Harnessing Interference with an Out-of-Band Relay: an Approximate Capacity Result

This work studies the Gaussian interference channel (IC) with an out-of-band relay (OBR) from an information-theoretical perspective. The relay works in bands orthogonal to the IC. The focus is on a symmetric channel model, in order to understand the fundamental impact of the OBR on the signal interaction of the IC, in the simplest possible setting. We classify the interference links as extremely strong, very strong, strong, moderate, weak, and very weak. We find that interference is useful to improve the achievable rates when it is extremely strong or very strong. Specifically, when interference is extremely strong, the channel acts as if there are two disjoint OBRs helping each source-destination pair. For strong interference, separate encoding, i.e., transmitting independent messages thorough the IC and OBR, is optimal, while for moderate interference, separate encoding is close to optimal. For weak or very weak interference, we show that the OBR can help improve the rates for common messages and further reduce the interference. It is shown that our strategy achieves the symmetric capacity to within 1.14625 bits per channel use for all channel parameters.