The development of multi-standard wireless communication systems with low cost and high integration is continuously requested and accompanied by the explosive growth of the wireless communication market. Although CMOS technology can provide most building blocks in RF transceivers, the implementation of CMOS RF power amplifiers is still a challenging task. The objective of this research is to develop design techniques to implement fully-integrated multi-mode power amplifiers using CMOS technology. In this dissertation, a load modulation technique with tunable matching networks and a pre-distortion technique in a multi-stage PA are proposed to support multi-communication standards with a single PA. A fully-integrated dual-mode GSM/EDGE PA was designed and implemented in a 0.18 um CMOS technology to achieve high output power for the GSM application and high linearity for the EDGE application. With the suggested power amplifier design techniques, fully-integrated PAs have been successfully demonstrated in GSM and EDGE applications. In Addition to the proposed techniques, a body-switched cascode PA core is also proposed to utilize a single PA in multi-mode applications without hurting the performance. With the proposed techniques, a fully-integrated multi-mode PA has been implemented in a 0.18 um CMOS technology, and the power amplifier has been demonstrated successfully for GSM/EDGE/WCDMA applications. In conclusion, the research in this dissertation provides CMOS RF power amplifier solutions for multiple standards in mobile wireless communications with low cost and high integration.

In this thesis, we consider an analog antenna combining architecture for a MIMO wireless transceiver, while pointing out its advantages with respect to the traditional MIMO architectures. In the first part of this work, we focus on the transceiver design, especially the calculation of the beamformers that must be applied at the RF. This analysis is performed in an OFDM system under different assumptions on the channel state information. As a result, several criteria and algorithms for the selection of the beamformers are proposed. In the second part, we address the FPGA design and implementation of a baseband processor for this architecture. This baseband processor is based on the standard IEEE 802.11a. Finally, some real-time tests of the implemented baseband processor are carried out both in stand-alone configuration and also with the whole physical layer setup. / En esta tesis consideramos una arquitectura de combinación analógica de antenas para una estación inalámbrica MIMO, señalando las ventajas de ésta con respecto a la arquitectura tradicional MIMO. En la primera parte de este trabajo analizamos el cálculo de los pesos que se deben aplicar en RF. Este análisis es realizado para un sistema OFDM bajo diferentes suposiciones sobre el conocimiento del canal en el transmisor. Como resultado, se ofrecen varios criterios y algoritmos para el cálculo de los pesos. La segunda parte se centra en el diseño y la implementación FPGA de un procesador banda base para esta arquitectura. Este procesador está basando en el estándar IEEE 802.11a. Finalmente se llevan a cabo algunos experimentos en tiempo-real del procesador banda base. Estos experimentos se han realizado tanto con el procesador aislado como integrado en el resto de la capa física del sistema.

The owned spectrum allocation policy, in use since the early days of modern radio communications, has been proven to effectively control interference among radio communication systems. However, the overwhelming proliferation of new operators, innovative services and wireless technologies during the last years has resulted, under this static regulatory regime, in the depletion of spectrum bands with commercially attractive radio propagation characteristics. An important number of spectrum measurements, however, have shown that spectrum is mostly underutilized, thus indicating that the virtual spectrum scarcity problem actually results from static and inflexible spectrum management policies rather than the physical scarcity of radio resources. This situation has motivated the emergence of Dynamic Spectrum Access (DSA) methods based on the Cognitive Radio (CR) paradigm, which has gained popularity as a promising solution to conciliate the existing conflicts between spectrum demand growth and spectrum underutilization. The basic underlying idea of DSA/CR is to allow unlicensed (secondary) users to access in an opportunistic and non-interfering manner some licensed bands temporarily unoccupied by the licensed (primary) users. Due to the opportunistic nature of this principle, the behavior and performance of a DSA/CR network depends on the spectrum occupancy patterns of the primary system. A realistic and accurate modeling of such patterns becomes therefore essential and extremely useful in the domain of DSA/CR research. The potential applicability of spectrum usage models ranges from analytical studies to the design and dimensioning of secondary networks as well as the development of innovative simulation tools and more efficient DSA/CR techniques. Spectrum occupancy modeling in the context of DSA/CR constitutes a rather unexplored research area. This dissertation addresses the problem of modeling spectrum usage in the context of DSA/CR by contributing a comprehensive and holistic set of realistic models capable to accurately capture and reproduce the statistical properties of spectrum usage in real radio communication systems in the time, frequency and space dimensions. The first part of this dissertation addresses the development of a unified methodological framework for spectrum measurements in the context of DSA/CR and presents the results of an extensive spectrum measurement campaign performed over a wide variety of locations and scenarios in the metropolitan area of Barcelona, Spain, to identify potential bands of interest for future DSA/CR deployments. To the best of the author's knowledge, this is the first study of these characteristics performed under the scope of the Spanish spectrum regulation and one of the earliest studies in Europe. The second part deals with various specific aspects related to the processing of measurements to extract spectrum occupancy patterns, which is largely similar to the problem of spectrum sensing in DSA/CR. The performance of energy detection, the most widely employed spectrum sensing technique in DSA/CR, is first assessed empirically. The outcome of this study motivates the development of a more accurate theoretical-empirical performance model as well as an improved energy detection scheme capable to outperform the conventional method while preserving a similar level of complexity, computational cost and application. The findings of these studies are finally applied in the third part of the dissertation to the development of innovative spectrum usage models for the time (in discrete- and continuous-time versions), frequency and space domains. The proposed models can been combined and integrated into a unified modeling approach where the time, frequency and space dimensions of spectrum usage can simultaneously be reproduced, thus providing a complete and holistic characterization of spectrum usage in real systems for the analysis, design and simulation of the future DSA/CR networks.

