A Direct Solution for Rate Balancing in MIMO Broadcast Channels with Per-Base-Station Power Constraints

It is well-known that the main capacity limitation in cellular communication systems is due to inter-cell interference. Multi-cell signal processing, for example joint transmission from multiple base stations to multiple terminals, is known to strongly improve spectral efficiency and system fairness by actively exploiting interference rather than treating it as noise. Specifically, we consider the scenario where downlink multi-cell beamforming is used to obtain perfect fairness, i.e. to provide all involved terminals with the same achievable rate. The aim is to find the power allocation and beamforming matrix achieving the highest possible common rate under per-base-station power constraints. This power-constrained optimization (PCO) problem has so far been solved by iteratively solving rateor SINR-constrained (SCO) transmit power optimization problems. In this paper, we derive a direct and therefore significantly less complex solution of a PCO problem with per-base-station power constraints.