A novel algorithm is proposed for estimating the direction of arrival (DOA) of linear frequency modulated (LFM) signals for the uniform circular array (UCA). Firstly, the UCA is transformed into an equivalent virtual uniform linear array (ULA) using the mode-space algorithm. Then, the short time Fourier transform (STFT) of each element's output is worked out. We can obtain the spatial time-frequency distribution matrix of the virtual ULA by selecting the single-source time-frequency (t-f) points in the t-f plane and then get the signal subspace of the array. The characteristics nature of the Bessel function allow us to obtain the multiple invariance (MI) of the virtual ULA. So the multiple rotational invariant equation of the array can be obtained and its closed-form solution can be worked out using the multi-least-squares (MLS) criterion. Finally, the two dimensional (2-D) DOA estimation of LFM signals for UCA can be obtained. Numerical simulation results illustrate that the UCA-STFT-MI-ESPRIT algorithm proposed in this paper can improve the estimation precision greatly compared with the traditional ESPRIT-like algorithms and has much lower computational complexity than the MUSIC-like algorithms.

This paper explores the potential of orbital angular momentum (OAM) multiplexing as a means to enable high-speed wireless transmission. OAM is a physical property of electro-magnetic waves that are characterized by a helical phase front in the propagation direction. Since the characteristic can be used to create multiple orthogonal channels, wireless transmission using OAM can enhance the wireless transmission rate. Comparisons with other wireless transmission technologies clarify that OAM multiplexing is particularly promising for point-to-point wireless transmission. We also clarify three major issues in OAM multiplexing: beam divergence, mode-dependent performance degradation, and reception (Rx) signal-to-noise-ratio (SNR) reduction. To mitigate mode-dependent performance degradation we first present a simple but practical Rx antenna design method. Exploiting the fact that there are specific location sets with phase differences of 90 or 180 degrees, the method allows each OAM mode to be received at its high SNR region. We also introduce two methods to address the Rx SNR reduction issue by exploiting the property of a Gaussian beam generated by multiple uniform circular arrays and by using a dielectric lens antenna. We confirm the feasibility of OAM multiplexing in a proof of concept experiment at 5.2 GHz. The effectiveness of the proposed Rx antenna design method is validated by computer simulations that use experimentally measured values. The two new Rx SNR enhancement methods are validated by computer simulations using wireless transmission at 60 GHz.

This paper deals with a beamforming and cell ID detection technique for a mobile station (MS) with multiple antenna arrays in millimeter wave (mm-wave) cellular communication systems. Multiple antenna arrays, required to cover the entire space around the MS, can be used to estimate the direction of arrivals (DoAs) and cell IDs, form beams in the direction of DoAs, select a serving cell in a cooperative manner, and improve BER performance by signal combining. However, a signal may enter the overlapped region formed by two adjacent arrays in the MS, resulting in a double-counting problem during the cell searching period. In this paper, a beamforming and cell detection technique without double-counting is proposed to handle this problem, and they are evaluated by simulation in a simple scenario of an mm-wave cellular system with spatial channel model (SCM).

The dramatic growth in wireless data traffic has triggered the investigation of fifth generation (5G) wireless communication systems. Small cells will play a very important role in 5G to meet the 5G requirements in spectral efficiency, energy savings, etc. In this paper, we investigate low complexity millimeter-wave communication systems with uniform circular arrays (UCAs) in line-of-sight (LOS) multiple-input multiple-output (MIMO) channels, which are used in fixed wireless access such as small cell wireless backhaul for 5G. First, we demonstrate that the MIMO channel matrices for UCAs in LOS-MIMO channels are circulant matrices. Next, we provide a detailed derivation of the unified optimal antenna placement which makes MIMO channel matrices orthogonal for 3×3 and 4×4 UCAs in LOS channels. We also derive simple analytical expressions of eigenvalues and capacity as a function of array design (link range and array diameters) for the concerned systems. Finally, based on the properties of circulant matrices, we propose a high performance low complexity LOS-MIMO precoding system that combines forward error correction (FEC) codes and spatial interleaver with the fixed IDFT precoding matrix. The proposed precoding system for UCAs does not require the channel knowledge for estimating the precoding matrix at the transmitter under the LOS condition, since the channel matrices are circulant ones for UCAs. Simulation results show that the proposed low complexity system is robust to various link ranges and can attain excellent performance in strong LOS environments and channel estimation errors.

