Robust Near-Field Adaptive Beamforming with Distance Discrimination

This paper proposes a robust near-field adaptive beamformer for microphone array applications in small rooms. Robustness against location errors is crucial for near-field adaptive beamforming due to the difficulty in estimating near-field signal locations especially the radial distances. A near-field regionally constrained adaptive beamformer is proposed to design a set of linear constraints by filtering on a low rank subspace of the near-field signal over a spatial region and frequency band such that the beamformer response over the designed spatial-temporal region can be accurately controlled by a small number of linear constraint vectors. The proposed constraint design method is a systematic approach which guarantees real arithmetic implementation and direct time domain algorithms for broadband beamforming. It improves the robustness against large errors in distance and directions of arrival, and achieves good distance discrimination simultaneously. We show with a nine-element uniform linear array that the proposed near-field adaptive beamformer is robust against distance errors as large as ±32% of the presumed radial distance and angle errors up to ±20⁰. It can suppress a far field interfering signal with the same angle of incidence as a near-field target by more than 20 dB with no loss of the array gain at the near-field target. The significant distance discrimination of the proposed near-field beamformer also helps to improve the dereverberation gain and reduce the desired signal cancellation in reverberant environments

Adaptive Interference Mitigation in GPS Receivers

Satellite navigation systems (GNSS) are among the most complex radio-navigation systems, providing positioning, navigation, and timing (PNT) information. A growing number of public sector and commercial applications rely on the GNSS PNT service to support business growth, technical development, and the day-to-day operation of technology and socioeconomic systems. As GNSS signals have inherent limitations, they are highly vulnerable to intentional and unintentional interference. GNSS signals have spectral power densities far below ambient thermal noise. Consequently, GNSS receivers must meet high standards of reliability and integrity to be used within a broad spectrum of applications. GNSS receivers must employ effective interference mitigation techniques to ensure robust, accurate, and reliable PNT service. This research aims to evaluate the effectiveness of the Adaptive Notch Filter (ANF), a precorrelation mitigation technique that can be used to excise Continuous Wave Interference (CWI), hop-frequency and chirp-type interferences from GPS L1 signals. To mitigate unwanted interference, state-of-the-art ANFs typically adjust a single parameter, the notch centre frequency, and zeros are constrained extremely close to unity. Because of this, the notch centre frequency converges slowly to the target frequency. During this slow converge period, interference leaks into the acquisition block, thus sabotaging the operation of the acquisition block. Furthermore, if the CWI continuously hops within the GPS L1 in-band region, the subsequent interference frequency is locked onto after a delay, which means constant interference occurs in the receiver throughout the delay period. This research contributes to the field of interference mitigation at GNSS's receiver end using adaptive signal processing, predominately for GPS. This research can be divided into three stages. I first designed, modelled and developed a Simulink-based GPS L1 signal simulator, providing a homogenous test signal for existing and proposed interference mitigation algorithms. Simulink-based GPS L1 signal simulator provided great flexibility to change various parameters to generate GPS L1 signal under different conditions, e.g. Doppler Shift, code phase delay and amount of propagation degradation. Furthermore, I modelled three acquisition schemes for GPS signals and tested GPS L1 signals acquisition via coherent and non-coherent integration methods. As a next step, I modelled different types of interference signals precisely and implemented and evaluated existing adaptive notch filters in MATLAB in terms of Carrier to Noise Density (\u1d436/\u1d4410), Signal to Noise Ratio (SNR), Peak Degradation Metric, and Mean Square Error (MSE) at the output of the acquisition module in order to create benchmarks. Finally, I designed, developed and implemented a novel algorithm that simultaneously adapts both coefficients in lattice-based ANF. Mathematically, I derived the full-gradient term for the notch's bandwidth parameter adaptation and developed a framework for simultaneously adapting both coefficients of a lattice-based adaptive notch filter. I evaluated the performance of existing and proposed interference mitigation techniques under different types of interference signals. Moreover, I critically analysed different internal signals within the ANF structure in order to develop a new threshold parameter that resets the notch bandwidth at the start of each subsequent interference frequency. As a result, I further reduce the complexity of the structural implementation of lattice-based ANF, allowing for efficient hardware realisation and lower computational costs. It is concluded from extensive simulation results that the proposed fully adaptive lattice-based provides better interference mitigation performance and superior convergence properties to target frequency compared to traditional ANF algorithms. It is demonstrated that by employing the proposed algorithm, a receiver is able to operate with a higher dynamic range of JNR than is possible with existing methods. This research also presents the design and MATLAB implementation of a parameterisable Complex Adaptive Notch Filer (CANF). Present analysis on higher order CANF for detecting and mitigating various types of interference for complex baseband GPS L1 signals. In the end, further research was conducted to suppress interference in the GPS L1 signal by exploiting autocorrelation properties and discarding some portion of the main lobe of the GPS L1 signal. It is shown that by removing 30% spectrum of the main lobe, either from left, right, or centre, the GPS L1 signal is still acquirable

