Low Ambiguity Zone: Theoretical Bounds and Doppler-Resilient Sequence Design in Integrated Sensing and Communication Systems

In radar sensing and communications, designing Doppler resilient sequences (DRSs) with low ambiguity function for delay over the entire signal duration and Doppler shift over the entire signal bandwidth is an extremely difficult task. However, in practice, the Doppler frequency range is normally much smaller than the bandwidth of the transmitted signal, and it is relatively easy to attain quasi-synchronization for delays far less than the entire signal duration. Motivated by this observation, we propose a new concept called low ambiguity zone (LAZ) which is a small area of the corresponding ambiguity function of interest defined by the certain Doppler frequency and delay. Such an LAZ will reduce to a zero ambiguity zone (ZAZ) if the maximum ambiguity values of interest are zero. In this paper, we derive a set of theoretical bounds on periodic LAZ/ZAZ of unimodular DRSs with and without spectral constraints, which include the existing bounds on periodic global ambiguity function as special cases. These bounds may be used as theoretical design guidelines to measure the optimality of sequences against Doppler effect. We then introduce four optimal constructions of DRSs with respect to the derived ambiguity lower bounds based on some algebraic tools such as characters over finite field and cyclic difference sets

NASA Tech Briefs, April 1992

Topics covered include: New Product Ideas; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Computer Programs; Mechanics; Machinery; Fabrication Technology; Mathematics and Information Sciences

Radio frequency interference detection and mitigation techniques for navigation and Earth observation

