Turbo-NFSK: iterative estimation, noncoherent demodulation, and decoding for fast fading channels

This paper considers the problem of communicating over fast fading channels, where the channel coherence time is only on the order of a few symbols. Since the fading is too fast for coherent reception, we employ M-ary frequency shift keying with soft noncoherent demodulation. Information is encoded by a binary turbo code. To improve performance, the soft demodulator and decoder work cooperatively through the iterative exchange of extrinsic information. During each iteration, the receiver estimates the channel state information (CSI), here defined to be the average received signal energy and noise spectral density for each block of symbols. The channel estimator uses the expectation maximization (EM) algorithm and exploits extrinsic information fed from the decoder. Simulation results show that for 16-NFSK in block independent Rayleigh fading, performance can be within 0.6 dB of the perfect CSI case by using blocks as small as 4 symbols.