Network coded retransmissions in wireless demodulate-and-forward relay channels

Consider the case where a source wishes to reliably deliver a number of data packets to the destination with the help of a demodulate-and-forward relay. We discuss two retransmission schemes, each of which can ensure that the packets from the source will be delivered to the destination without error. We first discuss a scheme that combines signals from the source and the relay at the destination using a weighed combiner. The use of network coding which facilitates the retransmission process is then proposed. The generation of network-coded retransmission packets takes the qualities of previously erroneously received packets at the destination into account, and we show that such network coding scheme can make the retransmission process more efficient even if there is only one destination, unlike the existing approaches where the gain of network coding must come from coding opportunities across multiple destinations. Simulation results show that while the first retransmission scheme performs well in the low signal-to-noise ratio (SNR) region, the second scheme has good performance in both low and high SNR regions, thanks to the use of network coding. Moreover, the second scheme induces smaller packet decoding delay than the first scheme.