20 research outputs found

    Modulation classification of digital communication signals

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    Modulation classification of digital communications signals plays an important role in both military and civilian sectors. It has the potential of replacing several receivers with one universal receiver. An automatic modulation classifier can be defined as a system that automatically identifies the modulation type of the received signal given that the signal exists and its parameters lie in a known range. This thesis addresses the need for a universal modulation classifier capable of classifying a comprehensive list of digital modulation schemes. Two classification approaches are presented: a decision-theoretic (DT) approach and a neural network (NN) approach. First classifiers are introduced that can classify ASK, PSK, and FSK signals. A decision tree is designed for the DT approach and a NN structure is formulated und trained to classify these signals. Both classifiers use the same key features derived from the intercepted signal. These features are based on the instantaneous amplitude, instantaneous phase, and instantaneous frequency of the intercepted signal, and the cumulates of its complex envelope. Threshold values for the DT approach are found from the minimum total error probabilities of the extracted key features at SNR of 20 to -5dB. The NN parameters are found by training the networks on the same data. The DT and NN classifiers are expanded to include CPM signals. Signals within the CPM class are also added to the classifiers and a separate decision tree and new NN structure are found far these signals. New key features to classify these signals are also introduced. The classifiers are then expanded further to include multiple access signals, followed by QAM, PSK8 and FSK8 signals. New features arc found to classify these signals. The final decision tree is able to accommodate a total of fifteen different modulation types. The NN structure is designed in a hierarchical fashion to optimise the classification performance of these fifteen digital modulation schemes. Both DT and NN classifiers are able to classify signals with more than 90% accuracy in the presence of additive white Gaussian within SNR ranging from 20 to 5dB. However, the performance of the NN classifier appears to be more robust as it degrades gradually at the SNRs of 0 and -5dB. At -5dB, the NN has an overall accuracy of 73.58%, whereas the DT classifier achieves only 47.3% accuracy. The overall accuracy of the NN classifier, over the combined SNR range of 20 to -5dB, is 90.7% compared to 84.56% for the DT classifier. Finally, the performances of these classifiers are tested in the presence of Rayleigh fading. The DT and NN classifier structures are modified to accommodate fading and again, new key features are introduced to accomplish this. With the modifications, the overall accuracy of the NN classifier, over the combined SNR range of 20 to -5dB and 120Hz Doppler shift, is 87.34% compared to 80.52% for the DT classifier

    Advanced Modulation and Coding Technology Conference

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    The objectives, approach, and status of all current LeRC-sponsored industry contracts and university grants are presented. The following topics are covered: (1) the LeRC Space Communications Program, and Advanced Modulation and Coding Projects; (2) the status of four contracts for development of proof-of-concept modems; (3) modulation and coding work done under three university grants, two small business innovation research contracts, and two demonstration model hardware development contracts; and (4) technology needs and opportunities for future missions

    Spectrum sensing for cognitive radio and radar systems

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    The use of the radio frequency spectrum is increasing at a rapid rate. Reliable and efficient operation in a crowded radio spectrum requires innovative solutions and techniques. Future wireless communication and radar systems should be aware of their surrounding radio environment in order to have the ability to adapt their operation to the effective situation. Spectrum sensing techniques such as detection, waveform recognition, and specific emitter identification are key sources of information for characterizing the surrounding radio environment and extracting valuable information, and consequently adjusting transceiver parameters for facilitating flexible, efficient, and reliable operation. In this thesis, spectrum sensing algorithms for cognitive radios and radar intercept receivers are proposed. Single-user and collaborative cyclostationarity-based detection algorithms are proposed: Multicycle detectors and robust nonparametric spatial sign cyclic correlation based fixed sample size and sequential detectors are proposed. Asymptotic distributions of the test statistics under the null hypothesis are established. A censoring scheme in which only informative test statistics are transmitted to the fusion center is proposed for collaborative detection. The proposed detectors and methods have the following benefits: employing cyclostationarity enables distinction among different systems, collaboration mitigates the effects of shadowing and multipath fading, using multiple strong cyclic frequencies improves the performance, robust detection provides reliable performance in heavy-tailed non-Gaussian noise, sequential detection reduces the average detection time, and censoring improves energy efficiency. In addition, a radar waveform recognition system for classifying common pulse compression waveforms is developed. The proposed supervised classification system classifies an intercepted radar pulse to one of eight different classes based on the pulse compression waveform: linear frequency modulation, Costas frequency codes, binary codes, as well as Frank, P1, P2, P3, and P4 polyphase codes. A robust M-estimation based method for radar emitter identification is proposed as well. A common modulation profile from a group of intercepted pulses is estimated and used for identifying the radar emitter. The M-estimation based approach provides robustness against preprocessing errors and deviations from the assumed noise model

