International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 2 Issue 2, February- 2013
Routing In Cognitive Radio Ad-Hoc Networks
Jitisha R. Patel, Sunita S. Barve
MIT Academy of Engineering, Alandi, Pune, Maharashtra, India.
Abstract
IIJJEE
RRTT
Cognitive radio (CR) is a promising technology used to
solve the problem of spectrum scarcity by identifying
the vacant portions of the spectrum opportunistically
and transmitting in them, and at the same time ensuring
that the licensed (also known as a primary user or
PUs) of the spectrum are not interfered or hindered
from using their licensed spectrum band. This
capability of cognitive radio calls for designing flexible
and dynamic spectrum access strategies in order to
opportunistically reuse portions of the spectrum
temporarily vacated by licensed primary users, but it is
not easy to design such communication protocols. This
paper focuses on designing effective routing solutions
for multi-hop CRNs, which is an important issue in
order to fully utilize the potentials and capabilities of
the cognitive networking paradigm. In cognitive radio
networks, multi-hop communication with efficient
routing can improve the connectivity and spectrum
efficiency for cognitive users. Many routing algorithms
have been proposed, but they may provide
unnecessarily long routing paths as the existence of
primary users and especially their locations have not
been explicitly taken into consideration. This paper
investigates distributed routing in cognitive radio
networks based on joint selection of the spectrum with
the choice of the next hop forwarder node, the location
information of the primary users, and joint spectrum
decision and re-configurability, where the route can be
adapted with local spectrum changes or by selecting a
different set of forwarding nodes altogether.
requests to regulatory authorities for spectrum
allocation. Recent studies by the Federal
Communications Commission (FCC) shows that the
use of licensed spectrum is quite uneven i.e., the
spectrum usage is concentrated on certain portions of
the spectrum while a significant amount of the
spectrum remains unutilized and that many spectrum
bands allocated through static assignment policies are
used only in bounded geographical areas or over
limited periods of time. According to Federal
Communications Commission (FCC) the average
utilization of such bands varies between 15% and 85%
[2].
The limited spectrum availability and the
inefficient spectrum usage calls for a new
communication paradigm to utilize the existing
wireless spectrum opportunistically which can adapt to
the dynamically changing spectrum resource, learn
about the spectrum occupancy, make decisions on the
quality of the available spectrum resource, including its
expected duration of use, probability or likelihood of
interference caused by the licensed users (PUs). As a
result, FCC has approved the use of unlicensed devices
(also called cognitive radio or secondary users) in
licensed bands i.e., unlicensed users or secondary users
(SUs)
may
use
licensed
spectrum
bands
opportunistically in a dynamic and non-interfering
manner. Consequently, dynamic spectrum access
(DSA) techniques are proposed to solve these current
spectrum inefficiency problems. The new networking
paradigm which employ these techniques is referred to
as NeXt Generation (xG) Networks as well as Dynamic
Spectrum Access (DSA) and cognitive radio
networks(CRNs) [3].
Keywords- Cognitive radio ad-hoc network (CRAHN),
Dynamic Spectrum Access (DSA), Multi-hop network,
Routing protocol.
1. Introduction
Wireless networks today are regulated by governmental
agencies i.e., the spectrum is characterized by a fixed
spectrum assignment policy. The spectrum is assigned
to license holders for vast geographical regions on a
long term basis [1]. The huge success of wireless
applications, in recent years has led to an increase in
www.ijert.org
2. Cognitive radio
Cognitive radio technology is the key technology that
allows a CRAHN to use spectrum in a dynamic
manner. The term, cognitive radio, can formally be
defined as follows [6]:
A „„Cognitive Radio” is a radio that can change its
transmitter parameters based on interaction with the
environment in which it operates.
1
International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 2 Issue 2, February- 2013
From this definition, two main characteristics of the
cognitive radio can be defined as follows [4,5]:
IIJJEE
RRTT
1) Cognitive capability: Cognitive capability refers
to the ability of the radio technology to capture or sense
the information from its radio environment. The
cognitive capability of a cognitive radio enables real
time interaction with its environment in order to
determine appropriate communication parameters and
adapt to the dynamic radio environment [1]. Through
this capability, the portions of the spectrum that are
unused at a specific time or location can be identified.
Consequently, the best spectrum and appropriate
operating parameters can be selected.
