Applying PDMA for Ground-HAPS Uplink Network with Hybrid FSO/RF Communication

Non-terrestrial network (NTN), especially HighAltitude Platform Station (HAPS), is a popular choice because of low latency, deployment flexibility, and disaster resilience. In order to increase the availability of HAPS, a new multiple access system that improves performance, such as throughput and user fairness, is required. Although Non-Orthogonal Multiple Access (NOMA) is a promising technology for improving spectral efficiency, there are significant differences in transmission rate according to power levels for NOMA. Since ground-HAPS links are the end part of NTN and the HAPS network is expected to be used for the mobile network, the user fairness of NOMA should be improved. Thus, to increase throughput guaranteeing user fairness and transmission accuracy for HAPS, we implement Power Level Division Multiple Access (PDMA) which power levels of users are randomly switched in each timeslot. In this paper, we propose a new system applying PDMA for ground-HAPS uplink network with hybrid Free Space Optics (FSO) / millimeter wave communication. In our system, it switches optical and millimeter waves according to the channel state of users. By simulation results, the throughput of PDMA is higher than that of Orthogonal Multiple Access (OMA), and its user fairness is significantly improved compared with NOMA.