Comparative Study of Different Non-Cooperative Techniques in Cognitive Radio

International Journal of Research and Scientific Innovation (IJRSI) | Volume IV, Issue IV, April 2017 | ISSN 2321–2705
www.rsisinternational.org Page 9
Comparative Study of Different Non-Cooperative
Techniques in Cognitive Radio
Khushbu, Purbasha Datta, Prof. Prabir Banerjee
ECE Dept., Heritage Institute of Technology, Kolkata
Abstract- Wireless technology is expanding its domain and with it
is growing the need for more frequencies for communication.
Cognitive radio offers a solution to this problem by using the
concept of Dynamic spectrum access instead of fixed spectrum
allocation. Such radios are capable of sensing the RF spectrum
for identifying idle frequency bands. It then transmits
opportunistically so as to avoid interference with primary user
over same band. In cognitive radio, intelligent spectrum sensing
forms the major and most important part. Out of the various
sensing techniques, we will give an overview of some of the
prominent non-cooperative techniques. The paper deals with
comparative study of these methods.
Keywords- Cognitive radio (CR), Spectrum sensing, Primary
User (PU), Secondary User (SU), Spectrum hole, Cognitive cycle
I. INTRODUCTION
adio spectrum is a limited natural resource. This resource
in today‟s era has become one of the most valuable
resources, with increase in use of wireless communication in
almost every field of technology. In most of the countries
specific frequency bands are allocated to users as licensed
bands in exchange for fixed amount. The licensed bands are
defined by frequency, transmission power, type of use etc.
The process is known as fixed spectrum access (FSA)
[2][10][15]. The authorised user of licensed band are known
as primary user (PU) [2][3][8]. They can neither change the
type of use nor transfer the right to other. This may lead to
congestion of frequency band.
Thus there was a need to adopt dynamic spectrum access
(DSA). This technique has been introduced to resolve the
congestion problem. Cognitive radio [8][10][11] is the
concept which is based on DSA [10][13][15][17]. This
technique was introduced by Joseph Mitola in late 1990‟s
[5][16]. It introduced the concept of opportunistic spectrum
access (OSA) [15][17] and it automatically exploits unused or
partially used band to provide new carrier for spectrum
access. In CR the secondary user [2][3][8] has to continuously
sense the environment to detect the presence of PU. PU has
first priority on the assigned spectrum. As soon as SU detects
that PU has become active it must switch to other available
spectrum. CR uses the key feature of spectrum sensing for
detecting the signal strength of available frequency and
presence of PU. Spectrum sensing is used to find out spectrum
hole [2][5][12][13] that can be utilized. Spectrum hole is
defined as part of the spectrum that can be utilized by SU,
which is a basic resource for CR. It is depicted as;
Fig.1 Spectrum hole
II. COGNITIVE RADIO
Cognitive radio is a solution to spectral congestion problem
caused due to increasing number of users and scarcity of
available spectrum. Cognitive radio was defined by FCC
[2][3][4][5][18] as „A radio or system that senses its
operational parameters to modify system operation such as
maximize throughput, mitigate interference, facilitate
interoperability and access secondary market.‟ Cognitive
Radio works on the principal of cognitive cycle
[11][18].Cognitive cycle consists of various steps; Spectrum
sensing, Spectrum decision, Spectrum sharing and Spectrum
mobility. Cognitive cycle can be shown as below;
Fig.2 Cognitive cycle
Spectrum Sensing- This is the process of detecting unused or
partially used spectrum. It makes the system aware of
parameters related to radio channel characteristics,
interference noise and transmit power [11][18].
R

