Towards the capacity of noncoherent orthogonal modulation: BICM-ID for turbo coded NFSK

This paper investigates coded communications using noncoherent orthogonal modulation and capacity-approaching binary channel codes. The focus is on the interface between the decoder and demodulator, which is critical for large modulation order M. While standard receivers for bit-interleaved coded-modulation (BICM) segregate the operations of demodulation and decoding, we follow the recently proposed paradigm of BICM with iterative decoding (BICM-ID) to approximate joint demodulation and decoding through the iterative exchange of soft information between demodulator and decoder. We focus our attention on the derivation of the soft-input/soft-output SISO demodulator for noncoherent M-FSK, and derive a log-domain SISO demodulator suitable for channels with uniform phase and fading amplitudes that are either known or constant (i.e. AWGN). Simulation results are shown for M = 2,4,16, and 64 using the well-known UMTS turbo code. It is found that feeding soft information from the decoder back to the demodulator improves performance by between 0.7-0.9 dB for 16-ary and 64-ary NFSK in AWGN and Rayleigh fading.