Wireless Sensor Network, 2009, 2, 61-121
doi:10.4236/wsn.2017 lished Online July 2009 (http://www.SciRP.org/journal/wsn/).
Copyright © 2009 SciRes. Wireless Sensor Network, 2009, 2, 61-121
009.12 Pub
A Novel DSA-Driven MAC Protocol for C o g ni t i v e
Radio Networks
Hua SONG, Xiaola LIN
School of Information Science and Technology, Sun Yat-sen University, Guangzhou, China
Email: songhua@mail2.sysu.edu.cn, linxl@mail.sysu.edu.cn
Received February 19, 2009; revised April 29, 2009; accepted May 6, 2009
Abstract
With the deployment of more wireless applications, spectrum scarcity becomes an issue in many countries.
Recent reports show that the reason for this spectrum shortage is the underutilization of some spectrum re-
sources. Fortunately, the emergence of open spectrum and dynamic spectrum assess (DSA) technology in
cognitive radio networks relieves this problem. In this paper, we propose a novel DSA-driven cognitive
MAC protocol to achieve highly efficient spectrum usage and QoS provisioning. In the proposed protocol,
secondary users are divided into several non-overlapping groups, and all leftover channels are allocated
among groups taking the groups’ bandwidth requirements into consideration. Moreover, the allocation of
vacant channels can be adjusted dynamically when members join/leave groups or primary users return/leave
the current network. Simulations show that the proposed MAC protocol greatly improves the quality of ser-
vice for secondary users and maximizes the utilization ratio of spectrum resources.
Keywords: Cognitive Radio, DSA-Driven MAC Protocol, QoS Provisioning, Dynamic Spectrum Access
1. Introduction
The deployment of wireless services and devices has
been increasing rapidly in recent years, but current us-
able spectrum has almost been allocated to various spec-
trum-based services, which greatly blokes the develop-
ment of wireless communication. However, extensive
reports indicate that the reason for this spectrum shortage
is not the scarcity of the radio spectrum, but the low
utilization (only 6%) of the licensed radio spectrum in
most of the time [1].
This underutilization of spectrum resources has prom-
pted the emergence of cognitive radio. In 2003, Federal
Communications Commission (FCC) suggested a new
concept/policy for dynamically allocating the spectrum
[2]. Thus, a promising implementation technique called
cognitive radio is proposed to alleviate the scarcity of
spectrum bandwidth. Based on cognitive radio, open
spectrum and dynamic spectrum access (DSA) technolo-
gies have shown great interest recently [3]. In this tech-
nology, primary users (licensed users) have high priority
to use their spectrum; secondary users (unlicensed users)
are allowed to opportunistically access the spectrum only
when the spectrum is not used by primary users.
Although the research community has proposed sev-
eral cognitive MAC protocols to address various issues
in cognitive network [4-8], commonly, they pay more
attention to save the number of transceivers, and improve
throughput of the whole system or decrease session de-
lays. However, all these protocols do not lay emphasize
on quality of service for secondary users and high usage
of leftover spectrum with dynamically adjusting alloca-
tion. For instance, in [5], a decentralized protocol, called
hardware-constrained cognitive MAC protocol (HC-
MAC), for managing and coordinating spectrum access
is proposed. Under HC-MAC, a pair of secondary users
can use several channels to communicate simultaneously
after they have sensed the vacant channels, but if the
leftover spectrum allocated to the pair of secondary users
is more than they can utilize, this part of surplus spec-
trum is wasted and cannot be used by other users whose
* This work was supported in part by NSFC under Projects 60773199,
U0735001, and 985 II fund under Project 3171310.
H. SONG ET AL.
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Copyright © 2009 SciRes. Wireless Sensor Network, 2009, 2, 61-121
bandwidth requirements have not been satisfied.
To effectively provide QoS for secondary users and
achieve highly efficient spectrum usage, in this paper, we
propose a novel DSA-driven MAC protocol which is of
significant importance in ad hoc cognitive network to
guarantee the QoS requirements of secondary users. Dif-
ferent from the existing cognitive MAC protocols, the
main advantages of the proposed MAC protocol include:
1) Maximizing the u tilization ratio of spectrum resources;
2) Using bonding/aggregation and dynamical channel
allocation techniques to guarantee the QoS requirements
of secondary users; 3) Ensuring the fairness of channel
allocation for groups.
The rest of the paper is organized as follows. Related
work is discussed in Section 2. The preliminaries and
system model is introduced in Section 3. The proposed
MAC protocol is presented in Section 4. Performance of
the proposed MAC protocol is evaluated by experiments
in Section 5. Fi nally, conclusi on is drawn in Secti o n 6.
