IEEE JOURNAL OF SELECTED TOPICS IN SIGNAL PROCESSING, VOL. 5, NO. 1, FEBRUARY 2011 5
Advances in Cognitive Radio Networks: A Survey
Beibei Wang and K. J. Ray Liu
Abstract—With the rapid deployment of new wireless devices
and applications, the last decade has witnessed a growing demand
for wireless radio spectrum. However, the fixed spectrum assign-
ment policy becomes a bottleneck for more efficient spectrum uti-
lization, under which a great portion of the licensed spectrum is
severely under-utilized. The inefficient usage of the limited spec-
trum resources urges the spectrum regulatory bodies to review
their policy and start to seek for innovative communication tech-
nology that can exploit the wireless spectrum in a more intelligent
and flexible way. The concept of cognitive radio is proposed to ad-
dress the issue of spectrum efficiency and has been receiving an in-
creasing attention in recent years, since it equips wireless users the
capability to optimally adapt their operating parameters according
to the interactions with the surrounding radio environment. There
have been many significant developments in the past few years on
cognitive radios. This paper surveys recent advances in research
related to cognitive radios. The fundamentals of cognitive radio
technology, architecture of a cognitive radio network and its appli-
cations are first introduced. The existing works in spectrum sensing
are reviewed, and important issues in dynamic spectrum allocation
and sharing are investigated in detail.
Index Terms—Cognitive radio (CR), platforms and standards,
radio spectrum management, software radio, spectrum sensing,
wireless communication.
I. INTRODUCTION
T
HE usage of radio spectrum resources and the regulation
of radio emissions are coordinated by national regulatory
bodies like the Federal Communications Commission (FCC).
The FCC assigns spectrum to licensed holders, also known as
primary users, on a long-term basis for large geographical re-
gions. However, a large portion of the assigned spectrum re-
mains under utilized as illustrated in Fig. 1. The inefficient usage
of the limited spectrum necessitates the development of dy-
namic spectrum access techniques, where users who have no
spectrum licenses, also known as secondary users, are allowed
to use the temporarily unused licensed spectrum. In recent years,
the FCC has been considering more flexible and comprehensive
uses of the available spectrum [1], through the use of cognitive
radio technology [2].
Cognitive radio is the key enabling technology that enables
next generation communication networks, also known as dy-
Manuscript received October 30, 2009 accepted October 24, 2010. Date of
publication November 18, 2010; date of current version January 19, 2011. The
associate editor coordinating the review of this manuscript and approving it for
publication was Dr. Sastri Kota.
B. Wang is with Corporate Research and Development, Qualcomm, Inc., San
Diego, CA 92121 USA (e-mail: beibeiw@qualcomm.com).
K. J. R. Liu is with the Department of Electrical and Computer Engi-
neering, University of Maryland, College Park, MD 20742 USA (e-mail:
bebewang@umd.edu; kjrliu@umd.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JSTSP.2010.2093210
Fig. 1. Spectrum usage [5].
namic spectrum access (DSA) networks, to utilize the spec-
trum more efficiently in an opportunistic fashion without inter-
fering with the primary users. It is defined as a radio that can
change its transmitter parameters according to the interactions
with the environment in which it operates [3]. It differs from
conventional radio devices in that a cognitive radio can equip
users with cognitive capability and reconfigurability [4], [5].
Cognitive capability refers to the ability to sense and gather in-
formation from the surrounding environment, such as informa-
tion about transmission frequency, bandwidth, power, modula-
tion, etc. With this capability, secondary users can identify the
best available spectrum. Reconfigurability refers to the ability
to rapidly adapt the operational parameters according to the
sensed information in order to achieve the optimal performance.
By exploiting the spectrum in an opportunistic fashion, cogni-
tive radio enables secondary users to sense which portion of the
spectrum are available, select the best available channel, coor-
dinate spectrum access with other users, and vacate the channel
when a primary user reclaims the spectrum usage right.
Considering the more flexible and comprehensive use of the
spectrum resources, especially when secondary users coexist
with primary users, traditional spectrum allocation schemes [6]
and spectrum access protocols may no longer be applicable.
