An Efﬁcient Cognitive Radio-EnAbled Multi-Channel MAC Protocol for Wireless Networks

7/26/2019 An Efficient Cognitive Radio-EnAbled Multi-Channel MAC Protocol for Wireless Networks

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978-1-4244-2100-8/08/$25.00 c2008 IEEE

CREAM-MAC: An Efficient Cognitive Radio-EnAbled

Multi-Channel MAC Protocol for Wireless Networks

Hang Su and Xi Zhang

Networking and Information Systems Laboratory

Department of Electrical and Computer Engineering

Texas A&M University, College Station, TX 77843, USA

Email:{hangsu, xizhang}@ece.tamu.edu

Abstract

Cognitive radio technology has emerged as the novel andeffective approach to improve the utilization of precious ra-dio spectrum. Employing the cognitive radio technology,secondary (unlicensed) users can opportunistically utilizethe unused licensed spectrum in a way that constrains thelevel of interference to the primary (licensed) users. How-ever, there are many new challenges associated with cog-nitive radio based wireless networks, such as the multi-channel hidden terminal problem and the fact that the time-varying channel availability is different for different sec-ondary users, in the medium access control (MAC) layer.To overcome these challenges, we propose an efficient Cog-nitive Radio-EnAbled Multi-channel MAC (CREAM-MAC)

protocol, which integrates the spectrum sensing at physi-cal layer and packet scheduling at MAC layer, over thewireless networks. Under the proposed CREAM-MAC pro-tocol, each secondary user is equipped with a cognitiveradio-enabled transceiver and multiple channel sensors.The proposed CREAM-MAC enables the secondary usersto best utilize the unused frequency spectrum while avoid-ing the collisions among secondary users and between sec-ondary users and primary users. In addition, we develop theanalytical models to quantitatively analyze our proposedCREAM-MAC protocol in the saturated network case. Wealso conduct simulation experiments to validate our devel-oped analytical models.

Keywords: Cognitive radios, multi-channel MAC proto-cols, opportunistic spectrum access, IEEE 802.11 DCF.

1 Introduction

As the requirements of the ubiquitous wireless serviceskeep growing, the number of variant wireless standards in-creases, which consequently imposes increasing stress on

The research reported in this paper was supported in part by the U.S.

National Science Foundation CAREER Award under Grant ECS-0348694.

the fixed and limited radio spectrum. However, extensivemeasurements reported indicate that large part of licensed

bands are in low utilization, for example, only 6% in mostof the time in U.S. [1]. Furthermore, even when a channel isactively used, the bursty nature of most data traffics still im-plies that a great amount of opportunities exist in using thespare spectrum. It is clear that the current frequency alloca-tion policies, under which each wireless service is assigneda fixed frequency band, need to be modified to better utilizethe licensed spectrum bands. In result, the Federal Com-munication Committee (FCC) has recently suggested a newconcept/policy fordynamicallyallocating the spectrum [2].This is the basis of the cognitive radio, which is proposedto take advantage of this more open spectrum policy for al-leviating the severe scarcity of spectrum bandwidth.

Cognitive radio is typically built on the software-definedradio (SDR) technology, in which the transmitters operat-ing parameters, such as frequency range, modulation type,and maximum transmission power, can be dynamically ad-

justed by software [3]. In the cognitive radio networks,thesecondary(unlicensed) users can dynamically tune theirtransceivers to the identified vacant channels in the spec-trum to communicate among themselves with the limited in-terference on the communications of the primary(licensed)users. Although the basic idea of cognitive radio is sim-ple, the efficient design of cognitive radio networks imposesthe new challenges that are not present in the conventionalwireless networks [47]. Specifically, the secondary usersmay encounter different characteristics of channel spectrumover frequency, time, and space because of variant behav-iors (i.e., channel usage patterns) of primary users, user mo-bilities, and wireless channel variations.

Accordingly, identifying the varying channel availabilityintroduces a number of nontrivial design problems to themedium access control (MAC) layer. One of the most dif-ficult, but important design problems is how the secondaryusers decide when and which channels they should use totransmit/receive the secondary users packets without af-fecting the communications among the primary users. Thisproblem becomes more challenging because there are no


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centralized controllers, such as basestations or accessingpoints, used in the wireless ad hoc networks.

