Optically Centralized Multi-beamforming Technology For Millimeter-wave Communications

Millimeter-wave communication is not only a key technology for 5G,but also it will continue to be used in the future 6G.However,millimeter-wave has huge path loss.To compensate for the huge path loss,a combination of millimeter-wave and multiple-input multiple-output techniques is needed to obtain strong gain to resist large signal attenuation by using beamforming techniques.Generally,phase shifters are used to achieve beamforming.However,in millimeter-wave communication systems,beamforming with phase shifters will be affected by beam squint due to the large bandwidth of the signal,i.e.,the direction of the beam will be skew,which degrades the system performance.Moreover,optical true time delay(OTTD)-based beamforming can not only overcome the beam squint issue but also achieve ultra-broadband microwave operating frequency up to terahertz order.Therefore,in this thesis,the OTTD-based beamforming deployment scheme and the corresponding algorithms are proposed for the single-cell scenario and the multi-cell scenario,respectively.For single-cell scenarios,an OTTD-based sub-connected hybrid beamforming is proposed,in which the weight control module(WCM)is centrally deployed in the control module to realize resource reuse.Unlike the conventional sub-connected hybrid beamforming,the number of deployed WCMs in the proposed scheme is less than the number of subarrays.In addition,the OTTD-based sub-connected hybrid precoding algorithm is proposed,where we obtained the optimal precoding matrix by using the idea of successive interference cancellation.Then,we selected the suitable beam pattern from the predesigned codebook by using the orthogonal matching pursuit.Moreover,in the proposed algorithm,the suboptimal mode is used to serve the subarray when the optimal beam pattern is occupied.Furthermore,the obtained simulation results show that although the proposed OTTD-based sub-connected hybrid beamforming scheme only uses half of the beamforming resources,it can achieve near-optimal performance.For the multi-cell scenario,an OTTD-based multi-cell hybrid beamforming is proposed,where the beamforming networks of remote radio heads(RRHs)are decoupled from all RRHs.Then,these beamforming networks of all RRHs are deployed centrally in the baseband unit(BBU)pool.Due to the centralized deployment feature,the number of deployed WCMs is less than the number of RRHs.In addition,considering the limited capacity of the fronthaul link,we utilize coordinated beamforming to overcome inter-cell interference.Therefore,we proposed an OTTD-based multi-cell interference coordination algorithm,in which each RRH only transmits data to the users who it serves,i.e.,the BBU pool does not need to transmit the information of all users to all RRHs through the fronthaul link.Thus,the proposed scheme can mitigate the capacity pressure on the fronthaul link.The obtained results show that the proposed OTTD-based multicell hybrid beamforming has great advantages in dense deployment and can effectively relieve the capacity pressure of the fronthaul link.