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.
Monday 14 November, 2005, 15:19 - BroadcastingLook at any Frequency Allocation Table (FAT) whether it's the ITU's radio regulations or a national one and there are lots of places where the use of the spectrum changes from one type to another (for example from mobile to fixed links). But what actually happens at the place where the transition takes place? The frequency which marks the transition point between two bands (or sub-bands) is unusable by the users who occupy the spectrum either side.
Let's take an example... A stereo FM broadcast transmission occupies a bandwidth of just over 250 kHz. A transmission on a frequency of 108 MHz (the band edge) would therefore have half, i.e. 125 kHz of its bandwidth inside the broadcast allocation and half inside the neighbouring (aeronautical radionavigation) band. Indeed even an FM transmission on 107.9 MHz would still occupy 25 kHz of spectrum above 108 MHz.
So who can or does use these 'no man's land' frequencies? Well in theory, no-one can, without contravening the band plan that has been put in place. But people do! In fact these frequencies (and those in the neighbourhood) are often occupied by some of the more 'interesting' signals you will find scattered across the radio spectrum.
Around the edges of the broadcasting bands is where you will often find pirate and clandestine stations: the frequency range 6200 - 6300 kHz has long been a short-wave pirate favourite. Such stations typically use much less power than traditional broadcast stations and as such could not compete if they had to battle alongside the MegaWatt signals from stations such as the BBC and Deustche Welle. However at the edge of the band where no-one lives and there are no signals, it's much easier to be heard. Take a look at some of the other frequencies used by pirate stations and you'll almost universally find that they sit roughly on the border between broadcast and non-broadcast users (Laser Hot Hits on 4025 and 9385 kHz for example).
However even the 'big boys' recognise the benefit of sitting in the quiet no man's land at the edge of the broadcast bands. Vatican Radio uses 1611 kHz (edge of the medium wave band); Radio Prague sit on 6200 kHz (edge of the 49 metre band); and Radio Pyongyang (North Korea) inhabit both 7100 and 15100 kHz (edge of the 41 and 25 metre bands respectively). Whilst in theory these stations are operating as 'illegally' as the pirates, spewing interference onto other services, they get away with it by the fact that they are 'internationally recognised'.
Take a tune around the band edges (not just the broadcast bands but other ones too) and see what you find...
Friday 4 November, 2005, 23:20 - Much Ado About NothingA lot has been made in the British press recently of a test which is being introduced by the Home Office to assess immigrants' knowledge of Britain - the 'Britishness Test'. Amongst the many peculiar questions raised in the test, newspapers and television channels alike have reported with surprise that in the UK there are two numbers which can be used to call the emergency services (police, fire, ambulance and coastguard): 999 and 112.
Most UK residents would be able to tell you that 999 is an emergency number, however few, if any, would be aware of the existence of the second option, 112. Which is quite amazing when you consider that 112 has been an emergency number in the UK for nearly 15 years. It stems from one of the (in my opinion) more sensible European Commission decisions (91/396/EEC) which requires all European Union (EU) Member States to adopt 112 as an emergency number by 31 December 1992. At the same time, the international access code was harmonised across EU countries to be 00 through a sister decision (92/264/EEC) to be implemented in the same time-frame.
The great thing about 112 is that you can use it in any EU country to get through to the emergency services, regardless of which country you are in. Of course if you don't happen to speak Czech, Slovenian or Estonian, this may be of limited use but it's a nice idea. And if you're visiting London you're in luck as the Metropolitan Police's call centre which handles emergency calls have been trialling a system which allows them to deal with calls in 150 different languages, particularly those which reflect the diverse ethnic make-up of this sceptred isle (Portuguese, Turkish, Punjabi, Spanish, French and Somali being the most common in London).
