Security-aware spectrum sharing for NOMA in cognitive radio networks with discrete-time energy harvesting

Abstract

In this paper, we propose a security-aware spectrum sharing scheme for wireless-powered cognitive radio networks (CRNs) with non-orthogonal multiple access (NOMA). In this system, a primary transmitter (PT) intends to send confidential signal to a primary receiver (PR) by assisting of a secondary transmitter (ST), while there is a potential eavesdropper (PE) within the scope of PR. It is assume that ST has no fixed energy supply but the NOMA cognitive transmission can be performed after harvesting sufficient radio frequency (RF) energy. To ensure that PT’s confidential signal is safety, PR will be equipped with two antennas and operating in full-duplex mode, so that while ST is performing cognitive transmission, one of the antennas sends artificial noise (AN) from PR to interfere the signal receiving of PE. According to whether the accumulated energy of ST is sufficient and whether ST can correctly decode the PT’s signal, the system can operate in three modes. Since ST needs to accumulate enough energy through several continuous transmission slots, the charging and discharging processes of the battery in the ST can be simulated as a discrete-time Markov chain. Based on these, we derive exact expressions of outage probabilities of the primary and secondary systems, then an approximate formula of secrecy outage probability (SOP) of primary system is also derived. In addition, in order to further improve the transmission performance of the system, optimal power allocation factor and power level of AN are obtained by maximizing the secrecy rate of the primary system, while guaranteeing the system energy efficiency. Compared with zero-forcing (ZF) technique, the proposed secure spectrum sharing scheme has a slight disadvantage in secrecy energy efficiency, but it is simple to deploy, low cost, and achieves a better secrecy rate, which can be applied for a variety of application scenarios.