Signal Transmission With Unequal Error Protection in Wireless Relay Networks

This paper studies a relaying technique based on the signal-to-noise (SNR) threshold in cooperative networks using hierarchical modulation. Hierarchical modulation is useful in applications that require different protection classes of the information. In particular, a cooperative network with one source, one relay, and one destination is considered. Two different protection classes are modulated by a hierarchical 2/4-amplitude shift keying (ASK) constellation at the source. Based on the instantaneous received SNR at the relay, the relay decides to retransmit both classes by using a hierarchical 2/4-ASK constellation, or the more-protection class by using a 2-ASK constellation, or remains silent. Optimal thresholds are chosen to minimize the bit error rate (BER) of the less-protection class, whereas the BER of the more-protection class meets a given requirement. Numerical and simulation results are provided to verify the analysis. The results show that the optimal thresholds significantly improve the performance.