Global System Mobile, GSM, 2G
7.3 Radio Specifications
7.3.5 GSM Adaptation to a Wideband Propagation Channel
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Figure 7.12 Structure of training sequence. 5 16 5
to ‘equalize’ the effects of multipath propagation. The impulse response can sometimes change even during a burst. It may happen if the terminal velocity is very high and also in a higher frequency band such as 1800 MHz. Therefore, if the setting of the channel equalizer was optimized with respect to the impulse response at the beginning of the burst, the equalization may be suboptimal for the last part of the burst. This results in an increased error rate. The degradation becomes larger for a large width of the Doppler spectrum (depending on the terminal speed and the radio frequency).
To avoid the complication of having to adapt the channel equalizer to variations in the impulse response of the propagation channel during a time slot, short slots are used and, in addition, the training sequence is placed in the middle of the burst. Consequently, the first section of the burst must be stored before demodulation can proceed. The training sequence consists of a 16-bit sequence extended in both directions by copying the first five bits at the end of the sequence and the last five bits at the beginning, see Figure 7.12.
The specifications [3] define eight different training sequences (‘colour codes’) for use in the normal burst, each with low cross-correlation properties following GMSK mod- ulation. This reduces the risk of synchronizing to a distant strong co-channel carrier.
7.3.5.2 The Channel Equalization
The normal burst, which contains both the data and the training sequence, is passed through a baseband modulator at the transmitter and then through the mobile radio channel. The received waveform will contain an ISI caused by the multipath propagation in the radio transmission channel.
At the receiver, the burst is de-multiplexed into two bit streams to separate the train- ing sequence and the data bits. The received training sequence is used to estimate the impulse response of the radio channel in the channel estimator. The channel estimator is used to produce waveforms for possible combinations of the sequence of data bits.
Since the channel estimator contains some ambiguity function due to a time windowing of estimation processing, further processing is done according to the Viterbi algorithm of soft decoding.
In practice, the time window for channel estimation is limited in size. On one hand, it has to contain the most significant multipath components; that is, it should accommo- date an excess delay spread of the channel. On the other hand, the size of the window corresponds to a number of states in a soft decoder, which directly affects the complexity of the Viterbi channel equalizer. A consecutive increase in delay (size of the processing window) corresponding to a bit duration doubles the number of states in the equalizer.
The maximum reasonable complexity at the time of GSM development was considered to be a limit equalizer window of up to four radio symbols.
An almost worst-case time dispersion in the mobile environment was estimated at about 16μs that approximately triples the delay spread of the urban channel, see
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Equaliser window (~ 3 x delay spread)
Rx sensitivity level
Propagation delay Figure 7.13 Equalizer window versus delay spread.
Figure 7.13. The equalizer window with a size of 16μs should accommodate four radio symbols, thus imposing a symbol length of about 4μs.
Recall that the training sequence consists of a 16-bit sounding sequence with five bits appended to either end. Those bits allow derivation of channel estimates for delay spreads (width of impulse response of the channel) of five bit periods. This is adequate for the specified urban and rural channels for GSM that have excess delay spreads of 5 and 0.5μs, respectively. As a consequence, perfect autocorrelation of the training sequence is ensured over a shift of up to five bits. Delays in excess of 5 bits of preamble may cause errors. These errors could be removed by the de-interleaver and FEC decoder.
It could be noted that GMSK signal may experience severe inter-symbol interference when the delay spread in the channel is greater than just 10% of a symbol duration. For speech transmission, a BER of order 0.3% is tolerable, so an equalizer with a resolution of about half a bit duration is appropriate for these purposes. However, requirements for data transmission are much greater and one of the possible strategies is to deploy Automatic Repeat Request protocol (ARQ).
7.3.5.3 Diversity Against Fast Fading
When either the receiver or scattering objects move with time, the received signal expe- riences time fluctuations caused by fading. At the input of the decoder/demodulator the signal is composed by slow fading+fast (Rayleigh) fading+noise components. The time scale of the noise fluctuationstN is determined by the receiver bandwidthB,tN = 1∕B, while the time scaletfof Rayleigh fading is given bytf = 𝜆∕2v, wherevis a speed of mobile unit or typical speed of scattering objects.
Roughly, the fading time scale is the time that elapses between two fading dips and gives an estimate of minimal duration between fades; that is, free of fade interval.
Actual duration between fades will depend on the fade’s amplitude. For the GSM-band (900 MHz), the spatial distance between two dips is about 15 cm so, if a mobile is travelling with a speed ofv=50 km∕h, the time between two dips is
tf = 0.3m
2•14m∕s =10.7ms
that lasts approximately two GSM frames (4.615 ms).
The fade duration is an important parameter for the design ofchannel coding and interleavingschemes, as well as ARQ protocol. Spatial correlation properties of the fades are needed for design of the space diversity systems.
