Multiple-input Multiple-output Architecture Research Articles

A Novel Monopole Ultra-Wide-Band Multiple-Input Multiple-Output Antenna with Triple-Notched Characteristics for Enhanced Wireless Communication and Portable Systems.

This study introduces a monopole 4 × 4 Ultra-Wide-Band (UWB) Multiple-Input Multiple-Output (MIMO) antenna system with a novel structure and outstanding performance. The proposed design has triple-notched characteristics due to CSRR etching and a C-shaped curve. The notching occurs in 4.5 GHz, 5.5 GHz, and 8.8 GHz frequencies in the C-band, WLAN band, and satellite network, respectively. Complementary Split-Ring Resonators (CSRR) are etched at the feed line and ground plane, and a C-shaped curve is used to reduce interference between the ultra-wide band and narrowband. The mutual coupling of CSRR enables the MIMO architecture to achieve high isolation and polarisation diversity. With prototype dimensions of (60.4 × 60.4) mm2, the proposed antenna design is small. The simulated and measured results show good agreement, indicating the effectiveness of the UWB-MIMO antenna for wireless communication and portable systems.

Iterative Variational Bayesian Inference Based Channel Estimation for TWR mmWave Systems

Millimeter wave (mmWave) with two way relaying (TWR) is an emerging paradigm towards cellular fifth/sixth generation (5G/6G) technology that can support various data hungry applications with improve coverage, network throughput, and reliability. The foreseen potential of mmWave TWR system can be further alleviated by using large-scale multiple input multiple output (MIMO) architecture. However, high power consumption, hardware complexity and cost of fully digital large-scale MIMO architecture restricts its application. As a countermeasure, hybrid precoding structures with reduced RF chains are utilized. Therefore, the amalgamation of TWR MIMO mmWave system with hybrid precoding structure demands an accurate channel state information (CSI) to avail the maximum gain of two way MIMO communications. Furthermore, the inherent self-interference in TWR systems in collusion with sparse mmWave channels imposes severe challenges in acquiring accurate CSI. The challenge is further aggravated in case of frequency-selective mmWave channel. To solve this problem, we propose a novel iterative variational Bayesian inference (IVBI)-based channel estimation (CE) scheme in the time domain for TWR system with reduced RF chain. In the proposed method, we follow the alternative minimization method involving variational Bayesian inference (VBI) to estimate all the channels of mmWave TWR system. The performance of the proposed algorithm are evaluated over both flat fading and frequency-selective channel. Extensive simulation results for symmetric and non-symmetric MIMO structure validate the proposed algorithm. The Bayesian Cramer-Rao bound (BCRB) of the proposed estimator is also derived. This novel scheme converges fast and achieves significant improvement in terms of normalized mean square error (NMSE) and bit error rate (BER) as compared to state-of-art.

Cooperative Power-Domain NOMA Systems: An Overview.

Interference has been a key roadblock against the effectively deployment of applications for end-users in wireless networks including fifth-generation (5G) and beyond fifth-generation (B5G) networks. Protocols and standards for various communication types have been established and utilised by the community in the last few years. However, interference remains a key challenge, preventing end-users from receiving the quality of service (QoS) expected for many 5G applications. The increased need for better data rates and more exposure to multimedia information lead to a non-orthogonal multiple access (NOMA) scheme that aims to enhance spectral efficiency and link additional applications employing successive interference cancellation and superposition coding mechanisms. Recent work suggests that the NOMA scheme performs better when combined with suitable wireless technologies specifically by incorporating antenna diversity including massive multiple-input multiple-output architecture, data rate fairness, energy efficiency, cooperative relaying, beamforming and equalization, network coding, and space-time coding. In this paper, we discuss several cooperative NOMA systems operating under the decode-and-forward and amplify-and-forward protocols. The paper provides an overview of power-domain NOMA-based cooperative communication, and also provides an outlook of future research directions of this area.

