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Technological advancements have enabled the active control of electromagnetic waves. Metalenses, known for their precision in wavefront shaping and functional versatility, represent a breakthrough in optical modulation. This study addresses the challenge of achieving dual-wavelength multifunctionality in metalens design. We developed and experimentally validated metalenses with polarization dual-function multiplexing at discrete mid-wave infrared wavelengths, demonstrating high phase fidelity and functional versatility. In addition, the proposed design method was extended to long-wave infrared wavelengths, showcasing its adaptability to different application scenarios. The application of spatial multiplexing significantly enhanced the performance of the metalenses, providing a promising solution for efficient and compact optoelectronic devices. [ABSTRACT FROM AUTHOR]

The capabilities of multi-antenna technology have recently been significantly enhanced by the proliferation of extra large array architectures. The high dimensionality of these systems implies that communications take place in the near-field regime, which poses some questions as to their effective performance even under simple line of sight configurations. In order to study these limitations, a uniform linear array (ULA) is considered here, the elements of which are three infinitesimal dipoles transmitting different signals in the three spatial dimensions. The receiver consists of a single element with three orthogonal infinitesimal dipoles and full channel state information is assumed to be available at both ends. A capacity analysis is presented when the number of elements of the ULA increases without bound while the interelement distance converges to zero, so that the total aperture length is kept asymptotically fixed. In particular, the total number of available spatial degrees of freedom is shown to depend crucially on the receiver position in space, and closed form expressions are provided for the different achievability regions. From the analysis it can be concluded that the use of three orthogonal polarizations at the transmitter guarantees the universal availability of at least two spatial streams everywhere.

The holographic interference of either same‐handed or orthogonal polarized beams is commonly employed in the fabrication of optical elements as it enables the development of an ideal sinusoidal surface modulation by inducing the mass migration of photochromic polymer. However, this approach encounters challenges in producing multiplexed optical elements with a complex surface topography, primarily due to only sinusoidal surface topography which limits control over the feature's shape and size. Here, a versatile approach is demonstrated for fabricating spatially multiplexed optical elements using ellipticity‐controlled orthogonally elliptically polarized (OEP) beams that offer distorted square‐like and rhombus‐like pillars on the azopolymer layer by asymmetric azopolymer migration driven by spatially varying ellipticity and azimuth angle of interference beam. These multiplexed distorted shapes enable selective energy transfer during light diffraction and also induce a π/8$\pi /8$ phase‐shift to polarization states. Ellipticity control of both writing and interference beams provides an additional degree of freedom for structuring the surface topography of optical elements. Furthermore, a quantitative analysis of the multiplexed OEP surface relief is performed to achieve distinct diffraction properties, thereby rendering its suitability for fabricating complex optical elements. [ABSTRACT FROM AUTHOR]

We review the progress in multiplexed single-photon sources, including overviews on heralded single-photon sources and photon-number-resolving detectors, the various approaches to multiplexing, and their successful experimental realizations. We also summarize the recent results on the theoretical description and optimization of multiplexed single-photon sources, focusing on the procedures and methods that enable the improvement of the performance of these sources. [ABSTRACT FROM AUTHOR]

Technological advancements have enabled the active control of electromagnetic waves. Metalenses, known for their precision in wavefront shaping and functional versatility, represent a breakthrough in optical modulation. This study addresses the challenge of achieving dual-wavelength multifunctionality in metalens design. We developed and experimentally validated metalenses with polarization dual-function multiplexing at discrete mid-wave infrared wavelengths, demonstrating high phase fidelity and functional versatility. In addition, the proposed design method was extended to long-wave infrared wavelengths, showcasing its adaptability to different application scenarios. The application of spatial multiplexing significantly enhanced the performance of the metalenses, providing a promising solution for efficient and compact optoelectronic devices.