Several decentralized cognitive MAC protocols [813]have been proposed recently. The authors of [8] proposed acognitive MAC with statistical channel allocation, in whichthe secondary users select the channel that has the highest

successful transmission probability to send packets basedon the channel statistics. However, the computational com-plexity determining the successful transmission probabili-ties increases quickly with the number of licensed chan-nels. The author of [9] proposed a multi-channel oppor-tunistic MAC protocol, which however targets only at theGlobal System for Mobile Communications (GSM) cellularnetworks. The authors of [10] developed a cognitive MACprotocol based on the partially observable Markov decisionprocesses (POMDPs) framework. In [11], we proposed op-portunistic MAC protocols with random and negotiation-based sensing policies for the time-slotted wireless net-works. In [12], we developed a channel hopping-based cog-

nitive MAC protocol which enables the secondary users toconduct channel negotiations at multiple rendezvous. How-ever, the schemes in [1012] require global synchronizationbetween primary and secondary users, which is not easy toimplement. The authors of [13] proposed a cognitive MACprotocol aiming to opportunistically utilize the TV broad-cast bands. However, the proposed protocol is costly andcomplicated as it requires not only a cognitive radio basedtransceiver but also a regular radio receiver, which operateon the unused TV channels and the control channel, respec-tively.

To amend the aforementioned problems of the exist-ing schemes, in this paper we propose an efficient Cogni-

tive Radio EnAbled Multi-channel MAC protocol, calledCREAM-MAC protocol, for wireless networks. Under theCREAM-MAC protocol, each secondary user is equippedwith a SDR-based transceiver that can dynamically uti-lize one or multiple licensed channels to receive/transmitthe secondary users packets, and multiple sensors thatcan detect multiple licensed channels simultaneously. TheCREAM-MAC protocol enables the secondary users to dy-namically utilize the unused licensed frequency spectrumin a way that confines the level of interference to the pri-mary users. With the help of the four-way handshakes ofcontrol packets, the CREAM-MAC protocol with a sin-gle transceiver can efficiently handle the traditional hid-den terminal and the multi-channel hidden terminal prob-

lems. In addition, the CREAM-MAC protocol does notneed any centralized controllers. We also study the aggre-gate throughput of CREAM-MAC based on our developedanalytical models.

The rest of this paper is organized as follows. Sec-tion II presents the system model. Section III develops theCREAM-MAC protocol. Section IV develops the analyti-cal models to study the CREAM-MAC protocol. Section Vevaluates our multi-channel MAC protocol by using our de-veloped analytical models and simulation experiments. Thepaper concludes with Section VI.

2 The System Model

Consider that there are two non-cooperating types ofusers, namely primary users and secondary users. The pri-mary users, for example, TVs, cellular phones, or wire-less microphones, are those to which an amount of wire-

less spectrum is licensed. On the other hand, the secondaryusers are referred to those without pre-assigned wirelessspectrum. However, the secondary users equipped with thecognitive radios can transmit their own packets by seizingthe opportunities that the primary users do not use the li-censed wireless spectrum. In this paper, the wireless spec-trum accessible to the secondary users is further dividedinto a number of channels, each with a fixed amount of fre-quency bandwidth.

2.1 Primary Users Behaviors

Suppose that a spectrum licensed to the primary usersconsists ofM channels. We assume that for each channel,

the channel usage pattern of the primary users follows inde-pendent and identically distributed (i.i.d.) ON/OFF randomprocess. An ON-period represents that the channel is occu-pied by the primary users. An OFF-period represents thatthe channel is vacant and thus can be opportunistically usedby the secondary users. Suppose that the ON- and OFF-periods on each channel are independent. Note that the av-erage ON- and OFF-periods depend on the channel usagepattern of the primary users. In this paper, we assume thatthe length of ON- and OFF-periods for i-th licensed chan-nel follows exponential distribution with means equal to iandi, respectively. If we denote i as the probability thati-th channel is occupied by the primary users, then we have

i = i

i+ i, (1)

where1 i M. Note thati also represents the channelutilization ofi-th channel with respect to primary users. Inpractice, the secondary users may impose interference onthe primary users when the secondary users opportunisti-cally access the licensed channels. To effectively limit theinterference, a maximum tolerable interference period, de-noted by Tmaxd , is employed as a hard protection criteria forthe primary users. Clearly, different kinds of primary usersmay imply differentTmaxd s [14].

