Research On Distributed Massive MIMO In Multi-Tier Heterogeneous Multi-Cell System

The novel network structure and massive network densification techniques have the potential to satisfy the development requriemnts for the next generation of mobile communications. Compared with the traditional MIMO system, the distributed MIMO system which is a new architecture of the future wireless access network has better performances of system capacity and coverage. The massive MIMO and small cell are the two typical massive network densifications techniques. In the thesis, we focus on the researches of the multi-cell, multi-user distributed MIMO, massive MIMO and dense deployment of small cells techniques. We mainly studied the system capacity, the precoding scheme for interference suppression and the newly proposed multi-tier heterogeneous network architecture in multi-cell, multi-user distributed MIMO system.Firstly, MIMO techniques in mobile communication systems are summarized; the characters, advantages and challenges of Massive MIMO and small cell technology are introduced. Besides, the channel modeling of the distributed MIMO is described in detail and several typical linear precoding schemes in the MU-MIMO system are discussed.Secondly, the downlink capacity of multi-user distributed MIMO system in the case of large-scale fading is studied. The theoretical channel capacity of distributed MIMO is analyzed combined with its influencing factors. A distributed MIMO channel modeling based on spatially correlated shadowing is proposed. The downlink capacities of the system with different power allocation algorithms are derived theoretically and then the impacts of power allocation and number of users on the performances of CAS and DAS are studied. Simulation results verify the feasibility of the proposed channel model and indicate that the impact of large-scale fading can be reduced greatly in the DAS so that the system capacity will be enhanced significantly. A better system performance can be achieved by using the water-filling power allocation algorithm at the transmitter. The system performance based on MMSE is improved to some extent compared with that based on ZFBF, especially in low SNR conditions.Thirdly, the downlink capacity of multi-cell distributed MIMO system is studied. Based on typical interference surpression skills at transmitter, the precoding schemes of single cell mode (SC-ZF) and multi-cell cooperative mode (JT-ZF) are provided. An improved precoding method based on the effective channel (EH-ZF) is proposed in this thesis. The expressions of multi-user system sum-rate of different precoding methods under different power allocation schemes are derived theoretically. Simulation results demonstrate that the performance of the multi-cell distributed MIMO system using EH-ZF is significantly enhanced compared with that of the system using SC-ZF; meanwhile, the feasibility of the MIMO system is also improved when EH-ZF method is adopted.Finally, the MIMO system with multi-tier heterogeneous network architecture which is the combination of massive MIMO and dense deployment of small cell techniques is researched detailedly. The researches focus on the capacity regions of multi-cell multi-user MIMO systems under the R-TDD and TDD protocols. Modified precoding schemes (M-RZF and MZF) based on RZF and ZFBF separately are proposed. The implementation algorithms of M-RZF and MZF depend on the local information and the channel reciprocity of TDD system. Simulation results show that the performance of the new two-tier network architecture adopting the proposed precoding scheme M-RZF and MZF is enhanced obviously without any forms of exchange of CSI and user data.