This paper investigates the joint channel estimation and localization problem for integrated sensing and communication (ISAC) systems assisted by a reconfigurable intelligent surface (RIS). The key technical challenges involve addressing the computational complexity of high-dimensional sparse recovery while effectively exploiting geometric relationships in parameter estimation. To begin with, we formulate the joint estimation problem as a geometrically-constrained sparse recovery task based on maximum a posteriori estimation principles, incorporating physical feasibility constraints. Then, we propose the geometrically-constrained separable multidimensional orthogonal matching pursuit (GSMOMP) algorithm, which iteratively executes three key procedures: (1) coarse candidate selection via separable projections to avoid high-dimensional tensor construction; (2) geometric feasibility filtering to eliminate physically implausible propagation paths; and (3) refined atom selection with joint parameter updates that concurrently optimize both channel estimates and location parameters. The convergence is ensured by analytically demonstrating the monotonic decrease of the residual error. Finally, numerical results verify the effectiveness of the proposed algorithm, which achieves substantial computational efficiency improvement compared to conventional MOMP while maintaining comparable localization and angular estimation accuracy.

Due to control limitations in the denoising process and the lack of training, zero-shot video editing methods often struggle to meet user instructions, resulting in generated videos that are visually unappealing and fail to fully satisfy expectations. To address this problem, we propose Align-A-Video, a video editing pipeline that incorporates human feedback through reward fine-tuning. Our approach consists of two key steps: 1) Deterministic Reward Fine-tuning. To reduce optimization costs for expected noise distributions, we propose a deterministic reward tuning strategy. This method improves tuning stability by increasing sample determinism, allowing the tuning process to be completed in minutes; 2) Feature Propagation Across Frames. We optimize a selected anchor frame and propagate its features to the remaining frames, improving both visual quality and semantic fidelity. This approach avoids temporal consistency degradation from reward optimization. Extensive qualitative and quantitative experiments confirm the effectiveness of using reward fine-tuning in Align-A-Video, significantly improving the overall quality of generated videos.

The communication security and covertness of legitimate vehicles in the sixth-generation (6G) mobile communication intelligent automatic transportation systems (IATS) face significant challenges, as the communication equipment and wireless signals are complex and susceptible to information leakage. To address the above issues, this paper proposes a novel IATS that integrates simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) with rate-splitting multiple access (RSMA). Concurrently, an eavesdropping vehicle, capable of monitoring and eavesdropping information from legitimate vehicles, is introduced to explore the joint security and covertness performance. A roadside device is also introduced to improve security and covertness performance by transmitting artificial noise. The closed-form expressions for outage probability (OP), detection error probability (DEP), and intercept probability (IP) are derived to characterize the reliability, covertness, and security of the proposed system. The impact of various system parameters on system performance has also been extensively conducted, including transmitted signal-to-noise ratio (SNR), interference power, the power allocation coefficients, and the number of elements in STAR-RIS. The simulation results reveal several key insights: 1) The OPs and IPs of the RSMA-IATS network gradually decrease and increase with the transmitted SNR, respectively; 2) Achieving an optimal balance in power allocation between monitoring and eavesdropping proves essential for effective eavesdropper management; 3) The energy efficiency (EE) of the proposed system exhibits dual peaks at higher STAR-RIS element numbers, contrasting with a single peak at lower element counts, underscoring the strategic deployment of STAR-RIS technology in RSMA-IATS network.

This article investigates the joint design problem of the hybrid/reflective beamforming for the dual-function radar-communication (DRC) system with the help of an intelligent reflecting surface (IRS). The degrees of freedom for beamformer design are the analog and digital beamformers (DBF) at the base station (BS), and the reflective beamformer (RBF) at the IRS. To begin with, we formulate the beamformer design problem as a weighted sum-rate (WSR) maximization problem subject to the transmit power and similarity between the designed beamformer and the reference one with desired beampattern property. Then, utilizing equivalent reformulation of the preceding design problem, we conceive an alternating direction sequential programming (ADSP)-based algorithm that iteratively updates the following low-complexity subproblems: 1) DBF at the BS through the Powell–Hestenes–Rockafellar (PHR) method; 2) analog beamformer at the BS using the Riemannian conjugate gradient method; and 3) RBF at the IRS by the successive convex approximation (SCA) technique. The convergence is ensured by analytically offering the nondecreasing and boundary of the WSR value. Finally, numerical results verify the effectiveness of the proposed algorithm, which achieves superior WSR performance while ensuring the radar requirement.

Device-to-device (D2D) communication is able to enhance the spectrum efficiency and coverage of wireless networks but its transmission secrecy is vulnerable since D2D terminals are usually resource/power constrained. Various existing endeavors propose physical-layer security (PLS) means to protect D2D transmissions, under the default prerequisite that D2D transmissions always improve the secrecy performance. In this article, we show that this assumption/conclusion is invalid. We investigate a so-called cellular transmission mode assisted by the base station (cellular mode) and compare its secrecy performance with that of the D2D direct transmission (D2D mode). In both modes, subject to the secrecy outage probability constraints, we optimize the transceiver parameters, such as the signal power, secrecy rates, and transmission thresholds, to maximize the secrecy throughputs. All of the optimal solutions are obtained by closed-form expressions in both modes. Numerical results show that neither of the two transmission modes is always the better but highly depends on the system parameters, such as the locations of nodes, transmit power, and secrecy requirements, and consequently provides the answer that D2D communication is not always beneficial for PLS. This immediately suggests that an adaptive switching transmission mode could achieve an enhanced secrecy performance, which is also validated by simulations.

