Line-of-sight Multiple-input Multiple-output Research Articles

Intelligent Reflecting Surface Aided MIMO With Cascaded LoS Links: Channel Modeling and Full Multiplexing Region

In this paper, we build up a new intelligent reflecting surface (IRS) aided multiple-input multiple-output (MIMO) channel model, named the cascaded LoS MIMO channel, that is applicable to both near-field and far-field scenarios. The proposed channel model consists of a transmitter (Tx) and a receiver (Rx) both equipped with uniform linear arrays (ULAs), and an IRS is used to enable communications between the transmitter and the receiver through the line-of-sight (LoS) links seen by the IRS. When modeling the reflection of electromagnetic waves at the IRS, we take into account the curvature of the wavefront on different reflecting elements. Based on the established channel model, we show that an IRS-assisted MIMO channel is able to support spatial multiplexing solely by the cascaded LoS links. We generalize the notion of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Rayleigh distance</i> originally coined for the single-hop MIMO channel to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">full multiplexing region (FMR)</i> for the cascaded LoS MIMO channel, where the FMR is the union of all the Tx-IRS and IRS-Rx distance pairs that enable full multiplexing communication. We derive an inner bound of the FMR under a special passive beamforming (PB) strategy named reflective focusing, and provide the corresponding orientation settings of the antenna arrays that achieve full multiplexing.

Achievable Spectral Efficiency of Line-of-Sight MIMO Fixed Links with Compact Planar Antenna Arrays and Power Amplifier Limitations

Fixed Point-to-Point microwave wireless systems with high spectral efficiency are needed to meet the pervasive and increasing demand for capacity in back-haul networks of mobile radio systems. In this context, spatially multiplexed LoS-MIMO (Line-of-Sight Multiple Input Multiple Output) systems have been studied for about twenty years, particularly in the millimeter wave frequency bands (above 15 GHz). However, their deployment in real networks has been really limited, to the authors’ knowledge. This has been due to several factors, i.e. the practical possibility of using extremely high-level modulation formats (nowadays up to 8192-QAM), the joint use of co-channel dual polarization, and the availability of wider channel bands in the new high frequency ranges (e.g. E-Band). In addition, a crucial reason has been the difficulty of installing multiple antennas spaced apart in order to maximize the MIMO spatial multiplexing so providing the maximum capacity gain. This optimal antenna separation, even for the classical MIMO M×N\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M \ imes N$$\\end{document} with M = 2 antennas at the receiver and N = 2 antennas at the transmitter, can be several meters, e.g. 5.71 m at 23 GHz on a 5 km link. In this article, we analyze the performance of LoS-MIMO systems where antenna separation is highly sub-optimal, for limiting the array size, and a satisfactory performance is made possible by the exploitation of specific bit loading and power allocation strategies and the setting of the working region of the RF transmitter power amplifiers to operate at a given Signal-to-Inter Modulation Distortion Ratio (SIMDR). The result is an overview of the advantages and drawbacks of compact LoS-MIMO from a wider perspective than in the existing literature, including fundamental aspects for the practical implementation of these systems. Performance is discussed in many cases of interest and compared with the state of the art SISO (Single Input Single Output) system.

LoS MIMO transmission for LEO satellite communication systems

To provide global service with low latency, the broadband low earth orbits (LEO) satellite constellation based communication systems have become one of the focuses in academic and industry. To allow for wideband access for user links, the feeder link of LEO satellite is correspondingly required to support high throughput data communications. To this end, we propose to apply line-of-sight (LoS) multiple-input multiple-output (MIMO) transmission for the feeder link to achieve spatial multiplexing by optimizing the antenna arrangement. Unlike the LoS MIMO applications for static scenarios, the movement of LEO satellites make it impractical to adjust the optimal antenna separation for all possible satellite positions. To address this issue, we propose to design the antenna placement to maximize the ergodic channel capacity during the visible region of the ground station. We first derive the closed-form probability distribution of the satellite trajectory in visible region. Based on which the ergodic channel capacity can be then calculated numerically. The antenna placement can be further optimized to maximize the ergodic channel capacity. Numerical results verify the derived probability distribution of the satellite trajectory, and show that the proposed LoS MIMO scheme can significantly increase the ergodic channel capacity compared with the existing SISO one.