New spectrum management approaches need to be developed
to solve new challenges in research related to cognitive radio,
specifically, in spectrum sensing and dynamic spectrum sharing.
As primary users have priority in using the spectrum, when
secondary users coexist with primary users, they have to per-
form real-time wideband monitoring of the licensed spectrum
to be used. When secondary users are allowed to transmit data
simultaneously with a primary user, interference temperature
limit should not be violated [7]. If secondary users are only al-
lowed to transmit when the primary users are not using the spec-
trum, they need to be aware of the primary users’ reappearance
through various detection techniques, such as energy detection,
feature detection, matched filtering and coherent detection. Due
to noise uncertainty, shadowing, and multipath effect, detection
performance of single user sensing is pretty limited. Coopera-
tive sensing has been considered effective in improving detec-
1932-4553/$26.00 © 2011 IEEE
6 IEEE JOURNAL OF SELECTED TOPICS IN SIGNAL PROCESSING, VOL. 5, NO. 1, FEBRUARY 2011
tion accuracy by taking advantage of the spatial and multi-user
diversity. In cooperative spectrum sensing, how to select proper
users for sensing, how to fuse individual user’s decision and ex-
change information, and how to perform distributed spectrum
sensing are issues worth studying.
In order to fully utilize the spectrum resources, efficient dy-
namic spectrum allocation and sharing schemes are very im-
portant. Novel spectrum access control protocols and control
channel management should be designed to accommodate the
dynamic spectrum environment while avoid collision with a pri-
mary user. When a primary user re-appears in a licensed band,
a good spectrum handoff mechanism is required to provide sec-
ondary users with smooth frequency transition with low latency.
In multi-hop cognitive wireless networks, intermediate cogni-
tive nodes should intelligently support relaying information and
routing through using a set of dynamically changing channels.
In order to manage the interference to the primary users and the
mutual interference among themselves, secondary users’ trans-
mission power should be carefully controlled, and their compe-
tition for the spectrum resources should also be addressed.
There have been many significant developments in the past
few years on cognitive radios. This article surveys recent ad-
vances in research related to cognitive radios. In Section II, we
overview the fundamentals of cognitive radio technology, ar-
chitecture of a cognitive radio network and its applications. In
Section III, we review existing works in spectrum sensing, in-
cluding interference temperature, different types of detection
techniques, and cooperative spectrum sensing. In Section IV,
we discuss several important issues in dynamic spectrum allo-
cation and sharing.
II. F
UNDAMENTALS
A. Cognitive Radio Characteristics
The dramatic increase of service quality and channel capacity
in wireless networks is severely limited by the scarcity of en-
ergy and bandwidth, which are the two fundamental resources
for communications. Therefore, researchers are currently fo-
cusing their attention on new communications and networking
paradigms that can intelligently and efficiently utilize these
scarce resources. Cognitive radio (CR) is one critical enabling
technology for future communications and networking that
can utilize the limited network resources in a more efficient
and flexible way. It differs from traditional communication
paradigms in that the radios/devices can adapt their operating
parameters, such as transmission power, frequency, modulation
type, etc., to the variations of the surrounding radio environment
[3]. Before CRs adjust their operating mode to environment
variations, they must first gain necessary information from the
radio environment. This kind of characteristics is referred to as
cognitive capability [4], which enables CR devices to be aware
of the transmitted waveform, radio frequency (RF) spectrum,
communication network type/protocol, geographical infor-
mation, locally available resources and services, user needs,
security policy, and so on. After CR devices gather their needed
information from the radio environment, they can dynamically
change their transmission parameters according to the sensed
Fig. 2. Cognitive cycle.
Fig. 3. Illustration of spectrum white space [5].
environment variations and achieve optimal performance,
which is referred to as reconfigurability [4].
B. Cognitive Radio Functions
A typical duty cycle of CR, as illustrated in Fig. 2, includes
detecting spectrum white space, selecting the best frequency
bands, coordinating spectrum access with other users and va-
cating the frequency when a primary user appears. Such a cog-
nitive cycle is supported by the following functions:
• spectrum sensing and analysis;
• spectrum management and handoff;
• spectrum allocation and sharing.