2) Reconfigurability: The cognitive capability
provides spectrum awareness whereas reconfigurability
enables the radio to be dynamically programmed
according to the radio environment. More specifically,
the cognitive radio can be programmed to transmit and
receive on a variety of frequencies and to use different
transmission access technologies supported by its
hardware design [7].
Thus, the ultimate objective of the cognitive radio
is to obtain the best available spectrum through
cognitive capability and reconfigurability as described
before. The most important challenge is to share the
licensed spectrum that too without interfering with the
transmission of other licensed users, as most of the
spectrum is already assigned. This scenario has been
illustrated in Fig. 1.
The temporarily unused spectrum vacated by PUs
that the cognitive radio use, is referred to as spectrum
hole or white space [4]. If this band is further utilized
by a licensed user, the cognitive radio moves to another
spectrum hole or stays in the same band, lowering its
transmission power level or modulation scheme in
order to avoid interference as shown in Fig. 1.
topology, and the time and location varying spectrum
availability are some of the key distinguishing factors.
The components of the cognitive radio ad-hoc network
(CRAHN) architecture, as shown in Fig. 2, can be
classified in two groups as the primary network and the
CR network components.
Figure 2. The CRAHN architecture.
The primary network is the one which already
exists and where the primary users (PUs) have an
exclusive right to a certain spectrum band. Examples
include the common cellular and TV broadcast
networks. The CR network consists of CR users. As the
CR network does not have a license to operate in a
desired band, additional functionality is required for
CR users to share the licensed spectrum band. These
CR users are mobile and can communicate with each
other in a multi-hop manner on both licensed and
unlicensed spectrum bands. Usually, CR networks do
not have direct communication channels with the
primary networks i.e., they are assumed to function as
stand-alone networks. Thus, in CRAHNs, each user
needs to have all CR capabilities and is responsible for
determining its actions based on the local observation,
as shown in Fig. 3.
Figure 1. Spectrum hole concept
3. CRAHN architecture
In CR ad-hoc networks (CRAHNs), the
distributed multi-hop architecture, the dynamic network
www.ijert.org
2
International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 2 Issue 2, February- 2013
Figure 3. Cognitive Radio Ad-Hoc Network
4. Routing schemes in CRAHNs
IIJJEE
RRTT
For CRAHNs, the establishment of end-to-end
route has various challenges as the CR users have to
find a route considering the presence of PUs.
According to [8] the existing works in CR routing
protocols are classified based on their support for: (i)
Spectrum decision, i.e., joint selection of the spectrum
with the choice of the next hop forwarder node (ii)
Joint spectrum decision with PU awareness, where the
CR users have the ability to identify the locations
where PUs are present and allow the routes to avoid
them, and (iii) Joint spectrum decision and reconfigurability, where the route can be adapted with
local spectrum changes or by selecting a different set of
forwarding nodes altogether. This classification is
summarized in Fig. 4.
while taking into consideration the spectrum that may
be used on the chosen link. Thus, the spectrum as well
as the path selection occurs jointly, which ensures that
the route remains connected during the network
operation as each link maintains a set of feasible
spectrum bands, which is called the channel set of a
node.
The CR users which form the route should be
able to assign the link spectrum so that the delay in
changing the spectrum at a node is minimized over the
path. However, the spectrum access time is shared by
the nodes on the link that are within range of each
other, if the same spectrum is used at consecutive links.
As only one node on the link can send or receive
packets at a time, the throughput is adversely affected.
The multi-hop single-transceiver CR routing protocol
(MSCRP) proposed in [9] balances these two
conflicting approaches. MSCRP is an on demand
protocol, based on ad-hoc on demand distance vector
(AODV) [10], doesn‟t base on control channel.
Therefore, routing protocol messages are being
exchanged without common control channel.
Due to the heterogeneous spectrum availability,
links may be available only a fraction of time in CRNs.
To set up multi-hop connections between node pairs
with heterogeneous spectrum availability, intermediate
bridge nodes have to switch between multiple channels
based on their availability that is, links on each path
need to communicate on different channels. This
introduced the new problem called “deafness”, which is
switching among available channel set to maintain the
route or avoid the interference on primary users. This
causes extra delay in CRNs communications [9]. To
avoid the deafness problem, the two consecutive nodes
in a flow cannot switch to different channel
simultaneously. Communicating with a switching node
is complicated, therefore MSCRP switching node uses
LEAVE or JOIN messages to inform its neighbours
about the channel it is working.