2.2 Channel Aggregating Technology

After the secondary users sensing the licensed channelsfor a period of time, they have information of the licensedchannel conditions. By using this information, the sec-ondary users can opportunistically utilize multiple unusedchannels simultaneously. However, in most cases, the un-used channels are discontinuous. Fortunately, the orthogo-nal frequency division multiplexing (OFDM) has been in-troduced to help the secondary users aggregate the discon-tinuous channels. In particular, the cognitive radios withOFDM can switch on and off the corresponding subcarri-ers based on channel availability, and thus access multiple


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continuous/discontinuous unused channels simultaneously.

3 The CREAM-MAC Protocol

3.1 Protocol Overview

There are many challenges imposing on the design ofMAC protocols for the cognitive radio networks. Out ofthem, the following three problems are most significant: (i)the problem when to transmit data packets in a way thatlimits the interference on the primary users, (ii) synchro-nization between the secondary sender and the secondaryreceiver due to the difference of the channel availability be-tween them, and (iii) the traditional hidden terminal prob-

lem and the multi-channel hidden terminal problem1. Keep-ing these in mind, we start to develop the CREAM-MACprotocol under which the secondary users can dynamicallyutilize the vacant licensed channels.

The CREAM-MAC protocol employs a common controlchannel as the rendezvous where secondary users exchangecontrol packets for multi-channel resource reservation. Thecontrol channel can be either statically assigned or dynam-ically selected. Under the statical case, the control chan-nel can be either specially licensed to the secondary usersby FCC or use the unlicensed spectrum band (e.g., 2.4GHzspectrum for IEEE 802.11b/g). On the other hand, for thedynamical case, the control channel can select the most re-liable one from the unused channels which are licensed tothe primary users [15]. In this paper, we will not delve intowhich way the control channel is selected and we only as-sume that control channel is alwaysreliable and available.

Under the CREAM-MAC protocol, each secondary useris equipped with n sensors, such that at most n licensedchannel can be sensed at one time. After sensing the li-censed spectrum for a period of time, each secondary userhas the information of the channel states in these spec-trum bands. Then, the secondary users can opportunisti-cally access the vacant channels which are not occupied bythe primary users. Since the interference from secondaryusers transmission must be constrained in a modest levelwhich the primary users can tolerate, we limit each chan-nel access time of secondary users to be not more than themaximum tolerable interference period (Tmaxd ). In practice,Tmaxd should be carefully designed such that for each chan-nel access duration of the primary users the average channel

occupation time is much larger than Tmaxd . Thus, the con-

straint that each opportunistically access of the secondaryusers does not exceed Tmaxd time unit ensures that the pri-mary users only experience the acceptable and limit inter-ference from the secondary users.

The key of the CREAM-MAC protocol is to employfour types (two pairs) of control packets, namely, Ready-to-

1In multi-channel systems, especially those with only one single

transceiver, the multi-channel hidden terminal problem emerges. The rea-

son is that a single transceiver may only operates on only one channel,

which makes it difficult to use virtual carrier sensing to handle the hidden

terminals [16].

Send/Clear-to-Send (RTS/CTS) packets and Channel-State-Transmitter/Channel-State-Receiver (CST/CSR) packets, toimplement the channel negotiation. All of the above fourtypes of control packets are exchanged over the controlchannel, as shown in Figure 1. First, the functions of theRTS/CTS control packets include (i) reserving the control

channel and (ii) solving the hidden terminal problem. Thesecondary senders start the channel negotiation by trans-mitting RTS packets over the control channel based in acontention way. Without loss of generality, we adopt bi-nary exponential backoff (BEB) based IEEE 802.11 Dis-tributed Coordination Function (DCF) [17] as the mediumcontention mechanism.

The handshakes of RTS/CTS can prevent the neighbor-ing secondary users from selecting the same channels totransmit data, resulting in no collisions between the sec-ondary users. Thus, exchanging the RTS/CTS control pack-ets can efficiently solve the hidden terminal. Second, thefunction of the CST/CSR handshakes is to synchronize thevacant channel information between the sender and the re-

ceiver, and thus to prevent the collisions between the sec-ondary users and the primary users. The CST packet in-cludes the lists of the vacant channels at the transmittersside, while the CSR packet includes the lists at the receiversside. The exchange of the CST/CSR packets ensures thatthe secondary sender and the secondary receiver select theset of the vacant channels, which are shared by both ofthem.