Wireless Fronthaul for 5G and Future Radio Access Networks: Challenges and Enabling Technologies

Wireless networks have been evolving at a frenetic pace to meet the ever increasing demands for higher capacity, lower latency, and massive number of connections. In the fifth generation (5G) radio access networks (RANS), millimeter-wave and higher band radio are instrumental in achieving such stringent requirements. The network deployment must go through significant densification to meet the extremely high demand of data traffic. This imposes a big challenge to the network architecture as well as to the design of fronthaul transport links in the network. Fronthaul is generally referred to as the connection link between baseband units and remote radio units. Fiber-based connections have been conventionally the mainstream technology for fronthaul. However, fiber-based fronthaul may not scale in a cost-effective way to meet dense and flexible deployment requirements. Wireless fronthaul could be a viable solution and an attractive alternative to fiber for such deployments. In this article, we first provide an overview of emerging fronthaul interfaces and requirements. We then highlight the enabling technologies and solutions that can be applied in wireless fronthaul to meet future fronthaul capacity and latency requirements in a robust manner. The discussed technologies include performing fronthaul compression, leveraging new higher band spectrum, optimizing high rank line-of-sight multiple-input multiple-output (LoS-MIMO) and applying forward-looking technologies like reconfigurable intelligent surfaces. Wireless fronthaul has the advantages of flexibility, scalability, and low cost compared to fiber-based fronthaul. However, it also raises challenges in reliably delivering the required key performance indicators. Simulation results of wireless fronthaul for a wide range of distances and point-to-multipoint deployment scenarios show that robust and reliable performance can be achieved using high rank LoS-MIMO at high millimeter-wave bands. The analysis of this article indicates that wireless fronthaul can be designed to meet the requirements and be an attractive option for future disaggregated RAN deployments.

How Friendly Are Building Materials as Reflectors to Indoor LOS MIMO Communications?

The tremendous popularity of Internet-of-Things (IoT) applications and wireless devices has prompted a massive increase of indoor wireless traffic. To further explore the potential of indoor IoT wireless networks, creating constructive interactions between indoor wireless transmissions and the built environments becomes necessary. The electromagnetic (EM) wave propagation indoors would be affected by the EM and physical properties of the building material, e.g., its relative permittivity and thickness. In this article, we construct a new multipath channel model by characterizing wall reflection (WR) for an indoor Line-of-Sight (LOS) single-user multiple-input–multiple-output (MIMO) system and derive its ergodic capacity in the closed-form. Based on the analytical results, we define the wireless friendliness of building material based on the spatially averaged indoor capacity and propose a scheme for evaluating the wireless friendliness of building materials. Monte Carlo simulations validate our analytical results and manifest the significant impact of the relative permittivity and thickness of building material on indoor capacity, indicating that the wireless friendliness of building materials should be considered in the planning and optimization of indoor wireless networks. The outcomes of this article would enable appropriate selection of wall materials during building design, thus enhancing the capacity of indoor LOS MIMO communications.

Line-of-Sight MIMO for High Capacity Millimeter Wave Backhaul in FDD Systems

Wireless backhaul is considered to be the key part of the future wireless network with dense small cell traffic and high capacity demand. In this paper, we focus on the design of a high spectral efficiency line-of-sight (LoS) multiple-input multiple-output (MIMO) system for millimeter wave backhaul using dual-polarized frequency division duplex (FDD). High spectral efficiency is very challenging to achieve for the system due to various physical impairments such as phase noise (PHN), timing offset (TO) as well as the poor condition number of the LoS MIMO. In this paper, we propose a holistic solution containing TO compensation, PHN estimation, precoder/decorrelator optimization of the LoS MIMO for wireless backhaul, and the interleaving of each part. We show that the proposed solution has a robust performance with end-to-end spectral efficiency of 60 bit/s/Hz for 8 &#x00D7; 8 MIMO.

Analog 28 GHz LoS MIMO Relay System Using a 90° Hybrid Coupler

To enlarge the range of wireless fronthaul and backhaul communication links operating close to the millimeter-wave region, this letter presents a novel line-of-sight (LoS) multiple-input–multiple-output (MIMO) relay system. The analog relay system consists of a simple 90° hybrid coupler to separate two uncorrelated transmit data streams realizing a low complex and cheap system. The functional capability is proven within measurements of two independent 2 × 2 LoS MIMO links at 28 GHz connected by the relay system. A high decoupling of the two independent transmitted data streams of more than 27 dB after the 90° hybrid coupler and a high modulation error rate at the last receiver of around 22 dB could be reached for quadrature phase shift keying (QPSK) and 16-quadrature amplitude modulation (QAM) modulated signals.