    Noise-based Transmit Reference Modulation:A Feasibility Analysis

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    Wireless sensor networks (WSNs) receive huge research interest for a multitude of applications, ranging from remote monitoring applications, such as monitoring of potential forest fires, floods and air pollution, to domestic and industrial monitoring of temperature, humidity, vibration, stress, etc. In the former set of applications, a large number of nodes can be involved which are usually deployed in remote or inaccessible environments. Due to logistic and cost reasons, battery replacement is undesired. Energy-efficient radios are needed, with a power-demand so little that batteries can last the lifetime of the node or that the energy can be harvested from the environment. Coherent direct-sequence spread spectrum (DSSS) based radios are widely employed in monitoring applications, due to their overall resilience to channel impairments and robustness against interference. However, a DSSS rake receiver has stringent requirements on precise synchronization and accurate channel knowledge. To obviate the complexity of a coherent DSSS receiver, particularly for low data rate sensor networks, a DSSS scheme that has fast synchronization and possibly low power consumption, is much desired. In this regard, this thesis studies a noncoherent DSSS scheme called transmit reference (TR), which promises a simple receiver architecture and fast synchronization. In traditional TR, the modulated information signal is sent along an unmodulated reference signal, with a small time offset between them. In this thesis, we present and investigate a variant of TR, called noise-based frequency offset modulation (N-FOM), which uses pure noise as the spreading signal and a small frequency offset (instead of a time offset) to separate the information and reference signals. The detection is based on correlation of the received signal with a frequency-shifted version of itself, which collects the transmitted energy without compromising the receiver simplicity. Analytical expressions on performance metrics, supplemented by simulation results, improve understanding of the underlying mechanisms and provide insights into utility of N-FOM in low-power WSNs. In point-to-point line-of-sight (LOS) communication, it was observed that the communication scheme has a minimal utility. The energy-detector type of receiver mixes all in-band signals, which magnifies the overall noise. Particularly, the self-mixing of the transmitted signal also elevates the noise level, which increases with a further increase in the received signal energy. Therefore, for a fixed set of system parameters, the performance attains an asymptote with increasing transmission power. The phenomenon also establishes a non-monotonic relation between performance and the spreading factor. It was observed that a higher spreading factor in N-FOM is beneficial only in a high-SNR regime. After developing an understanding of the performance degrading mechanisms, few design considerations are listed. It is found that a suitable choice of the receiver front-end filter can maximize the SNR. However, the optimal filter depends on received signal and noise levels. A practically feasible – albeit suboptimal – filter is presented which gives close to the optimal performance. Next, timing synchronization is considered. The implications of synchronization errors are analyzed, and a synchronization strategy is devised. The proposed synchronization strategy has little overhead and can be easily implemented for symbol-level synchronization. The N-FOM LOS link model is extended to assess the performance degradation due to interference. Performance metrics are derived which quantify the effects of multiple-user interference, as well as that from external interferers, such as WiFi. Since the correlation operation mixes all in-band signals, the total interfering entities are quadratically increased. The research shows the vulnerability of N-FOM to interference, which makes it optimistic to operate in a crowded shared spectrum (such as the ISM 2.4\,GHz band). We also observe an upper limit on the number of mutually interfering links in a multiple access (MA) network, that can be established with an acceptable quality. The scheme is further investigated for its resilience against impairments introduced by a dense multipath environment. It is observed that despite the noise enhancement, the N-FOM system performs reasonably well in a non-line-of-sight (NLOS) communication. The detection mechanism exploits the multipath channel diversity and leads to an improved performance in a rich scattering environment. An analytical expression for outage probability is also derived. The results indicate that a healthy N-FOM link with very low outage probability can be established at a nominal value of the received bit SNR. It is also found that the choice of the frequency offset is central to the system design. Due to multiple practical implications associated with this parameter, the maximum data rate and the number of usable frequency offsets are limited, particularly in a MA NLOS communication scenario. The analysis evolves into a rule-of-thumb criterion for the data rate and the frequency offset. It is deduced that, due to its limited capability to coexist in a shared spectrum, N-FOM is not a replacement for coherent DSSS systems. The scheme is mainly suited to a low data rate network with low overall traffic, operating in an interference-free rich scattering environment. Such a niche of sensor applications could benefit from N-FOM where the design goal requires a simple detection mechanism and immunity to multipath fading