The low-pass characteristics of front-end elements including light-emitting diodes (LEDs) and photodiodes (PDs) limit the transmission data rate of visible light communication (VLC) and Light Fidelity (LiFi) systems. Using multiplexing transmission techniques, such as spatial multiplexing (SMX) and wavelength division multiplexing (WDM), is a solution to overcome bandwidth limitation. However, spatial correlation in optical wireless channels and optical filter bandpass shifts typically limit the achievable multiplexing gain in SMX and WDM systems, respectively. In this paper, we consider a multiple-input multiple output (MIMO) joint multiplexing VLC system that exploits available degrees-of-freedom (DoFs) across space, wavelength and frequency dimensions simultaneously. Instead of providing a new precoder/post-detector design, we investigate the considered joint multiplexing system from a system configuration perspective by tuning system parameters in both spatial and wavelength domains, such as LED positions and optical filter passband. We propose a novel spatial clustering with wavelength division (SCWD) strategy which enhances the MIMO channel condition. We propose to use a state-of-the-art black-box optimization tool: Bayesian adaptive direct search (BADS) to determine the desired system parameters, which can significantly improve the achievable rate. The extensive numerical results demonstrate the superiority of the proposed method over conventional SMX and WDM VLC systems.

We review the progress in multiplexed single-photon sources, including overviews on heralded single-photon sources and photon-number-resolving detectors, the various approaches to multiplexing, and their successful experimental realizations. We also summarize the recent results on the theoretical description and optimization of multiplexed single-photon sources, focusing on the procedures and methods that enable the improvement of the performance of these sources.

This work discusses a cluster-based non-orthogonal multiple access (NOMA) structures aiming at improving the system energy efficiency (EE) and spectral efficiency (SE) in a Massive MIMO (M-MIMO) cell through user clustering and power allocation strategies. Effectively, this work proposes an improved clustering-based NOMA M-MIMO structure under the EE-SE perspective combining the EE and SE performance metrics. In the considered power-domain NOMA M-MIMO configuration, the users are grouped into several clusters; hence, each beamformer serves the users at a specific cluster. The EE-SE tradeoff, namely resource efficiency figure of merit in a cluster-based downlink NOMA M-MIMO is evaluated considering the application of two power allocation strategies. Based on two different channel cluster-head selection strategies, namely the equivalent channel vectors (Type I) and the channel matrix singular value decomposition (SVD) (Type II), the cluster-head in each cluster can tremendously mitigate the intra-cluster interference. Numerical results demonstrate that the best system performance regarding EE-SE tradeoff occurs for MIMO NOMA Type I cluster-head selection with two users per cluster. [ABSTRACT FROM AUTHOR]

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Underwater Wireless Optical Communication (UWOC) has the advantages of large capacity, high speed and etc. However, it suffers from limited communication range and data rate in practice due to the complex underwater environment and the limited bandwidth of optical devices. Multiple-Input Multiple-Output (MIMO) technology has received considerable attention since it converts spatial single-stream data into multiple-stream data for parallel transmission. Therefore it can extend the UWOC range and alleviate the performance degradation caused by underwater absorption, scattering, and turbulence. In this paper, focusing on the MIMO technology in UWOC systems, we analyzed the channel modeling, research progress, experimental setups and prototypes, and the development and future trend of MIMO-UWOC technology.

The pumping of Tm-doped crystal or fiber by a 792 nm semiconductor laser is an important way to generate a mid-infrared laser, which is widely used in various fields. In this paper, a high–power 792 nm fiber–coupled semiconductor laser module was successfully fabricated with the output power of 232 W at a 10 A continuous current and the electro-optic conversion efficiency of 48.6%. The laser module is coupled with 24 chips into a fiber by spatial multiplexing and polarization combination technology. For a single emitting laser chip, the continuous wave (CW) output power and threshold current are 10.45 W at 10 A and 1.55 A, respectively. A polarization as high as 94% can also be realized, which is more suitable for laser spatial beam combining. The laser module was aged for more than 4000 h at 12 A and 25 °C without obvious power degradation. [ABSTRACT FROM AUTHOR]

Fog screens offer a great advantage to be used as scattering screens in three-dimensional displays. In the absence of a fixed rigid form, fog screens work as screens without any physical obstruction. The density of water droplets can be regulated to adjust the screen’s transparency and scattering level. This study proposes a method for crosstalk reduction between adjacent viewing windows by decreasing the divergence of view of the projector, with concentration lenses. By applying the Scheimpflug principle, we also reduce the keystone effect, which occurs when the fog screen is slanted with respect to the optical axis. In this study, we have realized a tabletop multiview fog display without any structures on the table and established that the proposed method is feasible for decreasing crosstalk effectively.

