Noncoherent demodulation for cooperative diversity in wireless systems

This work develops a general framework for maximum likelihood (ML) demodulation in cooperative wireless systems with a demodulate-and-forward (DF) protocol at the relays. Although this general structure admits both coherent and noncoherent modulation, we analyze the performance of this demodulator mainly for noncoherent demodulation and compare it to existing results for coherent demodulation. The ML demodulator consists of a band of square-law devices followed by a nonlinear combiner. We develop an accurate approximation for this demodulator using a band of square-law devices followed by a piecewise-linear (PL) combiner. This approximation not only has certain implementation advantages, but also leads to a tight closed form approximation for the bit error rate (BER) of the ML demodulator. Based on this closed-form result, we derive a high signal-noise-ratio (SNR) approximation, that provides a simpler but tight approximation of the BER of the ML demodulator.