an efficient mac protocol for cooperative diversity in mobile ad hoc networks

WIRELESS COMMUNICATIONS AND MOBILE COMPUTINGWirel. Commun. Mob. Comput. 2008; 8:771–782Published online 27 April 2007 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/wcm.525

An efficient MAC protocol for cooperative diversityin mobile ad hoc networks

Md. Rajibul Islam∗,† and Walaa HamoudaDepartment of Electrical and Computer Engineering, Concordia University, Montreal, Quebec, H3G 1M8,Canada

Summary

Cooperative diversity is proposed to combat the detrimental effects of channel fading. In this paper, we investigatethe effectiveness of cooperative diversity in interference limited ad hoc networks. The negative effects due to relayblocking on the network throughput are investigated. We show that the relay blocking problem is mainly dependenton the relay selection criterion. To overcome this problem, we propose a new cooperative diversity technique basedon a modified IEEE 802.11 Medium Access Control (MAC) protocol. The throughput performance of the proposedMAC protocol is analyzed using a random structured network where nodes are assumed to be equipped with multipleantennas. In our simulations, we consider both single- and multiple-relay scenarios over fading channels. Copyright© 2007 John Wiley & Sons, Ltd.

KEY WORDS: ad hoc networks; medium access control; cooperative diversity

1. Introduction

Mobile ad hoc networks (MANET) are distributedas well as infrastructure-less networks where mobilestations can communicate with each other without thehelp of any centralized control or access point [1]. Inthis case, all the mobile stations within a MANETshare the same RF channel. This type of network iswidely used when an infrastructure-less network ispreferable. Recently, extensive research has focused onthe development of MANETs.

In wireless channels, signal fading is a majorimpairment that if not compensated for can cause se-rious system degradations. To mitigate the detrimentaleffects of signal fading, several diversity techniques

*Correspondence to: Md. Rajibul Islam, Department of Electrical and Computer Engineering, Concordia University, Montreal,Quebec, H3G 1M8, Canada.†E-mail: rajib is/[email protected]

have been proposed [2]. In MANETs, instead ofbeing idle, the neighboring stations to the source anddestination nodes can cooperate in a structured mannerto provide diversity gains at the destination node. Thespatial diversity in cooperative networks is providedby a relay station in the neighborhood of either thesource or destination node. Two proposed strategiescan be employed at the relay node. The first is referredto as Amplify-and-Forward (AF) mode, in which therelay amplifies the received signal (according to powerconstraints) before retransmission to the destinationnode. In the second cooperative strategy, known asDecode-and-Forward (DF) mode, the received signalfrom the source is first demodulated, decoded, andthen regenerated for subsequent transmission to the

Copyright © 2007 John Wiley & Sons, Ltd.

772 MD. R. ISLAM AND W. HAMOUDA

destination node. In these two strategies, the relayplays the role of virtual antenna at the source whereit re-sends the received signals from the source tothe receiving station over different independent fadingchannels. Subsequently, the destination node receivesthe same signals but over two independent fading links.

The distributed coordination function (DCF) of theIEEE 802.11 [1] is designed for wireless stationscommunicating in ad hoc scenarios. The carrier-sensemultiple-access with collision avoidance (CSMA/CA)of the IEEE 802.11 allows stations to communicatewith each other by sharing a single channel. In thiscase when two stations are granted access to thechannel, neighboring stations remain silent and waitfor future access to the channel. Within the context ofrelay networks, these silent neighboring nodes of thetransmitter and/or the receiving stations can be used asrelay stations to improve the received signal quality atthe destination node. This type of cooperative diversity[2] has been widely discussed in the literature (seeReferences [3–9] and references therein).

In this paper, we investigate the performance ofcooperative diversity in ad hoc networks. Without lossof generality, in our study, we consider the AF modeof operation [4,5]. We show that although the appli-cation of cooperative techniques can offer significantperformance gains at the physical layer, it can bringan overall throughput degradation in the network.To solve the relay blocking problem, we propose anew MAC protocol suited for cooperative networks.The proposed protocol employs adaptive antennas atthe relay stations. Our results show large throughputimprovements when considering both single- andmultiple-relay transmission. Other related works on theimplementation of MAC protocols in multiple antennasystems include the work in References [10–16].In these works, multiple antennas are used to improvethe overall network throughput but no cooperativetransmission is involved. Also, most previousworks only consider the performance over additive-white Gaussian noise (AWGN) channels whereashere we investigate the performance over fadingchannels.

