Extremely large-scale multiple-input multiple-output (XL-MIMO) has been viewed as a potential key technology for spectrum efficiency enhancement in 6G communications. With the spherical-wave channel model in XL-MIMO systems, the direct-link channels tend to be orthogonal as the number of antennas scales up. In this context, extra distance dimension can be utilized for multiple access and the location division multiple access (LDMA) was proposed for spectrum efficiency enhancement. However, the spectrum efficiency enhancement in near-field LDMA communications is limited when users are situated far away from the base station and the channels become highly correlated. To tackle this problem, we apply non-orthogonal multiple access (NOMA) in near-field communications to further enhance spectrum efficiency. Specifically, for two-user near-field NOMA, we analyse its closed-form solution for spectrum efficiency and the impact of channel correlation on its spectrum efficiency. Besides, we compare near-field NOMA with LDMA which employs zero-forcing (ZF) precoders. Furthermore, the criteria for applying near-field NOMA is formulated and the impact of near-field beamfocusing position on near-field NOMA performance is analyzed. Moreover, we extend the two-user near-field NOMA communications to the multi-user scenarios and give a overall framework of applying NOMA in near-field multi-user communications. Finally, simulation results verify the effectiveness of applying NOMA in near-field communications to enhance spectrum efficiency.

Near-field beam training is essential for acquiring channel state information in 6G extremely large-scale multiple input multiple output (XL-MIMO) systems. To achieve low-overhead beam training, existing method has been proposed to leverage the near-field beam split effect, which deploys true-time-delay arrays to simultaneously search multiple angles of the entire angular range in a distance ring with a single pilot. However, the method still requires exhaustive search in the distance domain, which limits its efficiency. To address the problem, we propose a distance-dependent beam-split-based beam training method to further reduce the training overheads. Specifically, we first reveal the new phenomenon of distance-dependent beam split, where by manipulating the configurations of time-delay and phase-shift, beams at different frequencies can simultaneously scan the angular domain in multiple distance rings. Leveraging the phenomenon, we propose a near-field beam training method where both different angles and distances can simultaneously be searched in one time slot. Thus, a few pilots are capable of covering the whole angle-distance space for wideband XL-MIMO. Theoretical analysis and numerical simulations are also displayed to verify the superiority of the proposed method on beamforming gain and training overhead.

Extremely large-scale multiple-input multiple-output (XL-MIMO) has been considered as a potential key technology for spectrum efficiency enhancement in 6G communications. The XL-MIMO systems introduce spherical- wave based near-field communications and a new multiple access scheme named location division multiple access (LDMA) is adopted. The near-field LDMA communications provides a novel distance dimension for enhancing spectrum efficiency. However, the spectrum efficiency enhancement in near-field LDMA communications is limited, when user are located far away from the base station and the channels become highly correlated. To solve this problem, we apply non-orthogonal multiple access (NOMA) in near-field communications to further enhance spectrum efficiency. Specifically, for two-user near-field NOMA communications, the closed-form solutions for spectrum efficiency in near-field LDMA and near-field NOMA systems and the impact of channel correlation on them are analyzed. Besides, the criteria for applying near-field NOMA is formulated. Moreover, we extend the two-user near-field NOMA to the multi-user scenarios and give a overall framework of applying NOMA in near-field multi-user communications. Finally, simulation results are provided to verify the feasibility and superiority of applying NOMA in near-field communications to enhance spectrum efficiency.

Abstract Near-field technology is increasingly recognized due to its transformative potential in communication systems, establishing it as a critical enabler for sixth-generation (6G) telecommunication development. This paper presents a comprehensive survey of recent advancements in near-field technology research. First, we explore the near-field propagation fundamentals by detailing definitions, transmission characteristics, and performance analysis. Next, we investigate various near-field channel models—deterministic, stochastic, and electromagnetic information theory based models, and review the latest progress in near-field channel testing, highlighting practical performance and limitations. With evolving channel models, traditional mechanisms such as channel estimation, beamtraining, and codebook design require redesign and optimization to align with near-field propagation characteristics. We then introduce innovative beam designs enabled by near-field technologies, focusing on non-diffractive beams (such as Bessel and Airy) and orbital angular momentum (OAM) beams, addressing both hardware architectures and signal processing frameworks, showcasing their revolutionary potential in near-field communication systems. Additionally, we highlight progress in both engineering and standardization, covering the primary 6G spectrum allocation, enabling technologies for near-field propagation, and network deployment strategies. Finally, we conclude by identifying promising future research directions for near-field technology development that could significantly impact system design. This comprehensive review provides a detailed understanding of the current state and potential of near-field technologies.

Millimeter-wave (mmWave) and terahertz (THz) communications with hybrid precoding architectures have been regarded as energy-efficient solutions to fulfill the vision of high-speed transmissions for 6G communications. Benefiting from the advantages of providing a wide scan-range and flat array gain, the uniform circular array (UCA) has attracted much attention. However, the growing bandwidth of mmWave and THz communications require frequency-dependent phase shifts, which can not be perfectly realized through frequency-independent phase shifters (PSs) in classical hybrid precoding architectures. This mismatch causes the beam defocus effect in UCA wideband communications, where high-gain beams could not form at non-central frequencies in any direction. In this paper, we first investigate the characteristics of the beam defocus effect distinguishing itself from the beam split effect in uniform linear array (ULA) systems. The beamforming gain in both frequency domain and angular domain is analyzed, characterizing the beamforming loss caused by the beam defocus effect. Then, the delay-phase precoding (DPP) architecture leveraging true-time-delays (TTDs) to generate frequency-dependent phase shifts is employed to mitigate the beam defocus effect. Finally, performance analysis and extensive simulation results are provided to evaluate the effectiveness of DPP architecture in UCA systems.

