Space-Time Coded OFDM with Low PAPR
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Space-Time Coded OFDM with Low PAPR Anand Venkataraman, Harish Reddy, and Tolga M. Duman Department of Electrical Engineering, Arizona State University, Tempe, AZ 85287-5706, USA Received 11 January 2005; Revised 25 July 2005; Accepted 1 September 2005 Recommended for Publication by Alex Kot Recently the use of multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems has been proposed for signaling over frequency-selective fading channels. Although various aspects of these systems have been considered in the literature, the problem of the inherent high peak-to-average power ratio (PAPR) is not examined. In this paper, we consider PAPR reduction for MIMO-OFDM systems and propose alternate low-complexity algorithms that can be used in conjunction with the trellis shaping method. We show that a PAPR reduction in the order of 4-5 dB can be achieved at the cost of a slight reduction in the spectral efficiency. Furthermore, we compare the trellis shaping technique with other PAPR reduction techniques such as tone reservation and partial transmit sequences. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
1.
INTRODUCTION
Single-carrier modulation together with equalization and multicarrier modulation, such as orthogonal frequency division multiplexing (OFDM), are used to overcome the challenges posed by dispersive channels. OFDM uses a number of subcarriers which are orthogonal to each other. Data is placed on each of the subcarriers and can be recovered at the receiver by exploiting the orthogonality among the subcarriers. Recently, in addition to the single-input single-output OFDM systems, space-time coded OFDM systems have been receiving significant attention. They were first proposed by Agrawal et al. in order to achieve data rates of 1.5–3 Mbps over a bandwidth of 1 MHz [1], and it is shown that spacetime coding can be used to achieve high data rates at low signal-to-noise ratios (SNRs) over different channels with different multipath delay profiles. In [2], the authors propose a space-time code for a Rayleigh flat fading channel which performs well for various wireless local area network (WLAN) applications. In [3], the authors present an algebraic design framework and propose two approaches for space-time codes in frequency-selective fading channels, one of which employs OFDM. In this scheme, a frequencyselective fading channel is converted into a set of flat block fading channels. Subsequently, an algebraic framework is employed to exploit the diversity available in the block fading channels so as to improve the performance of the system. Although OFDM has many advantages, it has limitations including high PAPR and carrier frequency offset sensitivity
[4]. Since the complex baseband OFDM signal is formed by the superposition of many sinusoids with different frequencies, the instantaneous power of the resulting signal may be larger than the average power of the OFDM signal exhibiting high peaks. It is important to reduce the PAPR because the high-power amplif
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