2. Related Work

The idea of cooperative relay networks was firstdiscussed in 1971 by Van Der Meulen [17] wherethe classical models for a class of three terminalcommunication channels were examined. Later, a novelwork on cooperative communication for relay channels

was presented by Cover and Gamal in Reference [18].In their work, the authors mainly focused on theinformation theoretic properties of the degraded relaychannels. Though there was some isolated works donein this field in the 80s and 90s, not until recentlycooperative communication networks have receiveda great deal of attention. The use of multiple relayshave been examined in many references [19–26].In multipath fading channels, Sendonaris et al. inReferences [3,27,28] were first to propose the conceptof user cooperation diversity where it was appliedto code-division multiple-access (CDMA) systems.In References [3] and [28], two mobile users act as‘partners’, each sending its own data as well as a portionof its partner’s to a common destination. It is shownthat, in an information theoretic sense, cooperationenlarges the rate region and increases the sum rate of thetwo mobiles [28]. In Reference [27], the same authorsdiscussed cooperative diversity as a mean to increasethe uplink capacity of cellular networks.

Recently, Laneman et al. [5] proposed possiblelow-complexity two-stage relay strategies consideringcertain design constraints. Three types of relayingstructures are discussed in Reference [5], (i) Fixedrelaying, (ii) Selective relaying, and (iii) Incrementalrelaying. Each of these relaying techniques can employAF or DF at the relay station. Other works on the use ofmultiple antennas in relay channels include the work inReferences [29] and [30]. Different from our work, inReferences [29] and [30] antennas are used for diversitygain and not for interference cancellation purposes.

It is clear from the above works that cooperativediversity can provide spatial diversity without the needof physical arrays. However, most of existing worksfocus on improving the peer-to-peer link quality in thesingle-user scenario by using coding or power and rateallocation. In ad hoc networks, how to efficiently andfairly allocate resources among multiple users and theirrelays is still a challenging task.

The DCF of the MAC layer protocol defined in theIEEE 802.11 is usually used in ad hoc networks. UsingCSMA/CA, all nodes in an ad hoc network contendfor a single channel access. In Reference [8], a MACprotocol based on user cooperation was proposed toimprove the performance of the IEEE 802.11 WLAN.The focus of this protocol was mainly to improve thedata rate of mobile stations far from the access pointby cooperation of an intermediate node. Beside thework in References [31] and [32], few other studieshave focused on the impact of cooperative diversity onthe system performance specially in ad hoc networks.In general, using cooperative transmission expands

Copyright © 2007 John Wiley & Sons, Ltd. Wirel. Commun. Mob. Comput. 2008; 8:771–782

DOI: 10.1002/wcm

MOBILE AD HOC NETWORKS 773

the range of signal radiation compared to directcommunication and hence increases the interferencerange.

In Reference [33], an analytical model for evaluatingthe Quality of Service (QoS) in wireless ad hocnetworks was developed. In this work, transmissionblocking probability is derived and chosen as a QoSfigure of merit. Following the same lines, Reference[6] has extended the analysis of Reference [33] to thecase of cooperative networks.

In this paper, we consider the negative effects of relayblocking in cooperative ad hoc networks for the caseof single- and multiple- relay channels. As a remedy,we propose an efficient MAC protocol that takes intoconsideration the relay selection criterion to improvethe overall network throughput.

The rest of this paper is organized as follows. InSection 3 we present the system model, including thePHY and MAC layers for conventional cooperativead hoc networks. In Section 4, we examine therelay blocking problem. In Section 5, we present ournew cooperative MAC protocol for both single- andmultiple-relay cases. Simulation results along withdiscussions are given in Section 6. Finally, in Section 7,conclusions are given.

3. System Model

In our simulation model, mobile stations are placedrandomly in a 200 m × 200 m area (as shown inFigure 1). To simplify the simulations, we considera network with 10 stations. The first one to fivestations are selected as transmitting stations with fixed

Fig. 1. Snapshot of a 10-station network topology with relayselection.

destination stations six to ten, respectively (1 → 6,2 → 7, and so on where x → y signifies transmissionfrom station x to station y). The radio range foreach station is assumed to be the same and equal to100 m. Stations positioned within the radio range ofany transmitting station are considered as neighbors tothat particular station. In what follows, we describethe operation of the physical and MAC layers inconventional cooperative networks.

