Power Angular Spectrum Research Articles

Switchable metasurface absorber used for enabling reconfigurable power angular spectrum in reverberation chamber

The reverberation chamber (RC) provides a fast and repeatable method for over-the-air (OTA) testing of wireless devices. Moreover, the RC-based method can reduce the OTA testing cost to a great extent. But the defect of the RC is also obvious. Compared to a multi-probe anechoic chamber, the channel spatial characteristics in RC are uncontrollable. The device under test in an RC usually sees a statistically isotropic channel and there is a strong impediment to control the channel in RC, which constrains the RC’s OTA applications. In this paper, we propose a method to realize reconfigurable RC enabling arbitrary channel power angular spectrum (PAS) by using a switchable metasurface absorber. Specifically, the unit cell of the metasurface can be switched between the reflection and absorption states by providing different bias voltages. By mounting the switchable metasurface absorber on the RC’s inside walls, the boundary conditions of the RC in the covered area could be switched electronically. Consequently, the channel’s PAS can be controlled by changing the local states of the switchable metasurface absorber. As a proof of concept, a prototype of the switchable metasurface absorber is made and comprehensive experiments are conducted to verify the effectiveness of the switchable metasurface applied to enable reconfigurable PAS in RC.

On the Feasibility of Out-of-Band Spatial Channel Information for Millimeter-Wave Beam Search

Prolonged beam alignment is the main source of overhead in mobile wireless communications at millimeter-wave (mm-wave) frequencies due to narrow beams following the requirement of high antenna gains. Out-of-band spatial information may be used in initial beam search when lower frequency band radios are operating in conjunction with mm-wave radios. The feasibility of using low-band channel information for coarse estimation of high-band beam directions strongly depends on the spatial congruence between the two frequency bands. In this work, we try to answer two related questions. First, how similar is the power angular spectrum (PAS) of propagation channels between two widely separated frequency bands? Then, what is the impact of practical antenna configurations on spatial channel similarity? We propose a beam directions based metric to assess the power loss and number of false directions if out-of-band spatial information is used instead of in-band information. This metric is more practical and useful than comparing the PASs directly. Point cloud ray tracing and propagation measurement results across multiple frequency bands and environments are used to show that the degree of spatial similarity of beamformed channels is related to antenna beam widths, frequency gap, and radio link conditions.

On dynamic channel emulation in sector MPAC for over-the-air testing of beamformed massive MIMO devices

In this article, novel emulation strategies for the sectored multiple probe anechoic chamber (SMPAC) are proposed to enable the reliable evaluation of the massive multiple-input multiple-output (MIMO) device operating at beamforming mode, which requires a realistic non-stationary channel environment. For the dynamic propagation emulation, an efficient closed-form probe weighting strategy minimizing the power angular spectrum (PAS) emulation errors is derived, substantially reducing the associated computational complexity. On the other hand, a novel probe selection algorithm is proposed to reproduce a more accurate fading environment. Various standard channel models and setup configurations are comprehensively simulated to validate the capacity of the proposed methods. The simulation results show that more competent active probes are selected with the proposed method compared to the conventional algorithms. Furthermore, the derived closed-form probe weighting strategy offers identical accuracy to that obtained with complicated numerical optimization. Moreover, a realistic dynamic channel measured in an indoor environment is reconstructed with the developed methodologies, and 95.6% PAS similarity can be achieved with 6 active probes. The satisfactory results demonstrate that the proposed algorithms are suitable for arbitrary channel emulation.

Dynamic MmWave channel reconstruction of MIMO OTA testing for handover scenarios

The evaluation of handover performance is essential for ensuring seamless user experience under innovative application scenarios in the fifth generation (5G) and beyond era, including autonomous driving, mobile augmented and virtual reality. However, due to the hardware constrains of a sectored multiprobe anechoic chamber (SMPAC), switching among multiple channel models is of low precision with a high cost in traditional over-the-air (OTA) test solutions. In this paper, we present an efficient and repeatable emulation strategy to reconstruct dynamic millimeter-wave (mmWave) channels in laboratories for multiple-input multiple-output (MIMO) mobile devices. Firstly, we propose a novel evaluation metric, called average power angular spectrum similarity percentage (APSP), which minimizes the unexpected impact induced by the indefinite condition of adaptive antenna arrays in mmWave terminals during handover process. Moreover, we propose a partitioned probe configuration strategy by designing a beam directivity-based switching circuit, which enables quick changes of probe configurations in SMPAC. Simulation results demonstrate the effectiveness of the proposed algorithms, thus providing a guideline for the reconstruction of the dynamic channel in different scenarios with resource limitation.

