Abstract-High altitude platform (HAP) drones can provide broadband wireless connectivity to ground users in rural areas by establishing line-of-sight (LoS) links and exploiting effective beamforming techniques. However, at high altitudes, acquiring the channel state information (CSI) for HAPs, which is a key component to perform beamforming, is challenging. In this paper, by exploiting an interference alignment (IA) technique, a novel method for achieving the maximum sum-rate in HAPbased communications without CSI is proposed. In particular, to realize IA, a multiple-antenna tethered balloon is used as a relay between multiple HAP drones and ground stations (GSs). Here, a multiple-input multiple-output X network system is considered. The capacity of the considered M ×N X network with a tethered balloon relay is derived in closed-form. Simulation results corroborate the theoretical findings and show that the proposed approach yields the maximum sum-rate in multiple HAPs-GSs communications in absence of CSI. The results also show the existence of an optimal balloon's altitude for which the sum-rate is maximized.

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Abstract-This paper investigates an interference alignment (IA) scheme suitable for a K-user multiple-input multipleoutput (MIMO) X network. The K-user MIMO X-network is a communication architecture where each transmitter, equipped with multiple antennas, has independent messages for each of the receivers, also equipped with multiple antennas. Earlier only 2 × N or M × 2 X networks was considered to be achievable. In this paper we remove this restriction by employing time division multiple access scheme. The proposed IA scheme allows to achieve a sum degree of freedom of, where A is the number of antennas at each transmitter and receiver. The capacity and bit error rate performance of the proposed scheme is compared to that of another scheme recently proposed by Park and Ko. Simulation results are reported to show the bit error-rate performance for K-user (for a toy example, K is set as 3) and two-user X channel IA in case of transmission over flat-fading Rayleigh channels.