Movable antenna (MA), which can flexibly change the position of antenna in three-dimensional (3D) continuous space, is an emerging technology for achieving full spatial performance gains. In this paper, a prototype of MA communication system with ultra-accurate movement control is presented to verify the performance gain of MA in practical environments. The prototype utilizes the feedback control to ensure that each power measurement is performed after the MA moves to a designated position. The system operates at 3.5 GHz or 27.5 GHz, where the MA moves along a one-dimensional horizontal line with a step size of 0.01λ and in a two-dimensional square region with a step size of 0.05λ, respectively, with λ denoting the signal wavelength. The scenario with mixed line-of-sight (LoS) and non-LoS (NLoS) links is considered. Extensive experimental results are obtained with the designed prototype and compared with the simulation results, which validate the great potential of MA technology in improving wireless communication performance. For example, the maximum variation of measured power in the considered scenario reaches over 40 dB and 23 dB at 3.5 GHz and 27.5 GHz, respectively, thanks to the flexible antenna movement. In addition, experimental results indicate that the power gain of MA system relies on the estimated path state information (PSI), including the number of paths, their elevation and azimuth angles of arrival (AoAs), as well as the complex gain of each path.

Integrated sensing and communications (ISAC) is a pivotal technology for low-altitude unmanned aerial vehicle (UAV) swarm. As the sensing targets for UAV swarm systems are usually densely located, the conventional compact multi-input multi-output (MIMO) with half-wavelength antenna spacing usually leads to prohibitive hardware, energy and signal processing costs when large array aperture is needed to achieve fine spatial resolution. By relaxing the traditional half-wavelength spacing constraint, sparse MIMO may achieve a larger array aperture without having to increase the number of antenna elements or radio frequency (RF) chains, which improves spatial resolution for both communication and sensing. Besides, sparse MIMO may also result in a larger near-field region. Therefore, in this paper, we study the near-field sparse MIMO bistatic orthogonal frequency division multiplexing (OFDM)-ISAC for low-altitude UAV swarm systems and propose the framework that utilizes physical array for communication while virtual array for sensing. The proposed method forms virtual arrays at the ISAC transmitter and sensing receiver simultaneously, while eliminating the angle and range coupling effect. As a result, high-resolution angle estimation based on virtual array is achieved. Simulation results demonstrate that sparse MIMO simultaneously improves communication sum rates and sensing resolution compared to the conventional compact MIMO.

In this paper, we investigate the far-field and near-field codebook-based beam training for multi-user modular extremely large-scale multiple-input multiple-output (XL-MIMO) communications, utilizing a modular extremely large-scale uniform linear array (XL-ULA) at the base station (BS). Unlike conventional collocated XL-ULA with all adjacent elements separated by signal wavelength scale, the modular XL-ULA has different inter-module and intra-module spacings, rendering the existing near-field polar-domain codebook design ineffective. To cater to the modular array architecture, one straightforward approach to beam codebook design is to remove those elements corresponding to the modular space in the conventional polar-domain codebook. However, such a naive polar-domain codebook for modular XL-ULA results in undesired grating lobes. To overcome this challenge, we propose a novel near-field optimization-based codebook design, by exploiting the priori knowledge about the users’ potential angular/distance range to minimize the levels of side lobes while maintaining the main lobe beamforming gain. Furthermore, based on the designed near-field codebooks, an efficient multi-beam training scheme enabled by grating lobes is devised for multi-user modular XL-MIMO communications. Numerical results verify the effectiveness of the proposed near-field optimization-based codebook for modular XL-MIMO in mitigating inter-user interference (IUI) for ultra-dense users, as well as the efficiency of the multi-beam training scheme.

This paper investigates the near-field spatial correlation (SC) for extremely large-scale multiple-input multiple-output (XL-MIMO) communications, where the positions of scatterers and/or users may be in the radiative near-field region due to the large size of the antenna array. Therefore, the conventional far-field uniform plane wave (UPW) assumption is no longer valid. By discarding the UPW assumption, we consider the generic non-uniform spherical wave (NUSW) model for accurate characterization of signal amplitude and phase variations on different antenna elements. A novel expression is derived for the near-field SC in terms of the scatterer distribution, which generalizes the conventional far-field SC. It is revealed that the developed near-field SC is determined by the power location spectrum (PLS), which is characterized not only by the scatterers’ directions but also by their distances from the array, while the conventional far-field model is only dependent on the power angular spectrum (PAS). In addition, the near-field SC no longer exhibits spatial wide-sense stationarity (SWSS), since the SC coefficient between each pair of transmit-receive antenna components is determined by their specific positions, not just by their relative locations. Furthermore, the developed near-field SC is utilized to obtain the optimal transmission strategy to maximize the ergodic spectral efficiency based on statistical channel state information (CSI). Besides, we consider the specific multi-ring scattering model for scatterer distribution, where semi-closed expressions for the near-field SC can be obtained. Simulation results are given to validate the developed near-field SC for XL-MIMO communications.

