Broadcast Coding and Successive Refinement for Layered UE Cooperation in Multi-User Downlink

A multi-user downlink system is considered, in which a base station (BS) serves multiple user equipments (UEs). It is assumed that, to improve the downlink performance, the UEs can exchange compressed versions of the downlink received signals on out-of-band device-to-device (D2D) communication links. Since the D2D links from a UE to the other UEs may have different channel qualities across the receiving UEs, this letter proposes a layered cooperation strategy which combines broadcast coding and successive refinement quantization techniques. With this approach, a UE can have a better reconstruction of the downlink signal of the transmitting UE than another UE with worse D2D channel quality. The problem of jointly optimizing the downlink precoding and layered UE cooperation strategies is tackled, and numerical results show the performance gains of the proposed layered scheme as compared to baseline schemes with no or single-layer cooperation.