Spectrum and Energy Aware Routing Protocol for Cognitive Radio Ad Hoc Networks

Abstract

Throughput maximization is one of the core challenges in cognitive radio ad hoc networks (CRANs), where local spectrum resources are changing over time and locations. This paper proposes a spectrum and energy aware routing (SER) protocol for CRANs, which involves spectrum aware, and energy-efficient route selection, and channel-timeslot assignment. A good routing protocol should be aware of the interference as well as the end-to-end delay. The proposed joint spectrum and energy aware routing with channel-timeslot assignment can balance the energy consumption, eliminates contention between users, and decompose contending traffics over different channels and timeslots. As a result, the proposed scheme leads to significant increases in network throughput and decreases the end-to-end delay. The simulation results show the effectiveness of our proposed approach with good generalization ability.

Full text

Spectrum and Energy Aware Routing Protocol
for Cognitive Radio Ad Hoc Networks
S. M. Kamruzzaman, Eunhee Kim, Dong Geun Jeong
Department of Electronics Engineering
Hankuk University of FS, Yongin-si, Kyonggi-do 449-791, Korea
E-mail: {smzaman, exeriment, dgjeong}@hufs.ac.kr
Abstract—Throughput maximization is one of the core challenges
in cognitive radio ad hoc networks (CRANs), where local
spectrum resources are changing over time and locations. This
paper proposes a spectrum and energy aware routing (SER)
protocol for CRANs, which involves route selection and channel-
timeslot assignment jointly. The proposed routing scheme with
channel-timeslot assignment can balance the energy
consumption, eliminate contention between users, and decompose
contending traffic over different channels and timeslots. As a
result, the proposed scheme leads to significant increases in
network throughput and decreases the end-to-end delay. The
simulation results show the effectiveness of the proposed
approach with good generalization ability.
Index Terms—Cognitive radio, routing protocol, ad hoc
networks, energy-aware, channel assignment.
I. INTRODUCTION
Cognitive radio (CR) technology has been proposed for
using the vacant spectrum of licensed frequency band
opportunistically without harmful interference or collisions to
the primary users (PUs). The CR networks refer to the
networks where CR users are equipped with spectrum agile
cognitive radios [1]. Furthermore, the CR ad hoc networks
(CRANs) are multi-hop self-organized and dynamic CR
networks where CR users can communicate with each other
through ad hoc connection.
In CR networks, the spectrum opportunity is varying over
time and locations. Hence, the integration of spectrum
discovery with route discovery is the key factor for spectrum-
aware routing to cope with the spectrum heterogeneity. By
multichannel routing the throughput of multi-hop wireless
networks can be significantly improved since the interference
can be avoided or reduced and the network load can be
balanced on different channels [2]. The temporal and spatial
spectrum variability caused by the PUs’ spectrum usage adds
complexity to the routing problems in the CR networks. Thus,
the efficient allocation of the channel-timeslot to each link in a
multichannel environment is an important issue to improve the
network performance.
In the ad hoc networks consisting of portable and battery-
powered devices (at least in part), energy management is of
prime importance and challenging. In such networks, energy-
efficient routing achieves higher performance than shortest
path routing and, at the same time, reduces the power
consumption of the relay users [3]. It also alleviates the
network partitioning problem caused by the energy exhaustion
of the relay users. On the other hand, the energy-awareness
may introduce additional complexities to the routing in
CRANs.
In this paper, we propose a routing protocol called the
spectrum and energy aware routing (SER) with channel-
timeslot allocation for multi-hop CRANs. Our goal is to
provide robust high throughput routing which involves not
only route selection but also channel-timeslot allocation. The
proposed SER protocol can select energy-efficient route and
assign channels and timeslots for a connection request. In
designing the SER protocol, we consider scheduled channel
access, which provides contention-free packet transmissions.
By combining routing with channel-timeslot allocation, the
SER protocol can improve network throughput by
decomposing the traffic over different channels and timeslots.
The rest of the paper is organized as follows. Section II
describes the related work. The system model is presented in
section III. The proposed routing protocol is described in
section IV. We present the performance evaluation in section
V, and finally in section VI we conclude the paper.
II. RELATED WORK
Routing protocols for CR networks should utilize the
flexibility of CRs and address the critical challenges that do
not exist in the traditional wireless networks. A number of
routing protocols for CR networks are available in the
literature. The routing protocols for the CR networks can be
categorized according to the number and the usage of radios.