For the intelligent reflecting surface (IRS) assisted sensing system, this letter investigates the joint design problem of active and passive beamforming with imperfect channel state information (CSI). To begin with, the beamformer design problem is formulated as a stochastic optimization problem with inner-layer variable (active beamforming vector) and outer-layer variable (passive phase-shift matrix), subject to the available transmit power and constant modulus constraints. Then, utilizing equivalent reformulation of the preceding design problem, we conceive a stochastic successive inner-outer programming (SSIOP) algorithm that updates the inner-layer and outer-layer variables in sequence and returns the corresponding closed-form solutions. Finally, numerical results assess the dependability of the SSIOP algorithm under an imperfect CSI scenario.

We consider that a transmitter covertly communicates with multiple receivers under the help of a friendly jammer. The messages intended for different receivers are transmitted in mutually orthogonal frequency bands. An adversary observes all these frequency bands aiming at detecting whether or not communication occurs, while the friendly jammer broadcasts jamming signals to degrade the detection performance of the adversary. We consider a block Rayleigh fading channel model and evaluate the performance of covert communication in two situations: 1) the wireless channels vary slowly such that the transmission ends within one channel coherent time block, and 2) the wireless channels vary fast such that the wireless channels have changed several times before the whole transmission is finished. In the former case, subject to a covertness constraint, we maximize the sum of the effective rates by optimizing the transmit power allocation and the transmission rate for each receiver. In the latter case, we take the channel training process into consideration, and subject to a covertness constraint, we maximize the sum of the ergodic rates by optimizing the power allocation and the pilot length. Though both of the two optimization problems are non-convex, we presented methods to find their global optimal solutions. Besides, we also present methods to find sub-optimal solutions with lower computational complexities. Numerical results are presented to evaluate the performance under the two situations.

In this paper, the outage performance of nonorthogonal multiple access (NOMA) is investigated in a downlink space division multiple access (SDMA) network with a multi-antenna base station and randomly deployed users. The NOMA technology is concurrently exploited with SDMA to further improve spectral efficiency. With limited channel state information (CSI) feedback taken into account, an analytical framework is proposed to evaluate outage performance for a given user. An expression for the outage probability is derived in closed form. Moreover, the diversity order and the effect of the number of feedback bits on the outage performance of NOMA are analyzed. Numerical results are demonstrated to verify our analytical findings and show that different from the perfect CSI case there always exists performance floor in the considered network due to limited feedback.

In this paper, we investigate secure communications uplink transmissions, where there are a base station (BS) with M receive antennas, K mobile users each with a single antenna, and an eavesdropper with N receive antennas. The closed-form expressions of the achievable ergodic secrecy sum rates (ESSRs) for a random k users selection scheme in the high and low signal-to-noise ratio regimes are presented. It is shown that the scaling behavior of ESSR with respect to the number served users k can be quite different under different system configurations, determined by the numbers of the BS antennas and that of the eavesdropper antennas. In order to achieve multiuser gain, two low-complexity user selection schemes are proposed under different assumptions on the eavesdropper's channel state information. The closed-form expressions of the achievable ESSRs and the multiuser secrecy gains of the two schemes are also presented in both low and high SNR regimes. observe that, as k increases, the multiuser secrecy gain increases, while the ESSR may decrease. Therefore, when N much larger than M, serving one user with the strongest channel (Time Division Multiple Address-like) is a favorable secrecy scheme, where the ESSR scales with √2 log K.

In this paper, we investigate the secrecy performance of matched-filter (MF) and zero-forcing (ZF) precoding in a multiuser massive multiple-input multiple-output (MIMO) system with a multi-antenna passive eavesdropper. For the two precoding schemes, we find that the equivalent wiretap channels are similar and thus the eavesdropper achieves the approximate same sum-rate. Closed-form expressions of ergodic secrecy sum-rate (ESSR) are derived for the both schemes in the low and high signal-to-noise ratio (SNR) regimes. We show that a positive ESSR of MF scheme may be not achieved when the BS simultaneously serves more than two users with high transmit power, while the ESSR of ZF scheme always grows with the number of the served users. On the contrast, MF scheme is better than ZF scheme in the low SNR regime. Simulation results corroborate the analytic results.

In this paper, we consider the problem of secure communications for a four-node system consisting of one source, one destination, one eavesdropper, and one helper. We investigate the question of which role should the helper act to improve the secrecy, to jam, or to relay. Two transmission schemes are investigated: (1) direct transmission scheme (DTS) with jamming and (2) relay transmission scheme (RTS). We consider both the path-loss and fading-in channel models and define a notion of distance normalized signal-to-noise-ratio (DN-SNR) to account for propagation. The ergodic secrecy rate (ESR) is adopted as the performance metric and semi-closed-form expressions of ESR for the two schemes are derived. Additionally, optimal power allocations in both low and high DN-SNR regimes are characterized analytically. We give the performance comparison of the two schemes from the perspective of energy efficiency in the low DN-SNR regime, and characterize the secrecy degree of freedom in the high DN-SNR regime. In the high DN-SNR regime, DTS provides higher secrecy rate compared with RTS, while in the low DN-SNR regime, RTS outperforms DTS. Furthermore, we show that eavesdropper's position impacts greatly on security.