International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 4 Issue 04, April-2015
A Survey on Routing Protocols for Cognitive
Radio Networks
Monisha Devi
Dipjyoti Deka
Department of Computer Science and Engineering
Tezpur University
Tezpur, Assam, India
Department of Computer Science and Engineering
Tezpur University
Tezpur, Assam, India
Abstract— In today’s world of communication, cognitive radio
(CR) has emerged as one of the most promising technology to
overcome the inefficient spectrum utilization problem. The basic
intention behind CR is to opportunistically allow unlicensed
users to utilize the vacant licensed spectrum without causing
much interference to licensed users, thereby providing a solution
to limited available spectrum. CR nodes are adapted to the
dynamics of spectrum availability. Although, routing is a
fundamental concept in ad hoc wireless communication, but in
CR network, spectrum scarcity makes routing a challenging
issue to carry out end-to-end data transmission. Change in
available channel list for a CR user firmly accounts to the
problem of routing. In this paper, a survey of different routing
protocols used in CR ad hoc network has been presented.
characterize the selected bands and choose the most
appropriate band from the availability list
In situations where a Primary user (PU)
returns to a
spectrum
band that is currently being utilized by some
Secondary user, the CR user experiences spectrum mobility
where in, the CR user immediately vacates the band and
moves to another spectrum hole. Therefore, the foremost aim
of CRN is to enhance the spectrum utilization efficiency for
better communication performance.
Keywords— Cognitive radio network (CRN), routing,
primary user (PU), secondary user (SU), spectrum
opportunity (SOP)
I.
INTRODUCTION
Use of radio frequency (RF) bands has shown a rapid
growth in the recent years all around the world. Users rely
on allocation of these spectrum bands for carrying out
various services. However, according to spectrum allocation
policy, certain frequency bands are allocated only to
licensed users, while others are open for all unlicensed
users. Some of the devices that can make efficient use of
unlicensed spectrum are like, Bluetooth, Wifi, digital
cordless phone etc. The frequency bands are regulated by
Communications
several standard bodies. Federal
Commission (FCC) of United States of America[2] is one
such body according to whom, plenty of space in licensed
spectrum remain underutilized for most of the time, and on
the other hand, unlicensed bands often remain congested
causing interference among the applications functioning in
this band. The spectrum utility has been illustrated in Figure
1. Hence, to rule out the spectrum usage problem and to
make the best use of RF spectrum, cognitive radio (CR) [1]
was brought into use.CR makes a flexible use of RF
spectrum by permitting access to the unused licensed
spectrum in an opportunistic manner. Unlicensed or
Secondary users (SU) make effective use of these
underutilized bands by reconfiguring the parameters
dynamically. A cognitive radio network (CRN) comprises
of nodes equipped with CR transceiver. In this net- work
environment, CR nodes determine the available spectrum
bands which are also known as spectrum holes,
IJERTV4IS041166
Fig 1: Spectrum Utilization [1]
The rest of the paper is organized as follows. In Section 2,
we discussed about the routing functionality in CR
network. In Section 3, the routing protocols designed for
CRN are classified accordingly. In Section 4, we tabled the
routing protocols by carrying out a comparison between
them. And finally we conclude our survey of routing
techniques in CRN in Section 5.
II.
ROUTING IN CRN : A CHALLENGE
Secondary users (SU) in a CR network communicate
among
themselves in a multi hop ad hoc manner. While performing
end-to end data transmission between any of these users,
certain complications are being faced because of the dynamic
nature of spectrum availability and data rates. This dynamic
behavior of the allocated channels arises because of some
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888
International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 4 Issue 04, April-2015
Primary user activity. Even if a route is being built between
the users, links on the route remain unstable and at any time
when dynamic channel on a link becomes unavailable, the
route breaks down calling off the data transmission. In a CRN
environment, the available spectrum bands may vary from
user to user with time and location. This characteristic of
CR nodes necessitates collaboration between spectrum
decision and route selection to design route in multi hop
CRN. Therefore, planning a routing protocol for CR
network needs to consider the above mentioned issues
[3].In our upcoming sections, we have presented some of
the routing protocols favored in this network.
III.
