Superposition coding and linear network coding for reliable multicast over fading channels

In many military applications, communication messages must be multicast to a group of receivers. When these transmissions are broadcast over a wireless channel, fading can result in the messages only being received by a subset of the intended receivers on any given transmission. The conventional approach of retransmission via automatic repeat request (ARQ) results in the performance being dominated by the single receiver that experiences the worst propagation conditions for each broadcast packet. Recently, it has been shown that linear network coding can be used to reduce the average number of transmissions required to deliver the multicast message by making it possible for a receiver to recover B packets that are coded together after correctly receiving any B transmissions. In this paper, we propose to further improve the performance by taking advantage of the diversity of fading gains that can exist during each message transmission. During each transmission, superposition coding (SPC) is used to simultaneously transmit multiple linear network-coded versions of the packets. The number of copies that a particular receiver can recover depends on the receiverpsilas fading process: the better the channel conditions, the more copies of the network-coded packet can be recovered. We show that by appropriately choosing the parameters of the SPC, the average number of transmissions can be reduced in comparison to previous approaches in some scenarios.