Non-constant Envelope Modulation Research Articles

A numerical assessment of an effective envelope-tracking semiconductor optical amplifier design for coherent-optical OFDM transmission

The inherent nonlinear effects associated to Semiconductor Optical Amplifiers (SOAs) may translate into a transmission performance loss for non-constant envelope modulation formats. However, a variety of linearization schemes may be adopted for coping with these impairments and offering an effective system design. In this paper, an envelope tracking (ET) technique is investigated for linearizing an SOA-based Coherent Optical OFDM transmitter. An optimized design of the ET subsystem is performed under various scenarios, with the eventual joint use of Peak-to-Average Power Ratio (PAPR) reduction either via hard-clipping or nonlinear companding. A thorough carrier density analysis is performed in the amplifier, for various target gain values, so as to assess the effectiveness of the proposed scheme. Moreover, we investigate the robustness of the proposed approach against some parameters variation both inside the ET path (DAC characteristics and bandwidth limited envelope generation). Extensive simulations performed with a precise SOA model show that up to 8 dB (resp. 7 dB) BER improvement can be achieved via the proposed scheme in the case of 4-QAM/OFDM (resp. 16-QAM/OFDM), compared to the conventional system with no linearization, and that even an envelope quantized with 2 bits still enables a significant performance increase.

Joint algorithm for burst detection and AGC improvement in high throughput software defined radio waveform

The SDR networking waveforms mostly use non-constant envelope modulation schemes at the physical layer requiring a stable AGC operation during envelope amplitude variations. Moreover, the size of network packets may vary form a single burst to multiple bursts requiring an AGC with fast convergence to reduce settling time overhead. In this paper, we propose a novel joint algorithm for both the burst detection and improvement of the convergence time and stability of a conventional AGC by using Zadoff-Chu sequence. A specifically designed repeated Zadoff-Chu sequence is used for the detection of burst and fixing the gain for the complete burst duration. The proposed algorithm is implemented on Field Programmable Gate Array (FPGA) on SDR platform. Results of both the simulation and implementation are shown to demonstrate the effectiveness of the proposed technique.

Frequency Reconfigurable mm-Wave Power Amplifier With Active Impedance Synthesis in an Asymmetrical Non-Isolated Combiner: Analysis and Design

A frequency reconfigurable millimeter-wave (mm-wave) power amplifier (PA), which can be programmed to operate efficiently for a wide swathe of the spectrum, approaching an universal transmitter, can enable a wide range of novel applications in high-speed communication, sensing, and imaging. Classical techniques to allow large operating range either rely on broadband higher order output combining networks or tunable passives, both of which tradeoff directly with output power and efficiency. In this paper, we present an active impedance synthesis methodology that exploits the interaction of multiple unit PA cells in an asymmetrical non-isolated combiner to synthesize complex mm-wave impedances in a programmable fashion. This allows the interacting power cells to see their optimal load-pull impedances for high-efficiency frequency-reconfigurable operation in an efficient combiner network with no lossy variable passives. Compared to a symmetrical combiner, this enables the network to break the strong tradeoffs between output power, efficiency, and frequency reconfigurability, allowing it to achieve N times the Bode-Fano bound compared with an N-way symmetrical architecture. As a proof of concept, an integrated PA, which is in a 0.13-μm SiGe process, is demonstrated to achieve P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> of 23.6 dBm at a power-added efficiency (PAE) of 27.7% at 55 GHz with a frequency reconfigurable P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> , <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sub> dB bandwidth of 25 GHz (40-65 GHz) and a PAE, <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sub> dB bandwidth of 20 GHz (40-60 GHz). Multi-Gbps data rates are demonstrated with non-constant envelope modulations across the frequencies 40-60 GHz.