New and future telecommunication networks are and will be broadband type. The existing terrestrial and space radio communication infrastructures might be supplemented by new wireless networks that make and will make use of aeronautics-technology. Our study/contribution is referring to radio communications based on radio stations aboard a stratospheric platform named, by ITU-R, HAPS (High Altitude Platform Station). These new networks have been proposed as an alternative technology within the ITU framework to provide various narrow/broadband communication services. With the possibility of having a payload for Telecommunications in an aircraft or a balloon (HAPS), it can be carried out radio communications to provide backbone connections on ground and to access to broadband points for ground terminals. The latest implies a complex radio network planning. Therefore, the radio coverage analysis at outdoors and indoors becomes an important issue on the design of new radio systems. In this doctoral thesis, the contribution is related to the HAPS application for terrestrial fixed broadband communications. HAPS was hypothesised as a quasi-static platform with height above ground at the so-called stratospheric layer. Latter contribution was fulfilled by approaching via simulations the outdoor-indoor coverage with a simple efficient computational model at downlink mode. This work was assessing the ITU-R recommendations at bands recognised for the HAPS-based networks. It was contemplated the possibility of operating around 2 GHz (1820 MHz, specifically) because this band is recognised as an alternative for HAPS networks that can provide IMT-2000 and IMT-Advanced services. The global broadband radio communication model was composed of three parts: transmitter, channel, and receiver. The transmitter and receiver parts were based on the specifications of the IEEE Std 802.16TM-2009 (with its respective digital transmission techniques for a robust-reliable link), and the channel was subjected to the analysis of radio modelling at the level of HAPS and terrestrial (outdoors plus indoors) parts. For the channel modelling was used the two-state characterisation (physical situations associated with the transmitted/received signals), the state-oriented channel modelling. One of the channel-state contemplated the environmental transmission situation defined by a direct path between transmitter and receiver, and the remaining one regarded the conditions of shadowing. These states were dependent on the elevation angle related to the ray-tracing analysis: within the propagation environment, it was considered that a representative portion of the total energy of the signal was received by a direct or diffracted wave, and the remaining power signal was coming by a specular wave, to last-mentioned waves (rays) were added the scattered and random rays that constituted the diffuse wave. At indoors case, the variations of the transmitted signal were also considering the following matters additionally: the building penetration, construction material, angle of incidence, floor height, position of terminal in the room, and indoor fading; also, these indoors radiocommunications presented different type of paths to reach the receiver: obscured LOS, no LOS (NLOS), and hard NLOS. The evaluation of the feasible performance for the HAPS-to-ground terminal was accomplished by means of thorough simulations. The outcomes of the experiment were presented in terms of BER vs. Eb/N0 plotting, getting significant positive conclusions for these kind of system as access network technology based on HAPS.

Disruption Tolerant Networks (DTNs) are mobile wireless networks that are designed to work in highly-challenged environments where the density of nodes is insufficient to support direct end-to-end communication. Recent efforts in DTNs have shown that mobility provides a powerful means for delivering messages in such highly-challenging environments. Unfortunately, many mobility scenarios depend on untethered devices with limited energy supplies. Without careful management, depleted energy supplies will degrade network connectivity and counteract the robustness gained by mobility. A primary concern is the energy consumed by wireless communications because the wireless interface is one of the largest energy consumers in mobile devices whether they are actively communicating or just listening. However, mobile devices exhibit a tension between saving energy and providing connectivity through opportunistic encounters. In order to pass messages, the device must discover communication opportunities with other nodes. At the same time, energy can be conserved by ``sleeping,' i.e., turning off or disabling the wireless interfaces. However, if the wireless interface is asleep, the node cannot discover other nodes for communication. Thus, power management in DTNs must balance the discovery of other nodes while aggressively sleeping the radio during the remaining periods. In this thesis, we first develop a power management framework for a single radio architecture that allows a node to save energy while discovering communication opportunities. The framework is tailored to the available knowledge about network connectivity over time. Further, the framework supports explicit trade-offs between energy savings and connectivity, so network operators can choose, for example, to conserve energy at the cost of reduced message delivery performance. We next examine the possibility of using a hierarchical radio architecture in which nodes are equipped with two complementary radios: a long-range, high-power radio and a short-range, low-power radio. In this architecture, energy can be conserved by using the low-power radio to discover communication opportunities with other nodes and waking the high-power radio to undertake the data transmission. However, the short range of the low-power radio may result in missing communication opportunities. Thus, we develop a generalized power management framework in which both radios support the discovery. In addition, we incorporate the knowledge of traffic load and network dynamics and devise approximation algorithms to control the sleep/wake-up cycling of the radios to provide maximum energy conservation while discovering enough communication opportunities to handle the expected traffic load. Finally, we investigate the Message Ferrying (MF) routing paradigm as a means to save energy while trading off data delivery delay. In MF, special nodes called ferries move around the deployment area to deliver messages for nodes. While this routing paradigm has been developed mainly to deliver messages in partitioned networks, here we explore its use in a connected MANET. The reliance on the movement of the ferries to deliver messages increases the delivery delay if a network is not partitioned. However, delegating message delivery to the ferries provides the opportunity for nodes to save energy by aggressively putting their radios to sleep when ferries are far away. To exploit this feature, we present a power management framework, in which nodes switch their power management modes based on the knowledge of ferry location.

