MU-MIMO Resource Optimization for Device-to-Device Underlay Downlink Cellular Networks

Device-to-Device (D2D) is an emerging technology that is typically employed as an underlay of the uplink (UL) cellular networks. The downlink cellular spectrum, however, is rarely reused by D2D links due to the strong interference from the base station to the D2D receivers. To this end, we propose to enable D2D transmissions in the downlink by multiplexing D2D and cellular users using beamforming techniques. In particular, a cross-layer design approach is adopted to jointly optimize beamforming, spectrum allocation, and power control, so that the total system transmission power is minimized. To deal with the non-convex and combinatorial nature of the problem, we transform the formulation into a convex optimization problem by identical deformation and relaxations, and propose a semidefinite relaxation (SDR) based algorithm to approximate the optimal solution. Moreover, we focus on the feasibility of the convex problem, and propose an improved algorithm which reduces the probability of users out of service. The simulation results show that the performance of our proposed algorithm is close to the optimal solution, and the improved algorithm gives a higher efficiency on user admission control.