Transmit power control of HAPS for two-way relay communication in space-air-ground integrated networks

Space-air-ground integrated networks (SAGINs) have attracted considerable attention as they provide a wide range of communication environments from outer space and the stratosphere. A SAGIN is a multi-layered integrated network of satellites, ground terminals (GTs) and aerial objects such as high-altitude platform stations (HAPSs), which are expected to improve the communication quality in relay transmission between satellites and GTs as relay base stations. However, the existing studies on relay transmission in SAGINs are based on the assumption that an aerial object assigned to the uplink and downlink has fixed the upper limit of transmit power; this causes either an excess or shortage of the transmit power allocable to each link when communication demand is unbalanced between bi-directional links. Therefore, to enable the integrated control of transmit power distributed to bidirectional links by HAPSs as well as maintain the number of GTs satisfying the communication demand at a high level, constructing a mathematical model is necessary. This study proposes a mathematical model for controlling the transmit power distributed by HAPS to bi-directional links according to the diverse communication requirements from each GT at different locations. The numerical analysis shows that our proposal improves the efficiency of two-way relay communication in the SAGIN.