A Novel DSA-Driven MAC Protocol for Cognitive Radio Networks

doi:10.4236/wsn.2009.12017

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Wireless Sensor Network, 2009, 2, 61-121

doi:10.4236/wsn.2017 lished Online July 2009 (http://www.SciRP.org/journal/wsn/).

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.

113

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-

H. SONG ET AL.

114

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.

115

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

12 m

…

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.

H. SONG ET AL.

116

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

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)

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

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:

max

min( ,(/))

(//)

cnM

iji iii

st cn

brrB

BnMn

























(1)

where (1,,)

ci M





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)

H. SONG ET AL.

117

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,,)

ri M

/ ))

i i

B r







{, ,}

Px x

} (1,,)

(1,,)

Bi n

(/ /

B n









()tt n

min( ,()

rr Mn





{0,1

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



it. If the value of i

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

0{0,0,P

,0}

(1,,)t

BB

{S

1k

,,jk

,jk

I















(2)

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

(1) {|PP

(2) {|PP

0,}

xxP

1,}

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

() k





B



(6)

() kt

ii i

iik

PxB





B



 (7)

The lower and upper bounds of the optimal solution in

the entire feasible region D are

max( )

kPS P







(8)

max( )

kPS P







(9)

4) Pruning: For k



, 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





.

In this step, many subsets are pruned to accelerate the

searching speed.

5) Convergence: In this step, examine whether it is the

iteration.

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.

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.

118

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.

Election

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.

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

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

40%

p

30%



, because with the

increasing of, although each group cannot gain an-

other channel, it has more opportunity to switch to a

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.

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.

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.

100

10 20 30 40 50 60 70 80 90 100

Unused Channels’ Usage (%)

Unused Channels (%)

Proposed MAC

OS-MAC

CO-MAC

Proposed MAC

(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%



. 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

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 (



) 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



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%

p

30% 60% 90%

Unused Channels (%)

Throughput (Mbps)

group1

group2

group3

group4

group5

(a) The number of secondary user gr oups M = 5.

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%



60%



and 90%



) 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%.

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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