Cooperative communications has gained a lot of attention in the research community recently. This was possible due to the fact that the broadcast nature of wireless networks, which was earlier considered a drawback, can now be used to provide spatial diversity to increase throughput, reduce energy consumption and provide network resilience. The main drawback of cooperative communication is that it requires more bandwidth compared to traditional communication networks. Decode and Forward (DF) is one of the cooperative communications forwarding strategy where the relay nodes first decodes the data and then retransmit it to the destination. DF requires advanced techniques, which can be used at the intermediate relay nodes to improve spectrum utilization.

Some well-known techniques for spectrum efficiency are Network Coding (NC) and Hierarchical Modulation (HM). In NC technique, nodes in a network are capable of combining packets for a transmission, thus reducing number of transmissions. HM is a technique which allows transmission of multiple data streams simultaneously. Both HM and NC are useful techniques for spectral efficiency.

In this work, we evaluate the performance of a joint HM and NC scheme for two-way relay networks. The relaying is based on the signal-to-noise (SNR) threshold at relay. In particular, a two way cooperative network with two sources and one relay is considered as shown in fig 1. Two different protection classes are modulated by a hierarchical 4/16 - Quadrature Amplitude Modulation (QAM) constellation at the source. Based on the instantaneous received SNR at the relay, the relay decides to retransmit both classes by using a hierarchical 4/16 - QAM constellation, or the more-protection class by using a Quadrature Phase Shift Keying (QPSK) constellation, or remains silent. These thresholds at the relay give rise to multiple transmission scenarios in a two-way cooperative network. Simulation results are provided to verify the analysis.


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