OTA Calibration Method And Implementation Of Uplink And Downlink Reciprocity In TDD-MIMO System Without Digital Transmission

In time division duplexing(TDD)wireless communication system,there is a very important characteristic called channel reciprocity.The basic idea of this feature is that in the time of channel coherence,the Base Station(BS)estimates the channel state information(CSI)of the received signal,and it can be considered that the estimated information is consistent with the downlink channel state information,so it can determine the method for downlink transmission.However,the actual channel is composed of the air channel and the physical channel.The air channel satisfies the reciprocity characteristic,while the physical channel is not reciprocal because of the differences in technology between the transceiver and receiver links,thus leading to the overall channel is not reciprocal.In order to calibrate the reciprocity of the actual channel,there are many in-depth studies at home and abroad,and there are many calibration algorithms.But these algorithms still need to be improved.To solve the above problems,we adopt the TDD-MIMO(Multiple-Input Multiple-Output)system upstream and downstream reciprocity air transmission(OTA)calibration method for channel reciprocity calibration,which has advantages.This method is also an OTA calibration algorithm based on Accessory Device(ACD),but it solves the shortcomings of the popular OTA calibration algorithm based on auxiliary devices.Firstly,the digital transmission in this method means that the auxiliary equipment does not need to estimate the downlink channel state information,so it does not need to transmit the downlink channel state information to the base station,which can reduce the overhead.Then,in order to reduce the calibration time,the calibration can also be completed in a very short time in the case of multiple antennas.This method uses Multi-Orthogonal carriers as pilot signals to make the base station communicate with auxiliary equipment,so the overall calibration time is one downlink slot and one upstream slot.For multi-antenna systems,the improvement of calibration efficiency is very obvious.Finally,the method uses frequency domain information to calibrate.It only needs FFT(Fast Fourier Transform)operation on any received time domain signal to calculate the relevantparameters of the corresponding frequency signal,so the frame synchronization operation of the time domain signal can be avoided and the calibration steps can be simplified.The OTA channel reciprocity calibration method is programmed on Universal Software Radio Peripheral(USRP)produced by NI Company.The experimental results verify the feasibility of calibration and the advantages mentioned above.