Some protocols assume a dedicated radio for control and
another radio for data. These protocols are based on the
concept of the existence of a common control channel (CCC)
and the control radio is used for exchanging the control
message only [4]−[7]. However, the additional control radio in
each CR user is likely to be harmful for energy-constrained
CR networks. The schemes using a single radio for both data
and control message exchange are available in [8] and [9], but
the key problem with those protocols is control message must
be broadcasted to all available channels, which is time
consuming and increase the network overhead.
Furthermore, routing scheme with multiple radios for CR
networks consisting of opportunistic links is available in [10].
Due to extremely dynamic nature of CR links, traditional
routing is not feasible to maintain end-to-end routing table for
CRANs. A local on-demand routing is presented for the
realistic at reasonable routing delay to route packets.
Although, throughput can be increased using multiple radios
but it is not suitable for energy-constraint CR networks.
Our protocol is different from the existing works as we
consider joint routing and channel-timeslot allocation in
energy-constraint CRANs with only a single radio transceiver
for data and control message. The proposed protocol enhances
the network lifetime and reduces the end-to-end delay.
978-1-61284-231-8/11/$26.00 ©2011 IEEE
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE ICC 2011 proceedings

III. SYSTEM MODEL
We consider a CR network consisting of M PUs and N CR
users. PUs are license holders for specific spectrum bands, and
can occupy their assigned spectrum any time and for any
duration. Consider the spectrum consisting of C non-
overlapping channels, which are licensed to PUs. The
bandwidth of channel c (c = 0, 1, 2, …, C−1) is denoted by Bc.
Considering the dynamic spectrum nature of the CR networks,
a common control channel (CCC) is used by all CR users for
spectrum access, which is always available. This CCC may be
owned by the CR service provider. We model each of C
channels as an ON-OFF source alternating between ON
(active) state and OFF (idle) state specifying whether the PU
signals are using a channel or not, respectively. The CR users
can utilize the OFF time to transmit their own traffic.
We assume that all CR users are equipped with a single half-
duplex CR transceiver. For accessing a channel, a CR user
must sense channels first, and can access the channels only if
any of these C channels is not being used by PUs. We assume
that each CR user has enough capability of accurately sensing
the presence of PU on any channel and keeping track of a list
of channels available for transmission.
A. Cognitive Radio MAC Model
We assume that system time is divided into fixed-length
frames and each frame consists of a sensing window, an ad
hoc traffic indication messages (ATIM) window, and a
communication widow as depicted in Fig. 1. The ATIM
window is contention-based and uses the same mechanism as
in the IEEE 802.11 DCF [11]. The ATIM window is divided
into the beacon and the control window. During the ATIM
window, control channel (i.e. CCC) is used for beaconing and
to exchange control message. The communication window is
time-slotted, each of which is called a timeslot. The duration
of each timeslot is the time required to transmit or receive a
single data packet including the time need to switch the
channel, and the acknowledgement (ACK). In the
communication window, CR users can send or receive packets
or go to sleep mode to save power. If a CR user has decided to
send or receive a packet in a specific timeslot on a specific
channel, it first switches to the decided channel and transmits
or waits for the data packet in that timeslot. If a receiver
receives a unicast packet, the receiver sends back an ACK in
the same timeslot as shown in the slot structure of Fig. 1.
All CR users are synchronized by periodic beacon
transmissions. In this MAC scheme, channel sensing is
performed in the starting of each frame not to disturb PUs. If
any chosen channel is found to be busy, that channel will not
be used in the ATIM window. If a sender does not hear an
ACK after it sends a unicast packet, because of the possible
collision with other transmissions, the sender may perform
random backoff before attempting its retransmission. If the
number of retransmissions exceeds the retry limit, the packet
is dropped. It is noted that along with other channels CCC can
also be used for data transmission in the communication
window, if needed. If a CR user has not decided to send or
receive a data packet in a specific timeslot, the CR user
switches to sleep mode for power saving.
A channel-timeslot pair (c, t) is defined as the
“communication segment.” To assure collision-free
communications, all neighborhood users of the intended
receiver except the intended transmitter should remain silent
on the particular channel during a given timeslot. With the
help of periodic beaconing, each CR user is aware of (1) the
identities and list of available communication segments within
its two-hop neighbor, and (2) existing transmission schedule
of communication segments of its one-hop neighbor. Based on
the collected neighbor information and its own information,
each CR user updates the status of its communication
segments as occupied or free. Free communication segment of
CR user v, free_segment(v), is the communication segments
for all available channels, which are not used by CR user v to
communicate with adjacent CR users.
The resource allocation problem in the MAC layer is
actually to determine how to assign available communication
segments to links subject to the interference constraints. For
each link in the network, the communication segment
assignment algorithm marks each communication segment as
one of the following:
• Occupied: this segment is being used by other
transmissions and hence can not be used.