Single pixel imaging as an alternative to traditional imaging methods, has attracted extensive attention in various research fields. Metasurfaces with subwavelength unit cells and compact footprint can be used as a substitute for traditional optical elements. In this work, we propose a single pixel imaging scheme based on metasurface composed of photon sieves, where spatial modulation is realized through shifting. Spatial multiplexing capability is demonstrated by this shifting mode, which can obtain more patterns in limited space and greatly increase the mask capacity. Benefited from the simple structure and easy manufacture of photon sieves, large capacity metasurface can be manufactured. Meanwhile, metasurfaces can simplify the single pixel imaging system, leading to the system miniaturization and integration. In addition, numerical and optical experiments prove that our proposal can operate at the range from the entire visible light to near-infrared light. Such scheme provides a new way for single pixel imaging and would be applied in microscopic imaging, dynamic imaging, hyperspectral imaging, and so on.

LiFi or networked optical wireless communication is likely to play an important role in offloading mobile data traffic from radio into the optical spectrum. As the number of Internet of Things (IoT) devices is growing, the RF spectrum becomes a rare resource. Imaging IoT sensors like cameras, ultrasonic devices, and Lidars have real-time requirements, need a high-capacity uplink, and operate in environments that cause or are sensitive against electromagnetic interference. In this paper, for the first time, we present realtime communication over an all-optical fixed-wireless LiFi link based on the distributed multiple-input multiple-output concept. For distributing the wireless signals, plastic optical fibers are used as an analog front-haul. We study the operation of the distributed multiple-input multiple-output link in two modes, i.e., spatial diversity and spatial multiplexing. For the diversity mode, a new combiner is presented, which can support equal gain as well as selection combining. We demonstrate that selection combining is highly effective and enables a similar LiFi performance in up- and downlink, as it is desirable for industrial applications. For the spatial multiplexing mode, we observe that the channel rank and the achievable throughput depend strongly on the user location. As effective solutions, we study the benefits of angular diversity and multiple-input multiple-output mode switching together with multi-user multiplexing and conclude that a dynamic switching between spatial diversity and spatial multiplexing is a practical approach.

A novel time division duplex massive MIMO technique is proposed based on performing a pilot-less channel estimation in the uplink (UL) utilizing reconstructed differentially encoded data. Spatial multiplexing and differentially encoded data is applied both in the UL and in the downlink (DL). In this system, a reference signal is the first one of the differentially encoded streams in the UL and DL, and the pilots for data estimation are avoided while maintaining spatial multiplexing capabilities. To improve the channel estimation we propose to use a linear Wiener filter and we also propose different symbols placing strategies in an OFDM grid. We also propose a detection improvement of the UL data utilizing the predicted channels. We perform an analysis of the MSE of the blind channel estimation using the differentially encoded data and analyze the symbol-error-rate for both the UL and the DL when channel aging is considered. The analysis is corroborated via numerical results and the proposed scheme is shown to outperform its pilot-based counterpart.

The advent of the fifth generation (5G) of mobile networks has introduced several new use cases that are pushing mobile networks in environments that are typically supported by wired technologies. The initial discussions around the sixth generation (6G) of mobile networks signalizes that different approaches are needed to address all contrasting requirements, where multiple-input multiple-output (MIMO) technique stands as a key technology for most future wireless systems. In this review, we present an introduction on classical linear estimators and coherent detectors along with an innovative and accurate complexity formulation within a common framework, allowing a fair comparison and providing an initial guideline for researchers that are looking for a general view of the main techniques available for spatial multiplexing (SM)-MIMO detection and estimation.