3.1. Physical Layer Design

At the transmitter, data frames are transmitted usingbinary phase shift keying (BPSK) modulation. Thechannel rate for all transmitting stations is fixed to1 Mbps. The channels between each pair of stationsare modeled as independent identically distributed(i.i.d.) slowly varying flat fading channels. That is,we assume that the fading coefficient is fixed for oneframe transmission but change independently from oneframe to another. Also, we define a threshold bit-error-rate (BER) as BER < 10−5. Packets received at thereceiving stations with BER less than this thresholdare marked as successful packets. For data frames,cooperative diversity is achieved by using the AF mode.In this case, the transmission scheme used is definedas follows. During the first time slot, the source sendsdata symbol x1 to both the destination and relay nodes.The received signals at the destination and relay nodesduring the first time slot, are given respectively by

ySD = hSD√

ESDx1 + z0 (1)

and

ySR = hSR√

ESRx1 + z1. (2)

At next time slot, the relay amplifies the received signalin Equation (2) for subsequent transmission to thedestination. The received signal at the receiver is thengiven by

yRD = αhRDySR + z2 (3)

where ESR = ESD = E represents the transmittedsymbol energy on the different links, zi, i = 0, 1, 2,is the AWGN with power spectral density N0/2. Then,hSD, hSR, hRD model the fading between the differentnodes, and are assumed to be independent complexGaussian random variables with variance 0.5 perdimension. Note that at the relay, we use an automaticgain control (AGC) where the average transmitted


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Fig. 2. BER performance comparison between IEEE 802.11and cooperative network with one relay.

energy per symbol, ERD, is controlled by the relay gainα =

√ERD/E(|ySR|2) with E denoting expectation.

Figure 2 shows a performance comparison betweenthe IEEE standard and the cooperative network in AFmode. As seen, the use of cooperative diversity cansignificantly improve the performance of the ad hocnetwork. Note that in these results, we only considera single-relay between the source and destinationnodes.

3.2. MAC Layer Design

At first, a station initiates the transmission by sendingrequest-to-send (RTS) packet. If the destination nodeis not engaged in communication with any other node,it replies back with a clear-to-send (CTS) packet (seethe timing diagram in Figure 3). During the exchangeof these control packets, if a neighboring station isable to assist by cooperation, it acknowledges thesource and destination nodes by transmitting a relayACK packet (RACK). Since more than one neighboringstation can reply by RACK, we consider a selectioncriterion based on the shortest distance from the source

Fig. 3. Timing diagram for cooperative strategy.

node. Now, the network allocation vector (NAV) forneighboring stations is delivered and renewed by theRTS, CTS, and relay acknowledgement packets. Inthis case, the neighboring stations of the transmitterand receiver have to defer their transmissions for theamount of time delivered by the NAV. Note that, theneighbors of the relay station also have to defer theirtransmission for a period equal to two data-packettransmissions (i.e., time = 2× frame length/channelrate). Upon selecting the relay node, the source notifiesthe corresponding relay using a relay ready-to-send(RRTS) packet. Following this, the source starts itstransmission by sending a data packet. The destinationand the relay stations receive this packet. When thesource finishes its transmission, the relay stationstarts forwarding (after amplification) the receivedpacket to the destination node. The destination thenacknowledges the source with an ACK if the achievedBER is less than the prescribed threshold (i.e., ≤10−5).

In Reference [9], different relay selection criteriahave been discussed where relays can be selectedby considering a number of decision constrains suchas: minimum distance, minimum load, and minimuminterference. Over fading channels, the channel statefor different links should also be included in theselection criteria. For simplicity, we assume that thestation closer to the transmitter and receiver stationsis selected as the relay. A snapshot of the 10-stationtopology with relay selection is shown in Figure 1.

4. Performance of CooperativeTransmission

In what follows, we investigate the performance ofthe cooperative ad hoc network strategy discussed inthe previous section. The throughput is defined as thetotal network throughput, calculated as the number ofsuccessful received frames per second (control framesare not included).