Rainfall Monitoring Based on Next-Generation Millimeter-Wave Backhaul Technologies in a Dense Urban Environment

High-resolution and accurate rainfall monitoring is of great importance to many applications, including meteorology, hydrology, and flood monitoring. In recent years, microwave backhaul links from wireless communication networks have been suggested for rainfall monitoring purposes, complementing the existing monitoring systems. With the advances in microwave technology, new microwave backhaul solutions have been proposed and applied for 5G networks. Examples of the latest microwave technology include E-band (71–76 and 81–86 GHz) links, multi-band boosters, and line-of-sight multiple-input multiple-output (LOS-MIMO) backhaul links. They all rely on millimeter-wave (mmWave) technology, which is the fastest small-cell backhaul solution. In this paper, we will study the rain attenuation characteristics of these new microwave backhaul techniques at different mmWave frequencies and link lengths. We will also study the potential of using these new microwave solutions for rainfall monitoring. Preliminary results indicate that all the test mmWave links can be very effective for estimating the path-averaged rain rates. The correlation between the mmWave link measurement-derived rain rate and the local rain gauge is in the range of 0.8 to 0.9, showing a great potential to use these links for precipitation and flood monitoring in urban areas.

Novel Unmanned Aerial Vehicle-Based Line-of-Sight MIMO Configuration Independent of Transmitted Distance Using Millimeter Wave

Recently, the millimeter-wave band has been used in outdoor wireless communications to improve the frequency utilization efficiency. In this study, we propose a method to realize line-of-sight multiple-input multiple-output (LOS-MIMO) transmission independent of the transmitted distance using a two-dimensional (2-D) fixed antenna element arrangement, assuming millimeter-wave-band communication by small autonomous unmanned aerial vehicles. In conventional LOS-MIMO transmission with uniformly spaced antenna elements, the theoretical upper bound of the channel capacity is obtained, which decreases at a certain transmitted distance. Therefore, we focus on the fact that the propagation channel characteristics in pure LOS are geometrically determined by the transmitted distance, and consider the optimization of the arrangement of antenna elements. The element arrangement is fixed without using antenna selection from the viewpoint of system simplification. In addition, from the viewpoint of array size, we study the optimization corresponding to a 2-D element arrangement. The proposed method approximates the optimal arrangement from a vast number of 2-D element arrangements using a genetic algorithm. We evaluate the channel capacity characteristics of the proposed method by computer simulation and show that the characteristic degradation due to change in the transmitted distance in conventional LOS-MIMO transmission is improved. In addition, we evaluate a $4\times4$ MIMO transmission in an outdoor environment. The $4\times4$ MIMO propagation channel is also measured by an actual measurement equipment in the 66-GHz band and it is shown that the measured and calculated results agree with each other.

Adaptive Beamforming Design for Millimeter-Wave Line-of-Sight MIMO Channel

In this letter, we propose dictionary learning (DL) algorithm to solve the hybrid beamforming of millimeter-wave (mmWave) line-of-sight (LoS) multiple-input multiple-output (MIMO), which is modeled by Ray-tracing principal. Additionally, the number of transmitted data streams is adaptively adjusted by the effective degree of freedom (EDoF). The numerical simulation shows that the performance of hybrid beamforming based on the proposed algorithm can approach the optimal performance of full digital beamforming. Moreover, the adaptive hybrid beamforming of mmWave LoS MIMO can achieve the maximum spectral efficiency regardless of any number of transmitted data streams.

Single‐carrier frequency domain equalisation for microwave LOS‐MIMO systems and its performance analysis

The authors analyse and investigate the effectiveness of frequency domain equalisation (FDE) for single-carrier (SC) transmission over a line-of-sight multiple-input multiple-output (LOS-MIMO) channel with frequency selective fading. For this purpose, they propose a 2 × 2 LOS-MIMO system model employing SC-FDE techniques together with a data frame structure having a cyclic prefix and postfix (CPP) overhead. The channel capacity of the proposed system is derived taking into account system design parameters, such as symbol rate and roll-off factor, as well as multipath fading channel parameters. An efficient channel estimation method is also proposed for implementing and simulating the proposed system. Signature curves estimated in simulations are consistent with those expected from the theoretical capacity analysis. The simulation results show that the SC-FDE approach to the LOS-MIMO system can, with small CPP overhead, effectively reduce intersymbol and interstream interference.

