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Performance Analysis and Modelling of Spectrum Handoff Schemes in Cognitive Radio Networks. Modelling and Analysis of Spectrum Handoff Decision Schemes in Cognitive Radio Networks using the Queuing Theory and Simulation for Licensed and Unlicensed Spectrum Bands.
Recently, wireless access has become an essential part of modern society. Consequently, the demand for new wireless applications and services, as well as the number of wireless users, are gradually increasing. Given that this amount of expansion is eventually controlled by the available radio frequency spectrum, government regulatory agencies have recently adopted a strict approach to the licensing of limited amounts of spectrum to different entities (e.g., public safety, military, service providers, unlicensed devices, and TV). All of them possess exclusive transmissions to their assigned frequency channels. A new study on spectrum efficiency revealed big geographic and temporal variations in spectrum utilisation, ranging from 15-85% in the bands below 3GHz. These variations were less at frequencies above this figure. Recently, the Cognitive Radio (CR) has risen as an encouraging piece of technology to improve spectrum efficiency and to solve the problem of spectrum scarcity. This is because CR allows the secondary (unlicensed) users to occupy unused licensed spectrum bands temporarily, given that the interference of the primary (licensed) users is prohibited or minimised.
In this thesis, various spectrum handoff management schemes have been proposed in order to improve the performance evaluation for CR networks. The proposed spectrum handoff schemes use the Opportunistic Spectrum Access (OSA) concept to utilise available spectrum bands. The handoff Secondary Users (SUs) have a higher priority to occupy available spectrum channels in the licensed and unlicensed spectrum bands without interfering with the legacy spectrum owner, i.e. primary users (PUs). However, existing spectrum handoff management schemes in CR networks do not provide high transmission opportunities for handoff secondary users to utilise the available radio spectrum resources. The first part of this thesis addresses the issue of spectrum handoff management in a licensed spectrum band environment. In this case, both reactive and proactive spectrum handoff schemes are proposed. Queuing theory or/and simulation experiments have been used to evaluate the performance of the proposed schemes and compare them with other existing schemes. Handoff delay has mainly been used to investigate the impact of successive handoff operations on the performance of the proposed CR networks. Implemented models have shown an improvement in the adopted performance measures. According to the achieved results, the improvement of the proposed, prioritised handoff schemes in some cases is approximately 75% when compared with existing schemes.
On the other hand, the second part of this research proposed a prioritised spectrum handoff scheme in a heterogeneous spectrum environment, which is composed of a pool of licensed and unlicensed spectrum channels. In general, the availability of substantial numbers of the licensed spectrum channels is the key benefit of using this type of radio spectrum channel. Whereas, accessing with equal rights for all types of users is the main advantage of using unlicensed spectrum channels. In this respect, no transmission interruptions occur once a user obtains a channel. In addition, the proposed schemes use only the unlicensed spectrum channels as their backup channels. This enables the user to resume interrupted transmission in the case of the spectrum handoff operation (mainly; due to the appearance of the primary users), and thus facilitates a SUs communication. The proposed principle is investigated using a retrial queuing theory as well as extensive simulation experiments, and is compared with another non-prioritised scheme which do not give any preference to handoff SUs over new SUs. The results indicate that the proposed model has improved on current average handoff delay.
This thesis contributes to knowledge by further enhancing the efficient utilisation of available radio spectrum resources and therefore subsequently provides an improvement in the spectrum capacity for wireless cognitive radio networks
Modeling and Analysis of Cognitive Radio Ad Hoc Networks
Eine Welt ohne drahtlose Ad-Hoc Netzwerke ist heute kaum noch
vorstellbar.Auf Grund der geringen Kosten und des minimalen
Installationsaufwands werden gegenwÀrtig immer mehr GerÀte in immer mehr
Anwendungsfeldern eingesetzt. Da die meisten dieser Netzwerke im
lizenzfreien ISM-Band operieren, ist dieses heute stark ausgelastet und
weist kaum noch freie KapazitÀten auf. Aktuelle Studien der Federal
Communication Commission (FCC) belegen allerdings, dass groĂe Teile (bis zu
70%) der lizenzbehafteten Frequenzen ungenutzt sind. Dieser Umstand zeigt,
dass das Problem weniger die generelle Knappheit an freien Frequenzen ist,
sondern vielmehr in der ineffizienten Verteilung bzw.Nutzung der
verfĂŒgbaren Resourcen zu suchen ist. Das Hauptaugenmerk der vorliegenden
Dissertation liegt in der Verbesserung der Spektrumsauslastung, um dadurch
die weitere Entwicklung von drahtlosen Ad-Hoc Netzwerken zu ermöglichen.In
dieser Arbeit wird ein neues Spektrum-Management-Konzept mit dem
Namen Opportunistic Spectrum Access with Backup channel (OSAB) entwickelt und
vorgestellt. Das hierbei zugrunde liegende Konzept gestattet Secondary
Users (SUs)dynamisch und flexibel auf Frequenzen unlizenzierter als auch
lizensierterFrequenzbÀnder zu zugreifen, wenn diese vom Primary User (PU)