2. Related Work
Over the past several years there have been increasing
interests in cognitive radio. In addition, wireless MAC
protocol has a principal part in spectrum reuse and effi-
ciency management. Therefore, various cognitive MAC
protocols have been proposed for more flexible and effi-
cient use of spectrum resources [4-8].
Hamdaoui and Shin [4] propose the OS-MAC protocol.
This protocol divides the secondary users into several
groups, at each Opportunistic Spectrum Period, the
channel used by a group can be adjusted dynamically
according to the channels' states of the whole system.
The OS-MAC protocol also discusses the non-coopera-
tive mode between primary users and secondary users,
but it does not give a feasible solution (we will present
our solution on this problem in subsequent paper). Fur-
thermore, as only one channel can be used in a group at
anytime, the spectrum resources cannot be used to the
maximum with taking secondary users’ QoS into consid-
eration.
Jia et al. [5] present the HC-MAC protocol. This pro-
tocol uses k-stage look-ahead method to sense unused
channels with high efficiency and takes hard-
ware-constraints into consideration (including sensing
constraint and transmission constraint). Besides the flaw
mentioned in Section 1, there is a problem called sensing
exposed terminal problem in this protocol. That is, if a
secondary pair senses unused channels while their
neighbors who didn't receive the C-RTS/C-CTS packets
are performing their operations freely, this pair of users
can not sense the vacant channels accurately.
Su and Zhang [6] propose the cross-layer based op-
portunistic MAC protocols. In the protocols two sensing
policies, the random sensing policy and the negotia-
tion-based sensing policy, are presented. Like [5], the
protocols also use bonding/aggregation technique to
transmit data through several channels. In essence, the
main contribution of their work is to reveal the tradeoff
between throughput and delay, which provides the
guidelines to support the different QoS requirements
over cognitive radio based wireless networks. But, at
anytime, only a pair of secondary users can use vacant
channels, thus the leftover channels cannot be used effi-
ciently.
Thoppian et al. [7] propose a CSMA-Based MAC
protocol. In the protocol, each node maintains a list of
favorable channels for each of its neighbors based on the
previous history of communication on each of the chan-
nels, and a secondary pair chooses the most favorable
channel for communication. As it does not consider the
channels’ utility of the whole system, it also cannot use
the radio spectrum resources efficiently.
Similarly, the methods in [9-12] all do not address the
issues of the QoS of secondary users and the spectrum
resources’ utility of the entire system.
3. Preliminaries and System Model
We first present an introduction to the channel bond-
ing/aggregation technique and the main framework of the
system.
3.1. Channel Bounding/Aggregation Technique
From the definition of channel bonding/aggregation tech-
nique in [13] we can see that ch annel bonding is used for
contiguous channels, but channel aggregation is for dis-
crete ones. So, if several contiguous channels can be
used, channel bonding is the appropriate technique. Oth-
erwise, we can adopt channel aggregation technique.
In MAC perspective, channel bonding incurs no addi-
tional overhead as all control messages are transmitted
only once, and an access point (AP) with channel bond-
ing also has much greater control and more freedom on
resource allocation and transmit power. In contrast, with
channel aggregation, the overhead increases considerably
with the number of channels used, and for an effective
channel aggregation solution, features such as sophisti-
cated scheduling, load balancing and channel manage-
ment are needed.
Figure 1(a) and Figure 1(b) compare the aggregate
throughput and efficiency of the two techniques respec-
tively. From Figure 1, we can see that channel aggrega-
tion incurs much more overhead than channel bonding
and these two techniques are designed for medium-high
loads. Also, this figure presents that ch annel bonding can
offer much better channel utilization with less overhead.
In [5], hardware con strains of bonding/aggreg ation are
pointed out: spect rum used by a secondary user has maxi-
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10
15
20
25
30
35
40
45
50
55
10 20 30 40 50 60
Aggregate throughput (Mbps)
Overall load (Mbps)
2 aggregated channels
3 aggregated channels
2 bonded channels
3 bonded channels
(a) Aggregate throughput against overload.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
10 20 30 40 50 60
Efficiency
Overall load (Mbps)
2 aggregated channels
3 aggregated channels
2 bonded channels
3 bonded channels
(b) Efficiency against overload.
Figure 1. Channel bonding vs. channel aggregation.
mum bandwidth limit and maximum fragmentation
number limit. For convenience, we will not take these
constrains into consideration presently, but they are in-
cluded in our future research plan.
