Linear Amplification With Nonlinear Components Transmitter Research Articles

Efficiency enhancement by reconfigurable matching networks in LINC transmitters

This paper proposes the use of a transmitter based on a linear amplification with nonlinear components (LINC) architecture, in which the reconfigurable matching networks (RMNs) are included. By varying the RMN active cell number, it is possible to change the load impedance at the power amplifier (PA) output, improving the amplifier drain efficiency and therefore the efficiency of the whole system. A long-term evolution (LTE) downlink signal with a bandwidth of 1.4 MHz and a peak-to-average power ratio (PAPR) of 11.48 dB is applied in order to carry out the experiments. Results show that the use of the RMNs in a LINC architecture improves the efficiency at all tested frequencies, especially at 927 MHz reaching an enhancement of 36.50%. Regarding the distortion, the adjacent channel leakage ratio (ACLR) values increase in all cases, with an improvement of 3.5 dB at 958 MHz. Finally, in terms of error vector magnitude (EVM), the proposed architecture offers a value of 1.96% at 927 MHz.

Ring-type Magnitude Modulation for LINC: a pragmatic approach to the efficiency challenge

This paper considers the use of the linear amplification with nonlinear components (LINC) technique for the power amplification of spectrally compact offset quadrature phase shift keying (OQPSK) signals allowing the use of highly efficient, low cost, and strongly nonlinear high power amplifiers (HPAs). However, the performance of the LINC signal separation and power combining procedures decreases with the rise of the signal’s peak-to-average power ratio (PAPR). A new ring-type magnitude modulation (RMM) method is proposed for OQPSK signals that limits both its maximum and minimum complex envelope excursions avoiding zero crossings, without spreading the transmitted signal’s spectrum. The performance results show that band-limited OQPSK signals whose envelope has low fluctuations produce LINC components with a narrower spectrum, with a considerable impact on the LINC transmitter regardless of the type of combiner chosen: when using a passive/matched combiner, the transmitter’s power efficiency is significantly increased without spreading the combined signal’s spectrum; for the highly efficient non-linear Chireix combiner, there is a reduction of the amount of spectral leakage produced by nonlinearly combining the LINC signal components. Finally, an iterative decoding scheme is also proposed, which employs estimates of the received symbols’ RMM coefficients to compensate the RMM distortion.

Design of high efficient 3-level LINC transmitter using a 9-point finite difference method

An efficiency of linear amplification with nonlinear components (LINC) system is degraded due to the low efficiency of a power combiner for high peak-to-average power ratio signals such as long-term evolution signal. A multi-level LINC system can be used to improve the performance of the conventional LINC system. In this paper, a novel 9-point finite difference method to determine the optimal threshold values for 3-level LINC system is suggested. Instead of solving the complicated differential equation, the proposed method can extract optimal threshold values efficiently by numerical method. The 3-level LINC system adopting the proposed scheme and dynamic biasing significantly improves the power efficiency and linear performance simultaneously. The proposed system is verified by comparing the performance of the 3-level system with those of the conventional and 2-level LINC systems.

Mismatch Calibration in LINC Power Amplifiers Using Modified Gradient Algorithm

One of the power amplifiers linearization technique is linear amplification with nonlinear components (LINC).The effects of phase and gain imbalances between two signal branches in LINC transmitters have been analyzed in this paper. Then a feedback path has been added to compensate this mismatches, using two complex gain in each path.This complex gains are controlled in a way to calibrate any gain and phase mismatches between two path using Modified Gradient Algorithm (MGA) adaptively. The main advantages of this algorithm over other algorithms are zero residual error and fast convergence time. In the proposedarchitecture power amplifiers in each path are modeled as a complex gain which its phase and amplitude depend on input signal level. Many simulations have been performed to validate the proposed self calibrating LINC transmitter. Simulation results have confirmed the analyticalpredictions. According to simulation results the proposed structure has around 40 dB/Hz improvement in the first adjacent channel of the output signal spectrum.

