Tarun Chaudhary

Im gegenw rtigen Szenario werden orthogonale Frequenzmultiplexsysteme (OFDM) aufgrund ihrer Robustheit gegen ber frequenzselektiven Fading-Kan len, der F higkeit, hohe Datenraten zu erzielen, und ihrer hohen spektralen Effizienz in den j ngsten Multimedia-Anwendungen und verschiedenen drahtlosen Standards stark genutzt. Das Multi-Input-Multi-Output (MIMO)-System in Kombination mit OFDM hat f r die n chste Generation von Breitband-Multimedia-Anwendungen aufgrund des zus tzlichen Potenzials einer erh hten Kanalkapazit t gro e Aufmerksamkeit auf sich gezogen. Trotz dieser Vorteile ist eine der gr ten Herausforderungen bei der Implementierung von OFDM-basierten Systemen das hohe Verh ltnis von Spitzen- zu Durchschnittsleistung (PAPR), das die Systemleistung durch die Einf hrung nichtlinearer Verzerrungen am Hochleistungsverst rker (HPA) verschlechtert. Um das Problem der hohen PAPR zu berwinden, werden verschiedene PAPR-Reduzierungstechniken analysiert und ihre Bitfehlerrate (BER) und PAPR-Reduzierungsf higkeit verglichen.

Dans le sc nario actuel, les syst mes de multiplexage par r partition orthogonale de la fr quence (OFDM) sont largement utilis s dans les applications multim dias r centes et dans diverses normes sans fil en raison de leur robustesse au canal d' vanouissement s lectif de la fr quence, de leur capacit atteindre un d bit de donn es lev et de leur grande efficacit spectrale. Les syst mes entr es et sorties multiples (MIMO) combin s l'OFDM ont attir l'attention sur les applications multim dias large bande de la prochaine g n ration en raison de leur potentiel suppl mentaire d'augmentation de la capacit des canaux. Malgr ces avantages, l'un des principaux d fis rencontr s dans la mise en oeuvre des syst mes bas s sur l'OFDM est le rapport lev entre la puissance de cr te et la puissance moyenne (PAPR), qui d grade les performances du syst me en introduisant une distorsion non lin aire au niveau de l'amplificateur de grande puissance (HPA). Pour surmonter le probl me du PAPR lev , diverses techniques de r duction du PAPR sont analys es et leur taux d'erreur binaire (BER) et leur capacit de r duction du PAPR sont compar s.

No cen rio actual, os sistemas de multiplexa o por divis o ortogonal de frequ ncia (OFDM) s o muito utilizados em aplica es multim dia recentes e em v rios padr es sem fios devido sua robustez ao canal de desvanecimento selectivo de frequ ncia, elevada capacidade de obten o de dados e elevada efici ncia espectral. O sistema multi-input multi-output (MIMO) em combina o com o OFDM chamaram significativamente a aten o para a pr xima gera o de aplica es multim dia de banda larga devido ao potencial adicional de fornecer uma maior capacidade de canal. Apesar destas vantagens, um dos maiores desafios enfrentados na implementa o do sistema baseado em OFDM, a elevada taxa de pico de pot ncia m dia (PAPR) que degrada o desempenho do sistema ao introduzir distor es n o lineares no amplificador de alta pot ncia (HPA). Para superar o problema do PAPR elevado, s o analisadas v rias t cnicas de redu o de PAPR e comparadas a sua taxa de erro de bit (BER) e a sua capacidade de redu o de PAPR.

Nello scenario attuale, i sistemi OFDM (orthogonal frequency division multiplexing) sono molto utilizzati nelle recenti applicazioni multimediali e in vari standard wireless, grazie alla loro robustezza nei confronti del canale a dissolvenza selettiva di frequenza, alla capacit di raggiungere un'elevata velocit di trasmissione dei dati e all'alta efficienza spettrale. I sistemi multi-input multi-output (MIMO) in combinazione con l'OFDM hanno attirato un'attenzione significativa per le applicazioni multimediali a banda larga di prossima generazione, grazie al potenziale aggiuntivo di fornire una maggiore capacit di canale. Nonostante questi vantaggi, una delle principali sfide affrontate nell'implementazione di un sistema basato su OFDM l'elevato rapporto tra picco e potenza media (PAPR), che degrada le prestazioni del sistema introducendo una distorsione non lineare nell'amplificatore ad alta potenza (HPA). Per superare il problema dell'elevato PAPR, vengono analizzate varie tecniche di riduzione del PAPR e vengono confrontati il loro tasso di errore di bit (BER) e la capacit di riduzione del PAPR.

