Estimation And Compensation Of IQ Imbalance In OFDM Systems

The orthogonal frequency division multiplexing (OFDM) has become the key technique in the broadband mobile communication system for its high spectral efficiency and robustness a-gainst multi-path fading. Direct-conversion receiver has drawn a lot of attention in recent years due to its small circuit-size and low power consumption. However, the IQ imbalance has severely limited its performance. As is known that OFDM system is very sensitive to the orthogonality between the sub-carriers, unfortunately, IQ imbalance will result in image interference and thus destroy the orthogonality between sub-carriers of the system. In addition, the IQ imbalance, either at transmitter (TX) or receiver (RX), will cause the CFO (Carrier Frequency Offset) esti-mation and compensation to be inaccurate. The CFO will in turn limit estimation performance of the IQ imbalance. Besides, the multi-path in the actual system will result in the frequency selective fading and thus entangle with the ICI (Inter-carrier-interference) caused by the CFO and IQ imbalance, so the performance of the OFDM system will be further reduced. This paper will focus on the research of IQ imbalance estimation algorithm for OFDM systems when CFO and channel fading both exist. The main contents are organized as follows:(1) To make the derivation of the proposed algorithm clear, the direct conversion transceiver model with both TX/RX I/Q imbalance and CFO (Carrier Frequency Offset) is described in this paper. By comparing the constellations of frequency-dependent (FD) and frequency-independent (FI) IQ imbalance, the difference between these two kinds of IQ imbalance is thoroughly ana-lyzed. The influence on CFO that is caused by the IQ imbalance is also investigated and the impact on the OFDM system is analyzed when IQ imbalance and CFO coexist.(2) For OFDM systems with only FI IQ imbalance in the receiver, a novel method is pro-posed to accurately estimate the channel and IQ imbalance separately. This method is based on the idea that without IQ imbalance, the channel has a smooth frequency response. The method performs well when there is no CFO in the system, but the performance decreases rapidly when CFO exists. In order to improve the performance when CFO is entangled with the IQ imbalance, an innovative pilot is proposed. The IQ imbalance and CFO can be estimated based on the pilot’s time-domain information and the channel parameters can be estimated with its frequency-domain information. Simulation results verify the effectiveness of the approach.(3) For OFDM systems with both FI and FD IQ imbalance in the transceiver, a pilot-based estimation and compensation scheme is proposed which can accurately estimate TX and RX IQ imbalances and CFO. The pilot is designed based on the property of the influence that is caused by the IQ imbalance. With the structure of the pilot’s time-domain, the RX IQ imbalance and CFO can be estimated without the influence of the TX IQ imbalance. With the structure of the pilot’s frequency-domain, the TX IQ imbalance and channel can be estimated separately. Simulation results show that the performance of the whole system is significantly improved, which is very close to the ideal case.