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An IQ Imbalance Compensation for OFDM and Quadrature Receivers

IP.com Disclosure Number: IPCOM000010607D
Original Publication Date: 2002-Dec-20
Included in the Prior Art Database: 2002-Dec-20
Document File: 3 page(s) / 58K

Publishing Venue

Motorola

Related People

Jing Fang: AUTHOR [+2]

Abstract

In quadrature transceiver system, IQ path mismatch exists and can cause severe signal degradation. The frequency offset between the receiver LO and the transmitter LO can add another complication to receiver IQ imbalance compensation. For systems such as OFDM in 802.11a and 802.11g, the number of sub-carriers used in the imbalance compensation has to be large enough to achieve an acceptable performance. The IQ imbalance compensation with such a large number of sub-carriers requires high cost of silicon gates and operation power. . Many approaches have been proposed and used to compensate IQ imbalance. Among them, low IF based, wideband based, adaptive LMS and RLS based, baseband digital frequency domain compensation are commonly used. For direct conversion OFDM receiver, these algorithms are either not applicable, or are associated with poor accuracy, or have high power consumption. Unlike traditional approaches, this invention compensates IQ imbalance in frequency domain with simple linear combination of a symbol generated from forward frequency offset correction and a symbol generated from backward frequency offset correction, where the weights in the linear combination can be generated from off-line calibration . The simulation indicates that this IQ imbalance provides an improved Packet Error Rate (PER) performance for the OFDM system in WLAN11a and 11.g and allows wide range of IQ imbalance range in the RF front-end.

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An IQ Imbalance Compensation for OFDM and Quadrature Receivers

 Jing Fang and Weizhong Chen

Abstract

In quadrature transceiver system, IQ path mismatch exists and can cause severe signal degradation.   The frequency offset between the receiver LO and the transmitter LO can add another complication to receiver IQ imbalance compensation. For systems such as OFDM in 802.11a and 802.11g, the number of sub-carriers used in the imbalance compensation has to be large enough to achieve an acceptable performance. The IQ imbalance compensation with such a large number of sub-carriers requires high cost of silicon gates and operation power. .  Many approaches have been proposed and used to compensate IQ imbalance.  Among them, low IF based, wideband based, adaptive LMS and RLS based, baseband digital frequency domain compensation are commonly used.  For direct conversion OFDM receiver, these algorithms are either not applicable, or are associated with poor accuracy, or have high power consumption. Unlike traditional approaches, this invention compensates IQ imbalance in frequency domain with simple linear combination of a  symbol generated from forward frequency offset correction and a symbol generated from backward frequency offset correction, where the weights in the linear combination can be generated from off-line calibration .   The simulation indicates that this IQ imbalance provides an improved Packet Error Rate (PER) performance for the OFDM system in WLAN11a and 11.g and allows wide range of IQ imbalance range in the RF front-end. 

 

The Problem

In the receiver, IQ  imbalance can occur due to the mismatch between devices such as the splitters, LO injections, mixers, baseband filters, and A/D converters.  The imbalance can cause severe signal degradation as in the WLAN OFDM system.  Since the imbalance is qu...