C LASSIFICATION OF R OUTING P ROTOCOLS
CRN routing protocols are comparatively different from
those used in traditional multi hop ad hoc networks.
These protocols have been classified into several
categories based on their operation, which includes, tree
based routing, location coordination based routing, on
demand routing, dynamic spectrum routing, and multipath
based routing. A figure representing this classification is
shown below:
than one spectrum tree and are recognized as overlapping
users. Framework for STOP-RP is shown in Figure 3.
In one spectrum tree, each node has a unique CR user
identifier (CRID) that basically stipulates a proactive
route to the root node. Accordingly, overlapping users will
have multiple CRIDs, one for each spectrum tree. This
protocol initializes with root selection procedure by sending
out a Root Request. Once the root user is selected, a Root
Announcement message is broadcast by the root node.
Furthermore, STOP- RP presents CR routing as intraspectrum routing and inter- spectrum routing where interspectrum happens to be in a single spectrum tree, whereas,
intra-spectrum operates in multiple trees. To construct a
route for end-to-end data transmission, STOP-RP
exchanges two types of control messages: Spectrum
Route REQuest (SRREQ) with fields: [CRIDS, CRIDD,
metric, intra/inter] and Spectrum Route REPly (SRREP)
with fields: [CRIDS, CRIDD, intra/inter], where, [CRIDS]
and [CRIDD] are the CRIDs of source node and destination
node, respectively, [metric] is the cumulative cognitive
route cost and [intra/inter] indicates whether the
destination and source nodes are in the same spectrum-tree
or not.
Routing protocols for
CRN
Dynamic
Spectrum
Routing
Tree
Based
Routing
On
Demand
Routing
Local
Coordination
Routing
Multipath
Based
Routing
Fig 2: Routing protocols for CRN
Fig 3: Framework for STOP-RP [10]
A. Tree Based Routing
Tree based routing is an approach where formation
of a spectrum-tree manages the collaboration between
route selection and spectrum decision in a well-organized
way. The following subsection summarizes two such
protocols.
1) Spectrum-Tree Based On-Demand Routing Protocol
(STOD-RP): While routing in CRN, users come across a
situation that requires spectrum awareness to get hold of
correct routing decisions. To secure this cooperation
between route selection and spectrum decision, STOP-RP
protocol [10] was designed that aims at constructing a
spectrum tree in each spectrum band by considering a new
route metric. Cognitive route cost is the metric preferred
for this solution as it considers both CR user’s QoS
requirements and Primary user activities to provide an
effective path. Each spectrum-tree picks out only one root
node that keeps information about the entire topology of
spectrum tree. However, some users may belong to more
IJERTV4IS041166
The Spectrum-Tree based On-Demand Routing algorithm
puts together tree-based proactive routing and on-demand
route discovery that is an extension of original AODV
protocol [7] for establishing route between source and
destination nodes. Moreover, it provides a fast ro ute recovery method for effectively dealing with situations
where a spectrum band being used by a Secondary user
for transmission gets occupied by a Primary user.
Ultimately, simulations have shown that STOP-RP has
reduced end-to-end delay with increase in the number of
nodes that belong to multiple spectrum trees and it has
also lessen the control overhead.
2) Cognitive Tree-based Routing (CTBR): CTBR [9]
works as an extension of TBR (tree based routing) protocol
proposed for wireless mesh networks. In CTBR protocol,
each Cognitive Transceiver (CT) in CR environment
maintains a routing table based on the tree structure, using
which data packets are forwarded across the CR network.
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International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 4 Issue 04, April-2015
This routing solution uses global and local decision
schemes for route calculation. A tree structured network is
formed by configuring cognitive radio base-station as
root node. As in TBR protocol, CTBR in cognitive
network also performs the routing procedure, where, root
node periodically sends Root Announcement (RANN)
message for forming the tree. When a CT receives a
RANN message, it caches the CT from whom it
received this announcement message as its potential
parent. Thereafter, the CT rebroadcasts the RANN with
updated link metric. In addition to this, the CT also selects
a parent CT from among all the potential parents based on
the best metric for the path to root node. For registering
with the root, each CT sends a RREP towards the root
node when it hears the RANN message from its parent CT.