Intelligent Processing in Wireless Communications Using Particle Swarm Based Methods

There are a lot of optimization needs in the research and design of wireless communica- tion systems. Many of these optimization problems are Nondeterministic Polynomial (NP) hard problems and could not be solved well. Many of other non-NP-hard optimization problems are combinatorial and do not have satisfying solutions either. This dissertation presents a series of Particle Swarm Optimization (PSO) based search and optimization algorithms that solve open research and design problems in wireless communications. These problems are either avoided or solved approximately before. PSO is a bottom-up approach for optimization problems. It imposes no conditions on the underlying problem. Its simple formulation makes it easy to implement, apply, extend and hybridize. The algorithm uses simple operators like adders, and multipliers to travel through the search space and the process requires just five simple steps. PSO is also easy to control because it has limited number of parameters and is less sensitive to parameters than other swarm intelligence algorithms. It is not dependent on initial points and converges very fast. Four types of PSO based approaches are proposed targeting four different kinds of problems in wireless communications. First, we use binary PSO and continuous PSO together to find optimal compositions of Gaussian derivative pulses to form several UWB pulses that not only comply with the FCC spectrum mask, but also best exploit the avail- able spectrum and power. Second, three different PSO based algorithms are developed to solve the NLOS/LOS channel differentiation, NLOS range error mitigation and multilateration problems respectively. Third, a PSO based search method is proposed to find optimal orthogonal code sets to reduce the inter carrier interference effects in an frequency redundant OFDM system. Fourth, a PSO based phase optimization technique is proposed in reducing the PAPR of an frequency redundant OFDM system. The PSO based approaches are compared with other canonical solutions for these communication problems and showed superior performance in many aspects. which are confirmed by analysis and simulation results provided respectively. Open questions and future Open questions and future works for the dissertation are proposed to serve as a guide for the future research efforts

