Compensation of IQ imbalance and phase noise in OFDM systems

Nowadays, a lot of effort is spent on developing inexpensive orthogonal frequency-division multiplexing (OFDM) receivers. Especially, zero intermediate frequency (zero-IF) receivers are very appealing, because they avoid costly IF filters. However, zero-IF front-ends also introduce significant additional front-end distortion, such as IQ imbalance. Moreover, zero-IF does not solve the phase noise problem. Unfortunately, OFDM is very sensitive to the receiver nonidealities IQ imbalance and phase noise. Therefore, we developed a new estimation/compensation scheme to jointly combat the IQ imbalance and phase noise at baseband. In this letter, we describe the algorithms and present the performance results. Our compensation scheme eliminates the IQ imbalance based on one OFDM symbol and performs well in the presence of phase noise. The compensation scheme has a fast convergence and a small residual degradation: even for large IQ imbalance, the overall system performance for an OFDM-wireless local area network (WLAN) case study is within 0.6 dB of the optimal case. As such, our approach greatly relaxes the mismatch specifications and thus enables low-cost zero-IF receivers.

I. Introduction

Orthogonal frequency-division multiplexing (OFDM) [1] is a widely recognized and standardized modulation technique for broadband wireless systems. Because of its ability to elegantly cope with a multipath environment, it is used for wireless local area network (WLAN) [2], [3], digital audio broadcasting (DAB) [4], digital video broadcasting (DVB) [5], and fixed wireless access [6], etc. Recently, a lot of effort is spent in developing integrated, cost and power efficient OFDM receivers. The zero intermediate frequency (zero-IF) or direct-conversion receiver [7], as shown in Fig. 1, is an attractive candidate, since it avoids costly IF filters and allows for easier integration than the super-heterodyne structure. However, due to the absence of a digital IF, IQ demodulation is no longer handled in the digital, but in the analog domain. Therefore, the zero-if receiver introduces IQ imbalance. Moreover, phase noise still remains a strong issue in zero-IF receivers. Fig. 1. Zero-IF receiver structure.