3.2. Secondary Users’ Devices
The whole available radio spectrum in a CR network is
divided into a number of non-overlapping data channels
(DCs) and a common control channel (CC), for transmit-
ting data information and control information respec-
tively. Further, we assume the CR network is a wireless
ad hoc network formed by a great many secondary users,
each of which is equipped with two wireless transceivers.
One is called data transceiver used to sense leftover DCs
and exchange data through these channels, but this
transceiver is not able to operate in sensing and transmit-
ting mode concurrently. The other transceiver is called
control transceiver used to send or receive messages
from CC. Note that, the transceivers are half-duplex, thu s
they are not able to send and receive messages simulta-
neously. We also assume that every secondary user can
use aggregated spectrum and has full spectrum sensing
ability.
3.3. Control Mechanism
Based on distributed environment, we divide secondary
users into several non-overlapping groups, and each gro-
up has a leader (the leader can be adjusted dynamically)
who is responsible for group members' management,
group channel's management and group channels' appli-
cation. Moreover, there is a manager in the whole wire-
less ad hoc network who is elected among leaders and is
used to manage all the leftover channels and allocate
these channels to groups fairly. The manager communi-
cates with leaders through CC, and the control messages
in a group are exchanged through DCs. So the common
control channel (CC) is light loaded and will not be the
bottleneck of the network. As the manager and leaders
can be changed dynamically, each of them is impossible
to become a single failure point. Thus, the system is pro-
vided good scalability and extensibility.
4. Our Proposed DSA-Driven MA C Protocol
In this section, we present the design of our proposed
DSA-driven MAC protocol for cogn itive radio networks.
4.1. Definitions and Notations
In order to present the proposed MAC protocol more
clearly, we would now like to introduce so me definitions
and notations here.
1) Tables: These tables are used for the management
of the whole network.
GroupTable: This table is maintained by the man-
ager. It contains the group number, the leader and
members in the group.
MemberTable: This table is maintained by the
leader. It contains the list of group members. Note
that each member has a MemberTable.
2) Control frames: These frames are used for the con-
trol of protocol's realization. The first five frames are
sent through CC, the LeaveReq frame is sent through DC
and the last one can be sent through CC or DC.
Invite: It contains the manager's id.
JoinReq: It contains the id of a secondary user
whom the request user wants to communicate with.
JoinACK: It contains the leader's id of chosen
group.
Sense: It contains the group number and the sensing
range of channels.
Allocate: It contains the group number and a list of
channels.
LeaveReq: A member uses this frame to apply for
leaving the current group.
Notify: It contains ManagerChanged and Leader-
Changed fields.
3) Timers: These timers are used for the maintenance
of the whole system.
ManagerHeartBeatTimer: This timer is started at
the manager when it broadcasts the Invite frame on
CC. In essence, ManagerHeartBeatTimer has two
functions, the first one is to let secondary users
H. SONG ET AL.
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Copyright © 2009 SciRes. Wireless Sensor Network, 2009, 2, 61-121
know who is the manager currently and the other
one is to make leaders assure the manager is still
alive.
ManagerFailureTimer: This timer is started at a
leader when it receives an Invite frame on CC. If
this timer expires before the leader receives another
Invite frame, it knows that the manager is failed.
LeaderHeartBeatTimer: This timer is started at a
leader when it broadcasts the LeaderHeartBeat
frame on DC in a group. As soon as members re-
ceive the LeaderHeartBeat frames they know that
the leader is alive.
LeaderFailureTimer: This timer is started at a
member when it receives a LeaderHeartBeat frame
from DC. If this timer expires before the member
receives another LeaderHeartBeat frame, it knows
that the leader is failed.
4.2. Overview
There are two kinds of control frames in our proposed
MAC protocol: inter -g roup co n trol fr ame and in tr a-gr oup
control frame. Inter-group control frames are transmitted
between the leaders and manager through CC, while in-
tra-group control frames are exchanged between the
leader and members through DCs in a group. If several
leaders want to send messages to the manager, they must
negotiate with each other via the contention-based algo-
rithms, such as IEEE 802.11 Distributed Coordination
Function (DCF) [14] and p-persistent Carrier Sense Mul-
tiple Access (CSMA) protocols [15]. In order to guaran-
tee reliable transmission, we use acknowledge mecha-
nism in control messages’ exchange.
