Energy-Efficient Distributed Estimation by Utilizing a Nonlinear Amplifier

abstract: Distributed estimation uses many inexpensive sensors to compose an accurate estimate of a given parameter. It is frequently implemented using wireless sensor networks. There have been several studies on optimizing power allocation in wireless sensor networks used for distributed estimation, the vast majority of which assume linear radio-frequency amplifiers. Linear amplifiers are inherently inefficient, so in this dissertation nonlinear amplifiers are examined to gain efficiency while operating distributed sensor networks. This research presents a method to boost efficiency by operating the amplifiers in the nonlinear region of operation. Operating amplifiers nonlinearly presents new challenges. First, nonlinear amplifier characteristics change across manufacturing process variation, temperature, operating voltage, and aging. Secondly, the equations conventionally used for estimators and performance expectations in linear amplify-and-forward systems fail. To compensate for the first challenge, predistortion is utilized not to linearize amplifiers but rather to force them to fit a common nonlinear limiting amplifier model close to the inherent amplifier performance. This minimizes the power impact and the training requirements for predistortion. Second, new estimators are required that account for transmitter nonlinearity. This research derives analytically and confirms via simulation new estimators and performance expectation equations for use in nonlinear distributed estimation. An additional complication when operating nonlinear amplifiers in a wireless environment is the influence of varied and potentially unknown channel gains. The impact of these varied gains and both measurement and channel noise sources on estimation performance are analyzed in this paper. Techniques for minimizing the estimate variance are developed. It is shown that optimizing transmitter power allocation to minimize estimate variance for the most-compressed parameter measurement is equivalent to the problem for linear sensors. Finally, a method for operating distributed estimation in a multipath environment is presented that is capable of developing robust estimates for a wide range of Rician K-factors. This dissertation demonstrates that implementing distributed estimation using nonlinear sensors can boost system efficiency and is compatible with existing techniques from the literature for boosting efficiency at the system level via sensor power allocation. Nonlinear transmitters work best when channel gains are known and channel noise and receiver noise levels are low.Dissertation/ThesisPh.D. Electrical Engineering 201

Wideband CMOS Data Converters for Linear and Efficient mmWave Transmitters

With continuously increasing demands for wireless connectivity, higher\ua0carrier frequencies and wider bandwidths are explored. To overcome a limited transmit power at these higher carrier frequencies, multiple\ua0input multiple output (MIMO) systems, with a large number of transmitters\ua0and antennas, are used to direct the transmitted power towards\ua0the user. With a large transmitter count, each individual transmitter\ua0needs to be small and allow for tight integration with digital circuits. In\ua0addition, modern communication standards require linear transmitters,\ua0making linearity an important factor in the transmitter design.In this thesis, radio frequency digital-to-analog converter (RF-DAC)-based transmitters are explored. They shift the transition from digital\ua0to analog closer to the antennas, performing both digital-to-analog\ua0conversion and up-conversion in a single block. To reduce the need for\ua0computationally costly digital predistortion (DPD), a linear and wellbehaved\ua0RF-DAC transfer characteristic is desirable. The combination\ua0of non-overlapping local oscillator (LO) signals and an expanding segmented\ua0non-linear RF-DAC scaling is evaluated as a way to linearize\ua0the transmitter. This linearization concept has been studied both for\ua0the linearization of the RF-DAC itself and for the joint linearization of\ua0the cascaded RF-DAC-based modulator and power amplifier (PA) combination.\ua0To adapt the linearization, observation receivers are needed.\ua0In these, high-speed analog-to-digital converters (ADCs) have a central\ua0role. A high-speed ADC has been designed and evaluated to understand\ua0how concepts used to increase the sample rate affect the dynamic performance

Modeling Approaches for Active Antenna Transmitters

The rapid growth of data traffic in mobile communications has attracted interest to Multiple-Input-Multiple-Output (MIMO) communication systems at millimeter-wave (mmWave) frequencies. MIMO systems exploit active antenna arrays transmitter configurations to obtain higher energy efficiency and beamforming flexibility. The analysis of transmitters in MIMO systems becomes complex due to the close integration of several antennas and power amplifiers (PAs) and the problems associated with heat dissipation. Therefore, the transmitter analysis requires efficient joint EM, circuit, and thermal simulations of its building blocks, i.e., the antenna array and PAs. Due to small physical spacing at mmWave, bulky isolators cannot be used to eliminate unwanted interactions between PA and antenna array. Therefore, the mismatch and mutual coupling in the antenna array directly affect PA output load and PA and transmitter performance. On the other hand, PAs are the primary source of nonlinearity, power consumption, and heat dissipation in transmitters. Therefore, it is crucial to include joint thermal and electrical behavior of PAs in analyzing active antenna transmitters. In this thesis, efficient techniques for modeling active antenna transmitters are presented. First, we propose a hardware-oriented transmitter model that considers PA load-dependent nonlinearity and the coupling, mismatch, and radiated field of the antenna array. The proposed model is equally accurate for any mismatch level that can happen at the PA output. This model can predict the transmitter radiation pattern and nonlinear signal distortions in the far-field. The model\u27s functionality is verified using a mmWave active subarray antenna module for a beam steering scenario and by performing the over-the-air measurements. The load-pull modeling idea was also applied to investigate the performance of a mmWave spatial power combiner module in the presence of critical coupling effects on combining performance. The second part of the thesis deals with thermal challenges in active antenna transmitters and PAs as the main source of heat dissipation. An efficient electrothermal modeling approach that considers the thermal behavior of PAs, including self-heating and thermal coupling between the IC hot spots, coupled with the electrical behavior of PA, is proposed. The thermal model has been employed to evaluate a PA DUT\u27s static and dynamic temperature-dependent performance in terms of linearity, gain, and efficiency. In summary, the proposed modeling approaches presented in this thesis provide efficient yet powerful tools for joint analysis of complex active antenna transmitters in MIMO systems, including sub-systems\u27 behavior and their interactions

