Code and constellation optimization for efficient noncoherent communication

Abstract

We consider noncoherent communication techniques with large signal alphabets, where explicit (i.e., pilot symbol based) channel estimates are not required by the receiver and iterative soft information exchange between noncoherent demodulator and channel decoder is employed for approaching Shannon-theoretic limits. Extrinsic Information Transfer (EXIT) functions are used to characterize convergence of the iterative receiver, enabling joint optimization of the inner modulation code constellation and symbol mapping with an outer binary channel code. We find that QAM constellations classically employed over the AWGN channel are inferior to modified constellation shapes based on aligned PSK rings with Gray-like amplitude/phase bit maps. EXIT analysis of turbo noncoherent communication further shows that standard convolutional codes are near optimal in serial concatenation with a unit-rate inner modulation code. The overall system is within 2.4 dB of Shannon capacity for the block fading channel at 1.8 bits/channel use, demonstrating that bandwidth-efficient noncoherent communication systems with reasonable complexity are now within reach.