Radio-Frequency Interference (RFI) signals are undesired signals that degrade or disrupt the performance of a wireless receiver. RFI signals can be troublesome for any receiver, but they are especially threatening for applications that use very low power signals. This is the case of applications that rely on the Global Navigation Satellite Systems (GNSS), or passive microwave remote sensing applications such as Microwave Radiometry (MWR) and GNSS-Reflectometry (GNSS-R). In order to solve the problem of RFI, RFI-countermeasures are under development. This PhD thesis is devoted to the design, implementation and test of innovative RFI-countermeasures in the fields of MWR and GNSS. In the part devoted to RFI-countermeasures for MWR applications, first, this PhD thesis completes the development of the MERITXELL instrument. The MERITXELL is a multi-frequency total-power radiometer conceived to be an outstanding platform to perform detection, characterization, and localization of RFI signals at the most common MWR imaging bands up to 92 GHz. Moreover, a novel RFI mitigation technique is proposed for MWR: the Multiresolution Fourier Transform (MFT). An assessment of the performance of the MFT has been carried out by comparison with other time-frequency mitigation techniques. According to the results, the MFT technique is a good trade-off solution among all other techniques since it can mitigate efficiently all kinds of RFI signals under evaluation. In the part devoted to RFI-countermeasures for GNSS and GNSS-R applications, first, a system for RFI detection and localization at GNSS bands is proposed. This system is able to detect RFI signals at the L1 band with a sensitivity of -108 dBm at full-band, and of -135 dBm for continuous wave and chirp-like signals when using the averaged spectrum technique. Besides, the Generalized Spectral Separation Coefficient (GSSC) is proposed as a figure of merit to evaluate the Signal-to-Noise Ratio (SNR) degradation in the Delay-Doppler Maps (DDMs) due to the external RFI effect. Furthermore, the FENIX system has been conceived as an innovative system for RFI detection and mitigation and anti-jamming for GNSS and GNSS-R applications. FENIX uses the MFT blanking as a pre-correlation excision tool to perform the mitigation. In addition, FENIX has been designed to be cross-GNSS compatible and RFI-independent. The principles of operation of the MFT blanking algorithm are assessed and compared with other techniques for GNSS signals. Its performance as a mitigation tool is proven using GNSS-R data samples from a real airborne campaign. After that, the main building blocks of the patented architecture of FENIX have been described. The FENIX architecture has been implemented in three real-time prototypes. Moreover, a simulator named FENIX-Sim allows for testing its performance under different jamming scenarios. The real-time performance of FENIX prototype has been tested using different setups. First, a customized VNA has been built in order to measure the transfer function of FENIX in the presence of several representative RFI/jamming signals. The results show how the power transfer function adapts itself to mitigate the RFI/jamming signal. Moreover, several real-time tests with GNSS receivers have been performed using GPS L1 C/A, GPS L2C, and Galileo E1OS. The results show that FENIX provides an extra resilience against RFI and jamming signals up to 30 dB. Furthermore, FENIX is tested using a real GNSS timing setup. Under nominal conditions, when no RFI/jamming signal is present, a small additional jitter on the order of 2-4 ns is introduced in the system. Besides, a maximum bias of 45 ns has been measured under strong jamming conditions (-30 dBm), which is acceptable for current timing systems requiring accuracy levels of 100 ns. Finally, the design of a backup system for GNSS in tracking applications that require high reliability against RFI and jamming attacks is proposed.Les interferències de radiofreqüència (RFI) són senyals no desitjades que degraden o interrompen el funcionament dels receptors sense fils. Les RFI poden suposar un problema per qualsevol receptor, però són especialment amenaçadores per les a aplicacions que fan servir senyals de molt baixa potència. Aquest és el cas de les aplicacions que depenen dels sistemes mundials de navegació per satèl·lit (GNSS) o de les aplicacions de teledetecció passiva de microones, com la radiometria de microones (MWR) i la reflectometria GNSS (GNSS-R). Per combatre aquest problema, sistemes anti-RFI s'estan desenvolupament actualment. Aquesta tesi doctoral està dedicada al disseny, la implementació i el test de sistemes anti-RFI innovadors en els camps de MWR i GNSS. A la part dedicada als sistemes anti-RFI en MWR, aquesta tesi doctoral completa el desenvolupament de l'instrument MERITXELL. El MERITXELL és un radiòmetre multifreqüència concebut com una plataforma excepcional per la detecció, caracterització i localització de RFI a les bandes de MWR més utilitzades per sota dels 92 GHz. A més a més, es proposa una nova tècnica de mitigació de RFI per MWR: la Transformada de Fourier amb Multiresolució (MFT). El funcionament de la MFT s'ha comparat amb el d'altres tècniques de mitigació en els dominis del temps i la freqüència. D'acord amb els resultats obtinguts, la MFT és una bona solució de compromís entre les altres tècniques, ja que pot mitigar de manera eficient tots els tipus de senyals RFI considerats. A la part dedicada als sistemes anti-RFI en GNSS i GNSS-R, primer es proposa un sistema per a la detecció i localització de RFI a les bandes GNSS. Aquest sistema és capaç de detectar senyals RFI a la banda L1 amb una sensibilitat de -108 dBm a tota la banda, i de -135 dBm per a senyals d'ona contínua i chirp fen un mitjana de l'espectre. A més a més, el Coeficient de Separació Espectral Generalitzada (GSSC) es proposa com una mesura per avaluar la degradació de la relació senyal a soroll (SNR) en els Mapes de Delay-Doppler (DDM) a causa del impacte de les RFI. La major contribució d'aquesta tesi doctoral és el sistema FENIX. FENIX és un sistema innovador de detecció i mitigació de RFI i inhibidors de freqüència per aplicacions GNSS i GNSS-R. FENIX utilitza la MFT per eliminar la interferència abans del procés de correlació amb el codi GNSS independentment del tipus de RFI. L'algoritme de mitigació de FENIX s'ha avaluat i comparat amb altres tècniques i els principals components de la seva arquitectura patentada es descriuen. Finalment, un simulador anomenat FENIX-Sim permet avaluar el seu rendiment en diferents escenaris d'interferència. El funcionament en temps real del prototip FENIX ha estat provat utilitzant diferents mètodes. En primer lloc, s'ha creat un analitzador de xarxes per a mesurar la funció de transferència del FENIX en presència de diverses RFI representatives. Els resultats mostren com la funció de transferència s'adapta per mitigar el senyal interferent. A més a més, s'han realitzat diferents proves en temps real amb receptors GNSS compatibles amb els senyals GPS L1 C/A, GPS L2C i Galileo E1OS. Els resultats mostren que FENIX proporciona una resistència addicional contra les RFI i els senyals dels inhibidors de freqüència de fins a 30 dB. A més a més, FENIX s'ha provat amb un sistema comercial de temporització basat en GNSS. En condicions nominals, sense RFI, FENIX introdueix un petit error addicional de tan sols 2-4 ns. Per contra, el biaix màxim mesurat en condicions d'alta interferència (-30 dBm) és de 45 ns, el qual és acceptable per als sistemes de temporització actuals que requereixen nivells de precisió d'uns 100 ns. Finalment, es proposa el disseny d'un sistema robust de seguiment, complementari als GNSS, per a aplicacions que requereixen alta fiabilitat contra RFI.Postprint (published version