Figure 2 shows the principle of our proposed MAC
protocol. It divides time into Periods and each of which
consists of three consecutive phases: Sensing Phase, Al-
locating Phase and Transmitting Ph ase. At any time, only
one pair of members in a group can exchange informa-
ti on. In order to let all membe rs in the same group use ch-
Period
Sensing
Phase
Allocating
Phase
Transmitting
Phase
DC
DC
CC
12 m
1
n
t
Data Channels unused
by primary users
Common Control
Channel
Managing group members’
joining and leaving
Reelecting the leaders and
manager if needed
Manager
allocates
channels for
each group
Manager
coordinates all
groups sensing
vacant channels
Each leader divides this phase
into several slots according to
the number of users in the
group, and only a pair of users
can communicate in a slot
Each group
switches to
the allocated
channels
Secondary
users start
sensing vacant
channels
Figure 2. The principle of our proposed MAC protocol.
annels fairly, the Transmitting Phase is divided into sev-
eral time slots according to the number of users. Fur-
thermore, a few of time slots are reserved for intra-group
control frames exchang e.
In Sensing Phase, manager coordinates all groups
sensing vacant channels (it's an efficient method to check
all unused channels in a short time). Then, based on the
feedback of sensing results and bandwidth requirements
from all groups, the manager calculates out a best alloca-
tion scheme and allocates the available channels to each
group in Allocating Phase. When groups gain their new
allocated channels, they switch to these channels imme-
diately and begin to transmit information in Transmitting
Phase.
4.3. Details of the Proposed MAC Protocol
In the proposed MAC protocol, each secondary user in
the network will be in one of the follo wing phases at any
given time.
1) Initialization Phase: If a secondary user is not in-
volved in any group and seeks channels to transmit in-
formation, it will listen to CC.
a) If the secondary user receives an Invite frame from
CC, it sends a JoinReq control frame to apply for joining
a group. After receiving the JoinReq frame, the manager
looks up its GroupTable to decide which group this user
should join in according to the user's request. If the
manager finds an appropriate group, it updates its
GroupTable with a JoinACK control frame to send back.
When the secondary user receives the JoinACK frame, it
communicates with the leader of the chosen group
through CC and tunes its data transceiver to the accord-
ing DCs. However, the manager may not find a suitable
group for the secondary user to join in, then it checks
whether it's possible to establish a new group for this
user. If so, the manager creates a new group and makes
this secondary user a leader; otherwise a REJ control
frame is responded.
b) If the ManagerFailureTimer expires before the sec-
ondary user receives an Invite frame, the user knows that
it is the only secondary user in the network. Then it es-
tablishes a new group, makes itself a leader and manager,
creates a MemberTable and a GroupTable, and broad-
casts an Invite frame on CC.
2) Sensing Phase: all secondary users cooperate to
sense the channels unused by primary users.
a) The manager allocates sensing channels among all
groups according to GroupTable, and sends a Sense
frame to each leader via CC.
b) Each group senses vacant channels whose range is
indicated by the Sense frame.
c) Leaders report sensing results to the manager
through CC.
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Copyright © 2009 SciRes. Wireless Sensor Network, 2009, 2, 61-121
3) Allocating Phase: When the manager gets the va-
cant channel information from all groups, it allocates
these channels among groups.
a) Each leader sends bandwidth requirement of its
group to manager via CC.
b) The manager determines a channel allocation
scheme according to the bandwidth requirement of each
group, and sends this scheme to leaders.
c) Each group switches to the allocated channels.
4) Transmitting Phase: As soon as groups switch to
the allocated channels, they can use bonding/aggr egation
technique to transmit data.
Send Invite frame
Perform
operation
Manager allocate
vacant channels
Receive Sense
Reply frames
from leaders
Compute vacant
channels for
each group
Send Allocate
frame to
leaders
Leave the group
The
coordinator is
manager?
Yes
Assign a new
leader and a
new manager
Send GroupTable
to the new
manager
Send Notify frame
to members and
leaders
Receive frames
1
No
Assign a new
leader of the
group
Send Notify frame
to members and
the manager
Manager organize
sensing process
Compute sensing
channes for
each group
Send Sense
frame to
leaders
(a)
1
Member Where are the
frames from?
Analyze the
type of the
frames
LeaveReq
Modify the
GroupTable
Send ACK frame
Allocate Sense
Manager
Tell members to
switch to the
allocated channels
Coordinate all
members to
sense vacant
channels
Report sensing
results and
bandwidth
requirement to
the manager
Analyze the
type of the
frames
New comer
Analyze the
type of the
frames
JoinReq
JoinGrp
Approved by
the manager?