Adaptive Transmit Antenna Selection Based on PCA for Millimeter Wave LOS MIMO Channel

Uniform rectangular antenna array (URA) is a commonly used antenna array in multiple-input multiple-output (MIMO) system. However, the existing works rarely concentrate on URA in millimeter-wave (mmWave) line-of-sight (LOS) MIMO channels. In this paper, we conduct research on the point-to-point mmWave LOS MIMO channel based on URA. We derive the optimal antenna deployment for URA to obtain the maximum multiplexing gain in mmWave LOS MIMO channel. For the URA with fixed parameters, as the distance between transmitter and receiver increases, some singular values of the channel matrix tend to be 0. In this case, it is not necessary to use all of the transmit antennas. Therefore, according to the two kinds of channel-state information feedback schemes, we propose two adaptive transmit antenna selection (ATAS) schemes based on the principal component analysis for this mmWave LOS MIMO channel. The computational complexity for the proposed algorithm is also derived. Numerical results show that the channel capacity of this proposed ATAS system can approach or even surpass that of the full transmit antenna system with equal power allocation in mmWave LOS MIMO channel. Compared with transmit antenna selection based on greedy search and exhaustive search algorithm, when the distance is longer, the proposed algorithm can achieve the near-optimal performance with much lower computational complexity.

Optimal Design of Uniform Circular Antenna Array in mmWave LOS MIMO Channel

The capacity of millimeter-wave (mmWave) line-of-sight (LOS) multiple-input multiple-output (MIMO) channel can be reduced due to the absence of rich scattering. However, the optimization of antenna array deployment can enable the maximum spatial multiplexing gain in the mmWave LOS MIMO channel, which can be applied to the long distance airborne data transmission. In this paper, we first establish the channel model of mmWave LOS MIMO based on a uniform circular antenna array (UCA) and derive the upper/lower bound of channel capacity. Aiming at achieving the maximum capacity of mmWave LOS MIMO channel, we propose the Newton-iterative method to obtain the optimal parameters of UCA. To simplify the optimization algorithm, the 2-D search can be transformed into 1-D search. Then, the analysis of bit error rate (BER) of the mmWave LOS MIMO system based on the UCA has been provided to quantify the mmWave LOS MIMO channel. Finally, numerical results demonstrate that the optimal parameters of UCA can be obtained by the Newton-iterative method and the BER of mmWave LOS MIMO communication system based on the corresponding optimized UCA can be minimized.

Line-of-Sight $2 \times n_r$ MIMO With Random Antenna Orientations

Line-of-sight (LoS) multiple-input multiple-output (MIMO) gives full spatial-multiplexing gain when the antenna array geometry and orientation are designed based on the interterminal distance. These known design methodologies that hold for antenna arrays with fixed orientation do not provide full MIMO gains for arbitrary array orientations. In this paper, we study LoS MIMO channels with random array orientations when the number of transmit antennas used for signaling is 2. We study the impact of common array geometries on error probability, and identify the code design parameter that describes the high signal-to-noise ratio (SNR) error performance of an arbitrary coding scheme. For planar receive arrays, the error rate is shown to decay only as fast as that of a rank 1 channel, and no better than SNR $^{-3}$ for a class of coding schemes that includes spatial multiplexing. We then show that for the tetrahedral receive array, which uses the smallest number of antennas among nonplanar arrays, the error rate decays faster than that of rank 1 channels and is exponential in SNR for every coding scheme. Finally, we design a LoS MIMO system that guarantees a good error performance for all transmit/receive array orientations and over a range of interterminal distances.

Performance Evaluation of LoS MIMO Microwave Radio Systems Over Frequency Selective Multipath Fading Channels

Although spectrally efficient microwave LoS-MIMO (Line of Sight-Multiple Input Multiple Output) systems seem to be practically attractive mainly at higher frequency bands (above 15 GHz), also at lower frequency bands, where frequency selective multipath fading is the dominant impairment for the outage time, they could play a significant role. Under this perspective, the paper analyzes performance of LoS-MIMO systems affected by frequency selective fading, taking into account the necessary equalization for satisfying the outage objectives. For the purpose of the analysis, we adopted a frequency selective $$2 \times 2$$ LoS-MIMO channel model extrapolated from the 2-ray model historically used in SISO (Single Input Single Output) systems and, exploiting the full statistical behaviour of the propagation channel model, we described a methodology for the evaluation of frequency selective LoS-MIMO outage performance. Then the proposed methodology is used to investigate the effectiveness of equalization in the intersymbol and mutual interference reduction of two equalizer structures, linear MMSE and MMSE Partial Response, using as a practical reference the target outage probability predicted on the same link for a SISO system. Finally, it is discussed the opportunity of using the Condition Number of the LoS-MIMO channel matrix transfer function as an exhaustive indicator of the channel quality.