Through spectrum sensing and analysis, CR can detect the
spectrum white space (see Fig. 3), i.e., a portion of frequency
band that is not being used by the primary users, and utilize the
spectrum. On the other hand, when primary users start using the
licensed spectrum again, CR can detect their activity through
sensing, so that no harmful interference is generated due to sec-
ondary users’ transmission.
After recognizing the spectrum white space by sensing,
spectrum management and handoff function of CR enables
secondary users to choose the best frequency band and hop
among multiple bands according to the time varying channel
characteristics to meet various Quality of Service (QoS) re-
quirements [5]. For instance, when a primary user reclaims
his/her frequency band, the secondary user that is using the
licensed band can direct his/her transmission to other available
frequencies, according to the channel capacity determined by
the noise and interference levels, path loss, channel error rate,
holding time, and etc.
In dynamic spectrum access, a secondary user may share
the spectrum resources with primary users, other secondary
users, or both. Hence, a good spectrum allocation and sharing
WANG AND LIU: ADVANCES IN COGNITIVE RADIO NETWORKS: A SURVEY 7
Fig. 4. Network architecture of dynamic spectrum sharing.
mechanism is critical to achieve high spectrum efficiency. Since
primary users own the spectrum rights, when secondary users
co-exist in a licensed band with primary users, the interference
level due to secondary spectrum usage should be limited by
a certain threshold. When multiple secondary users share a
frequency band, their access should be coordinated to alleviate
collisions and interference.
C. Network Architecture and Applications
With the development of CR technologies, secondary users
who are not authorized with spectrum usage rights can utilize
the temporally unused licensed bands owned by the primary
users. Therefore, in a CR network architecture, the components
include both a secondary network and a primary network, as
shown in Fig. 4.
A secondary network refers to a network composed of a
set of secondary users with/without a secondary base station.
Secondary users can only access the licensed spectrum when it
is not occupied by a primary user. The opportunistic spectrum
access of secondary users is usually coordinated by a secondary
base station, which is a fixed infrastructure component serving
as a hub of the secondary network. Both secondary users and
secondary base stations are equipped with CR functions. If
several secondary networks share one common spectrum band,
their spectrum usage may be coordinated by a central network
entity, called spectrum broker [8]. The spectrum broker col-
lects operation information from each secondary network, and
allocates the network resources to achieve efficient and fair
spectrum sharing.
A primary network is composed of a set of primary users and
one or more primary base stations. Primary users are authorized
to use certain licensed spectrum bands under the coordination of
primary base stations. Their transmission should not be inter-
fered by secondary networks. Primary users and primary base
stations are in general not equipped with CR functions. There-
fore, if a secondary network share a licensed spectrum band with
a primary network, besides detecting the spectrum white space
and utilizing the best spectrum band, the secondary network is
required to immediately detect the presence of a primary user
and direct the secondary transmission to another available band
so as to avoid interfering with primary transmission.
Because CRs are able to sense, detect, and monitor the sur-
rounding RF environment such as interference and access avail-
ability, and reconfigure their own operating characteristics to
best match outside situations, cognitive communications can in-
crease spectrum efficiency and support higher bandwidth ser-
vice. Moreover, the capability of real-time autonomous deci-
sions for efficient spectrum sharing also reduces the burdens of
centralized spectrum management. As a result, CRs can be em-
ployed in many applications.
8 IEEE JOURNAL OF SELECTED TOPICS IN SIGNAL PROCESSING, VOL. 5, NO. 1, FEBRUARY 2011
First, the capacity of military communications is limited by
radio spectrum scarcity because static frequency assignments
freeze bandwidth into unproductive applications, where a large
amount of spectrum is idle. CR using dynamic spectrum access
can alleviate the spectrum congestion through efficient alloca-
tion of bandwidth and flexible spectrum access [2]. Therefore,
CR can provide military with adaptive, seamless, and secure
communications.