Yes
Send group
channels
information
No No
Send REJ frame
No
The
coordinator
is manager?
Yes
Look for a
group for the
new comer
Find an
appropriate
group?
Yes
Update GroupTable
and send JoinACK
frame
Possible
to establish a
new group?
Create a new
group and make
the new comer
a leader
NoYes
(b)
Figure 3. The operation flow chart of a coordinator.
a) As leaders divide the whole Transmitting Phase into
several time slots, in each group, the members use chan-
nels in turn.
b) At the end of Transmitting Phase, the manager and
leaders exchange group information through CC in order
to update GroupTable and MemberTable respectively.
Also, in each group the leader broadcasts GroupTable to
all members.
c) If a member wants to leave a group, the following
operations will be done:
If the member is not a leader or manager, it sends a
LeaveReq frame to the leader through DC, then the
leader modifies the GroupTable with an ACK frame is
replied.
If the member is a leader, it assigns a new leader
from rest members of this group and broadcasts a Notify
frame on DCs in the group; it also sends this frame to the
manager via CC. In case this leader is the last one in the
group, it sets leader's id NULL in the LeaderChanged
field of Notify frame.
If the member is a manager, first, it selects a new
leader from rest members in its group and a new man-
ager from all leaders; second, it sends GroupTab le to the
new manager; last, it broadcasts a Notify frame on DCs
in the group, as well as posts this frame to leaders via
CC.
d) If a secondary user wants to jo in a group, it will do
the operation as shown in Initialization Phase.
In the following, we call the manager or leader coor-
dinator. Figure 3 delineate the operation flow of a coor-
dinator according to the protocol's details mentioned
above. For conciseness, we only give the foremost con-
trol operat i o ns.
4.4. Channel Allocation Mechanism
We suppose a radio spectrum system consisting of M
groups which apply for n vacant channels. For each
group, the allocation optimization problem can be de-
scribed as:
1
11
1
max
..
min( ,(/))
(//)
i
i
c
ij
j
i
cnM
iji iii
ji
n
i
i
b
st cn
brrB
BnMn



1
i
r

(1)
where (1,,)
i
ci M
th
i
is the number of leftover chan-
nels allocated to the group, denotes
the bandwidth of the channel which is allocated to
the group (In this paper, we assume that all chan-
nel’s bandwidths are not the same and they are uniformly
(1,,
ij i
bj c)
th
j
th
i
H. SONG ET AL.
117
Copyright © 2009 SciRes. Wireless Sensor Network, 2009, 2, 61-121
distributed), is the bandwidth require-
ment of the group, and is the band-
width of the vacant channels. The last constraint
indicates that the bandwidth allocated to each group
cannot exceed a certain upper bound which is defined as
. The
augend of the upper bound represents each group’s de-
served bandwidth according to its requirement and the
addend is an adjusting factor which guarantees the chan-
nel allocation is accurate and even.
(1,,)
i
ri M
th
i
th
i
11
/ ))
nM
i i
ii
B r


1
{, ,}
t
Px x
} (1,,)
(1,,)
i
Bi n
1
(/ /
n
i
i
B n
()tt n
min( ,()
ii
rr Mn
{0,1
i
The object of problem (1) is to maximize the utility of
vacant channels. For this kind of problems, branch and
bound algori thm [16 , 17 ] is an eff ectiv e meth od to search
the optimal solutions. In this paper, we propose a branch
and bound algorithm, which is more suitable for the
channel allocation optimization problem in cognitive
radio networks than the general one. For each group, the
detail steps are as follows:
1) Initialization: The feasible solution can be pre-
sented as, Whereis the number of
available vacant channels for the current group,
and
x
it. If the value of i
x
is 1, it
means that the channel is allocated to the current
group. We define. In this algorithm, the
channels are sorted by which is the band-
width of the channel, that is,;
because the channel with higher bandwidth has larger
impact to the optimal solution. Let to indicate
the initial feasible set and initialize . The initial
lower and upper bound of the optimal solution can be
represented as
th
i
0{0,0,P
i
Bi
th
i
00
,0}
(1,,)t
jk
BB
0
{S
1k
,,jk
0
}P
,jk
I
01
t
i
1k
PS
k
(2)
i
B (3)
2) Branching: Based on the characteristics of the chan-
nel allocation optimization problem, the branching
method puts each in duplicate to setand set
the value of
k
S
x
.
