Cooperative FSO/RF Space-Air-Ground Integrated Network System With Adaptive Combining: A Performance Analysis

To improve the performance of the cooperative free space optical/radio frequency (FSO/RF) space-air-ground integrated network (SAGIN) system, a SAGIN system with an adaptive combining scheme is proposed. Specifically, when the instantaneous signal-to-noise ratio of the FSO relay link exceeds the threshold, only the FSO relay link with the amplify-and-forward protocol exists. Otherwise, the RF and FSO links are in operation, and two signals are combined at the receiver using the maximal ratio combining approach. Moreover, the alpha-eta-kappa-mu distribution is employed to characterize the RF channel, and the effects of atmospheric absorption, atmospheric turbulence with the M & aacute;laga fading model, pointing errors, and angle-of-arrival fluctuations on the optical signal are considered. The closed-form expressions of the outage probability, symbol error rate (SER), and ergodic capacity were derived. Furthermore, asymptotic expressions for the outage probability and SER are provided. The findings indicate that the performance of the considered SAGIN system with an adaptive combining scheme outperforms that of other schemes.