Virtual Uniform Circular Array Research Articles

Modeling Multi-Frequency Characteristics for Classroom and Hall Scenarios at 2-4, 9-11 and 27-29 GHz Bands

This paper investigates the wideband channel characteristics obtained in a classroom and a hall indoor scenarios at 2-4, 9–11 and 27-29GHz. A virtual uniform circular array (UCA) based channel sounding system was utilized to capture wideband spatial channel characteristics. The propagation parameters of multipath components (MPCs) were estimated by using a high resolution parameter estimation (HRPE) algorithm. The estimated MPCs are further grouped into clusters via a novel clustering identification algorithm based on the KPowerMeans algorithm. The comparison of the composite and cluster-level characteristics at multiple frequency bands in both scenarios is investigated. Moreover, the impact of the indoor room environment, i.e. dimension and furniture, on the propagation channel is also analyzed. The statistics of channel parameters at multiple frequency bands extracted constitute a stochastic clustered spatial channel model.

Dynamic Channel Modeling for Indoor Millimeter-Wave Propagation Channels Based on Measurements

In this contribution, a recently conducted measurement campaign for indoor millimeter-wave (mm-wave) propagation channels is introduced. A vector network analyzer (VNA)-based channel sounder was exploited to record the channel characteristics at the frequency band from 28-30 GHz. A virtual uniform circular array (UCA) with a radius of 0.25 m was formed using a rotator with 360 steps. Moreover, by taking advantage of fiber-optic technique applied in the channel sounder, measurements at 50 positions were performed from an indoor hall to an indoor corridor along a long pre-defined route. A low-complexity high-resolution propagation estimation (HRPE) algorithm is exploited to estimate the propagation parameters of multipath components (MPCs). Based on the HRPE estimation results, a novel clustering identification and tracking algorithm is proposed to trace clusters. Composite channel characteristics, cluster-level characteristics and dynamic (or birth-death) behaviours of the clusters are investigated, which constitute a dynamic model for the indoor mm-wave channel.

Trajectory-Aided Maximum-Likelihood Algorithm for Channel Parameter Estimation in Ultrawideband Large-Scale Arrays

Millimeter-wave with ultrawide bandwidth available and ability to pack massive number of antennas in a small area is considered the key enabler for future generation communication systems. Accurate understanding and modeling of the ultrawideband propagation channel with large-scale array configuration is essential. In this contribution, a realistic spherical-propagation signal model considering the spatial nonstationarity of path gain across the array elements is proposed. A novel trajectory-aided maximum-likelihood (TAMax) algorithm is proposed to extract propagation parameters from the measured data, since the existing high-resolution propagation parameter estimation algorithms are not applicable due to either prohibitively high computation loads or assumption violations. In the proposed TAMax algorithm, the high-dimensional maximum-likelihood estimation (MLE) problem is first decomposed into a subproblem where delays and amplitudes of multipath components (MPCs) are estimated at individual array elements. A novel transform is then proposed to identify multiple MPC trajectories in the delay-element domain. With interference cancellation and fast initialization obtained in the proposed transform, spherical propagation parameters are finally acquired via joint MLE with significantly decreased searching spaces. Moreover, a measurement campaign conducted at the frequency band of 27-29 GHz using a virtual uniform circular array is introduced, where the proposed TAMax algorithm is applied and validated.

Underdetermined DOA Estimation via Covariance Matrix Completion for Nested Sparse Circular Array in Nonuniform Noise

This paper proposes a covariance matrix completion based algorithm for underdetermined direction of arrival (DOA) estimation in the presence of unknown nonuniform noise using nested sparse circular array (NSCA) with only N sensors. The proposed algorithm provides a systematic procedure to complete a covariance matrix for a virtual uniform circular array (UCA) with M sensors (M > N). Compared with the covariance matrix of the NSCA, the completed covariance matrix is capable of increasing degrees of freedom (DOFs), and is noise-free to mitigate the effect of nonuniform noise. The elements of the completed covariance matrix are from three steps: (1) elements from covariance matrix of the NSCA; (2) elements generated from the properties of the UCA; (3) elements produced from output of oblique projection operator based on initial DOAs. Then compressive sensing (CS) method is used to estimate DOAs based on the completed covariance matrix for better performance. The computational complexity of the proposed algorithm, and CRB are also given. Simulation results demonstrate that the proposed algorithm outperforms the state-of-the-art methods in estimation accuracy.

