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.

This paper proposes a novel multi-antenna architecture, termed ray antenna array (RAA), which practically enables flexible beamforming and also enhances wireless communication performance for high-frequency systems in a cost-effective manner. RAA consists of a large number of inexpensive antenna elements and a few radio frequency (RF) chains. These antenna elements are arranged in a novel ray-like structure, where each ray corresponds to onesimple uniform linear array(sULA) with a carefully designed orientation. The antenna elements within each sULA are directly connected, so that each sULA is able to form a beam towards a direction matching the ray orientation without relying on any analog or digital beamforming. By further designing a ray selection network (RSN), appropriate sULAs are selected and connected to the RF chains. Compared to conventional multi-antenna architectures such as the uniform linear array (ULA) with hybrid analog/digital beamforming (HBF), the proposed RAA enjoys four appealing advantages: (i) finer and uniform angular resolution for all signal directions; (ii) enhanced beamforming gain by using antenna elements with higher directivity, as each sULA is only responsible for a small portion of the total angle coverage range; (iii) dramatically reduced hardware cost since no phase shifters are required, which are expensive and difficult to design in high-frequency systems such as millimeter wave (mmWave) and Terahertz (THz) systems; (iv) beam squint free, enabling stable beamforming performance in high-frequency wideband communications. To validate such advantages, we first present the input-output mathematical model for RAA-based wireless communications. Efficient algorithms for joint RAA beamforming and ray selection are then proposed for single-user and multi-user RAA-based wireless communications. Simulation results demonstrate that RAA achieves superior performance compared to the conventional ULA with HBF, while significantly reducing hardware cost.

Extremely-large scale MIMO (XL-MIMO) is a key technology for 6G networks, thanks to its superior spatial resolution and beamforming gain. While the recently proposed ray antenna array (RAA) with directly-connected ULAs enables cost-effective beamforming without phase shifters, it may suffer from signal blockage due to coplanar element placement. To address this issue, we propose a cylinder directly-connected antenna array (DCAA) composed of multiple simple uniform circular arrays (sUCAs) in a layered three-dimensional (3D) structure. Each sub-array in sUCA forms a beam aligned with its orientation to achieve full spatial coverage. We characterize the structural design, provide an overall design guideline, and analytically analyze its performance. An input-output signal model is developed for uplink and downlink, along with optimization algorithms to maximize the achievable sum rate. Simulations under 3GPP channels validate the superior performance of the proposed structure, showing that the cylinder DCAA achieves uniform spatial resolution, higher achievable rate, lower hardware cost and power consumption than the conventional ULA with hybrid beamforming (HBF), highlighting its potential for XL-MIMO in millimeter wave (mmWave) and Terahertz (THz) systems.

This paper proposes a novel multi-antenna architecture, termed ray antenna array (RAA), which aims to enhance wireless communication performance in a cost-effective manner. RAA is composed of massive cheap antenna elements and a few radio frequency (RF) chains. The massive antenna elements are arranged in a novel ray-like structure, with each ray corresponding to a simple uniform linear array (sULA) with a carefully designed orientation. The antenna elements of each sULA are directly connected to an RF combiner, so that the sULA in each ray is able to form a beam towards a direction matching the ray orientation without relying on any analog or digital beamforming. By further designing a ray selection network (RSN), appropriate sULAs are selected to connect to the RF chains for further baseband processing. Compared to conventional multi-antenna architectures like hybrid analog/digital beamforming (HBF), the proposed RAA has two major advantages. First, it can significantly reduce hardware cost since no phase shifters, which are usually expensive especially in high-frequency systems, are required. Besides, RAA can greatly improve system performance by configuring antenna elements with higher directionality, as each sULA only needs to be responsible for a portion of the total coverage angle. To demonstrate such advantages, in this paper, we first present the input-output model for RAA-based wireless communications, based on which the ray orientations of the RAA are designed. Furthermore, efficient algorithms for joint ray selection and beamforming are proposed for single-user and multi-user RAA-based wireless communications. Simulation results demonstrate the superior performance of RAA compared to HBF while significantly reducing hardware cost.

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.