Thursday 15 December, 2005, 02:00 - Spectrum ManagementThe concept of 'spread spectrum' dates back to World War II when the concept was put to use as countermeasures against jamming; against detection by radar; against detection of navigation beacons and as a way to make interception of communications more difficult. The idea is to take a signal and spread it across a much wider range of frequencies than the originating (data) signal would have otherwise occupied; indeed this is one of the two requirements of a system to be officially designated as spread spectrum. The other requirement is that some function of the original data signal has an impact on the final spread spectrum signal - a technicality that we won't bother with here as it's not relevant to the ensuing discussion.
But what is the point of this? Why occupy more spectrum than is necessary? Isn't this a highly inefficient use of spectrum?
Well spreading a signal has two noticeable effects:
1. It makes the signal much more difficult to detect;
2. It makes the signal much less susceptible to narrow-band interference (such as traditional jamming signals).
Clearly these effects have immediate military connotations and it is no surprise that (as with many other radio techniques) the idea was first put to use in this context, however these effects also have great benefits for other uses too. Let's examine them one by one.
Reduced likelihood of detection
From a civil perspective, reducing the likelihood that your transmissions will be heard isn't of major concern except perhaps in public telecommunications networks, however the use of encryption is a much more straightforward way of going about making your transmissions private. For 'reduced likelihood of detection' we could alternatively write 'reduced likelihood of causing interference', as if a signal is difficult to detect, it is unlikely to cause interference to another transmission. In fact, all modern cellular phone networks use some kind of spread spectrum technique to try and reduce the interference that a handset communicating with one base-station causes to other base-stations in the network. Also, for technologies such as Wireless LAN and Bluetooth where many users share the same piece of spectrum, using spread spectrum techniques vastly reduces the probability of users causing each other interference.
Of course spread spectrum is not the panacea of interference mitigation as if two spread spectrum signals of the same type and on the same frequency clash they still cause each other interference, but it can go a long way to reducing interference between users, especially in de-regulated radio environments where it's 'every user for themselves'.
Reduced suscepitibility to narrow-band interference
Spreading a signal across a wide-range of frequencies means that the amount of information transmitted in any portion of those frequencies is only a fraction of the overall information being transmitted. As such, if a portion of the spread signal is interfered with, the received data will only be fractionally affected and with data correction techniques it is possible to stop narrow-band signals from interfering with spread spectrum signals at all. Of course one spread spectrum signal could easily interfere with another using the same frequencies as if all the frequencies in use are affected there is generally no way to 'tune-out' the interfering signal.
Spread spectrum systems are therefore largely immune to narrow-band interference such as might be caused by traditional AM or FM signals. As well as the obvious anti-jamming military uses, this still has great benefit in a civil situation where a system has to share spectrum with lots of potentially interfering sources.
Spread Spectrum Techniques
There are two commonly deployed spread-spectrum techniques plus two uncommon ones:
Frequency Hopping Spread Spectrum (FHSS) is a commonly deployed technique whereby a signal changes frequency or 'hops' (usually very rapidly) across a wide band of available frequencies. The receiver has to be synchronised with the transmitter so that it knows what hopping pattern to follow. If a few of the frequencies onto which the system hops have interference, the overall transmission will still be received as communication continues takes places on the clear frequencies (indeed some systems are intelligent and cease to use frequencies on which interference is present).
Hopping across a wide range of frequencies has one additional benefit: the propagation conditions on one frequency will be different to those on another frequency, such that, especially in mobile environments, FHSS signals provide a way to mitigate against the frequency selective fading that occurs. Even changes in frequency of a few percent can be useful and this is one of the main reasons that GSM networks often employ FHSS from the handsets. Bluetooth also uses FHSS but in this case to dodge between the large number of interfering signal that appear in the shared frequency band it inhabits.
Direct Sequence Spread Spectrum (DSSS) is a commonly deployed technique whereby a radio signal is spread by mixing it with a high bit-rate data spreading signal. At the receiver, the same spreading signal can be used to 'de-spread' the received signal and return it to its original state. This de-spreading results in any narrow-band interference being spread and effectively turned into background noise. Equally, the effect of frequency selective fading (as found in mobile environments) is to null out reception on some frequencies. with DSSS these nulls will be spread to appear as 'anti-noise', or in other words, a small reduction in the overall reveived signal.
Unlike FHSS signals which only occupy one frequency at once, DSSS signals occupy all the frequencies over which they have been spread at the same time. CDMA systems use DSSS, as do Wireless LAN's conforming to many of the 802.11 series of standards.
Chirp is an uncommon technique in which a transmission begins on one frequency and ends on another, changing from one to the other during the period of the transmission. It is difficult to receive such signals as almost perfect synchronisation is needed between receiver and transmitter in terms of time and frequency, however with modern signal processing techniques it is becoming possible.
Chirp, however, is often used for radar systems as it is a spectrally efficient way of generating the wide-bandwidth transmissions that are required to ensure the accuracy of the radar.