In Figure 4, we simulate the 10-user system overan ensemble of 20 different random topologies andobtain the average throughput at different system loads.According to Figure 4, it is clear that cooperativenetworks can achieve better throughput performancethan the IEEE 802.11 standard.

To examine the effect of fading on the throughputperformance of the IEEE 802.11 with and withoutcooperative diversity, in Figure 5, we perform acomparison with the AWGN channel. Note that thelarge throughput degradation over fading channelsis due to the large packet error rate compared to


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Fig. 4. Average throughput versus load for 20 randomtopologies.

the performance on AWGN channels. Although theperformance of the cooperative network is better thanthe IEEE 802.11 over fading channels, it is still far fromthe AWGN channel.

We observed that the throughput achieved usingcooperative networks is limited by two major factors(beside the effect of fading): (i) The same data packetis transmitted twice which requires at least two framelength/channel rate amount of time for one successfulframe transmission. (ii) As mentioned earlier, to avoidcollision at the receiving station, only the relay or thesource station is allowed to transmit at a time. Basedon this transmission scheme, the neighboring nodesto the relay are blocked for a period of at least twoframe transmissions before they can contend again

Fig. 5. Throughput performance for the network topology inFigure 1.

Fig. 6. Cooperative scenario with relay blocking.

for the channel. To illustrate, Figure 6(a) shows acooperative scenario where by using station 3 as arelay to the destination station 6 results in transmissionblocking for stations 2 → 7. This, in turn, resultsin a decrease in the overall network throughput. Onthe other hand, if neighboring stations to the relayare allowed to communicate then interference will bepresent as depicted in Figure 6(b) and (c).

To examine the effect of relay blocking on theoverall throughput performance, let us consider thescenario of fixed topology in Figure 1. By analyzingthe topology of Figure 1, one can see the existenceof the relay blocking problem (see Table I). As seen,station 1 has stations 4, 5, 6, 9, and 10 as its neighbors.Also, station 6 has stations 1, 4, 5, 9, and 10 as itsneighbor. If station 9 is selected as a relay for 1 → 6transmission, then station 9 will block transmissionsfrom stations 2 to 7 (recall that we assume stations 1to 5 to be the transmitting stations and stations 6 to10 as the receiving ones). Note that although station2 is not in the neighborhood of 1 and 6, it stillcannot transmit to its destination station 7 because ofthe relay node 9. Similar case occurs when station2 communicates with station 7 or station 3 withstation 8.

From the above, one can see that relay blocking,if not compensated for, can cause major throughputdegradation in ad hoc networks. As a remedy, in whatfollows, we propose a cooperative strategy designed tosolve the relay blocking problem.

Table I. Transmitting, receiving, relay, and blocking stations.

Tx Rx Relay Blocking stations

1 6 9 2–72 7 4 1–63 8 5 1–64 9 10 —5 10 4 —


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5. Proposed Cooperative Protocol

We consider an ad hoc network where each station isequipped with a global positioning system (GPS) todetermine its position, and all stations are equippedwith directional antennas [13–16]. These directionalantennas are only used when a station is acting as arelay where in this case, it retransmits signals to thedestination node in a directional mode. This will bediscussed in more details in the following section.

5.1. Channel Reservation

Generally, cooperative relay networks require morenetwork resources relative to conventional networkswith direct transmissions. With the half duplex natureof most radio implementations, terminals cannottransmit and receive at the same time using the samefrequency band. By considering this limitation, onemay use a separate frequency channel or a differenttime slot for relay transmission.

In an ad hoc network scenario, cooperativetransmission brings the relay blocking problem asdiscussed earlier. That is, if two different cooperativetransmissions have to be performed side by side, thenrelay blocking should be considered in the design andthe assignment of network resources.

Considering the half duplex nature of the transceiver,the relay transmission blocking, and channel resourceallocation, we propose a two-channel cooperativeprotocol that incorporates adaptive antennas at therelay. As will be explained later, by allocatingtwo channels for the proposed protocol, the relayblocking problem can be solved and the cooperativecommunication can be formed in a more efficient way.Note that at any given time, a node is only allowed touse one of the two available frequency channels.

