Predeinterleaving soft decision performance for MFSK detection on the Rayleigh fading channel

The authors consider MFSK (M-ary frequency shift keying) communications systems that achieve time diversity by using convolutional interleaving of L repeated chips per symbol. Two system processing options are investigated. The first is that of conventional deinterleaving followed by demodulator magnitude accumulation and soft decision bit recovery. The second is predeinterleaver soft decision, in which demodulator magnitudes are soft detected before recombination. The soft decision metric used in each case is the difference of maximums algorithm. The advantage of soft decision before deinterleaving is a significant deinterleaver memory reduction at the cost of a performance degradation. This degradation is quantified for an ideal Rayleigh fading channel by simulation for a range of chip repeats and bit energy to noise ratios. Preliminary results indicate an implementation loss of 1-4 dB or less for diversities up to 16 over the signal-to-noise ratios of interest. System implementation complexity reductions for several deinterleaver cases are also given