Bandwidth Efficient OFDM Transmitter Diversity Techniques
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Bandwidth Efficient OFDM Transmitter Diversity Techniques 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 30332-0250, USA Email: [email protected] Received 17 December 2002; Revised 2 September 2003 Space-time block-coded orthogonal frequency division multiplexing (OFDM) transmitter diversity techniques have been shown to be efficient means of achieving near-optimal diversity gain in frequency-selective fading channels. However, these known techniques all require a cyclic prefix to be added to the transmitted symbols, resulting in bandwidth expansion. In this paper, iterative space-time and space-frequency block-coded OFDM transmitter diversity techniques are proposed that exploit spatial diversity to improve spectral efficiency by eliminating the need for a cyclic prefix. Keywords and phrases: space-time coding, space-frequency coding, transmitter diversity, OFDM, channel estimation, pilot symbols.
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INTRODUCTION
The last decade has witnessed an explosive growth of wireless communications, especially in mobile communications and personal communications services (PCS). With the continuing expansion in both existing and new markets and the introduction of exciting new services such as wireless internet access and multimedia applications, the wireless communications market is expected to continue to grow at a rapid pace. Furthermore, the ever-increasing demand for faster and more reliable services to support new applications has created strong interests in developing high data rate wireless communications systems. With existing and emerging wireless applications, all competing for a limited radio spectrum, the development of high data rate wireless communications systems that are spectrally efficient is especially important. The main challenge in developing reliable high data rate mobile communications systems is to overcome the detrimental effects of frequency-selective fading in mobile communications channels. A number of space-time coded orthogonal frequency division multiplexing (OFDM) transmitter diversity techniques have recently been proposed for high data rate wireless communications [1, 2, 3, 4]. It has been shown in [3, 4] that space-time and space-frequency block-coded OFDM (STBC-OFDM and SFBC-OFDM) sys-
tems are efficient means of achieving near optimum diversity gain in frequency-selective fading channels. These previously proposed OFDM transmitter diversity systems all require a cyclic prefix to be added to the transmitted symbols to avoid intersymbol interference (ISI) and interchannel interference (ICI) in the OFDM symbols, and the number of cyclic prefix symbols has to be equal to or greater than the order of the wireless channels [5]. The addition of the cyclic prefix causes bandwidth expansion if a desired data rate is to be maintained or a reduction in data rate if the transmission bandwidth is fixed. For many high d
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