Recent portion of the licensed spectrum referredRecent portion of the licensed spectrum referred

Recent
advances in technology have resulted in rapid increase of wireless devices
operating in ISM band (900MHZ- 2.4GHZ). With ever increasing demand for mobile
applications, these unlicensed band are getting congested leading to spectrum
scarcity problem. On the other hand, frequency bands that are licensed for
operator services in 400-700MHZ range remain underutilized. In this scenario, Cognitive
radio has opened up as the enabling technology to solve the problem of spectrum
scarcity by considering DSA as a paradigm for opportunistic access of the
temporarily unused portion of the licensed spectrum referred to as spectrum holes
or white spaces. The prominent solution for spectrum efficiency remains in the
development of CRN considering legacy users as Primary users and CR users as Secondary
users. However, unique challenges are imposed on CR network due to dynamic
nature of the available spectrum. In particular, typical challenges of CR Adhoc
networks (CRAHN) are its self organizing capability due to varying spectrum in
both time and space. Therefore, exploration of the spectrum access
opportunities and addressing the communication problems of CR user in the
meantime prevention of its interference to legacy users is considered as the
most critical issue.

1.1.1       
COGNITIVE
CONCEPT

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             The
term CR was coined by Joseph Mitola and by its definition it enables CRAHN to
access radio spectrum dynamically by its capability of sensing the environment,
adapting to the changes by learning the interactions and reconfiguring the
operating parameters to enhance the diverse QOS parameters. The spectrum access
capability and dynamic programming through reconfigurability are considered as
two main characteristics of cognitive radio.

 

1.1.2       
 ARCHITECTURAL OVERVIEW

                        In the context of standard OSI (Open System
Interconnection) model, the architecture of CR network is viewed as illustrated
in Fig.1.

Fig.1 Architecture of
CR in layered OSI model.

 

                     Spectrum sensing plays a vital role in
discovering spectrum hole as well as preventing PU from harmful interference in
both physical and link layers. Physical layer sensing is done by various signal
detection methods and Medium access control (MAC) sensing improves the
detection performance of PU by the process of co-operative sensing. The
function of network layer is to opportunistically route in a multihop
environment such that the best route is discovered through cross layer
interactions taking into account hop-by-hop spectrum availability. Protocol
design is carried out in transport layer for improving traditional routing
protocols or to completely create a new protocol to improve the overall
spectrum availability. Spectrum access rights is transferred between PU and SU by
mechanisms such as spectrum auction and leasing. This process of usage right
transfer is termed as spectrum trading which is the function of application
layer. Therefore, to evaluate the performance of CR, quality of service (QOS)
awareness and security are considered as the vital functions for efficient
spectrum utilization.  

 

1.1.3       
SPECTRUM
MANAGEMENT FUNCTIONS

 

                In
order to adapt to DSA environment, CRAHN necessitate spectrum aware operation
termed as cognitive cycle. The main functions of this cycle are : Spectrum
sensing, Spectrum decision, Spectrum sharing and Spectrum mobility  Since it is a infrastructure less network,
cross layer protocols are in need for incorporating these functionalities. A
brief description about the functions :

                             

Spectrum sensing: identifies the
spectrum hole by observing the RF environment without causing harmful
interference against PU.

 

Spectrum decision: Recognize
appropriate frequency bands according to diverse QOS requirements.

 

Spectrum sharing : Sharing of
available spectrum resource among multiple CR users.

 

Spectrum mobility: Process of
vacating the spectrum band and to switch to another if PU is in the band of
interest.

 

1.1.4       
ISSUES
AND CHALLENGES IN CRAHN 

The coexistence of CR user with PUs
impose some of the challenges through different functions of the cognitive
cycle. They are optimization and coordination of sensing, joint detection of
spectrum and reconfiguration, PU activity modelling, spectrum decision model,
distributing topology information, flexible spectrum handoff, efficient design
of routing algorithms.

 

1.2     
COGNITIVE
RADIO: A ROUTING PERSPECTIVE

 

1.2.1       
MOTIVATION
FOR CROSS LAYER DESIGN

                Most of the research work in CRN
has its focus on single hop wireless access networks. The researchers later
realized CR as a paradigm shift for multihop networks to exploit the unexplored
services with wide network coverage. To fully harvest the advantage of multihop
CRN, it is necessary to study routing by considering the unique properties of
CRN.  Existing research works on CR routing
mainly focuses on PHY and MAC layer issues but in some cases due to link
failure, there is a need for re-routing at the network layer leading to performance
degradation of MAC protocols. Thereby, to deliver reliable traffic in DSA
system all communication protocols are closely related. In addition, multihop
CRAHN requires cross layer interactions to develop routing protocols
incorporating the best spectrum management functions.  However, the major breakthrough of this
research lies in utilizing the diversity gains in all dimensions namely time,
frequency and space of the scarce spectrum to enhance transmissions among CR
components.

 

1.2.2       
ROUTING
CHALLENGES IN CRAHN

                 In a nutshell, some of the
design challenges of routing of multihop CRN include: dynamic channel
availability, diversity of operating channel, lack of fixed common control
channel, joint routing and channel decision, minimizing delay, heterogeneity of
SUs. By considering this as open research challenges, various routing protocols
in different approaches namely proactive, reactive and hybrid approaches are
studied.

 

1.3     
OBJECTIVE
OF THE RESEARCH WORK

1.4     
RESEARCH
CONTRIBUTIONS

 

·        
Implementation of
adaptive spectrum sensing technique in CRN.

·        
Design of cooperative
sensing based spectrum access in CR networks using game theory.

·      
A spectrally efficient
network coded two-path relayed cognitive radio network.

·      
Design of SOP based
cross layered opportunistic routing protocol for CR Ad-hoc networks.