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 DLIUL 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.
Multiple antennas techniques have been widely adopted in broadband wireless communication systems to enhance spectral efficiency and achieve space diversity gain. Transmitter beam forming is one of promising multiple antenna techniques for IMT -Advanced system, which takes advantage of directional signal transmission or reception. For efficient transmitter beamforming, TDD (Time Division Duplex) system has an inherit advantage to exploit the channel reciprocity between uplink and downlink, and no additional channel feedback is needed. However, the mismatches of actual RF (ratio frequency) channel may cause channel non-reciprocity. This paper presents the latest progress on multiple antenna calibration, including the analysis of the hardware error characteristics, typical statistical models of the calibration error (CE), and the impacts on downlink beamforming due to CEo Furthermore, we present the performance analysis and comparison for widely used Base Station (BS) based calibration schemes. Finally, several possible terminal based calibration schemes are summarized to compensate the actual RF mismatches.
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