Time Hopping Spread Spectrum (THSS) is an undeveloped technique in which, the transmitted signal is turned on and off in a psuedo-random fashion (thus hopping on and off with time). Interest has been raised in this technique as a method for sharing spectrum for Ultra Wide Band devices.
Spread spectrum techniques are now a part of every-day radio usage both for military and civil systems and are allowing more effective use of spectrum to be made as well as providing a means of using otherwise difficult to use spectrum (such as that shared with microwave ovens). What will be the next military technique to make the cross-over to every-day civil usage?
Wednesday 30 November, 2005, 06:55 - Spectrum ManagementNapoleon and the Mathematician
Before going any further we need to take a brief side-step and remember Jean Baptiste Joseph Fourier (1768-1830), one time governor of Lower Egypt under Napoleon and master mathematician. Fourier developed an equation (the Fourier Transform), which allows us to transform waveforms in the time-domain into waveforms in the frequency domain. Fourier shows us that a pure sine-wave in the time-domain transforms into a single line in the frequency domain, representing the frequency at which the sine-wave oscillates (simple eh?). The converse is also true, in that a single line in the time-domain (an impulse such as is caused when lightning strikes), converts to a sine-wave in the frequency-domain – having content at virtually ALL frequencies – which is why a nearby lightning strike, which looks like an impulse in the time-domain, can affect reception on every radio receiver in a house from medium wave (~1 MHz) to television (~500 MHz) and even satellite (~10,000 MHz).
Spark Gap Transmitters
Let's now move forward a hundred years to the work of Guglielmo Marconi (1874-1937), father of modern-day radio. During Marconi's experiments into radio transmission, oscillators producing a nice sine-wave on a single frequency weren't available. Instead, Marconi used a spark gap transmitter. This device produced an impulse (much like a lightning strike) by making a spark between two contacts and thus produced a waveform with content on all frequencies. Then, using a crude tuned-circuit, the frequency that was wanted was selected. It was an inefficient way of transmitting but using high-powered sparks, high-powered signals could easily be developed – which were needed because of the poor sensitivity of the receivers. Also, due to the way in which spark-gap signals were generated, they produced a great deal of interference on many radio frequencies other than the one to which they were tuned.
As the use of radio grew, there was an increased need to separate one transmission from another to reduce interference and to that end, in 1927, the International Radio Consultative Committee (CCIR) was formed in Washington DC (these days part of the ITU). The 1927 conference also decided that the use of spark-gap transmitters would be outlawed (and they would have been completely phased-out by 1939 had World War II not started which slightly delayed their demise).
Ultra Wide Band
Ultra Wide Band (UWB) is a relatively new term to describe a technology which had been descrined in the early 1960's as 'carrier-free', 'baseband' or 'impulse' technology. The basic concept is to develop, transmit and receive a burst of radio frequency (RF) which represent from one to only a few cycles of an RF carrier wave – effectively an impulse of RF. The resultant waveforms are extremely wide, so much so that it is often difficult to determine the actual RF centre frequency. Recent implementations of UWB do away with the idea of using an impulse to generate the signal due to difficulties in selecting the wanted frequencies, but the concept of using a transmission spread out over a very wide frequency range remains. In this sense, UWB is the modern-day equivalent of spark-gap transmitters!
The difference, however, is the power levels concerned. Whereas the spark-gap transmitters of old often used powers of 10s of kilowatts, UWB uses powers of only 10s of milliwatts (around a million times less). Further, the fact that UWB signals are purposefully spread over a very wide range of frequencies means that the actual power transmitted on any given frequency is miniscule and in many cases is below the background noise (caused by man-made, natural and even galactic sources). Conversely, spark gap transmitters were few and far between but the intention is that UWB devices will be ubiquitous. Over a country the size of the UK, where there may have been only a handful of spark gap transmitters, there may be millions of UWB devices so the potential for interference is, arguably, almost as great.
In order to try and prevent UWB from causing interference to other radio users, 'masks' have been set by a number of organisations from the Federal Communications Commission (FCC) in the United States to the European Telecommunications Standards Institute (ETSI) in Europe. A radio mask can be considered as similar to a mask used when printing, it prevents radio signals from being transmitted over certain areas (or reduces them) and allows them to be transmitted over other areas, in this way sensitive services (such as aeronautical radar) can have increased protection from interference. The fact that masks are necessary indicates that spectrum administrations are concerned about the interference potential from UWB, despite the very low signal levels involved.
Counterbalancing the interference problems, however, are the benefits that UWB technology claims to offer over other, competing technologies (such as, for example, WiFi and Bluetooth). UWB claims to provide:
· Extremely high bandwidth data links (over very short ranges)
· Precision measurement and location identification (in radar applications)
· Immunity to multi-path reflections of the signal
· Low complexity and therefore (in theory) low cost
UWB is therefore well suited to applications such as high bandwidth LANs (100Mb/s+), collision avoidance sensors, precision location systems, radio tags (cargo, personnel etc), radio imaging and even low powered handheld radios. The benefits are potentially large ranging from life-saving applications to quality of life enhancing applications, but this must be balanced against the potential for interference to other radio users.
