Channel State Information Acquisition And Calibration In Massive MIMO

Massive MIMO has grown up to be a hot topic in both academia and in-dustry as one of the foremost candidate technologies for 5G.Recent studies have shown that massive MIMO systems with tens or hundreds of antennas at the base station have new features,such as the energy efficiency could be made proportional to the number of antennas and the channel capacity can be increased by supporting a large number of users simultaneously.Channel state information plays a critical role in a massive MIMO system.However,im-perfect channel state information can limit the performance of massive MIMO systems.In order to exploit the full potential of massive MIMO based on TDD,we focus on channel state acquisition and channel reciprocity calibration in this paper.The main innovations and contributions of the paper are as follows.1)We propose a new iterative grouping calibration algorithm for channel recip(?)city in MU-MIMO systems.We set a base station with one antenna to be the reference user,then we divide the MIMO BS antennas into N groups.We assume users and every antenna group reform a new user-antenna group,the scope of transceiver circuitry coherence time in different user-antenna group is different.We only calibrate one user-antenna group in one free-space electro-magnetic channel coherence time.The iterative grouping calibration algorithm can improve the channel capacity by transmitting pilot and signal at the same time slot.And the iterative grouping calibration algorithm goes especially well with massive MIMO systems and high-speed mobile environment for its flexi-ble antenna grouping.2)We introduce an improved relative channel reciprocity calibration method for TDD massive MIMO systems.The improved calibration method divides the massive MIMO system into one SISO system and two SIMO systems instead of calibrating the massive MIMO directly.We select the channel with high channel gain to be the reference channel.The antennas of the SISO system are the reference antennas.At the same station the reference antenna and other antennas consist the SIMO system.The three systems include all the receiver circuitry and the transmitter circuitry information about the massive MIMO sys-tem.We calibrate the three systems and acquire the calibration coefficients.Then we use the calibration coefficients of the three systems to calibrate the massive MIMO system.The proposed calibration method in this paper can re-duce the amount of feedback and the computation complexity compared with calibrating the massive MIMO directly.What' s more,the improved calibra-tion method outperforms the traditional calibration method under high-speed mobile environment.3)This paper studies the channel reciprocity calibration problem based on minimum mean square error in a 7-Cell network MIMO system.Network MIMO system can mitigate the inter-cell interference by joint precoding tech-niques.However,mismatches between the transceiver modules of the antennas destroy the channel reciprocity in TDD mode.Therefore the mismatches should be calibrated before channel reciprocity can be used.An effective method for calculating the calibration coefficients in the network MIMO system whose base stations are equipped with multiple antennas is proposed.The 7-Cell net-work MIMO system is divided into 7 independent centralized MIMO systems and one distributed MIMO system,then the calibration coefficients of the eight systems can be brought together to calculate the calibration coefficients of the 7-Cell network MIMO system.The validity of the method is evidenced by the simulation results.