In the present scenario, orthogonal frequency division multiplexing (OFDM) systems are greatly utilized in recent multimedia applications and various wireless standards due to their robustness to frequency selective fading channel, high data rate achieving capability, and high spectral efficiency. The multi-input multi-output (MIMO) system in combination with OFDM has drawn significant attention for the next-generation broadband multimedia applications due to the additional potential of providing increased channel capacity. Despite these advantages, one of the major challenges faced in implementing an OFDM-based system is the high peak-to-average power ratio (PAPR) that degrades the system performance by introducing non-linear distortion at the high-power amplifier (HPA). To overcome the problem of high PAPR, various PAPR reduction techniques are analyzed and their bit error rate (BER) and PAPR reduction capability are compared.

The main objective of this book is to analyze the performance of spectrum decision schemes. In this book probability-based spectrum decision and sensing based spectrum decision schemes are compared in term of the overall system time spent by secondary user in the system. The preemptive resume priority (PRP) M/G/1 queuing model is implemented in both approaches for balancing the incoming traffic in the spectrum. Each channel in the spectrum maintains two queues one for primary user and other for low priority secondary user. The optimal number of candidate channels are calculated from the long-term observation according to the time taken by CR to find out the idle channel. The effects of sensing errors like false alarm and missed detection on overall system time also considered. This analysis shows that the effect of missed detection is more severe than the false alarm, because due to missed detection the primary user has to resend its data and it will take more time.

Spektrumsb nder sind die Kommunikationsautobahn der Natur, die heutzutage stark berlastet sind. Um dieses Problem zu l sen, wird eine dynamische Zuweisung des Spektrums in Erw gung gezogen, die die derzeitige feste Zuweisungspolitik ersetzt. Bei der dynamischen Zuteilung nutzen unlizenzierte Benutzer den Kanal, wenn dieser frei ist, und geben ihn sofort frei, wenn ein lizenzierter Benutzer zur ckkehrt. Handoff ist das Hauptmerkmal der dynamischen Frequenzzuteilungspolitik. Es wurde auch beobachtet, dass mehrere Handoffs die Dienstqualit t f r die unlizenzierten Benutzer verschlechtern k nnen. Aus diesem Grund k nnen verschiedene Optimierungsmethoden angewandt werden, um die bestm gliche Spektrumszuteilung w hrend des Handoffs zu erreichen, so dass den kognitiven Nutzern eine bessere Dienstqualit t bei gleichzeitiger Verbesserung der Spektrumsnutzung geboten werden kann. Das Handoff-Verfahren kann pro-aktiv oder reaktiv sein. Pro-aktiv besch ftigt sich haupts chlich mit der vorherigen Vorbereitung des Satzes der Zielkan le, w hrend reaktives Handoff ein On-Demand-Ansatz ist. Beide Arten haben ihre eigenen Vor- und Nachteile. Je nach Anwendung kann jede der bergabetechniken verwendet werden.

Spectrum bands are nature's communication highway which are highly congested now-a-days. To overcome this problem, dynamic allocation of spectrum is under consideration over the current fixed spectrum allotment policy. In dynamic allotment, unlicensed users will be using the channel, whenever they find the channel idle and will immediately vacate the channel if licensed user returns. Handoff is the main feature of dynamic spectrum allotment policy. Also, it has been observed that multiple handoffs may degrade the service quality given to the unlicensed users. For that reason, different optimization methods may be applied for achieving the best possible spectrum allotment during handover so that the better quality of service can be given to the cognitive users with the simultaneous improvement in spectrum utilization. The handoff procedure can be pro-active or reactive type. Pro-active mainly deals with the beforehand preparation of the set of the target channels, while reactive handoff is on-demand approach. Both the styles have their own pros and cons. Depending on the application, any of the handoff technique can be used.

The book mainly focuses on the problem of high leakage which contributes to excessive standby power consumption. As it is known that the entire semiconductor industry is struggling with the heat of chips increasing exponentially as the number of transistors increases. The large leakage current may thus become a scaling showstopper, if alternative solutions are not provided to maintain it at an acceptable level. Although, to extend the scenario of transistor scaling and Moore's Law, a lot of research has been done in the recent past. Here in the book it is categorized as - research on alternative gate dielectrics like hafnium dioxide (HfO2), zirconium dioxide (ZrO2) and titanium dioxide (TiO2) and alternative device geometries like Double Gate MOSFET (DGMOSFET), tri-gate, gate all around) structures, silicon nanowire transistors (SNWT) etc. Effective gate control can be achieved by the multigate structures. High leakage current in deep-sub micrometer regimes is coming a significant contributor to power dissipation of CMOS circuits as threshold voltage, channel length, and gate oxide thickness are reduced.