The phenomenon of fading constitutes a fundamental problem in wireless communications. Researchers have proposed many methods to improve the reliability of communication over wireless channels in the presence of fading. Many studies on this topic have focused on diversity techniques. Transmit diversity is a common diversity type in which multiple antennas are employed at the transmitter. Space-time coding is a technique based on transmit diversity introduced by Tarokh et alii in 1998. In this thesis, various types of space-time codes are examined. Since they were originally introduced in the form of trellis codes, a major part is devoted to space-time trellis codes where the fundamental design criteria are established. Then, space-time block coding, which presents a different approach, is introduced and orthogonal spacetime block codes are analyzed in some detail. Lastly, rank codes from coding theory are studied and their relation to space-time coding are investigated.

The future success of communication networks hinges on the ability to overcome the mismatch between requested quality of service (QoS) and limited network resources. Spectrum is a natural resource that cannot be replenished and therefore must be used efficiently. On the other hand, energy efficiency (EE) is also becoming increasingly important as battery technology has not kept up with the growing requirements stemming from ubiquitous multimedia applications. The qualities of wireless channels vary with both time and user. We use channel state information (CSI) to dynamically assign wireless resources to users to improve spectral and energy efficiency. We first investigate a series of general treatments of exploiting CSI in a distributed way to control the medium access to maximize spectral efficiency for networks with arbitrary topologies and traffic distributions. As the first step, we propose decentralized optimization for multichannel random access (DOMRA), which uses local CSI and two-hop static neighborhood information to achieve performance comparable with the global optimal channel-aware Aloha. The generic framework developed in DOMRA proved to be very useful in improving cellular networks as well. We develop cochannel interference avoidance (CIA) medium access control (MAC), which is optimized by DOMRA, to mitigate the downlink severe cochannel interference that is usually experienced by cell-edge users. Aloha-based schemes have low channel utilization efficiency because of the collision of entire data frames. We further develop channel-aware distributed MAC (CAD-MAC), which avoids collision through signaling negotiation ahead of data transmission. CAD-MAC completely resolves the contention of networks with arbitrary topologies, achieves throughput close to centralized schedulers, and is robust to any channel uncertainty. Then we address energy-efficient wireless communications while emphasizing orthogonal frequency multiple access (OFDMA) systems. We first discover the global optimal energy-efficient link adaptation in frequency-selective channels using the strict quasiconcavity of energy efficiency functions. This link adaptation optimally balances the power consumption of electronic circuits and that of data transmission on each subchannel. The global optimal energy-efficient transmission can be obtained using iterative operations, which may be complex to be implemented in a practical system. Besides, running iterative algorithms consumes additional energy. Hence, we further develop a closed-form link adaptation scheme, which performs close to the global optimum. Besides, since subchannel allocation in OFDMA systems determines the energy efficiency of all users, we develop closed-form resource allocation approaches that achieve near-optimal performance too. In an interference-free environment, a tradeoff between EE and spectral efficiency (SE) exists, as increasing transmit power always improves SE but not necessarily EE. We continue the investigation in interference-limited scenarios and show that since increased transmit power also brings higher interference to the network, SE is not necessarily higher and the tradeoff is improved. Especially, in interference-dominated regimes, e.g., local area networks, both spectral- and energy-efficient communications desire optimized time-division protocols and the proposed DOMRA, CIA-MAC, and CAD-MAC can be used to improve both spectral and energy efficiency.

The objective of the proposed research is to develop interference-aware resource management techniques for CR networks that opportunistically operate within the licensed primary networks spectrum and to investigate the application of such CR techniques to emerging wireless networks. In this thesis, we report on a set of laboratory experiments that we undertook to analyze the interference between the CR-based wireless regional-area network (WRAN) standard and the digital television (DTV) broadcasting system. We determined the tolerable levels of WRAN interference into DTV receivers and studied the effect of these interference levels on WRAN deployment. Based on the need for efficient utilization of the primary network spectrum, we propose efficient interference-aware radio resource allocation (RRA) techniques for orthogonal frequency-division multiple access (OFDMA) CR networks. These RRA techniques aim to maximize the CR network throughput and to keep the CR interference to the primary network at or below a predefined threshold, known as the "interference temperature" limit. Moreover, we propose a joint spectrum-sensing design and power control algorithm that lead to increased CR network throughput and efficient protection of the PUs from undue interference. Interference coordination (IC) is considered a key technique for capacity maximization in emerging heterogeneous wireless networks. We propose a CR-based IC and RRA algorithm for OFDMA femtocell deployments to achieve efficient spectrum utilization and maximum network throughput. CR is envisioned as a key enabling technology for future wireless networks; our novel CR techniques will provide other researchers useful tools to design such networks.