In Reference [9], the authors discussed the problemof resource allocation in cooperative ad hoc networks,where they considered a channel allocation algorithmfor N nodes if M independent channels are available.Our protocol, on the other hand, assumes a minimumof two independent channels available to form cooper-ative communication and to avoid relay blocking.

We assume that there are two available frequencydivision multiplexing channels; one channel is assignedto the transmitter (labeled CH-1) whereas the secondchannel is assigned to the relay (labeled CH-2).Figure 7 shows a simple diagram depicting the channelassignment of our protocol. As shown in this figure, thesender transmits data using an omnidirectional mode(single-antenna transmission) over CH-1 and the relay

Fig. 7. Sender-relay-destination links.

for any source node n and destination rIf CheckResources( ) = = available

GetChannel( )If channel = = IDLE

Case(Start of communication):send omnidirectional RTS and initiate NAV

End IfEnd IfIf RTS transmission = = success

send omnidirectional CTS and initiate NAVEnd IfIf CTS transmission = = success and number of neighbors ofsource and destination > 0

SelectRelay()End IfIf relay selection = = success

send omnidirectional Reack and initiate Relay NAVEnd IfIf Reack transmission = = success

select transmission protocol = cooperative modeElse

select transmission protocol = non-cooperative modeEnd IfIf transmission protocol = = cooperative

If data channel = CH1select Relay channel = CH2

Elseselect Relay channel = CH1

End Ifsend Data frame using omnidirectional antenna in data channelrelay Data frame using directional antenna in Relay channel

End IfIf transmission protocol = = non-cooperative

send Data frame using omnidirectional antennaEnd IfIf data transmission = = success and relay data = = success

send omnidirectional Ack and CREnd IfCase(End of communication)for any other source station Kstation n and r = out of range of K and station r = within range of KIf CheckResources( ) = = available

GetChannel( )If Relay NAV(CH1) > 0

select Channel = CH2End IfIf Relay NAV(CH2) > 0

select Channel = CH1End If

End If

uses CH-2 in a directional mode (multiple-antennatransmission) to transmit to the destination node.Note that the relay uses directional antennas mainlyfor beamforming to the destination and hence


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minimizing the effect of interference on nearbyreceivers.

One key parameter of the proposed MAC protocolis that the positions of the neighboring stations areknown to each station. This can be easily satisfiedby sending this position information using a separatecontrol packet or through the ongoing RTS and CTSpackets. Following is a pseudo-code describing theproposed protocol. The function CheckResources( )is used to check the available frequency channelwhereas GetChannel( ) executes the channelsensing for transmission. Finally, the functionSelectRelay is used to select the relay according tothe shortest distance criterion (from the source anddestination).

Figure 8 shows the conditions on frequency channelassignments for two neighboring cooperative links. Toexplain, suppose station A is transmitting to stationB and C is selected as the corresponding relay. Alsoassume that station A is using CH-1 for transmissionto B. Given this, relay C will be assigned CH-2 forrelaying the signal to station B. Now assume thatstations D and E are neighbors to C, but out of theradio range of both A and B. Also consider the casewhere station D has data to send to E and according tothe relay selection criterion station F has been selectedas their relay. In this case, D will be assigned CH-2for transmission to E and relay F will be assigned CH-1 for relaying to station B. The reason behind this isthat, relay C has to receive signals from A over CH-1 and hence, any other transmission over this channel(within the radio range of C) may cause interferenceat the relay C. Note that if station D uses CH-2 fortransmission to E, there will be no interference at the

Fig. 8. Sender-relay-destination links.

receiver of station C. Also the use of adaptive antennasat the relay will reduce the interference level at allneighboring receivers. This of course will improve theoverall system throughput.

In order to clearly discuss the proposed MACprotocol, let us consider the network topology ofFigure 1 where station 1 sends an RTS to station 6over CH-1. In response, station 6 replies back by CTS.From Figure 1, stations 4, 5, 9, and 10 are shown tobe within the range of both stations 1 and 6. If weconsider a single-relay transmission scenario, then oneof these stations (4, 5, or 10) will be selected as the relayfor 1 → 6 transmission. Let us assume that station 9has been selected as the relay station. Station 9 willthen send a relay acknowledgement (Reack) packetacknowledging the agreement on relay assignment.Following this, station 1 starts transmitting its datapackets using CH-1 to both the destination and the relaystations. Upon receiving these data packets, station 9amplifies and retransmits the received data to station6 using CH-2. After successfully receiving the data,station 6 replies with an ACK packet at which the relaysends a clear-relay (CR) packet to inform its neighborsof the released connection.