Understanding space weather to shield society: A global road map for 2015-2025 commissioned by COSPAR and ILWS

There is a growing appreciation that the environmental conditions that we call space weather impact the technological infrastructure that powers the coupled economies around the world. With that comes the need to better shield society against space weather by improving forecasts, environmental specifications, and infrastructure design. [...] advanced understanding of space weather requires a coordinated international approach to effectively provide awareness of the processes within the Sun-Earth system through observation-driven models. This roadmap prioritizes the scientific focus areas and research infrastructure that are needed to significantly advance our understanding of space weather of all intensities and of its implications for society. Advancement of the existing system observatory through the addition of small to moderate state-of-the-art capabilities designed to fill observational gaps will enable significant advances. Such a strategy requires urgent action: key instrumentation needs to be sustained, and action needs to be taken before core capabilities are lost in the aging ensemble. We recommend advances through priority focus (1) on observation-based modeling throughout the Sun-Earth system, (2) on forecasts more than 12 hrs ahead of the magnetic structure of incoming coronal mass ejections, (3) on understanding the geospace response to variable solar-wind stresses that lead to intense geomagnetically-induced currents and ionospheric and radiation storms, and (4) on developing a comprehensive specification of space climate, including the characterization of extreme space storms to guide resilient and robust engineering of technological infrastructures. The roadmap clusters its implementation recommendations by formulating three action pathways, and outlines needed instrumentation and research programs and infrastructure for each of these. [...]Comment: In press for Advances of Space Research: an international roadmap on the science of space weather, commissioned by COSPAR and ILWS (63 pages and 4 figures

Proof-of-concept of a single-point Time-of-Flight LiDAR system and guidelines towards integrated high-accuracy timing, advanced polarization sensing and scanning with a MEMS micromirror