Full-Duplex Massive Multiple-Input, Multiple-Output Architectures: Recent Advances, Applications, and Future Directions

The increasingly demanding objectives for next-generation wireless communications have spurred recent research activities on multiantenna transceiver hardware architectures and relevant intelligent communication schemes. Among them belong full-duplex (FD) multiple-input, multiple-output (MIMO) architectures, which offer the potential for simultaneous uplink (UL) and downlink (DL) operations in the entire frequency band. However, as the number of antenna elements increases, the interference signal leaking from the transmitter (Tx) of the FD radio to its receiver (Rx) becomes more severe. In this article, we present a unified FD massive MIMO architecture comprising analog and digital transmit and receive beamforming (BF) as well as analog and digital self-interference (SI) cancellation, which can be jointly optimized for various performance objectives and complexity requirements. The performance evaluation results for applications of the proposed architecture to fully digital and hybrid analog and digital-BF operations using recent algorithmic designs as well as simultaneous communication of data and control (SCDC) signals are presented. It is shown that the proposed architecture, for both small and large numbers of antennas, enables improved spectral-efficient FD communications with fewer analog-cancellation elements compared to various benchmark schemes. The article concludes with a list of open challenges and research directions for future FD massive MIMO communication systems and their promising applications.

On the Convenience of Perfect Channel Knowledge for Spatial Equalization of Correlated MIMO-VLC Links: Is it Really Worth it?

Visible Light Communication has risen as a promising technology to support indoor wireless connectivity and provide illumination services as well. Moreover, high communication performance can be achieved by implementing Multiple-Input Multiple-Output architectures. However, the crosstalk characterizing spatially correlated channels scenarios may significantly impact on reliability. Such issue is commonly addressed by means of spatial equalization, with zero forcing and minimum mean square error representing the most widespread approaches. These methods allow the mitigation of crosstalk, even though they may cause undesired noise amplification as side effect, inducing errors during signal detection and decoding. In this regard, by focusing on a more realistic approach with respect to the literature, this work overturns the belief that having perfect channel knowledge is always better. Hence, we show that, when applying spatial equalization, imperfect channel knowledge is sometimes preferable to ideal channel state information. As a result, noise amplification is reduced at the expense of a reasonably less accurate crosstalk mitigation, with better communication performance being achieved.

Polarization Converter Surface Integrated MIMO Antenna for Simultaneous Reduction of RCS and Mutual Coupling

This letter provides an effective solution to deal with the radar cross section (RCS) and mutual coupling of the multiple-input–multiple-output (MIMO) antenna simultaneously. A wideband reflective type polarization converter (PC) is designed and its strategic placement in between the antenna elements of the two-element MIMO antenna has been conceived to mitigate these two issues concurrently. The antenna elements of the proposed MIMO antenna are placed at a proximal distance of 0.088 λ from each other. The proposed MIMO architecture ensures a maximum coupling reduction of 46 dB compared to the conventional MIMO across 4.4–8 GHz. The PC surface reduces the reflection of copolarized waves and enhances cross-polarized reflection. A supercell formed by a unit cell of PC and its 90° rotated counterpart causes phase cancellation of cross- polarized reflected waves, which contributes to wideband RCS reduction by a maximum value of 7 dB. The alignment of PC cells in a column between the adjacent antenna elements causes a reduction in the mutual coupling due to introduction of an extra coupling path and cross-polarization conversion. A good agreement between the simulated and measured results validates this concept of the proposed MIMO design with low RCS and mutual coupling.

A Tb/s Indoor MIMO Optical Wireless Backhaul System Using VCSEL Arrays

In this paper, the design of a multiple-input multiple-output (MIMO) optical wireless communication (OWC) link based on vertical cavity surface emitting laser (VCSEL) arrays is systematically carried out with the aim to support data rates in excess of 1 Tb/s for the backhaul of sixth generation (6G) indoor wireless networks. The proposed design combines direct current optical orthogonal frequency division multiplexing (DCO-OFDM) and a spatial multiplexing MIMO architecture. For such an ultra-high-speed line-of-sight (LOS) OWC link with low divergence laser beams, maintaining alignment is of high importance. In this paper, two types of misalignment error between the transmitter and receiver are distinguished, namely, radial displacement error and orientation angle error, and they are thoroughly modeled in a unified analytical framework assuming Gaussian laser beams, resulting in a generalized misalignment model (GMM). The derived GMM is then extended to MIMO arrays and the performance of the MIMO-OFDM OWC system is analyzed in terms of the aggregate data rate. Novel insights are provided into the system performance based on computer simulations by studying various influential factors such as beam waist, array configuration and different misalignment errors, which can be used as guidelines for designing short range Tb/s MIMO OWC systems.

MIMO architecture for fast convergence of distributed online optimization in smart grids

This paper proposes an architectural solution to enhance the resilience of distributed decision-making and control in microgrids and smart grids with respect to communication delays. The paper develops a resource allocation framework to optimally place the multiple-input multiple-output (MIMO) communication technology on critical channels to reduce the communication bottle neck and expedite the convergence speed of distributed online optimization algorithms, which require convergence in the cyber network before the solution can be implemented on the physical grid. The paper shows that optimal placement of the MIMO technology can decrease communication transmission delays, which in turn improves the reliability of power system operational protocols, such as distributed frequency regulation. Illustrative case studies using data from real-world power systems show that only upgrading the critical communication channels is sufficient to achieve system-wide performance improvement.