• Free: unassigned idle segment.
• Tentatively assigned: this segment might be used for data
transmission on a specific link.
B. Network Model
We consider energy-constrained multi-hop CRANs where
each CR user has limited battery energy. We assume CR users
are stationary or moving very slowly. In our CR network, PUs
are also assumed to be stationary and they coexist with the CR
users. There is a link l = (u, v)
∈
E between CR users u and v, if
two users are in the transmission range and there is an available
channel ,
uv
cH H∈∩ where u
H
and v
H
represent list of
available channels at CR users u and v, respectively, and E is
the set of communication links each connecting a pair of CR
users. We model the impact of interference by using the well-
known protocol model proposed in [12]. A transmission on
channel c through link l is successful if all interferers of both
CR users u and v are silent on channel c for the duration of the
transmission. Two wireless links (u, v) and (x, y) interfere with
other if they work on the same channel and any of the given
expression is true: v = x, u = y, v
∈
Nb(x), or u
∈
Nb(y), where
Nb(v) represents the set of neighbors of CR user v. If links
(u, v) and (x, y) are conflicting, CR users u and y are within
two-hops of each other [13].
ATIM
Window
Communication Window
Frame
Sensing
Window
B: Beacon
DATA ACK
Guard Periods
Control PacketsB
Control
Channel
Multiple Data Channels
Slot
123 ..T..
ATIM
Window
Communication Window
Frame
Sensing
Window
B: Beacon
DATA ACK
Guard Periods
Control PacketsB
Control
Channel
Multiple Data Channels
Slot
123 ..T..
Fig. 1. Structure of MAC protocol.
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE ICC 2011 proceedings

IV. PROPOSED ROUTING PROTOCOL
The proposed spectrum-aware and energy-efficient routing
is an on-demand routing proposed for multi-hop CRANs. The
basic operations of the proposed SER protocol include route
discovery, data transmission, and route maintenance. We now
describe below the components of SER in further detail.
A. Routing Algorithm
The proposed route request (RREQ) broadcast procedure is
based on DSR [14] protocol. When the source CR user has
packets to send to the destination CR user, it will initiate a
route discovery process by broadcasting a spectrum aware
RREQ message on the CCC to all of its neighbors. The fields
of RREQ packet are shown in Table I.
The target of the energy-efficiency is to balance the nodal
energy consumption to prevent one or some critical users
depleting their energy supplies and drop out from the network.
For selecting the energy-efficient path, we consider maximal
minimal nodal residual energy with lower hop count as routing
metric. Initially, at the source CR user, the value of minimum
residual nodal energy, mEres equals the initial energy of the
battery. To avoid CR users having very poor energy in a route;
intermediate CR users should have a threshold energy, Eth.
Furthermore, because of the opportunistic properties of
CRANs, a CR user should also have at least one channel
common with its previous hop. When the forwarding
intermediate CR users receive the RREQ packet, they compare
their own residual battery energy, Eres with the mEres and
update accordingly in order to keep the value of mEres lowest
among all the CR users in this route. This will ensure the route
that has the minimal nodal residual battery energy.
In the proposed protocol, an intermediate CR user v starts a
timer whenever it receives the first RREQ. For each RREQ
received from a neighbor CR user u before the timer expires,
CR user v runs the communication segment assignment
algorithm, discuss later on, to find out good feasible
communication segment for the link l = (u, v). If it fails to find
a feasible assignment, it drops the corresponding RREQ.
Otherwise, it attaches itself to the current partial path, includes
SSeg in the header, updates the other information and
rebroadcast it by appending its own information to route_seq
and FSeg. Consequently, the CR user increases the value of
hop count (HC) and decreases the value of time to live (TTL).
Similarly, the destination CR user sets up a timer to collect
multiple RREQs for a connection request. It will run the same
algorithm to select feasible communication segment for the
last hop. After receiving the first RREQ packet, destination
CR user waits for a time period to get more RREQs before it
makes route reply. Destination CR user then computes route
utility using (1) and selects the route using (2).
,,
res k
k
k
mE
Uk
HC
=∀
; (1)
max{ },
k
P
Uk=∀
; (2)
where k
U is the route utility of path k = 1, 2, …, K, the number
of received RREQ by the destination. k
Uis used to evaluate
the selected routes to find the maximum value of P for which
the value of ,res k
mE is maximized and k
H
Cis minimized.
Maximum value of ,res k
mE ensures that the selected path has
high energy i.e. energy-efficient path; on the other hand, the
smaller hop count gives the packet transmission with
comparatively low delay.
