Protection-Level-Exchanging Hierarchical Modulation for Multiresolution Services Under Decode-and-Forward Cooperative Networks

In the two-level hierarchical modulation (HM), two data streams—the base layer and refinement layer—are transmitted with different degrees of protection according to their importance. In general, the refinement layer suffers from insufficient protection for users experiencing poor channel conditions. Although cooperative transmission can improve the reception performance via diversity combining the signals from the source node and relay node, the refinement layer may still have unacceptable performance for users with very poor signal quality. To improve the overall performance of multiresolution services, we propose protection-level-exchanging HM for multiresolution services under decode-and-forward cooperative networks. At the relay node, it exchanges the protection levels of the decoded base bits and refinement bits, and remodulates the data based on a redesigned optimal HM constellation mapping for transmission. Hence, the overall bit error rate (BER) of the refinement bits is significantly improved at the destination node. We also propose the weighted combined BER (WCB) to fairly and reasonably evaluate the overall service performance. Based on the WCB, we investigate the optimization of the constellation priority parameters used at the source node and relay node to optimize system performance. According to the simulation results, our proposed scheme achieves better performance than other existing schemes.