1,721,371 research outputs found
Wavelet based design of digital multichannel communications systems
Full text linkThe huge penetration of the personal communications systems in the market is constantly presenting new challenges to the research, aimed at satisfying people's needs and requirements for effective communication systems. At present, the cellular telephone network is perhaps the most evident example of communication system that has had a great impact on the lives of ordinary people and, at the same time, is the subject of interest of many researchers both at academic and industrial level. For the future, one of the main challenges in telecommunications will be the provision of ubiquitous broadband tetherless integrated services to mobile users. Such a pretentious goal cannot be achieved without a continuous research facing such problems as service quality, complete mobility support, and affordable complexity that are still open problems. However, present telecommunication problems are not only a matter of implementation or development of new services, exploiting a totally assessed doctrine. In order to respond to the mobility of the users personal communication systems have to deal with the wireless communication channel whereby mobility and non-stationarity of the propagation conditions require a stochastic description of the channel parameters. While this fact can be viewed as strong limitation to the development of a solid theory whose validity can be assesed in practice, on the other hand allows for an investigation and study of novel communication schemes, sometimes encompassing basic aspects of digital communications. This thesis, is the result of a research work that has investigated one of the basic building block of every communication systems, the modulation scheme, and the design of the pulse shape carrying the digital information. We have studied the design of multichannel communication scheme exploiting the mathematical theory of wavelets. Such a theory, developed recently, has had a great impact in many fields of engineering and of other scientific disciplines. In particular, wavelet theory has become very popular in the signal processing area; in fact it is a flexible toolbox for signal analysis allowing effective representation of signals for features extraction purposes. The main features that make wavelet waveforms suitable to be used as shaping pulses for modulation are their substantial compact support both in the time and frequency domains, and the fact that they are ISI-free pulses over frequency flat channels. The study presented in this thesis is focused on application of wavelet theory to design high-efficiency multichannel communication schemes and to the performance evaluation over linear and non-linear channels. We present a general method to design wavelet based multichannel communication schemes that we denoted Wavelet Orthogonal Frequency Division Multiplexing (WOFDM). We show that such schemes, having a largerspectral efficiency for a small number of channels, are a valid alternative to the classical OFDM. Potential advantage of wavelet modulation are shown presenting other applications examined in this thesis: a joint use of WOFDM and Trellis Coded Modulation to shape the power spectrum in order to match a frequency selective channel and minimize distortion, and application to spread spectrum modulation. Particular attention has been devoted to the timing recovery problem in multichannel communication schemes, exploiting the timing information of the different subchannels to improve the error variance in estimation of the sampling instant leading to a reduction of the adjacent channels interferenc
The Interference Threat
No full textIt is well known that several phenomena may affect the quality of the pseudor- ange estimation that is based on the measurement of the propagation time of a signal from a satellite to the user. Any electromagnetic source interacting with the signals is interfering with the process of estimating the propagation time. This book focuses on artificial sources of RFI generated either intentionally or unintentionally by some communication system. The following chapters address such sources of artificial interference and the receiver-based techniques used to detect and mitigate their effects. Note, however, that other kinds of interference might be a threat to GNSS positioning performance. They are discussed in the following subsections, but are not be specifically addressed in this book since their detection and mitigation follows specific approache
GNSS Interference Threats and Countermeasures
No full textReliable positioning and navigation is becoming imperative in more and more applications for public services, consumer products, and safety-critical purposes. Research for finding pervasive and robust positioning methodologies is critical for a growing amount of societal areas while making sure that navigation is trustworthy and the risks and threats of especially satellite navigation are accounted for. This book provides a comprehensive survey of the effect of radio-frequency interference (RFI) on the Global Navigation Satellite Systems (GNSS) as well as of the spoofing threats.
Through case studies and practical implementation/applications, this resource presents engineers and scientists with a better understanding of interference and spoofing threats, ultimately helping them to design and implement robust systems
Un modello di canale per comunicazionivia piattaforma stratosferica
No full textIn recent years there has been a great interest in the development of HAPs for telecom applications. This paper deals with the derivation of the theoretical channel model for the communication lin
Comparative Studies on BOC Signal Performance in Multipath Environment
No full textIt is well known that in indoor environment performance of GNSS-based positioning are strongly limited by several factors. Acquisition of weak signals is critical, visibility of a sufficient number of satellite is not granted, and the SIS received at the antenna propagates via multiple paths. Indoor signal acquisition can be aided by external elements providing augmentation signals through dedicated links or by means of communication channels (e.g. Assisted GPS) but the fine pseudorange estimation cannot be improved by external aids. Even differential corrections are not effective since a user few meters far away by a reference station can experience very different multipath conditions. The navigation receiver must embed multipath rejection strategies in order to improve the signal tracking and identify and minimize or correct the multipath effects on the final position estimatio
Comparative Performance Study of Linear and Gaussian Kernel SVM Implementations for Phase Scintillation Detection
No full textThe aim of this paper is to analyze the design of support vector machine (SVM) algorithm that belongs to the class of supervised machine learning algorithms for phase scintillation detection and to discuss the performance comparison of linear and Gaussian kernel implementations by considering the design parameter’s effects. The algorithm processes the phase scintillation indices computed for GPS L1 signals through the designed linear and Gaussian kernel SVM models. The study is based on the real GNSS signals which are affected by phase scintillations, collected at South African Antarctic research base (SANAE IV)
Use of the Wavelet Transform for interference detection and mitigation in Global Navigation Satellite Systems
Full text linkRadio frequency interference detection and mitigation are becoming of paramount importance due to the increasing number of
services and applications based on the position obtained by means of Global Navigation Satellite Systems. A way to cope with such
threats is the implementation in the receiver of advanced signal processing algorithm able to raise proper warning or improve the
receiver performance. In this paper, we propose a method based on the Wavelet Transform able to split the useful signal from the
interfering component in a transformed domain. The wavelet packet decomposition and proper statistical thresholds allow the
algorithm to show very good performance in case of multiple pulse interference as well as in the case of narrowband interference,
two scenarios in which traditional countermeasures might not be effectiv
- …