Occupying more channels increases transmission rate, however, crosstalk increases to an unacceptable level long before all 24 channels can be exploited. The crosstalk is not uniform between modes, hence occupying different subsets of channels leads to vastly different achievable transmission capacities. In addition to optimizing occupied subsets of channels, we adopt a bit loading approach. We examine several resource allocation strategies in a coherent detection system, starting with typical OAM mode group granularity (all channels in a group occupied) and ending with single channel granularity. By exhaustive search at mode group granularity, we find a bit load increasing the total capacity by $\sim$ 15% compared to a minimax solution for mode group allocation. Single channel granularity imposes great computational effort to optimize bit loading. We propose search algorithms that are computationally tractable and improve capacity by $\sim$ 30% vis-à-vis the minimax solution for mode groups. Finally, we examine the impact of signal-to-noise ratio (SNR) and receiver digital signal processing (DSP) complexity on the overall capacity. We include a discussion of DSP with limited or with no multi-input, multi-output (MIMO) processing.Our algorithms could be applied to any mode multiplexing fiber, as it only relies on knowledge of the crosstalk matrix across modes. [ABSTRACT FROM AUTHOR]

Fog screens offer a great advantage to be used as scattering screens in three‐dimensional displays. In the absence of a fixed rigid form, fog screens work as screens without any physical obstruction. The density of water droplets can be regulated to adjust the screen's transparency and scattering level. This study proposes a method for crosstalk reduction between adjacent viewing windows by decreasing the divergence of view of the projector, with concentration lenses. By applying the Scheimpflug principle, we also reduce the keystone effect, which occurs when the fog screen is slanted with respect to the optical axis. In this study, we have realized a tabletop multiview fog display without any structures on the table and established that the proposed method is feasible for decreasing crosstalk effectively. [ABSTRACT FROM AUTHOR]

The free-space optical (FSO) communication represents a promising futuristic technology due to several benefits, which has attracted significant attention recently for various applications. FSO communication is susceptible to fading due to atmospheric turbulence. FSO channel, being a slow fading channel, inherently does not allow the efficient use of error-correcting codes (ECCs). ECCs have shown to have good performance in a fast fading channel. A way to convert the slow fading channel into a fast fading channel is by using techniques like diversity. Diversity makes the bits pass through many channels simultaneously, and thus the fast fading channel effect is observed at the receiver. In this paper, we first derive the capacity and bit error rate performance of the low-density parity-check (LDPC) codes in a fast fading channel. We then study the performance of LDPC codes in receiver diversity systems. Finally, we see the impact of using LDPC coded multiple-input multiple-output (MIMO) systems in the FSO channel. We use regular (3,6) LDPC code and irregular LDPC codes of rate 0.5, where the desired degree distribution is optimized for the channel condition.

To overcome the inherent bandwidth limitation of non-line-of-sight (NLOS) ultraviolet communication (UVC) channel, a novel space division multiplexing (SDM) framework based on non-coplanar rotation angle is established. The influence of SDM structure parameters on inter-channel interference (ICI) is studied to obtain the optimal antenna configuration. And a Gaussian approximate distribution (GAD) signal detection model is presented, which simplifies the calculation of the Bit error rate (BER) of the Photon multiplier tube (PMT) reception and is suitable for complex ICI analysis. Furthermore, a low ICI scheme based on the relationship between BER and the interference factor is proposed to increase the data transmission rate or reduce the alignment configuration. As a result, the maximum data rate increases to 34 Mbps, or the alignment demand decreases by 33.33% at most. Moreover, these analysis results and techniques can be applied to other NLOS scattering optical wireless communication systems.

In this paper we discuss the design of antenna arrays to be used for multiplexing applications. In particular, we introduce a suitable performance index to analyze the effect of the antenna geometry and the distribution of users for the overall performance of Multi-User Multiple Input Multiple Output systems. By means of such performance index, antenna arrays can be designed so as to increase the number of multiplexed parallel sub-channels. Numerical results show that a proper design could allow to double the contemporary served users and the overall system throughput.

This paper proposes an approach for acoustic field imaging using simultaneous multi-view digital holography based on three-color digital off-axis holography. Considering spatio-chromatic multiplexing and the recording with a monochrome sensor, the numerical processing of time-sequences of holograms yields both the amplitude and phase of the acoustic field along three different directions of observation. Distortion analysis is presented and the acousto-optic interaction along the optical beam is discussed using a theoretical modelling. Experimental results with an emitter at 40 kHz establish the proof-of-concept of the proposed multi-view imaging for acoustic fields. [ABSTRACT FROM AUTHOR]