Active Noise Filtering for $X$ -Band GaN Transmitters With Bitstream Modulations

Bitstream transmitters based on bandpass or envelope delta–sigma modulation promise high power efficiency for broadband communications with nonconstant-envelope modulations, but at the price of elevated quantization noise. An active noise filtering technique is proposed in this paper to address the quantization noise issue of such transmitters. The essential concept of active noise cancellation is to utilize multiple power amplifiers (PAs) with identical or similar transfer characteristics fed with the same input, but with different time delays. The difference in time delay determines whether the outputs of the PAs are combined in phase or out of phase at a certain frequency, which forms the passband or the stopband of the active filter. The active noise filtering technique can potentially replace the external high-Q passive bandpass filter typically needed for such transmitters. A two-channel bitstream transmitter with active noise cancellation operating at the $X$ -band is fabricated with 0.25- $\mu \text{m}$ discrete GaN HEMT devices. It has been demonstrated experimentally that the proposed active noise filtering architecture effectively suppresses quantization noise as expected and delivers a 34.5-dBm output power with 37.6% drain efficiency and 30% power added efficiency for a single-channel WCDMA signal with 100-MHz bandwidth and 5.3-dB peak-to-average ratio. Adjacent channel leakage power ratio (ACLR) at 100-MHz offsets is measured to be 36.6/35.4 dBc without using digital predistortion.

A Novel ML-2D-LUT Based Adaptive Predistorter of High Power Amplifier Using New Improved RLS Algorithm

This paper introduces a Multilevel two dimensional Look-Up-Table (ML-2D-LUT) based adaptive predistortion (APD) technique to reduce gain distortion in the saleh modeled power amplifier (HPA). The quick initialization of the ML-2D-LUT, a new improved RLS algorithm is implemented to compensate the time-varying problem in the HPA. The proposed method distinguishes the coefficients successfully. Moreover, additional convergence in steady state and less steady state error (SSE) will be accomplished for the power amplifiers with profound impacts in the memory by using the modified step-size parameter. Simulation results demonstrate that the proposed technique offers enhanced the PA efficiency for non-constant envelope modulations and nonlinear distortions can be extraordinarily decreased with the NIRLS algorithm. The BER and MSE results conclude that the proposed technique gives better results than conventional RLS.

Mitigation of Oscillator Pulling in SoCs

This paper presents a novel method to mitigate undesirable oscillator pulling effects in wireless transmitter architectures. In practice, coupling of the power amplifier (PA) signal caused by inductive or electromagnetic crosstalk, inevitably leads to injection pulling. Particularly, for nonconstant envelope modulation schemes such as Bluetooth enhanced data rate (EDR), GSM-EDGE, or WiFi, the pulling is extremely troublesome due to the amplitude-to-frequency (AM–FM) conversion effect. The difficulty to cope with this undesirable effect stems from the fact that the pulling is largely dependent on process spread and environmental factors like temperature and antenna load mismatch. This paper proposes a novel method based on stochastic-gradient algorithms to resolve the pulling effect. It can be realized inexpensively in digital design and does not rely on knowledge of the unpredictable crosstalk path which makes it extremely efficient to mitigate the pulling effect. To verify the novel idea, a prototype IC has been fabricated in 65 nm CMOS technology.

A mm-Wave Segmented Power Mixer

The segmented power-mixer array based mm-wave power generation architecture is demonstrated to be an energy-efficient technique for generating high-speed nonconstant envelope modulations. High output power levels are achieved by efficiently combining power from several power mixers using an area efficient dual-primary distributed active transformer. The segmented scheme leads to back-off efficiency improvements while simultaneously providing direct envelope modulation eliminating the need for high-speed high-efficiency supply modulators. The power mixer is implemented in a 32-nm silicon-on-insulator CMOS process and provides a peak output power of 19.1 dBm at 51 GHz with a drain efficiency of 14.2% and a peak power-added efficiency of 10.1%. High-speed constant (binary phase-shift keying, quadrature phase-shift keying), as well as nonconstant envelope modulations ( $m$ -amplitude shift keying, quadrature amplitude modulation) show the versatility of the architecture towards spectrally efficient modulation schemes. Reliability against segment breakdown over long periods of time at 30% higher supply voltages has also been demonstrated.

