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Cognitive radio based smart grid communications Chapter 6 213

changing its system configurations such as waveform, bandwidth, transmission
power level to prevent inference with PUs. At the same time, the OSA of spec-
trum holes and altering of the operating frequency bands by a CR device can be
shown from the Fig. 6.1.
The CRN improves wireless network services by adjusting system param-
eters, autonomously assigns spectrum bands to the users, and authorizes CR
devices for adapting to wireless communication demands. Two important tar-
gets correlated the CR devices cover [22]:

l High accuracy, originality and communication feasibility on space-time
effects
l Sufficient use of existing radio spectrum
Generally, spectrum bands can be classified into two categories as official and
unofficial bands. Users employing the official bands with licenses have the
highest channel access priority while users exploiting the unofficial bands
are assumed to benefit from the band without affecting official users. While
the number of available channels in conventional wireless network is constant,
the number of available channels for the CR systems changes depending on the
time and spectrum holes. The active number of PUs directly determines the
number of accessible channels for the CR devices. The varying spectrum case
of accessible channels may cause several challenges for the CR spectrum man-
agement systems. These challenges can be summarized as quality-of-service
(QoS) awareness, reliable interference management and ensuring reliable com-
munication for CRs independent of primary appearance. In order to overcome
these issues, the CRNs should detect which parts of the frequency bands are
accessible for CR devices, manage access to prevent collision and interferences,
choose the best channel to reach the required QoS, and clear out the channel if
the PUs return. A typical cognitive cycle of CR devices that contains detection
of spectrum holes, choosing the best channel, managing spectrum access and
clearing out the exploited frequency band when a PU returns as depicted in
Fig. 6.2. This cycle is composed of the following stages: spectrum sensing,
spectrum decision, spectrum sharing and spectrum mobility.
Spectrum Sensing: Spectrum sensing (SS) is a transaction realized in the CR
devices to acquire radio or frequency information at a certain time and in a spe-
cific place. Sensing and analyzing processes are performed to detect signal
strength over a particular spectrum, and CR devices decide whether there are
available spectrum holes/white spaces. The acquired data thanks to the SS pro-
cess contain information regarding channel status of PUs, interference rate
affecting PUs, data traffic of PUs and channel gains among CR devices. In addi-
tion, the CRNs make spectrum access and spectrum sharing decision depending
on these information. Depending on the SS time of transmission frame, the SS
process can be performed by two methods called proactive method and reactive
method [23]. While the reactive sensing is only accomplished on demand, the
proactive sensing is realized periodically. On the other hand, CR devices