Apples and Oranges?
Is it really fair to compare modern UWB technology with 100 year old spark-gap transmitters? Yes. And no. Yes, both transmit over a wide range of frequencies. Yes, both are based on the notion of using an impulse to generate radio signals. Yes, both have the potential to cause interference to many other radio users. No, the technology employed is fundamentally different. No, the power levels are several orders of magnitude different. No, the intended usage is different.
So what are the implications for radio users? Large numbers of UWB transmitters working together have the potential to cause interference to users across the spectrum (from around 1 to around 10 GHz for most currently planned devices). This frequency range includes a number of economically and politically important users such as mobile phones (GSM 1800, CDMA 1900 and UMTS), aeronautical and maritime radar, wireless LAN and HiperLAN, governmental and private fixed links, satellite links and in some countries wireless cable services. Despite much research in both corners of the UWB boxing-ring, there is no conclusive evidence to indicate that UWB will not cause interference to these services, then again there is no evidence that it will. Most administrations are being cautious about allowing the introduction of UWB as it's unlikely that once introduced, it would be able to be taken out of service as operation will typically be licence-exempt and thus controlling future usage will be near impossible.
For any country therefore, the decision currently exists as to whether to allow UWB services to be introduced today, delivering the kind of user benefits that the technology can provide, or whether to hold back on making that decision until there is a greater weight of international experience. Is the bright spark worth the spark gap?
Tuesday 15 November, 2005, 07:44 - Spectrum ManagementCDMA - Code Division Multiple Access - is probably one of the most complex methods for accessing the radio spectrum that is in use today. Let's first take a look at the easier to understand methods to see what the differences are.
FDMA - Frequency Division Multiple Access - is a system whereby different users use different frequencies at the same time. Imagine tuning up and down the FM dial, there are lots of stations you can hear but you can separate one from the other by tuning into the different frequencies that each uses. If two occupy the same frequency at the same time, they cause interference to each other and the whole thing falls to pieces.
TDMA - Time Division Multiple Access - is a system whereby different users use the same frequency at different times. So a number of users messages are scheduled one after the other, so that they can all share the same frequency. Imagine a lot of letters passing along the conveyor belt in a postal sorting office. Each message passes after the other along the same conveyor and can be sorted out back into individual messages for delivery. If two letters were to be in the same place at the same time they would crash into each other and the system would fail. From a radio perspective this works only if you take the message concerned and 'squash' or 'compress' it into less time than it would normally take so that at the receiving end it can be de-compressed to fill in the gap that is produced whilst the other messages are being sent.
CDMA is a system whereby all the users use the same frequency at the same time. How can this possibly work without interference being caused? It works by assigning each message a unique code. Think of it as assigning each message a different language and then place yourself in a room where you can hear all the messages together - the one you immediately understand is the one in the language you speak. So if you hear 'Je m'appelle Colin', 'Ich heisse Lyon' and 'My name is Derek', all spoken at the same time, the clearest message is the one in your native language. However this only works if all the messages are roughly the same volume (and in fact works best if all the messages are exactly the same volume), if one language is shouted a lot louder than the others, it inevitably swamps the listener so that he can no longer hear his natural language.
As such, one element of a CDMA radio system is that the receiver (listener) needs to inform all the transmitters (speakers) how strong their signal is (how loud they are being heard) so that the transmitters can adjust their transmitter power (volume) to allow the receiver to hear them all at the same strength. This system of 'power control' is one of the key technologies that allows CDMA systems to operate.
CDMA receivers also suffer from something called 'cell breathing'. Imagine that you can hear 4 different languages, the chances are that you will hear the one you understand quite easily. Now imagine you can hear 64 languages, it becomes much more difficult to hear your native language. So the more transmitters sharing the same frequency, the harder the receiver has to listen. To compensate for this, receivers move closer together and further apart each and every second to make sure that they are sufficiently near to those transmitters they are receiving to hear them well. Clearly this is not possible and what happens is that the coverage of the cell shrinks when it is heavily loaded, complicating coverage planning.
All this being said, CDMA is reckoned to be around twice as spectrally efficient as either TDMA or FDMA techniques, meaning twice as much traffic (data) can be carried in a given amount of spectrum. It is no surprise, therefore, that all of the competing third generation (3G) mobile phone technologies use CDMA: the US-based CDMA-1x series of specifications designed by Qualcomm; the European W-CDMA (Wideband-CDMA) or UMTS (Universal Mobile Telecommunications System) system; and the Chinese TD-SCDMA (Time Division-Synchronised CDMA) system.
What's interesting is that although all 3G mobile technologies use CDMA, other mobile technologies (such as Digital Audio Broadcasting, Wireless LAN and HiperLAN) have forgone the use of CDMA and instead use another spectrum access technique called OFDM - Orthogonal Frequency Division Multiplex. But that's a story for another day.