Received power modelling in ultra‐dense networks

In received power modelling for system level simulations, the relative orientation of the transmitter and receiver antennas are not generally considered. By doing that, a constant gain of the receiver antenna, independently of the angle of arrival, is implicitly assumed. This is usually of minor relevance for small or low-dense scenarios. However, in ultra-dense deployments, especially those based on cell-free massive multiple-input–multiple-output architectures where users interact with all access points, the consideration of the antennas relative orientation comes to the forefront. In this paper, the impact of whether to take the receiver antenna gain into account or not for ultra-dense indoor environments with ceiling-mounted antennas are studied. A simple approach to take this gain into account using the antenna effective area is provided. Finally, this approach is compared with that based on antenna radiation patterns.

The impact of intentional interference on the performances of ML detector in MIMO systems

Here we address the problem of performing the resilience of Multiple Input Multiple Output (MIMO) architecture against intentional and unintentional interferences. We investigate the performance of a non-linear receiver based on the Maximum Likelihood (ML) detector in MIMO systems over Gaussian fading channels in the presence of interfering signals. Using the properties of the Gaussian matrix, a finite expression of the Probability Density Function (PDF) for the Signal to Interference plus Noise Ratio (SINR) is obtained as a function of the brute jammer power budget and the number of affected antennas. By considering a particular closed-form of intelligent jamming strategies against MIMO architecture presented in the literature, approximated upper limits of the Bit Error Rate (BER) are performed under different jamming scenarios depending on the Channel State Information (CSI) availability. These results enable us to characterize the consequences of such conflicting attacks on the quality of the legitimate link. Furthermore, extensive simulations are carried out to justify the performance of the ML detector and validate the obtained results.

Sparse Bayesian Learning Kalman Filter-based Channel Estimation for Hybrid Millimeter Wave MIMO Systems: A Frequency Domain Approach

In this paper, channel estimation for Millimeter Wave (mmWave) Multiple Input Multiple Output (MIMO) communication system is presented. In the frequency domain, the Sparse Bayesian Learning-based Kalman Filter (SBL-KF) scheme is developed to estimate the sparse channel vector of hybrid mmWave MIMO architecture. The mmWave MIMO hybrid architecture provides reduced power loss and achievable data rates at mmWave frequencies. Frequency-selective channels are sparse at mmWave frequencies. The recovery of the sparse channel is estimated using different channel estimation methods. The hybrid precoding technique achieves both multiplexing gain and array gain in mmWave channels. Simulation results of the proposed SBL-KF scheme show low estimation error over the existing Orthogonal Matching Pursuit, Simultaneous Orthogonal Matching Pursuit, Simultaneous Weighted Orthogonal Matching Pursuit algorithm and SBL scheme. The complexity of the online estimation SBL-KF algorithm is low and it is ideally suited for implementation in an efficient mmWave MIMO system.

Energy Efficient Hybrid Beamforming for Multi-User Millimeter Wave Communication With Low-Resolution A/D at Transceivers

Millimeter wave (mmWave) multiple-input multiple-output (MIMO) communication systems with a large number of antennas are power hungry when using conventional high-resolution analog-to-digital/digital-to-analog converters (A/Ds). To reduce the power consumption of mmWave MIMO systems, existing studies have considered hybrid structures with a reduced number of high-resolution or low-resolution A/Ds at either the transmitter or the receiver side. In this paper, we propose and investigate a multi-user hybrid architecture with low-resolution A/Ds equipped at both the transmitter and the receivers. To mitigate the impact of utilizing low-resolution A/Ds at the transceivers, we propose a novel data transmission scheme, which exploits a weighted phased-array to synthesize the beamforming matrix in the analog domain so as to mitigate inter-user interference. Under the scheme proposed, we derive the achievable rate and the energy efficiency to establish guidelines on the optimal resolution choice of A/Ds for hybrid mmWave systems. For a typical total transmit power at the BS, e.g., 30 dBm, the proposed scheme with 5~6-bit A/Ds can significantly improve the energy efficiency by as much as 100% over that of the conventional hybrid MIMO architecture with high-resolution A/Ds (10-bit A/Ds), without significant degradation in data rate performance.