Compared with intensity imaging, more inherent information of objects can be obtained by polarization and multispectral imaging. Here, a broadband and highly integrated single‐chip full Stokes polarization‐spectral (FSPS) imaging is demonstrated using all‐dielectric spatially multiplexed metalens (SMM) in the wavelength band from 1400 to 1700 nm. The proposed FSPS‐SMM is composed of three sets of off‐axis sub‐metalens simultaneously working for pairs of 0°/90°, 45°/135° linear polarization (LP), and left‐handed/right‐handed circular polarization (CP), respectively. Experimental results show that the optical resolution of each sub‐metalens of the fabricated FSPS‐SMM reaches the diffraction limit of the full‐aperture metalens although the area of each sub‐metalens is only 1/3 of the entire metalens, the averaged polarization extinction ratio of both LP and CP reaches 32.8:1, and the energy efficiency reaches 81.8% at the design wavelength and 60% averagely in the whole 300 nm bandwidth, which breaks the 50% limit of conventional polarizer‐based methods. The proposed design provides a new idea for high‐efficiency and high‐resolution full Stokes polarization‐spectral imaging and multichannel information processing. [ABSTRACT FROM AUTHOR]

For user terminals, energy-detection based multiple-input-multiple-output (MIMO) has the advantages of low complexity and anti-Doppler capability. However, the energy detector makes it difficult to linearly separate the signals in spatial multiplexing. In this letter, the linearized model is introduced in $2\times 2$ M-ary frequency-shifted keying (MFSK) V-BLAST system with energy detection. To avoid rank deficiency of the model, we propose nonlinear virtual antennas (NVA) to increases both the dimension and receive diversity gain of MIMO system. Then the full-rank condition is discussed. In multiple-antenna systems, NVA is further generalized by spatial index modulation with dual antennas selection. By simulations, linear detectors are verified to be feasible for non-coherent spatial-multiplexing MIMO, and its diversity gain is obviously increased by the proposed NVA. [ABSTRACT FROM AUTHOR]

We propose a new interference cancellation (IC) approach designed for coded uplink multiple-input multiple-output (MIMO) systems that support a massive number of Internet-of-Things (IoT) devices. The performance of MIMO spatial multiplexing depends on the symbol detection schemes, which in general offer a trade-off between the error rate performance and computational complexity. Previous studies have shown that a simple matched-filter (MF)-based detector with powerful channel coding can achieve near-optimal performance as the number of base station (BS) antennas increases even without IC. However, if the number of BS antennas is the same order as that of the IoT devices supported by the BS, its achievable rate will be significantly degraded. Furthermore, channel estimation errors due to the lack of orthogonal pilot sequences should also become a dominant factor. This paper overcomes this problem by introducing a MF detector with quadrature interference cancellation (MF-QIC), which can improve error performance and spectral efficiency. Taking polar codes as our channel coding example, we propose a code design based on mutual information tailored for MF-QIC. Our simulation results demonstrate that the proposed low-complexity polar-coded MF-QIC outperforms other conventional approaches even in the presence of imperfect channel state information. [ABSTRACT FROM AUTHOR]

Generalized quadrature spatial modulation (GQSM) seamlessly amalgamates the generalized spatial modulation (GSM), quadrature spatial modulation (QSM) and vertical Bell Laboratories layered space-time (V-BLAST) techniques. In contrast to traditional multiple-input multiple-output (MIMO) schemes transmitting information bits only through the constellation defined on the complex plane, GQSM transmits additional information bits implicitly by selecting the transmit antenna (TA) activation pattern. The philosophy of QSM is that of separating the indices of the real and the imaginary parts of the transmit symbols, which increases the attainable throughput. In this paper, we analyze the inherent benefits of GQSM in terms of throughput. More specifically, we first derive the achievable rate expression of the recent GQSM scheme and unveil the condition for GQSM to approach the maximum rate. Furthermore, we compare the rate of the GQSM scheme to that of other transmission schemes and reveal the conditions for the GQSM’s maximum throughput to exceed that of the benchmark schemes. Simulation results show that when the number of TAs is 40 and quadrature phase shift keying (QPSK) is adopted, the rate of the GQSM scheme may reach 150% of that of V-BLAST. Additionally, a pair of low-complexity detectors are conceived-one based on ordered successive interference cancellation (OSIC) and another one on the orthogonal matching pursuit (OMP) algorithm. [ABSTRACT FROM AUTHOR]

