvements in receiver electronics are able to tackle some of the problems, but as pressure increases to make efficient and effective use of the spectrum, the problem of intermodulation isn't going away and in the end may prove one of the major limiting factors in maximising the density with which radio services can be packed together.
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