Performance Enhancement of Mixed FSO/RF based HAPS Relaying Communication Network using MP-PPM Modulation

Information and communication technology have significantly increased and advanced in recent years. But the ever-inflating data and numerous media services requirements has resulted in congestion in the conventionally used RF spectrum, necessitating a move from radio frequency (RF) carriers to optical carriers. Free Space Optical (FSO) communication systems are becoming a feasible choice for many applications due to their low power consumption, wide bandwidth broadband availability, and low implementation costs. This paper examines a mixed FSO/RF-based HAPS-assisted communication network, in which HAPS works as a decode & forward relay. Since FSO follows line of sight (LOS) communication, the Source Ground Station (SGS) to HAPS link is presumed to be an FSO link. Consequently, the HAPS to Destination Ground Station (DGS) link is regarded as a nakagami-m distributed RF link, allowing it to serve many ground destinations. Operating at a height of 20 kilometers in the stratosphere, a High Altitude Platform Station (HAPS) can significantly advance the concept of a 6G network. However, since the HAPS is quasi-stationary, the FSO link connecting SGS to HAPS suffers from atmospheric turbulences and pointing error losses. In this paper, MP-PPM modulation is taken into account which enhances the performance of the FSO link compared to the conventional On-off keying (OOK) modulation and the results of both are compared. The performance is evaluated by obtaining the expressions of outage probability and Error rate by considering all the situations stated above.