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. For strong and moderate interference, separate encoding is close to optimal. For very strong and extremely strong interference, the interference links provide side information to the destinations, which can help the transmission through the OBRC. In particular, when interference is extremely strong, the channel acts as if there are two disjoint OBRC helping each source-destination pair. For moderate or weak interference, we use the Han-Kobayashi scheme for the IC, where we split the messages into common and private messages. We find that it is beneficial to further split the common message into two parts, and the OBRC plays an important role in decoding the common messages. It is shown that our strategy achieves the symmetric capacity to within 1.14625 bits per channel use for all channel parameters.