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The incorporation of cognitive radio (CR) capability in wireless sensor networks yields a promising network paradigm known as CR sensor networks (CRSNs), which is able to provide spectrum efficient data communication. However, due to the high energy consumption results from spectrum sensing, as well as subsequent data transmission, the energy supply for the conventional sensor nodes powered by batteries is regarded as a severe bottleneck for sustainable operation. The energy harvesting technique, which gathers energy from the ambient environment, is regarded as a promising solution to perpetually power-up energy-limited devices with a continual source of energy. Therefore, applying the energy harvesting (EH) technique in CRSNs is able to facilitate the self-sustainability of the energy-limited sensors. The primary concern of this study is to design sensing-transmission policies to minimize the long-term outage probability of EH-powered CR sensor nodes. We formulate this problem as an infinite-horizon discounted Markov decision process and propose an ϵ-optimal sensing-transmission (ST) policy through using the value iteration algorithm. ϵ is the error bound between the ST policy and the optimal policy, which can be pre-defined according to the actual need. Moreover, for a special case that the signal-to-noise (SNR) power ratio is sufficiently high, we present an efficient transmission (ET) policy and prove that the ET policy achieves the same performance with the ST policy. Finally, extensive simulations are conducted to evaluate the performance of the proposed policies and the impaction of various network parameters.

A novel secrecy scenario of uplink non-orthogonal multiple access (NOMA) along with cooperative jammer(s) to improve secrecy performance is studied in this work. We first characterize secrecy performance in terms of positive secrecy rate probability, secrecy outage probability (SOP) and effective secrecy throughput (EST), and derive the closed-expressions of individual secrecy performance. The analytical results not only show an improvement on secrecy performance with the aid of cooperative jamming, but also clearly illustrate how each jammer affects the secrecy performance. We then continue the study of the individual secrecy performance for its asymptotic behaviors, and reveal it is only dependent on its relative distance to the eavesdropper over the desired receiver in a high signal-to-noise ratio (SNR) regime. Finally, the maximization of overall secrecy performance is covered by investigating two optimization problems: 1) optimal selection of jammer(s) under the same transmit power; 2) optimal power allocation to each transmitter, including jammers, under the constraint of limited total transmit power. The solutions to these two problems are presented and demonstrated by an appropriate case. INDEX TERMS Cooperative jamming, non-orthogonal multiple access (NOMA), successive interference cancellation (SIC), secrecy performance, zero-forcing (ZF). I. INTRODUCTION A. BACKGROUND

We investigate individual secrecy performance in a K-user quasi-static Rayleigh fading homogeneous multiple access wiretap channel (MAC-WT), where a legitimate receiver employs successive interference cancellation (SIC) decoding. We first evaluate individual secrecy performance under an arbitrary SIC order by deriving closed-form expressions with respect to secrecy outage probability and effective secrecy throughput (EST) as main metrics. The resulting closed-form expressions disclose a significant impact on the secrecy performance from the order of SIC decoding. Therefore, we propose three SIC decoding order scheduling schemes: (1) round-robin scheme, absolutely fair and served as a benchmark; (2) suboptimal scheme, based on each user's main channel condition; and (3) optimal scheme, based on each user's achievable secrecy rate. Comparison results show that the last two schemes outperform the first one with regard to both the EST and the multi-user diversity gain, whereas the performance of the suboptimal scheme is highly close to that of the optimal scheme which is usually impractical due to a requirement for the eavesdropper's channel state information (CSI).

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