Channel Calibration Methods For Massive MIMO BDMA Transmission Systems

The massive multiple-input multiple-output(MIMO)technology,which can significantly improve both the spectral efficiency and energy efficiency,is believed to be a promising technology for the next genera-tion of mobile communication systems.Based on the sparsity of the beam domain channel,different users are allocated with different beam sets for transmission via user scheduling in massive MIMO beam division multiple access(BDMA)transmission.Therefore,the multi-user MIMO channel can be equivalently decom-posed into multiple single-user MIMO channels,which reduces the transmission complexity.However,for practical transmission systems,the whole communication channel consists of not only the wireless propaga-tion channel,but also the radio frequency circuits of the antennas at both sides of the link.The radio frequency gain mismatch caused by radio frequency circuit results in amplitude mismatch and phase mismatch between different channels,which degrades the performance of BDMA transmission.To solve this problem,we in-vestigate the calibration methods for channel mismatch in massive MIMO BDMA transmission systems in this thesis.Firstly,the influences of channel mismatch at both ends of the transceiver on energy distribution of beam domain channel and ergodic achievable sum-rate upper bound of the transmission system are analyzed respectively.Analyses show that,on the one hand,the energy of beam domain channel does not spread under the influence of channel mismatch at the user terminals.The user-side phase mismatch does not affect the sum-rate upper bound,while the influence of amplitude mismatch is negligibly small.On the other hand,channel mismatch at the base station spreads the energy of beam domain channel and decreases the sum-rate upper bound.Within the normal range of amplitude and phase gains,phase mismatch at the base station is the main factor degrading the performance of BDMA transmission.Secondly,we propose a phase calibration method based on minimizing the sparseness of beam domain channel for receiving channel mismatch at the base station.According to the characteristics of BDMA trans-mission,it is unnecessary to obtain the accurate radio frequency gains in calibration.Instead,calibration criteria can be changed to minimize the sparseness of the beam domain channel.On this basis,the relation-ship between channel sparseness and l1-norm of corresponding vector is derived.Then,by utilizing the uplink channel state information obtained from channel estimation,the problem of minimizing channel sparseness is transfonned into the problem of l1-norm minimization of channel vector.The solution of the minimization problem is also presented.Simulation results demonstrate that the calibration matrix obtained by the pro-posed phase calibration method can reduce the energy spread of uplink beam domain channel and improve the sum-rate performance of BDMA uplink transmission.Thirdly,we propose a phase calibration method based on reciprocity calibration for transmitting channel mismatch at the base station.The phase calibration matrix of transmitting channels at the base station is obtained by multiplying the calibration matrix of receiving channels at the base station and the reciprocity calibration matrix.By utilizing the mutual coupling between antennas in a large-scale antenna array,a group-by-group reciprocity calibration algorithm is proposed.At first,the antennas are divided into several groups according to the form of antenna arrays as well as the strength of mutual coupling and then the calibration parameters of each group are calculated respectively.After that,the calibration parameters of each group are connected and the final reciprocity calibration matrix is obtained.Simulation results show that the proposed reciprocity calibration method can obtain high accuracy by choosing proper group size.After calibrating the transmitting channels at the base station,the performance of BDMA downlink transmission is significantly improved.Finally,we propose a hardware calibration method based on channel estimation.Calibration circuit is configured at the base station and control signals make the calibration system operate in three different modes.When the calibration system works under the receiving channel calibration mode,phase compensation is required at first and then receiving channel calibration matrix can be obtained from the receiving signals.When the calibration system works under the transmitting channel calibration mode,the signals sent by differ-ent antennas are transmitted to corresponding receiving channels through directional couplers.Based on the receiving signals,calibration matrix of transmitting channels can be obtained.After the channel calibration is completed,the system is operated under normal operating mode.Simulation results demonstrate that the hardware calibration method is of high accuracy.