Page 255 - From Smart Grid to Internet of Energy

Cognitive radio based smart grid communications Chapter 6 227

trains, sinusoidal carriers, and spreading codes may be typically exploited to
detect periodicity of signals [16, 62, 63]. A process can be considered as
wide-sense stationary when autocorrelation and mean functions of this process
are time-independent. In addition, if a process periodically indicates wide-sense
stationary characteristic, it is also cyclic stationary. The periodicity of a PU sig-
nal and wide-sense stationary of an uncorrelated noise make it possible for the
detector to simply distinguish noise from the PU signal. Nonetheless, compu-
tational complexity of this method is very high since the detector should com-
pute two functions that are also depend on both frequency and cyclic frequency.
The cyclic spectral density (CSD) or cyclic periodogram of a received signal
y(t) can be calculated by using following equation [16].
∞
X α j2πfτ
Sf, αÞ ¼ R τðÞe (6.10)
ð
y
τ¼ ∞
α
where α shows the cyclic frequency (or frequency separation), R y (τ) denotes the
α
cyclic autocorrelation function of the received signal. The R y (τ) can be also
given as follows.
α j2παt
∗
ð
R τðÞ ¼ Ey t + τð Þy t τÞe (6.11)
y
∗
where E[ ] shows the expectation operation while ( ) is the complex conjuga-
tion. If there exist active PUs in the channel, it is shown that the CSD function
expressed in Eq. (6.10) generates peaks in the event of cyclic frequency is same
with the main frequency. Therefore, the availability of the channel may be
determined by investigating peaks at the main frequency. A typical block dia-
gram of the CFD detector is depicted in Fig. 6.13.
This detection method provides excellent detection results at low SNR
values and it has a noise insensitive characteristic. On the other hand, the dis-
advantages of the CFD method contain the long observation period requirement
and high computational complexity. A comparison table for non-cooperative
detection methods are listed in Table 6.1.

6.4.4 Cooperative spectrum sensing based spectrum detection
The non-cooperative detection methods suffer from wireless communication
problems such as shadowing, fading, multi-path propagation effects and hidden
terminal problem. Unlike the use of non-cooperative methods, the CR devices
can cooperate to provide better spectrum detection reliability and to handle hid-
den terminal problem that occurs in the presence of multipath fading and

FIG. 6.13 Block diagram of cyclostationary system.