A Rateless Coded Protocol for Half-Duplex Wireless Relay Channels

We propose a rateless coded protocol for a half-duplex wireless relay channel where all links experience independent quasi-static Rayleigh fading and the instantaneous channel realizations are unavailable at the transmitters. We assume that the network does not have a stringent delay constraint-thus the source and/or the relay continue transmitting until the destination acknowledges successful decoding. We identify rateless coded relaying as the natural choice, where each transmission from the source and/or the relay adds incremental redundancy to help the destination recover the original message. Our proposed protocol utilizes, in conjunction with rateless coding, a combination of the two popular relay cooperation schemes, namely decode-forward and compress-forward. Assuming very limited feedback from the destination, we derive the theoretical performance bounds specifically with binary phase-shift keying. We then implement the rateless coded relaying protocol using Raptor codes. The degree profiles for the Raptor codes are designed to maximize the average throughput-with the design formulated as a convex optimization problem. Using discretized density evolution for asymptotically large block lengths, the optimized codes lose approximately 5% in performance from the theoretical limit, whereas with practical finite block lengths, the performance loss is approximately 9%.