Design of Energy Efficient Multi-HAPS Assisted Hybrid RF/FSO Satellite Communication Systems with Optimal Placement

Satellites and high altitude platform station (HAPS) systems are expected to become the key elements of non-terrestrial networks with the recent advances on sixth-generation (6G) wireless networks. In this article, motivated by the mounting interest in HAPS systems both from academia and industry, we propose a multi-HAPS aided optical satellite communication architecture using hybrid radio frequency (RF)/free space optical (FSO) communication to provide enhanced throughput, and reliability. The proposed architecture can be used in railways, transcontinental highways or maritime communications to provide enhanced coverage, throughput and reliability. To quantify the overall performance of the proposed scenario, outage probability, ergodic capacity, throughput, energy efficiency are obtained and validated. Additionally, the impact of outdated channel state information and channel estimation errors are considered, severely affecting the system performance by causing signal-to-noise ratio loss and outage floors. Furthermore, we find optimum HAPS distance and obtain the number of HAPS systems that is required for reliable communications. The results show that serial placement of HAPS systems at optimum distances can enhance the system performance and energy efficiency.