Impact and Mitigation of Multiantenna Analog Front-End Mismatch in Transmit Maximum Ratio Combining
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Impact and Mitigation of Multiantenna Analog Front-End Mismatch in Transmit Maximum Ratio Combining ´ 1 Andre´ Bourdoux,1 and Alain Barel4 Jian Liu,1, 2 Nadia Khaled,1, 3 Frederik Petre, 1 Interuniversity
Microelectronics Center (IMEC), Wireless Research, Kapeldreef 75, 3001 Leuven, Belgium ELEC-ETRO, Vrije Universiteit Brussel, 1050 Brussel, Belgium 3 E.E. Department, KULeuven, ESAT/INSYS, Kapeldreef 75, 3001 Leuven, Belgium 4 Department of ELEC, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussel, Belgium 2 Department
Received 20 December 2004; Revised 23 May 2005; Accepted 27 May 2005 Transmit maximum ratio combining (MRC) allows to extend the range of wireless local area networks (WLANs) by exploiting spatial diversity and array gains. These gains, however, depend on the availability of the channel state information (CSI). In this perspective, an open-loop approach in time-division-duplex (TDD) systems relies on channel reciprocity between up- and downlink to acquire the CSI. Although the propagation channel can be assumed to be reciprocal, the radio-frequency (RF) transceivers may exhibit amplitude and phase mismatches between the up- and downlink. In this contribution, we present a statistical analysis to assess the impact of these mismatches on the performance of transmit-MRC. Furthermore, we propose a novel mixed-signal calibration scheme to mitigate these mismatches, which allows to reduce the implementation loss to as little as a few tenths of a dB. Finally, we also demonstrate the feasibility of the proposed calibration scheme in a real-time wireless MIMO-OFDM prototyping platform. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
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INTRODUCTION
High-throughput (HT) wireless local area networks (WLANs) of the fourth-generation, the physical (PHY), and medium access control (MAC) layer of which is currently being standardized in the IEEE 802.11n task group [1] aim to significantly increase the data rate, to significantly improve the quality-of-service (QoS), and to significantly extend the range, compared to existing IEEE 802.11a/g type of WLANs. To satisfy these ambitious requirements over the highly space- and frequency-selective indoor propagation channel, multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) techniques perform low-complexity space-frequency processing to boost the spectral efficiency (and, hence, the data rate), as well as the performance (and, hence, the QoS and/or the range), compared to their single-antenna counterparts [2–4]. In this perspective, transmit maximum ratio combining (TX-MRC) is a simple yet powerful antenna diversity technique that allows to significantly extend the range by exploiting both (transmit) spatial diversity and (transmit) array gain [5, 6]. It is particularly attractive in multiple-input single-output (MISO) downlink scenarios, where the multiple-antenna access point would optimally weigh the transmit data stream across its antennas, such that
channel filtering leads to maximum receive signal
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