Integrated Self-Healing for mm-Wave Power Amplifiers

Self-healing as a technique for improving performance and yield of millimeter-wave power amplifiers (PAs) against process variation and transistor mismatch, load impedance mismatch, and partial and total transistor failure is described and investigated. A 28-GHz PA is presented with three types of sensors, two types of actuators, data converters, and a digital algorithm block that are all integrated on a single chip to show the validity of the technique. Two algorithms are implemented to either maximize output power or to minimize dc power for a desired output power. Measurements from 20 chips show increased RF output power up to 3 dB or reduced dc power by 50% in backoff with a 50- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\Omega$</tex></formula> load. Self-healing with up to 4-1 voltage standing-wave ratio load impedance mismatch is verified and linear operation under nonconstant envelope modulation is shown to improve with healing. Self-healing after laser cutter induced transistor failure is verified and increases RF output power by up to 5.4 dB. The aggregate yield of the PA across several representative specifications is increased from 0% to 80% with self-healing.

An energy-efficient polar transmitter for IEEE 802.15.6 body area networks: system requirements and circuit designs

This article discusses the radio system requirements and design trade-offs of a low-power 2.3/2.4 GHz IEEE 802.15.6-narrowband transmitter for body area network applications. The IEEE 802.15.6 narrowband PHY adopts nonconstant envelope DPSK modulations. Transmitter architecture selection is a major challenge because it needs to provide both amplitude and phase modulation, with good efficiency and strict requirements on modulation accuracy and linearity for non-constant envelope modulations. A PLL-based polar transmitter with a Sigma-Delta digitally controlled power amplifier (ΣΔ-DPA) is employed for its excellent energy efficiency. Finally, this article discusses the circuit design considerations and implementation results. With balance between analog and digital implementation, and careful planning of the radio system, the polar transmitter presented in this article can meet all of the specifications defined in IEEE 802.15.6 with abundant margin, while having extremely low power consumption of 5.4 mW and excellent energy efficiency of 5.4 nJ/bit.

Efficiency Enhancement of Pico-cell Base Station Power Amplifier MMIC in GaN HFET Technology Using the Doherty Technique

1 Abstract With the growth of smart-phones, the demand for more broadband, data-centric technologies are being driven higher. As mobile operators worldwide plan and deploy 4 generation (4G) networks such as LTE to support the relentless growth in mobile data demand, the need for strategically positioned pico-sized cellular base stations known as ‘pico-cells’ are gaining traction. In addition to having to design a transceiver in a much compact footprint, pico-cells must still face the technical challenges presented by the new 4G systems, such as reduced power consumptions and linear amplification of the signals. The RF power amplifier (PA) that amplifies the output signals of 4G pico-cell systems face challenges to minimize size, achieve high average efficiencies and broader bandwidths while maintaining linearity and operating at higher frequencies. 4G standards as LTE use nonconstant envelope modulation techniques with high peak to average ratios. Power amplifiers implemented in such applications are forced to operate at a backed off region from saturation. Therefore, in order to reduce power consumption, a design of a high efficiency PA that can maintain the efficiency for a wider range of radio frequency signals is required. The primary focus of this thesis is to enhance the efficiency of a compact RF amplifier suitable for a 4G pico-cell base station. For this aim, an integrated two way Doherty amplifier design in a compact 10x11.5mm monolithic microwave integrated circuit using GaN device technology is presented. Using non-linear GaN HFETs models, the design achieves high efficiencies of over 50% at both back-off and peak power regions without compromising on the stringent linearity requirements of 4G LTE standards. This demonstrates a 17% increase in power added efficiency at 6 dB back off from peak power compared to conventional Class AB amplifier performance. Performance optimization techniques to select between high efficiency and high linearity operation are also presented. Overall, this thesis demonstrates the feasibility of an integrated HFET Doherty amplifier for LTE band 7 which entails the frequencies from 2.62-2.69GHz. The realization of the layout and various issues related to the PA design is discussed and attempted to be solved.