A Hybrid Polar-LINC CMOS Power Amplifier With Transmission Line Transformer Combiner

This paper demonstrates the implementation of a hybrid polar linear amplification with nonlinear components (LINC) power amplifier (PA) that uses the combination of polar and LINC schemes. The hybrid polar-LINC operation is based on the idea of representing envelope information with the combination of the two schemes. The high bandwidth burden of a supply modulator in a polar transmitter can be effectively relieved, and the back-off efficiency of a LINC transmitter is improved. The implementation is accomplished with a CMOS PA and a CMOS supply modulator. The designed PA includes a class-E amplifier and the proposed output combiner, which is based on a transmission line transformer with isolation resistors. Since the combiner keeps the operating condition of PAs intact regardless of the phase differences, out-phased signals for LINC can be linearly combined. The implemented hybrid polar-LINC architecture, including the PA and a supply modulator, has a 28.5-dBm maximum linear power and 29.6% efficiency without any pre-distortions.

Adaptive bias LINC transmitter design using EDA software for power efficiency improvement

In this article, a novel adaptive bias LInear amplification with Nonlinear Components (LINC) transmitter is introduced and analyzed. Predistortion is applied to the baseband signal and the bias of the high-efficiency power amplifier is adaptively changed according to the envelope distribution of the baseband signal and the power amplifier itself. This novel transmitter can simultaneously achieve high average efficiency and linearity even with a high peak to average ratio signal. A comprehensive simulation framework has been developed using EDA software to validate this adaptive bias scheme with 16, 32, and 64 quadrature amplitude modulation signals. The simulation results show that this novel scheme can help in improving the average overall efficiency of the LINC transmitter by 15% while sustaining expected linearity. © 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2013.

A Sub-mW All-Digital Signal Component Separator With Branch Mismatch Compensation for OFDM LINC Transmitters

Linear amplification with nonlinear components (LINC) is an attractive technique for achieving linear amplification with high efficiency. This paper presents a sub-mW all-digital signal component separator (SCS) design for OFDM LINC transmitters, including a phase calculator and a digital-control phase shifter (DCPS) pair. In addition, a digital mismatch compensation scheme is proposed and integrated into the SCS to reduce the design complexity of the power amplifier. This chip is manufactured in a 90 nm standard CMOS process with an active area of 0.06 mm2. The DCPS can generate phase-modulated signals at 100 MHz with 8-bit resolution and RMS error 9.33 ps (0.34°). The phase calculation can be performed at a maximum speed of 50 MHz using a 0.5 V supply voltage, resulting in a 73.88% power reduction. Comparing to state-of-the-art, the power consumption of the overall SCS is only 949.5 μW which minimizes the power overhead for an LINC transmitter. This SCS with the branch mismatch compensation provides a 0.02 dB gain and 0.15° phase fine-tune resolution without adding additional front-end circuits. Considering 1 dB gain and 10° phase mismatch, the system EVM of - 29.81 dB and ACPR of - 34.56 dB can still be achieved for 5 MHz bandwidth 64-QAM OFDM signals.

Nonlinear signal processing schemes for OFDM modulations within conventional or LINC transmitter structures

AbstractThis paper considers two classes of digital signal processing schemes, recently proposed by the authors, both combining a nonlinear operation in the time domain with a linear operation in the frequency domain, so as to reduce the envelope fluctuations and allow good power/bandwidth tradeoffs with Orthogonal Frequency Division Multiplexing (OFDM) transmission. These classes can be adopted within both conventional and two‐branch, LInear amplification with Nonlinear Components (LINC) transmitter structures . The paper addresses the statistical characterisation of the transmitted frequency‐domain blocks, so as to evaluate performances by analytical means, in a unified manner. A set of numerical results, which have been obtained with the help of this analytical method, is presented and discussed. These results allow a comparison between the two classes of signal processing schemes, namely for nonlinear operations of the ‘clipping’ type, when a conventional transmitter structure is employed. They also allow an accurate evaluation of the phase and gain imbalance effects in the two‐branch power amplification case. Copyright © 2008 John Wiley & Sons, Ltd.