    Proceedings of the Third International Mobile Satellite Conference (IMSC 1993)

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    Satellite-based mobile communications systems provide voice and data communications to users over a vast geographic area. The users may communicate via mobile or hand-held terminals, which may also provide access to terrestrial cellular communications services. While the first and second International Mobile Satellite Conferences (IMSC) mostly concentrated on technical advances, this Third IMSC also focuses on the increasing worldwide commercial activities in Mobile Satellite Services. Because of the large service areas provided by such systems, it is important to consider political and regulatory issues in addition to technical and user requirements issues. Topics covered include: the direct broadcast of audio programming from satellites; spacecraft technology; regulatory and policy considerations; advanced system concepts and analysis; propagation; and user requirements and applications

    Proceedings of the Second International Mobile Satellite Conference (IMSC 1990)

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    Presented here are the proceedings of the Second International Mobile Satellite Conference (IMSC), held June 17-20, 1990 in Ottawa, Canada. Topics covered include future mobile satellite communications concepts, aeronautical applications, modulation and coding, propagation and experimental systems, mobile terminal equipment, network architecture and control, regulatory and policy considerations, vehicle antennas, and speech compression

    Διάδοση Σήματος στις Επίγειες Οπτικές Ασύρματες Ζεύξεις με Πολυπλεξία και Τεχνικές Διαφορικής Λήψης