Η έννοια της ψηφιακής αναμετάδοσης έχει μελετηθεί σαν ένα ϑεωρητικό πρόβλημα από την πλευρά της ϑεωρίας της πληροφορίας στις δεκαετίες του ’70 και ’80 όπου υπολογίστηκε η χωρητικότητα συστημάτων με ένα αναμεταδότη. Το σενάριο στο οποίο η επικοινωνία δύο τερματικών σταθμών πραγματοποείται με τη ϐοήθεια ενός τρίτου το οποίο αναλαμβάνει να αναμεταδόσει την πληροφορία, υλοποιήθηκε αρχικά στους δορυφόρους με ανακλαστική επανεκπομπή. Ενδιαφέρον, σύμφωνα με όσα γνωρίζουμε, προκαλεί το γεγονός ότι δεν υπήρχε έως πολύ πρόσφατα καμία εργασία σχετική με την ϑεωρία της αναμετάδοσης - πιο πιθανή αιτία η μη ύπαρξη σχετικών εφαρμογών. Με την ωριμότητα των ψηφιακών ασύρματων επικοινωνιών και λόγω των ϱαγδαία αυξανόμενων αναγκών για επικοινωνίες υψηλής απόδοσης, ϕαίνεται ότι έφτασε η ώρα για την ανάπτυξη ‘‘έξυπνης’’ αναμετάδοσης στο άμεσο μέλλον. Η παρούσα διδακτορική διατριβή (DD) έρχεται να συμβάλλει προς την κατεύθυνση αυ- τή και να δώσει στους μελετητές των ψηφιακών επικοινωνιών νέες τεχνικές και εργαλεία αναλύοντας και μελετώντας στο ϕυσικό επίπεδο τα ασύρματα τηλεπικοινωνιακά συστήματα (ή δίκτυα) πολλαπλών αλμάτων με μη-αναγεννητικούς αναμεταδότες δηλ. συστήματα στα οποία το σήμα εκπέμπεται από τον πηγαίο κόμβο και ϕθάνει στον κόμβο προορισμού διαμέσου ενός ή περισσοτέρων αναμεταδοτών. Με την τεχνική αυτή καταπολεμούμε τους δύο ϐασικούς παράγοντες υποβάθμισης του εκπεμπόμενου σήματος που είναι η μειωμένη μέση ισχύς λήψης λόγω απόστασης ή/και εμποδίων, και οι διακυμάνσεις του σήματος στη λήψη του, εξαιτίας του ϕαινομένου των διαλείψεων. Βασικός σκοπός της DD είναι η αξιολόγηση των επιδόσεων των συστημάτων πολλαπλών αλμάτων, που λειτουργούν σε περιβάλλον διαλείψεων Rayleigh ή Nakagami-m, με δείκτες επιδόσεων όπως ο μέσος λό- γος σήματος προς ϑόρυβο (ΛΣΘ), η πιθανότητα διακοπής της επικοινωνίας (ΠDΕ) και η μέση πιθανότητα σφάλματος bit (ΜΠΣΒ). Για να επιτευχθούν τα παραπάνω στα πολυαλ- ματικά συστήματα, απαιτείται η στατιστική μελέτη του απ’ άκρο-σε-άκρο ΛΣΘ στην έξο- δο του κόμβου προορισμού για την εξαγωγή σημαντικών στατιστικών συναρτήσεων όπως της συνάρτησης πυκνότητας πιθανότητας (ΣΠΠ), της συνάρτησης αθροιστικής πιθανότητας (ΣΑΠ), της ϱοπο-γεννήτριας συνάρτησης (ΡΓΣ) και των ϱοπών. Οι δείκτες επίδοσης παρουσιάζονται είτε σε κλειστές μαθηματικές μορφές οι οποίες εξάγουν άμεσα αποτελέ- σματα, είτε σε αναλυτικές μορφές για τις οποίες η χρήση της αριθμητικής ολοκλήρωσης είναι απαραίτητη μέσω γνωστών μαθηματικών λογισμικών όπως MATHEMATICA ή MAPLE. Παράλληλα, όπου απαιτείται, παρουσιάζονται αποτελέσματα προσομοίωσης Monte Carlo για την επαλήθευση των αριθμητικών αποτελεσμάτων. Λαμβάνοντας υπόψην όλα τα παραπάνω, ερευνάται ένα σύστημα N αλμάτων σε περι- ϐάλλον διαλείψεων Nakagami-m και παράγονται τόσο σημαντικά αποτελέσματα για την επίδοσή τους όσο και ένα καινοτόμο στατιστικό ϑεώρημα για τη ΡΓΣ τυχαίων μεταβλητών Γάμμα. Η επίδοση των συστημάτων αυτών αξιολογείται για μη-αναγεννητικούς αναμε- ταδότες και συγκεκριμένα, τόσο για αναμεταδότες μεταβλητού κέρδους (ΑΜΚ) δηλ. το κέρδος τους εξαρτάται από την εκτίμηση της κατάστασης του καναλιού (ή κέρδος κανα- λιού) στην είσοδό τους, όσο και για αναμεταδότες σταθερού κέρδους (ΑΣΚ). Επίσης στη DD προτείνεται ένα νέος τύπου μη-αναγεννητικού αναμεταδότη, ο αναμεταδότης συνδυα- σμένου κέρδους (ΑΣυΚ), του οποίου η επίδοση εξετάζεται σε ένα σύστημα διπλού άλματος (δηλ. σ’ ένα σύστημα με έναν αναμεταδότη). Η επίδοσή του συγκρίνεται με αυτή του ΑΜΚ. Η επίδοση του ΑΣυΚ εμφανίζεται να είναι καλύτερη σε κάποιες περιπτώσεις απ’ αυτήν του ΑΜΚ. Ιδιαίτερη ϐαρύτητα δίνεται τέλος και στη μελέτη πολυαλματικών συστημάτων επικοινω- νίας με διαφορική συνεργασία χρηστών. Τα συστήματα αυτά διαφέρουν από τα συμβατικά πολυαλματικά διότι εκμεταλλεύονται τα πλεονεκτήματα της ‘‘εικονικής’’ διαφορικής εκ- πομπής στον πηγαίο κόμβο και τη διαφορική λήψη στον