An intermediate CT on receiving RREP, forwards this
RREP message to its parent CT, and at the same time,
updates its routing table by selecting source CT of the
RREP as its destination node. Finally at the end, root node
constructs a tree structure to reach any node in the network,
as the root has now knowledge about all CTs.
B. On Demand Based Routing
This approach for routing in CRN is a modification of
on demand routing protocols used in wireless ad hoc
networks. In such protocols, a path is established only when
an active communication is required. Following subsection
describes two such routing protocols.
1) Spectrum Aware On Demand Routing Protocol
(SORP): SORP [6] is a modified form of ad hoc on demand
distance vector (AODV) routing protocol to adapt in CR
environment. Such modifications are needed in CRN
because of the dynamics in spectrum availability. In
SORP, modifications are made by inserting spectrum
related information of CR users into control packets, such as
RREQ and RREP, while they are being forwarded across
the network. This scenario for routing in CRN assumes a
common control channel (CCC) for the entire network to
exchange control messages. For performing end- to-end
data transmission, source node broadcasts a RREQ over the
CCC. This RREQ will now have spectrum opportunity
(SOP) of the source node piggybacked on it. However, each
relay node on forwarding this RREQ appends its SOP on
the RREQ. This continues until destination is reached,
where, a spectrum band for data transfer is chosen from
the received SOP. Thereafter, the destination assigns the
spectrum and sends back a RREP along with a Choice list
containing the selected spectrum band towards the source
node, over the reverse path of RREQ. Likewise, the
intermediate nodes assign the spectrum bands on forwarding
the RREP. One such scenario is shown in Figure 4.
IJERTV4IS041166
Fig 4: Inclusion of SOP into RREQ and Choice list into RREP
while they are forwarded [14]
As such , on forwarding of RREQ and RREP messages, the
size of the packets increases with agreement with the hop
or with the distance between source and destination.
Moreover, SORP defines cumulative delay caused by
existing flows at a node as the sum of switching delay and
backoff delay. Switching delay is caused by switching
among frequency bands, whereas, backoff delay is
caused by multi-flow interference within a frequency
band. Since, SORP focuses on delays, it is suitable for
delay-sensitive applications.
2) Multi-hop Single-transceiver Cognitive Radio
Networks Routing Protocol (MSCRP): MSCRP [5] uses
on demand routing based on ad hoc on-demand distance
vector(AODV) routing protocol. This scheme disallows
the use of common control channel (CCC) [15] for
exchanging control messages across the CR network. No
node in the network is aware of the channel availability
list of the other network nodes. As such, AODV needs
to be modified so that spectrum related information can
be exchanged among the nodes. Since, two nodes may
be listening on different channels in CRN, they may not
be able to communicate with each other. In order to
avoid this problem, two consecutive nodes in a flow cannot
be in the switching state simultaneously. As the
communication with a switching node is difficult,
therefore, the switching node uses LEAVE or JOIN
messages to inform its neighbor nodes about its working
channel.
In MSCRP, route discovery is initiated by broadcasting
RREQ message on all available channels of the sender
node. The channel availability related information is
piggybacked on RREQ message. An intermediate node on
receiving a RREQ, appends its state and channel
availability list to the RREQ message and thereafter
forwards the RREQ.As the RREQ is forwarded among
the nodes, a reverse path gets established from
destination to the source node. When RREQ message
reaches destination node, destination comes to know
about channel availability list of every node on the path,
and then, it assigns a channel for this flow. In response to
RREQ, destination node sends back a RREP that includes
the assigned channel, along the reverse path of RREQ to
the source node. Hence, a path is set for end-to-end
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transmission of data packets. This approach is well
suitable for multi-hop and single transceiver CRNs.
However, it somewhat introduces extra overhead for
broadcasting RREQ messages on all available channels
instead on a single channel.
This functionality helps to reduce workload at the
intermediate node by interacting with the neighbors. As
such, this routing solution results in a path with low
cumulative delay.
C. Local Coordination Based Routing
Dynamic spectrum aware routing enables CR users to
forward the data traffic through paths having high
spectrum availability in an opportunistic way. One such
routing protocol has been discussed in the following
subsection.
Local coordination based approach for routing in CRN is
an extended work of SORP. It aims at providing good
adaptability for handling situations that arise due to
inconsistency in available spectrum bands.
