An Adaptive Channel Estimation Algorithm Using Time-Frequency Polynomial Model for OFDM with Fading Multipath Channels
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An Adaptive Channel Estimation Algorithm Using Time-Frequency Polynomial Model for OFDM with Fading Multipath Channels Xiaowen Wang Wireless Systems Research Department, Agere Systems, Murray Hill, NJ 07974, USA Email: [email protected]
K. J. Ray Liu Electrical and Computer Engineering Department, University of Maryland, College Park, MD 20742, USA Email: [email protected] Received 1 August 2001 and in revised form 7 March 2002 Orthogonal frequency division multiplexing (OFDM) is an effective technique for the future 3G communications because of its great immunity to impulse noise and intersymbol interference. The channel estimation is a crucial aspect in the design of OFDM systems. In this work, we propose a channel estimation algorithm based on a time-frequency polynomial model of the fading multipath channels. The algorithm exploits the correlation of the channel responses in both time and frequency domains and hence reduce more noise than the methods using only time or frequency polynomial model. The estimator is also more robust compared to the existing methods based on Fourier transform. The simulation shows that it has more than 5 dB improvement in terms of mean-squared estimation error under some practical channel conditions. The algorithm needs little prior knowledge about the delay and fading properties of the channel. The algorithm can be implemented recursively and can adjust itself to follow the variation of the channel statistics. Keywords and phrases: channel estimation, OFDM, polynomial approximation.
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INTRODUCTION
The 3G wireless communication system is the next generation mobile cellular system that aims to provide high rate data communications of a bit rate up to 2 Mbit/s. Among many technical challenges in this broadband system, the severe intersymbol interference (ISI) caused by multipath effect of wireless channels is an essential one. One effective technique to deal with this problem is the orthogonal frequency division multiplexing (OFDM) [1, 2]. In OFDM systems, the entire bandwidth is partitioned into parallel subchannels by dividing the transmit data into several parallel low bit rate data streams to modulate the carriers corresponding to those subchannels. By doing so, the OFDM system has a relatively longer symbol duration, thus provides a great resistance to ISI and impulse noise. When the number of subchannels is large enough, the subchannels can be treated as independent of each other and only a one-tap equalizer is needed for each subchannel. Because of these advantages, OFDM has become a promising technique for broadband wireless communications. Channel estimation is a key issue in a communication
system, as is the case for the OFDM system. Without the knowledge of channel information, noncoherent detection, such as differential modulation, has to be used and results in some performance loss compared to the coherent detection. The channel estimation problem becomes more important for the 3G systems because many sophisticated signal processing techniques that require the knowled
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