Par reduction in ofdm via active constellation extension

Abstract

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 5 symbol-clip probability is obtained for 256-channel QPSK OFDM. The results show great promise for use in commercial systems. Index Terms-Multicarrier communications, OFDM, PAR reduction, peak power reduction, peak-to-average ratio. I. INTRODUCTION M ULTICARRIER 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. An OFDM signal is the sum of independent QAM signals modulated onto subchannels of equal bandwidth with frequency separation of between adjacent subcarriers, where is the time duration of the OFDM symbol. An input bit stream Manuscript

Evaluation of Performance Improvement Capabilities of PAPR-reducing Methods

“…Active constellation extension (ACE) [6], tone reservation (TR) [7], partial transmit sequences (PTS) [8] and selected mapping (SLM) [9] PAPR-reduction techniques are used.…”

“…ACE and TR techniques are implemented as described in Refs. [6] and [10], respectively. PTS and SLM are implemented as in Ref.…”

Selected basis for PAR reduction in multi-user downlink scenarios using lattice-reduction-aided precoding

“…PAR reduction techniques for single-antenna OFDM systems have been well analyzed in the literature. The most prominent are selected mapping (SLM) [2], partial transmit sequences (PTS) [3], active constellation extension (ACE) [4] or tone reservation (TR) [5].…”

An efficient crest factor reduction technique for wideband applications

“…The various PAPR techniques can be categorized into two groups depending on whether they use linear techniques (modulation-and-coding-dependent) or nonlinear techniques (modulation-and-coding-independent). Methods that use linear techniques for OFDM systems do not distort the signal in the time domain so that the spectral properties are not altered [1][2][3]. On the other hand, nonlinear techniques modify the envelope of the time domain signal and are mainly based on clipping-filtering (CF) and windowing [4,5].…”