1)
Local Coordination Based Routing and Spectrum
Assignment in Multi-hop Cognitive Radio Networks:
This approach [4] is a continuous work of SORP that
involves a local coordination scheme to be applied on
intersecting nodes to perform load balancing.
Basically, this solution consists of two parts: a joint
on-demand Routing and Spectrum Assignment
Protocol for constructing a multi-hop path between
source and destination nodes, and, a local
coordination scheme for balancing the traffic load at
intersecting nodes along the path in order to achieve
minimal end-to-end delay. The proposed protocol
looks for exchanging SOP information among the
network nodes and assigning a suitable spectrum band
to each link on the established route. The local
coordination scheme, on the other hand, gets
invoked at a node as soon as the node transforms
into an intersecting node. This scheme allows the
intersecting node to decide whether to accommodate
the ongoing flow or to redirect it so as to distribute
the workload among neighbor nodes.
As shown in Figure 5, node A is serving flow 2
while
another node B is serving flow 3.When a new flow 1 appears, it
takes nodes A and B as its intermediate nodes. These two
relay nodes perform a local coordination to identify its
neighbors so as to redirect the flow appropriately. And as a
result of this scheme, node A redirects flow 1 to node C
whereas node B redirects flow 3 to node D.
D. Dynamic Spectrum Aware Routing
1)
Spectrum Aware Mesh Routing in Cognitive Radio
Net- works (SAMER): SAMER [11] is a routing solution
for mesh networks in the CR environment where it
allows the traffic to route along the paths having high
spectrum availability so as to utilize the spectrum holes
efficiently. This scheme uses a new routing metric based
on the computation of spectrum availability of path.
This new metric constructs a route that is optimal in
terms of hop-count, thus, providing long-term stability
routes. SAMER builds a runtime forwarding mesh through
which the data packets are forwarded opportunistically.
The constructed mesh is periodically updated according
to the dynamics of spectrum availability and also it
offers a set of candidate routes to the destination node.
For building a mesh around the optimal hop-count
path, a cost, given by Costi, is computed for each node i
in the net- work. SAMER calculates all the routes from a
node i to destination node D having length less than H
hops and thereafter it selects the path with highest
spectrum availability. As such, the computed cost
represents the spectrum availability of a path from node
i to node D whose length is at most H hops and also has
the highest spectrum availability. However, it is a tough
decision to look for the value of H that needs to be
computed for estimating the cost. Ultimately, SAMER
provides a balance between long-term route stability and
short-tern opportunistic performance by carrying out data
transmission across a mesh that is centered around the
shortest hop-count path having highest spectrum
availability. Hence, SAMER achieves high end-to-end
throughput by utilizing long term stability and short term
opportunistic spectrum access.
E. Multipath Based Routing
Multipath based routing protocols enable multiple
paths to exist between source and destination nodes.
They have the ability to reduce route discovery
frequency and also provide load balancing to satisfy
Q o S r e q u i r e m e n t s o f CR users. Following
subsection presents two such routing protocols.
Fig 5: Local coordination scheme applied for load balancing [14]
IJERTV4IS041166
1)
Multipath
Routing
and
Spectrum
Access
(MRSA): MRSA[8] is the first multipath routing
protocol for CR environment that aims at minimizing
inter path contention and interference. In this approach,
spectrum wise disjointness concept is revised, which
specifies that, if multiple paths do not have any
interfering bands between them than these paths are
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Vol. 4 Issue 04, April-2015
considered as spectrum wise disjoint. MRSA assumes that
there are N bands for sending out data traffic in the
spectrum. However, signaling traffic can also be delivered
over these bands, but this requires broadcasting
signaling traffic on all available bands because of
which a separate band is being assumed for signaling
traffic. The route discovery procedure of MRSA uses
dynamic source routing (DSR)[13] mechanism according
to which source node broadcasts a RREQ message
with new RREQ ID and attaches its band radio usage
table (BRT) to the message. An intermediate node on
receiving RREQ checks whether the RREQ ID in the
message is new or is an old one, before forwarding
the RREQ. In case, RREQ ID is not new, the relay node
counts the hop count from source node. If current RREQ
has fewer hop count than the previous RREQ, the relay
node appends its BRT to the message and then forwards
the RREQ. Proceeding in this way, the destination node
receives the same RREQ from multiple paths. Thus it first
assigns band and radio to each link and then evaluates all
the candidate paths by their available bandwidth. Hence,
this routing technique p r o v i d e s e n d - t o - e n d d a t a
t r a n s m i s s i o n b y constructing multiple paths to
minimize contention and interference and maximize
spectrum wise disjointness.
