Frequency-domain Equalization of Single Carrier Transmissions over Doubly Selective Channels

Wireless communication systems targeting at broadband and mobile transmissions commonly face the challenge of fading channels that are both time and frequency selective. Therefore, design of effective equalization and estimation algorithms for such channels becomes a fundamental problem. Although multi-carrier transmissions demonstrate prominent potential to combat doubly selective fading, several factors may retard their applications, such as: high peak-to-average power ratio, sensitivity to phase noise, etc. Meanwhile, single-carrier transmission is a conventional approach and has important applications, such as HDTV broadcasting, underwater acoustic communication. In this dissertation, we focus on receiver design for single-carrier transmissions. Our goal is to design and develop a group of channel estimation and equalization algorithms in the frequency-domain, which enable high performance and low complexity reception of single-carrier transmissions through doubly selective channels. For single-carrier transmissions over moderately fast fading channels with longdelay spread, we present an improved iterative frequency-domain equalization (IFDE) algorithm based on soft-interference-cancellation (SIC) and propose a novel adaptive frequency-domain channel estimation (AFDCE) based on soft-input Kalman filter, where soft information feedback from the IFDE can be exploited in the channel estimator. Simulation results show that, compared to other existing schemes, the ii proposed scheme offers lower MSE in channel prediction, lower BER after decoding, and robustness to non-stationary channels. We extend the IFDE/AFDCE scheme to accommodate the application of digital television (DTV) signal reception. Compared with the traditional joint decision feedback equalization (DFE) /decoding plus frequency-domain least-mean-square (FDLMS) channel estimation approach, the proposed scheme achieves better performance at a fraction of the implementation cost. For very fast fading large-delay-spread channels, traditional FDE methods fail, because channel variation within a FFT block induces significant off-main-diagonal coefficients in the frequency domain. To conquer the problem, we apply Doppler channel shortening to shape the energy distribution of those coefficients and derive a pilot-aided MMSE estimator to estimate them for SIC. We also propose a novel IFDE by leveraging both the sparse structure of shortened channel and finite-alphabet property of transmitted symbols. Numerical results show that the proposed scheme has advantages over the well-known FIR-MMSE-DFE/RLS-CE scheme in both performance and complexity.