Non-constant Envelope Signals Research Articles

A novel method for PAPR reduction of the OFDM signal using nonlinear scaling and FM

Orthogonal frequency division multiplexing (OFDM) has been adopted as standard beginning with the 4th generation mobile communication system because of its high-bit-rate transmission capability under frequency selective fading channel conditions. However, a major disadvantage of OFDM is the non-constant envelope signal with a high peak-to-average power ratio (PAPR). The high peak signal in OFDM is distorted through a nonlinear amplifier, which causes bit error ratio (BER) reduction. Many techniques have been developed for reducing PAPR at the cost of inefficient bandwidth usage or throughput because of the additional information about PAPR reduction. We propose a novel method, in which the high peak signal above the threshold of the nonlinear amplification region is nonlinearly downscaled to lower the PAPR. The time slot location and scaling ratio for where and how the high peak baseband OFDM signal is downscaled are transmitted using frequency modulation (FM) combined with OFDM, which requires less additional bandwidth than the previously proposed methods. Simulation results show that the proposed novel method provides a lower PAPR and elicits a better BER performance compared with other conventional methods, because it reduces the PAPR by nonlinear scaling and restores the pre-distorted signal using FM.

Reconditioning Equalizer Filter for Non-Constant Envelop Signals

A technique for a reconditioning equalizer filter for non-constant envelope signals is described. The input to a transmitter chain is modified by a reconditioning equalizer filter, prior to being applied to the transmitter. The reconditioning equalizer filter modifies and smoothens the amplitude of the signal. The modified and smoothened signal has its peaks reduced which results in lower Crest Factor. The input to the reconditioning equalizer filter could be a baseband, intermediate frequency (IF) or radio frequency (RF) signal. When the signal is an IF or RF signal, it needs to be down-converted to baseband before being applied to the reconditioning equalizer filter. The reconditioning equalizer filter could be performed in a digital or analog domain.

Polar Antenna Impedance Detection and Tuning for Efficiency Improvement in a 3G/4G CMOS Power Amplifier

An antenna load impedance detection and automatic tuning technique is presented. An antenna load impedance detection circuit as well as burst detector enabling autonomous tuning is integrated on a CMOS-PA chip. A low-loss impedance tuner with an SOI switch is also demonstrated. The proposed impedance detection circuit employs polar detection technique, so that the antenna impedance is tuned in a vector fashion to track the time-varying antenna impedance promptly. Also, it can operate with nonconstant envelope signal thanks to an introduced relative detection technique. At the back-off operation, the impedance can be automatically tuned to optimum value. The circuit is time shared with PA linearization loop, minimizing the overhead of the circuit. When the VSWR is approximately 2.5, the peak PAE is kept more than 40% with tuning, whereas it varies from 28% to 45% without tuning. The linear efficiency without the tuning is 15% as the worst case, while more than 30% of PAE is maintained when the tuning and PA-closed loop are enabled. The drain efficiency at 6 dB back-off is improved from 30% to 40%. The tuner can cover a VSWR of around 6, while achieving a loss ranging from 0.3 to 4.6 dB.

An all-digital synthesizable baseband for a delay-based LINC transmitter with reconfigurable resolution

The linear amplification with nonlinear component transmitter is a promising solution to high efficiency and high linearity amplification for non-constant envelope signals. An all-digital synthesizable baseband for a delay-based LINC transmitter is implemented. This paper proposes a standard-cell based synthesizable methodology which can be applied in the ASIC process efficiently without performance degradation compared to the manual layout. A scheme to overcome the limited resolution of conventional phase detectors is proposed. It employs alternative phase detector structures to provide reconfigurability for higher resolution after fabricating, resulting in an 11 ps resolution improvement. Due to the PVT variation, an adaptive calibration scheme focusing on the inherent imbalance between two delay lines is depicted, which reveals an effective EVM enhancement of 5.37 dB. This baseband chip is implemented in 0.13 μm CMOS technology, and the transmitter with the baseband has an EVM of −28.96 dB and an ACPR of −29.51 dB, meeting the design requirement.

Nonlinear modeling and analysis of a Doherty power amplifier driven by non-constant envelope signals

A circuit-level CAD method based on circuit envelope (CE) simulation is presented to analyze the nonlinear behavior of Doherty power amplifiers (DPAs) in LDMOS technology, when driven by non-constant envelope signals. In this paper, a general nonlinear model has been extracted for an RF LDMOS power transistor. Then, a complete CE analysis is applied to a designed DPA using the extracted model at 3.5 GHz. To solve the CE equation at specific envelope samples, analytic expressions are developed for the Jacobian matrix and an effective method for quick calculation is suggested. Employing this nonlinear analysis gives insight for optimum design of DPAs and provides an accurate tool to precisely model the nonlinearity of the amplifier in order to predict its distortion effects in modern communication systems.

