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 simultaneously enhancing the downlink signal strength and mitigating interference. We first consider the ideal case when perfect channel state information (CSI) is available for determining the beamforming vectors and then extend to the case of imperfect CSI. For both cases, the beamforming optimization problems are non-convex. Assuming perfect CSI, we obtain the optimal joint transmit (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 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赋形设计。我们首先考虑完美信道状态信息（CSI）可获取的理想情况以确定波束赋形向量，继而扩展至非完美CSI场景。针对这两种情况，波束赋形优化问题均为非凸问题。在完美CSI假设下，我们基于上下行对偶性获得最优联合传输（JT）波束赋形向量；在存在未知CSI误差时，采用结合伯恩斯坦型不等式的半定松弛（SDR）方法推导鲁棒联合传输波束赋形。最后通过数值结果评估所提方案的性能。