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Turbo NFSK Iterative Estimation Noncoherent Demodulation and Decoding for Fast Fading Channels Shi Cheng and Matthew C Valenti Don Torrieri West Virginia University U S Army Research Laboratory Morgantown WV Adelphi MD shic mvalenti csee wvu edu dtorrieri arl army mil ABSTRACT 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 INTRODUCTION Bit interleaved coded modulation BICM 1 combines binary forward error correcting coding with M ary modulation It has become a standard method for signaling over fading channels forming the basis of third generation cellular and 802 11a g wireless networks The performance of BICM can be improved by feeding back a priori information in the form of bit likelihoods from the decoder back to the demodulator Such iterative demodulation and decoding schemes were independently developed by ten Brink 2 Benedetto et al 3 and Li and Ritcey 4 The latter reference terms this technique bit interleaved coded modulation with iterative decoding BICM ID other option is to use orthogonal signaling and noncoherent reception This is more appropriate when either dealing with fast fading



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