PAR reduction in OFDM via active constellation extension

The high peak-to-average power ratio (PAR) in orthogonal frequency division multiplexing (OFDM) modulation systems can significantly reduce power efficiency and performance. Methods exist which alter or introduce new signal constellations to combat large signal peaks. We present a new PAR-reduction method that dynamically extends outer constellation points in active (data-carrying) channels, within margin-preserving constraints, in order to minimize the peak magnitude. This scheme simultaneously decreases the bit error rate slightly while substantially reducing the peak magnitude of an OFDM transmit block. Furthermore, there is no loss in data rate and, unlike other methods, no side information is required. PAR reduction for an approximated analog signal is considered, and about a 4.6 dB reduction at a 10/sup -5/ symbol-clip probability is obtained for 256-channel QPSK OFDM. The results show great promise for use in commercial systems.

I. Introduction

MULTICARRIER modulation has become a key communication systems technology; for example, coded OFDM schemes are used for wireless LAN (802.11a and Hiperlan2) and terrestrial digital television (DVB-T) and audio broadcasts (DAB-T) in Europe, and discrete multitone modulation (DMT) [1]–[4] is used in the Asymmetric Digital Subscriber Line (ADSL) standard. Multicarrier communication systems are competing well with single-carrier systems, but suffer from a serious drawback: the approximately Gaussian-distributed output samples cause a high peak-to-average power ratio (PAR) [5]–[14]. The analog hardware at the transmitter requires an expensive high-power amplifier (HPA) to avoid clipping and/or soft thresholding that causes nonlinear output. The power consumption of a HPA depends largely on its peak power output rather than the average output power, and thus handling occasional large peaks leads to low power efficiency.