The potential approaches to achieve channel reciprocity in FDD system with frequency correction algorithms

Channel reciprocity is an inherit feature of time division duplex (TDD) system, which is widely used to get uplink (UL)/downlink (DL) channel knowledge from DL/UL channel measurements without additional feedback. While in frequency division duplex (FDD) system, the transmitter usually obtains the DL channel state information (CSI) through a variety of feedback strategies in order to enhance the system performance. However, the impacts of feedback overhead and delay on Multiple-Input-Multiple-Output (MIMO) performance can´t be ignored. The paper focuses on the potential approaches to realize FDD channel reciprocity so as to reduce the feedback channel overhead. Compared with TDD system, UL and DL are allocated by different frequencies in FDD system thus the frequency correction is needed to achieve channel reciprocity. The paper presents the performance analysis and comparison of typical frequency correction algorithms, especially the algorithms based on the long-term statistical channel characteristics, which mainly include the channel covariance matrix, the channel space-time correlation, and the direction of arrival (DoA). The potential application scenario for each approach is presented as conclusion.