(1) {|PP
(2) {|PP
0,}
kk
xxP
1,}
kk
xxP
(4)
(5)
3) Bound ing: The lower bound of the optimal solu tion
in a set can achieve by a greedy method. In this method,
the channel with high bandwidth has the priority to be
chosen in order to achieve the optimal capacity. Thus,
the lower and upper bound of the optimal value in P is
1
() k
ii
i
Px
B
(6)
11
() kt
ii i
iik
PxB

B
(7)
The lower and upper bounds of the optimal solution in
the entire feasible region D are
max( )
k
kPS P
(8)
max( )
k
kPS P
(9)
4) Pruning: For k
PS
, it should be pruned if one of
the following conditions is satisfied:
a) The constraints are violated.
b) The upper bound of this set is smaller than the
maximum lower bound, which is () k
P
.
In this step, many subsets are pruned to accelerate the
searching speed.
5) Convergence: In this step, examine whether it is the
iteration.
th
ta) If ,1ktk k
 and go to the branching step.
b) Ifkt
, the optimal solutions of all the unpruned
subsets are confirmed.
In the convergence step of the iteration, end the
iteration and enumerate the optimal solutions of all un-
pruned subsets in to search the global optimal solu-
tion.
th
t
t
P
6) Return: After getting the optimal solution for cur-
rent group, the algorithm returns to calculate for the next
group. Thus, the channels allocated to the current group
will be excluded in the next round, and the number of
available channels t will also be modified.
4.5. Election Mechanism
If the coordinator is crashed, members must elect a new
one. In our proposed MAC protocol, we use a simple and
high efficient algorithm to realize the election mecha-
nism.
When any member notices the coordinator is not func-
tioning, it sends an Election frame on CC/DCs (manager
election frame on CC and leader election frame on DCs)
to apply for becoming the new coordinator. If several
members detect the coordinator's malfunction concur-
rently, they have to compete for the CC/DCs through the
IEEE 802.11 random access scheme. Recall that for a
successful transmission, an ACK frame will be sent back
to the sender. Under our proposed MAC protocol, as all
members are listening to CC/DCs at all times, each
member is able to receive this Election frame, and if
anyone replies, the ACK frame will be heard by all
members. The member who is the first to successfully
H. SONG ET AL.
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Copyright © 2009 SciRes. Wireless Sensor Network, 2009, 2, 61-121
deliver an Election frame is automatically appointed as
the new coordinator. Therefore, upon receiving an ACK
frame notifying a successful reception, this member con-
siders itself the new leader. Also, any other members
who hear the ACK frame, and hence would know that
someone else is appointed to be the new leader and need
not send its own acknowledgem ent.
In Figure 4, an example of election algorithm is given.
The group consists of eight processes, numbered from 0
to 7. Member 7 is the coordinator which has ju st crash ed.
Member 4 is the first one to notice this, so it sends an
Election frame on CC/DCs. After hearing the Election
frame from Member 4, Member 6 affirms Member 7’s
malfunction and replies with an ACK frame. Upon re-
ceiving this ACK frame, Member 4 knows that it has
been permitted to be the new coordinator. Furthermore,
because the rest members in the same group all can hear
the Election and ACK frames, they are also aware of
Member 7’s crash, as well as the new coordinator's gen-
eration.
Note that the election mechanism is very efficient, it
exploits the already existing ACK mechanism and does
not require any extra message exchange, thus incurring
no bandwidth overhead.
5. Simulation and Performance Evaluation
In this section, we present the simulation results for the
performance evaluation of the protocol. The simulations
are performed using the network simulator ns-2 [18]. The
used parameters are presented in Table1.
In the simulation, the proposed MAC protocol is com-
pared with the following existing MACs: OS-MAC and
CO-MAC.
1) OS-MAC: OS-MAC [4] is an opportunistic spec-
trum MAC. It divides time into periods each of which is
called OSP (Opportunistic Spectrum Period) and consists
of three consecutive phases: Select, Delegate, and Up-
date.
Select Phase: Each SUG (Secondary User Group)
selects a “best” DC, and uses it for communication
until the end of the current OSP.
Delegate Phase: On each DC, a DSU is appointed
among members to represent the group during the
Update Phase.
1
7
03
46
5
2
Election
1
7
03
46
5
2
ACK
(a) (b)
Figure 4. Election algorithm.
Table 1. Parameters used in the simulations.
Parameter Value
Number of secondary user group 5, 8
Number of DCs 30
Bandwidth of each DC (not the same) 1~1.5Mbps
Required bandwidth of eac h group 4.5Mbps
Transmit power 0.01W
Transport protoc o l UDP
Update Phase: All DSUs switch to CC to update
each other about their channel conditions while all
non-DSUs continue communicating on their DCs.
