Zhuangzhuang Tie

This paper investigates covert communication in the presence of an eavesdropper employing stochastic resonance (SR)–based detection. A unified analytical model is developed to characterize nonlinear noise-matching detectors, with SR serving as a representative example. Closed-form expressions for the achievable covert rate are derived under a KL-divergence-based covertness constraint, revealing that SR-enabled eavesdroppers can significantly enhance detection performance under perfectly matched noise conditions. More importantly, this work establishes a fundamental performance–robustness tradeoff: although stochastic resonance can amplify weak signals, its detection gain is highly sensitive to noise-level mismatch and interference uncertainty. We prove that under random or unknown interference, the worst-case detection performance of SR-based detectors is strictly inferior to that of conventional linear detectors, such as energy detection. This result demonstrates that the detection advantage offered by stochastic resonance is not free and inevitably comes at the cost of robustness. Numerical results validate the theoretical analysis and show that random, fast-varying interference constitutes a minimax-optimal strategy for covert transmission when the eavesdropper's detection model is unknown. These findings provide new insights into the fundamental limits of covert communication against advanced nonlinear detectors.

This letter investigates physical-layer security for orthogonal time frequency space (OTFS) systems under a discriminatory channel estimation (DCE) scheme that exploits the block-circulant structure of the delay–Doppler channel and coarse prior knowledge. We derive a fixed-point characterization of the linear minimum mean square error (LMMSE) channel-estimation error that captures the impact of pilot power, interference and multipath richness. A closed-form expression for the secrecy rate is obtained, showing that secrecy improves with pilot power at low-to-moderate levels but deteriorates in the high-power regime. Numerical results validate the analysis and reveal trade-offs among pilot allocation, path diversity and secrecy performance.

Orthogonal Time Frequency Space (OTFS) technique has received significant research attentions as a potential solution for ensuring robust and reliable communication in high-mobility mobile communication scenarios in recent years. In this paper, we propose a model-driven deep learning based orthogonal approximate message passing (OAMP) detector to harvest the time and frequency diversity promised by OTFS modulation and improve the detection performance in the presence of channel estimation error. The intelligent detector is designed by unfolding the OAMP detector with adding several learnable parameters. Simulation results demonstrate that, with much reduced complexity and much fewer parameters, the proposed detector can achieve better error performance under channel estimation errors compared with other nonlinear detectors.

With the advantages of low cost and flexible deployment, low earth orbit satellite (LEO-Sat) communications have received widespread attentions, such as providing global wireless access with enhanced data rates in future 6G networks. However, due to the feature of high-mobility, LEO-Sat communications are facing severe Doppler effects, which heavily degrades the link reliability. The existing approaches for coping with Doppler effects, such as phase locked loop, mainly carry out the prediction and compensation for Doppler frequency shifts, which demands high implementation complexity, as well as accurate channel estimation. Against this background, this article portrays a new paradigm of reliable LEO-Sat communications with the aid of orthogonal time frequency space (OTFS) modulation scheme, which can average out the channel dynamics through modulating information in the delay-Doppler (DD) domain, thereby overcoming severe Doppler effects. In particular, we start with studying the Doppler effect characteristics of LEO-Sat communications, which serves as the motivations of using OTFS. Then, we conceive a case study to show the feasibility of OTFS enabled LEO-Sat communication by evaluating the reliability performance. Finally, we discuss a range of promising research opportunities and potential challenges.

In this paper, we investigate the covert communication for a cooperative cognitive radio (CCR) system, where the secondary transmitters (STs) help the primary transmitter (PT) to transmit information, while covertly transmitting their own information to the secondary receiver (SR) against the warden of PT. In particular, the different ST scheduling schemes are proposed for covert communcations, namely covert rate maximization (CRM), best channel gain (BCG), and fairness oriented (FO). The closed-form expression is derived for the expected detection error probability at the warden PT. Based on that, we respectively analyze the covert rate and covert outage probability (COP) of the proposed ST scheduling schemes. The simulation results show that, under the both effects of channel uncertainty and noise uncertainty, the CRM scheme achieves the best covert rate performance, the BCG and FO schemes ensure the quality of service (QoS) and the STs’ fairness as the cost of covert performance degraded.

Orthogonal time frequency space (OTFS) is a promising candidate for high-mobility communications that has the potential to overcome the challenges brought by severe Doppler effects. In this letter, we investigate the optimal power allocation (OPA) for OTFS transmissions employing frequency-domain linear equalizers, including frequency-domain zero-forcing (FD-ZF) and/or minimum mean squared error (FD-MMSE) equalizers. More specifically, for FD-ZF, we derive a closed-form expression for the received signal-to-noise ratio (SNR). Leveraging this, we develop the OPA method under using FD-ZF by applying the Lagrange multipliers to solve the optimization problem of maximizing the received SNR. Similarly, for FD-MMSE, we give a closed-form expression of its signal-to-signal-plus-interference-plus-noise ratio (SSINR), and obtain the OPA to maximize the SSINR. Our analytical results show that, more transmit power will be allocated to the poorer subchannels in time frequency (TF) domain of OTFS, which is contrary to the water-filling (WF) method for orthogonal frequency division multiplexing (OFDM).

This paper investigates the security performance of a joint unicast-multicast streaming system, where different users present heterogeneous mobilities. The orthogonal time frequency space (OTFS) scheme is employed to overcome severe Doppler effect caused by high mobility. The closed-form expression is derived for the maximum secrecy rate of unicast transmission with high privacy. Furthermore, the positive secure capacity probability (PSCP) of unicast transmission is also obtained and analyzed. Our analytical results show that compared with high-mobility eavesdroppers, low-mobility eavesdroppers pose a greater threat to unicast secrecy. Moreover, when the outage probability of unicast is greater than 1/2, more time frequency (TF) resources should be allocated to unicast, in order to guarantee the security performance of unicast.

This paper investigates the performance analysis for a novel covert communication system, where a friendly UAV-jammer is used to protect the covert transmission from Alice to Bob against the eavesdropping from Dave. In particular, by exploiting the spatial diversity, the UAV can emit artificial noise (AN) to complicate the noise uncertainty at Dave. In this letter, two scenarios are studied, where the channels from Alice to Bob and Dave experience 1) AWGN and 2) Nakagami- $m$ fading. The closed-form expression for privacy rate expression under AWGN is derived, where evaluating the condition for achieving a positive gain of using UAV-jammer. Furthermore, under the Nakagami- $m$ fading scenario, an approximate expression of ergodic privacy rate is obtained by using Taylor expansion. In the end, simulation results show that significant performance gain in terms of privacy rate can be achieved by employing UAV-jammer for covert communication.