High altitude platform (HAP) drones can provide broadband wirelessconnectivity to ground users in rural areas by establishing line-of-sight (LoS)links and exploiting effective beamforming techniques. However, at highaltitudes, acquiring the channel state information (CSI) for HAPs, which is akey component to perform beamforming, is challenging. In this paper, byexploiting an interference alignment (IA) technique, a novel method forachieving the maximum sum-rate in HAP-based communications without CSI isproposed. In particular, to realize IA, a multiple-antenna tethered balloon isused as a relay between multiple HAP drones and ground stations (GSs). Here, amultiple-input multiple-output X network system is considered. The capacity ofthe considered M*N X network with a tethered balloon relay is derived inclosed-form. Simulation results corroborate the theoretical findings and showthat the proposed approach yields the maximum sum-rate in multiple HAPs-GSscommunications in absence of CSI. The results also show the existence of anoptimal balloon's altitude for which the sum-rate is maximized.
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机译：高海拔平台（HAP）无人机可以通过建立视距（LoS）链路并利用有效的波束成形技术来向农村地区的地面用户提供宽带无线连接。然而，在高海拔地区，获取HAP的信道状态信息（CSI）是执行波束成形的关键组成部分，这具有挑战性。在本文中，通过利用干扰对齐（IA）技术，提出了一种在没有CSI的情况下实现基于HAP的通信中最大和速率的新方法。特别地，为了实现IA，将多天线系留气球用作多个HAP无人机和地面站（GS）之间的中继。在此，考虑多输入多输出X网络系统。考虑的具有系留气球中继的M * N X网络的容量以封闭形式导出。仿真结果证实了理论结论，并表明所提出的方法在没有CSI的情况下在多个HAP-GSs通信中产生了最大的求和率。结果还表明存在最优气球高度，对于该高度，总速率最大。
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