According to MSCRP, a route request (RREQ)
message is broadcast on all the possible channels to the
destination for route discovery. The channels
availability information is appended in the RREQ
message and is forwarded in broadcast process. All
intermediate nodes append their state and available
channel set to RREQ message. The reverse path to the
source node is established as RREQ is forwarded.
Destination node receives the channel information and
number of nodes on each channel at the end and
decides on the spectrum selection for the shortest path
based on analytical estimates of the time for spectrum
switching, channel contention, and data transmission
and thus assigns channel for this flow. It binds the
assigned channel information in route reply (RREP)
Fig.4. Classification of CR routing protocol.
4.1 Routing with spectrum decision
Under this scheme, the routing protocol selects the
next hop node among the possible candidate forwarders
www.ijert.org
3
International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 2 Issue 2, February- 2013
message, which is sent back to the source on the
decided route.
MSCRP incurs an extra overhead of
broadcasting RREQ message on all available channels
rather on single channel and this overhead becomes
may adversely affect the efficiency, in case there are
many channels available for each node in the network.
path formed can be guaranteed to be close to the
shortest visibility path between CS and CD.
4.2 Routing with joint spectrum decision with
primary user (PU) awareness
Figure 5. System model.
4.3 Routing with joint spectrum decision and
re-configurability
IIJJEE
RRTT
Here, the routes in a CR network must explicitly
take into consideration the on going communication of
the PUs and provide a measure to protect it. For this,
the route may entirely avoid the regions where the PUs
frequently use their licensed band.
A Location-Aware Distributed Routing in
Cognitive Radio Networks proposed in [12], can avoid
routing through those areas where PUs are located and
thus has the potential to improve the utilization of
spectrum holes at different locations in cognitive radio
networks.
Consider the cognitive radio network as shown in
Fig. 5, where PUs as well as CUs exist in the same
area. Among the CUs, there is a cognitive sourcedestination pair (CS-CD) and all others are cognitive
relays (CRs). Every CU has the same transmission
range R, within which other CUs can successfully
receive the transmitted message and the transmission
range of PU is called guard zone, within which no CU
can transmit. PU guard zones have been shown as
hexagons with the same sizes in the Fig. 5. It has been
assumed that all the PUs work in the same frequency
band and there is no overlap between any PU guard
zones. The CUs have full knowledge of where the PU
guard zones as well as their neighbours in their
transmission range are located. A distributed routing
algorithm is developed based on this information.
According to it, a navigation direction is planned first,
which is assumed to be a narrow strip from the CS to
the CD at the same time avoiding the PU guard zones.
The shortest geometric path between the CS and
CD, which doesn‟t pass through the PU guard zones
can be determined by the CUs. This path, shown in Fig.
6(a) is termed as the guide path. The green lines are the
edges between the CUs that can see each other. As the
CRs may lie anywhere, it is not possible to route on this
path exactly. However, the guide path can help in
routing and plan the navigation direction. Instead of a
guide path, a guide strip can be followed for routing. It
is a narrow strip formed starting from CS to the CD in
the navigation direction by expanding the guide path
equally as shown in Fig. 6(b). The guard zones are also
expanded or grown. With this narrow strip followed the
www.ijert.org
The routing protocols falling under this class has
the key ability to recover from changes in the spectrum
caused by PU arrival. The route re-configuration can be
done either by locally choosing a new spectrum on the
affected links or reconstructing the whole path.
The spectrum-aware routing protocol (SPEAR)
identifies all possible routes during the route-setup
stage [12]. A SPEAR source node does so by
broadcasting a RouteRequest (RREQ) message on the
dedicated control channel, with a list of available
channels which are not occupied by PUs and not
reserved by other cognitive users nearby. This list is
maintained by every node. It ensures that a single
spectrum is used throughout the route and takes
measures to avoid broadcast congestion by limiting the
number of paths discovered in the network. One of the
way it uses is to keep a per-flow counter and allow only
some fixed number of RREQ messages to be
forwarded. The other way is that the destination node,
after receiving the first RREQ, starts a per-flow timer
and when it expires makes selection of the best possible
route out of the routes received based on hop count,
delay, maximum end-to-end throughput, and other
potential routing metrics and does the channel
assignment. It then frames a RouteReply (RREP)
message and sends it back to the source on the selected
route. The intermediate nodes reserve the channel
assigned as well as the timeslots when they receive the
RREP and forward channel reservation message to all
4
International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 2 Issue 2, February- 2013
(a)
(b)
Figure 6. (a) The guide path. (b) The guide strip.