κόμβο προορισμού. Οι δείκτες επίδοσης εξετάζονται είτε για συστήματα διαφορικής συνεργασίας ενός χρήστη, είτε για πολλούς χρήστες με διαφορικό δέκτη συνδυασμού επιλογής ή μεγίστου λόγου στον κόμβο προορισμού, αντίστοιχα. / The concept of digital relaying has been studied as a theoretical problem from a network information theory perspective in 70’s and 80’s, and capacity regions of simple relaying channels have been evaluated. The scenario where the communication between two terminal is achieved with the help of a third terminal, was firstly implemented in bentpipe satellites. To the best of our knowledge, there has been no other analytical study on this concept until very recently due to the fact that there were no foreseeable applications at that time. With the maturity of digital wireless technologies and with the overwhelming demand for high data rate coverage, it seems that time has finally arrived for the implementation of intelligent relaying. The present PhD thesis contributes in that direction and gives to researchers of digital communications new techniques and smart tools by analyzing and studying multihop wireless communications systems in physical layer. In multihop systems, the source terminal transmits a signal which arrives to the destination terminal via a number of relays. By applying relayed transmission we mitigate two main reasons that causes Quality of Service degradation of the received signal, which are the low average receive power, induced by the presence of obstacles or by long distance, or due to the fluctuations in the received signal induced by fading effects. The main subject of this PhD is the evaluation of the performance metrics of multihop wireless communications systems, operating over Rayleigh or Nakagamim fading channels such as average signaltonoise ratio (SNR), outage probability and average biterror rate. To achieve this, the statistics of the equivalent endtoend SNR at the output of the destination terminal is required and specifically the probability density function, the cumulative distribution function, the moments and moment generating function (MGF). The performance metrics are presented either in closed forms giving direct results or in analytical ones where the results are extracted via numerical integration with the use of standard mathematical software packages such as MATHEMATICA or MAPLE. Additionally, Monte Carlo simulation results are also presented for the verification of the numerical results. Considering all the above, a multihop system with Nhops over Nakagamim environment is studied, and important performance results are derived. Also, a novel statistical result regarding the MGF of the product of rational powers of Gamma random variables is presented for the first time in the technical literature. The performance metrics of the multihop system are evaluated for nonregenerative relays and specifically, for both channel state information (CSI)based and fixed gain relays. Moreover, in this PhD thesis a new type of relay is presented, the combined gain relay (CGR), and its performance is studied in dualhop systems. The performance of the CGRs is compared to CSIbased ones whose amplification gain depends on the channel state at their input. It is interesting that CGR performs better in some situations than CSI ones. Finally, we concentrate on multihop wireless communications systems with cooperative diversity. Cooperative diversity systems differ from conventional multihop ones since exploits the benefits of virtual transmit diversity in the source terminal and diversity reception in the destination terminal. Performance analysis is conducted for cooperative diversity systems with one or more users (relays) and selection combiner or maximal ratio combiner at the destination terminal, correspondingly.