Glasses-free augmented reality is of great interest by fusing virtual 3D images naturally with physical world without the aid of any wearable equipment. Here we propose a large-scale spatial multiplexing holographic see-through combiner for full-color 3D display. The pixelated metagratings with varied orientation and spatial frequency discretely reconstruct the propagating lightfield. The irradiance pattern of each view is tailored to form super Gaussian distribution with minimized crosstalk. What’s more, spatial multiplexing holographic combiner with customized aperture size is adopted for the white balance of virtually displayed full-color 3D scene. In a 32-inch prototype, 16 views form a smooth parallax with a viewing angle of 47°. A high transmission (>75%) over the entire visible spectrum range is achieved. We demonstrated that the displayed virtual 3D scene not only preserved natural motion parallax, but also mixed well with the natural objects. The potential applications of this study include education, communication, product design, advertisement, and head-up display.

The pumping of Tm-doped crystal or fiber by a 792 nm semiconductor laser is an important way to generate a mid-infrared laser, which is widely used in various fields. In this paper, a high–power 792 nm fiber–coupled semiconductor laser module was successfully fabricated with the output power of 232 W at a 10 A continuous current and the electro-optic conversion efficiency of 48.6%. The laser module is coupled with 24 chips into a fiber by spatial multiplexing and polarization combination technology. For a single emitting laser chip, the continuous wave (CW) output power and threshold current are 10.45 W at 10 A and 1.55 A, respectively. A polarization as high as 94% can also be realized, which is more suitable for laser spatial beam combining. The laser module was aged for more than 4000 h at 12 A and 25 °C without obvious power degradation.

This paper presents a new compact four‐port component with dual‐functional dual‐mode characteristics for spatial diversity and multiplexing applications. The component can achieve the 3‐dB hybrid coupler mode at the lower frequency and the transmission line mode at the higher frequency, corresponding to spatial diversity and multiplexing applications, respectively. The proposed component is designed based on a three‐section crossover and analyzed based on the Even‐odd model analysis. It features good S‐parameter characteristics, compact size, and none of the RF switches. A component working at 4.5‐/5.4‐GHz has been fabricated and measured to prove its design theory. To practice, the proposed component is applied to a 1 × 2 wideband Yagi antenna array. The measured results show diversity beams and multiplexing beams at two operating bands respectively, suitable for complex wireless communication systems. [ABSTRACT FROM AUTHOR]

This article presents a 71–86 GHz 16-element array receiver application-specified integrated circuit (ASIC) for Massive multiple-input–multiple-output (MIMO) wireless uplink, featuring a multiple-output analog beamformer (BF) supporting up to 16 spatially multiplexed users at the same time. The ASIC includes 16 direct-conversion mixer-first RX front-ends, local oscillator (LO) generation and distribution, and a $16\times 16$ fully connected baseband analog beamformer, which derives each user stream as a linear combination of all the 16 antennas. The 16 mm2 28 nm CMOS ASIC is packaged on an organic interposer including a linear patch antenna array. Over-the-air measurements demonstrate up to 2 Gb/s single-user data-rate, and four simultaneous links at 500 Mb/s each, with number of users and data rate only limited by setup constraints. Circuits are optimized for low power consumption in order to enable scaling to massive arrays, and consumes 1.7 W total power, for a power figure of 7 mW/antenna/user. [ABSTRACT FROM AUTHOR]

The increased competition for digital systems and the development of Internet-related material in practical environments has sparked interest in high-speed technologies. Multiple transmit and receive antennas are used to create multiple input multiple output (MIMO) channels to accomplish these objectives. MIMO systems significantly increase system capacity by manipulating the spatial dimension via spatial multiplexing, reliability through diversity and reduction of interference in multiple users. Massive MIMO becomes a main fifth generation (5G) mobile communications technology that will enhance huge performance like higher data rates, higher reliability, lower latency and more channel capabilities MIMO diversity systems are mixed with orthogonal frequency division multiplexing (OFDM) to create MIMO-OFDM systems for further increase the device efficiency and protect toward fading. Diversity is a multipath-fading strategy used to counter it. Space Diversity method employing various number of antenna are used at transmitter and receiver side. [ABSTRACT FROM AUTHOR]

