As an emerging reconfigurable antenna technology, fluid antenna system (FAS) has the capability of improving both sensing and communication (S&C) performance by switching the antenna position over the available ports. This increased spatial degree-of-freedom (DoF) by FAS can be translated into enlarging the trade-off region for integrated sensing and communication (ISAC). In this letter, we propose a signal model for FAS-enabled ISAC and jointly optimize the transmit beamforming and port selection of FAS. In particular, our objective is to minimize the transmit power, while satisfying both communication and sensing requirements. To tackle this, an efficient iterative algorithm based on sparse optimization, convex approximation, and a penalty approach is developed. Our simulation results illustrate that the proposed scheme can attain 33% reductions in the transmit power with guaranteed S&C performance, showing the great potential of FAS for striking a balance between S&C in ISAC systems.

In this paper, we investigate the design of energy-efficient beamforming for an ISAC system, where the transmitted waveform is optimized for joint multi-user communication and target estimation simultaneously. We aim to maximize the system energy efficiency (EE), taking into account the constraints of a maximum transmit power budget, a minimum required signal-to-interference-plus-noise ratio (SINR) for communication, and a maximum tolerable Cramér-Rao bound (CRB) for target estimation. We first consider communication-centric EE maximization. To handle the non-convex fractional objective function, we propose an iterative quadratic-transform-Dinkelbach method, where Schur complement and semi-definite relaxation (SDR) techniques are leveraged to solve the subproblem in each iteration. For the scenarios where sensing is critical, we propose a novel performance metric for characterizing the sensing-centric EE and optimize the metric adopted in the scenario of sensing a point-like target and an extended target. To handle the nonconvexity, we employ the successive convex approximation (SCA) technique to develop an efficient algorithm for approximating the nonconvex problem as a sequence of convex ones. Furthermore, we adopt a Pareto optimization mechanism to articulate the tradeoff between the communication-centric EE and sensing-centric EE. We formulate the search of the Pareto boundary as a constrained optimization problem and propose a computationally efficient algorithm to handle it. Numerical results validate the effectiveness of our proposed algorithms compared with the baseline schemes and the obtained approximate Pareto boundary shows that there is a non-trivial tradeoff between communication-centric EE and sensing-centric EE, where the number of communication users and EE requirements have serious effects on the achievable tradeoff.

Integrated sensing and communications (ISAC) systems employ dual-functional signals to simultaneously accomplish radar sensing and wireless communication tasks. However, ISAC systems open up new sensing security vulnerabilities to malicious illegitimate eavesdroppers (Eves) that can also exploit the transmitted waveform to extract sensing information from the environment. In this paper, we investigate the beamforming design to enhance the sensing security of an ISAC system, where the communication user (CU) serves as a sensing Eve. Our objective is to maximize the mutual information (MI) for the legitimate radar sensing receiver while considering the constraint of the MI for the Eve and the quality of service to the CUs. Then, we consider the artificial noise (AN)-aided beamforming to further enhance the sensing security. Simulation results demonstrate that our proposed methods achieve MI improvement of the legitimate receiver while limiting the sensing MI of the Eve, compared with the baseline scheme, and that the utilization of AN further contributes to sensing security.

To expand the degrees of freedom and achieve additional gains beyond multiple-input multiple-output (MIMO), the fluid antenna system (FAS) has been regarded as a potential approach for exploiting spatial diversity by reconfigurable position-switchable antennas. In the operation of FAS, port selection is one of the key challenges, demanding substantial channel training overhead. In this paper, we propose a novel online learning framework to address the port selection issue without the instantaneous full channel state information (CSI) in time-varying channel conditions. Based on bandit learning, the proposed approach learns the optimal port selection by dynamically interacting with the varying channels. The proposed algorithm is proved to achieve guaranteed performance with a bounded deviation and simulation results confirm comparable performance by comparing with the ideal optimal selections.

In order to maximize the spectral efficiency (SE) in multicarrier-division duplex (MDD) enabled cell-free massive MIMO (CF-mMIMO), a heterogeneous graph neural network (HGNN), referred to as CF-HGNN, is specifically introduced to optimize the power allocation (PA). To efficiently manage the interference invoked, a meta-path based mechanism is applied in CF-HGNN to enable individual access point (AP) and mobile station (MS) nodes to aggregate information from the interfering and communication paths with different priorities during message passing. Moreover, the proposed CF-HGNN employs the adaptive node embedding layer and adaptive output layer to make it scalable to the various numbers of APs, MSs and subcarriers. For comparison, a quadratic transform and successive convex approximation (QT-SCA) algorithm is proposed to solve the PA problem in classic way. Numerical results show that CF-HGNN is capable of achieving 99% of the SE achievable by QT-SCA but using only 10 ^-4 times of its operation time, and it can outperform the conventional learning-based and greedy unfair methods in terms of SE performance. Furthermore, CF-HGNN exhibits good scalability to the CF networks with various numbers of nodes and subcarriers, and also to the large-scale CF networks when assisted by user-centric clustering.

Unmanned aerial vehicles (UAVs) have demonstrated exceptional capabilities in various applications such as vehicle tracking, military monitoring and communication assistance. With their advantages in mobile communication and integrated multiple sensors, UAVs have great potential to obtain the integration gain through integrated sensing and communications (ISAC) designs. In this paper we focus on the beamforming algorithm for UAV-to-vehicle communications. To reduce the communication overhead of block transmission caused by the high mobility of the target vehicles and the UAV, we utilize the inherent vision functionality of the UAV platform and propose a vision-assisted beamforming framework. Specifically, to obtain the position of to-be-served vehicles, we employ YOLOv5, a deep learning model, for vehicle detection, followed by a Sage-Husa filter for multiple target vehicle tracking. Based on the predicted positions of the vehicles, we propose a lightweight beamforming algorithm to save communication overhead of beam tracking. To validate the effectivess of this work, we perform experiments on the UAVDT dataset for vehicle detection and tracking. The simulation results show that the proposed algorithm achieves significant performance gains on both the effective achievable rates and the energy efficiency.