Στην απαρχή της καινούριας χιλιετίας, έχει γίνει πλέον ξεκάθαρος ο σπουδαίος ρόλος και η χρησιμότητα των ασυρμάτων τηλεπικοινωνιών και των εφαρμογών τους, η εξέλιξη των οποίων επικεντρώνεται σε μεγάλο βαθμό στις Ασύρματες Προσωπικές Τηλεπικοινωνίες (ΑΠΤ). Εξαιτίας της μεγάλης δημοτικότητας που έχουν τα συστήματα αυτά, υπάρχει συνεχής ανάγκη για αύξηση της χωρητικότητας και αναβάθμιση των παρεχόμενων υπηρεσιών ϕωνής και δεδομένων. Μια πολύ σημαντική περιοχή έρευνας για τις ΑΠΤ, που έχει προκύψει από τις παραπάνω απαιτήσεις, είναι η μελέτη και η αντιμετώπιση των προβλημάτων που δημιουργούνται στην επίδοση των δικτύων αυτών από διάφορα ϕαινόμενα που επιδρούν στο ασύρματο κανάλι. Σε αυτήν την ερευνητική περιοχή εστιάζεται και η παρούσα διδακτορική διατριβή, που έχει ως στόχο τη ϑεωρητική μελέτη της επίδοσης ασύρματων ψηφιακών τηλεπικοινωνιακών συστημάτων τα οποία υπόκεινται σε διαλείψεις. Για την καταπολέμηση των αρνητικών συνεπειών των διαλείψεων έχει υιοθετηθεί η τεχνική του διαφορισμού στο δέκτη. Σύμφωνα με αυτήν, ο δέκτης διαθέτει περισσότερες από μια κεραίες λήψης με αποτέλεσμα να λαμβάνει πολλαπλά αντίγραφα του εκπεμπόμενου σήματος, τα οποία συνδυάζει κατάλληλα ανάλογα με τις απαιτήσεις του διαχειριστή του δικτύου. Η διεθνής έρευνα για τη χρήση δεκτών διαφορισμού σε κανάλια Διαλείψεων Μικρής Κλίμακας (ΔΜΙΚ) είναι εκτενής και αφορά κυρίως κανάλια που βασίζονται στις κατανομές Rayleigh, Rice και Nakagamim. Δύο άλλες σημαντικές κατανομές για τη μοντελοποίηση των ΔΜΙΚ είναι η Weibull και, η γενίκευση της, η Γενικευμένη-Γάμα (ΓG), οι οποίες σε σύντομο χρονικό διάστημα έχουν κερδίσει εκτεταμένο επιστημονικό ενδιαφέρον στην περιοχή των ασύρματων κινητών τηλεπικοινωνιών. ΄Ομως, παρά το ενδιαφέρον αυτό, η μελέτη της επίδοσης δεκτών διαφορισμού σε κανάλια που μοντελοποιούνται με τις κατανομές αυτές, δεν έχει λάβει τις διαστάσεις που έχουν λάβει οι αντίστοιχες έρευνες για τις άλλες κατανομές. Αυτό οφείλεται σε μεγάλο βαθμό στη μεγάλη πολυπλοκότητα, από μαϑηματικής άποψης, που παρουσιάζει η στατιστική αναπαράσταση του σήματος της εξόδου πολλών τύπων δεκτών διαφορισμού που σε συνδυασμό με τη σχετικά σύνθετη μαθηματική έκφραση των κατανομών Weibull και ΓG, δημιουργεί δυσεπίλυτα προβλήματα. Στην παρούσα διατριβή, προσπερνώντας αυτές τις δυσκολίες, αρχικά μελετήθηκε η απόδοση μιας ειδικής κατηγορίας δεκτών διαφορισμού, του δέκτη Συνδυασμού Γενικευμένης Επιλογής (ΣΓΕ), ο οποίος λειτουργεί σε Ανεξάρτητα αλλά Μη-Ομοιόμορφα Κατανεμημένα (ΑΜΟΚ) κανάλια διαλείψεων Weibull. Σε αυτήν τη μελέτη εξετάζεται η περίπτωση όπου επιλέγονται οι δύο κλάδοι με το υψηλότερο λόγο σήματος προς ϑόρυβο, Signal to Noise Ratio (SNR), από τους L διαθέσιμους, ΣΓΕ(2, L). Κατά συνέπεια, με την παρουσίαση μιας καινούριας ιδιότητας για το γινόμενο των ροπών ταξινομημένων Τυχαίων Μεταβλητών (ΤΜ) Weibull, γίνεται δυνατή η εξαγωγή εκφράσεων κλειστής μορφής για τις ροπές του SNR στην έξοδο των δεκτών ΣΓΕ(2, L). Χρησιμοποιώντας αυτές τις εκφράσεις δίνονται, σε κλειστή μορφή επίσης, σημαντικά κριτήρια επίδοσης όπως είναι ο Μέσος Λόγος Σήματος προς Θόρυβο (ΜΛΣΘ) και η Ποσότητα των Διαλείψεων (ΠΔ). Επιπλέον, με την εφαρμογή της ϑεωρίας των προσεγγιστών Pad´e και της προσέγγισης μέσω της Ροπο-Γεννήτριας Συνάρτησης (ΡΓΣ), γίνεται δυνατή η μελέτη της Πιθανότητας Διακοπής της Επικοινωνίας (ΠΔΕ) και της Μέσης Πιθανότητας Σφάλματος Bit (ΜΠΣΒ) του συστήματος. Στη συνέχεια, υποθέτοντας Ανεξάρτητα και Ομοιόμορφα Κατανεμημένα (ΑΟΚ) κανάλια διαλείψεων ΓG, παρουσιάστηκε σε κλειστή μορφή έκφραση για τις ροπές του SNR στην έξοδο των δεκτών ΣΓΕ(2, L), ενώ η ΡΓΣ και η Αθροιστική Συνάρτηση Κατανομής (ΑΣΚ) εξήχθησαν υπό τη μορφή απειροσειρών. Χρησιμοποιώντας αυτές τις εκφράσεις απλοποιείται σημαντικά η μελέτη των κριτηρίων επίδοσης της ΠΔΕ και της ΜΠΣΒ. Επιπλέον, παρουσιάστηκε η χωρητικότητα του καναλιού ΓG με τη χρήση ή όχι δεκτών διαφορισμού, με βάση πολύ γνωστές τεχνικές προσαρμογής της εκπομπής. Μια άλλη πολύ σημαντική ερευνητική περιοχή που αφορά τα κανάλια διαλείψεων και επηρεάζει σημαντικά την επίδοση των ασύρματων ψηφιακών τηλεπικοινωνιακών συστημάτων που λειτουργούν σε αυτά, είναι το σύνθετο περιβάλλον διάδοσης που δημιουργείται όταν οι ΔΜΙΚ συνυπάρχουν με Διαλείψεις Μεγάλης Κλίμακας (ΔΜΕΚ). Στην παρούσα διδακτορική διατριβή, αυτό το περιβάλλον διάδοσης μοντελοποιείται με τις σύνθετες κατανομές K και γενικευμένη-K (KG). Αυτές οι κατανομές είναι αρκετά γενικές, αλλά ταυτόχρονα και μαθηματικά ευέλικτες, και έχει διαπιστωθεί ότι είναι κατάλληλες για τη μοντελοποίηση των συνθηκών του καναλιού που προκύπτει όταν οι ΔΜΙΚ συμβαίνουν ταυτόχρονα με ΔΜΕΚ. Στην περίπτωση της κατανομής KG, εξάγονται σημαντικά στατιστικά χαρακτηριστικά του SNR στην έξοδο των δεκτών διαφορισμού Συνδυασμού Μεγίστου Λόγου (ΣΜΛ), Συνδυασμού ΄Ισης Απολαβής (ΣΙΑ), Συνδυασμού Μεταγωγής και Παραμονής (ΣΜΠ) και του δέκτη Διαφορισμού Επιλογής (ΔΕ). Χρησιμοποιώντας αυτές τις εκφράσεις και υποθέτωντας ΑΜΟΚ συνθήκες διαλείψεων του καναλιού KG, εξάγονται σε κλειστή μορφή τα κριτήρια επίδοσης ΜΛΣΘ, ΠΔΕ και ΠΔ. Επιπλέον, ακολουθώντας την προσέγγιση που βασίζεται στη ΡΓΣ για τους δέκτες ΣΜΛ και ΣΜΠ και τους προσεγγιστές Pad´e για τους δέκτες ΣΙΑ και ΔΕ μελετάται η ΜΠΣΒ. Για την περίπτωση της κατανομής K, υποθέτωντας επιπλέον συσχετισμένες ΤΜ, παρουσιάζεται και μελετάται η συσχετισμένη (δι-μεταβαλλόμενη) κατανομή K, η οποία χαρακτηρίζεται από μη ομοιόμορφους παράγοντες μορφοποίησης και κλίμακας. Η σύνθετη αυτή κατανομή είναι βολική για τη μοντελοποίηση του συσχετισμένου περιβάλλοντος διαλείψεων/σκίασης, όταν η συσχέτιση μεταξύ των πλατών των σημάτων και της ισχύος τους είναι διαφορετική. Για αυτήν την κατανομή, εξάγονται γενικές εκφράσεις με απειροσειρές για τη Συνάρτηση Πυκνότητας Πιθανότητας (ΣΠΠ), την ΑΣΚ και τις από κοινού ροπές. Υποθέτοντας ομοιόμορφους παράγοντες κλίμακας και ασυσχέτιστη ισχύ σημάτων, παρέχονται απλούστερες εκφράσεις για τη ΣΠΠ, την ΑΣΚ και τη χαρακτηριστική συνάρτηση, ενώ οι από κοινού ροπές εξάγονται σε κλειστή μορφή. Επιπλέον, βασιζόμενοι σε αυτές τις ϑεωρητικές εκφράσεις της δι-μεταβαλλόμενης κατανομής K, παρουσιάζεται η ανάλυση της επίδοσης σημαντικών δεκτών διαφορισμού, όπως είναι οι δέκτες ΣΜΛ, ΣΙΑ και ΔΕ οι οποίοι λειτουργούν σε συσχετισμένα κανάλια διαλείψεων K. Για το δέκτη ΔΕ μελετάται η ΠΔΕ, ενώ για τους δέκτες ΣΜΛ και ΣΙΑ η ΜΠΣΒ. Γενικά η προτεινόμενη ανάλυση συνοδεύεται από αριθμητικά αποτελέσματα, τα οποία επιδεικνύουν ξεκάθαρα τη χρησιμότητα και την ευελιξία της ϑεωρητικής προσέγγισης. Τα αριθμητικά αυτά αποτελέσματα περιλαμβάνουν σύγκριση της επίδοσης διαφορετικών δεκτών διαφορισμού, συνθηκών καναλιού και τεχνικών διαμόρφωσης. / At the beginning of the new millennium, it becomes totaly clear the important role and usefulness of the wireless communication systems and their applications, which continue to evolute in several areas including Personal Communication Services (PCS). In this context there is a continuing demand for increased capacity and integration of the provided services, such as voice, data and other types of traffic, over radio channels. One of the key technical problem areas that has emerged out is the requirement for determining the fading effects on network performance. This also the research area of the current Philosophy diploma, which aims to the theoretical study of the performance analysis of wireless communication systems, which are subject to fading. The current study has employed diversity reception as a fading mitigation technique. According to this technique, the receiver employs more than one antennae, in order to receive multiple copies of the transmitted signal. Hence, these copies are appropriate combined in order to satisfy network administrator demands. The technical literature concerning diversity reception techniques over fading channels is quite extensive. However, it deals mainly with fading channels, which are based on Rayleigh, Rice, and Nakagami-m distributions. Additionally, important distributions for modeling small scale fading are also the Weibull and, its generalization, the generalized-gamma (ΓG), which have recently gained an increased scientific interest in the wireless mobile communication area. However, the performance analysis of diversity receivers operating over fading channels, modeled by these distributions, has not been thoroughly investigated. Reasons for that include the composite statistical representation of several diversity receivers output in