Massive multiple-input multiple-output (MIMO) for 5G is evolving into the extremely large-scale antenna array (ELAA) to increase the spectrum efficiency by orders of magnitude for 6G communications. ELAA introduces spherical-wave-based near-field communications, where channel capacity can be significantly improved for single-user and multi-user scenarios. Unfortunately, the near-field region at large incidence/emergence angles is greatly reduced with the widely studied uniform linear array (ULA). Thus, many randomly distributed users may fail to benefit from near-field communications. In this paper, we leverage the rotational symmetry of uniform circular array (UCA) to provide uniform and enlarged near-field regions at all angles, enabling more users to benefit from near-field communications. Specifically, by exploiting the geometrical relationship between UCA and users, the near-field beamforming technique for UCA is developed. Based on the analysis of near-field beamforming, we reveal that UCA is able to provide a larger near-field region than ULA in terms of the effective Rayleigh distance. Moreover, a concentric-ring codebook is designed to realize efficient codebook-based beamforming in the near-field region. In addition, we find out that UCA could generate orthogonal near-field beams along the same direction when the focal point of the near-field beam is exactly the zeros of other beams, which has the potential to further improve spectrum efficiency in multi-user communications compared with ULA. Simulation results are provided to verify the effectiveness of theoretical analysis and feasibility of UCA to enable more users to benefit from near-field communications by broadening the near-field region.

Millimeter-wave (mmWave) and terahertz (THz) communications with hybrid precoding architectures have been regarded as energy-efficient methods to fulfill the vision of high-speed transmissions for 6G communications. Benefiting from the advantages of providing a wide scan range and uniform array pattern, uniform circular array (UCA) has attracted much attention. However, the growing bandwidth of mmWave and THz communications require frequency-independent phase shifts to perform beamforming, which can not be perfectly realized through frequency-independent phase shifters (PSs) in hybrid precoding schemes. This mismatch causes the beam defocus effect in UCA systems, where high-gain beams disappear at non-central frequencies. In this paper, we first investigate the characteristics of the beam defocus effect distinguishing from beam split effect in uniform linear array (ULA) systems. The beam pattern of UCA in both frequency and angular domain is analyzed, characterizing the beamforming loss caused by beam defocus effect. Then, the delay-phase precoding (DPP) architecture leveraging true-time-delay (TTD) is employed to mitigate the beam defocus effect. Finally, performance analysis and simulations are provided to evaluate the performance improvement with DPP architectures.

Spatial division multiple access (SDMA) is essential to improve the spectrum efficiency for multi-user multiple-input multiple-output (MIMO) communications. The classical SDMA for massive MIMO with hybrid precoding heavily relies on the angular orthogonality in the far field to distinguish multiple users at different angles, which fails to fully exploit spatial resources in the distance domain. With dramatically increasing number of antennas, extremely large-scale antenna array (ELAA) introduces additional resolution in the distance domain in the near field. In this paper, we propose the concept of location division multiple access (LDMA) to provide a new possibility to enhance spectrum efficiency. The key idea is to exploit extra spatial resources in the distance domain to serve different users at different locations (determined by angles and distances) in the near field. Specifically, the asymptotic orthogonality of beam focusing vectors in the distance domain is proved, which reveals that near-field beam focusing is able to focus signals on specific locations to mitigate inter-user interferences. Simulation results verify the superiority of the proposed LDMA over classical SDMA in different scenarios.

Spatial division multiple access (SDMA) is essential to improve the spectrum efficiency for multi-user multiple-input multiple-output (MIMO) communications. The classical SDMA for massive MIMO with hybrid precoding heavily relies on the angular orthogonality in the far field to distinguish multiple users at different angles, which fails to fully exploit spatial resources in the distance domain. With the dramatically increasing number of antennas, the extremely large-scale antenna array (ELAA) introduces additional resolution in the distance domain in the near field. In this paper, we propose the concept of location division multiple access (LDMA) to provide a new possibility to enhance spectrum efficiency compared with classical SDMA. The key idea is to exploit extra spatial resources in the distance domain to serve different users at different locations (determined by angles and distances) in the near field. Specifically, the asymptotic orthogonality of near-field beam focusing vectors in the distance domain is proved, which reveals that near-field beam focusing is able to focus signals on specific locations with limited leakage energy at other locations. This special property could be leveraged in hybrid precoding to mitigate inter-user interferences for spectrum efficiency enhancement. Moreover, we provide the spherical-domain codebook design method for LDMA communications with the uniform planar array, which provides the sampling method in the distance domain. Additionally, performance analysis of LDMA is provided to reveal that the asymptotic optimal spectrum efficiency could be achieved with the increasing number of antennas. Finally, simulation results verify the superiority of the proposed LDMA over SDMA in different scenarios.

Extremely large-scale MIMO (XL-MIMO) communication is a promising technology to improve the capacity for future 6G networks. With a very large number of antennas, the near-field property of XL-MIMO systems becomes significant. Unlike the classical far-field line-of-sight (LoS) channel with only one available data stream, significantly increased degrees of freedom (DoFs) are available in the near-field LoS channel. However, limited by the small number of radio frequency (RF) chains, the existing hybrid precoding architecture widely used for 5G is not able to fully utilize the extra DoFs in the near-field region. In this paper, to exploit the near-field effect as a new possibility for capacity improvement, the distance-aware precoding (DAP) architecture is developed, where each RF chain can be flexibly configured to active or inactive according to the distance-related DoFs. Moreover, based on the developed DAP architecture, a DAP algorithm is proposed to optimize the number of activated RF chains and precoding matrices to match the increased DoFs. Finally, simulation results verify that, the proposed DAP scheme can efficiently utilize the extra DoFs in the near-field region to improve the spectrum efficiency.