A Pattern Correlation Decomposition Method for Analysis of ESPAR in Single-RF MIMO Systems

A systematic and general method for obtaining orthogonal far-field radiation patterns using any electronically steerable parasitic array radiator (ESPAR) embedded in a propagation environment with an arbitrary power angular spectrum (PAS) is described. The method is useful in designing single-RF multiple-input multiple-output (MIMO) systems where the orthogonal patterns can be utilized as a basis set for beamspace MIMO. The method utilizes the correlation matrix of the open-circuit radiation patterns and is termed the pattern correlation decomposition method (PCDM). PCDM is applicable to any antenna type (including realistic antennas with losses), array configuration, PAS and overcomes the limitations of previously proposed techniques. PCDM also provides closed-form solutions for the antenna currents to generate the orthogonal patterns. It also provides an estimate of the effective aerial degrees-of-freedom of the ESPAR. In addition, efficient optimization approaches to finding the optimal reactive loads that excite the required orthogonal radiation patterns are provided. Simulation examples using a 5-element and a 8-element planar inverted-F antenna array including material losses in various PAS are provided. These show that the proposed approach increases system capacity by up to 7 bits/s/Hz or equivalently reduces the required transmit power by up to 73% compared with previous approaches.

Near-Field Spatial Correlation for Extremely Large-Scale Array Communications

Extremely large-scale array (XL-array) communications correspond to systems whose antenna sizes are so large that the scatterers and/or users may no longer be located in the far-field region. By discarding the conventional far-field uniform plane wave (UPW) assumption, this letter studies the near-field spatial correlation of XL-array communications, by taking into account the more generic non-uniform spherical wave (NUSW) characteristics. It is revealed that different from the far-field channel spatial correlation which only depends on the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">power angular spectrum</i> (PAS), the near-field spatial correlation depends on the scattered power distribution not just characterized by their arriving angles, but also by the scatterers’ distances, which is termed as <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">power location spectrum</i> (PLS). A novel integral expression is derived for the near-field spatial correlation in terms of the scatterers’ location distribution, which includes the far-field spatial correlation as a special case. The result shows that different from the far-field case, the near-field spatial correlation no longer exhibits <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">spatial stationarity</i> in general, since the correlation coefficient for each pair of antennas depends on their specific positions, rather than their relative distance only. To gain further insights, we consider a generalized one-ring model for scatterer distribution, where the ring center may be flexibly located. Numerical results are provided to show the necessity of the near-field spatial correlation modelling for XL-array communications.

Angular Clustering of Millimeter-Wave Propagation Channels With Watershed Transformation

An angular clustering method based on image processing is proposed in this paper. It is used to identify clusters in 2D representations of propagation channels. The approach uses operations such as watershed segmentation and is particularly well suited for clustering directional channels obtained by beam-steering at millimeter-wave. This situation occurs for instance with electronic beam-steering using analog antenna arrays during beam training process or during channel modeling measurements using either electronic or mechanical beam-steering. In particular, the proposed technique is used here to cluster two-dimensional power angular spectrum maps. The proposed clustering is unsupervised and is well suited to preserve the shape of clusters by considering the angular connection between neighbor samples, which is useful to obtain more accurate descriptions of channel angular properties. The approach is found to outperform approaches based on K-Power-Means in terms of accuracy as well as computational resources. The technique is assessed in simulation using IEEE 802.11ad channel model and in measurement using experiments conducted at 60 GHz in an indoor environment.