2) CO-MAC: Lik e OS-MAC, CO-MAC [6], an oppor-
tunistic MAC protocol, uses two transceivers, a dedi-
cated CC, and N DCs. The time is divided into a number
of periodical time slots and each slot is divided into two
phases, namely, Reporting Phase and Negotiating Phase.
Reporting phase can be further divided into n mini-slots,
each of them corresponding to one of the n licensed
channels.
Reporting Phase: Each secondary user is equipped
with one SDR transceiver, and by using this trans-
ceiver only one of n licensed channels can be
sensed. Thus the secondary user is unable to accu-
rately know the states of all the channels by itself.
However, the goal of the Reporting Phase is to em-
power the secondary users to have a large picture of
all the channels’ states through their cooperatio n. In
particular, each secondary user senses a channel in
corresponding mini-slot, if the channel is idle, the
user sends a beacon during this mini-slot over the
control channel. Otherwise, no beacon is posted.
Negotiating Phase: the secondary users use the
control transceivers to negotiate about the data
channels among the secondary users by exchanging
request-to-send (RTS) and clear-to-send (CTS)
packets over the control channel. Meanwhile, the
only secondary user which is the winner in con-
tending for the data channels during the last time
slot uses the SDR transceiver to transmit data pack-
ets over all the unused licensed channels in the cur-
rent time slot.
For conciseness, we define the unused channels’ per-
centage in the entire system as, where N is the
number of channels licensed to primary users.
/
n
pnN
5.1. Throughput Analysis
In this section, we analyze the throughput with different
numbers of unused channels under three protocols: the
proposed MAC, OS-MAC and CO-MAC. The through-
put allows us to evaluate the protocols’ performance, that
is, the higher throughput, the higher performance.
H. SONG ET AL.
119
Copyright © 2009 SciRes. Wireless Sensor Network, 2009, 2, 61-121
Figure 5 shows the throughput comparison for our
proposed MAC protocol with the other two MAC proto-
cols under each of the two network scenarios: (Figure
5(a)) the number of secondary user groups with M=5 and
(Figure 5(b)) the number of secondary user groups with
M=8. First note that with the increase of unused chan-
nels' percentage (), the system throughput of these
three protocols all increases. Also, observe that the
throughput of the proposed MAC protocol increases
sharply while CO-MAC attains a bound rapidly and
OS-MAC increases little after reaching a fixed value.
This demonstrates that our proposed protocol can suffi-
ciently use the vacant channels with the increment of.
In OS-MAC, as only one channel can be used by a group
and all channel’s bandwidths are uniformly distributed
between 1Mbps and 1.5Mbps, when each group monopo-
lizes a channel (i.e., this group does not share the channel
with other groups), the overall throughput won't increase
much with the increment of but with the number of
secondary user groups (M). For example, in Figure 5 (a),
the system throughput of OS-MAC in is a
litter higher than which in
n
p
n
p
n
p
n
p
40%
n
p
30%
, because with the
increasing of, although each group cannot gain an-
other channel, it has more opportunity to switch to a
n
p
0
5
10
15
20
25
10 20 30 40 50 60 70 80 90 100
Transmission Throughput (Mbps)
Unused Channels (%)
proposed MAC
OS-MAC
CO-MAC
(a) The number of secondary user gr oups M = 5.
0
5
10
15
20
25
30
35
10 20 30 40 50 60 70 80 90 100
Transmission Throughput (Mbps)
Unused Channels (%)
Proposed MAC
OS-MAC
CO-MAC
(b) The number of secondary user groups M = 8.
Figure 5. The throughput with different number of unused
channels.
10
20
30
40
50
60
70
80
90
100
10 20 30 40 50 60 70 80 90 100
Unused Channels’ Usage (%)
Unused Channels (%)
Proposed MAC
OS-MAC
CO-MAC
(a) The number of secondary user gr oups M = 5.
10
20
30
40
50
60
70
80
90
100
10 20 30 40 50 60 70 80 90 100
Unused Channels’ Usage (%)
Unused Channels (%)
Proposed MAC
OS-MAC
CO-MAC
Proposed MAC
o
O
S
d
A
A
A
(b) The number of secondary user groups M = 8.
Figure 6. The unused channels’ usage with different num-
ber of unused channels.