In short, the goal of the RTS/CTS control packets is toprevent the collisions among the secondary users, while theobject of the CST/CSR control packets is to avoid the col-lisions between the secondary users and the primary users.We will detail the process how the control packets are ex-

changed over the control channel in Section 3.3.

3.2 Selection of Licensed Channels

Since the licensed channels are sometimes utilized by theprimary users unevenly, some licensed channels may be uti-lized more than the others. Because the secondary users canonly sense a limited number of licensed channels simulta-neously, in order to fully utilize the licensed channels, thesecondary users need to select those which are used less in-tensively to sense. At the beginning, the secondary usersrandomly select a number (n) of channels to sense. Theycan update the statistical utilization information of licensedchannels in either the non-cooperation way or the cooper-

ation way. In the non-cooperation way (e.g., the POMDPscheme [10]), the secondary users update its channel bythemselves without exchanging information. On the otherhand, the cooperation based channel selection allows thesecondary users to exchange information such that the sec-ondary users can learn the global channel states. Comparedwith the non-cooperation based schemes, the cooperationbased schemes allow the secondary users to obtain the up-dated channel states more accurately and quickly, but needmore communication overheads. We adopt the cooperation-based scheme for the CREAM-MAC protocol. In particu-lar, the channel state information is embedded in the control


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RTS CTS CST CSR

Opportunistic Spectrum Access by A Secondary UserOcc upied by Primary Users

Occu pied b y Primary Users

ChannelGroup

CH 1

CH 2

CH 3

...

Control

Channel

...Channel Negotiation Contention

Backoff

Time

Frequency

Contention

Opportunistic Spectrum Access by A Secondary User

Backoff

DIFS SIFS SIFS SIFS

RTS

DIFS

Figure 1. Illustrations of the CREAM-MAC protocol.

packets. Thus, the secondary users can obtain the neigh-bors channel state information by overhearing the controlpackets over the common control channel . After obtainingthe statistical utilization information of the licensed chan-nels, the secondary users select n of the licensed channels,termed achannel group, to sense by using then sensors.

3.3 Channel Contention

The CREAM-MAC does not require global synchroniza-tion among all the primary users and the secondary users.Under the CREAM-MAC, the contention mechanism overthe control channel is similar to the IEEE 802.11 DCF, asshown in Figure 1. In particular, the secondary users re-serve time for the following transmission operations withinthe neighborhood through the control channel by exchang-ing RTS/CTS control packets with the destinations. When asecondary user wants to send packets to another secondaryuser, it first transmits a RTS packet including its channelgroup list to the destination over the control channel. Uponreceiving the RTS packet, if at least one channel in the chan-nel group is currently not used by its neighboring secondaryusers, the destination secondary user replies to the sourcewith a CTS packet and uses its sensors to detect the channelgroup indicted in the RTS packet. The other neighboringsecondary users overhear the RTS/CTS control packets toupdate the list of available channels.

After the pair of secondary users successful reserve thecontrol channel by successfully exchanging RTS/CTS pack-ets, they negotiate on the licensed channels which are va-cant for both the sender and the receiver. More precisely,the source secondary user first sends the CST packet whichincludes the vacant channel list at the senders side. Uponreceiving the CST packet, the destination secondary userreplies with the CSR packet telling the source which com-mon channels are vacant and how long the communicationwill last over these common channels. Since the communi-cation interval can be less than or equal to Tmaxd , the otherneighboring secondary users can overhear the CST/CSR

packets to precisely predict when the channels used by thispair of secondary users will be released.

In order to decrease the collision probability of the con-trol packets, the secondary user, which attempts to send aRTS packet, selects a backoff counter within a contentionwindow and maintains a contention window size. At theinitial state, the contention window size, denoted byCW isset to a predefined value, denoted by C Wmin. The backoffcounter is randomly chosen from[1, CW]and deducted byone after a time slot during which both the control chan-nel and at least one channel in the channel group are idle.Otherwise, the backoff counter holds until the channels be-come idle again. Once the backoff counter reaches zero, the

secondary user tries to reserve the control channel by send-ing a RTS to the destination. The binary exponential back-off mechanism is employed when collisions occur. That is,after each unsuccessful transmission, CW is doubled, upto the maximum value C Wmax = 2

mCWmin, wherem iscalled maximum backoff stage. After the successful trans-mission, the value ofCWis reset to beC Wmin.

