Research On Channel Reciprocity Of TDD System In 5G Background

In order to meet the increasing demand for mobile data in the future,the next generation mobile communication system must have higher spectral efficiency,wider transmission bandwidth and more intensive cells.The fifth generation wireless communication system will use large-scale MIMO technology to achieve higher spectrum efficiency and energy efficiency by configuring more antennas at BS,wider bandwidth by using higher frequency to avoid crowded low frequency band,and ultra-dense networking to achieve higher flow density.The TDD system can utilize the channel reciprocity to estimate the downlink CSI through the uplink pilot signal in the coherent time of the physical channel,which can reduce the overhead of feedback signaling and improve the throughput of the system,and will continue to play a role in the next generation communication system.However,in actual,the system's own hardware impairment,RF front-end mismatch,mutual coupling between antennas and channel estimation errors,and natural factors such as atmospheric ducts make TDD systems unable to effectively utilize channel reciprocity.Aiming at the system's own problems,the generalized channel model is established by modeling the hardware impairment of the transmitter and receiver,the mismatch of the RF front end,and the mutual coupling between multiple antennas.The influence of channel estimation on the system performance is analyzed,the mean square error of channel estimation is derived,and the closed expression of the uplink and downlink channel capacity is obtained.For the issue of reciprocity calibration,in order to overcome the shortcomings of traditional reciprocity calibration schemes,a calibration scheme with on-line calibration,small impact on system performance and high accuracy is designed based on the established mutual coupling model and the power control function of base stations.Among natural factors,the cross-link interference between long-distance base stations in atmospheric duct environment makes the TDD system unable to estimate the uplink channel accurately,thus making the system unable to utilize channel reciprocity.In order to analyze the impact of the atmospheric ducts on the communication system,the modeling problems of terrain,backward reflection and atmospheric absorption are solved,and the electromagnetic wave propagation simulation platform is built.The simulation results show that whether the system can utilizes channel reciprocity effectively depends on meteorological parameters,atmospheric underlying surface and the antenna configuration.