Dissertação de mestrado integrado em Engenharia Física (área de especialização em Dispositivos, Microssistemas e Nanotecnologias)The core focus of the work reported herein is the fulfillment of a functional Light Detection and Ranging (LiDAR) sensor to validate the direct Time-of-Flight (ToF) ranging concept and the acquisition of critical knowledge regarding pivotal aspects jeopardizing the sensor’s performance, for forthcoming improvements aiming a realistic sensor targeted towards automotive applications. Hereupon, the ToF LiDAR system is implemented through an architecture encompassing both optical and electronical functions and is subsequently characterized under a sequence of test procedures usually applied in benchmarking of LiDAR sensors. The design employs a hybrid edge-emitting laser diode (pulsed at 6kHz, 46ns temporal FWHM, 7ns rise-time; 919nm wavelength with 5nm FWHM), a PIN photodiode to detect the back-reflected radiation, a transamplification stage and two Time-to-Digital Converters (TDCs), with leading-edge discrimination electronics to mark the transit time between emission and detection events. Furthermore, a flexible modular design is adopted using two separate Printed Circuit Boards (PCBs), comprising the transmitter (TX) and the receiver (RX), i.e. detection and signal processing. The overall output beam divergence is 0.4º×1º and an optical peak power of 60W (87% overall throughput) is realized. The sensor is tested indoors from 0.56 to 4.42 meters, and the distance is directly estimated from the pulses transit time. The precision within these working distances ranges from 4cm to 7cm, reflected in a Signal-to-Noise Ratio (SNR) between 12dB and 18dB. The design requires a calibration procedure to correct systematic errors in the range measurements, induced by two sources: the timing offset due to architecture-inherent differences in the optoelectronic paths and a supplementary bias resulting from the design, which renders an intensity dependence and is denoted time-walk. The calibrated system achieves a mean accuracy of 1cm. Two distinct target materials are used for characterization and performance evaluation: a metallic automotive paint and a diffuse material. This selection is representative of two extremes of actual LiDAR applications. The optical and electronic characterization is thoroughly detailed, including the recognition of a good agreement between empirical observations and simulations in ZEMAX, for optical design, and in a SPICE software, for the electrical subsystem. The foremost meaningful limitation of the implemented design is identified as an outcome of the leading-edge discrimination. A proposal for a Constant Fraction Discriminator addressing sub-millimetric accuracy is provided to replace the previous signal processing element. This modification is mandatory to virtually eliminate the aforementioned systematic bias in range sensing due to the intensity dependency. A further crucial addition is a scanning mechanism to supply the required Field-of-View (FOV) for automotive usage. The opto-electromechanical guidelines to interface a MEMS micromirror scanner, achieving a 46º×17º FOV, with the LiDAR sensor are furnished. Ultimately, a proof-of-principle to the use of polarization in material classification for advanced processing is carried out, aiming to complement the ToF measurements. The original design is modified to include a variable wave retarder, allowing the simultaneous detection of orthogonal linear polarization states using a single detector. The material classification with polarization sensing is tested with the previously referred materials culminating in an 87% and 11% degree of linear polarization retention from the metallic paint and the diffuse material, respectively, computed by Stokes parameters calculus. The procedure was independently validated under the same conditions with a micro-polarizer camera (92% and 13% polarization retention).O intuito primordial do trabalho reportado no presente documento é o desenvolvimento de um sensor LiDAR funcional, que permita validar o conceito de medição direta do tempo de voo de pulsos óticos para a estimativa de distância, e a aquisição de conhecimento crítico respeitante a aspetos fundamentais que prejudicam a performance do sensor, ambicionando melhorias futuras para um sensor endereçado para aplicações automóveis. Destarte, o sistema LiDAR é implementado através de uma arquitetura que engloba tanto funções óticas como eletrónicas, sendo posteriormente caracterizado através de uma sequência de testes experimentais comumente aplicáveis em benchmarking de sensores LiDAR. O design tira partido de um díodo de laser híbrido (pulsado a 6kHz, largura temporal de 46ns; comprimento de onda de pico de 919nm e largura espetral de 5nm), um fotodíodo PIN para detetar a radiação refletida, um andar de transamplificação e dois conversores tempo-digital, com discriminação temporal com threshold constante para marcar o tempo de trânsito entre emissão e receção. Ademais, um design modular flexível é adotado através de duas PCBs independentes, compondo o transmissor e o recetor (deteção e processamento de sinal). A divergência global do feixe emitido para o ambiente circundante é 0.4º×1º, apresentando uma potência ótica de pico de 60W (eficiência de 87% na transmissão). O sensor é testado em ambiente fechado, entre 0.56 e 4.42 metros. A precisão dentro das distâncias de trabalho varia entre 4cm e 7cm, o que se reflete numa razão sinal-ruído entre 12dB e 18dB. O design requer calibração para corrigir erros sistemáticos nas distâncias adquiridas devido a duas fontes: o desvio no ToF devido a diferenças nos percursos optoeletrónicos, inerentes à arquitetura, e uma dependência adicional da intensidade do sinal refletido, induzida pela técnica de discriminação implementada e denotada time-walk. A exatidão do sistema pós-calibração perfaz um valor médio de 1cm. Dois alvos distintos são utilizados durante a fase de caraterização e avaliação performativa: uma tinta metálica aplicada em revestimentos de automóveis e um material difusor. Esta seleção é representativa de dois cenários extremos em aplicações reais do LiDAR. A caraterização dos subsistemas ótico e eletrónico é minuciosamente detalhada, incluindo a constatação de uma boa concordância entre observações empíricas e simulações óticas em ZEMAX e elétricas num software SPICE. O principal elemento limitante do design implementado é identificado como sendo a técnica de discriminação adotada. Por conseguinte, é proposta a substituição do anterior bloco por uma técnica de discriminação a uma fração constante do pulso de retorno, com exatidões da ordem sub-milimétrica. Esta modificação é imperativa para eliminar o offset sistemático nas medidas de distância, decorrente da dependência da intensidade do sinal. Uma outra inclusão de extrema relevância é um mecanismo de varrimento que assegura o cumprimento dos requisitos de campo de visão para aplicações automóveis. As diretrizes para a integração de um micro-espelho no sensor concebido são providenciadas, permitindo atingir um campo de visão de 46º×17º. Conclusivamente, é feita uma prova de princípio para a utilização da polarização como complemento das medições do tempo de voo, de modo a suportar a classificação de materiais em processamento avançado. A arquitetura original é modificada para incluir uma lâmina de atraso variável, permitindo a deteção de estados de polarização ortogonais com um único fotodetetor. A classificação de materiais através da aferição do estado de polarização da luz refletida é testada para os materiais supramencionados, culminando numa retenção de polarização de 87% (tinta metálica) e 11% (difusor), calculados através dos parâmetros de Stokes. O procedimento é independentemente validado com uma câmara polarimétrica nas mesmas condições (retenção de 92% e 13%)

