Spectral Efficient Cooperative Transmission over Broadband Wireless Channels

Abstract

This thesis discusses spectral efficient cooperative transmission over broadband wireless channels. As the data traffic demand in communication networks gets larger, there is a need for increasing homo/heterogenous node deployment density. Also, signal processing techniques for such nodes'cooperation are evolved to deal with the high-throughput transmission and its reliable reception. In this way, recent advances in cooperative communications have been significantly improve the network throughput. However, in general, the advances such as protocol design, adaptation techniques, and detection methods are based on the assumption of perfect channel state information (CSI) at the transmitter as well as at the receiver. This may not be a practical assumption. To estimate such information, we present a channel estimation scheme for single-carrier frequency domain equalization/frequency division multiple access (SC-FDE/FDMA), which is an attractive air interface scheme for broadband wireless communications. For the coherent demodulation of SC-FDMA systems, the receiver performs channel estimation to obtain the channel frequency response by using the reference signals multiplexed in the transmitted signal. In this thesis, we propose an iterative frequency domain channel estimation scheme for SC-FDMA systems. To prevent severe noise enhancements due to the Gaussianity of the frequency domain data signal, we consider two low complexity algorithms, namely, hard and soft frequency replacements. We first analytically derive the mean square error (MSE) to characterize the effects of frequency replacement and post iterative filtering. By analyzing MSE performance, we present a design framework to develop the optimal frequency replacement. Based on the design framework, hard and soft frequency replacement algorithms are optimized in the MSE sense. Then, their low complexity adaptation methods are proposed for practical SC-FDMA systems, where the proposed frequency replacement is performed according to the reliability of the data estimates. Simulation results show that the proposed iterative channel estimation techniques effectively compensate for noise enhancements, and thus give good MSE and frame error rate performances.Cooperative diversity overcomes the limitation of size and complexity of the mobile equipment without the additional complexity of multiple antennas. The cooperation with the other nodes, so called relay, benefits from pathloss savings, additional power from relays, and diversity in the presence of fading. In this thesis, we propose a pilot position selection/detection technique for channel estimation of distributed space-frequency block coded (D-SFBC) SC-FDE. Unlike the conventional block-type channel estimation techniques, the proposed scheme superimposes pilots on data-carrying tones whose positions are selected to minimize the distortion of original signals.Without additional pilot overhead, the proposed technique can track the CSI even when the mobile equipment speed is high. The corresponding destination structure and frequency domain equalization are also presented, where the pilot positions are blindly detected and the distorted data symbols are iteratively reconstructed. Simulation results show that the proposed method gives better BER performance than the block-type channel estimation for the D-SFBC SC-FDE over fast fading relay channels, without the loss of spectral efficiency.Network coding is an attractive solution for improving spectral efficiency in cooperative transmission systems. The relay employs network coding to transmit the packets from the source nodes simultaneously. The cooperative transmission based on network coding usually works on decode-and-forward (DF) protocols. However, detection errors at the relay cause error propagation, which degrades the performance of cooperative communications. To overcome this problem, we model the error propagation effect of the DF-based system at the destination as the addition of virtual noise, and then design a low complexity detection method. We derive the achievable diversity gain to evaluate the proposed model and corresponding detection scheme. To extend the proposed model to network-coded systems, we first express the channel conditions between the sources and relay as a single equivalent channel gain. Then, we develop low complexity detection schemes for the network-coded systems. Simulation results show that the proposed model and detection scheme effectively reduce the error propagation effects.From the error propagation model, we propose a dual mode network coding technique, which exploits different network coding schemes adaptively according to channel qualities. The proposed dual mode network coding has gains for all channel conditions and thus gives better BER performance than conventional methods. Then, as an anther approach to deal with the error propagation, we consider the opportunistic relay selection and proportional combining schemes performed with the equivalent channel and the relay-destination channel. Simulation results show that all cooperative users can obtain a diversity gain of order two by using single network-coded transmission, even when there are detection errors at the relay.