Cognitive Polar Receiver Using Two Injection-Locked Oscillator Stages

A novel cognitive polar receiver that utilizes two injection-locked oscillator stages to extract the modulation envelope and phase components of a received nonconstant envelope modulation signal without using phase-locked-loop-based carrier recovery circuitry is presented. The paper begins with a theoretical analysis of injection locking and pulling phenomena based on the discrete-time computation approach, and then develops the principles of the proposed receiver. The implemented prototype can cover a sensing bandwidth of 140 MHz at a central frequency of 2.43 GHz and perform π/4 differential quadrature phase-shift keying and quadrature phase-shift keying demodulation with the best error vector magnitudes of 6.6% and 7.9%, respectively, both at a symbol rate of 2 Ms/s. Due to its simplicity, the proposed receiver has great potential as an energy-efficient architecture with low complexity for short-range wireless communications.

Design and Experimental Evaluation of a LINC Transmitter for OFDM Systems

The LInear amplification using Nonlinear Components (LINC) technique is a well-known power amplifier linearization method to reduce out-of-band interferences in a nonconstant envelope modulation system. Its major drawback is the inherent sensitivity to gain and phase imbalances between the two amplifier branches. In this paper a digital correction method, which corrects these gain and phase imbalances, is implemented and verified. Commercial analog devices and a digital platform for signal processing purposes are used. The experimental results applied in an OFDM transmitter show a linearity improvement close to 20 dB without reducing the output power and without increasing the in-band distortion.

Linearizing CMOS Switching Power Amplifiers Using Supply Regulators

In high-speed communications systems, the power amplifier (PA) is the dominant source of power consumption from the battery and, thus, one of the main limitations in increased mobility. Switching PAs are more efficient than their linear counterparts and, thus, demand less power from the battery; however, they do not have sensitivity to the amplitude variations seen in nonconstant-envelope modulation. This brief discusses supply modulation, a technique that enables use of switching PAs for improved power efficiency, by providing a means of linearizing the PA.

An Octave-Range, Watt-Level, Fully-Integrated CMOS Switching Power Mixer Array for Linearization and Back-Off-Efficiency Improvement

Non-constant envelope modulation schemes have become commonplace in cellular applications due to their higher spectral efficiency. Linear PAs driven by an RF transmitter are usually used to generate these signals faithfully at the expense of power efficiency. Separate processing of amplitude and phase information (e.g., EER or polar modulation) has been proposed [1] as a way of improving the system power efficiency. However, many of these schemes require an efficient high-power low-frequency supply modulator to reconstruct the amplitude information. This can be done, for instance, using a switching DC-to-DC converter with its own limitations in efficiency, bandwidth, and area that also requires an external inductor [2]. Even for an ideal supply modulator, the amplitude dynamic range of the PA itself is limited by the gate-to-drain feedthrough and its associated AM-to-AM and AM-to-PM conversion. (For example, in [3] a 10dB change in the supply results in a 5° phase shift at the output.) Although a digitally-modulated polar PA [4] has been shown as a possible solution at lower power levels, its implementation in a wideband watt-level fully-integrated CMOS PA for non-constant modulation has not yet been demonstrated.

Accurate Characterization of TWTA Distortion in Multicarrier Operation by Means of a Correlation-Based Method

Complex payload architectures, together with the use of high-order digital modulations and advanced coding schemes, call for an accurate evaluation of traveling-wave tube amplifier (TWTA) nonlinearity impairments. To assess in a simple yet accurate manner the level of signal distortion generated by the TWTA nonlinearity, we adopt a correlation-based methodology. The proposed method provides an accurate evaluation of the TWTA intermodulation, particularly in the presence of multicarrier operation with nonconstant envelope modulations.

Design and Linearization of Class-E Power Amplifier for Nonconstant Envelope Modulation

This paper presents design and linearization techniques for a highly efficient, but nonlinear Class-E power amplifier (PA) applied to linear RF transmitters. To achieve maximum efficiency for Class-E PA operating above gigahertz frequencies, the proposed design theory modifies the classic Class-E condition and considers switch-on and switch-off resistances in deriving the analytical solution. For linearization, a digital envelope predistorter (DEP) is used to provide predistorted envelope modulation for the RF-input and supply-voltage signals fed to the Class-E PA. This DEP can successfully compensate for AM/AM and AM/PM distortions such that the Class-E PA, although classified as a switching-mode PA, can act as a linear PA. A cdma2000 1 times transmitter incorporating this Class-E PA and DEP is demonstrated to simultaneously achieve high efficiency and high modulation quality.