Multiple Antenna-based GPS Multipath Mitigation using Code Carrier Information

학위논문 (박사)-- 서울대학교 대학원 : 전기공학부, 2013. 8. 최진영.여러 응용분야에서 수 억대의 GPS(Global Positioning System) 수신기가 사용되고 있지만, GPS을 기반으로 하는 위치기반 서비스(LBS: Location Based Services)에서는 여전히 다중경로 오차와 같은 전파 방해가 발생하고 있으며, 이러한 오차들로 인하여 상관함수의 왜곡은 거리 오차가 발생에 영향을 미치고 있다. 이러한 이유로 인하여 GPS을 이용한 항법 시스템에서의 위치 정확도 향상을 위하여, 다중경로 오차를 효과 적으로 줄이기 위한 강인하고 현실적인 방법이 요구된다. 다중경로는 GPS 신호가 장애물에 의해 반사나 회절 되어 수신기에 도착할 때 잘 일어난다. 가시경로 신호에 결합된 다중경로 신호는 GPS 수신기의 상관함수의 변형을 일으키며 궁극적으로 차별함수에 영향을 미치므로 거리오차를 발생시킨다. 그러므로 다중경로 오차는 위성항법 시스템에서의 위치정확도 향상을 위해 해결 되어야 될 문제로 쟁점이 되어왔다. 최근에는 이러한 전파 간섭신호를 줄이기 위하여 다중개의 안테나(Multiple Antenna)를 이용하는 방법이 GPS 항법 시스템에서 이용되고 있다. 현 시점에서, 다중개의 안테나를 사용하는 응용분야는 주로 학술적인 연구 및 복잡한 군사용 연구로 주로 진행 되었다. 그러나 안테나 제작 방법 및 전기적 시스템의 급격한 발전으로 인해 이전의 하드웨어 및 소프웨어적인 문제를 쉽게 해결 됨에 따라 가까운 미래에는 다중 안테나 기반의 수신기가 민간 상용분야로 확대 될 것으로 예상이 된다. 또한 안테나 수신기 RF단의 소형화로 인하여 다중 안테나 시스템에서의 안테나 크기 문제점 또한 해결 가능하다. 그러므로 본 논문에서는 다중 GPS 안테나를 이용하여 GPS 항법에서의 전파 간섭 및 다중경로 오차 감쇄에 대한 연구를 목적으로 한다. 본 연구는 강한 전파 간섭 및 다중경로 신호에 대하여 공간 처리 기법을 적용한다. 제안된 새로운 방법은 다중 안테나를 기반의 코드 케리어 정보를 이용한 공간처리 기법으로 전파 간섭 및 다중경로 오차를 완화시키며, 또한 빔형성 기법을 이용하여 신호 대 잡음 비율을 최대로 한다. 제안된 성능을 검증하기 위하여 소프트웨어 GPS 수신기를 사용된다. 소프트웨어 GPS 수신기를 이용한 신호처리 기법은 새로운 장비의 제품화 및 GPS 신호 분석에 장점을 가지고 있다. 또한 GPS 알고리즘 분석 및 수신기 성능 향상 검증 등 여러 연구분야에서 널리 이용되고 있다. 본 논문에서는 제안된 방법의 성능 검증을 위하여 컴퓨터 시뮬레이션 및 가공 IF 데이터를 이용한 소프트웨어 수신기 결과를 제시한다. 그 결과 제안된 방법은 전파 간섭 및 다중경로 오차 감쇄에 강인하며, GPS 항법시스템에서의 위치정확도 향상에 가능성을 보여준다. 그로므로 제안된 방법은 차량 항법 응용분야에서 방해신호 감쇄에 사용될 것으로 예상된다.Although hundreds of millions of receivers are used all around the world, the performance of location-based services(LBS) provided by GPS is still compromised by interference which includes unintentional distortion of correlation function due to multipath propagation. For this reason, the requirement for proper mitigation techniques becomes crucial in GPS receivers for robust, accurate, and reliable positioning. Multipath propagation can easily occur when environmental features cause combinations of reflected and diffracted replica signals to arrive at the receiving antenna. These signals which are combined with the original line-of-sight (LOS) signal can cause distortion of the receiver correlation function and ultimately distortion of the discrimination functionhence, errors in range estimation occur. Therefore, multipath error in the satellite navigation system to improve location accuracy is an important issue to be addressed. Recently, interference mitigation techniques utilizing multiple antennas have gained significant attention in GPS navigation systems. Although at the time of this dissertation, employing multiple antennas in GPS applications is mostly limited to academic research and possibly complicated military applications, it is expected that in the near future, antenna array-based receivers will also become widespread in civilian commercial markets. Rapid advances in antenna design technology and electronic systems make previously challenging problems in hardware and software easier to solve. Furthermore, due to the significant effort devoted to miniaturization of RF front-ends and antennas, the size of antenna array based receivers will no longer be a problem. Given the above, this dissertation investigates multiple antenna-based GPS the interference suppression and multipath mitigation. Firstly, a modified spatial processing technique is proposed that is capable of mitigating both high power interference and coherent and correlated GPS multipath signals. The use of spatial-temporal processing for GPS multipath mitigation is studied. A new method utilizing code carrier information based on multiple antennas is proposed to deal with highly correlated multipath components and to increase the signal to noise ratio of the beamformer by synthesizing antenna array processing. In order to verify the proposed method, a software defined GPS receiver is used. Software-based GPS signal processing technique has already produced benefits for prototyping new equipment and analyzing GPS signal quality. Not only do such receivers provide an excellent research tool for GPS algorithm verification, they also improve GPS receiver performance in a wide range of conditions. In this dissertation, the enhancement of the proposed method is presented in terms of the simulations and software defined