DOA Estimation for UCA in the Presence of Gain-Phase Errors

In this letter, a direction-of-arrival (DOA) estimation method for the uniform-circular array (UCA) in the presence of gain-phase errors is proposed. Thanks to UCA properties, the virtual steering vector, which is constructed with the Hadamard product of the steering vector and its conjugate to eliminate phase errors, is considered to correspond to virtual 2-D sources impinging on the virtual UCA. This finding indicates that the azimuths and elevations of virtual sources, which are related to DOAs of original sources, can be decoupled and estimated via UCA-estimating-signal-parameter-via-rotational-invariance-techniques with the help of the recursive relationship of the Bessel function in the mode space of the virtual steering vector. We also propose the spatial filtering method, which is based on the linear independence of steering vectors in different directions, to eliminate one component in the virtual covariance matrix, which affects the DOA estimation of adjacent sources. This method can avoid the 2-D multiple-signal-classification search and cope with the one-component problem. Simulations verify its effectiveness.

Channel Sounding System for MM-Wave Bands and Characterization of Indoor Propagation at 28 GHz

The aim of this work is to present a vector network analyzer based channel sounding system capable of performing measurements in the range from 2 to 50 GHz. Further, this paper describes an indoor measurement campaign performed at 26–30 GHz. The sounding system is capable of receiving two channels and transmitting one. Using this feature a channel measurement has been performed using both a directional horn antenna and a virtual uniform circular array (UCA) at the same time. This allows for comparative studies of measured channels with two different antennas in a simultaneous way. The measurement has been conducted with 42 measurement positions distributed along a 10 m long path through an indoor laboratory environment. The transmitter was positioned such that measurements were conducted both in line-of-sight and non-line-of-sight scenarios. The measurements showed good agreement between the measurement data collected with the horn antenna and the data collected with the UCA. The propagation environment was found to be sparse both in delay and angular domain for the given scenario. Based on the performed measurement campaign together with validation measurements of the system stability, it is found that the system works as expected.

Measured wideband characteristics of indoor channels at centimetric and millimetric bands

Accurate characterization of spatial multipath channels at millimeter wave bands has gained significant interest both in industry and academia. A channel measurement was conducted at three different frequency bands, i.e., 2−4, 14−16, and 28−30 GHz in a line-of-sight (LOS) and an obstructed-LOS (O-LOS) scenarios in an empty room environment. A vector network analyzer connected to a virtual uniform circular array and to a rotational directional horn antenna was used in the measurements, respectively. Angle-of-arrivals and delay-of-arrivals of the multipath components were obtained from the measurements for the three frequency bands. Room electromagnetic properties for the three different frequencies at different propagation scenarios were investigated as well.

A fast computation method for MUSIC spectrum function based on circular arrays

The large computation amount of multiple signal classification (MUSIC) spectrum function seriously affects the timeliness of direction finding system using MUSIC algorithm, especially in the two-dimensional directions of arrival (DOA) estimation of azimuth and elevation with a large antenna array. This paper proposes a fast computation method for MUSIC spectrum. It is suitable for any circular array. First, the circular array is transformed into a virtual uniform circular array, in the process of calculating MUSIC spectrum, for the cyclic characteristics of steering vector, the inner product in the calculation of spatial spectrum is realised by cyclic convolution. The computational amount of MUSIC spectrum is obviously less than that of the conventional method. It is a very practical way for MUSIC spectrum computation in circular arrays.

Estimating Discrete Power Angular Spectra in Multiprobe OTA Setups

The letter discusses over-the-air (OTA) testing for multiple-input-multiple-output (MIMO) capable terminals with emphasis on estimating discrete power angular spectrum modeled at the receiver (Rx) side in the test zone. Two techniques based on a uniform circular array (UCA) are proposed to obtain accurate direction-of-arrival estimates as well as power estimates of the impinging signals in the test zone. Simulation results match well with the target, as expected. Measurement results based on a virtual UCA in a practical 3-D multiprobe setup further support the simulation results. Possible reasons for the deviations between simulations and measurements are also investigated.

