Estimation of Fast-Fading Channels for Turbo Receivers With High-Order Modulation

This paper proposes an efficient, low-complexity, and near-optimal approach to pilot-assisted fast-fading channel estimation for single-carrier modulation with a turbo equalizer and a decoder. The proposed method is applicable to higher order modulation schemes, where the detector is sensitive to the estimation error. A doubly selective fast-fading channel is estimated using a fixed-lag Kalman filter (KF). The Kalman filtering is followed by a zero-phase low-pass filter, functioning as a smoother. A method for designing the smoother is introduced. Block processing is utilized to reduce the transient effects of the zero-phase filter (ZPF) at the edges of the symbol blocks. A first-order autoregressive (AR) model is fitted to the channel variations. For the case of 64-quadrature-amplitude modulation (QAM), a complex-exponential basis expansion model (CE-BEM) is exploited to capture the varying gains, and an AR(1) process is used to model the time evolution of the BEM coefficients. By virtue of the long memory of the smoother, the error-rate floor, which is commonly associated with low-order channel estimation models, is avoided. The potential for parallelism makes our approach well suited to multicore field implementations. The applicability of the method to 4-QAM, 16-QAM, and 64-QAM schemes is shown through simulation experiments.