conjunction with the quite complicated mathematical expressions of the Weibull and/or ΓG distributions. In this thesis, by overcoming these difficulties the performance of a class of Generalized-Selection Combining (GSC) receivers operating over independent but non identically distributed Weibull fading channels is studied. The case where the two branches with the largest instantaneous Signal-to-Noise Ratio (SNR) are selected, from a total of L available GSC(2, L), is considered. By introducing a novel property for the product of moments of ordered Weibull Random Variables (RV), convenient closed form expressions for the moments of the GSC(2, L) output SNR are derived. Using these expressions, important performance criteria, such as average output SNR and amount of fading (AF ), are obtained in closed form. Furthermore, employing the Pad´e approximants theory and the Moment-Generating Function (MGF)-based approach, outage and bit-error rate performances are studied. Additionally, by considering independent and identical distributed (iid) ΓG fading channels, the moments of the output SNR of the GSC(2, L) receivers were derived, also, in closed form, while infinite series representations were obtained for the MGF and the Cumulative Distribution Function (CDF). Using these expressions, a simplified study of the Outage Probability (OP) and Average Bit Error Probability (ABEP) performance criteria can be performed. Moreover, the capacity of the ΓG fading channel, using different adaptation policies, is also studied, with and without diversity reception. Another important research issue concerning the performance analysis of wireless digital communications systems operating over fading channels, is the composite propagation environment arising in cases where fading is superimposed on shadowing. In the current thesis this propagation environment is modeled by the K and generalized-K (KG) composite distributions. These distributions are quite general and mathematical versatile, and have been found to be appropriate for accurately modeling the channel conditions when fading occurs simultaneously with shadowing. For the KG distribution, expressions for important statistical metrics of the output SNR for the Selection Diversity (SD) receiver and Maximal Ratio Combining (MRC), Equal Gain Combining (EGC), and Switch and Stay Combining (SSC) diversity receivers are derived. Using these expressions and by considering independent but not necessarily identical distributed fading channel conditions, performance criteria, such as average output SNR, AF and OP are obtained in closed form. Moreover, following the MGFbased approach for MRC and SSC receivers, and the Pad´e approximants theory for SD and EGC receivers, the ABEP is also studied. For the case of the K distribution, assuming correlated RVs, the correlated (bivariate) K distribution, with not necessarily identical shaping and scaling parameters, is introduced and studied. This composite distribution is convenient for modeling multipath/shadowing correlated fading environments when the correlations between the signal envelopes and their powers are different. For this distribution, generic infinite series representations are derived for the Probability Density Function (PDF), the CDF and the joint moments. Assuming identical shaping parameters and uncorrelated signal powers, simpler expressions for the PDF, CDF and the Characteristic Function (CF) are provided, while the joint moments are derived in closed form. Capitalizing on these theoretical expressions for the statistical characteristics of the correlated K distribution, the performance analysis of various diversity reception techniques, such as MRC, EGC and SD, over bivariate K fading channels is presented. For the SD, the outage probability is studied, while for the MRC and EGC the ABEP is obtained. In general the proposed analysis is accompanied by several numerical evaluated results, clearly demonstrating the usefulness and flexibility of the theoretical approach. These numerical results include performance comparisons of several diversity receiver structures, employing various modulation formats and different channel conditions.