Statistics of the Effective Massive MIMO Channel in Correlated Rician Fading

Massive multiple-input multiple-output base stations use multiple spatial diversity branches, which are often assumed to be uncorrelated in theoretical work. Correlated branches are seldom considered since they are mathematically less tractable. For correlated Rician fading, only the first- and second-order moments have been explored. To describe propagation environments more accurately, full distribution functions are needed. This manuscript provides distribution functions for the maximum ratio combining effective channel, a quadratic form of a random complex normal channel vector. Its mean vector and covariance matrix are based on a plane wave model incorporating array geometry, antenna element pattern, power angular spectra and power delay profiles. Closed-form approximations of the distribution functions are presented, to allow the fast evaluation of many real-world scenarios. The statistical framework is used to show that low-directivity antenna elements provide better performance in angular constricted Rician fading with off-axis incidence than high-directivity elements. Moreover, two base station array layouts are compared, showing that a half-circle array illuminates a cell more evenly than a uniform linear array. With the full distribution functions available, performance can be compared over the full range of received powers and not only based on the average signal-to-noise ratio.

Time-Domain Nonstationary Channel Emulation in Multiprobe Anechoic Chamber Setups for Over-the-Air Testing

ThThis letter proposes a novel four-dimensional multi-probe anechoic chamber (4D-MPAC) method for emulating time-domain non-stationary channel models of over-the-air (OTA) testing. The proposed method adds time dimension to the 3D-MPAC system, so that the channel model with spatial parameters changing over time can be emulated. In addition, 4D power angular spectrum similarity percentage (4D-PSP) as a figure of merit to evaluate the accuracy of the channel reconstruction is proposed for the first time. Based on the 4D-PSP, genetic algorithm (GA) is designed to reduce the number of active probes and testing cost. Furthermore, a dynamic motion scenario in a street is modeled by ray tracing simulation, where the time-domain non-stationary channel model is reconstructed by the proposed method with a small number of probes.

A study of uplink and downlink channel spatial characteristics in an urban micro scenario at 28 GHz

This paper presents an empirical study of the uplink and downlink azimuth angle of arrival (AoA) in an urban micro (UMi) scenario at 28 GHz. At present, most UMi measurements are conducted in the downlink and then the uplink situation is inferred assuming channel reciprocity. Although the channel correlation coefficient of the uplink and downlink can be as high as 0.8, this does not mean that they are the same. Only a real uplink measurement can accurately describe its channel conditions, and this is what this study does. A receiver equipped with a rotatable horn antenna is mounted at the base station and the user terminal, respectively, in simulating the uplink and downlink. To improve the angular resolution, we extract the multipath components (MPCs) using the space-alternating generalized expectation-maximization algorithm. Also, a spatial lobe approach is used to cluster the MPCs in the power angular spectrum. By matching MPCs with objects in the environment, we find that direct propagation and first-order reflections are dominant in line-of-sight and non-line-of-sight cases. By comparing our measurement with those in standard channel models, we verify that the AoA of clusters follows a Gaussian distribution in the uplink and downlink. In addition, a two-dimensional Gaussian distribution for ray AoA and power is established to reflect their correlation.

On Angular Sampling Intervals for Reconstructing Wideband Channel Spatial Profiles in Directional Scanning Measurements

Directional scanning measurement is the dominant strategy taken to measure channel spatial profiles for millimeter wave (mmWave) frequency bands. The focus of the paper is on wideband channel spatial profile reconstruction and angular sampling interval (ASI) selection for directional scanning measurement. We propose to employ the trigonometric interpolation technique to reconstruct the spatial channel profile and to use Nyquist sampling theorem and spherical wave mode theory to determine the ASI. Our analysis demonstrated that the half-power beam width (HPBW) of the antenna is a good approximation for the ASI setting to ensure accurate directional scanning measurements. The reconstructed power-angle-delay profile (PADP), composite power angular spectrum (PAS), delay and angle characteristics match well with those of the target in the measured data when the ASI is set to the antenna HPBW.

