A novel angle of arrival tracking method in large-scale-array systems

In this paper, we investigate the problem of angle of arrival (AOA) tracking for the large-scale array in terahertz (THz) communication, which has a large size and a narrow beam, highly demanding an accurate angle estimation. On the one hand, the system usually adopts a hybrid structure with limited radio-frequency (RF) chains, which increases the difficulty of angle estimation; on the other hand, the rapid mobility of users also brings new challenges to angle estimation. To address the above challenges, a two-stage tracking framework is proposed in this paper, which employs the random phase matrix and orthogonal long pilots in the first stage to reduce the complicated multi-user estimation to multiple single-user estimations, followed by using both wide and narrow beams in the second stage to serve high-speed and low-speed users. Furthermore, a generalized-approximated-message-passing (GAMP) method is proposed for facilitating a low-accuracy estimation of the angles, followed by adopting a modified expectation-maximization (EM) algorithm based phase estimation to unbiased estimate the instantaneous angle with the help of high-gain characteristics of the beams. The proposed structure can not only simplify the estimation complexity, but also improve the estimation accuracy due to its capability of transferring the non-linear problem of angle observation into a linear gaussian model. In addition, the Kalman tracking framework is employed for performing a continuous angle tracking. Numerical results show that the angle estimation based on the random phase matrix in the initial stage can obtain a high enough estimation accuracy, while the GAMP algorithm implemented in the second stage can quickly capture the angle range under the Rayleigh limit. The performance of the proposed EM-based tracking method is shown to outperform the traditional extended Kalman filter (EKF) method.