Precoder Design for Cooperative Multi-User Downlink MISO Channels with Finite Side-Link Capacity

For a pair of closely located and cooperating user equipments (UEs), a precoder is designed at the base station (BS) which considers the UE cooperation. The UEs collaborate over an out-of-band side-link with a finite capacity. The side-link may be used for interference cancellation (IC) or for relaying decoded symbols. The precoder has two parts, a cooperative part based on channel inversion and a non- cooperative part based on the minimum mean squared error (MMSE) metric. It is the interference from the former part which is managed at the UEs in the cooperative IC phase. The latter part is included at the BS to further improve the performance if there is transmit power remaining at the BS after the cooperative phase; this due to the assumption of finite capacity side-link. To study the performance of the precoder design, various joint optimization problems that maximize a sum-rate objective are considered. The effects of power imbalance and correlation between the UEs are included into the problems. From the numerical results it is observed that the sum-rate increases with cooperation. Further, with an increase in the side-link capacity the objective value approaches the single-user water-filling capacity limit. By modifying the precoder structure and the user cooperation mechanism, a unidirectional side-link case, where only one of the users accesses the side-link, is also discussed.