A block-based predistortion for high power-amplifier linearization

A predistortion (PD) technique has been proposed to linearize the power amplifier (PA) and improve the PA efficiency for nonconstant envelope modulation schemes. In this paper, a new PD scheme based on block estimation suitable for burst-type communications is described. The purpose of the PD is to increase the efficiency of the PA while fulfilling the error vector magnitude and spectrum mask system requirements. Piecewise (i.e., segmented) regression with continuity constraints is applied to estimate the AM/AM and AM/PM distortions. Analytical results using pth-order regression within each segment is presented. With this method, the training period is minimized. Further, the method do not suffer from bad performance during training. Simulation results with 16 quadrature amplitude modulation (QAM) show significant reduction in adjacent channel interference and considerable improvement in constellation quality for a typical class-AB high PA. The method is implemented on a hardware platform containing a digital signal processor, a field programmable gate array, a modulator, and a 1.6-GHz class-AB PA. Experimental results with a 16-QAM modulated signal and a symbol rate of 151.2 kHz are presented

A transmitter architecture for nonconstant envelope modulation

This paper presents a transmitter architecture which is less sensitive to power amplifier AM/AM and AM/PM conversions. Indeed, most radio communication standards employ nonconstant envelope modulation such as in the 802.11 series. Owing to power amplifiers' nonlinearities, distortions on the modulations degrade the overall transmitter performance. As a consequence, linearization methods, such as envelope elimination and restoration, have to be employed. The architecture relies on the transformation of a nonconstant envelope modulation to a constant or switched one. This signal coding enables the efficient amplification of the signal without undergoing power amplifier distortions. The envelope variation is restored by bandpass filtering just after the power amplifier

An Adaptive Digital Method of Imbalances Cancellation in LINC Transmitters

The linear amplification using nonlinear components (LINC) technique is a well-known power amplifier linearization method to reduce adjacent channel interference in a nonconstant envelope modulation system. Its major drawback is the inherent sensitivity to gain and phase imbalances between the two amplifier branches. In this paper, a novel full-digital base band method is described which corrects any gain and phase imbalances in LINC transmitters mainly due to the unmatching of the two amplifier paths. Amplifiers are characterized by a level-dependent complex gain using a memoryless model. The method uses adaptive signal processing techniques to obtain the optimal complex coefficient to adjust gain and phase imbalances. Its main advantage is the ability to track the input signal variations and adapt to the changes of amplifier nonlinear characteristics. Other effects are included in the analysis such as quadrature modulator and demodulator imbalances and loop delay. A computer simulation has been carried out to verify the method functionality.

Nonlinear Distortion Cancellation Using LINC Transmitters in OFDM Systems

The LInear amplification using Nonlinear Components (LINC) technique is a well-known power amplifier linearization method to reduce out-of-band interferences in a nonconstant envelope modulation system, such as Digital Video Broadcasting (DVB) system, which is based on a very sensitive to nonlinear distortions OFDM modulation scheme. The major drawback of LINC transmitters is the inherited sensitivity to gain and phase imbalances between the two amplifier branches. In this paper, a novel full-digital base band method, which corrects any gain and phase imbalances in LINC transmitters mainly due to the un-matching of the two amplifier paths, is described. Amplifiers are characterized by a level-dependent complex gain using a memoryless model. The method uses adaptive signal processing techniques to obtain the optimal complex coefficients to correct gain and phase imbalances. Its main advantage is the ability to track the input signal variations and adapt to the changes of amplifier nonlinear characteristics. Other effects are included in the analysis such as quadrature modulator and demodulator impairments. A computer simulation has been carried out to verify method functionality.