System Design and Parameter Optimization for Remote Coverage from NOMA-Based High-Altitude Platform Stations (HAPS)

Stratospheric solar-powered high-altitude platform station (HAPS) can provide line-of-sight (LoS) communications to the ground users in its ultra-wide coverage area. This paper addresses the challenge of HAPS communication system design especially the access link. We propose to divide the ground users into multiple user-groups and serve each group by a high-density dynamically steerable spotbeam, generated by the phased array antennas mounted on HAPS. We employ time-division multiplexing (TDM) to serve different user groups and non-orthogonal multiple access (NOMA) to simultaneously serve all users within a usergroup. We formulate user grouping problem as an equivalent geometric disk cover (GDC) problem and beam optimization problem as a minimum enclosing circle (MEC) problem. We present the optimization framework to jointly design user grouping, user association, beam optimization, and power allocation aiming at sum rate maximization while guaranteeing the quality-of-service (QoS) with limited power budget. System performance is assessed using the key metrics such as signal-to-interference noise ratio (SINR), achievable data rate, average energy efficiency (AEE), average spectral efficiency (ASE), user fairness and outage probability. We observe upto 42% reduction in required groups, 5dB increase in received SINR, 37.5% improvement in energy efficiency, 57.9% rise in spectral efficiency, 22% enhanced user fairness, 65% surge in achievable data rates and ten-folds reduction in outage with the proposed optimization framework over conventional schemes using system-level simulations. Our findings reveal the significance of joint design of system parameters for enhanced performance, optimum energy utilization, and resource allocation.