Hybrid Transceiver Design for Beamspace MIMO-NOMA in Code-Domain for MmWave Communication Using Lens Antenna Array

As a hybrid MIMO architecture, beamspace multiple input multiple output (MIMO) can significantly reduce the number of required radio frequency (RF) chains in millimeter wave (mmWave) massive MIMO systems without obvious performance loss, in which, however, the number of users supported cannot be larger than that of RF chains. To break this fundamental limit, we introduce the concept of code-domain non-orthogonal multiple access (NOMA) into beamspace MIMO. A beam selection scheme is proposed first to maximize the sum-rate by utilizing the quasi-orthogonality of the beamspace channel. Furthermore, a low-complexity detection algorithm is developed to realize the transceiver design in mmWave communications using lens antennas. Finally, numerical results of the decoding complexity at receiver side are analyzed. Simulation results show that the proposed beamspace MIMO-NOMA in code domain can achieve higher spectrum and energy efficiency compared with the existing beamspace MIMO.

Lens Antenna Subarrays in mmWave Hybrid MIMO Systems

Data rate and energy efficiency performance of mmWave wireless communication systems consisting of a new lens antenna subarray (LAS) based hybrid multiple input multiple output (MIMO) architecture is investigated. The LAS architecture degenerates into traditional hybrid MIMO architecture (TA) or single lens antenna array architecture (SLA) when the lens diameter of the subarray is reduced or enlarged, respectively. It is shown that under mmWave channel scattering conditions, LAS architecture operates with a data rate approaching that of TA while significantly exceeding the data rate of the SLA. Due to the reduced hardware complexity and power consumption, LAS architectures operate with a significantly improved energy efficiency. In an example system consisting of 64 and 16 transmitter and receiver antennas with 2 RF chains, it is shown that the LAS architecture operates with $2\times $ energy efficiency and 98% spectrum efficiency. In a larger hybrid MIMO antenna systems, LAS architecture is expected to benefit even more.

Multi-Gigabit/s OFDM real-time based transceiver engine for emerging 5G MIMO systems

This paper presents a highly scalable multi-Gigabit/s Real-Time (RT) Wideband (WB) Orthogonal Frequency Division Multiplexing (OFDM) processing chain for 5G applications. It is aimed to significantly reduce the Hardware (HW) footprint of Multiple Input Multiple Output (MIMO) systems. In this context, implementation complexity results for MIMO configurations of 2 × 2, 4 × 4 and 8 × 8, enabled at sampling rates of 245.76, 122.88, 61.44 and 30.72 MHz, respectively, are compared with the ones from conventional MIMO architectures. For example, for a 8 × 8 MIMO–OFDM configuration implementation, savings of up to 87% are achieved when compared with conventional design methods, in terms of DSP48E1 slices. Moreover, in a Xilinx Virtex 7 XC7VX485T, it is shown that a second processing branch might be implemented, leading to a 5 Gbps (using 1024 Quadrature Amplitude Modulation (QAM)) Single-Input Single-Output (SISO) link or a 16 × 16 MIMO configuration. Finally, in order to validate the proposed architecture, the impact of its inclusion on a complete Field Programmable Gate Array (FPGA) 8 × 8 OFDM system, at LTE’s highest sampling rate of 30.72 MHz, is evaluated. Subsequently, it is demonstrated that this does not affect the performance of OFDM, even in the presence of Carrier Frequency Offset (CFO).

A Space-Time Coded Mills Cross MIMO Architecture to Improve DOA Estimation and Its Performance Evaluation by Field Experiments

Conventional Mills Cross architecture suffers from poor direction-of-arrival angle estimation accuracy in the dimension that the transmitter is aligned. To improve the estimation accuracy, a space-time coded, multiple-input multiple-output (MIMO) direction finding method, with complementary codes is presented. The performance of the suggested MIMO Mills Cross architecture has been evaluated by underwater field experiments. Field experiments confirm the feasibility of the proposed approach and illustrate the performance gains. Since the proposed approach does not require any changes in the conventional Mills Cross hardware, the performance improvement can also be realized by a software reconfiguration for existing legacy systems.

