Robert J. Baxley

Comparing Selected Mapping and Partial Transmit Sequence for PAR Reduction

Selected mapping (SLM) and partial transmit sequence (PTS) are two existing distortionless peak-to-average power ratio (PAR) reduction schemes that have been proposed for orthogonal frequency division multiplexing (OFDM). Previously, it was argued that SLM and PTS have comparable PAR reduction performance but that the latter has lower computational complexity because it uses fewer IFFTs. In this paper, we show that the overall computational complexity of PTS is only lower than that of SLM in certain cases, and that SLM always has better PAR reduction performance. We compare the two schemes using three different performance metrics by assuming a given amount of computational complexity that can be afforded. Using the metrics, we show that SLM outperforms PTS for a given amount of complexity.

Constrained Clipping for Crest Factor Reduction in OFDM

In this paper, we propose a constrained clipping method for reducing the peak to average power ratio (PAR) or crest factor of an orthogonal frequency division multiplexing (OFDM) signal. This is a transmitter-side processing technique that does not impose any modification at the receiver. Specifically, constrained clipping achieves PAR reduction while simultaneously satisfying spectral mask and error vector magnitude (EVM) constraints which are specified by most modern communications standards. Our proposed constrained clipping method consists of two independent processing units, one to satisfy the in-band EVM constraint and the other to satisfy the out-of-band spectral constraint. Achievable PAR reduction results vary depending on the particular standards requirements, but we show that by using constrained clipping on a QPSK WiMax signal with 256 subcarriers, a 4.5 dB PAR reduction at the 10-2 complementary cumulative distribution function (CCDF) level can be obtained

MAP metric for blind phase sequence detection in selected mapping

Selected mapping (SLM) is a promising technique to reduce the high peak-to-average power ratio (PAR) of orthogonal frequency division multiplexing (OFDM) signals. In this paper, we derive the optimal phase sequence detection metric for blind selected mapping (BSLM) and show that the minimum distance metric is suboptimal. We then define a certain selection criterion in the transmitter which assigns different probabilities to different mapping sequences. These probabilities are integrated into the optimal metric to achieve several dBs of SNR improvement in blind phase sequence detection.

Pilot Design for IEEE 802.16 OFDM and OFDMA

In this paper we apply a new pilot design optimization technique to the IEEE 802.16a wireless MAN standard. Using the specifications from 802.16a, we demonstrate that significant SER improvements are possible for 802.16a in a Rayleigh fading channel by judiciously choosing where the pilots are placed and the power contained in each pilot. Specifically, for the OFDM mode, we show that up to 13 dB SNR improvement is possible by simply modifying the pilots. For the OFDMA mode we demonstrate that a more modest but still significant improvement of 1.8 dB is possible with proper pilot design.

Error Vector Magnitude Analysis for OFDM Systems

Error vector magnitude (EVM) is a popular figure-of-merit adopted by various communication standards for evaluating in-band distortions introduced in a communication system. In this paper, we regard EVM as a random variable and investigate its statistical distributions as the result of the following distortion mechanisms: phase noise, amplitude clipping, power amplifier nonlinearities, and gain/phase imbalances in orthogonal frequency division multiplexing (OFDM) systems. We relate key parameters characterizing the various distortion mechanisms to the statistical behavior of EVM; such statistical behavior can be used to verify compliance of the transmit signals to the requirements of the standard.

Error vector magnitude optimization for OFDM systems with a deterministic peak-to-average power ratio constraint

Orthogonal frequency division multiplexing (OFDM) has been adopted by several wireless transmission standards. A major disadvantage of OFDM is the large dynamic range of its time-domain waveforms, making OFDM vulnerable to nonlinearities (including clipping effects) of the power amplifier (PA) and causing the PA to yield low efficiency on the RF to dc power conversion. A commonly used metric to characterize a signals dynamic range is the peak-to-average power ratio (PAR). To suppress the nonlinear effects, one may want to reduce the signal PAR. However, this results in the increase of error vector magnitude (EVM), and may violate the spectral mask. In this paper, we formulate the problem as an EVM optimization task subject to a deterministic PAR constraint and a spectral mask constraint. A low-complexity customized interior-point algorithm is developed to solve the optimization problem. We also discuss extensions of the optimization framework, whereby we optimize the parameters with respect to two metrics on signal-to-noise-and-distortion ratio (SNDR) and mutual information, respectively.

Embedded Synchronization/Pilot Sequence Creation Using Pocs

In this paper we build on the orthogonal frequency division multiplexing (OFDM) peak-to-average power ratio (PAR) reduction work by Chen and Zhou. It has been demonstrated that pilot sequences that are constant modulus in the time domain can lead to an ensemble PAR-reduction across all data realizations. However, the problem of creating constant modulus sequences from arbitrary frequency domain power profiles has never been addressed. Often, it is desirable to have a some freedom of choice in how pilot and, possibly synchronization, energy is allocated in the frequency domain. In this paper we present a projection on to convex sets (POCS) method for creating low-PAR synchronization/pilot (S/P) sequences with arbitrary frequency-domain power profiles

Assessing peak-to-average power ratios for communications applications

High peak-to-average ratio (PAR) is a significant drawback for orthogonal frequency division multiplexing (OFDM). Accordingly, many authors have proposed different PAR reduction methods that rely on different trade-offs. It is important to quantify the effectiveness of an individual PAR reduction method. The PAR at a certain probability level (10/sup -4/, 10/sup -5/, etc) has proven to be a popular but inconsistent metric. We propose to use the expected PAR, E[PAR], as a metric to compare the different PAR reduction methods, in this paper we derive a closed form expression for the E[PAR] for the selected mapping (SLM) approach. We also evaluate SLM in terms of its computational cost as characterized by the expected number of different phase mappings to be tried.

Comparison of Selected Mapping and Partial Transmit Sequence for Crest Factor Reduction in OFDM

Selected mapping (SLM) and partial transmit sequence (PTS) are two existing distortionless crest factor reduction (CFR) schemes that have been proposed for orthogonal frequency division multiplexing (OFDM). Previously, it was argued that SLM and PTS have comparable CFR performance but that the latter has lower computational complexity because it uses fewer IFFTs. In this paper, we show that the overall computational complexity of PTS may be higher than that of SLM, but that SLM always has better CFR performance as quantified through a per unit of complexity metric

Magnitude-Scaled Selected Mapping: A Crest Factor Reduction Scheme for OFDM Without Side-Information Transmission

Selected mapping (SLM) is a distortionless crest factor reduction (CFR) method for orthogonal frequency division multiplexing (OFDM) transmission. With SLM, it is possible to reduce the peak-to-average power ratio (PAR) of an OFDM symbol by several decibels. In this paper, we propose a method for SLM phase sequence detection that does not require side information transmission. We refer to this method as magnitude-scaled SLM, in the sense that it scales the frequency-domain power profile of the OFDM symbol with an envelope function from a set of pre-determined envelope functions. From the envelope of the received symbol, the receiver can detect which envelope and thus which phase sequence was used in the transmission. Also presented in this paper are the theoretical characterizations of the detection error rate (DER) and symbol error rate (SER) in a magnitude-scaled SLM system. Compared with ordinary OFDM without CFR, magnitude-scaled SLM can achieve an order of magnitude SER improvement in a peak-power-limited channel.