The handshakes of RTS/CTS can only solve the tradi-tional hidden terminal problem, but not the multi-channelhidden terminal problem. Specifically, the secondary userswhich just finished the data transmission over the licensedchannels may miss their neighbors control packets whiletheir transceivers worked over the licensed data channels.

They may probably win the control channel contention, andthen enter the licensed channels over which their neighborsare receiving data. Consequently, these secondary users be-come the hidden terminals interrupting their neighbors on-going communications. To avoid this happening, we needto put additional rules on CREAM-MAC. In particular, thesecondary users which just finished the data transmissioncan only select the same channel group which they just re-leased within a waiting periodofTmaxd . After the waitingperiod ofTmaxd , these secondary can select any other chan-nel groups to use. It allows these secondary users to haveenough time to observe the current spectrum activities be-


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fore they start packet transmissions and prevent them frominterfering the neighbors ongoing communications, sincethe maximum time interval that the secondary users can oc-cupy the licensed channels each time is Tmaxd . During thewaiting period, if these secondary users receive any controlpackets, they can obtain the updated channel state from the

control packets. Otherwise, it is safe for these secondaryusers to assume that all the licensed channels are not be-ing used by any secondary users after the waiting period,which can efficiently solve the multi-channel hidden termi-nal problem. Note that the same rules also apply to the newsecondary users which first join the network.

3.4 Data Transmission

As shown in Figure 1, after the four-way handshakesof the control packets, the secondary users which win thechannel groups start their data transmission over these chan-nel groups by using the channel aggregating technology asdiscussed in Section 2.2.

4 The Analytical Models

In this section, we develop the analytical models to an-alyze the aggregate throughput of our proposed CREAM-MAC protocol under the saturated network case, whereeach secondary user has always unlimited data packets tosend.

4.1 Analysis for The Licensed Data Chan-nels

Suppose that there are M licensed data channels andeach secondary user is equipped with n sensors. Based

on the CREAM-MAC protocol, the secondary users can re-serve at most n licensed channels, and utilize all of themwith the help of channel aggregating technology if n li-censed channels are unused by the primary users. ThereareM/nchannel groups that can be utilized by the sec-ondary users.

Denote discrete random variableHas the number of va-cant channels in a specified channel group with n licensedchannels. To make the model tractable, we assume that eachchannel is evenly utilized by the primary users. Thus, weapply the sameto all licensed channels, i.e., = i =jfor 1 i, j M, where i is given in Eq. (1). Sincethe channel states among different channels are indepen-dent with each other, the probability that the number (H)of vacant channels in a specified channel group isi followsthe binomial distribution, that is,

Pr{H=i}=

n

i

(1 )ini. (2)

Thus, the average number, denoted by E[H], of vacantchannels can be derived by

E[H] =ni=0

i Pr{H=i}= n(1 ). (3)

4.2 Analysis for The Control Channel

In order to analyze the saturation throughput of the pro-posed CREAM-MAC, we need to study the contention be-havior over the control channel where the control packetsare transmitted based on the IEEE 802.11 DCF. We developthe analytical model based on the work of [18, 19], whichuses a two-dimensional Markov chain model to analyze thebackoff operations for IEEE 802.11 DCF. Following theprevious work, if we denote the probability that a given sec-ondary user transmits in a randomly chosen slot time by ,and the probability that a transmitted packet collides by p,respectively, then we obtain the following equations:

(p) = 2(12p)(12p)(CWmin+1)+CWminp[1(2p)m]

p() = 1(1 )u1(4)

wherem is the maximum backoff stage, u is the number ofthe contending secondary users, C Wmin is the initial con-tention backoff window size. Solving simultaneously the

two equations in Eq. (4), we can obtain the numerical so-lution of and p. Obviously, 0 < , p < 1. ObservingEq. (4), we can learn thatp only depends on the number ofthe contending secondary users (u), the maximum backoffstage (m), and the initial contention backoff window size(CWmin).

