Energy-efficient secure multiuser scheduling in energy harvesting untrusted relay networks

In this paper, we investigate opportunistic secure multiuser scheduling in energy harvesting untrusted relay networks, where an energy-constrained amplify-and-forward (AF) relay harvests energy from the received radio frequency signal via using the power splitting protocol. To exploit the potential advantages (e.g., multiuser diversity and direct links) for secrecy improvement, we examine three opportunistic user scheduling schemes, i.e., optimal scheduling scheme, maximum scheduling scheme, and minimum scheduling scheme. Specifically, the optimal scheduling scheme performs user selection by maximizing the ratio between the signal-to-noise-ratio (SNR) at the destination and the corresponding SNR at the untrusted relay. The maximum scheduling scheme is conventional user selection that maximizes the channel gain of the direct links. For the minimum scheduling scheme, we try to reduce the overheard information at the untrusted relay by selecting the user with the lowest channel gain of the relay links. For these three schemes, we comprehensively examine the achievable secrecy performance including probability of positive secrecy rate, secrecy outage probability (SOP), asymptotic SOP, secrecy throughput (ST), as well as secure energy efficiency (SEE) to help facilitate an energy-efficient secure transmission design. Simulation results demonstrate that the maximum scheduling scheme outperforms the minimum scheduling scheme in terms of SOP at relatively low SNRs and target secrecy rates.