Traditional antenna calibration methods for time division duplex (TDD) systems assume that the free-space channel remains the same during calibration, which is unreasonable under the high-speed rail and other time-varying channel scenarios, and will cause calibration error due to time variability. This paper proposes an antenna calibration method for time-varying channels. In the proposed method, the transceiver first sequentially sends a pilot signal to obtain equivalent downlink and uplink channel responses. Then, by predicting the downlink (uplink) channel response fed back from the receiver using the channel prediction algorithm, the transmitter obtains the channel response corresponding to the channel response on uplink (downlink). Finally, the transmitter calculates the transmission calibration factor through the prediction value. Compared with the traditional antenna calibration method, this method can improve the accuracy of the calibration factor. Simulation results show that the performance degradation of antenna calibration can be caused by time-varying channels and the proposed method can well compensate for the performance loss and significantly improve the antenna calibration performance for time-varying channels.
For the uplink in the long-term evolution(LTE)systems,the physical random access channel(PRACH)adopts Zadoff–Chu(ZC) sequences as preamble sequences,and each user can choose a unique cyclic shift for user identification.In high-mobility scenario,the Doppler frequency shift raises up the false alarm probability by inducing the correlation peak offsetting at the receiver side.Traditional detection method utilizes multiple search windows for searching the potential uplink users.When the Doppler frequency shift is larger than the subcarrier spacing,the energy of correlation peak leaks outside the search window and increases the false alarm probability.In this paper,we present a novel design for PRACH preamble sequence for high-speed railway scenario.Through numerical observation,we find out that the offset of the correlation peaks only depends on the initial root sequence number and the length of ZC sequences.Therefore,the root number of ZC sequences should be carefully designed to generate a preamble sequence to fight against the Doppler frequency shift.Consequently,those offsets of the correlation peaks are concentrated within a single search window,resulting in lower complexity of detection and lower probability of false alarm for random access in LTE uplink.
