Pilot-Symbol-Assisted Channel Estimation for Space-Time Coded OFDM Systems
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Pilot-Symbol-Assisted Channel Estimation for Space-Time Coded OFDM Systems King F. Lee Multimedia Architecture Lab, Motorola Labs, Schaumburg, IL 60196, USA Email: [email protected]
Douglas B. Williams School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA Email: [email protected] Received 31 May 2001 and in revised form 5 March 2002 Space-time coded orthogonal frequency division multiplexing (OFDM) transmitter diversity techniques have been shown to provide an efficient means of achieving near optimal diversity gain in frequency-selective fading channels. For these systems, knowledge of the channel parameters is required at the receivers for diversity combining and decoding. In this paper, we propose a low complexity, bandwidth efficient, pilot-symbol-assisted (PSA) channel estimator for multiple transmitter OFDM systems. The pilot symbols are constructed to be nonoverlapping in frequency to allow simultaneous sounding of the multiple channels. The timevarying channel responses are tracked by interpolating a set of estimates obtained through periodically transmitted pilot symbols. Simulations are used to verify the effectiveness of the proposed estimator and to examine its limitations. It is also shown that the PSA channel estimator has a lower computational complexity and better performance than a previously proposed decisiondirected minimum mean square error MMSE channel estimator for OFDM transmitter diversity systems. Keywords and phrases: transmitter diversity, OFDM, channel estimation, pilot symbols, interpolation.
1.
INTRODUCTION
The mobile wireless channel suffers from multipath fading that severely attenuates the received signal during periods of deep fades. Spatial diversity is a well-known technique for improving the performance and reliability of wireless communications over fading channels. Traditionally, spatial diversity has been implemented at the receiver end by using multiple antennas at the receiver and then combining signals to improve the quality of the received signal. Unfortunately, receiver diversity requires multiple, widely-spaced antennas and multiple radio frequency (RF) front-end circuits at the receiver. This multiplicity of receiver front-end hardware is undesirable and impractical for portable receivers, such as pagers or cellular handsets, where physical size and current drain are important constraints. Transmitter diversity, on the other hand, can be implemented with multiple spatially separated antennas at the transmitter and requires only a single antenna and front-end circuit at the receiver. Transmitter diversity techniques are, therefore, very suitable for paging, cellular, and portable wireless data services, where a small number of base stations serve a large number of mobile users and where spatially separated antennas can be easily implemented at the base stations. Hence, transmitter
diversity has received strong interest in recent years, especially in the mobile communications research community. Furthermore,
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