CFO and channel estimation in single user and multi-users OFDM systems
Orthogonal frequency division multiplexing (OFDM) systems are sensitive to the carrier frequency offset (CFO) caused by mismatch of the local oscillator in transceivers and Doppler shifts due to mobility. The inter-carrier interference (ICI) induced by CFO destroys the orthogonality between subcarriers and causes significant performance degradation, so it is critical to estimate the CFO at the receiver and compensate its effects.
For single user OFDM system, CFO estimation has been extensively studied. However, few works considered the effect of unknown timing offset in the signal. Without accurate timing, most of the existing CFO estimators fail. In this topic, we derived and analyzed joint CFO and channel estimators that are robust to timing error, under Maximum Likelihood and Bayesian framework, respectively. We have also studied the effect of I/Q imbalance on the estimation of CFO and channel. Furthermore, a joint CFO and channel estimator for OFDM-based two-way relay network has also been proposed in a recent work.
For multi-users OFDM systems (OFDMA / MIMO-OFDM), the main difficulty lies in the estimation of multiple CFOs from different users. Conventionally, alternative projection, and EM-algorithms are frequently used for multi-parameters estimation. However, there is no guarantee that these algorithms will converge to the global optimal solution. In this work, we propose an importance sampling based estimator, which can guarantee to obtain the global optimal solution in a multi-parameter estimation problem.
Xun Cai, Yik-Chung Wu, Hai Lin and Katsumi Yamashita, “Estimation and Compensation of CFO and I/Q Imbalance in OFDM Systems under Timing Ambiguity,” IEEE Trans. on Vehicular Technology, Vol.60, no.3, pp.1200-1205, Mar 2011. [Preprint, IEEE Xplore]
Gongpu Wang, Feifei Gao, Yik-Chung Wu and Chintha Tellambura, “Joint CFO and Channel Estimation for OFDM-Based Two-Way Relay Network,” IEEE Trans. on Wireless Communications, Vol.10, no.2, pp.456-465, Feb 2011. [IEEE Xplore]
Kun Cai, Xiao Li, Jian Du, Yik-Chung Wu and Feifei Gao, “CFO Estimation in OFDM Systems under Timing and Channel Length Uncertainties with Model Averaging,” IEEE Trans. on Wireless Communications, Vol. 9, no. 3, pp. 970-974, Mar 2010. [PDF]
Jianwu Chen, Yik-Chung Wu, S. C. Chan and Tung-Sang Ng, “Joint maximum-likelihood CFO and channel estimation for OFDMA uplink using importance sampling,” IEEE Trans. on Vehicular Technology, vol. 57, no. 6, pp.3462-3470, Nov. 2008. [PDF]
Jianwu Chen, Yik-Chung Wu, Shaodan Ma and Tung-Sang Ng, “Joint CFO and channel estimation for multiuser MIMO-OFDM systems with optimal training sequences,” IEEE Trans. on Signal Processing, vol. 56, no. 8, pp. 4008-4019, Aug 08. [PDF]
Jianwu Chen, Yik-Chung Wu, Shaodan Ma and Tung-Sang Ng, “ML Joint CFO and Channel Estimation in OFDM systems with Timing Ambiguity,” IEEE Trans. on Wireless Communications, vol. 7, no. 7, pp. 2436-2440, Jul 08. [PDF]
Yik-Chung Wu and E. Serpedin, ``Comments on ``Class of Cyclic-Based Estimators for Frequency-Offset Estimation of OFDM Systems'' '', IEEE Trans. on Communications, vol. 53, no. 3, pp. 413-414, Mar. 2005. [PDF]