The concept of massive multiple input multiple output (MIMO) has recently been proposed. It has been reported that using linear or planar arrays to implement massive MIMO yields narrow beams that can mitigate the interference signal even if interference cancellation techniques such as zero forcing (ZF) are not employed. In this work, we investigate the interference reduction performance achieved by circular array implemented massive MIMO in a real micro cell environment. The channel state information (CSI) is obtained by using a wideband channel sounder with cylindrical 96-element array in the 2-GHz band in an urban area. Circular arrays have much larger beamwidth and sidelobe level than linear arrays. In this paper, when considering the cylindrical array, the interference reduction performance between ZF and maximum ratio combining is compared when one desired user exists in the micro cell while the interference user moves around the adjacent cell. We show that ZF is essential for reducing the interference from the adjacent cell in the circular array based massive MIMO. The required number of antennas in the vertical and horizontal planes for the interference reduction is evaluated, in order to simplify the burden of signal processing for the ZF algorithm in massive MIMO. Because there are elements with low signal to noise power ratio (SNR) when considering cylindrical 96-element array, it is shown that the degradation of the signal to noise plus interference power ratio (SINR) when the number of antennas is reduced is smaller than that by ideal antenna gain reduction with a linear array. Moreover, we show that the appropriate antennas should be selected when a limited number of antennas is assumed, because the dominant waves arrive from certain specific directions.

In this paper, two receive beamforming techniques (Method 1 and Method 2) are proposed for a mobile station (MS) with multiple antenna arrays in an OFDM-based millimeter-wave (mm-wave) cellular communication system. Since the MS in mm-wave cellular communication requires fast processing due to its frequent movement and rotation, a receive beamforming technique with reduced computation complexity and processing time is proposed in Method 2. Of particular interest, estimation techniques for 2-dimensional (2D) direction-of-arrivals (DoAs) corresponding to each cell ID are proposed for uniform circular arrays (UCAs) and uniform rectangular arrays (URAs). Also, a cell selection technique for MSs with multiple antenna arrays is described that use the candidate cell IDs and parameters estimated for all antenna arrays to provide combining gain in addition to array gain in multipath channels. The proposed beamforming techniques are evaluated by computer simulation using a simple model of amm-wave cellular communication system with 3-dimensional spatial channel model (3D SCM).

A novel algorithm is presented for estimating the 3-D location (azimuth angle, elevation angle, and range) of multiple sources with a uniform circular array (UCA). Based on its centrosymmetric property, a UCA is divided into two subarrays. The steering vectors for these subarrays then yield a 2-D direction of arrival (DOA)-related rotational invariance property in the signal subspace, which enables 2-D DOA estimations using a generalized-ESPRIT algorithm. Based on the estimated 2-D DOAs, a range estimation can then be obtained for each source by defining the 1-D MUSIC spectrum. Despite its low computational complexity, the proposed algorithm can almost match the performance of the benchmark estimator 3-D MUSIC.

A novel closed-form algorithm is presented for estimating the 3-D location (azimuth angle, elevation angle, and range) of a single source in a uniform circular array (UCA) with a center sensor. Based on the centrosymmetry of the UCA and noncircularity of the source, the proposed algorithm decouples and estimates the 2-D direction of arrival (DOA), i.e. azimuth and elevation angles, and then estimates the range of the source. Notwithstanding a low computational complexity, the proposed algorithm provides an estimation performance close to that of the benchmark estimator 3-D MUSIC.

Mainly, a uniform linear array (ULA) has been used for DOA estimation of coherent signals because we can apply the spatial smoothing preprocessing (SSP) technique. However, estimation by a ULA has ambiguity due to the symmetry, and the estimation accuracy depends on the DOA. Although these problems can be solved by using a uniform circular array (UCA), we cannot estimate the DOA of coherent signals because the SSP technique cannot be applied directly to the UCA. In this paper, we propose to estimate 2-dimensional DOA (polar angles and azimuth angles) estimation of coherent signals using a cylindrical array which is composed of stacked UCAs.

The MUSIC (Multiple Signal Classification) technique with the circular array can estimate both elevational and azimuthal direction-of-arrival (DOA). This conventional method can not distinguish coherent signals, therefore, it can not estimate proper DOA in the presence of coherent signals. On the other hand, limited as to uniformly spaced linear arrays, the spatial smoothing technique is shown to be effective approach in decorrelating coherent signals. This scheme can not be applied directly to the nonlinear arrays. To overcome the coherent signal nonseparation problem in the nonlinear arrays, the approach using a linear interpolation technique has been proposed. However, this approach provides DOA estimates in one dimensional. In our proposed method, we use not only a linear interpolation technique for the circular array but also the symmetry of the circular array. The computer simulation is performed to demonstrate the usefulness of our method. As its result shows, the method can perform well even in the presence of coherent signals.

In this paper, we propose a new algorithm of enhancing covariance matrix estimate to be used for estimating the directions-of-arrival (DOAs) of multiple incoherent signals incident on a uniform circular array. The underlying covariance matrix possesses a special theoretical property such as having spatial stationarity. The proposed enhancement approach based on the use of this property is found to provide improved DOA estimates in comparison to the unenhanced MUSIC for narrowband incoherent signals.