The line-of-sight (LoS) dominant air-ground channels have posed critical interference issues in cellular-connected unmanned aerial vehicle (UAV) communications. In this paper, we propose a new base station (BS) cooperative beamforming (CB) technique for the cellular downlink to mitigate the strong interference caused by the co-channel terrestrial transmissions to the UAV. Besides the conventional CB by cooperatively transmitting the UAV’s message, the serving BSs of the UAV exploit a novel CB-based interference transmission scheme to effectively suppress the terrestrial interference to the UAV. Specifically, the co-channel terrestrial users’ messages are shared with the UAV’s serving BSs and transmitted via CB so as to cancel their resultant interference at the UAV’s receiver. To optimally balance between the CB gains for UAV signal enhancement and terrestrial interference cancellation, we formulate a new problem to maximize the UAV’s receive signal-to-interference-plus-noise ratio (SINR) by jointly optimizing the power allocations at all of its serving BSs for transmitting the UAV’s and co-channel terrestrial users’ messages. First, we derive the closed-form optimal solution to this problem in the special case of one serving BS for the UAV and draw useful insights. Then, we propose an algorithm to solve the problem optimally in the general case. As the optimal solution requires centralized implementation with exorbitant message/channel information exchanges among the BSs, we further propose a distributed algorithm that is amenable to practical implementation, based on a new divide-and-conquer approach, whereby each co-channel BS divides its perceived interference to the UAV into multiple portions, each to be canceled by a different serving BS of the UAV with its best effort. Numerical results show that the proposed centralized and distributed CB schemes with interference transmission and cancellation (ITC) can both significantly improve the UAV’s downlink performance as compared to the conventional CB without applying ITC.

中文翻译：

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蜂窝连接无人机的协作下行链路干扰传输和消除：一种分而治之的方法

视距 (LoS) 占主导地位的空地信道在蜂窝连接的无人机 (UAV) 通信中造成了严重的干扰问题。在本文中，我们为蜂窝下行链路提出了一种新的基站 (BS) 协作波束成形 (CB) 技术，以减轻由同信道地面传输对无人机造成的强干扰。除了通过协作传输无人机消息的传统CB之外，无人机的服务基站还利用一种新颖的基于CB的干扰传输方案来有效地抑制对无人机的地面干扰。具体而言，同信道地面用户的消息与 UAV 的服务 BS 共享并通过 CB 传输，以消除它们在 UAV 接收器处产生的干扰。为了在 UAV 信号增强和地面干扰消除的 CB 增益之间实现最佳平衡，我们制定了一个新问题，通过联合优化其所有服务的功率分配来最大化 UAV 的接收信号与干扰加噪声比 (SINR)。用于传输 UAV 和同信道地面用户消息的 BS。首先，我们在 UAV 的一个服务 BS 的特殊情况下推导出该问题的封闭形式最优解，并得出有用的见解。然后，我们提出了一种算法来在一般情况下最优地解决问题。由于最佳解决方案需要集中实施，BS之间的消息/信道信息交换过多，我们进一步提出了一种基于新的分而治之的分布式算法，适合实际实施，因此，每个同信道 BS 将其对 UAV 的感知干扰分成多个部分，每个部分都由 UAV 的不同服务 BS 尽最大努力消除。数值结果表明，与不应用 ITC 的传统 CB 相比，所提出的具有干扰传输和消除 (ITC) 的集中式和分布式 CB 方案都可以显着提高无人机的下行链路性能。