Research On Broad Beamforming Technologies In Massive MIMO Systems

As one of the key technologies in the 5th generation mobile communication systems(5G),massive multiple-input multiple-output(MIMO)has great advantages in improving spectrum efficiency and energy efficiency.As an important way to improve the performance of the physical layer in wireless communications,massive MIMO beamforming has been widely studied in 5G communications.However,there are still many challenges of massive MIMO beamforming applied in 5G communication scenarios,such as the beam squint problem of narrow beam caused by frequency variation and the application problem of narrow beam in specific scenarios.This dissertation studies broad beamforming technologies for massive MIMO systems.Main work and contributions are summarized as follows.· For the problem of unfair communications along the position caused by Doppler shift and path loss in massive MIMO high-speed railway communication systems,a location-fair transmission scheme based on sectorized broad beam is proposed.First,an optimization problem for the broad beam is formulated to maximize the average received power within the beam coverage with limited variation in the received power at different locations.The beam optimization problem can be solved by linear programming.Subsequently,the broad beamforming design is extended to the scenario where two trains are located in different sectors simultaneously,an optimization problem for the broad beam pair is formulated to maximize the sum minimum transmission rate with the suppressed average interference and limited variation in the received signal-to-interference-plus-noise ratio.Alternatively,the objective function is shifted to minimizing the power consumption,for which a greedy iterative algorithm to derive the optimal solution is proposed.Through numerical results,compared with the discrete Fourier transformation precoding,the proposed scheme can effectively improve the communication fairness and system throughput in the presence of Doppler shift,and effectively reduce the channel quality indicator feedback overhead.· For the dedicated coverage in massive MIMO high-speed railway communica- tions,a joint optimization scheme of elliptical cell based broad beamforming and power allocation is proposed.An optimization problem for the broad beamforming design is formulated to maximize the received power at the elliptical cell edge subject to the equal received power at different location of cell edge.The optimal beam can be obtained by selecting the appropriate eccentricity.Subsequently,an improved power allocation is proposed,the parameter adjustment allows the system coverage and energy efficiency to be adjusted across all power allocation schemes.Numerical results shows that the proposed beamforming design covers longer railway than the linear cell beamforming,and the coverage of elliptical cell based beamforming can be increased with the eccentricity.Moreover,proposed broad beamforming design with appropriately chosen BS location can achieve the larger coverage distance.In addition,beamforming with the improved power allocation can further improve the railway coverage and guarantee the energy efficiency.· For the beam squint compensation problem in massive MIMO millimeter wave wideband communications,a broad analog beamforming scheme is proposed.A broad beamforming optimization problem is formulated to maximize the average beam gain in the bandwidth with little variation of the beam gain over frequencies within the bandwidth.The Semidefinite Relaxation(SDR)is developed to solve the optimization problem.When the rank of SDR solution is not equal to 1,an efficient rank reduction algorithm is proposed to obtain the beam vectors,and a space-time block code(STBC)based beamforming transmission scheme is proposed.Through numerical results,the proposed STBC based broad beamforming transmission scheme can compensate for the beam squint effectively and improve system throughput under some conditions.