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    Οι ασύρματες οπτικές επικοινωνίες ελευθέρου χώρου (Free-Space Optical, FSO) έχουν κερδίσει σημαντικό εμπορικό και ερευνητικό ενδιαφέρον τα τελευταία χρόνια ως αποτέλεσμα των διαφόρων πλεονεκτικών χαρακτηριστικών τους. Είναι σε θέση να ανταποκριθούν στις σημαντικά αυξανόμενες ανάγκες μεταφοράς τεράστιου όγκου πληροφοριακών δεδομένων στα υφιστάμενα και μελλοντικά τηλεπικοινωνιακά δίκτυα. Τα συστήματα FSO λειτουργούν στη ζώνη συχνοτήτων μεταξύ 300 GHz - 430 THz η οποία δεν απαιτεί ειδικές άδειες για τη χρήση της, προσφέροντας ένα σημαντικό οικονομικό πλεονέκτημα σε σύγκριση με τα αντίστοιχα συστήματα ραδιοσυχνοτήτων (RF). Τα FSO συστήματα δεν επηρεάζονται από ηλεκτρομαγνητικές παρεμβολές και παρουσιάζουν υψηλό επίπεδο ασφάλειας λόγω των στενών οπτικών δεσμών laser. Επίσης, θεωρούνται φιλικές προς το περιβάλλον λόγω της χαμηλής κατανάλωσης ηλεκτρικής ενέργειας κατά τη λειτουργία τους. Σε αντίθεση με τα ευεργετικά χαρακτηριστικά τους, οι επίγειες οπτικές ασύρματες ζεύξεις είναι ευάλωτες στις ατμοσφαιρικές επιδράσεις. Το φαινόμενο της ατμοσφαιρικής τυρβώδους ροής (atmospheric turbulence) είναι ένας από τους σημαντικότερους επιβλαβείς παράγοντες κατά τη διάδοση του οπτικού ηλεκτρομαγνητικού κύματος διαμέσου της ατμόσφαιρας. Η ατμοσφαιρική τυρβώδης ροή δημιουργείται ως αποτέλεσμα των ανομοιογενειών στον δείκτη διάθλασης μεταξύ των αέριων μαζών στην ατμόσφαιρα, οδηγώντας σε διακυμάνσεις της λαμβανόμενης έντασης και φάσης και τελικώς σε απώλεια ισχύος στην πλευρά του δέκτη. Λόγω των ραγδαίων διακυμάνσεων που προκαλούνται στο λαμβανόμενο οπτικό σήμα, η επίδραση της ατμοσφαιρικής τυρβώδους ροής μελετάται μέσω στατιστικών μοντέλων για την συνάρτηση της πυκνότητας πιθανότητας της λαμβανόμενης οπτικής έντασης, φαινόμενο γνωστό και ως σπινθηρισμός. Τα συστήματα FSO συνήθως εγκαθίστανται στις στέγες υψηλών κτιρίων ή σε μεγάλα υψόμετρα πάνω από το έδαφος. Έτσι, αυτά τα συστήματα είναι ευάλωτα σε ριπές ανέμου, σε πιθανή ταλάντευση των κτιρίων π.χ. λόγω μικρών σεισμών και σε θερμικές συστολές και διαστολές. Κατά αυτό το τρόπο, μπορούν να προκληθούν επιπρόσθετες διακυμάνσεις στο οπτικό σήμα. Αυτό το φαινόμενο είναι γνωστό στην τεχνική βιβλιογραφία ως σφάλματα σκόπευσης (pointing errors) και μελετάται με κατάλληλα στατιστικά μοντέλα σε σύνδεση με το φαινόμενο της ατμοσφαιρικής τυρβώδους ροής. Αξίζει να σημειωθεί ότι στην πλειονότητα των περιπτώσεων χρησιμοποιείται ένα προσεγγιστικό μοντέλο της κατανομής του Beckmann, η οποία λαμβάνει υπόψη την πιθανή σταθερή μη μηδενική απόκλιση του κέντρου της οπτικής δέσμης από το κέντρο του δέκτη και διαφορετικές τυπικές αποκλίσεις για την ακτινική μετατόπιση στους κατακόρυφους άξονες στο επίπεδο του δέκτη. Εκτός από τα προαναφερθέντα στατιστικά φαινόμενα, οι FSO ζεύξεις υποφέρουν από διάφορες προκαθοριστικές επιπτώσεις, όπως ο θόρυβος περιβάλλοντος, απώλειες οπτικής ισχύος λόγω διαφόρων ατμοσφαιρικών συστατικών (σωματιδίων, μορίων) και από ποικίλες καιρικές συνθήκες όπως ομίχλη, βροχή, χαλάζι κλπ., και απώλειες διάδοσης ελευθέρου χώρου. Όλα τα μαθηματικά μοντέλα που περιγράφουν την επίδραση αυτών των φαινομένων, με πολύ υψηλή ακρίβεια, περιλαμβάνονται στη διατριβή και ο αντίκτυπός τους μελετάται στην τελική αξιολόγηση των επιδόσεων των ασύρματων οπτικών ζεύξεων. Οι τεχνικές διαφορικής λήψης έχουν αποδειχθεί πολύ αποτελεσματικές στην καταπολέμηση διαλείψεων και εξασθενίσεων στα RF τηλεπικοινωνιακά συστήματα. Στην παρούσα διατριβή μελετάται η εφαρμογή διαφορικής λήψης στα FSO συστήματα. Συγκεκριμένα, διερευνάται η διαφορική λήψη στο δέκτη μαζί με τη βέλτιστη περίπτωση χρήσης του συνδυαστή μέγιστης αναλογίας (MRC). Η διαφορική λήψη δεκτών μελετάται για ένα FSO σύστημα μονής εισόδου πολλαπλής εξόδου (SIMO) με χρήση τεχνικών ψηφιακής διαμόρφωσης. Μελετώνται οι πιο συχνά εφαρμοζόμενες τεχνικές ψηφιακής διαμόρφωσης στα συστήματα οπτικών επικοινωνιών, όπως η κωδικοποίηση on-off (OOK), η διαμόρφωση πλάτους παλμού (PAM ) και η διαμόρφωση θέσης παλμού (PPM). Η απόδοση της SIMO FSO ζεύξης με διαφορική λήψη εκτιμάται με βάση την μέτρηση του μέσου ρυθμού σφάλματος δυαδικών ψηφίων (average BER), υπό την επίδραση της ατμοσφαιρικής τυρβώδους ροής που μοντελοποιείται είτε μέσω της Gamma-Gamma (GG) κατανομής είτε μέσω της εκθετικής κατανομής (NE). Ο ρυθμός σφάλματος μπλοκ πληροφορίας (BLER) αποτελεί μια βασική μετρική απόδοσης για κάθε τηλεπικοινωνιακή ζεύξη που λειτουργεί σε σχετικά υψηλούς ρυθμούς μετάδοσης. Είναι μια μετρική που έχει ερευνηθεί κυρίως στις RF