Millimeter‐wave propagation modeling and characterization at 32 GHz in indoor office for 5G networks

Since it has a great bandwidth that supports gigabit communication, it is considered to use the millimeter-wave (mmWave) band in the fifth generation (5G) wireless communication. Therefore, an efficient, reliable, and accurate channel model is of vital importance in mmWave bands for indoor environments, especially in the 31.8 to 33.4 GHz band allocated by ITU for 5G communications. In this article, we performed modeling and characterization campaign at the 32 GHz in a typical indoor office environment on fourth floor of the Engineering Faculty in University of Karabuk, Turkey. The obtained results provide large-scale fadings such as path loss, shadowing, root mean square (RMS) delay spread, RMS angular spread, power angular spectrum, number of clusters, and Ricean K-factor in an open-plan indoor environment. Power angular spectrum is used to comprehend the propagation structure. We propose that the results obtained in this study will play a key role in simulating and planning systems at 32 GHz for 5G wireless communication.

Probe Selection for 5G Massive MIMO Base Station Over-the-Air Testing

A sectored multiprobe anechoic chamber is seen as a promising method to carry out the radiated testing of massive multiple-input–multiple-output (MIMO) base station (BS), and realistic channel environment reconstruction is crucial for the performance assessment of wireless devices, especially for the testing of beamforming procedures, such as beam acquisition, tracking, and so on. This letter proposes a novel probe selection algorithm and corresponding device rotation scheme to enable more accurate channel emulation with specific probe antennas. On the other hand, within the acceptable threshold of error, the number of required probe antennas can be reduced, which will mitigate setup complexity and the number of hardware resources. In the simulation, to verify the robustness of the proposed probe configuration, several metrics are examined with different numbers of selected probes, in terms of spatial correlation and power angular spectrum emulation. The simulation results show that the proposed selection and rotation algorithms perform well in synthesizing the accurate target propagation environment on the BS side.

Machine Learning-Enabled LOS/NLOS Identification for MIMO Systems in Dynamic Environments

Discriminating between line-of-sight (LOS) and non-line-of-sight (NLOS) conditions, or LOS identification , is important for a variety of purposes in wireless systems, including localization and channel modeling. LOS identification is especially challenging in vehicle-to-vehicle (V2V) networks since a variety of physical effects that occur at different spatial/temporal scales can affect the presence of LOS. This paper investigates machine learning techniques for LOS identification in V2V networks using an extensive set of measurement data and then develops robust and efficient identification solutions. Our approach exploits several static and time-varying features of the channel impulse response (CIR), which are shown to be effective. Specifically, we develop a fast identification solution that can be trained by using the power angular spectrum. Moreover, based on the measurement data, we also compare three different machine learning methods, i.e., support vector machine, random forest, and artificial neural network, in terms of their ability to train and generate the classifier. The results of our experiments conducted under various V2V environments, which were then validated using $K$ -fold cross-validation, show that our techniques can distinguish the LOS/NLOS conditions with an error rate as low as 1%. In addition, we investigate the impact of different training and validating strategies on the identification accuracy.

OTA Testing for Massive MIMO Devices Using Cascaded APM Networks and Channel Emulators

This paper proposes an over-the-air (OTA) testing setup for millimeter-wave (mmWave) massive multiple-input multiple-output (MIMO) equipment using cascaded amplitude and phase modulation (APM) network and channel emulator. Compared with the existing test setup with mechanical switch, the proposed testing setup enables more accurate reconstruction of the radio channel environment under the multiprobe anechoic chamber (MPAC) setup without increasing the number of channel emulators (CEs) to control the system cost. The constructed MPAC testing setup for mmWave and massive MIMO equipment is composed of an anechoic chamber, a sectored probe wall containing a number of probes, an APM network, a fading channel emulator, and a user emulator (UE). In this paper, the structural model and the performance advantages of the proposed radiated testing setup are described, and a fully connected APM network for radiated testing is more prominent than the existing switch. Moreover, the angular spectrum is selected as the performance metric for the reconstructed channel. The ability of the proposed system to reconstruct the power angular spectrum (PAS) of the target channel is studied under both static and dynamic channel models, which can reflect the performance of beamforming procedures of the massive MIMO antenna arrays, e.g., beam acquisition, tracking, and refinement. The simulation results for angular spectrum support the superiority of the proposed OTA testing setup. Furthermore, the simulations for average channel capacity also show that radiated testing setup using cascaded APM network and channel emulator is valid.