Ship target recognition

Includes bibliographical references.In this report the classification of ship targets using a low resolution radar system is investigated. The thesis can be divided into two major parts. The first part summarizes research into the applications of neural networks to the low resolution non-cooperative ship target recognition problem. Three very different neural architectures are investigated and compared, namely; the Feedforward Network with Back-propagation, Kohonen's Supervised Learning Vector Quantization Network, and Simpson's Fuzzy Min-Max neural network. In all cases, pre-processing in the form of the Fourier-Modified Discrete Mellin Transform is used as a means of extracting feature vectors which are insensitive to the aspect angle of the radar. Classification tests are based on both simulated and real data. Classification accuracies of up to 93 are reported. The second part is of a purely investigative nature, and summarizes a body of research aimed at exploring new ground. The crux of this work is centered on the proposal to use synthetic range profiling in order to achieve a much higher range resolution (and hence better classification accuracies). Included in this work is a comprehensive investigation into the use of super-resolution and noise reducing eigendecomposition techniques. Algorithms investigated include the Principal Eigenvector Method, the Total Least Squares Method, and the MUSIC method. A final proposal for future research and development concerns the use of time domain averaging to improve the classification performance of the radar system. The use of an iterative correlation algorithm is investigated

NASA Tech Briefs, December 1989

Topics include: Electronic Components and Circuits. Electronic Systems, Physical Sciences, Materials, Computer Programs, Mechanics, Machinery, Fabrication Technology, Mathematics and Information Sciences, and Life Sciences

Characterising Spatial and Temporal Ionospheric Variability with a Network of Oblique Angle-of-arrival and Doppler Ionosondes