Now suppose station 2 wishes to communicate withstation 7. Note that stations 2 and 7 both are outsidethe radio range of station 1 and 6, but station 2 iswithin the range of relay station 9. Station 2 willreceive the Reack packet sent by relay station 9. Uponreceiving this packet, station 2 will learn that 9 hasbeen selected as a relay for the time of two successiveframe transmissions. Based on this information, station2 adjusts its relay network-allocation-vector (Relay-NAV) for a period of two frame transmissions. Duringthis time, if station 2 wishes to transmit then it willuse CH-2. Station 2 then starts its transmission to7 by sending RTS using CH-2. In response, station7 replies back by a CTS. Now the communication2 → 7 has only station 3 as its neighboring stationfor relaying data. Station 3 will then be selectedfollowing the same protocol used for the relay station9. Following the Reack sent by the relay station 3,station 2 commences its data transmission over CH-2 whereas station 3 relays the received data over CH-1using the directional mode. The destination station 7then sends an ACK packet after successfully receivingthe data. Also, relay station 3 sends a CR packet toindicate the end of the Relay-NAV to its neighboringstations.

Figures 9 and 10 depict the timing diagrams forthe proposed cooperative protocol for single-relay andmultiple-relay transmissions, respectively. There are


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Fig. 9. Timing diagram for the single-relay new cooperativeMAC protocol.

two types of NAV considered; (i) one is used by thesender or destination station to inform its neighborsof the time required for the current communicationon the channel, (ii) the second is a Relay-NAV wherethe neighbors of the relay station can learn the activeduration of the relay station. Also given the Relay-NAVpacket, the neighboring stations to the relay will beinformed to use a certain frequency channel (CH-1 orCH-2) if they wish to communicate.

One should note that the proposed MAC protocolis based on the CSMA/CA similar to the IEEE802.11 with RTS/CTS handshaking mechanism. Alsothe RTS, CTS, and Reack packets in our MAC protocolare transmitted using omnidirectional antennas toovercome the hidden terminal problem.

In the case of multi-channel environment, a differenthidden terminal problem can occur, as shown in thescenario of Figure 11. Note that this type of hiddenterminal problem arises only if a node misses theReack packet. Here nodes B and C are using CH-1for communication but node A wishes to communicatewith node B over the same frequency channel. In our

Fig. 10. Timing diagram for the multiple-relay newcooperative protocol.

Fig. 11. Hidden terminal in relay communications.

protocol, this situation can occur at the relay regionwhen any of the stations in the territory of the relay usesthe same frequency channel as the relay. To solve thisproblem, in our protocol we use the relay-NAV packetto deliver the information of available channels in therelay transmission region.

Another common problem in networks wheredirective antennas are used is the deafness problem.This problem arises when a source node fails to com-municate with a desired receiver that is beamformingin different direction of on-going communication. Inour protocol, this problem is avoided since all receivingstations employ omnidirectional antennas for receptionand only the relay uses adaptive antennas to thedestination.

6. Simulations

In the following, we use MATLAB simulations toexamine the performance of the proposed cooperativeprotocol relative to existing protocols. The simulationparameters used in our study are summarized inTable II.

We randomly generate 20 different topologies andevaluate the average network throughput. Similar tothe results in Section 3, we consider a transmissionscenario where station 1 → 6, 2 → 7, 3 → 8, 4 → 9,and 5 → 10. All stations (sender/relay/destination)receive signals in omnidirectional mode. We assumethat the relay station knows the position of thedestination station. Through this information, thedirection-of-arrival (DOA) angle is estimated atthe relay station and then used for antennabeamforming. We also assume that we have a perfect

Table II. Simulation parameters.

