Signal design for multipath fading channels-Doppler and ISI barriers

Signal design for digital communications in linear multipath fading channels is discussed. We define two extremes for the symbol rate in single-carrier systems, i.e., the Doppler barrier and the intersymbol interference (ISI) barrier and we note that it is difficult to go beyond these two barriers. The Doppler barrier is roughly equal to the Doppler spread of the channel. The ISI barrier is roughly equal to the inverse of the multipath spread of the channel. We use the time-frequency duality and show, that above the ISI barrier it is advantageous to use frequency multiplexed signals like orthogonal frequency division multiplexing (OFDM) signals, and below the Doppler barrier it is advantageous to use the corresponding time-multiplexed signals. An important parameter defining the measurability of the channel is the spread factor or the product of the Doppler spread and the multipath spread. Communications in overspread channels in which the spread factor is larger than unity is also discussed. All the results should be useful for system designers.