2) Ad Hoc On-Demand Multipath Distance Vector
(AOMDV) Routing Protocol: AOMDV [12] protocol
extended the ad hoc on-demand distance vector (AODV)
routing protocol to calculate multiple loop-free and disjoint
paths between source and destination. However, the main
difference between AODV and AOMDV lies in the
number of routes computed in each route discovery. When
a RREQ is broadcast from source towards the destination,
multiple
reverse routes get established
both
at
intermediate nodes as well as at destination node. In
response to the RREQ, multiple RREPs traverse back
along the reverse paths to the source node. Furthermore,
this routing solution ensures that the multiple paths
discovered are disjoint and loop-free by applying route
update rules at each node locally. Figure 6 shows an
example where source node S broadcasts a RREQ.
Fig 6: Multipath between source and destination [12]
copies even though reverse path is formed only via node
X (assuming that the first copy reaches D via X
earlier).Thereafter, two RREPs, one for each copy of
RREQ, are sent by node D and they get merged at node
I which forwards them along two disjoint paths (via A
and B). As such, S obtains two link disjoint paths to D.
IV.
C OMPARISON OF R OUTING P ROTOCOLS IN CRN
Table 1 presents a comparison among
some of the routing solutions used in cognitive radio
network. The table specifies category of the protocol to
which in belongs, route discovery procedure initiated by the
protocol and method applied by the protocol for selecting
best route to perform end-to-end data transmission.
V.
C ONCLUSION
This survey paper presents a number of routing protocols used in ad hoc cognitive radio networks with their
efficiency. However, routing in CRN in a challenging
issue because of the dynamics in spectrum availability of
CR users.Therefore, routing in CRN has attracted a lot of
attention in recent years as a result of which, researchers all
around the world are focusing to introduce some novel
routing solutions for CR environment. Our paper has
analyzed some of the routing protocols in CRN by
classifying them into tree based protocols, on demand
based protocols, local coordination based protocols,
dynamic spectrum aware based protocols and multipath
based protocols.
Table 1: Comparison of routing protocols in CRN
Routing
Protocols
Protocol Type
Route Discovery
Best Path
Selection
CTBR
Hybrid (Tree
based
and On
Demand)
Tree based
SRREQ with
fields [CRIDS,
CRIDD, metric,
intra/inter]
Broadcast Root
Announcement
(RANN)
SORP
On demand
Broadcast RREQ
message
Establishing a
spectrum tree
at each
spectrum band
Based on
global and
local decision
schemes
Switching
delay and back
off delay
MSCRP
On demand
RREQ message on
all available
channels
Number of
flows on each
channel
Local
Coordination
Local
coordination
based
Broadcast RREQ
message
Dynamic
spectrum aware
Link state packets
Based on
cumulative
delay of the
path
Minimum hop
count and
spectrum
availability
STOP-RP
SAMER
Intermediate node I duplicates the first RREQ copy via
node A and suppresses the second RREQ copy via node
B.As such, two copies of the first RREQ copy via A
reaches destination node D.D replies to both the RREQ
IJERTV4IS041166
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International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 4 Issue 04, April-2015
MRSA
Multipath based
Broadcast RREQ
message with new
RREQ_ID
Minimum hop
count
AOMDV
Multipath based
Broadcast RREQ
message
Route carrying
minimum hop
count
We have also summarized the routing operations
related to the protocols we presented. Looking at existing
works and discussions on routing protocols in CRN, it
can be stated that most of these protocols use the same
routing metric as used in traditional wireless networks. As
such, there is a need to design new routing metric that can
exploit all dynamic characteristics of CRN. Therefore, it
is believed that by analyzing the existing routing
techniques used in CR network, various new open
issue could be exposed and also some novel routing
solutions could be developed by enhancing the current
routing schemes.
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