Interference mitigation with joint beamforming and common message decoding in multicell systems

Conventional multicell wireless systems operate with out-of-cell interference treated as noise—interference detection is infeasible as intercell interference is typically weak. This paper considers the benefit of designing decodable interference signals by allowing common-private message splitting at the transmitter and common message decoding by users in adjacent cells. In particular, we consider a downlink scenario, where base-stations are equipped with multiple transmit antennas, the remote users are equipped with a single antenna, and multiple remote users are active simultaneously via spatial division multiplexing. We solve a network optimization problem of jointly determining the appropriate users in adjacent cells for rate splitting, the optimal beamforming vectors for both common and private messages, and the optimal common-private rates to minimize the total transmit power across the base-stations subject to service rate requirements for remote users. We observe that for fixed user selection and fixed common-private rate splitting, the optimization of beamforming vectors can be performed using a semidefinite programming approach. Further, this paper proposes a heuristic user-selection and rate splitting strategy to maximize the benefit of common message decoding. Simulation results show that common message decoding can significantly improve both the total transmit power and the feasibility region for cell-edge users.