επικοινωνίες. Στο πλαίσιο της παρούσας διδακτορικής διατριβής, διερευνάται η μέση απόδοση BLER ενός OOK FSO συστήματος σε συνθήκες ατμοσφαιρικής τυρβώδους ροής που μοντελοποιείται μέσω των κατανομών GG και NE με σφάλματα σκόπευσης μη-μηδενικής απόκλισης. Η τεχνική αναλογικής διαμόρφωσης έντασης (AIM) έχει διερευνηθεί εκτενώς στις επικοινωνίες οπτικών ινών μέσω των πεδίων της μικροκυματικής φωτονικής (MWP) και των ραδιοσυχνοτήτων μέσω οπτικών ινών (RoF). Ωστόσο, η εφαρμογή της στις ασύρματες οπτικές συνδέσεις βρίσκεται ακόμη σε πρώιμο στάδιο. Σε αυτή τη διατριβή διεξάγεται εκτενής έρευνα για την εφαρμογή των τεχνικών AIM στις FSO ζεύξεις και ειδικότερα στη τεχνική μεταφοράς RF σήματος μέσω των FSO συστημάτων, μια τεχνική γνωστή ως Radio-over-FSO (RoFSO). Έτσι, οι συνδέσεις RoFSO εξετάζονται για τη μετάδοση σημάτων με πολυπλεξία όπως OFDM και CDMA σε κανάλια ατμοσφαιρικής τυρβώδους ροής με σφάλματα σκόπευσης. Αξίζει να σημειωθεί ότι για την περίπτωση της CDMA RoFSO μετάδοσης, η απόδοση ενός τέτοιου συστήματος διερευνάται για πρώτη φορά στις κατευθύνσεις της εμπρόσθιας και της αντίστροφης ζεύξης σε συνθήκες τυρβώδους ροής που μοντελοποιούνται από το ενοποιητικό μοντέλο της M(alaga) κατανομής. Μια από τις πιο ελπιδοφόρες λύσεις, προκειμένου να βελτιωθεί η απόδοση, να ξεπεραστούν οι επιβλαβείς ατμοσφαιρικές επιπτώσεις και να επεκταθεί τελικά η απόσταση κάλυψης των FSO συστημάτων, είναι η χρήση αρχιτεκτονικών αναμετάδοσης. Εξετάζεται η εφαρμογή αρχιτεκτονικής πολλαπλών αλμάτων με σειριακούς κόμβους αποκωδικοποίησης και προώθησης (DF) για ένα σύστημα OFDM RoFSO. Οι συγκεκριμένοι DF κόμβοι δρουν ως αναγεννητές για το σήμα πληροφορίας και έτσι επιτυγχάνεται μια βέλτιστη απόδοση. Η βελτίωση της απόδοσης για το σύστημα πολλαπλών αλμάτων αξιολογείται μέσω του μέσου ρυθμού σφάλματος δυαδικών ψηφίων και της εκτίμησης της πιθανότητας διακοπής. Τέλος, μελετάται η διαφορική λήψη δεκτών για OFDM και CDMA RoFSO ζεύξεις, όπου οι συγκεκριμένες χωρικά ποικιλόμορφες ζεύξεις χρησιμοποιούν πολλαπλές πηγές laser. Σε αυτό το σύστημα διαφορικής λήψης, κάθε μία από τις πηγές laser συνδέεται με ένα συγκεκριμένο δέκτη, μέσω της χρήσης πολύ στενών οπτικών δεσμών. Τα αποτελέσματα που προκύπτουν αποκαλύπτουν την αποτελεσματικότητα αυτής της διαμόρφωσης καθιστώντας αυτά τα συστήματα RoFSO με διαφορική λήψη δεκτών πολύ αξιόπιστα ακόμη και στις πιο δυσμενείς συνθήκες λειτουργίας τους υπό ισχυρή επίδραση της ατμοσφαιρικής τυρβώδους ροής, των σφαλμάτων σκόπευσης και μη γραμμικών φαινομένων που σχετίζονται με τα συστήματα RoFSO.Free-Space Optical (FSO) communication systems have been gaining significant commercial and research interest in the last few years as a result of their various advantageous features. They are capable of meeting the fast-paced growing needs for transferring huge amounts of data in the existing and future telecommunications networks. FSO systems operate in the unlicensed band of frequencies between 300 GHz – 430 THz, offering a significant economic advantage compared to their radio frequency (RF) counterparts. They are immune to electromagnetic interference and exhibit high-security level due to their narrow optical laser beams. Also, they are considered as environmental-friendly due to their low electrical energy consumption. Unlike their beneficial characteristics, the terrestrial FSO links are vulnerable to atmospheric effects. The atmospheric turbulence phenomenon is one of the main degradation factors for the electromagnetic optical-wave propagation in the atmospheric medium. Atmospheric turbulence arises as a result of inhomogeneities in the refractive index between air masses in the atmosphere, leading to intensity and phase fluctuations and eventually to amplitude loss on the receiver side. Due to the rapid fluctuations induced to the optical signal, the atmospheric turbulence effect is studied in a statistical manner through probability density functions for the characterization of irradiance fluctuations or the commonly referred to as scintillations. FSO systems are usually installed at the rooftops of tall buildings or at high altitudes above the ground. Thus, these systems are susceptible to gusts of wind, potential sway of the buildings e.g. due to small earthquakes and thermal contraction and expansion. In a similar vein, additional irradiance fluctuations can be provoked to the optical signal. This phenomenon is well-known in the technical literature as pointing errors and is studied statistically in conjunction with the