5. Conclusion
Due to heterogeneous spectrum availability and
data rates, routing in CRN is a great challenge. In
routing multi-hop cognitive network, hops learn
information from neighbours and then find the route by
learning the dynamism. This paper first discusses the
CRAHN architecture and then studies classification of
CR routing techniques based on their support for
spectrum decision, joint spectrum decision and
awareness about PU location and joint spectrum
decision and re-configurability in order to change
routes whenever there is change in local spectrum.
Multi-hop communication with efficient routing which
fully exploit the dynamic characteristics of CRN, can
improve the connectivity and spectrum efficiency for
cognitive users. Therefore, researches are going on to
design some more novel routing techniques.
IIJJEE
RRTT
neighbours coming under their transmission range.
They also discard the additional RREQ messages for
this flow, if they further receive them.
As and when there is a change in channel
availability due to the actions of PUs, nodes perform
Local Adaptation by modifying their local channel
usage in order to maintain flow connectivity. If this
fails, then a new and fresh route is formed from the
source.
The various operations of SPEAR can be
explained using Fig. 7. The source S sends the RREQ
on two possible routes, Route 1 and Route 2.
Destination D selects route 2 as the optimal route based
on some routing parameters and sends RREP appended
with channel assignment information to it on route 2.
References
[1] I.F. Akyildiz, W.-Y. Lee, M.C. Vuran, S. Mohanty, NeXt
generation dynamic spectrum access cognitive radio wireless
networks: a survey, Computer Networks Journal (Elsevier),
Issue 13, 50, September 2006, pp. 2127–2159.
[2] F.C. Commission, Spectrum policy task force, Technical
report, November 2002.
Figure 7. Setting up a multi-hop route between
source S and destination D using the SPEAR
protocol.
www.ijert.org
[3] I.F. Akyildiz, Y. Altunbasak, F. Fekri, R. Sivakumar,
AdaptNet: adaptive protocol suite for next generation
wireless internet, IEEE Communications Magazine 42 (3)
(2004) 128–138.
5
International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 2 Issue 2, February- 2013
[4] S. Haykin, Cognitive radio: brain-empowered wireless
communications, IEEE Journal on Selected Areas in
Communications 23 (2) (2005) 201–220.
[5] R.W. Thomas, L.A. DaSilva, A.B. MacKenzie, Cognitive
networks, in: Proc. IEEE DySPAN 2005, November 2005,
pp. 352–360.
[6] FCC, ET Docket No 03-222 Notice of proposed rule
making and order, December 2003.
[7] F.K. Jondral, Software-defined radio-basic and evolution
to cognitive radio, EURASIP Journal on Wireless
Communication and Networking 2005.
[8] Ian F. Akyildiz *, Won-Yeol Lee, Kaushik R.
Chowdhury, CRAHNs: Cognitive radio ad-hoc networks, AdHoc Networks 7 (Elsevier)
[9] H. Ma, L. Zheng, X. Ma, Y. Luo, Spectrum-aware routing
for multi-hop cognitive radio networks with a single
transceiver, in: Proceedings of the Cognitive Radio Oriented
Wireless Networks and Communications (CrownCom), 15–
17 May 2008, pp. 1–6.
IIJJEE
RRTT
[10] Amjad Ali, Muddesar Iqbal, Adeel Baig, Xingheng
Wang, Routing techniques in cognitive radio networks: a
survey, International Journal of Wireless & Mobile Networks
(IJWMN) Vol. 3, No. 3, June 2011
[11] Fangyong Li, Jun Zhang, and Khaled B. Letaief,
Location-Aware Distributed Routing in Cognitive Radio
Networks, IEEE
[12] A. Sampath, L. Yang, L. Cao, H. Zheng, B.Y. Zhao,
High throughput spectrum-aware routing for cognitive radio
based ad-hoc networks, in: Proceedings of the International
Conference on Cognitive Radio Oriented Wireless Networks
and Communications (CROWNCOM), May 2008.
www.ijert.org
6
International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
IIJJEE
RRTT
Vol. 2 Issue 2, February- 2013
www.ijert.org
7