Asymmetric Modulation Gains in Network Coded Relay Networks

Wireless relays have usually been considered in two ways. On the one hand, a physical layer approach focused on per-packet reliability and involving the relay on each packet transmission. On the other, recent approaches have relied on the judicious activation of the relay at the network level to enhance data transmission of groups of data packets, generally using some packet-level coding approach. Network coding's recoding capabilities at intermediate nodes make it particularly suitable for the later. This paper's idea falls in between these two research lines. We aim to increase the end-to-end throughput by (i) using network coding to control the use of the relay in an effective manner, and (ii) leveraging asymmetric modulation to enable the source to efficiently allocate its resources. We provide mathematical analysis and a simple optimization mechanism. Numerical results for the case of fading channels and standard error correcting codes at the physical layer show that gains of up to four-fold gains with respect to the use of the direct link are possible for a wide range of locations of the relay in between the source and the destination.