Securing Multiuser Underlay Cognitive Transmissions with Hardware Impairments and Channel Estimation Errors

We investigate physical-layer security for a multiuser underlay cognitive system with hardware impairments (HIs) and channel estimation errors (CEEs), which comprises multiple cognitive users (CUs), a cognitive base station (CBS), and an eavesdropper (E). Considering the quality-of-service of primary transmissions, the cognitive transmit power is limited by interference threshold at the primary receiver. To safeguard the cognitive transmissions, a selection combining (SC) scheme and a switch-and-examine combining with post-selection (SECPS) scheme are proposed to improve the security-reliability tradeoff (SRT). As a baseline, we also analyze the SRT performance of round-robin scheduling (RRS) scheme. The closed-form expressions of outage probabilities (OPs) and intercept probabilities (IPs) are derived for these schemes. Additionally, we study the diversity gains of the RRS, SC, and SECPS schemes and it shows that the CEEs cause the diversity gain drop to zero while the HIs have no impacts on the diversity gain if the outage event does not occurs. Numerical results are provided to show that the SC scheme achieves the best SRT performance at the cost of much computation overhead. Moreover, the SRT performance of the SECPS scheme outperforms that of the RRS scheme and it consumes less computation overhead than the SC scheme when a suitable threshold is set.