Moreover, a CR network can also be implemented to en-
hance public safety and homeland security. A natural disaster or
terrorist attack can destroy existing communication infrastruc-
ture, so an emergency network becomes indispensable to aid the
search and rescue. As a CR can recognize spectrum availability
and reconfigure itself for much more efficient communication,
this provides public safety personnel with dynamic spectrum
selectivity and reliable broadband communication to minimize
information delay. Moreover, CR can facilitate interoperability
between various communication systems. Through adapting to
the requirements and conditions of another network, the CR de-
vices can support multiple service types, such as voice, data,
video, and etc.
Another very promising application of CR is in the com-
mercial markets for wireless technologies. Since CR can intel-
ligently determine which communication channels are in use
and automatically switches to an unoccupied channel, it pro-
vides additional bandwidth and versatility for rapidly growing
data applications. Moreover, the adaptive and dynamic channel
switching can help avoid spectrum conflict and expensive re-
deployment. As CR can utilize a wide range of frequencies,
some of which has excellent propagation characteristics, CR
devices are less susceptible to fading related to growing fo-
liage, buildings, terrain and weather. When frequency changes
are needed due to conflict or interference, the CR frequency
management software will change the operating frequency au-
tomatically even without human intervention. Additionally, the
radio software can change the service bandwidth remotely to ac-
commodate new applications. As long as no end-user hardware
needs to be updated, product upgrades or configuration changes
can be completed simply by downloading newly released radio
management software. Thus, CR is viewed as the key enabling
technology for future mobile wireless services anywhere, any-
time and with any device.
III. S
PECTRUM SENSING AND ANALYSIS
Through spectrum sensing, CR can obtain necessary obser-
vations about its surrounding radio environment, such as the
presence of primary users and appearance of spectrum holes.
Only with this information can CR adapt its transmitting and
receiving parameters, like transmission power, frequency, mod-
ulation schemes, and etc., in order to achieve efficient spec-
trum utilization. Therefore, spectrum sensing and analysis is
the first critical step towards dynamic spectrum management. In
this section, we will discuss three different aspects of spectrum
sensing. First is the interference temperature model, which mea-
sures the interference level observed at a receiver and is used to
protect licensed primary users from harmful interference due to
unlicensed secondary users. Then we will talk about the spec-
trum hole detection to determine additional available spectrum
resources and compare several detection techniques. Finally, we
will discuss cooperative sensing with multiple users’ help.
A. Interference Temperature
In opportunistic spectrum access, secondary users need to de-
tect primary users’ appearance and decide which portion of the
spectrum is available. Such a decision can be made according to
different metrics. Traditional approach is to limit the transmitter
power of interfering devices, i.e., the transmitted power should
be no more than a prescribed noise floor at a certain distance
from the transmitter. However, due to the increased mobility
and variability of radio frequency (RF) emitters, constraining
the transmitter power becomes more problematic, since unpre-
dictable new source of interference may appear. To address this
issue, FCC Spectrum Policy Task Force [9] has proposed a new
metric on interference assessment, the interference temperature,
to enforce an interference limit perceived by receivers. The in-
terference temperature is a measure of the RF power available
at a receiving antenna to be delivered to a receiver, reflecting the
power generated by other emitters and noise sources [10]. More
specifically, it is defined as the temperature equivalent to the RF
power available at a receiving antenna per unit bandwidth [11],
i.e.,
(1)
where
is the average interference power in Watts cen-
tered at
, covering bandwidth measured in Hertz, and Boltz-
mann’s constant
is 1.38 Joules per degree Kelvin.
With the concept of interference temperature, FCC further
established an interference temperature limit, which provides
a maximum amount of tolerable interference for a given fre-
quency band at a particular location. Any unlicensed secondary
transmitter using this band must guarantee that their transmis-
sion plus the existing noise and interference must not exceed the
interference temperature limit at a licensed receiver.
Since any transmission in the licensed band is viewed to be
harmful if it would increase the noise floor above the interfer-
ence temperature limit, it is necessary that the receiver have a re-
liable spectral estimate of the interference temperature. This re-
quirement can be fulfilled by using the multitaper method to es-
timate the power spectrum of the interference temperature with
a large number of sensors [4]. The multitaper method can solve
the tradeoff between bias and variance of an estimator and pro-
vide a near-optimal estimation performance. The large number
of sensors can account for the spatial variation of the RF energy
from one location to another. Subspace-based method has also
been proposed to gain knowledge of the quality and usage of a
spectrum band [12], where information about the interference
temperature is obtained by eigenvalue decomposition.