Spectrum decision- Based on the sensing result best available
spectrum that meets user communication requirements is
selected [11][18].
Spectrum sharing- It provides a fair spectrum scheduling
method to users [11][18].
Spectrum mobility- In this step CR users exchange their
frequency of operation [11][18].
III. SPECTRUM SENSING
Spectrum sensing [2][13] is the basic mechanisms employed
by Cognitive radios (CR) to find the unused spectrum.
Spectrum sensing is the task of obtaining awareness about the
spectrum usage and existence of PU in a geographical area.
The cognitive user can then decide whether it can use that
frequency or not. It helps in determining the parameters that
need to be adjusted inorder to avail the free or idle spectrum.
By adjusting the parameters, this technique reduces the
interference with PU. Spectrum sensing techniques are still in
their early stage of development and number of methods has
been proposed for identifying the presence of signal
transmission. Some major techniques are [5][14]:
1. Cooperative sensing
The primary signal for spectrum opportunities are detected
reliably by interacting or cooperating with other users. The
method can be implemented in 2 ways. Either centralized
access to spectrum coordinated by a spectrum server or
distributed approach implied by the load smoothing algorithm
or external detection.
2. Interference based sensing
In this type of sensing, CR system operates in an environment
where the SU coexists with PU and is allowed to transmit at
low power. The power is restricted by the interference
temperature level so as not to cause harmful interference to
primary users [12][14].
3. Non-cooperative sensing
It is also known as Transmitter detection method. This is
called non-cooperative method as there is no cooperation or
sharing among terminals. In his method CR user detect the
presence or absence of PU through sensing by itself. The data
obtained is used to decide on channel state that is idle or busy.
Whereas in cooperative sensing CR shares its sensed data
with others. The sensing outcome of others is utilized for
decision. There are various methods for non-cooperative
sensing. Some of the well-known methods are energy
detector, matched filter, Cyclostationary based sensing and
waveform based sensing [12][13][17].
a) Energy Detection: Energy detector based approach is also
known as Radiometry or Periodogram. It is the most common
way of spectrum sensing because of its low computational and
implementation complexities. It is a type of non-coherent
detection. In this process primary user detection is done by
comparing energy of received signal with a predefined
threshold and declares band as busy if measured energy is
greater than threshold. The process can be depicted as follows
[1][7][9][14][18],
Fig.3 Block diagram of energy detector
In the above process the received signal is filtered with a band
pass filter to select preferred bandwidth. Then it is squared
and integrated over sensing intermission. The result which is
the test static is obtained and is compared with decision
threshold to test presence or absence of primary user. The test
static or decision metric can be written as;
T(y) = ∑ ( ( ))
T(y) is Energy detector test metric. λ is defined as the
threshold which distinguishes between two test hypothesis.
: y(n) = w(n)
: y(n) = s(n)+w(n)
n→ number of samples , w(n) → AWGN noise, s(n) → signal
to be detected
When, T(y) > λ /
T(y) < λ /
Hypothesis depicts absence of PU whereas depicts
presence of PU.
Advantage- This method is more generic as receiver do not
need to know any knowledge of PU and thus it can be used
when PU information is not available [2][6].
Disadvantage- This method cannot differentiate interference
from PU and noise. The performance is poor SNR is low
[2][6].
Spectrum
sensing
Interference
based sensing
Cooperative
sensing
Non-
Cooperative
sensing
Energy
Detection
Matched
filter
Cyclostationary
feature based
detection
Wavefor
m based
detection
PSD BPF ∫
Integrat
or
H1
H0

