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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.

Sum-Rate Analysis for High Altitude Platform (HAP) Drones with Tethered Balloon Relay