Thanks to its low transceiver complexity, mode division multiplexing (MDM) using orbital angular momentum (OAM) has been recently investigated as a new physical layer wireless transmission technique. This is due to the fact that different OAM modes are spatially orthogonal to each other, thus, perfectly suitable for spatial multiplexing and/or diversity. The orthogonality amongst different OAM modes is achieved in the absence of any mode offset between the transceivers’ phase-shifting feeding networks. However, the inherent presence of OAM mode offset destroys the orthogonality between different modes, which gives rise to inter-mode interference (IMI). Therefore, in this paper, in order to theoretically analyze the negative impact of OAM mode offset of MDM-OAM systems on the spectral efficiency of line-of-sight free-space wireless communication systems, an explicit signal-to-interference-plus-noise ratio expression for each OAM mode is derived, through which the spectral efficiency degradation is evaluated. Furthermore, in order to support our theoretical analysis, a $9\times 9$ MDM-OAM practical communication experiment for different OAM mode settings is carried out at a microwave frequency of 5.8 GHz. In addition, by leveraging the fact that all circulant matrices always have the same eigendirections, regardless of the entries of the matrix, a low-complexity optimal OAM mode selection scheme is proposed which significantly improves the spectral efficiency of the system. Finally, in order to completely null-out the impact of OAM mode offset, a baseband zero-forcing stage is embedded at the receiver. [ABSTRACT FROM AUTHOR]

We propose a deep learning-based channel estimation, quantization, feedback, and precoding method for downlink multiuser multiple-input and multiple-output systems. In the proposed system, channel estimation and quantization for limited feedback are handled by a receiver deep neural network (DNN). Precoder selection is handled by a transmitter DNN. To emulate the traditional channel quantization, a binarization layer is adopted at each receiver DNN, and the binarization layer is also used to enable end-to-end learning. However, this can lead to inaccurate gradients, which can trap the receiver DNNs at a poor local minimum during training. To address this, we consider knowledge distillation, in which the existing DNNs are jointly trained with an auxiliary transmitter DNN. The use of an auxiliary DNN as a teacher network allows the receiver DNNs to additionally exploit lossless gradients, which is useful in avoiding a poor local minimum. For the same number of feedback bits, our DNN-based precoding scheme can achieve a higher downlink rate compared to conventional linear precoding with codebook-based limited feedback. [ABSTRACT FROM AUTHOR]

In this work, we focus on the implementation of reconfigurable distributed multiple-input multiple-output (RD-MIMO) schemes in practical communication environment. Considering RD-MIMO schemes, we raise an original research question, i.e., under what scenario whether we should recruit-then-transmit to multiple mobile receivers simultaneously or just recruit-then-transmit to one best selected mobile receiver. To address this question, we design a spatial multiplexing RD-MIMO (SM-RD-MIMO) scheme and an opportunistic RD-MIMO (O-RD-MIMO) scheme for the scenario of multiuser communication and that of one single best user communication, respectively. Aiming at the practical implementation of the RD-MIMO schemes, we choose zero-forcing (ZF) and maximal ratio transmitting (MRT) as beamforming strategies for the SM-RD-MIMO scheme and the O-RD-MIMO scheme, respectively. To evaluate the performance of the proposed schemes, we formulate sum rate maximization problems and solve the formulated problems analytically under some specific network settings. Through analytical and numerical examples, we show that the SM-RD-MIMO scheme outperforms the O-RD-MIMO scheme at moderate and high transmit signal-to-noise ratio (SNR) regimes, and vice versa at low transmit SNR regime. In addition, via numerical results, we provide a new finding that the ZF precoding under dynamic transmit power threshold-based per transmitter power constraint (PTPC) gives the same performance as that of the ZF precoding under sum power constraint (SPC).

