GSM cellular phones use a combination of Time and Frequency Division Multiple Access. TDMA simply means that several stations can share a frequency by taking turns. The difficult bit is making them think they’ve all got the channel to themselves. FDMA simply means that the spectrum is split into channels spaced 200 kHz apart. The TDMA works by transmitting taking place during 0.577mS burst periods. There are eight of these bursts in a TDMA frame which lasts 4.615 mS. One channel consists of a single burst per frame. Two channels are needed for two-way communications of course. The result is that four duplex conversations can take place on the same frequency at the same time.
Channels are either common channels, which are used by mobiles in “idle” mode, or dedicated channels which are used for calls in progress. Remember though that these are “virtual channels” – they are simply slots for data to be exchanged, not RF channels!
Dedicated channels contain sequences of 26 TDMA frames. Each sequence lasts 120mS and contains one TDMA frame used for control purposes, 24 for calls, the last being unused. The channels for uplinking and downlinking a call are separated by 3 bursts, so there is maximum separation between transmit and receive periods.
Common channels are used to set up calls and arrange handover between base stations. There are six different types of common or control channel. The Broadcast Control Channel sends base station ID, frequency plan and frequency hopping schedule. The Frequency Correction Channel together with the Synchronisation Channel define when the burst periods happen and the time slot numbering. Another channel is the Random Access Channel which is used by mobiles to log onto the network. The Paging Channel is used to inform mobiles about incoming calls. Finally the Access Grant Channel is used to give a mobile permission to use the signalling channel to set up a call.
The modulation technique is called Gaussian-Filtered Minimum Shift Keying (GMSK). It’s designed to minimise spuraii while still having good spectral efficiency and not needing too much complexity in the modulator.
The digital speech coding is called Regular Pulse Excited – Linear Predictive Code (RPE-LPC). This works in a similar way to MPEG video. The coder predicts the next sample based on the last few. The decoder can also predict the sample the same way. Most predictions are quite close to the actual sample in which case the system will send just the difference between prediction and sample. This needs much less data than sending the complete sample. Speech is divided into 20 mS samples each of 260 bits, giving a data rate of 13kbit/s.
Error correction is added to the speech data with the bits likely to cause the worst errors having better correction. If the sample is too corrupt to correct it is replaced with an attenuated version of the previous sample. Radio channels suffer from bad burst errors, typically caused by impulse interference, so the samples are interleaved (ie shuffled around in time). Each burst in fact carries data from two different samples. The result of interference on an interleaved signal is a reduction in quality rather than complete drop-out for the duration of the interference.
The system has auto equalisation to minimise the effect of multipath fading. A known 26 bit “training sequence” is transmitted in the middle of every burst. After checking the sequence for errors the effect of multipath on the off-air signal can be calculated and an inverse filter applied.
The final twist to this amazingly complicated system is frequency hopping. The mobiles have to be frequency agile because transmit, receive and adjacent base station monitoring all take place on different frequencies. The GSM system makes use of the ability by applying slow frequency hoppping, where each TDMA frame is transmitted on a different carrier frequency. The algorithm for this is sent on the control channel. There are two reasons for frequency hopping – reducing co-channel interference and multipath effects. Both effects will still impair signals but will only cause momentary problems rather than complete drop-outs.
“Digital transmission systems are like hovercraft. They fly – but only just.” – John Wilkinson
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