Resource Allocation Strategy For Three-Dimensional Massive MIMO Systems

As a key technique in fifth generation(5G)communications,massive multiple input multiple output(MIMO),equipped with amounts of antennas,can obtain enough degree of freedom(DoF),and improve the spectral efficiency and reliability of data link.Compared with the traditional uniform linear array(ULA)in massive MIMO systems,by deploying uniform planar array(UPA)at base stations(BSs),three-dimensional(3D)massive MIMO systems can not only save the space of BSs,but further improve the system performance by utilizing the DoF in elevation.However,there are many problems in 3D massive MIMO systems,e.g.,the cell-wide coverage of public signals in the broadcasting channel,the channel esti-mation in uplink transmission,the inter-user interference and user scheduling in downlink transmission,et al.In this thesis,we emphasis on the resource allocation for 3D massive MIMO systems.The main contributions obtained in this thesis are summarized as follows.1.Aiming at the problem that broadcasting techniques in single antenna and MIMO sys-tem are not suitable for 3D massive MIMO systems,the omnidirectional transmission for 3D massive MIMO systems is investigated.Combined with 3D spatial channel charac-teristics,departing from three aspects as achievable rate maximization,constant received power in angle domain and the sufficient utilization of power amplifiers,several criteria are introduced to realize the omnidirectional transmission.On this basis,we propose two omni-directional space-time block codes(STBCs),a discrete omnidirectional STBC based on the Zadoff-Chu(ZC)sequences and a consecutive omnidirectional STBC design based on the two-dimensional orthogonal complementary codes.The discrete omnidirectional STBC can realize constant received power at finite discrete angles,and the number of discrete angles is the same as the number of transmit antennas,so the antenna number at BSs should be large to ensure enough angles with constant received power.The consecutive omnidirectional STBC can realize constant received sum power at any angle in one STBC transmission and the number of BS antennas does not impact the broadcasting performance of it apparent-ly.Simulation results verify that both designs can realize full diversity order,of which the consecutive one has better omnidirectionality.2.Aiming at problems that the pilot contamination(PC)is severe in TDD mode and channel feedback overhead is large,we investigate the pilot assignment and channel estimate for 3D massive MIMO systems.Firstly,a soft pilot reuse scheme based on the geographical loca-tions is proposed for TDD mode.By exploiting the elevation DoF,the users with different elevation channel state information(CSI)are regarded as center users or edge users,respec-tively.Edge users in adjacent cells are assigned orthogonal pilot sequences to eliminate the PC,while center users in all cells reuse the pilot sequences and suppress the PC through improved channel estimate method.On this basis,to further decrease the pilot overhead,a pilot reuse scheme based on 3D spatial correlation is proposed.Same pilots are reused for cell center users who have little spatial correlation with each other,while multi-cell coordi-nated pilot reuse are conducted for cell edge users.In this way,the pilot contamination can be suppressed,and the accuracy of channel estimate can be improved.In addition,we fur-ther investigate the downlink CSI estimation in FDD mode.By utilizing the partial channel reciprocity between uplink and downlink channel,we extract the reciprocity features,i.e.,the directions of main beams,from the uplink channel statistical information.Then,the BS sends pilots in these beams,and users feed the equivalent channel matrix,i.e.,the power and phase of main beams,back to the BS based on the pilot information.Finally,the BS can achieve complete downlink CSI with smaller feedback and pilot overhead.3.Aiming at the problem that the complexity and overhead are high for multi-cell user scheduling and precoding in 3D massive MIMO systems,user scheduling and precoding with lower complexity and overhead in downlink transmission are investigated.Multi-cell edge user scheduling scheme is proposed to decrease the inter-edge-user interference.By re-allocating edge users who have similar channel information to one cell,or reallocating edge users to a cell in which the interference to other users is least,the interference suppression of edge users can be realized.Moreover,we present three categories of edge user schedul-ing strategies,based on K-Means algorithm,signal to leakage and noise ratio(SLNR)and geographical locations,respectively.On this basis,we investigated the downlink precod-ing scheme for 3D massive MIMO systems.A multi-layer precoding scheme based on joint spatial division and multiplexing(JSDM)is further proposed,which can realize inter-cell in-terference elimination,user grouping and intra-group interference elimination through each layer precoding.Meanwhile,with the precoding scheme,the dimension of channel matrix can be reduced,and the feedback and downlink pilot overhead can be further decreased.