Secrecy Performance of Wireless Powered Communication Networks With Multiple Eavesdroppers and Outdated CSI

In this paper, we present a comprehensive secrecy performance analysis of wireless powered communication networks with multiple eavesdroppers, where an energy-limited information source with multiple antennas harvests the radio frequency energy from a dedicated power beacon (PB) before transmission. To exploit the benefits of multiple antennas at source, two popular multi-antenna transmission schemes, i.e., maximal ratio transmission and transmit antenna selection, are investigated for two intercepting ways at Eves, i.e., non-colluding and colluding scenarios, respectively. Specifically, adopting the time-switching protocol at PB, we derive exact closed-form expressions of the secrecy outage probability and the average secrecy rate for both two transmission schemes taking into account the outdated channel state information. Furthermore, in order to deeply extract insights on the system design, we further present tractable asymptotic secrecy outage probability and ergodic secrecy capacity at high signal-to-noise ratio regimes, which easily enable us to obtain the secrecy diversity order and coding gain of two transmission schemes, respectively. From our analysis, several important concluding remarks are obtained as follows: 1) full secrecy diversity order can be achieved by both two transmission schemes with no feedback delay, however, it reduces to one in the presence of feedback delay; 2) as the number of eavesdroppers increases, the secrecy performance gap between non-colluding and colluding scenarios becomes large; and 3) maximal ratio transmission (MRT) scheme always outperforms transmit antenna selection (TAS) scheme with no feedback delay. However, TAS scheme achieves a similar performance as MRT scheme or even better in moderate and even serious feedback delay conditions.