This work considers covert communication in downlink NOMA systems, where a public action (communication from Alice to the NOMA-weak user U1) is used to provide a cover for a secret action (communication from Alice to the NOMA-strong user U2). To aid system design, we first derive the detection error probability (DEP) at Willie and the connect outage probabilities (COPs) at U1 and U2. Then, considering the covertness and outage constraints, the optimization of power allocation factor ratio between U1 and U2 is tackled in order to maximize the effective covert rate. The results demonstrate that the minimum DEP is the power allocation factor for U1. In addition, for a given covert constraint, relaxing the outage constraint will effectively increase the maximum effective covert rate within a certain range of the maximum transmit power of U1.

This paper investigates covert communications with finite blocklength in the presence of multiple randomly distributed wardens, which are modeled as a Poisson point process (PPP). Specifically, the protection area, which is guarded by the transmitter against the wardens, is introduced in order to enhance the covert communication performance. First, the closed-form expressions of covert communication constraint are derived under both the non-colluding and colluding detection strategies over the channels suffering both path-loss and small scaling fading. Then, considering the constraint of maximum available blocklength, the transmit power and the blocklength are jointly optimized to maximize the average effective covert throughput (AECT). Differing from the existing literature with only one warden, it is demonstrated that the increasing of maximum available blocklength causes the improvement of maximum AECT only when the density of warden is small. In addition, our examinations show that the expansion of the protection area indeed improves the maximum AECT, and the improvement under the non-colluding detection strategy is more significant than that under the colluding detection strategy.

This letter investigates covert communication in uplink NOMA systems assisted by an energy harvesting jammer (EHJ), where a reliability-required user (RRU) and a covertness-required user (CRU) are paired to perform NOMA to transmit messages to Bob, in the presence of a warden (Willie) who tries to detect CRU’s transmission behavior. In particular, the EHJ with time switching (TS) scheme has been employed for the purpose of confusing Willie and compensating for the energy supply. We derive the closed-form expressions of expected minimum detection error probability (EMDEP) and connection outage probabilities (COPs) at RRU and CRU, then we analyse the maximum effective covert rate (ECR) under the covertness and RRU’s quality of service (QoS) constraints. Our analysis indicates that there exist an optimal transmit power at Bob and time switching factor to maximize the maximum ECR, and RRU’s QoS constraint has a remarkable effect on the maximum ECR as the transmit power at Bob or time switching factor increases.

The millimeter wave (mmWave) ultra-dense networks are more suitable for wireless power transfer since the short-distance transmissions experience less pathloss and the base station (BS) packed with large-scale antenna arrays can achieve significant array gains. However, the secrecy performance of the SimultaneousWireless Information and Power Transfer (SWIPT) mmWave ultra-dense networks has not been investigated so far. In this paper, we consider the secure communications in downlink SWIPT mmWave ultra-dense networks, where the energy-constrained users extract energy and information from the mmWave signals in the presence of multiple eavesdroppers. First, the analytical expressions of the energy-information coverage probability (EICP) are derived for both power splitting (PS) and time switching (TS) policies using stochastic geometry. Then, we derive closed-form expressions of the secrecy probability (SP) in the presence of multiple independent or colluding eavesdroppers. Finally, the effective secrecy throughput (EST) which can measure the network energy coverage, secure and reliable transmission performance in a unified manner is derived. Theoretical analysis and simulation results reveal that, the EST first increases and then decreases with the increasing of the transmit power, power/time splitting ratio, codeword transmission rate and confidential information rate, respectively. Furthermore, reducing beamwidth of the signal at BSs can decrease the information leakage and improve the EST.

In this letter, we jointly investigate packet generation and covert communication in delay-intolerant status update systems. For the first time, we establish a bridge between the prior transmission probability and the finite block-length in covert communication by modeling the stochastic status packet generation as a Poisson process. Considering a general prior transmission probability model, we derive closed-form expressions for the covertness constraint, the average packet error probability, and the effective covert throughput (ECT) to reveal the non-trivial tradeoff between the communication covertness and reliability. An optimization problem to maximize ECT is also formulated, and then the optimal transmit power and block-length are obtained, respectively. In contrast to exiting results, our examination shows that the longest block-length and the widely adopted equal prior probabilities are generally suboptimal when jointly considering the packet generation and covert communication.