Relay Selection for Improving Physical-Layer Security in Hybrid Satellite-Terrestrial Relay Networks

In this paper, a hybrid satellite-terrestrial relay network (HSTRN) interconnecting a satellite and multiple terrestrial nodes is considered, where communication is achieved by the satellite transmitting information to a destination through multiple relays at the appearance of an eavesdropper attempting to intercept the transmissions from both the satellite and relays. We present the single-relay selection and multi-relay selection as well as round-robin scheduling schemes to investigate the physical-layer security of this considered HSTRN by adopting the decode-and-forward relay strategy. Specifically, in a single-relay selection scheme, a relay is chosen as the “best”relay which has the maximum instantaneous capacity of relay-destination channel out of the decoding relay set, which is composed of all the relays capable to decode the received signals from the satellite successfully. By contrast, in a multi-relay selection scheme, all relays of the decoding relay set are invoked simultaneously to aid the satellite communicating with the destination. Moreover, suppose that only the main channels' state information is known while the wiretap channels' state information is unavailable due to the passive eavesdropper, we analyze the secrecy performance in accordance with secrecy outage probability of the HSTRN by driving out the closed-form expressions for the single-relay selection and baseline round-robin scheduling schemes, as well as by computer simulations for multi-relay selection scheme. Numerical results show that the two relay selection schemes generally outperform the round-robin scheduling baseline scheme in the light of improving the secrecy performance of HSTRN even when the legitimate links are inferior to the wiretap links.