An efficient beam-training scheme for the optimally designed subarray structure in mmWave LoS MIMO systems

This paper studies the fundamental relations between key design parameters of millimeter wave (mmWave) multiple input multiple output (MIMO) communication systems and subarray structures deployed at both the transmitter and the receiver with each radio frequency (RF) chain connected to only a specific subset of the antennas. The concept of effective degrees of freedom (EDoF) is introduced to measure the maximum spatial multiplexing gain available for the MIMO system. An analytical expression for the EDoF with respect to the parameters of antenna configuration and transmission distance is obtained for the line of sight (LoS) scenario. In addition, the upper and lower bounds of the EDoF are further obtained for some special cases. A fast beam training algorithm based on the codebook is developed to reduce the number of training for the designed mmWave system. Extensive simulation results indicate that the proposed scheme reduces the computational load of the exhaustive approach with only minimal loss in performance. Moreover, the proposed design is robust to the geometrical change and misplacement.

Non-Uniform Linear Antenna Array Design and Optimization for Millimeter-Wave Communications

In this paper, we investigate the optimization of non-uniform linear antenna arrays (NULAs) for millimeter wave (mmWave) line-of-sight (LoS) multiple-input multiple-output (MIMO) channels. Our focus is on the maximization of the system effective multiplexing gain (EMG), by optimizing the individual antenna positions in the transmit/receive NULAs. Here the EMG is defined as the number of signal streams that are practically supported by the channel at a finite SNR. We first derive analytical expressions for the asymptotic channel eigenvalues with arbitrarily deployed NULAs when, asymptotically, the end-to-end distance is sufficiently large compared to the aperture sizes of the transmit/receive NULAs. Based on the derived expressions, we prove that, the asymptotically optimal NULA deployment that maximizes the achievable EMG should follow the groupwise Fekete-point distribution. Specifically, the antennas should be physically grouped into K separate uniform linear antenna arrays (ULAs) with the minimum feasible antenna spacing within each ULA, where K is the target EMG to be achieved; in addition, the centers of these K ULAs follow the Fekete-point distribution. We numerically verify the asymptotic optimality of such an NULA deployment and extend it to a groupwise projected arch type (PAT) NULA deployment, which provides a more practical option for mmWave LoS MIMO systems with realistic non-asymptotic configurations. Numerical examples are provided to demonstrate a significant capacity gain of the optimized NULAs over traditional ULAs.

On Spatial Multiplexing of Strong Line-of-Sight MIMO With 3D Antenna Arrangements

In recent works, it has been shown that specific 2D antenna arrangements for multiple-input multiple-output (MIMO) systems can achieve similarly high spatial multiplexing gains under deterministic line-of-sight (LOS) conditions as non-line-of-sight channels with strong scattering considered in classical papers. However, the question whether 3D antenna arrays could provide an additional advantage was not addressed. In this work we show that the capacity of dominant LOS MIMO channels is invariant w.r.t. small offsets of the antenna elements along the transmit direction. This proves that the optimal 2D arrangements for point-to-point communication of LOS MIMO arrays are equivalent to 3D arrangements, whose projections of the antenna positions into a plane perpendicular to the transmit direction reproduce the optimal 2D arrangements. This insight also leads directly to the optimal designs for antenna arrays that communicate with each other along a transmit direction that is oblique w.r.t. the array plane(s).

Tens of Gigabits Wireless Communications Over E-Band LoS MIMO Channels With Uniform Linear Antenna Arrays

This paper studies the fundamental characteristics of point-to-point E-band channels with uniform linear antenna arrays (ULAs) deployed at both the transmitter and receiver. We model the channels as line-of-sight (LoS) multiple-input multiple-output (MIMO) ones and focus on the channel eigenvalue characterization when the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Rayleigh distance criterion</i> cannot be fulfilled due to limited physical sizes of the transmitter and receiver. We first derive explicit expressions for some channel eigenvalues at certain discrete system settings. Asymptotic analyses are then developed when the antenna numbers at the transmitter and receiver or the distance between them goes to infinity. Based on these analytical results, the maximum eigenvalue and the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">effective multiplexing distance</i> (EMD) of the E-band channel are investigated, where EMD is defined as the end-to-end distance at which the channel can support a certain number of simultaneous spatial streams at a given signal-to-noise ratio (SNR). We analytically show that the EMD for a given number of parallel signal transmissions is mainly determined by the product of the aperture sizes of the transmit and receive ULAs. Numerical results are provided to validate the analyses.

Mutual information assessment of LOS MIMO systems with reconfigurable antenna arrays

In presence of line-of-sight (LOS) propagation, multiple-input multiple-output (MIMO) systems can achieve maximum capacity over the whole signal-to-noise ratio (SNR) region by deploying reconfigurable antenna arrays. In this paper, the moment generating function (MGF) formulas of the mutual information (MI) is obtained for LOS MIMO systems with reconfigurable arrays. Then the exact expression of the mean MI is derived in an easily evaluated form. The results show an excellent match between the theoretical curves and the Monte-Carlo simulations.