Performance Bounds for MLSD Reception of OFDM Signals in Fast Fading

For OFDM systems in fast fading, it is difficult to obtain a closed form expression for the OFDM symbol error probability. Thus, tight analytical bounds on actual performance are extremely useful for performance prediction and verification. Additionally, the structure of the bound provides insight into system behavior. A new expurgated 2-dimensional union bound is proposed and applied to an OFDM system in fast fading. This bound becomes extremely tight as the rate of fading L increases. Performance comparison of the new bound to a lower bound on a known expurgated union bound demonstrates a gain of up to 1.5 dB at P(e) = 10 2 for channel implicit diversity order of L = 4. A new simulated upper bound that uses a reduced state vector in a modified trellis search algorithm and a simulated lower bound that assumes limited knowledge of the ICI are also presented. Both bounds are derived from a "time-varying" finite state machine model of the received signal. Performance results for these bounds are extremely tight for small to large values of N, where N is the number of OFDM signal tones. Also, the reduction in computational complexity achieved for N = 512 is from 2⁵¹² to 2⁶ for both bounds.