LetPtr be the probability that there is at least one trans-mission in a given time. Since each contending secondaryuser transmits with probability at any given time, giventhere are u contending secondary users, Ptr can be ex-pressed as:

Ptr = 1(1 )u (5)

The probability, denoted byPs, that a secondary user trans-

mits successfully without collisions, given that at least onesecondary user transmits, can be written as:

Ps =u(1 )u1

Ptr=

u(1 )u1

1(1 )u (6)

Denote the duration of a time slot by. Under the IEEE802.11 DCF, the backoff counter of the contention node de-creases by 1 when the sensed channel is idle in a time slot.However, it is worth noting that that under the CREAM-MAC protocol, only when both the control channel and atleast one channel in the channel group is idle, the backoffcounters of the contending secondary users decrease by 1.That is, if all of channels in the channel group are busy, the

backoff counter should remain the same until the time slotwhere control channel and at least one channel in channelgroup are idle. Therefore, we introduce the effective du-ration of a time slot, denoted by , which represents theaverage duration of a time slot after taking the above caseinto account. The effective duration of a time slot includesthe duration of the normal time slot and the average dura-tion of time slots where the backoff counter holds due to allthe channels in the channel group being busy. Thus, canbe derived by

=(1 + E[Nbusy]), (7)


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Table 1. The parameters for design and anal-ysis of the CREAM-MAC protocol.

RTS 2 0 B The leng th of RTS packet

CTS 2 0 B The leng th of C TS packet

CST 2 0 B The leng th of CST packet

CSR 2 0 B The leng th of CSR p acket

9 s Time-slot intervalSIFS 15 s Short interframe space

DIFS 34 s DCF interframe space

Rc 1 Mbps Transmission rate of the control channel

Rd 1 Mbps Transmission rate of a licensed channel

n The number of sensors each secondary user has

u The number of contending secondary users

Channel utilization of primary users

M The number of licensed channels

E[Tc] Avg. time for successful four-way handshakesTmaxd

10 ms Max.tolerable interference-time of primary users

CWmin 256 The minimum size of contention window

whereNbusy is the random number of time slots in which

all the channels in a channel group are busy between thetwo immediate time slots where the backoff counter de-creases, and E[Nbusy] is the mathematical expectation ofNbusy. Since the channel states in different time slots areindependent,Nbusy follows the geometric distribution, andthus its probability mass function (pmf) can be obtained by

Pr{Nbusy= i}= Pi

busy(1 Pbusy), (8)

wherePbusy = Pr{H = 0} is the probability that all thechannels in a channel group are busy. According to Eq. (2),we havePr{H= 0}= n. Then, we can get E[Nbusy]by

E[Nbusy] =

i=0

i Pr{Nbusy= i}=

n

1 n . (9)

Hence, substituting Eq. (9) into (7), we can calculate theeffective duration () of a time slot.

Let Tsuccand Tcollbe the time used for successful trans-mission and the time spent when collisions happen, respec-tively. Then,Tsuccand Tcoll can be expressed as:

Tsucc= RTS+CTS+CST+CSR

Rc+ 3SIFS+DIFS,

Tcoll = RTSRc

+DIFS,(10)

whereRc is the transmission rate of control channel, SIFS

is the duration of the short interframe space, DIFS is theduration of DCF interframe space, RTS, CTS, CST, andCSR are the lengths of RTS, CTS, CST, CSR control pack-ets, respectively. Then, the average time, denoted by E[Tc],that is spent for the successful four-way handshakes ofRTS/CTS/CST/CSR can be given by:

E[Tc] = (1 Ptr) + PsPtrTsucc+ Ptr(1 Ps)Tcoll

PsPtr

= Tsucc+1 Ptr

PsPtr +

1 PsPs

Tcoll. (11)

Exchange Data o ver ChannelGroup 1

Contention

Period

Exchange Data overChannel Group 3

Exchange Data over

Channel Group 4

Contention

Period

Contention

Period

Contention

Peri od

Exchange Data o ver Channel

Group 2

Cont rol

Channel

ChannelGroup 1

ChannelGroup 2

ChannelGroup 3

Channel

Group 4

t

t

t

t

t

ChannelNegotiation

for CG 1

ChannelNegotiation

for CG 2

ChannelNegotiation

for CG 1

Contention

Peri odContention

PeriodIdle

Channel

Group 1

Channel

Group 2

Cont rolChannel t

t

t

Idle ContentionPeriod

Exchange Data over

Channel Group 1

Exchange Data over Channel

Group 2

Exchange Data over Channel

Group 1

Exchange Data o ver Channel

Group 2

Data

(a)

