Multi-cell cooperation is an effective means to improve service quality to cellular users. Existing work primarily focuses on interference cancellation using all the degrees of freedom (DoF). This leads to low service quality for some users with poor channel quality to its serving base station. This work investigates the multi-cell beamforming design for enhancing the downlink signal strength and mitigating interference simultaneously. We first consider the ideal case when perfect channel state information (CSI) is available for determining the beamforming vectors and then extend the problem to the case with imperfect CSI. For both cases, the beamforming optimization problems are non-convex. Assuming perfect CSI, we obtain the optimal JT beamforming vectors based on the uplink-downlink duality. In the presence of unknown CSI errors, we use the semidefinite relaxation (SDR) with Bernstein-type inequality to derive the robust JT beamforming. Numerical results are presented to evaluate the performance of the proposed JT beamforming schemes.

翻译：多小区协作是提升蜂窝用户服务质量的有效手段。现有研究主要利用全自由度进行干扰消除，这导致部分信道质量较差的用户与其服务基站之间的服务质量低下。本文研究同时增强下行信号强度与抑制干扰的多小区波束成形设计。我们首先考虑理想情况，即可获取完美信道状态信息以确定波束成形向量，随后将问题扩展至非完美信道状态信息场景。针对这两种情况，波束成形优化问题均为非凸问题。在完美信道状态信息假设下，我们基于上下行对偶性获取最优联合传输波束成形向量。当存在未知信道状态信息误差时，采用结合伯恩斯坦不等式的半定松弛方法推导鲁棒联合传输波束成形。仿真结果验证了所提联合传输波束成形方案的性能。