Research On Transmission Schemes For 3D Channel Based MU-MIMO Systems

Multiple input multiple output (MIMO) is a key technique in future wireless communication systems. By setting up multiple antennas at the transmitter and/or receiver, we can exploit the spatial dimension out of the time and frequency dimensions and improve data transmission rate effectively. If we exploit new spatial degrees of freedom and schedule multiple users on a same time-frequency resource block, then it forms the multi-user (MU) transmission network which further enhances the resource efficiency. Traditional MU-MIMO techniques generally study on the 2-Dimensional (2D) horizontal plane. By adjusting the direction of transmit beams in the horizontal domain and employing beamforming techniques, multiple users can be served by the base station (BS) simultaneously. While there lies the limit that the all the transmit beams have the same downtilt. With the modernization of big cities, more and more high rises appear and users may be distributed within a wide vertical range. Therefore exploiting the vertical domain is essential. During recent years the 3-Dimensional (3D) MIMO techniques have attracted massive attention.3D techniques mean to utilize the whole spatial information and to deeply exploit the spatial degrees of freedom. Compared with 2D MU-MIMO,3D MU-MIMO system enables the simultaneous scheduling of users who stays in a same building but different floors and increase the multiuser transmission efficiency. This thesis studies the 3D channel based MU-MIMO transmission schemes, especially the downlink precoding techniques.Firstly, we study the physical layer of 3D MU-MIMO systems. The physical layer transmission model is described detailedly, including the downlink transmission model, the channel model, the system capacity and the beamforming technique. Having established the multiuser received signal model, we analyse the inter-user interference (IUI) and two general IUI elimination method. One is the downlink precoding, including the non-linear dirty paper coding which maximizes the system capacity and the linear precoding schemes which overcome the computation complexity. The other method is multiuser scheduling. Generally used scheduling principles include the maximum sum-rate, the round-robin, the proportional fair and etc.Secondly, this thesis introduces a double codebook based transmission scheme for 3D dual-polarized MU-MIMO systems. The scheme focuses on the downlink two-user transmission scenario and the BS de-ploys dual-polarized uniform planer antenna array. We introduce the Kronecker product based precoding codebook as the basic beam set and adopt the double codebook structure in the design of precoding matrix. The inner matrix and the outer matrix are long-term and short-term, separately. Then we propose a multiuser transmission scheme based on the double codebook structure, including the best companion cluster feedback scheme and the way to calculate the feedbacks. To save the calculation time of searching long-term codebook, we further propose a fast codebook searching method based on the quartering algorithm. The performance of the double codebook based transmission scheme is evaluated by simulation and numerical results show that this 3D MU scheme produces more throughput than 2D MU transmission schemes.Finally, we propose a joint space division multiple access (SDMA) and interference suppression mul-tiuser transmission scheme for millimeter-wave massive MIMO system. Multiple single-antenna users are served by BS simultaneously. We establish the physical and beamspace channel model and investigate the beamspace sparsity. Based on this sparsity, we propose a joint SDMA and zero-forcing (ZF) precoding scheme to eliminate the IUI in millimeter-wave massive MU-MIMO systems. The user set is divided into two subsets according to the beam overlap between each user pair. One user subset adopts a double precod-ing based SDMA scheme and the other subset adopts a ZF precoding based multiuser transmission scheme. We also investigate the extension of the proposed scheme to the multi-path broadcasting scenario. Simulation results show that the joint SDMA and interference suppression multiuser transmission scheme is an effective method to eliminate IUI and enhance multiuser system sum-rate with a small amount of computation and feedback.