Minimizing Spectral Leakage of Nonideal LINC Transmitters by Analysis of Component Impairments

LInear amplification with Nonlinear Components (LINC) is a distortion-free transmitter architecture under ideal conditions. However, in a realistic transmitter, imbalances such as dc offset, differential gain, and differential phase shift present in the quadrature modulators result in generation of out-of-band spectral components, even assuming that both amplifiers have identical characteristics. The level of distortion not only increases with the magnitude of the imbalances but also depends, to a large extent, on their relative phase arguments. The contribution of this paper is to provide a simplified characterization of the spectral leakage in the LINC transmitter in terms of quadrature modulator imbalances, with identical or mismatched amplifiers. This characterization allows identification of the transmitter with minimum spectral leakage under the constraint that the quadrature modulators perform equally well when used individually, i.e., in a transmitter composed of just the quadrature modulator followed by an amplifier. With reasonably small imbalances in the quadrature modulators and realistic amplifier mismatch, the spectral leakage at the output of a LINC transmitter driven by typical modulation schemes can be reduced by up to 10 dB, simply by a wise pairing of modulators

A New Mode-Multiplexing LINC Architecture to Boost the Efficiency of WiMAX Up-Link Transmitters

This paper proposes a new amplifier architecture based on the outphasing technique intended for the efficiency enhancement of linear amplification with nonlinear components (LINC) transmitters. The proposed mode-multiplexing linear amplification with nonlinear components (MM-LINC) scheme operates according to the LINC concept for input signal magnitude drive levels below a certain threshold and as a balanced amplifier beyond this threshold. The setting of this threshold level influences the performance of the amplifier in terms of average power-added efficiency and linearity. A 2-W up-link transmitter prototype for worldwide interoperability for microwave access (WiMAX) applications was designed using this new architecture and optimized for a WiMAX signal with an 11.3-dB peak-to-average power ratio. The experimental results revealed a significant increase in power-added efficiency, from 6% for a LINC transmitter to 21% for the MM-LINC amplifier, while maintaining an error vector magnitude value under 8%, which is compliant with the standard requirement

A 200-MHz IF BiCMOS signal component separator for linear LINC transmitters

The linear amplification with nonlinear components (LINC) transmitter is an architecture that provides linear amplification using nonlinear but power efficient amplifiers. The signal component separator (SCS) is a crucial signal processing function of LINC. It forms two constant-amplitude phase-modulated signal components from the input signal. Due to the nonlinear signal processing involved, digital signal processing (DSP) implementation of the SCS at baseband has so far been assumed to be the best choice although it suffers from matching, bandwidth and power consumption problems. In this paper a new SCS architecture based on analog integrated circuit techniques is presented to avoid the disadvantages in a DSP based realization. A 200-MHz IF SCS chip using the proposed architecture was designed and fabricated in a 0.8 /spl mu/m BiCMOS process. An experimental LINC transmitter was built with the SCS chip, nonlinear amplifiers and a power combiner. Test results showed that spurious levels around -50 dBc could be obtained with a /spl pi//4-shifted DQPSK modulated North American Digital Cellular (NADC) signal. This implies a high degree of linearity in the implemented LINC transmitter.

The effect of quantization in a digital signal component separator for LINC transmitters

Spectrally efficient modulation schemes have been chosen for the second generation of cellular systems in the United States and Japan, as well as for private mobile radio (PMR) systems. This paper analyzes the effect of quantization in a digital signal component separator (SCS) for linear amplification with nonlinear components (LINC) transmitters. The analysis relies on the fact that the two signal components can be obtained as the source signal plus/minus a signal in quadrature to the source signal. The equations derived are vital because they allow the designer to optimize digital signal processor (DSP) power consumption and bandwidth. The word lengths that are required for the source signal and the quadrature signal can be calculated from the amplifier gain characteristic, the probability density function for the modulation scheme, and the specified adjacent channel interference (ACI). The validity of the analysis has been verified by means of simulations.

Effect of modulation scheme on LINC transmitterpower efficiency

The authors describe how the power efficiency of an LINC (linear amplification with nonlinear components) transmitter is affected by the modulation scheme when using a conventional power combiner to recombine the constant envelope signals. The efficiency originating from the recombining peak at 60% for low-level modulation schemes but decreases to below 20% for high level schemes.