“best” one, Moreover, in Figure 5(b), the throughput of
OS-MAC is much higher than the counterpart in Figure
5(a), because more vacant channels are utilized by
groups as M is increasing. In CO-MAC, since only a pair
of secondary users uses multiple vacant channels at any-
time and each pair's required bandwidth is the same in
our “best” one, Moreover, in Figure 5(b), the through-
put of OS-MAC is much higher than the counterpart in
Figure 5(a), because more vacant channels are utilized by
groups experiment, as soon as the required bandwidth
is satisfied, the entire throughput will not change. An-
other point that requires attention is that, in Figure 5(a),
the curve represented for the system throughput of our
proposed protocol is closed to a fixed value when
80%
n
p
. This is because of the throughput of the en-
tire system maintains a certain value and more vacant
channels are not needed when all groups’ bandwidth
requirements are satisfied.
5.2. Ratio of Unused Channels’ Utilization
Analysis
Figure 6 depicts the variation of unused channels’ utili-
zation rate (
) of the three MAC protocol with the
change of unused channels’ quantity (pn) under each of
H. SONG ET AL.
120
Copyright © 2009 SciRes. Wireless Sensor Network, 2009, 2, 61-121
the two network scenarios: (Figure 6(a)) the number of
secondary user groups with M=5 and (Figure 6(b)) the
number of secondary user groups with M=8. There are
three observations to make. First, the unused channels’
usage (
) of our proposed MAC protocol fluctuates
between 0.9 and 1.0, while the other two protocols’ un-
used channels' usage (
) decreases with the increase
of . Second, we find that, in the proposed protocol,
unless the secondary users’ bandwidth requirements are
satisfied, they utilize the vacant channels to their fullest
extent and conduct high channel usage. However, the
unused channels usage (
n
p
) in OS-MAC drops rapidly
with due to the utilization of fixed number of vacant
channels except for the increment of M. Third, in Figure
6 (a), the curve represented for
n
p
under our proposed
MAC protocol declines quickly after caused
by the satisfaction of all groups’ bandwidth requirements.
However, unlike CO-MAC, in the proposed protocol,
with the increment of , part of unused channels will
not be occupied by some secondary users whose band-
width requirements are already satisfied, thus these
channels are able to be allocated to other users who need
them. This is an important feature.
80%
n
p
n
p
0
1
2
3
4
5
30% 60% 90%
Unused Channels (%)
Throughput (Mbps)
group1
group2
group3
group4
group5
(a) The number of secondary user gr oups M = 5.
0
1
2
3
4
30% 60% 90%
Throughput (Mbps)
group1
group2
group3
group4
group5
group6
group7
group8
Unused Channels (%)
(b) The number of secondary user groups M = 8.
Figure 7. Group throughput with different pn in our pro-
posed protocol.
5.3. QoS and Fairness Analysis
In this subsection, we evaluate QoS and fairness of the
proposed MAC protocol.
Figure 7 presents each group’s throughput against
the different number of unused channels (30%
n
p
,
60%
n
p
and 90%
n
p
) under each of the two net-
work scenarios: (Figure 7(a)) the number of secondary
user groups with M = 5 and (Figure 7(b)) the number of
secondary user groups with M = 8. Commonly, the
throughput comparison of all groups with certain
reflects the fairness of the whole system, and the com-
parison of certain group’s throughput with different
shows the QoS support of the network. In Figure 7, we
observe that, with certain, the throughputs of all
groups are much closer. Especially when = 90%, the
groups’ throughputs ar e almost the same as those in Fig-
ure 7(a). We also note that the throughput of certain
group increases monotonically with the increment of.
For instance, in Figure 7(b), the throughput of Group1
raises from 1.365Mbps to 3.671Mbps as increases
from 30% to 60%.
n
p
n
p
n
p
n
p
n
p
n
p
Based on the simulation results, we can make the fol-
lowing conclusions. First, our proposed MAC protocol is
shown to be more efficient than the other two protocols
not only from throughput aspect, but also from unused
channels’ utilization aspect. Second, our proposed MAC
protocol greatly improves the quality of service for sec-
ondary users and guarantees the fairness of channels al-
location for groups.
6. Conclusions
In this paper, we have proposed a novel DSA-driven
MAC protocol for cognitive radio networks, In which,
secondary users are divided into several non-overlapping
groups, and each group uses bonding/aggregation tech-
nique to transmit data. All leftover channels are allocated
among groups taking the groups’ bandwidth require-
ments into consideration. Moreover, the allocation of
vacant channels can be adjusted dynamically when
members join/leave groups or primary users return/leave
the current network. The simulations indicate that our
proposed MAC protocol greatly improves the quality of
service for secondary users and maximizes the utilization
ratio of spectrum resources.
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