Number of nodes 10Node coverage radius 100 mNetwork area 200 m × 200 mDisplacement step 1 mSimulation time 1 secNode traffic generation rate 0.1–1 MbpsMaximum packet length 8000 bitsLength of RTS/CTS/ACK 20/15/14 octetsLength of REACK/CR 20/14 octetsDIFS/SIFS/slot time 50/10/20 �secChannel rate 1 Mbps


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Fig. 12. Throughput versus load at SNR = 25 dB, using four-antenna elements for the new cooperative protocol and a

single-relay transmission.

position information delivered by the GPS. In this case,the relay can accurately perform antenna beamforming.

We employ a simple single-hop routing protocol.In this routing protocol, if the destination node isout of range then transmission can only take placeif the distance between the sender and destinationinvolves a single hop. For fair comparison, the samerouting protocol is used for both IEEE 802.11 andconventional cooperative networks. In what follows,we investigate the throughput performance of thenew cooperative protocol using the 10-user simplifiednetwork in Figure 1. We also consider single- andmultiple-relay cooperative scenarios.

Figure 12 examines the throughput performanceas a function of the system load (per station) ata signal-to-noise ratio (SNR) = 25 dB for (i) IEEE802.11 over AWGN and fading channels; (ii) An adhoc cooperative network where relay blocking exists;(iii) The new cooperative protocol for the single-relay channel; (iv) the new cooperative protocol withsingle-relay and full-rate transmission mode (i.e., theextra time delay required to relay signals is eliminatedusing a full-rate transmission mode). One can seethat, using our cooperative protocol (single-relay) thethroughput is improved relative to the conventionalcooperative network with relay blocking. Note thatthe BER performance improves through cooperativediversity gain, where at 25 dB the packet error rate islow and hence the throughput increases.

Let us now examine the effect of interferencedue to cooperative transmission. This interference ismainly caused by different mobile terminals sharing

Fig. 13. Throughput performance of the new cooperativeprotocol with four-antenna elements at SNR = 13 dB for thesingle-relay and multiple-relay scenarios and using the fixed

topology of Figure 1.

the same frequency band. In traditional IEEE 802.11-based ad hoc networks, only one station can transmitat a time. On the contrary, our cooperative protocolallows for transmission and/or reception in the relayregion. Therefore, in this relay region, one mayexpect the throughput performance to degrade due tointerference/collisions. To overcome this limitation, weuse adaptive antennas at the relay stations to allow theneighboring nodes in the relay region to communicatewithout blocking and with low interference levels.

At first, we considered the case where single-relayis employed. From Figure 1, we can see that station 7has no interference from any neighboring station (i.e.,stations 1, 6, and 9 are out of the radio range of 7).Let us now consider the case when multiple relays areused. In this case, all the neighboring stations availablefor a sender–destination pair are considered as relaycandidates for that particular link. In Figure 1, we cansee that stations 4, 5, 10, and 9 are possible relaycandidates for the 1 → 6 link. Then, the interferencefrom relay 4, 5, and 10 will be received at station 7.Even though the use of multiple relays can improve thereceived signal quality, it can also increase the levelof interference at the neighboring stations. Figure 13shows the throughput performance at different systemloads using single and multiple relays.

Figure 14 shows the throughput performance as afunction of the SNR using our cooperative protocolfor single- and multiple-relays at a fixed load of0.1 Mbps/station. To examine the effect of interference,we also consider the case when no interference is


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Fig. 14. Throughput performance at a fixed load of1 Mbps/station, using four-antenna elements per relay node.

present. As seen from our results, when interference isignored, the multiple-relay case always achieves betterthroughput performance than the single-relay case.However, when considering the effect of interference,the single-relay case is shown to offer better throughputat relatively large SNRs (SNR > 15 dB). This also jus-tifies the throughput results shown in Figure 13. Notethat to improve the throughput for the multiple-relaycase, a larger number of relay antennas should be usedto reduce the effect of interference caused by multiple-relay transmissions. This is noted in Figure 14, wherethe use of six-antenna elements is shown to improvethe throughput relative to the four-antenna case.

In Figure 15, one can see that using multiplerelays for cooperative networking causes throughput

Fig. 15. Throughput versus system load at SNR = 25 dB.