Linearization techniques to suppress optical nonlinearity

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis is shown the implementation of the linearization techniques such as feedforward and pre-distortion feedback linearization to suppress the optical components nonlinearities caused by the fibre and semiconductor optical amplifier (SOA). The simulation verified these two linearization techniques for single tone direct modulation, two tone indirect modulation and ultra wideband input to the optical fibre. These techniques uses the amplified spontaneously emission (ASE) noise reduction in two loops of SOA by a feed-forward and predistortion linearizer and is shown more than 6dB improvement. Also it investigates linearization for the SOA amplifier to cancel out the third order harmonics or inter-modulation distortion (IMD) or four waves mixing. In this project, more than 20 dB reductions is seen in the spectral re-growth caused by the SOA. Amplifier non-linearity becomes more severe with two strong input channels leading to inter-channel distortion which can completely mask a third adjacent channel. The simulations detailed above were performed utilizing optimum settings for the variable gain, phase and delay components in the error correction loop of the feed forward and Predistortion systems and hence represent the ideal situation of a perfect feed-forward and Predistortion system. Therefore it should be consider that complexity of circuit will increase due to amplitude, phase and delay mismatches in practical design. Also it has describe the compatibility of Software Defined Radio with Hybrid Fibre Radio with simulation model of wired optical networks to be used for future research investigation, based on the star and ring topologies for different modulation schemes, and providing the performance for these configurations

Analysis and design of ΣΔ Modulators for Radio Frequency Switchmode Power Amplifiers

Power amplifiers are an integral part of every basestation, macrocell, microcell and mobile phone, enabling data to be sent over the distances needed to reach the receiver’s antenna. While linear operation is needed for transmitting WCDMA and OFDM signals, linear operation of a power amplifier is characterized by low power efficiency, and contributes to unwanted power dissipation in a transmitter. Recently, a switchmode power amplifier operation was considered for reducing power losses in a RF transmitter. A linear and efficient operation of a PA can be achieved when the transmitted RF signal is ΣΔ modu- lated, and subsequently amplified by a nonlinear device. Although in theory this approach offers linearity and efficiency reaching 100%, the use of ΣΔ modulation for transmitting wideband signals causes problems in practical implementation: it requires high sampling rate by the digital hardware, which is needed for shaping large contents of a quantization noise induced by the modulator but also, the binary output from the modulator needs an RF power amplifier operating over very wide frequency band. This thesis addresses the problem of noise shaping in a ΣΔ modulator and nonlinear distortion caused by broadband operation in switchmode power amplifier driven by a ΣΔ modulated waveform. The problem of sampling rate increase in a ΣΔ modulator is solved by optimizing structure of the modulator, and subsequent processing of an input signal’s samples in parallel. Independent from the above, a novel technique for reducing quan- tization noise in a bandpass ΣΔ modulator using single bit quantizer is presented. The technique combines error pulse shaping and 3-level quantization for improving signal to noise ratio in a 2-level output. The improvement is achieved without the increase of a digital hardware’s sampling rate, which is advantageous also from the perspective of power consumption. The new method is explored in the course of analysis, and verified by simulated and experimental results. The process of RF signal conversion from the Cartesian to polar form is analyzed, and a signal modulator for a polar transmitter with a ΣΔ-digitized envelope signal is designed and implemented. The new modulator takes an advantage of bandpass digital to analog conversion for simplifying the analog part of the modulator. A deformation of the pulsed RF signal in the experimental modulator is demonstrated to have an effect primarily on amplitude of the RF signal, which is correctable with simple predistortion

Digital predistortion of RF amplifiers using baseband injection for mobile broadband communications

Radio frequency (RF) power amplifiers (PAs) represent the most challenging design parts of wireless transmitters. In order to be more energy efficient, PAs should operate in nonlinear region where they produce distortion that significantly degrades the quality of signal at transmitter’s output. With the aim of reducing this distortion and improve signal quality, digital predistortion (DPD) techniques are widely used. This work focuses on improving the performances of DPDs in modern, next-generation wireless transmitters. A new adaptive DPD based on an iterative injection approach is developed and experimentally verified using a 4G signal. The signal performances at transmitter output are notably improved, while the proposed DPD does not require large digital signal processing memory resources and computational complexity. Moreover, the injection-based DPD theory is extended to be applicable in concurrent dual-band wireless transmitters. A cross-modulation problem specific to concurrent dual-band transmitters is investigated in detail and novel DPD based on simultaneous injection of intermodulation and cross-modulation distortion products is proposed. In order to mitigate distortion compensation limit phenomena and memory effects in highly nonlinear RF PAs, this DPD is further extended and complete generalised DPD system for concurrent dual-band transmitters is developed. It is clearly proved in experiments that the proposed predistorter remarkably improves the in-band and out-of-band performances of both signals. Furthermore, it does not depend on frequency separation between frequency bands and has significantly lower complexity in comparison with previously reported concurrent dual-band DPDs