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.
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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.
Millimeter-wave (mmW) communication has great potential in expanding channel capacity, increasing transmission rates, enhancing anti-interference capabilities, and reducing delay. Therefore, it has been the subject of much attention and research, and it has been adopted as a core technology of 5G. So far, researchers have experimentally verified that high throughput can be achieved with 5G mmWs. However, compared to those of traditional medium-and low-frequency base stations, the propagation characteristics of mmWs have caused link instability and interference problems in large-scale networks, which brings new challenges to the industrial capability of the mmW band and puts forward new requirements for the performance evaluation of mmW base stations. In this paper, based on the signal characteristics of the mmW band and applicable scenarios of mmW base stations, a performance evaluation method for mmW base stations in commercial networks is comprehensively presented. Using the evaluation system described in this paper, we conduct measurements on commercial mmW base stations, evaluate the performance of these stations, and provide comprehensive guidance for their design and actual deployment.
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