atmospheric turbulence effect. It is worth noting that an approximation of the Beckmann’s distribution model is employed in most cases, which takes account of the potential fixed non-zero deviation of the optical beam centre from the receiver centre and different standard deviations for the radial displacement for the vertical axes at the receiver. Apart from the foregoing statistical phenomena, FSO links suffer from various deterministic effects such as background noise, optical power losses due to various atmospheric constituents and weather conditions such as fog, haze, rain, hail etc and free-space loss. All the mathematical models that describe the behavior of the aforementioned effects, with very high accuracy, are included in the thesis and their impact is studied to the final performance evaluation of the wireless optical links. Spatial diversity techniques have been proved very effective in combating fading in RF wireless communication systems. In the present thesis, the application of spatial diversity to the FSO systems is studied. Specifically, the spatial diversity of the receivers is investigated along with the optimum case of using the maximum ratio combiner (MRC). The spatial diversity of the receivers is studied for a single-input multiple-output (SIMO) FSO link employing some of the most widely used modulation schemes in optical communications, such as on-off keying (OOK), pulse amplitude modulation (PAM) and pulse position modulation (PPM). The performance of the link is assessed in terms of the average bit error rate (BER) metric estimation, under the influence of the atmospheric turbulence effect modeled either by the gamma-gamma (GG) or the negative exponential (NE) distribution with pointing errors. The block error rate (BLER) constitutes an essential performance measure for every communication link operating at relatively high throughput conditions. It’s a metric which has been investigated mostly in RF communications. In the context of the current thesis, the average BLER performance of an OOK FSO link is investigated over atmospheric turbulent conditions modeled by the GG and NE distributions with non-zero boresight pointing errors. Analogue intensity modulation (AIM) technique has been extensively researched in optical fibre communications through the fields of microwave photonics (MWP) and radio over fibre (RoF). However, its application to the wireless optical links is at an immature stage. In this thesis, extensive research is conducted for the application of AIM techniques to the FSO links and especially of the RF signal transport scheme over FSO links, a technique known as Radio-over-FSO (RoFSO). Thus, RoFSO links are examined for transmission of OFDM and CDMA RF signals over atmospheric turbulence channels with pointing errors. It is worth mentioning that for the case of the CDMA RoFSO link, the performance is investigated for the first time for both directions of the forward and the reverse link over atmospheric turbulent conditions modeled by the M(alaga) distribution. One of the most promising solutions, in order to enhance the performance, overcome the harmful atmospheric effects and eventually extend the distance coverage of FSO systems, is the use of relay architectures. The application of multi-hop architecture with serial decode-and-forward (DF) relay nodes to an OFDM RoFSO system is investigated. The specific DF relay nodes act as regenerators for the information signal and thus an optimum performance is achieved. The performance improvement for the multi-hop system is evaluated through the average bit error rate and the outage probability estimation. Finally, the spatial diversity of the receivers is studied for OFDM and CDMA RoFSO links, where the specific spatially diverse links employ multiple laser sources. In this scheme, each one of the laser sources is linked to a specific receiver, through the use of very narrow optical beams. The derived results reveal the effectiveness of this configuration, rendering the links very reliable even in the most adverse operating conditions under the strong influence of the atmospheric turbulence and the pointing errors and the enhanced impact of the nonlinear distortion effects related to the RoFSO systems

    Cooperative Radio Communications for Green Smart Environments

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    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
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