Destination CR user then reply to the source CR user with
route reply (RREP) packet through CCC. Accordingly, the
communication segment selected for each link along the path
is reserved for data transmission. Moreover, all the other CR
users in the neighborhoods of the CR users in the selected
paths are notified that these communication segments are
going to be used by the new incoming connection. While
RREP is forwarding towards source CR user, intermediate CR
users reserve the communication segments in accordance with
the information in the RREP packet. However, if the
communication segments that have been tentatively selected
during route discovery phase may be reserved by another
RREP during the reservation phase. Hence, instead of
discovering new route, communication segment assignment
scheme is initiated to coordinate the conflicting CR users with
its one-hop neighbors’ scheduling to recover the route
reservation. Finally, after receiving the RREP packet by the
source CR user, the data transmission begins. In addition,
local route recovery (RREC) and route error (RERR) based
scheme is used for route maintenance.
B. Assignment of Communication Segment
In this subsection, we present a heuristic algorithm to assign
communication segment for the link l = (u, v). To ensure the
collision-free transmissions, the following conditions must be
satisfied in selecting the communication segment (c, t) for the
link l = (u, v):
• Timeslot t is not assigned to any link incident
(connected) on CR user u,
• Timeslot t is not assigned to any outgoing link from the
CR user v,
• Timeslot t is not used on channel c by any link
l′, () ()
x
Tl Nbv
′∈ and
• Timeslot t is not used on channel c by any link
l′, () ()
x
R
lNbu
′∈.
here Tx(·)/Rx(·) represent the set of transmitters and receivers,
respectively, of the given link. Note that one of the necessary
constraints for collision-free communication is that no two
links incident at CR user can be assigned same timeslot. If all
the above conditions are satisfied, communication segment
(c, t) is assigned to the link l = (u, v).
Table I. Fields of RREQ Packet
req_id Unique route request sequence number determined by the
source CR user
src_id Address of the source CR user i
dst_id Address of the destination CR user i
mEres Minimum residual nodal energy in a route i
route_seq List of the address of CR users from the source to the
current traversed user
FSeg List of free communication segments (channel-timeslot)
of each user from the source to the current traversed user
SSeg List of the selected communication segment of each user
from the source to the current traversed user
HC Hop count i
TTL The limitation of hop-length of the search path i
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE ICC 2011 proceedings

V. PERFORMANCE EVALUATION
The effectiveness of the SER protocol is validated through
computer simulation. To evaluate SER, we have developed a
packet-level discrete-event simulator written in C
programming language, which implements the features of the
protocol stack described in this paper. The simulated network
is composed of 50 CR users deployed randomly within a 500
m × 500 m square region. The transmission and interference
range of each CR user is approximately 150 m and 300 m,
respectively. We set an initial energy of 100 Joules per CR
user. We consider the transmitting energy of each CR user:
ETx = (1.65 × packet size in bits)/2 × 106 Jules and receiving
energy: ERx = (1.15 × packet size in bits)/2 ×106 Jules [15].
We ignore the energy consumptions of CR users in PUs
sensing sessions.
We vary the number of channels from 4 to 12, each of
which has a data rate of 2 Mbps. Among them, one channel is
CCC and the others are data channels. The packet size is set to
1000 bytes. Based on our simulation experience, we set the
frame interval for the MAC scheme to 45 ms where the
communication window is 34 ms and the sensing window is 3
ms. The number of timeslots in the communication window is
set to 8 and each slot duration is 4.25 ms, which is calculated
for a 1000 bytes packet to be sent through the channel of data
rate 2 Mbps. The length of the ATIM window is 8 ms where
2.5 ms is assigned for beacon period. Channel switching delay
is set to 80 μs. We initiate the RREQ between randomly
selected but disjoint source-destination pairs. We place 5 or 10
PUs at some random locations in the region. The active and
idle time duration of PUs are exponentially distributed with
mean value 100 seconds, respectively. Each of the active PUs
randomly chooses a channel to use, which is then considered
to be unavailable for all the CR users within their coverage
range, which is set to 300 m.
We impose the best effort traffic with message generation
time exponentially distributed with mean value 1/ {(message
generation rate)/(number of CR users)} second. Average
message length is geometrically distributed with mean value
1000 packets. We vary the message generation rate to vary the
offered load to the network. Each data point in the plots is an
average of 5 runs where each run uses a different random
network topology. The simulation time of each run is set to
500 seconds.