Development of single‐stage and doherty GaN‐based hybrid RF power amplifiers for quasi‐constant envelope and high peak to average power ratio wireless standards

AbstractThis article describes the design, realization, and characterization of a set of hybrid medium‐power RF power amplifiers, based on a commercial packaged GaN HEMT and developed through a low‐cost microstrip process. Two different design solutions suitable for wireless applications are presented: the first, intended for a constant envelope modulation (with reference to the GSM standard) is a Class F amplifier exhibiting at 900 MHz an efficiency of 72% with an output power of 37.5 dBm; the second, optimized for nonconstant envelope signals with high dynamics (with reference to the UMTS WCDMA standard) is a Doherty amplifier showing, at 2.14 GHz, an efficiency higher than 40% at 6 dB of output power back‐off with a maximum output power of 40 dBm. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:206–210, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26459

Error performance of PAM systems using energy detection with optimal and suboptimal decision thresholds

In this paper, we consider the error probability for multilevel pulse amplitude modulated (PAM) signals, which are used with a multipath diversity combiner based on energy detection (ED). A distinctive feature of ED combined with nonconstant envelope signals is that the symbol decision thresholds need to be determined from a non-Gaussian distributed decision variable. We first present a novel theoretical analysis framework which provides a basis to predict the error performance of ED-PAM systems with optimal maximum likelihood decision thresholds in general multipath fading channels with arbitrary number of degrees of freedom. We then use frequency-selective lognormal fading channels as a case example and evaluate simple approximations for the error probability. Moreover, the analysis is extended to include the joint effect of systematic and random threshold deviation on the error probability with suboptimal but practical data-aided threshold estimation.

A Class-E PA With Pulse-Width and Pulse-Position Modulation in 65 nm CMOS

A class-E power amplifier (PA) utilizes differential switches and a tuned passive output network improves power-added efficiency (PAE) and insensitivity to amplitude variations at its input. A modulator is introduced that takes outphased waveforms as its inputs and generates a pulse-width and pulse-position modulated (PWPM) signal as its output. The PWPM modulator is used in conjunction with a class-E PA to efficiently amplify constant envelope (e.g., GMSK) and non-constant envelope (e.g., QPSK, QAM, OFDM) signals with moderate peak-to-average ratios (PAR). The measured maximum output power of the PA is 28.6 dBm with a PAE of 28.5%, and the measured error vector magnitude (EVM) is 1.2% and 4.6% for GMSK and pi/4-DQPSK (PAR ap 4 dB) modulated signals, respectively.

A Polar Modulator Using Self-Oscillating Amplifiers and an Injection-Locked Upconversion Mixer

A polar modulator for use with non-constant envelope signals is presented. Techniques for efficient behavior of both the amplitude (AM) and phase (PM) path are introduced. The amplitude modulation technique is based on asynchronous pulse-width modulation of a phase-modulated RF signal. The digital signal generated in this way is suited for amplification using a nonlinear, efficient type of PA, while preserving the AM modulation, since this information is present in the signal. The efficiency of the switching amplifier is less dependent of the output power, compared to class B operation. Using a self-oscillating, asynchronous type of pulse-width modulator, the spurs are concentrated in narrow bands well separated from the signal band. These switching spurs are filtered out by the filters already present in the transmitter. The feasibility of this RF-pulse-width-modulation is proven by measurements with amplitude modulated signals on a prototype in 0.18 mum CMOS. Unmodulated, the circuit is able to deliver a peak output power of 8.26 dBm with a drain efficiency of 35%. In the phase-modulation path of the polar modulator, injection locking of an oscillator is used as a single-stage high-gain amplifier. Avoiding multiple stages and related power loss improves the overall power efficiency.

The Effects of Repeater Hard-Limiting, Filter Distortion, and Noise on a Pseudo-Noise, Time-Of-Arrival Estimation System

Employment of a repeater in a communication system may lead to the introduction of several sources of potential performance degradation. They are hard-limiting, IF filter group delay distrotion and center frequency offset, and additional noise in the links from transmitter-to-repeater and repeater-to-receiver. Specifically, the hardlimiter and IF filter may introduce signal waveform distortion while the former may also introduce signal-to-noise ratio suppression. In this paper the impact of the above on a pseudo-noise (PN) time-of-arrival (TOA) estimation system is considered. The assumed modulation scheme is differential phase shift keying (DPSK) and realistic nonconstant envelope signals are considered; the performance criterion is TOA estimate standard deviation. One surprising result is that the presence ot the limiter tends to degrade performance; this is contrary to corresponding results that have been derived for data detection. It was also found that performance degrades quite gradually with increasing IF filter differential group delay over the signal bandwidth. Finally, although analytical results presented here pertain specifically to the hard-limiter, they may be straightforwardly extended to determine the effects of repeater <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</tex> th law devices.