Frequency Invariant Uniform Concentric Circular Arrays with Directional Elements

A new approach for designing frequency invariant (FI) uniform concentric circular arrays (UCCAs) with directional elements is proposed, and their applications to direction-of-arrival (DOA) estimation and adaptive beamforming are studied. By treating the sensors along the radial direction of the UCCA as linear subarrays and using appropriately designed beamformers, each subarray is transformed to a virtual element with appropriate directivity. Consequently, the whole UCCA can be viewed as a virtual uniform circular array (UCA) with desired element directivity for broadband processing. By extending the approach for designing FI-UCAs, the frequency dependency of the phase modes of the virtual UCA is compensated to facilitate broadband DOA and adaptive beamforming. Both the linear array beamformers (LABFs) and compensation filters can be designed separately using second- order cone programming (SOCP). Moreover, a new method to tackle the possible noise amplification problem in such large arrays by imposing additional norm constraints on the design of the compensation filters is proposed. The advantages of this decoupled approach are 1) the complicated design problem of large UCCAs can be decoupled into simpler problems of designing the LABFs and compensation filters, and 2) directional elements, which are frequently encountered, can be treated readily under the proposed framework. Numerical examples are provided to demonstrate the effectiveness and improvement of the proposed methods in DOA estimation, adaptive beamforming, and elevation control over the conventional FI-UCCA design method.

Research on DOA Estimation of Multi-Component LFM Signals Based on the FRFT

A novel algorithm for the direction of arrival (DOA) estimation based on the fractional Fourier transform (FRFT) is proposed. Firstly, using the properties of FRFT and mask processing, Multi-component LFM sig-nals are filtered and demodulated into a number of stationary single frequency signals. Then the one-dimensional (1-D) direction estimation of LFM signals can be achieved by combining with the tradi-tional spectrum search method in the fractional Fourier (FRF) domain. As for the multi-component LFM signals, there is no cross-term interference, the mean square error (MSE) and Cramer-Rao bound (CRB) are also analyzed which perfects the method theoretically, simulation results are provided to show the validity of our method. The proposed algorithm is also extended to the uniform circular array (UCA), which realizes the two-dimensional (2-D) estimation. Using the characteristics of time-frequency rotation and demodulation of FRFT, the observed LFM signals are demodulated into a series of single frequency ones; secondly, operate the beam-space mapping to the single frequency signals in FRF domain, which UCA in array space is changed into the virtual uniform circular array (ULA) in mode space; finally, the DOA estimation can be realized by the traditional spectral estimation method. Compared with other method, the complex time-frequency cluster and the parameter matching computation are avoided; meanwhile enhances the esti-mation precision by a certain extent. The proposed algorithm can also be used in the multi-path and Doppler frequency shift complex channel, which expands its application scope. In a word, a demodulated DOA esti-mation algorithm is proposed and is applied to 1-D and 2-D angle estimation by dint of ULA and UCA re-spectively. The detailed theoretical analysis and adequate simulations are given to support our proposed al-gorithm, which enriches the theory of the FRFT.

On the Use of Uniform Circular Arrays for Characterizing UWB Time Reversal

Time reversal (TR) is a promising technique for the improvement of future telecommunication systems. A better understanding of this phenomenon requires deeper investigation of real propagation channels. This paper presents a new scheme to characterize the space-time focusing characteristics of time reversal. The originality of this technique relies on the use of a virtual uniform circular array, which allows to simply derive the space-time focusing characteristics of the channel. An extensive ultrawideband (UWB) propagation experiment is reported, and the method is applied to the experimental data. Different focusing parameters are computed in different configurations. In particular, the paper shows that the delay spread parameter is inappropriate to study time focusing, and a new parameter characterizing the maximum symbol rate is introduced. In a UWB single-input single-output (SISO) configuration, time reversal increases the power of the strongest path by up to 10 dB and the total received power by up to 5 dB. Our analysis shows that when the received signal is appropriately focused in time, the data rate can be increased by a factor of three. The received signal is also focused in space, and the average size of the focus area is approximately 3lambda , where lambda is the wavelength of the central frequency. We observed that the focusing spot is larger than the ideal case and is often directed inline with the transmitter direction. At 9 lambda away from the target, the received signal is attenuated by up to 10 dB in a non line-of-sight environment, which demonstrates the high focusing capabilities of the TR-UWB scheme.