Optimization of Hierarchical Modulation for Decode-and-Forward Wireless Relay Networks

This paper presents two designs of optimal nonuniformed constellations for decode-and-forward wireless relay networks with an orthogonal space-time block code (STBC). The first design is concerned with the unequal error protection issue, in which two data streams to be transmitted from a source to a destination are protected at two different levels. The design is to minimize the bit error rate (BER) of one stream while maintaining the BER of the other stream to be no larger than a given threshold. Based on a simple approximation of the average BER, a design parameter is obtained in closed form. The second design focuses on minimizing the average BER of the combined data stream at the destination. Developed are two near-optimal designs for 4/16-quadrature amplitude modulation (QAM) and 4/64-QAM hierarchical modulation (HM) schemes. The designs are also extended to two-way relay networks where both the source and the destination have data to transmit to each other. Simulation results confirm the superiority of the proposed designs and the advantages of relay-assisted transmission employing HM over conventional point-to-point transmission. A comparison with the existing relay-assisted transmission model is also discussed.