gerade nicht genutzt werden - es also keine Interferenzen geben kann.Da der
Zugriff auf das Frequenzspektrum heute existierender Systeme noch sehr
unflexibel ist, soll dieser in Zukunft durch Cognitive Radios (CR)weit
flexibler und dynamischer gestaltet werden können. Bei der Entstehung von
OSAB wurden speziell die unterschiedlichen Eigenschaften verschiedener
FrequenzbĂ€nder berĂŒcksichtigt.Der Hauptvorteil von lizenzbehafteten BĂ€ndern
ist, dass diese in hoher Anzahl verfĂŒgbar sind. Der Hauptvorteil von
lizenzfreien Frequenzen ergibt sich hingegen aus der Gleichstellung aller
Nutzer. Sobald ein SU einmal einen Kanal belegt hat, kann er nicht mehr aus
selbigem verdrÀngt werden.Kommuniziert OSAB in lizenzierten BÀndern, so
wird stets ein Backup Channel (BC)vorgehalten um auf das plötzliche
Auftreten des PUs reagieren zu können.Das vorgeschlagene Konzept wurde in
dieser Arbeit auĂerdem einer intensiven Analyse mittel Markov-Ketten
unterzogen. Die dabei erzielten Ergebnisse zeigen,dass OSAB den
Paketverlust und die erwartete Anzahl an Spektrum-Hand-Offs um 60% bzw. 17%
reduzieren kann.Um den Nutzen und die Vorteile von OSAB praktisch unter
Beweis zu stellen, wurde in der vorliegenden Arbeit weiterhin das
MAC-Protokoll SWITCH (opportunisticSpectrum access WITh backup CHannel)
entwickelt.SWITCH ist ein dezentrales, asynchrones, verbindungsbasiertes
MAC-Protokoll, welchesdurch das Backup-Channel-Konzept in der Lage ist,
effektiv auf das plötzliche Eintreffen von PUs zu reagieren.Jeder SU ist
dabei mit zwei Transceivern ausgestattet, wobei einer davon stets fĂŒr
die Kommunikation auf dem gemeinsam genutzten Kontroll-Kanal (Common Control
Channel) verantwortlich ist. Der zweite Transceiver ist so ausgelegt, dass
dieser periodisch alle ungenutzten KanÀle absucht und dynamisch auf diese
zugreifen kann. Um den Zustand eines Kanals (belegt/nicht belegt) korrekt
erkennen zu können wird in dieser Arbeit eine einfache aber effektive Form
des kooperativen Sensings genutzt. Die Performanz des Protokolls wurde mit
Hilfe von Simulationen evaluiert. Die Ergebnisse zeigen, dass SWITCH im
Vergleich zu anderen CR-MAC-Protokollen eine Verbesserung des Durchsatzes
von bemerkenswerten 91,7% erzielen konnte. Zusammenfassend kann gesagt
werden, dass die vorgeschlagenen BeitrÀge einen Schritt hin zu einer
effektiveren Nutzung der verfĂŒgbaren Funkressourcen und zur Erhöhung der
KapazitÀt von drahtlosen Ad-Hoc Netzwerken darstellen.Wireless ad hoc networks are becoming more ubiquitous in terms of devices,
application areas, etc. due to their low cost and minimal deployment
effort. Since all these networks operate in the unlicensed band, the
problems of congestion and spectrum scarcity have arisen. On the other
hand, a recent study by Federal Communications Commission (FCC) has
revealed that swathes of licensed bands, measured by 70%, are unutilized.
This highlights that the actual problem is not the scarcity of spectrum but
inefficient allocation policies and usage. Therefore, this dissertation is
focused on improving spectrum utilization and efficiency to tackle the
spectrum scarcity problem and support further wireless ad hoc networks.This
thesis proposes a new spectrum management concept called opportunistic
spectrum access with backup channel (OSAB). The proposed concept provides
secondary users (SUs) (e.g. ad hoc users) with the ability to adaptively
and dynamically exploit channels from both licensed and unlicensed bands
without interfering the legacy users of licensed bands, i.e. the so called
primary users (PUs). Since existing radio systems offer very limited
flexibility, cognitive radios (CR), which can sense and adapt to radio
environments, are exploited to support such a dynamic concept. For the
development of OSAB, the channels' characteristics from each band are taken
into consideration. The main advantage of licensed channels is their
availability in significant numbers, whereas, the main advantage of
unlicensed channels is that all users have the same rights to channel
access and thus no preemption occurs once a user obtains a channel. In
addition, OSAB uses a backup channel (BC) to handle the appearance of PUs
and thus facilitates SU communication. The proposed concept is extensively
evaluated using a Markov chain model and compared to existing spectrum
management approaches such as opportunistic spectrum access (OSA). The
results indicate that OSAB decreases the dropping probability and the
expected number of spectrum handoffs for SUs compared to OSA by 60% and 17%
respectively.In order to apply OSAB practically, we develop a MAC protocol
that reacts efficiently to sudden appearance of PUs. The new protocol is
named opportunistic Spectrum access WITh backup CHannel (SWITCH) protocol.
SWITCH is a decentralized, asynchronous, and contention-based MAC protocol.
The BC's concept makes SWITCH extremely robust to the appearance of PUs.
Each SU is equipped with two transceivers, one is tuned to a common control
channel for the negotiation purpose with other SUs while the other is
designed specifically to periodically sense and dynamically use the
identified unused channels. To obtain the channel state accurately, we
propose an efficient spectrum sensing strategy. This strategy is based on
cooperative spectrum sensing among SUs. The performance of proposed
protocol is evaluated through simulations. The results show that SWITCH
accomplishes a remarkable 91.7% throughput gain over other CR-MAC
protocolsTo conclude, the proposed contributions are a step forward towards
efficient use of available radio resources and improve the spectrum
capacity for wireless ad hoc networks