Robust Adaptive OFDM with Diversity for Time-Varying Channels
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Research Article Robust Adaptive OFDM with Diversity for Time-Varying Channels Erdem Bala and Leonard J. Cimini, Jr. Department of Electrical and Computer Engineering, University of Delaware, Newark, DE 19716, USA Received 23 November 2006; Accepted 17 April 2007 Recommended by Sangarapillai Lambotharan The performance of an orthogonal frequency-division multiplexing (OFDM) system can be significantly increased by using adaptive modulation and transmit diversity. An accurate estimate of the channel, however, is required at the transmitter to realize this benefit. Due to the time-varying nature of the channel, this estimate may be outdated by the time it is used for detection. This results in a mismatch between the actual channel and its estimate as seen by the transmitter. In this paper, we investigate adaptive OFDM with transmit and receive diversities, and evaluate the detrimental effects of this channel mismatch. We also describe a robust scheme based on using past estimates of the channel. We show that the effects of the mismatch can be significantly reduced with a combination of diversity and multiple channel estimates. In addition, to reduce the amount of feedback, the subband approach is introduced where a common channel estimate for a number of subcarriers is fedback to the transmitter, and the effect of this method on the achievable rate is analyzed. Copyright © 2007 E. Bala and L. J. Cimini, Jr. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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INTRODUCTION
The performance of wireless communication systems can be significantly improved by adaptively matching the transmission parameters such as rate, power level, or coding type to the channel frequency response [1–3]. When OFDM is used in a wideband frequency-selective channel, different subchannels usually experience different channel gains. The system capacity can then be maximized if the data rate and power level of each subcarrier are adjusted according to the channel gain of that subcarrier. To take advantage of this property of OFDM, bit-loading algorithms that adaptively distribute the input bits over the available subcarriers have been proposed [4–6]. To achieve the performance gain of adaptive modulation, an accurate estimate of the channel response, as seen at the receiver, is required at the transmitter. One approach to accomplish this is to measure the channel state at the receiver and feedback the estimate to the transmitter. Due to the time-varying nature of the wireless channel, however, if the Doppler is large enough, this information may be outdated by the time it is used for detection resulting in an imperfect channel state information (CSI) at the transmitter. Channel estimation errors or errors introduced in the feed-
back channel might also cause imperfections in the CSI. The performance of adaptive loading algorithms degrades when imperfect CSI is used to compute the bit
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