MUI-Free Joint Transmission Under Per-Base Station Power Constraints

Networks employing multiple base stations (BSs) to serve multiple users show great potential in enhancing spectrum efficiency and supporting dense user loads. In such networks, BSs may have different power budgets, for example, drones with limited battery capacity or BSs operating under specific radio emission restrictions near hospitals or within Flight Information Regions. These scenarios require the development of precoding techniques for cooperative BSs under individual per-BS power constraints. This task is challenging, as the system must design the column- wise coefficients of a joint precoding matrix while satisfying row- wise transmit power constraints associated with different power budgets at each BS. To address this challenge, this study proposes two cooperative multi-user interference (MUI)-free precoding schemes, in which each BS is subject to an individual power budget. In the first approach, we prove that the problem is locally concave maximization over a convex feasible set when the value of SNR exceeds 0 dB. Subsequently, we apply the semidefinite relaxation (SDR) to convexify the problem locally and leverage semidefinite programming (SDP) solvers to yield a suboptimal solution. Since the first solution demands high computational complexity, the second approach proposes a closed-form asymptotically suboptimal solution utilizing Lagrange multipliers, significantly reducing computational overhead. Simulation results show that both proposed solutions greatly outperform conventional schemes in terms of sum rate. Furthermore, the proposed closed-form solution achieves performance closely approaching that of the proposed numerical solution, while substantially reducing computational complexity.