Thursday 3 November, 2005, 10:36 - Pirate/ClandestineAny illegal radio transmission runs the risk of being tracked down by the relevant authorities who use sophisticated equipment to monitor the spectrum for unlicensed transmissions. Such equipment is capable of pin-pointing the location of a given transmitter to within a few hundred metres in under a second and together with mobile and hand-held equipment can track down an illegal transmitter with no difficulty. This is easier at VHF and UHF frequencies than it is at frequencies below 30 MHz, as the line-of-sight nature of propagation at these higher frequencies simplifies the tracking operation. At frequencies below about 30 MHz, the accuracy of most of the equipment begins to fall and large, fixed stations are needed to produce any level of detail. Such stations do exist though, and given the fact that MF and HF signals travel large distances, a few of these fixed monitoring stations in two or three neighbouring countries can work together to identify the location of an illegal transmitter (which the mobile teams can then home in on).
It amazes me, therefore, that there are so many pirate radio stations who continue to operate 24 hours per day, 7 days per week. A quick tune across the FM dial in London at lunchtime in the middle of the week yielded the following stations happily pumping out pumping tunes:
87.9 Shine 879
91.8 Passion FM
93.8 Vibes FM
95.4 Roots FM
97.9 Bassline FM
101.9 The Beat
Now it's fair to say that many of these stations are regularly closed down by the authorities, but have sufficient funding and expertise to be back on the air in short shrift. They also tend to use a complicated set of links to connect the studio to the transmitter site such that if the transmitter site is identified, the studio location (where the more expensive equipment and the presenter are located) remains secret. A studio hit on a pirate costs much more to recover from and often leads to prosecution.
It's also fair to say that these staitons have, on the whole, managed to settle on frequencies that tend to minimise the amount of interference they cause to the legal stations. Many pirate stations in London operate only 200 kHz away from legal stations which is far too close to prevent interference (the reasons for which are something to talk about on another day perhaps), but all these stations sit sufficiently far enough away from legal stations (typically a minimum of 400 kHz) that the interference they cause is relatively small. The fact that they are all in mono also improves the situation (and has the bonus for the pirate station that their coverage area is increased). Nonetheless, these stations do cause interference, especially close to the transmitters concerned where the sheer power of the illegal signal overwhealms nearby receivers and the authorities tend to act more virulently where a complaint of interference has been received.
However these aren't the only long-term illegal transmissions that can be found. A recent scan of the short-wave broadcast bands brought to my attention 'Laser Hot Hits' - a pirate station that broadcasts 24/7 on a number of short-wave frequencies (try 4025, 6219, 6285, 7460 and 9385 kHz, though the 7460 kHz signal is often wiped out by a strong BBC DRM transmission from Kvitsoy in Norway on 7465 kHz in the afternoons). What makes this more intriguing than the London pirate situation is that many of the frequencies they are using will cause significant interference not, in this case, to other broadcasters, but to maritime, aeronautical and defence spectrum users, yet their transmissions have remained on-air, seemingly unchecked for many months now. In particular, the 6219 kHz transmission will directly interfere with safety-of-life, emergency maritime communications on 6215 kHz.
So how come they are still on-air? One possibility is that their transmitter site location (not needing to be near to the listeners due to the propagation at HF) is somewhere so remote that the authorities either can't, or can't be bothered, to get to it. A persistant pirate of the late 1980's, 'Radio Fax' used to transmit from a rural location in Ireland and the Irish authorities who have always been much more tolerant to pirate transmissions than many other countries, were very sluggish to step into action. Maybe Laser Hot Hits' transmitter site is on one of the mountains of the Netherlands, which the local mountain rescue teams can't reach! Wherever it is (or they are - there is no reason to suspect that all the frequencies are transmitted from the same site) they have obviously escaped scrutiny so far.
So what conclusions can we draw from this? Perhaps those in power accept 'pirate radio' as a problem that will never go away and for stations who act responsibly turn a blind-eye (remember that pirate radio has had a greater impact on the shape of the UK radio industry than possibly any other factor, and even has a major impact today). Perhaps they do not have the resources to track down every station and concentrate on those who cause the biggest sores (e.g. those with outspoken political views or whose transmitting equipment is of low quality and is causing lots of interference). Perhaps they wait for compliants to be raised before tackling the problem. Whatever the reason, one must doff one's cap to those who continue to flout the law for our entertainment. Many are outlets for music which ordinary stations turn their back on but which has a large, cult, following. Some are there just for the fun of it (don't tell me that doing something naughty isn't at least a little bit thrilling...?) Gentlemen (and ladies), I raise my glass to you.