Ionospheric variability exists on a broad range of scales, and routinely impacts skywave propagation modes of high frequency radio waves, to the detriment of radar and communication systems. In order to better understand the electron density structures associated with such variability at mid-latitudes, a network of oblique angle-of-arrival (AoA) and Doppler ionosondes were installed in central and northern Australia as part of the ELOISE campaign in 2015. This thesis analyses observations from the ELOISE AoA ionosondes, with a focus on characterising the influence of medium- to large- scale gradients and signatures of travelling ionospheric disturbances (TIDs). Following an overview of the experiment, the design and calibration of the new ionosonde system is described. With multi-channel receivers connected to each element of two twin-arm arrays, a total of eleven AoA paths of between 900 and 2700 km were collected, including nine with interleaved Doppler measurements using a special channel scattering function (CSF) capability. On-board signal processing was developed to perform real-time clear channel evaluation and CSF scheduling, and generate the AoA ionograms and delay-Doppler images with fitted electron density profiles. In further offline analysis, peak detection and mode classification was carried out, to support reflection point mapping and tilt estimation. Significant testing and validation of the new ionosonde before and after the experiment revealed AoA uncertainties on the scale of 0.2–0.5° in bearing and 0.4–0.9° in elevation. Having identified a low-elevation bias, models of tropospheric refraction and antenna mutual coupling effects were considered as possible correction strategies, but ultimately an empirical approach based on aggregated ionospheric returns was implemented. Small-scale (intra-dwell) ionospheric variability also has the potential to compromise results, through unresolved multi-mode mixing, and this has been investigated using a combination of spatial and temporal variability metrics derived from the CSF data. The analysis of large quantities of F2 peak data shows persistent diurnal patterns in the oblique AoA observables that are also well-captured by a conventional data-assimilative ionospheric model, even without the benefit of AoA and Doppler inputs. Furthermore, Doppler measurements are reproduced remarkably well using just the midpoint fitted profiles. A statistical study has quantified the level of consistency between observations and model, to provide greater confidence in the results. Many of the geophysical features can be interpreted as ionospheric gradients, as evident in the tilt estimates, and horizontally moving structures such as TIDs, using a form of Doppler-based drift analysis. While signatures of TIDs vary considerably, two simple wave-like perturbation models have been evaluated to help classify quasi-periodic behaviour in the AoA observations, as well as understand the directional filtering effect imposed by the path geometry. In some cases, a set of TID parameters can be determined by eye, but in others automatic parameter inversion techniques may be more viable. Two such techniques were implemented but results using both real and synthetic data demonstrated some significant limitations. Finally, attempts to relate TID signatures across multiple paths shows promise, but there still appears to be a strong dependence on path geometry that is difficult to eliminate.Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 202

Cooperative Radio Communications for Green Smart Environments

The demand for mobile connectivity is continuously increasing, and by 2020 Mobile and Wireless Communications will serve not only very dense populations of mobile phones and nomadic computers, but also the expected multiplicity of devices and sensors located in machines, vehicles, health systems and city infrastructures. Future Mobile Networks are then faced with many new scenarios and use cases, which will load the networks with different data traffic patterns, in new or shared spectrum bands, creating new specific requirements. This book addresses both the techniques to model, analyse and optimise the radio links and transmission systems in such scenarios, together with the most advanced radio access, resource management and mobile networking technologies. This text summarises the work performed by more than 500 researchers from more than 120 institutions in Europe, America and Asia, from both academia and industries, within the framework of the COST IC1004 Action on "Cooperative Radio Communications for Green and Smart Environments". The book will have appeal to graduates and researchers in the Radio Communications area, and also to engineers working in the Wireless industry. Topics discussed in this book include: • Radio waves propagation phenomena in diverse urban, indoor, vehicular and body environments• Measurements, characterization, and modelling of radio channels beyond 4G networks• Key issues in Vehicle (V2X) communication• Wireless Body Area Networks, including specific Radio Channel Models for WBANs• Energy efficiency and resource management enhancements in Radio Access Networks• Definitions and models for the virtualised and cloud RAN architectures• Advances on feasible indoor localization and tracking techniques• Recent findings and innovations in antenna systems for communications• Physical Layer Network Coding for next generation wireless systems• Methods and techniques for MIMO Over the Air (OTA) testin