If a regulatory body sets an interference temperature limit
for a particular frequency band with bandwidth , then the sec-
ondary transmitters has to keep the average interference below
. Therefore, the interference temperature serves as a cap
placed on potential RF energy that could appear on that band,
and there are some previous efforts about how to implement
efficient spectrum allocation with the interference temperature
limit.
WANG AND LIU: ADVANCES IN COGNITIVE RADIO NETWORKS: A SURVEY 9
Fig. 5. Summary of main spectrum sensing techniques.
Spectrum shaping has been proposed to improve spectrum
efficiency [13] in CR networks. More specifically, using inter-
ference fitting, a CR senses the shape of the interference power
spectrum and create spectra inversely shaped to the current in-
terference environment to take advantage of gaps between the
noise floor and the cap of the interference temperature limit.
Interference temperature dynamics in a CR network were
investigated in [14] using a hidden Markov model (HMM),
where the trained HMM can be used as a sequence generator
for secondary nodes to predict the interference temperature of
the channel in the future and aid their channel selection for
transmission.
A comprehensive analysis has been presented in [15], which
quantifies how interference temperature limits should be se-
lected and how those choices affect the range of licensed sig-
nals. It is shown that the capacity achieved is a simple function
of the number of nodes, the average bandwidth, and the frac-
tional impact to the primary signal’s coverage area. However, as
observed by [15], the achievable capacity from the interference
temperature model is low, compared to the amount of interfer-
ence it can cause to primary users.
1
B. Spectrum Sensing
Spectrum sensing enables the capability of a CR to measure,
learn, and be aware of the radio’s operating environment, such
as the spectrum availability and interference status. When a cer-
tain frequency band is detected as not being used by the primary
licensed user of the band at a particular time in a particular posi-
tion, secondary users can utilize the spectrum, i.e., there exists a
spectrum opportunity. Therefore, spectrum sensing can be per-
formed in the time, frequency, and spatial domains. With the
1
It is also argued by other commenting parties of the FCC that the interference
temperature approach is not a workable concept and would result in increased
interference in the frequency bands where it would be used. Therefore, in May
2007 the FCC terminated the proceedings of rule making implementing the in-
terference temperature model.
recent development of beamforming technology, multiple users
can utilize the same channel/frequency at the same time in the
same geographical location. Thus, if a primary user does not
transmit in all the directions, extra spectrum opportunities can
be created for secondary users in the directions where the pri-
mary user is not operating, and spectrum sensing needs also to
take the angle of arrivals into account [16]. Primary users can
also use their assigned bands by means of spread spectrum or
frequency hopping, and then secondary users can transmit in
the same band simultaneously without severely interfering with
primary users as long as they adopt an orthogonal code with re-
spect to the codes adopted by primary users [17]. This creates
spectrum opportunities in code domain, but meanwhile requires
detection of the codes used by primary users as well as multi-
path parameters.
A wealth of literature on spectrum sensing focuses on pri-
mary transmitter detection based on the local measurements of
secondary users, since detecting the primary users that are re-
ceiving data is in general very difficult. According to the a priori
information they require and the resulting complexity and ac-
curacy, spectrum sensing techniques can be categorized in the
following types, which are summarized in Fig. 5.
1) Energy Detector: Energy detection is the most common
type of spectrum sensing because it is easy to implement and
requires no prior knowledge about the primary signal.
Assume the hypothesis model of the received signal is
(2)
where
is the primary user’s signal to be detected at the local
receiver of a secondary user,
is the additive white Gaussian
noise, and
is the channel gain from the primary user’s trans-
mitter to the secondary user’s receiver.
is a null hypothesis,
meaning there is no primary user present in the band, while
means the primary user’s presence. The detection statistics of
![Fig. 1. Spectrum usage [5].](/figures/fig-1-spectrum-usage-5-3l0y8cja.webp)
![Fig. 3. Illustration of spectrum white space [5].](/figures/fig-3-illustration-of-spectrum-white-space-5-3r2kdpqp.webp)
