In comparison to a FDMA system supporting the same user data rate the transmitted data rate in a TDMA system is larger by a factor equal to the number of users sharing the frequency band. This factor is eight in the pan-European GSM systems and three in the D-AMPS system. Thus, the symbol rate is reduced by the same factor and severe intersymbol interference results, at least in the cellular environment.
To illustrate, consider the example from above where each user transmits 25 K
symbols per second. Assuming eight user per frequency band leads to a symbol
duration of 5 
sec.
Even in the cordless application with delay spreads of up to 1 sec, an
equalizer may be useful to combat the
resulting interference between adjacent symbols.
In cellular systems, however, the delay spread of up to 20 sec introduces
severe intersymbol interference spanning up to 5 symbol periods.
As the delay spread often exceeds the symbol duration the channel can be
classified as frequency selective, emphasizing the observation that the channel
affects different spectral components differently.
The intersymbol interference in cellular TDMA systems can be so severe that linear equalizers are insufficient to overcome its negative effects. Instead more powerful, non-linear decision feedback or maximum-likelihood sequence estimation equalizers must be employed [Proakis, 1991]. Furthermore, all these equalizers require some information about the channel impulse response which must be estimated from the received signal by means of an embedded training sequence. Clearly, the training sequence carries no user data and, thus, wastes valuable bandwidth.
In general, receivers for cellular TDMA systems will be fairly complex. On the positive side of the argument, however, the frequency selective nature of the channel provides some ``built-in'' diversity which makes transmission more robust to channel fading. The diversity stems from the fact that the multi-path components of the received signal can be resolved at a resolution roughly equal to the symbol duration and the different multi-path components can be combined by the equalizer during the demodulation of the signal. To further improve robustness to channel fading coding and interleaving, slow frequency hopping, and antenna diversity can be employed as discussed in connection with FDMA.
