Adaptive-Combining-Based Hybrid FSO/RF Satellite Communication With and Without HAPS

Abstract

In recent years, a growing interest has been witnessed in the usage of free space optics (FSO) link for satellite communication (SATCOM) scenarios, as it offers much higher data rates up to gigabits per second (Gbps) compared to existing radio frequency (RF) link. However, FSO links are sensitive to beam scintillation and pointing errors. In this paper, we consider a hybrid FSO/RF communication between ground station (GS) and satellite, where the RF link will act as a backup link to improve the reliability of FSO communication. In addition, we also consider high-altitude platform station (HAPS), which will act as a relay station, between GS and satellite to improve the end-to-end system performance. This has led to the development of space-air-ground integrated hybrid FSO/RF SATCOM networks. We analyse the performance of the proposed hybrid network considering an adaptive-combining-based switching scheme for both uplink and downlink scenarios with and without using HAPS as a relay station. In case of adaptivecombining-based switching scheme, the data is continuously transmitted over the FSO link, while maximalratio-combining (MRC) of RF and FSO links is performed when the quality of FSO link deteriorates. The performance analysis of adaptive-combining-based switching scheme in terms of outage and average symbol error rate (SER) is carried out and the same is compared with the single-link FSO SATCOM and single-threshold-based switching scheme proposed in the literature for hybrid FSO/RF SATCOM. In addition, the performance gain obtained by the proposed adaptive combining scheme over single-link FSO system for different channel conditions is also reported. Further, the asymptotic analysis is also carried out to obtain the diversity gain of the proposed system.INDEX TERMS Adaptive combining, free space optics, high-altitude platform station (HAPS), hybrid FSO/RF, performance analysis.

Towards a Hybrid RF/Optical Lunar Communication System (LunarComm)

“…In fact, similar methodology has been used to improve other systems such as 5G backhaul links [13]. There is also active work in hybrid RF/FSO communication strategies for ground to satellite communications [5]. However, the authors are aware of no existing effort to combine these technologies to achieve Earth-to-Lunar communications using Lunar Gateway and SmallSat constellations.…”

Performance Analysis of Hybrid SAG-FSO/RF Satellite Communication System

“…For both [14] and [15], the switching scheme for hybrid FSO/RF systems was based on single-threshold hard-switching. The adaptive-combining switching was considered in [16] for extra improvement. The maximal-ratio-combining took place at the receiver when the Fig.…”

“…The introduction of a HAP relay effectively divides the single FSO link into two shorter hops. According to [15], [16], the SAG network achieved a performance gain of about 4 dB over traditional single-hop Satcom networks in the uplink scenario. On the other hand, it achieves a limited performance improvement in the downlink transmission, where the beamwandering effect is negligible.…”

“…Despite the disparity in the performance, the achieved gain in the uplink highlights a promising direction toward improved FSO-link performance. With the SAG-FSO design in [15], [16], the HAP relay is situated on the line between the ground station and satellite. The ground-to-HAP link will experience a similar amount of atmospheric turbulence as the direct ground station to satellite (ground-to-SAT) transmission.…”

“…This minimum distance will be independent of the zenith angle of the satellite, which is especially important when working with medium earth orbit (MEO) and LEO satellites. Note that with the SAG-FSO transmission design in [15], [16], the HAP needs to continuously follow the satellite in order to be always situated between the ground station and satellite, which can be technically challenging due to the fast movement of the LEO and MEO satellites. The proposed SAG-FSO design will lead to a longer HAP-to-SAT hop, especially when the satellite has a large zenith angle.…”