Instead of antenna diversity in handheld terminals, a combination of channel coding, interleaving and coordinated frequency hopping is used to obtain a diversity gain in respect to the multipath fading. A necessary condition is that the propagation channel has fairly large time dispersion. Together, these features of GSM give such high diversity and coding gains that the required protection ratio (the local mean over the fast fading) will typically be 9–10 dB.
Interleaving a full-rate traffic channel means that the 456 bits in a 20-ms speech frame are split up into 57-bit sequences that are spread out over eight TDMA frames;
that is, over 40 ms (see Section 7.9.2). If the duration of a fading dip is not more than few milliseconds, typically only one user time slot (one TDMA frame) is affected. The de-interleaver will then change the error burst to a relatively random error sequence spread over eight code words. Thus, one-eighth of the bits in each code word will be subject to a BER of about 50%. In a most situations, Forwards Error-Correction coding is sufficient to mitigate these errors.
7.3.5.4 Frequency Hopping
Fading dips that are longer than channel coding with interleaving can cope with, occur- ring over quasi-stationary propagation paths, something that affects portable terminals in a pedestrian environment. In this case, a fading dip could affect many consecutive TDMA frames reducing the effect of interleaving. In other words, time diversity does not work while frequency diversity may help for slow-moving terminal.
GSM’s method of frequency diversity is frequency hopping, whereby each physical channel is switched between different radio channels. For each TDMA frame the carrier frequency is changed, see Figure 7.14. The size of a typical frequency hop should ideally be large enough to give almost uncorrelated fast fading in the different frequency slots.
In practice, coherence bandwidth of GSM channels can vary from several hundred kHz to a few MHz. Given that an operator typically owns only a few MHz of spectrum, and only a subset of frequencies can be used instantly in each cell, fading in channels used for hopping can still be correlated.
Nonetheless, frequency hopping provides certain a advantage even in this case:
co-channel interference from other cells, in particular, is whitened and leads to less interference on average and tighter frequency reuse becomes possible.
While frequency hopping was specified only as an option, it is a norm in commercial mobile networks. Without frequency hopping, a far higher dB protection ratio would be needed for portable terminals. Frequency hopping cannot be used for the main sig- nalling radio channel, the Broadcast Carrier. The Broadcast Carrier must be on a fixed frequency known by the terminals. In some system implementations, a separate and larger cluster size is used for the Broadcast Carrier compared with the rest of the fre- quency hopping traffic channels. A traffic channel or dedicated control channel may hop into the broadcast carrier frequency as part of its hopping sequence, but their power is then adjusted to the level of Broadcast Carrier.
Frequency hopping could be implemented in two ways: as a baseband or RF synthe- sizer hopping. In baseband hopping, each transmitter operates on a fixed frequency.
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0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
5 6 7
Frame n Frame n + 1 Frame n + 2
Frame n Frame n + 1 Frame n + 2
Downlink (mobile receives)
Uplink (mobile transmits)
0 1 2 3 4 5
0 1 2
1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5
0 1 2 3 4 5 6 0 1 2 3 2 5 6 7 0 1 2 3 5
MS monitoring neighbor adjacent channels f1
f2 f3 f4
f5 f6 f1ʹ f2ʹ f3ʹ f4ʹ
fmʹ = fm– 45 MHz
4 7
2 42
0 2 02
02
Figure 7.14 Frequency hopping concept.
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Tx,f0
Tx,f1
Tx,f2
Tx,f3
Filter Combiner TRX 1
TRX 2
TRX 3
TRX 4
Bus for routing of bursts Figure 7.15 Baseband frequency hopping.
At transmission, all bursts, irrespective of which connection they belong to, are routed to the transmitter of the proper frequency, see Figure 7.15.
The advantage with this mode is that narrowband filter combiners can be used with very small insertion loss. This makes it possible to use many transceivers without having to connect several combiners in cascade. The disadvantage is that it is not possible to use a larger number of frequencies in the hopping sequence than the number of available transmitters in the cell.RF Synthesizerhopping means that one transmitter handles all bursts that belong to a specific connection. The bursts are sent ‘straight on forwards’
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Tx, f0...fn TRX 1
TRX 2
TRX 3
TRX 4
Tx, f0...fn
Tx, f0...fn
Tx, f0...fn
Hybrid combiner
Hybrid combiner
Figure 7.16 RF synthesizer frequency hopping.
and not routed by the bus in contrast to baseband hopping. The transmitter tunes to the correct frequency at transmission of each burst, see Figure 7.16.
The advantage is that the number of frequencies that can be used for hopping is not dependent on the number of transmitters. It is possible to hop over a lot of frequen- cies even if only a few transceivers are installed. The gain from frequency hopping can thereby be increased. This concept is often called fractional loading. A disadvantage with synthesizer hopping is that wideband hybrid combiners have to be used. This type of combiner has approximately 3 dB loss making more than two combiners in a cascade impractical.