In this paper, we investigate multi-user modular extremely large-scale multiple-input multiple-output (XL-MIMO) communication systems, where modular extremely large-scale uniform linear array (XL-ULA) is deployed at the base station (BS) to serve multiple single-antenna users. By exploiting the unique modular array architecture and considering the potential near-field propagation, we develop sub-array based uniform spherical wave (USW) models for distinct versus common angles of arrival/departure (AoAs/AoDs) with respect to different sub-arrays/modules, respectively. Under such USW models, we analyze the beam focusing patterns at the near-field observation location by using near-field beamforming. The analysis reveals that compared to the conventional XL-MIMO with collocated antenna elements, modular XL-MIMO can provide better spatial resolution by benefiting from its larger array aperture. However, it also incurs undesired grating lobes due to the large inter-module separation. Moreover, it is found that for multi-user modular XL-MIMO communications, the achievable signal-to-interference-plus-noise ratio (SINR) for users may be degraded by the grating lobes of the beam focusing pattern. To address this issue, an efficient user grouping method is proposed for multi-user transmission scheduling, so that users located within the grating lobes of each other are not allocated to the same time-frequency resource block (RB) for their communications. Numerical results are presented to verify the effectiveness of the proposed user grouping method, as well as the superior performance of modular XL-MIMO over its collocated counterpart with densely distributed users.

For extremely large-scale array (XL-array) communications, the scatterers and/or user equipments (UEs) may be located in the near-field region and only visible to some portions of the XL-array. This paper studies the near-field spatial correlation function (S-CF) of multi-path XL-array communications with mixed line-of-sight (LoS) and non-LoS (NLoS) links. The generic near-field non-uniform spherical wave (NUSW) characteristic and the partial visibility property are considered. For the LoS link, a novel near-field S-CF is derived, which is in terms of the correlation of UE's visibility and location. It is found that the near-field S-CF depends on the UE's angle of arrival (AoA) and distance, which differs from the far-field result that only depends on the UE's AoA. For the NLoS links, we derive a novel integral expression for the near-field S-CF in terms of the correlation of the scatterers' visibility and location distribution. The near-field result is shown to depend on the scatterers' partial visibility and the power location spectrum (PLS) characterized by the AoAs and distances of scatterers, in contrast to the far-field model, which relies on the power angular spectrum (PAS). The result demonstrates that the near-field S-CF of the LoS/NLoS component no longer exhibits spatial wide-sense stationarity (SWSS) and is more generic than the far-field model. To gain further insights, we consider a specific scatterer's location distribution, namely the multi-ring scatterer model. Numerical results show the necessity of modeling near-field S-CF for XL-array communications with partial visibility.

In this paper, we investigate the near-field modelling and analyze the beam focusing pattern for modular extremely large-scale array (XL-array) communications. As modular XL-array is physically and electrically large in general, the accurate characterization of amplitude and phase variations across its array elements requires the non-uniform spherical wave (NUSW) model, which, however, is difficult for performance analysis and optimization. To address this issue, we first present two ways to simplify the NUSW model by exploiting the unique regular structure of modular XL-array, termed sub-array based uniform spherical wave (USW) models with different or common angles, respectively. Based on the developed models, the near-field beam focusing patterns of XL-array communications are derived. It is revealed that compared to the existing collocated XL-array with the same number of array elements, modular XL-array can significantly enhance the spatial resolution, but at the cost of generating undesired grating lobes. Fortunately, different from the conventional far-field uniform plane wave (UPW) model, the near-field USW model for modular XL-array exhibits a higher grating lobe suppression capability, thanks to the non-linear phase variations across the array elements. Finally, simulation results are provided to verify the near-field beam focusing pattern and grating lobe characteristics of modular XL-array.

This paper investigates a dual-unmanned aerial vehicle (UAV)-enabled secure communication system, in which, a UAV moves around to send confidential messages to a mobile user while another cooperative UAV transmits artificial noise signals to confuse malicious eavesdroppers. Both UAVs have energy constraints and the location information of eavesdroppers is imperfect. We consider a worst-case secrecy rate maximization problem of the mobile user over all time slots. This optimization problem is solved by jointly designing the three-dimensional (3D) trajectory of UAVs and the time allocation (recharging and service or jamming) under practical constraints including maximum UAV speed, UAV collision avoidance, UAV positioning error, and UAV energy harvesting. Specifically, we adopt a more practical UAV-ground channel model with both large-scale and small-scale fading components. Due to the non-convex feasible region constructed by the complicated constraints, directly finding the optimal solution of the original problem is intractable. To address this issue, we decouple the original optimization problem into three subproblems and develop an iterative algorithm to find its suboptimal solution by using the block coordinate descent technique. To solve each subproblem, certain advanced optimization tools, such as integer relaxation, S-procedure, and successive convex approximation techniques, are utilized. Numerical simulation results are provided to corroborate the theoretical derivations and to evaluate the performance of the proposed algorithm. Additionally, the numerical results assist to draw new insights on the 3D UAV trajectory by comparing the performance with conventional two-dimensional (2D) schemes.

In this paper, we consider an energy-constrained unmanned aerial vehicle (UAV)-enabled mobile relay assisted secure communication system in the presence of a legitimate source-destination pair and multiple eavesdroppers with imperfect locations. The energy-constrained UAV employs the power splitting (PS) scheme to simultaneously receive information and harvest energy from the source, and then exploits the time switching (TS) protocol to perform information relaying. Furthermore, we consider a full-duplex destination node which can simultaneously receive confidential signals from the UAV and cooperatively transmit artificial noise (AN) signals to confuse malicious eavesdroppers. To further enhance the reliability and security of this system, we formulate a worst case secrecy rate maximization problem, which jointly optimizes the position of the UAV, the AN transmit power, as well as the PS and TS ratios. The formulated problem is non-convex and generally intractable. In order to circumvent the non-convexity, we decouple the original optimization problem into three subproblems; this facilitates the design of a suboptimal iterative algorithm. In each iteration, we propose a multi-dimensional search and numerical method to handle the subproblem. Numerical simulation results are provided to demonstrate the effectiveness and superior performance of the proposed joint design versus the conventional schemes in the literature.