Layer-2 retransmission and combining for network coding-based forwarding in wireless networks

Network coding is an effective method to improve channel efficiency by reducing the number of transmissions. However, most existing network coding schemes are based on network layer broadcast. As a result packets become vulnerable to channel errors due to lack of retransmissions. In this paper we present a cross-layer signal processing technique and show that network coded signal mixed from different flows can be separated in the physical layer by exploiting a priori knowledge. Then we propose the soft network coding framework for multi-hop forwarding. This framework enables layer-2 retransmission and further reduces packet errors by combining retransmitted copies. It supports both conventional unicast and the proposed network coding based bi-cast. Simulation results show that compared with plain Decode-and-Forward (DF), the proposed scheme can reduce air-time by about 30% for Poisson arrival traffic while retaining almost the same packet error rate as DF with frame combining.