Dynamic Wireless Network Coding With Overhearing and Variable Channel Rates

We study a one-hop broadcast channel with two receivers. The receivers have side information obtained by overhearing wireless channels. The relay takes control decisions by coding transmissions based on its knowledge of side information in the receivers. We consider two control mechanisms. In the ACK system, the relay has definite knowledge of side information announced via overhearing reports. In the NACK system, the relay has statistical knowledge of side information and receives feedback after every decoding failure. Our contribution is as follows. We provide the minimal evacuation times for the two systems and obtain analytical expressions of the throughput region for the ACK and the code-constrained region for the NACK system. When the transmission rates are the same (r₁ = r₂) or when the receiver with the highest transmission rate has perfect side information (p_f=1), we show that the two regions are equal. We then provide simple joint xor coding and scheduling policies that achieve those regions and, thus, are throughput optimal. Subsequently, we evaluate the report overhead performance for both mechanisms and reflect on the involved tradeoff with throughput. Ultimately, we demonstrate by simulations that the proposed throughput optimal policies can be appropriately enhanced to have good delay properties, particularly for protocols that utilize sequenced packet delivery.