Inter-Access Point Synchronization and Over-The-Air Reciprocal Calibration for Cell-Free Massive MIMO Systems
The performance of coherent joint transmission(CJT)techniques with multiple access points in wireless communication systems is highly dependent on known channel state information(CSI)at the transmitter.To avoid CSI feedback,it is generally assumed that systems operate in time division duplex(TDD)mode,and then the downlink CSI is obtained according to the uplink sounding signals by using the reci-procity of the uplink and downlink channels.However,due to the synchronization error between access points(AP)and the non-ideal hard-ware factors,the uplink and downlink channels are not reciprocal.In this paper,the time-frequency synchronization of the wireless network and over-the-air(OTA)calibration technologies for cell-free massive MIMO are studied.Based on the commercial remote radio unit(RRU)and synchronization technology,synchronization and calibration schemes for large-scale network are discussed.Finally,based on the ex-perimental system,the performance requirements of CJT for synchronization and calibration are discussed,as well as the implementation of cell-free massive MIMO and the antenna configuration of the AP,and the prediction of the phase of the calibration coefficient is studied.
NETL
NSTL
万方数据
