Noncircular Measurement and Mitigation of $I/Q$ Imbalance for OFDM-Based WLAN Transmitters

Abstract

In future high-speed communication networks, the in-phase/quadrature (I/Q) imbalance mitigation and oscillator drift compensation is a key issue in the design of orthogonal frequency division multiplexing (OFDM)-based wireless LAN (WLAN) transmitters. To this end, we propose a two-stage I/Q imbalance measurement method, where by virtue of the WLAN standard-compliant training sequences, a coarse I/Q imbalance estimation is initially performed jointly with channel equalization. This makes it possible to decouple the effects of frequency-selective channels from the exact amplitude and phase imbalances induced by the local oscillator. Next, the so recovered symbols in DATA field of standardized OFDM systems, such as the IEEE 802.11ac, are recalibrated using a decision-directed scheme; this facilitates least squares-based fine I/Q imbalance estimation. For rigor, augmented complex statistics is employed to account for the effects of data noncircularity and widely linear natures of communication channels. Computer simulations and real world experiments based on the IEEE 802.11ac compliant signals demonstrate the high accuracy of the proposed technique for OFDM-based WLAN transmitters.