Yingguan Wang

A low-complexity LMMSE channel estimation method for OFDM-based cooperative diversity systems with multiple amplify-and-forward relays

Orthogonal frequency division multiplexing- (OFDM-) based amplify-and-forward (AF) cooperative communication is an effective way for single-antenna systems to exploit the spatial diversity gains in frequency-selective fading channels, but the receiver usually requires the knowledge of the channel state information to recover the transmitted signals. In this paper, a training-sequences-aided linear minimum mean square error (LMMSE) channel estimation method is proposed for OFDM-based cooperative diversity systems with multiple AF relays over frequency-selective fading channels. The mean square error (MSE) bound on the proposed method is derived and the optimal training scheme with respect to this bound is also given. By exploiting the optimal training scheme, an optimal low-rank LMMSE channel estimator is introduced to reduce the computational complexity of the proposed method via singular value decomposition. Furthermore, the Chu sequence is employed as the training sequence to implement the optimal training scheme with easy realization at the source terminal and reduced computational complexity at the relay terminals. The performance of the proposed low-complexity channel estimation method and the superiority of the derived optimal training scheme are verified through simulation results.

Two-hop time synchronization protocol for sensor networks

One of the critical middleware services for sensor networks is the time synchronization, which provides supports to different applications. Synchronization protocols used for Internet and LANs are not appropriate in the sensor networks for the high-density and limited energy resource. This paper describes two-hop time synchronization (TTS) that aims at reducing the synchronization overhead and providing more accurate network-wide synchronization. The synchronization message exchanges are minimized by making full use of sensors’ broadcast domain and enlarging the common node synchronization range in multi-hop scenarios. By halving synchronization hops, the TTS achieves high multi-hop synchronization precision. The proposed protocol contains single-hop synchronization model, multi-hop synchronization algorithm, and a power control scheme. We prove that the extension of single-hop TTS to network-wide synchronization is NP-complete. The complexity and convergence time of multi-hop TTS are analyzed in detail. We simulate TTS on MATLAB and show that it requires minimal overhead and convergence time compared with other protocols. We also implement TTS on common sensors and its multi-hop synchronization error is less than that of receiver-receiver synchronization (R-RS).

A Simple Single-Carrier Space-Time Transmission Scheme for Asynchronous Cooperative Communications over Frequency-Selective Channels

This paper presents a simple single-carrier (SC) distributed space-time block coded transmission scheme for asynchronous cooperative communications over frequency-selective channels. The cyclic prefix (CP) at the source node and relay nodes is used to combat inter-symbol interference from multi-path fading and the timing errors from the relay nodes. In this scheme, the relay nodes only implement very simple operations on their received signals to construct distributed space-time block code, and decoding, DFT and IDFT operations are only needed at the destination node. The low complexity SC zero-forcing frequency domain equalization (ZF-FDE) is applied at the destination node to recover the transmitted signals. The analysis and simulation results demonstrate that this simple SC scheme can achieve order-two spatial diversity gain for amplify-and-forward based cooperative communication over frequency-selective channels in the presence of asynchronization between relay nodes.This paper proposes a simple orthogonal frequency -division multiplexing (OFDM) transmission scheme for asynchronous cooperative communications over frequency-selective fading channels. In this scheme, the relay nodes only implement very simple operations on their received signals, and no decoding, DFT or IDFT operation is needed. With our proposal, the received signals have the Alamouti code structure on each subcarrier, and thus, it has the fast symbol-wise ML decoding. The analysis and simulation results demonstrate that our simple scheme can achieve order-two spatial diversity gain for amplify-and-forward based cooperative communications over frequency-selective fading channels in the presence of asynchronization between relay nodes.