Photonics-Assisted Technologies for Extreme Broadband 5G Wireless Communications

We summarize the enabling technologies for photonics-assisted broadband millimeter-wave (mm-wave) communication, which is a promising candidate for the enhanced mobile broadband (eMBB) communications, one of the three main typical application scenarios of 5G wireless networks. These enabling technologies, mainly focusing on the improvement of the system structure, include broadband mm-wave signal generation with simple and cost-effective schemes, multiple-input multiple-output architecture with polarization-multiplexing optical mm-wave signal, advanced multilevel modulation, optical or electrical multicarrier modulation, antenna polarization multiplexing and the employment of the high-gain mm-wave antenna, multi-band multiplexing, and broadband mm-wave signal detection. We also review the advanced digital signal processing (DSP) for heterodyne coherent detection, which can be applied into the photonics-assisted mm-wave communication systems, to further enhance the system performance for a given system structure and certain available devices. Based on these enabling technologies and advanced DSP, we have realized over 1 Tb/s wireless signal transmission at D-band and over 2.5 km wireless transmission with a bit rate up to 54 Gb/s at W-band. Our work verifies the photonics-assisted broadband mm-wave communication can meet the high-data-rate demand of eMBB.

Elevation Beamforming With Full Dimension MIMO Architectures in 5G Systems: A Tutorial

Full dimension (FD) multiple-input multiple-output (MIMO) technology has attracted substantial research attention from both wireless industry and academia in the last few years as a promising technique for next-generation wireless communication networks. FD-MIMO scenarios utilize a planar 2-D active antenna system (AAS) that not only allows a large number of antenna elements to be placed within feasible base station (BS) form factors, but also provides the ability of adaptive electronic beam control over both the elevation and the traditional azimuth dimensions. This paper presents a tutorial on elevation beamforming analysis for cellular networks utilizing FD massive MIMO antenna arrays. In contrast to existing works that focus on the standardization of FD-MIMO in the 3rd generation partnership project (3GPP), this tutorial is distinguished by its depth with respect to the theoretical aspects of antenna array and 3-D channel modeling. In an attempt to bridge the gap between industry and academia, this preliminary tutorial introduces the relevant array and transceiver architecture designs proposed in the 3GPP Release 13 that enable elevation beamforming. Then it presents and compares two different 3-D channel modeling approaches that can be utilized for the performance analysis of elevation beamforming techniques. The spatial correlation in FD-MIMO arrays is characterized and compared based on both channel modeling approaches and some insights into the impact of different channel and array parameters on the correlation are drawn. All these aspects are put together to provide a mathematical framework for the design of elevation beamforming schemes in single-cell and multi-cell scenarios. Simulation examples associated with comparisons and discussions are also presented. To this end, this paper highlights the state-of-the-art research and points out future research directions.

4 × 4 MIMO architecture supporting IFoF-based analog indoor distributed antenna system for 5G mobile communications.

We demonstrate the analog indoor distributed antenna system (DAS) for 5G mobile communications that supports 4 × 4 multiple-input multiple-output (MIMO) configuration. For this, we exploit a pair of intermediate frequency over fiber (IFoF)-based analog optical links, transporting 32 frequency allocation (FA) 5G mobile signals (effectively ~4 GHz bandwidth). The analog optical link manifests its high fidelity in the measured characteristics: small gain variation (< ± 1 dB for the entire transmission bandwidth), low noise (<-136 dBm/Hz), and large dynamic range (spurious free dynamic range of >106 dB∙Hz2/3), subsequently providing superior error vector magnitude (EVM) performance (~2%) for a wide range of ambient temperatures (-20 ~60°C). Consequently, the IFoF-based 4 × 4 MIMO supporting analog indoor DAS is capable of providing record high peak throughput of 5.3 Gb/s for millimeter wave based 5G mobile communication system.

Wideband millimeter wave communication: Single carrier based hybrid precoding with sparse optimization

In millimeter wave (mmWave) communication systems, hybrid analog/digital precoding with single-carrier transmission and time delay compensation is a promising technique for realizing the cost-effective large-scale multiple-input multiple-output (MIMO) communications with limited radio frequency (RF) chains. This paper studies the mmWave downlink with fully connected MIMO architecture system for the general frequency-selective channels. By leveraging the fact that the mmWave channels are likely to be sparse in the directions and the signal propagation is usually dominated by the line of sight path, we investigate the low-complexity single-carrier transmission scheme with time delay precompensation applied at the base station (BS). The resulting signal-to-interference-plus-noise ratio (SINR) is derived in terms of the precoding vectors by taking into account both the residual intersymbol interference and interuser interference. Based on the derived SINR expression, we first propose a codebook based precoding scheme, then develop an effective joint codeword selection and precoding algorithm by utilizing the group sparse approaches to accommodate a limited number of RF chains at the BS. Numerical results are provided to validate the effectiveness of the proposed schemes, showing that the proposed scheme achieves a performance close to the fully digital system.