GPS receiver using simulated IF data. From the result, the proposed method is robust to interference suppression, and multipath mitigation, and shows a strong possibility for use in improving location accuracy. Thus, this method can be employed to mitigate interference signals in vehicular navigation applications.Contents Abstract i Acknowledgements iv Contents v List of Figures x List of Tables xiv Chapter 1.Introduction 1 1.1 Introduction 1 1.2 Background and Motivation 2 1.2.1 Strong Narrowband and Wideband Interference 6 1.2.2 Multipath 7 1.3 Antenna Array Processing in GPS 11 1.3.1 Interference Suppression 11 1.3.2 Multipath Mitigation 13 1.4 Software-Defined GPS Receiver 15 1.5 Objective and Contribution 17 1.6 Dissertation Outline 18 Chapter 2. Global Positioning System 21 2.1 GPS System Overview 21 2.2 Basic Concept of GSP 25 2.3 Determining Satellite to User 28 2.4 Calculation of User Position 33 2.5 GPS Error Sources 40 2.5.1 Receiver Clock Bias 41 2.5.2 Satellite Clock Bias 42 2.5.3 Atmospheric Delay 43 2.5.4 Ephemeris Delay 46 2.5.5 Multipath Error 47 2.5.6 Receiver Noise 55 2.6 Summary 55 Chapter 3. Antenna Array Processing and Beamforming 56 3.1 Background on Antenna Arrays and Beamformers 56 3.1.1 Signal Model 59 3.2 Conventional Optimum Beamformers 69 3.2.1 Minimum Variance Distortionless Response Beamformer 69 3.2.2 Maximum Likelihood Estimator 71 3.2.3 Maximum Signal to Noise Interference Ratio Beamformer 72 3.2.4 Minimum Power Distortionless Response Beamformer 75 3.2.5 Linear Constrained Minimum Variance and Linear Constrained Minimum Power Beamformers 76 3.2.6 Eigenvector Beamformer 77 3.3 Space-Time Processing 81 3.4 Array Calibration 85 3.5 Summary 86 Chapter 4. Multipath Mitigation using Code-Carrier Information 87 4.1 Introduction 87 4.2 Interference Suppression and Multipath Mitigation 88 4.2.1 Signal Model 88 4.2.2 Interference Suppression by Subspace Projection 90 4.2.3 Multipath Mitigation by Subspace Projection 93 4.3 Determination of Multipath Satellites using Code-carrier Information 95 4.4 MSR Beamformer 100 4.5 Simulation Results 102 4.5.1 Subspace Projection and Beamforming 102 4.5.2 Performance Comparison 109 4.6 Summary 111 Chapter 5. Performance Verification using Software-Defined GPS Receiver 113 5.1 Introduction 113 5.2 Software-Defined GPS Receiver Methodology 114 5.2.1 Software-Defined GPS Receiver Signals 115 5.2.2 Software-Defined GPS Receiver Modules 116 5.3 Architecture of Software-Defined GPS Receiver 120 5.3.1 GPS Signal Generation 120 5.3.2 Interference Signal Generation 124 5.3.1 Front-End Signal Processing 125 5.4 Experimental Results 126 5.3.1 Static Environments 128 5.3.2 Dynamic Environments 133 5.5 Summary 136 Chapter 6. Conclusions and Future Work 138 6.1 Conclusions 138 6.2 Future Work 139 Bibliography 142 Appendix 168 Abstract in Korean 170 Acknowledgments 173Docto

Aeronautical engineering: A continuing bibliography with indexes, supplement 190

This bibliography lists 510 reports, articles and other documents introduced into the NASA scientific and technical information system in July 1985

Aeronautical engineering: A continuing bibliography, supplement 122

This bibliography lists 303 reports, articles, and other documents introduced into the NASA scientific and technical information system in April 1980

Potential impacts of advanced aerodynamic technology on air transportation system productivity

Summaries of a workshop held at NASA Langley Research Center in 1993 to explore the application of advanced aerodynamics to airport productivity improvement are discussed. Sessions included discussions of terminal area productivity problems and advanced aerodynamic technologies for enhanced high lift and reduced noise, emissions, and wake vortex hazard with emphasis upon advanced aircraft configurations and multidisciplinary solution options

Aeronautical Engineering: A continuing bibliography with indexes (supplement 182)

This bibliography lists 475 reports, articles and other documents introduced into the NASA scientific and technical information system in December 1984

Aeronautical Engineering: A continuing bibliography with indexes, supplement 153, October 1982

This bibliography lists 535 reports, articles and other documents introduced into the NASA Scientific and Technical Information System in September 1982