Mixed FSO/RF based HAPS Relaying Communication Network for Ground Users

In the past several years, there has been a significant rise and advancement in information and communication technologies. However, the necessity to switch from radio frequency (RF) carriers to optical carriers has arisen due to congestion in the traditionally employed RF spectrum caused by the ever-inflating demand for data and multiple media services. Wide-bandwidth broadband availability, low power consumption, and inexpensive implementation have made Free Space Optical (FSO) communication systems a viable option for numerous applications. Furthermore, data transmission between the transmitter and receiver enables strong link security. Operating at a height of 20 km in the stratosphere, a High Altitude Platform Station (HAPS) maintains a quasi-stationary location and can make a substantial contribution to the idea of a 6G network. This study considers a mixed FSO/RF-based HAPS-assisted communication network, wherein HAPS is a relay of type decode-and-forward (DF). Source Ground Station (SGS) to HAPS link is assumed to be an FSO link, and since FSO follows line of sight (LOS) communication therefore, HAPS to Destination Ground Station (DGS) is considered to be an RF link so that it can cater to multiple ground destinations. For the uplink, atmospheric turbulence is modeled using the Gamma-Gamma fading model, and path loss is modeled using Beers Lambert's law. As the HAPS is quasi-stationary, a pointing error has been considered, keeping nonzero boresight error and identical jitters parameters. The RF link is deemed to be nakagami-m distributed. The performance of the model is investigated by deriving the analytical expression of outage probability and bit error rate (BER) by considering all the conditions stated above.