(b)

ChannelNegotiation

for CG 1

ChannelNegotiation

for CG 2

ChannelNegotiation

for CG 3

ChannelNegotiation

for CG 4

Figure 2. Illustrations of the CREAM-MAC protocol forthe saturated network case. (a) The number of channel

groups is larger than (Nc+ 1). (b) The number of channelgroups is less than(Nc+ 1). Here CG is short for channelgroup.

4.3 Aggregate Throughput

For convenience of presentation, Table 1 lists the im-portant parameters for the design and analysis of the pro-posed CREAM-MAC protocol. LetNc be the maximumnumber of secondary users that successfully reserve the li-censed channel groups during the length ofTmaxd on av-erage. Clearly Nc is inversely proportional to E[Tc], andthus we obtain Nc = T

maxd /E[Tc]. Note that on average

the number of secondary users, denoted by Nd, that can si-multaneously utilize the licensed channels depends on notonly the number of secondary users that can win the con-tention over control channel in duration ofTmaxd , but alsothe number of licensed channel groups. Then, we obtain

Nd = min(Nc+ 1),

M

n . (12)

That is, there are at mostNdsecondary users opportunis-tically transmitting data over the licensed channels at onetime from the global viewpoint. Note that we can predictif the control channel get saturated by identifying the rela-tionship between (Nc + 1) and (M/n). Figures 2(a) and(b) show the cases when the number of channel groups islarger than and less than (Nc + 1), respectively. In par-ticular, when Nd = (Nc + 1), it indicates that the con-trol channel is saturated and the aggregate throughput onlydepends on the pairs secondary users that can successfullyexchange RTS/CTS/CST/CSR packets during the period of


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16 32 64 128 256 512 10249.8

10

10.2

10.4

10.6

10.8

11

11.2

11.4

Size of Contetion Window

AggregateThroug

hput(Mbps)

u=20u=30u=50

Figure 3. The aggregate throughput against the size ofcontention window (CWmin). The number (n) of sensors

is 4. The channel utilization () of primary users is 0.5. Rc

and Rdare equal to 1 Mbps.

Tmaxd . On the other hand, when Nd = M/n, that is(Nc + 1) > (M/n), the control channel is not the bot-tleneck, and thus the secondary users can fully utilize thevacant licensed channels.

Since the aggregate throughput, denoted by , is propor-tional toNd, we can derive as

=Tmaxd NdE[H]Rd

E[Tc] + Tmaxd=

Tmaxd Ndn(1 )RdE[Tc] + Tmaxd

, (13)

whereRd is the data rate of a licensed channel, E[H]is the

average number of vacant channels and is given by Eq. (3).

5 Performance Evaluations

The parameters used to evaluate the CREAM-MAC pro-tocol are summarized in Table 1. We first investigate theaggregate throughput for the saturated network case. Letthe number (n) of sensors of each secondary user be 4, thechannel utilization () of primary users be fixed at 0.5, andRc be equal to 1 Mbps. Using Eq. (13), we plot the aggre-gate throughput () against the size of the contention win-dow (CWmin) in Figure 3. In Figure 3, we observe thatthe optimal C Wmin, denoted by C Wmin, which achieves

the highest aggregate changes with the different number (u)of contending secondary users. This is expected becausegiven there are sufficient licensed channels, the aggregatethroughput only depends on the time spent to accomplishthe RTS/CTS/CST/CSR four-way handshakes over the con-trol channel, which is ultimately determined by the IEEE802.11 DCF parameters, such as CWmin and Rc. If wecan obtain the number of contending secondary users in ad-vance, we can preselect the optimal CWminwhich achievesthe highest aggregate throughput. On the other hand, if thenumber of contending secondary users dynamically fluctu-ates, we can adopt the algorithms proposed in [18] to adjust

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

2

4

6

8

10

12

14

16

18

20

Channel Utilization of Primary Users ()

AggregateThroughp

utinMbps()

n=1, Rc=1 Mbps

n=1, Rc=2 Mbps

n=2, Rc=1 Mbps

n=2, Rc=2 Mbps

Figure 4. The aggregate throughput against the channelutilization of primary users, when the number (M) of li-

censed channels is 30, the number (u) of contending sec-

ondary users is 30, and CWmin= 256.

the value ofC Wmin on the fly. In the rest of our paper, weassume that in each scenario the number of contending sec-ondary users is fixed, and thus we can preselect CWminfordifferent scenarios with differentus.

