Article

# On Optimum Pilot Design for Comb-Type OFDM Transmission over Doubly-Selective Channels.

IEEE Transactions on Communications (Impact Factor: 1.98). 04/2011; 59:930-935. DOI: 10.1109/TCOMM.2011.020411.100151

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**ABSTRACT:**Clipping is one of the simplest peak-to-average power ratio (PAPR) reduction schemes for orthogonal frequency division multiplexing (OFDM). Deliberately clipping the transmission signal degrades system performance, and clipping mitigation is required at the receiver for information restoration. In this work, we acknowledge the sparse nature of the clipping signal and propose a low-complexity Bayesian clipping estimation scheme. The proposed scheme utilizes a priori information about the sparsity rate and noise variance for enhanced recovery. At the same time, the proposed scheme is robust against inaccurate estimates of the clipping signal statistics. The undistorted phase property of the clipped signal, as well as the clipping likelihood, is utilized for enhanced reconstruction. Further, motivated by the nature of modern OFDM-based communication systems, we extend our clipping reconstruction approach to multiple antenna receivers, and multi-user OFDM. We also address the problem of channel estimation from pilots contaminated by the clipping distortion. Numerical findings are presented, that depict favourable results for the proposed scheme compared to the established sparse reconstruction schemes.IEEE Access. 10/2014; 2. -
##### Conference Paper: Minimum Error Transmissions with Imperfect Channel Information in High Mobility Systems

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**ABSTRACT:**One of the main challenges faced by communications in high mobility systems, such as unmanned aerial vehicle (UAV), is the fast time-varying fading. This paper studies the optimum system design that can minimize the error probability of a high mobility wireless communication system with imperfect channel state information (CSI). We consider a system that employs pilot-assisted minimum mean square error (MMSE) channel estimation. The channel estimation error is usually non-negligible in high mobility systems and might have significant impacts on system performance. With asymptotic analysis, the channel estimation mean square error (MSE) is expressed as a closed-form expression of the percentage of pilot symbols in the transmitted symbols. An optimum receiver for systems with imperfect CSI and the corresponding symbol error rate (SER) are then derived by analyzing the statistical properties of the channel estimation error. The SER is expressed as an explicit function of a number of parameters, such as pilot percentage, maximum Doppler spread, and the signal-to-noise ratio (SNR), etc. The optimum pilot percentage that can minimize the SER is analytically identified, and the impacts of the imperfects CSI on the system performance are studied through both theoretical analysis and simulations.MILCOM 2013 - 2013 IEEE Military Communications Conference; 11/2013 - [Show abstract] [Hide abstract]

**ABSTRACT:**As high-speed railway is booming worldwide, the communication system with fast-time varying channel has drawn great attention. The comb pilot based linear minimum mean square error (LMMSE) channel estimator is proved to be an effective method for fast time-varying channel estimation. In this paper, the clustered comb pilot-aided channel estimation for orthogonal frequency-division multiplexing (OFDM) system is discussed, where the time varying channel is approximated by a basis expansion model (BEM). A modified clustered comb pilot structure is proposed and justified to improve the estimation performance compared with the clustered comb pilot proposed by Tang. Based on the complex-exponential BEM (CE-BEM) model, a suboptimal-pilot structure is proposed. In addition, optimal pilot length is analyzed and simulated with a predefined total number of pilots. The simulation results show that the modified clustered comb pilot can greatly reduce the estimation error especially with high Doppler spread. The suboptimal-pilot structure with guard pilot approximation is proven to be competitive. Optimal nonzero pilot lengths for different Doppler spread are obtained by simulation with a predefined channel order and fixed pilot subcarriers.Journal of Modern Transportation. 12/2013; 20(4):220-226.

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