Fig. 16. Effect of increasing the number of antennas onthe throughput performance at SNR = 25 dB for the fixed

topology in Figure 1 and at fixed load/station = 1 Mbps.

degradation due to the relay blocking problem whichcan be more severe than the single-relay case. Thisis simply due to the fact that as the number ofrelays increases, the blocking region increases resultinghigher probability of transmission blocking. The resultsin Figure 15 clearly show that both the single-and multiple-relay cases, with four-antenna elements,achieve significant throughput improvement relativeto conventional cooperative networks (single/multiple-relay). For instance, the single-relay (four antennas)compared to multiple-relay (six antennas) offer almostthe same throughput. For larger number of relayantennas (see eight-antenna case), the throughput forthe multiple-relay case can be further improved byreducing the level of interference.

Finally in Figure 16, we examine the throughputperformance as a function of the number of relayantennas (for both the single- and multiple-relay). Wehave noted that the use of large number of antennasis more effective in the multiple-relay case when therelay blocking region is large. For the single-relay case,where the interference level in the network is very low,the saturation throughput can be achieved using smallnumber of antennas. This is different from the caseof multiple relays where the saturation throughput isreached using large number of relay antennas.

7. Conclusion

The relay blocking problem in existing cooperativead hoc networks was discussed. A new cooperativeprotocol based on the MAC layer of the IEEE 802.11


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has been proposed and analyzed for both single-and multiple-relay channels. The proposed protocol isshown to solve the relay blocking problem. Our resultsshow that the use of directional antennas becomesadvantageous for multiple-relay communications. Itwas also shown that using multiple relays can improvethe QoS (BER) of the particular link, at the expense ofincreasing the level of interference of other neighboringlinks. We noted that by increasing the number ofantennas (at the relay station) one can reduce theeffect of interference and hence, improve the overallnetwork throughput specially in the case of multiple-relay channels.

Acknowledgement

This research was supported by the Natural Sciencesand Engineering Research Council of Canada(NSERC) Grant N00861 and P-FQRNT/NATEQ GrantF00482.

References

1. International standard ISO/IEC 8802-11; ANSI/IEEE Std802.11, 1999 Edn. Part 11: wireless LAN Medium AccessControl (MAC) and physical Layer (PHY) specifications.

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DOI: 10.1002/wcm


Authors’ Biographies

Md. Rajibul Islam received the Bach-elor of Science degree in Electricaland Electronic Engineering from Is-lamic University of Technology, Dhaka,Bangladesh, in 2003. He received hisMasters of Applied Science degree inElectrical and Computer Engineeringfrom Concordia University, Montreal,Canada in 2007. His research interests

are in the general areas of wireless communications,cooperative network, mobile ad hoc networks, MIMOsystems, space-time coding and smart antenna techniques.

Walaa Hamouda received his M.A.Sc. and Ph.D. degrees from Queen’sUniversity, Kingston, Ontario, Canada,in 1998 and 2002, respectively, all inElectrical Engineering. From 1994 to1995, he joined Siemens, Cairo, Egypt,in the Telecommunications Division,where he focused on the design oftelecommunication cables and fiber

optic transmission systems. Since July 2002, he has beenappointed as an Assistant Professor with the Department ofElectrical and Computer Engineering, Concordia University,Montreal, Quebec, Canada. Starting June 2006, he hasbeen appointed as a Concordia University Chair inCommunications. He has served/is serving on the TechnicalProgram Committee/chaired sessions of several IEEEconferences, including IEEE International Conference onCommunications (ICC 06/07), Globecom 2006/07, IEEE-LCN-MVN2007 Mobile and Vehicular Networking, 2007,IEEE IPCCC 2003, IEEE International Conference onInformation & Communication Technologies, 4th ACS/IEEEInternational Conference on Computer Systems and Applica-tions (AICCSA-06): Ad hoc and sensor networks workshop,and VTC 2001–2003, Fall 2005, Fall 2007. He has servedas a track Chair: Radio Access Techniques, IEEE VehicularTechnology Conference (VTC-Fall) 2006, and technical co-chair Signal Processing Symposium, IEEE WirelessCom2005, Poster Chair for ACS/IEEE International Conferenceon Computer Systems and Applications (AICCSA) 2008.In Sept. 2005, has been appointed as the Chair, IEEEMontreal chapter, Communications and Information Theory.His current research interests are in wireless and mobilecommunications, space-time processing, MIMO systems,CDMA, channel coding.


DOI: 10.1002/wcm

an efficient mac protocol for cooperative diversity in mobile ad hoc networks

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