A Novel Random Angular Sampling Method for Spatial and Temporal Channel Emulation

In this letter, a novel angular sampling method–random offset sampling (ROS) is proposed for the simulation of ray-based physical reference channel models. This new method employs an asymmetrical random angular offset to the equal power sampling method, which makes the simulated channel match the spatial–temporal characteristics of the reference channel model more exactly. It is shown that this new method can make better utilization of the symmetrical property of power angular spectrum distributions with very limited sampling sinusoids, especially performing pretty well in case of small angular spread. With small modifications, ROS can be directly embedded in the state-of-the-art standardized channel simulation models.

Plane wave compensation technique for multiple‐input multiple‐output over‐the‐air testing in small multi‐probe anechoic chamber

Multiple-input multiple-output (MIMO) over-the-air (OTA) testing is often executed in multi-probe anechoic chamber (MPAC), the size of which is generally large enough so that the waves radiated from the probes can be considered plane waves in test area. The physical dimension of test setups is a key cost factor. In order to save laboratory space and reduce the cost, a compensation technique for spherical waves synthesising plane waves is proposed, which can be applied in small anechoic chamber. The object is to investigate the performances of the proposed plane wave compensation technique on MIMO terminal OTA testing in small MPAC. In the simulation, this technique presents good performances on various metrics, e.g. field distribution, spatial correlation, power angular spectrum estimation. As demonstrated in simulation, the ring radius of anechoic chamber can be reduced and the conventional far field criteria can be relieved.

Approximation Algorithm Based Channel Estimation for Massive MIMO Antenna Array Systems

This paper presents a three-dimensional (3D) massive multiple-input and multiple-output (MIMO) antenna array model, which includes the spherical array assumption and geometric properties for future fifth generation (5G) wireless communications. A parametric approximation algorithm is developed for estimating the spatial fading correlations (SFCs) and channel capacities of the 3D massive MIMO antenna array systems under different power angular spectrum (PAS). The relationship between correlation with the spacing of antenna arrays and angular parameters was classified. The results show that the simulation values of the approximate method fit the theoretical calculation very well, thereby validating the feasibility of the proposed 3D large-scale massive MIMO model.

Online Learning for Position-Aided Millimeter Wave Beam Training

Accurate beam alignment is essential for the beam-based millimeter wave communications. The conventional beam sweeping solutions often have large overhead, which is unacceptable for mobile applications, such as a vehicle to everything. The learning-based solutions that leverage the sensor data (e.g., position) to identify the good beam directions are one approach to reduce the overhead. Most existing solutions, though, are supervised learning, where the training data are collected beforehand. In this paper, we use a multi-armed bandit framework to develop the online learning algorithms for beam pair selection and refinement. The beam pair selection algorithm learns coarse beam directions in some predefined beam codebook, e.g., in discrete angles, separated by the 3 dB beamwidths. The beam refinement fine-tunes the identified directions to match the peak of the power angular spectrum at that position. The beam pair selection uses the upper confidence bound with a newly proposed risk-aware feature, while the beam refinement uses a modified optimistic optimization algorithm. The proposed algorithms learn to recommend the good beam pairs quickly. When using $16\times 16$ arrays at both transmitter and receiver, it can achieve, on average, 1-dB gain over the exhaustive search (over $271\times 271$ beam pairs) on the unrefined codebook within 100 time steps with a training budget of only 30 beam pairs.

Evaluation of angle spread and power balance for design of radio links with directional antennas in multipath environment

Development of spatial multiplexing techniques and co-channel operation of different wireless systems require a way to assess the mutual influence of emitted signals. This paper focuses on the signal level evaluation as a function of the path loss that depends on the parameters of a transmitting and receiving antenna in the multipath propagation conditions. The developed procedure uses a multi-elliptical propagation model to estimate a power angular spectrum, which is the basis for evaluation of the received signal power. This procedure provides adaptation of the close-in free space reference distance path loss propagation model by considering parameters of directional parameters. Simulation studies are the basis for assessing the impact of the antenna parameters on the path loss and consequently on the received signal power. The results of these studies show the influence of the width of the main lobe and, most importantly, the direction of the antennas on the level and angular dispersion of the received signals. A comparison of the results for multi-path and free-space propagation conditions demonstrates the viability of the developed procedure for the purposes of predicting power distribution and reception direction.