Design of OFDM systems for frequency-selective and time-variant channels

We show that frequency-selectivity and time-variance represent adverse constraints for the design of wireless OFDM systems. This observation is important since OFDM is a promising candidate for the realisation of broadband wireless systems, which by definition transmit over frequency-selective and time-variant channels. We present a convenient design criterion for the OFDM symbol duration. The criterion is relating the two basic channel parameters, the maximum excess delay time and the maximum Doppler frequency, with two design parameters reflecting the bandwidth efficiency and the robustness of the system. The introduction of the latter two parameters allows the efficient design of OFDM systems by providing information on the system reserves. Furthermore, we show how the use of multiple antenna arrays exploiting spatial diversity can be incorporated in the system design.