Meta‐holograms have been investigated widely because of their capabilities that are not feasible in conventional holograms, such as full‐color, broadband, and bifacial image generation performance. In particular, a high degree of freedom in designing meta‐atoms provides opportunities to multiplex holograms by using light properties, such as angle, polarization, and wavelength. However, further development of techniques for meta‐holograms recording multiple wavefronts is still necessary to expand the amount of optical information and improve the security level of holographic encryption. In this study, detour phase holograms are combined with the spatial multiplexing method to record four phase profiles in a single metasurface device. Independent wavefront modulation, which can process four channels as a bijective function, is realized by engineering the scattering properties of the meta‐atom. Based on the concept, meta‐holograms are demonstrated experimentally, independently generating four different images depending on the incident angles with high fidelity. Because of the expanded capacity and independence, the proposed device provides a potential platform for optical devices in various fields, such as holography, multifunctional flat optics, and off‐axis optical devices. [ABSTRACT FROM AUTHOR]

To get channel state information (CSI) at a base station (BS), most of researches on massive multiple-input multiple-output (MIMO) systems consider time division duplexing (TDD) to get benefit from the uplink and downlink channel reciprocity. Even in TDD, however, the BS still needs to transmit downlink training signals, which are referred to as channel state information reference signals (CSI-RSs) in the 3GPP standard, to support spatial multiplexing in practice. This is because there are many cases that the number of transmit antennas is less than the number of receive antennas at a user equipment (UE) due to power consumption and circuit complexity issues. Because of this mismatch, uplink sounding reference signals (SRSs) from the UE are not enough for the BS to obtain full downlink MIMO CSI. Therefore, after receiving the downlink CSI-RSs, the UE needs to feedback quantized CSI to the BS using a pre-defined codebook to support spatial multiplexing. In this paper, possible approaches to reconstruct full downlink MIMO CSI at the BS are proposed by exploiting both the SRS and quantized downlink CSI considering practical antenna structures with reduced downlink CSI-RS overhead. Numerical results show that the spectral efficiencies by spatial multiplexing based on the proposed downlink MIMO CSI reconstruction techniques outperform the conventional methods solely based on the quantized CSI. [ABSTRACT FROM AUTHOR]

In this paper, we address a multiple-input multiple-output (MIMO) communication system based on generalized spatial modulation (GSM) and dual-polarized (DP) antennas in sub-TeraHertz (sub-THz) bands to improve the Spectral Efficiency (SE) while reducing the spatial correlation effect and the space occupancy. Moreover, the joint Maximum Likelihood (ML) detector and a Modified Ordered Block Minimum Mean-Squared Error (MOB-MMSE) detector algorithm are proposed to detect jointly the complex symbols and the virtual bits conveyed by the activated polarization and the transmit antenna combination index. MOB-MMSE can achieve near-ML performance with low complexity. The performance of DP-GSM system over correlated Rayleigh/Rician fading and indoor sub-THz channels is studied. Provided results show that DP-GSM system is robust to the spatial correlation effects under Rician fading channel. In addition, an average bit-error probability (ABEP) upper bounding is derived and analyzed for the DP-GSM system over correlated Rayleigh/Rician fading channels. Since the sub-THz band suffers from many technological limitations and severe RF-impairments such as low transmit output power and important phase noise (PN), the DP-GSM system is studied by considering the sub-THz impairments. Moreover, the Dual Polarized Spatial Multiplexing (DP-SMX) and the Uni-Polarized GSM (UP-GSM) are compared with the DP-GSM system. The results reveal that DP-GSM with low order modulation schemes as QPSK outperforms the UP-GSM. Besides, DP-GSM and DP-SMX provide a good performance up to medium PN, which is interesting for sub-THz bands, but DP-GSM can reduce the transceiver cost by using a lower number of RF chains when the used transmitter architecture is based on RF-switching. [ABSTRACT FROM AUTHOR]

To overcome the inherent bandwidth limitation of non-line-of-sight (NLOS) ultraviolet communication (UVC) channel, a novel space division multiplexing (SDM) framework based on non-coplanar rotation angle is established. The influence of SDM structure parameters on inter-channel interference (ICI) is studied to obtain the optimal antenna configuration. And a Gaussian approximate distribution (GAD) signal detection model is presented, which simplifies the calculation of the Bit error rate (BER) of the Photon multiplier tube (PMT) reception and is suitable for complex ICI analysis. Furthermore, a low ICI scheme based on the relationship between BER and the interference factor is proposed to increase the data transmission rate or reduce the alignment configuration. As a result, the maximum data rate increases to 34 Mbps, or the alignment demand decreases by 33.33% at most. Moreover, these analysis results and techniques can be applied to other NLOS scattering optical wireless communication systems. [ABSTRACT FROM AUTHOR]