Research On Hybrid Beamforming Technologies Of Large-scale Antenna Array

The technology of multi-antenna is important for the current wireless communication system.With the development of demand for communication quality and the limitation of the available spectrum,the large-scale antenna array is inevitable for the future communication system.The increased number of antennas makes the hardware requirement and algorithms design of the large-scale antenna are different from the traditional multi-antenna technology.Therefore,the research on the large-scale antennas emerges.Because of the limitation on hardware,the large-scale antennas cannot employ the fully digital beamforming technologies directly.Hybrid beamforming,however,can reduce the number of radio frequency chains by digital beamforming combined with analog beamforming.The reduced cost of hardware in the large-scale antenna system makes the hybrid beamforming become necessary.Therefore,the thesis focuses on the hybrid beamforming in large-scale antenna array systems for different application scenario.The major contributions are as follows.(1)In multi-user systems,the hybrid beamforming with large-scale antenna array is often assigned in the base stations because of the limitation of the hardware and the computational complexity.Considering the adverse effect of ill-conditioned channel matrix,the thesis proposes two algorithms to improve the condition by analog beamforming.The first algorithm chooses a group of vectors from all array response vectors(ARVs).The choosed vectors have the smallest correlation with each other.Compared with the instantaneous channel state information(CSI),the ARVs are changed slowlier,which can reduce the cost of CSI estimation.The algorithm is based on the channel model of high-frequency communication systems.The second algorithm is more situable for the traditional multi-antenna systems.The virtual channel that combines analog processing and the physical channel is optimized to be well-conditioned for multi-user communication.The two proposed algorithms can be used in both uplink and downlink.They have their specific advantages in practical application.The second algorithm has slightly better performance,about 1 dB,in uplink,while both of the two algorithms can realize significantly better sum rates than the current algorithms in downlink,with less computational complexity.(2)Furthermore,the proposed two algorithms may not achieve the good performance enough for multi-cell systems.The inter-cell interference suppression in multi-cell systems may affect the optimized degree of ill-conditioned channel matrix.The method of user grouping before digital processing is proposed to improve the performance of hybrid beamforming further.The virtual channel vectors before digital processing with the smallest correlation with each other are chosen in one group for the digital zero-forcing(ZF)precoding.The interference between groups is suppressed by orthogonal projection.The groups are constituted one by one.The latter groups have no interference to the previous groups.The sum rates of the whole system can be improved.The proposed grouping algorithm can achieve better performance than the current algorithms with less computational complexity.Especially for the terribly ill-conditioned channel matrix,the performance can be improved about 10 dB.(3)The quantization of analog processing matrix can weaken the performance of hybrid beamforming.Oversampling is considered for digital beamforming to offset such the deficiency.However,oversampling will increase the computational complexity.For the single-cell and the multi-cell systems,two algorithms are proposed to reduce the complexity by hybrid beamforming.The proposed algorithms have the similar performance with the optimal digital beamforming,with less computational complexity by the design of parameter.(4)Finally,self-interference(SI)is the key factor for the performance of full duplex(FD)systems.The FD technology is also a hot area of research.Next,the base station is employed by a large-scale antenna array combined with FD communication.For the traditional FD communication,the fully digital beamforming technology can suppress SI and improve the performance well.But the hardware limitation and the high power consumption also make digital processing impractical for the practical large-scale antenna systems.In such a system,a hybrid beamforming transceiver is designed.Moreover,a corresponding algorithm is proposed to achieve the SI suppression and the multi-user communication.Compared with the fully digital processing scheme,the proposed design has significantly less computational complexity with the similar performance.