In Fig. 2, we measured the aggregate throughput (the sum
of individual routes throughput), which is defined as the data
traffic received by the destinations in Mbps by varying the
offered load. We can see from the figure, when the offered
load increases, aggregate throughput increases up to message
generation rate 1.25 and then slightly decreases. SER without
PU outperforms in all level of offered load compare to the 5
PUs and 10 PUs cases. Throughput without PU achieves 6%
more throughput than with 5 PUs and 9% more than with 10
PUs. This figure shows the impact of PUs on aggregate
throughput. When the number of PUs increases throughput
decreases because of available resources are decaying. Figure
2 also shows the effect of channels and PUs on throughput.
When the number of channels decrease and that of PUs
increase aggregate throughput decreases. One point is noted
here that even though we have multiple channels with 2 Mbps
data rate each but the obtained aggregate throughput is closed
to 2 Mbps. This is because of the multi-hop routing nature.
Another reason is that for the PUs activities, we are unable to
utilize the whole resources of the multichannel we have.
In any cases, if a link supports lower data rate, then the
aggregate throughput of all routes passing through that link is
decreased.
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
2.25
0 0.25 0.5 0.75 1 1.25 1 .5 1.75 2 2.25 2.5 2.75
Message generation rate (p er sec)
Aggregate throughput (Mbps)
4 Chls, 5 PUs
4 Chls, 10 PUs
8 Chls, 5 PUs
8 Chls, 10 PUs
Fig. 2. Aggregate throughput with different number of channels and PUs.
Fig. 3. Average end-to-end packet delay with different number of PUs and
12 Channels.
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.11
0.12
0.13
0.14
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75
M essage generation rate (p er sec)
End-to-end packet delay (sec)
Without PU
5 PUs
10 PUs
Fig. 4. Normalized routing overhead per data packet.
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2 .25 2.5 2.75
Message generation rate (per sec)
Normalized routing o verhead
Without PU
5 PUs
10 PUs
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE ICC 2011 proceedings

We define the end-to-end packet delay as the latency
incurred by a data packet between the generation time at the
source and the arrival time at the destination. Figure 3 shows
the average end-to-end packet delay under the proposed
protocol by varying the network offered load with 12
channels. The effect of PUs on the average end-to-end packet
delay is shown in this figure. SER without PU shows lower
delay in all level of the offered load because of the network
recourses are static in this scenario. SER with 10 PUs shows
comparatively higher delay because of the limited spectrum
recourses availability.
The normalized routing overhead which is defined as the
number of routing packets transmitted per data packet
delivered at the destinations is shown in Fig. 4. When the PUs
increase, the routing overhead increases due to the increase of
RREQ, RREP, RREC, and RERR. The result shows that our
proposed protocol needs lower routing overhead per data
packet delivery. The energy-efficiency that is measured in
received data packets by destinations per Joule shown in Fig.
5. The graph shape is identical with aggregate throughput. It is
shown in the figure that received packets at the destinations
per Joule increases up to the saturation level of the offered
load and then slightly decreases.
The network lifetime which is defined as the duration from
the beginning of the simulation to the first time a CR user runs
out of energy is shown in Fig. 6. The effect of PUs on the
network lifetime is shown in this figure. SER without PU
shows lower lifetime because all CR users are participated in
the routing tasks in whole simulation time. When the number
of PUs increase the routings are distributed to the other CR
users having higher residual nodal energy; thus prolongs the
lifetime of individual CR users and overall network. When the
offered load increases the network lifetime decreases because
of the increasing of the number of routes.
VI. CONCLUSION
We have proposed a spectrum and energy aware on-demand
protocol for routing and channel-timeslot assignment in multi-
hop CRANs. The proposed SER protocol combines the
integration of spectrum and route discovery to establish
communications across areas of spectrum heterogeneity. Our
protocol balances the traffic load among different CR users
according to their nodal residual battery energy and prolongs
the lifetime of individual CR user and the overall networks.
Simulation results show that the proposed SER protocol can
provide a lower end-to-end packet delay and routing overhead
but ensure the higher throughput and longer network lifetime.
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Fig. 5. Energy-efficiency measured in data packets/Joule with different
number of PUs.
0
5
10
15
20
25
30
35
40
45
50
0 0.25 0.5 0 .75 1 1.25 1.5 1 .75 2 2.25 2.5 2.75
M ess age generation rat e (p er sec)
Packets/Joule
Without PU
5 PUs
10 PUs
Fig. 6. Network lifetime till first CR user die.
150
175
200
225
250
275
300
325
350
375
400
0 0.25 0.5 0.75 1 1.25 1 .5 1.75 2 2.2 5 2.5 2.75
Message generat ion rate (per sec)
Netw ork lifetime (s ec)
Without P U
5 PUs
10 PUs
This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE ICC 2011 proceedings