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Method for transmitter I/Q imbalance correction with digital down-conversion

IP.com Disclosure Number: IPCOM000010030D
Publication Date: 2002-Oct-09
Document File: 3 page(s) / 703K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for transmitter in-phase/quadrature (I/Q) imbalance correction with digital down-conversion. Benefits include improved performance.

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Method for transmitter I/Q imbalance correction with digital down-conversion

Disclosed is a method for transmitter in-phase/quadrature (I/Q) imbalance correction with digital down-conversion. Benefits include improved performance.

Background

        � � � � � Direct-conversion transmitters and receivers are increasingly used as their low cost provides up and down conversion for applications such as cellular, wireless LAN, and broadband wireless. As these standards increasingly use more sophisticated modulation techniques, such as 16 and 64 QAM, the sensitivity of the receivers to small imbalances between the In-phase (I) and Quadrature (Q) arms of the conversion becomes a significant issue.

        � � � � � Amplitude and phase mismatch between the I and Q arms can affect performance. In addition, the mismatch between the I and Q arm filters can cause imbalance that is also a function of frequency.

        � � � � � Conventional modem approaches utilize digital frequency and phase correction in the digital domain after the analog-to-digital conversion. I/Q imbalance correction should implement this digital down-conversion approach, if possible. With digital down-conversion, a large frequency offset occurs that flows through the I/Q imbalance (mixer detuning) of the receiver. The transmitter imbalance becomes irrecoverable when the frequency offset is ~25 ppm.

General description

        � � � � � The disclosed method of correcting remote transmitter I/Q imbalance uses a VCO technique to recover phase directly at the I/Q converter. In practice, the use of a VCO to provide rapid frequency and phase feedback to the converter can be difficult during the preamble (which is several µseconds long). Because of the sensitivity of the VCO input to noise sources, extensive filtering is required to smooth the noise. This filtering results in a relatively slow response time (~milliseconds), which is not fast enough for tracking during the 11a preamble (~µseconds).

Advantages

        � � � � � The disclosed method provides an I/Q imbalance correction procedure, which results in improved performanc...