After setting optimal CWmin to be 256 for the casewhere u= 30, we utilize Eq. (13) to get numerical results ofthe aggregate throughput against the channel utilization ofprimary users as shown in Figure 4. The aggregate through-put () decreases as the channel utilization () of primaryusers increases, which implies that the secondary users getless opportunities to transmit their own packets if the pri-mary users utilize the licensed channels more intensively.Figure 4 also shows that more number of sensors equippedin a secondary user can lead to higher aggregate through-put. However, the more sensors per secondary user, thehigher the hardware cost. An alternative way to improvethe aggregate throughput with the rigor hardware cost con-straint is to increase the data rate of the control channel. Forexample, consider Scenario I where each secondary user isequipped with a sensor and the data rate of the control chan-nel is 2 Mbps and Scenario II where each secondary useris equipped with two sensors and the data rate of the con-trol channel is 1 Mbps. As shown in Figure 4, the aggre-gate throughputs achieved by Scenario I and Scenario II are

close regardless of the channel utilization of primary users.Then, we evaluate our proposed CREAM-MAC proto-

col in the saturated network case using the Matlab basedsimulator. In the simulations, the secondary users form anone-hop cognitive radio network. Figure 5 shows the sim-ulation and analytical results given the number (M) of li-censed channels is 30. Each point in the simulation plots ofFigure 5 is the mean of the results of 500 simulations. FromFigure 5, we conclude that the simulation results agree wellwith the analytical results.

As shown in Figure 5, the aggregate throughput linearlyincreases as the number of sensors equipped in a secondary


8/8

0 1 2 3 4 5 60

2

4

6

8

10

12

14

The number of sensors (n)

AggregateThroug

hputinMbps()

Rc=2 Mbps, Analytical

Rc=2 Mbps, Simulation

Rc=1 Mbps, Analytical

Rc=1 Mbps, Simulation

Rc=2 Mbps

Rc

=1 Mbps

Figure 5. The aggregate throughput against the numberof sensors in each secondary user, when the channel uti-

lization () of the primary users is 0.5, the number (M) of

licensed channels is 30, the number (u) of contending sec-

ondary is 30, and CWmin= 256.

user increases before they reach the state where all the li-censed channels are saturated. This is expected by Eqs. (12)and (13). More precisely, when the secondary users canonly access small number of channels simultaneously withless sensors, there are sufficient channel groups for the sec-ondary users to access. As a result, the control channel be-comes saturated, which means that the average number ofwinning secondary users during a fixed amount of time isconstant. Thus, increasing the number of sensors can ef-ficiently increase the aggregate throughput. On the other

hand, when the secondary users are equipped more sensors,they can access more channels at one time, which imply-ing that the control channel is not the bottleneck any more.In this case, the licensed data channels become saturated,and thus further increasing the sensors cannot enlarge theaggregate throughput. However, as shown in Figure 5, wecan still increase the aggregate throughput by increasingthe data rate of the control channel because higher controlchannel data rate means less time spent to accomplish theRTS/CTS/CST/CSR four-way handshakes.

6 Conclusions

We proposed and analyzed the CREAM-MAC protocol

for the cognitive radio based wireless ad hoc networks.Under the CREAM-MAC protocol, each secondary user,which is equipped a cognitive radio and multiple channelsensors, seizes the opportunity where vacant licensed chan-nels are available to exchange their own packets while caus-ing insignificant interference to the primary users. Althoughour proposed CREAM-MAC protocol does not need anycentralized controllers, it can solve both the traditional andmulti-channel hidden terminal problems by introducing thefour-way handshakes of control packets over the controlchannel. Applying the IEEE 802.11 DCF based model, wedeveloped analytical models to evaluate the performance of

our proposed CREAM-MAC protocol. The simulation re-sults verified our developed analytical model.

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An Efﬁcient Cognitive Radio-EnAbled Multi-Channel MAC Protocol for Wireless Networks

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