A communications system comprising a base station, and a plurality of terminals served by that base station which may include an ad-hoc network of terminals. Information from the base station or transmitting terminal is transported to the receiving terminals in the form of symbols in an orthogonal frequency division multiplexed format. Each terminal has means for measuring a channel dispersion and rate of phase variation corresponding to multipath signal reception for that terminal and for reporting that dispersion and variation measure to the transmitting station. In response to the channel dispersion and phase change measurements, the base station or transmitting terminal is arranged to provide, for groups of terminals having a similar channel dispersion, respective symbol and prefix lengths on the basis of the dispersion measurement of that group of terminals.