b) Matched Filter: Matched filter method is a type of non
blind spectrum sensing [10][16]. It is used to sense PU when
information about transmitter working with parameters such
as frequency, bandwidth, modulation type, pulse shaping and
frame format are known.
Fig.4 Block diagram of matched filter based sensing
The signal received is passed through BPF. This will
determine energy of received signal. The signal is then given
to matched filter. The signal correlated with a time reversed
version of itself. The signal is fed to comparator whose output
is utilized to take decision whether signal is acceptable or not
[3][7][14].
Advantage- This technique maximizes SNP of received signal
and is best suited if CR has prior information of PU signal.
Also another advantage is short time to achieve a certain
probability of misdetection [6].
Disadvantage- If PU information is not accurate then matched
filter operates weakly. CR users usually need to observe a
wide spectrum band with multiple primary systems and thus,
they need the receiver to estimate different types of PU
signals which leads to high complexities in implementation
and large power consumption [6].
c) Cyclostationary: This type of detection is done by
exploiting Cyclostationary feature of received signal i.e. it
exploits periodicity of received signal to identify PU. A signal
is said to be Cyclostationary if its mean and autocorrelation
are periodic function. It requires prior knowledge of signal. It
can distinguish PU signal from noise and can be used at lower
SNR by using the information embedded in PU signal that are
absent in noise [5][7][9][13][14].
Fig.5 Block diagram of Cyclostationary based sensing method
Advantage- It typically allows difference among dissimilar
signal or waveform or PU and noise. This is a result of the
fact that noise is in a wide-sense stationary with no correlation
while modulated signals are Cyclostationary with spectral
correlation due to redundancy of signal periodicities [6]
.Disadvantage- The major drawback of this method is
complexity of calculation as it deals with all frequencies in
order to generate spectral correlation function [2].
d) Waveform based sensing: This type of sensing is also
known as coherent based sensing. This is applicable to signals
with known signal patterns such as preamble, midambles etc.
Preamble is a known sequence transmitted before each burst
or slot. Sensing is performed by correlating received signal
with a known copy of itself. It requires short measured time.
The decision metric can be constructed as [2],
M = Re [∑ ( ) ( )]
The decision metric is compared to fixed threshold to
determine if transmitted signal is detected. When the PU is
transmitting the decision metric become,
M = ∑ | ( )| + Re [∑ ( ) ( )]
Where n(i)-noise present, s(i)- PU transmitted signal.
When PU is not transmitting then decision metric become,
M = Re [∑ ( ) ( )]
Advantage- Number of samples required to form decision
metric is less [3][6].
Disadvantage- The drawback of this method is that it requires
transmitted signal to contain known pattern. The exact
position of pattern may not be known. It calls for estimation
of position which in turn, increases complexity [3].
IV. COMPARISION BETWEEN VARIOUS NON-COOPERATIVE
TECHNIQUES
After knowing about different methods of non-cooperative
sensing, selecting a method for detection require some
tradeoffs. Characteristics of PUs are main factor in selecting a
method. Comparison based on certain characteristics is done
in the following table:
Parameters
Energy
detector
Wavefo
rm
based
sensing
Matched
filter
Cyclostation
ary based
filter
Accuracy in
detection
based on
SNR value
Preferred
if SNR is
high but
at low
SNR the
performa-
nce
degrades.
Performs
well at
all SNR.
If receiver
has prior
knowledge
of
transmitter
then it
performs
well at all
SNR.
Overall
performance
is good at all
SNRs.
Design
pattern
Threshold
selection
forms an
important
deciding
factor
Transmi-
ssion
pattern
can be
increas-
ed to
improve
accuracy
.
Transmissi-
on
characteristic
can be
appropriately
chosen to
improve
accuracy.
Cyclostation-
ary
characteristic
can be
induced to
improve
accuracy.
Robustness
Informati-
on of
transmitte
d signal is
not
required
but it is
not
suitable
for spread
spectrum
Transmi-
tted
signal
should
contain,
known
pattern
but exact
position
of
pattern
Requires
near perfect
transmitted
information
at receiver.
Receiver
must know
transmitted
signal
fundamental
frequency.
BPF Matched
filter
Comparator
BPF N
point
FFT
Correlatio
n
Avg.
over
T
Feature
detector

signals. of
pattern
may not
be
known at
receiver.
Complexity
Simple to
impleme-
nt but
requires
large
number of
samples to
converge.
Requires
small
number
of
samples
and also
not so
complex.
Requires
small
number of
samples and
is complex.
Requires
small number
of samples
and is
complex in
nature.
Sensing
time
Takes
large
sensing
time to
achieve a
given
probabili-
ty of
detection.
Takes
compar-
atively
less
time.
Requires less
time to
achieve high
processing
gain due to
coherent
detection.
Takes longer
sensing time.
The above table describes various parameters on which we
can choose a sensing method. Based on our priority of
application, we may select any one of the given methods.
Most of the time we consider the accuracy with respect to
complexity of method under consideration. Based on the
accuracy and complexity we can plot different methods on a
plane as given below [2][13][14][18];
Accuracy
Complexity
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Matche
d filter
Cyclostationar
y based
sensing
Waveform
based
Sensing
Energy
detector

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Comparative Study of Different Non-Cooperative Techniques in Cognitive Radio