WLC37-3: Diversity Benefits of OFDM in Fast Fading

Time-selective fading, which introduces interchannel interference in a received orthogonal frequency division multiplexing (OFDM) signal, is often viewed negatively as an impairment which needs to be cancelled or removed. This paper shows that time-selectivity is desired. The fast fading process provides an implicit diversity which an OFDM signalling scheme can most effectively take advantage of. Results show a performance gain in signal-to-noise ratio (SNR) of about 11 dB at an error rate of 10^-2 for an 8-tone system in fast fading with diversity order 4, relative to that in slow fading. Performances for optimal and suboptimal OFDM receivers are compared. The optimal receiver generates the sufficient statistics input to an optimal search algorithm, A* to determine the resultant symbol vector. The suboptimal receiver, uses the fast Fourier transform (FFT) of the time samples of the ideal bandpass filtered continuous time signal in the A* algorithm. The sampling process is lossy with respect to information and degrades performance. For N = 4, the optimal receiver outperforms the suboptimal by an SNR of more than 6 dB in a fast fading channel with diversity order 4 at an error rate of 10^-2 dB. and for N = 8, by more than 5.