In this paper, we design the wideband hybrid-analog-digital (HAD) beamforming for integrated sensing and communication (ISAC) systems. Specifically, we incorporate the phase shifters (PSs) and true-time delay lines (TTDs) to combat the wideband beam squint effect, which are able to provide frequency-dependent phase shift in the analog beamforming stage. The fully-digital (FD) beamformers with guaranteed sensing and communication signal-to-interference-plus-noise ratios (SINRs) are first designed. Then, the HAD beamforming is formulated as a least squares (LS) problem to approximate the designed FD beamformers with constant-modulus constraints, whose main challenges are the complicated objective function and the non-convex constraints. To tackle these issues, we majorize the objective function to decouple the optimization variables. Then, the beamformer for PSs can be solved with a closed-form solution, whereas the beamformer for TTDs can be obtained by a simple grid-search. Finally, we adjust the PS and TTD beamformers by the Riemannian conjugate gradient method (RCGM) to improve the performance. Simulation results demonstrate the superior performance of the proposed algorithm over the conventional algorithm.

Bo Lin, Huanming Zhang, Yuhua Jiang, Yucong Wang, Tengyu Zhang, Shaoqiang Yan, Hongyao Li, Yihong Liu, Feifei Gao. Proceedings of the 2025 Conference on Empirical Methods in Natural Language Processing. 2025.

mmWave networks offer wide bandwidth for high-speed wireless communication but suffer from limited range and susceptibility to blockage. Existing coverage provisioning solutions not only incur high costs but also require significant expert knowledge and manual efforts. In this paper, we present AutoMS, an automated service framework to optimize mmWave coverage by strategically designing and placing low-cost passive metasurfaces. Our approach consists of three key components: (1) joint optimization of metasurface phase configurations and placement as well as access point beamforming codebooks. (2) a fast 3D ray-tracing simulator for accelerated large-scale metasurface channel modeling. (3) a metasurface design amenable to ultra-low-cost hot stamping fabrication, featuring high reflectivity, near 2π phase control, and wideband support. Simulation and testbed experiments show that AutoMS can increase the median received signal strength by 11 dB in target rooms and over 20 dB at previous blind spots, and improve the median throughput by over 3× in real-world scenarios.

Reconfigurable intelligent surface (RIS) has been identified as a promising disruptive innovation to realize a faster, safer and more efficient communication system. In this paper, we study the broad beamwidth design of RIS. A problem is formulated to achieve broadbeam with maximum and equal power gain within a pre-defined angular region given constraints of the unit modulus weights of RIS. Since the formulated problem is non-convex, where the optimal solution cannot be analytically obtained, we propose the difference-of-convex-based semi-definite programming (DC-SDP) algorithm. In addition, as important guidance of signal coverage for arbitrary angular regions, we mathematically derive the relationship between the angular range of the spatial sector and the maximum average received power. The upper bounds of the average received power with different RIS configurations are also obtained, where uniform rectangular array (URA) and uniform linear array (ULA) are considered. Simulation results demonstrate the effectiveness of our derivations and verify that our proposed DC-SDP algorithm is applicable in practical applications and outperforms other baseline methods. Overall, this work can be viewed as a foundation for the practical implementation of RIS on coverage enhancement and can also be seen as an initial step towards achieving channel estimation.

Secret key generation in physical layer security exploits the unpredictable random nature of wireless channels. The millimeter-wave (mmWave) channels have limited multipath and channel randomness in static environments. In this paper, for mmWave secret key generation of physical layer security, we use a reconfigurable intelligent surface (RIS) to induce randomness directly in wireless environments, without adding complexity to transceivers. We consider RIS to have continuous individual phase shifts (CIPS) and derive the RIS-assisted reflection channel distribution with its parameters. Then, we propose continuous group phase shifts (CGPS) to increase the randomness specifically at legal parties. Since the continuous phase shifts are expensive to implement, we analyze discrete individual phase shifts (DIPS) and derive the corresponding channel distribution, which is dependent on the quantization bit. We then derive the secret key rate (SKR) to evaluate the randomness performance. With the simulation results verifying the analytical results, this work explains the mathematical principles and lays a foundation for future mmWave evaluation and optimization of artificial channel randomness.

In this paper, we model, analyze and optimize the multi-user and multi-order-reflection (MUMOR) intelligent reflecting surface (IRS) networks. We first derive a complete MUMOR IRS network model applicable for the arbitrary times of reflections, size and number of IRSs/reflectors. The optimal condition for achieving sum rate upper bound with one IRS in a closed-form function and the analytical condition to achieve interference-free transmission are derived, respectively. Leveraging this optimal condition, we obtain the MUMOR sum rate upper bound of the IRS network with different network topologies, where the linear graph (LG), complete graph (CG) and null graph (NG) topologies are considered. Simulation results verify our theories and derivations and demonstrate that the sum rate upper bounds of different network topologies are under a$K$-fold improvement given$K$-piece IRS.

Intelligent Reflecting Surfaces (IRS) show a revolutionary potential for wireless communications. In this paper, a single IRS is used to achieve distributed multi-user beamforming and interference-free transmission. We first establish the IRS assisted multi-user system model and formulate an optimization problem called multi-user linearly constrained minimum variance (MU-LCMV) beamformer, under the criterion of minimizing the overall received signal power subject to a certain level of power response (e.g., unit power response) at desired signal directions and arbitrary low power response (e.g., zero power response) at the interference directions. A closed-form amplitude-unconstrained phase-continuous (AUPC) solution is derived first, then an amplitude-constrained phase-continuous (ACPC) solution is obtained by using sequential quadratic programming (SQP). Given the solutions, the IRS beam pattern shows that to achieve multi-user ($N$pairs of transceivers,$N > 1$) transmission through a single surface, up to$N-1$redundant beams are generated, significantly affecting power efficiency. The directions of the redundant beams are mathematically derived. The effect of mutual coupling on IRS is also analyzed to show the characteristic of side lobes. Simulation results verify the existence and accuracy of the redundant beam directions. This work can potentially enhance state-of-the-art wireless communication systems ranging from transceiver design, system and architecture design, network deployment and self-organizing-network operations.

Recent advances in meta-materials offer the prospect of deploying smart surfaces, or intelligent reflecting surfaces (IRS), that can manipulate electromagnetic (EM) channels and expand their achievable capacity. In this paper, we investigate the programmable channel of multi-user multiple input and multiple output (MU-MIMO) transmission and beamforming using meta-surface with multiple elements. We first model the MU-MIMO channel, and the optimal solution is derived based on the proposed multi-user Linearly Constrained Minimum Variance (MU-LCMV) beamformer. The beam pattern is analyzed, which shows that a single set of optimal weights can form multiple interference-free beams with redundant beams to be formed to achieve the multi-stream MIMO transmission in typical configurations. The mathematical relationships of the beams are derived with different surface configurations. Extensive simulations verify the results. This work is fundamental and can potentially enhance any state-of-the-art wireless communication systems ranging from transceiver design, system and architecture design, network deployment, and self-organizing-network operations.