New CDMA-Based MAC Protocol for Ad HocNetworks

Thamer Al-Meshhadany and Wessam AjibComputer Sciences Department, Universite du Quebec a Montreal

201, avenue du President-Kennedy Montreal, Quebec, Canada H2X 3Y7Phone 1 514 987 3000 # 3227; fax 1 514 987 8477

Email: thamer@labunix.uqam.ca, ajib.wessam@uqam.ca

Abstract— It is widely accepted that Ad Hoc networks areat the leading edge of the research in the domain of wirelessnetworking. These networks are not supported by infrastructureto connect the mobile hosts, thereby they have to be selfconfigured, self organized and the resources have to be allocatedin a distributed manner. The medium access control (MAC)layer is seen as the bottleneck for the throughput in wirelessAd hoc networks. Hence, we propose in this work a newMultichannel MAC protocol. The proposed protocol can be basedon Code Division Multiple Access (CDMA) or Frequency DivisionMultiple Access (FDMA). A channel can be represented by onespreading code in CDMA systems or by one frequency band inFDMA case. In our analysis and simulations, we assume that theprotocol is based on CDMA technique. We consider one channelfor control packets and multiple channels for transmitting datainformation. We propose that the reservation of a data cannelis done implicitly using the common channel. We show throughcomputer simulations that our proposition of Multichannel MACprotocol improves significantly the communication performancein wireless Ad Hoc networks, even when the introduced overheadis considered.

I. INTRODUCTION

Ad Hoc networks consist of terminals that are distributedin a geographical zone and connected together without in-frastructure. Hence, They have to be self configured, selforganized and the resources have to be allocated in a dis-tributed manner. The medium access control (MAC) protocolin these networks encounters two major phenomena that limitthe network capacity [1]. These phenomena are the hiddenand exposed terminals. The first one causes higher rate ofpacket collisions; whereas, the second phenomena causes aloss in the resources. Many MAC protocols are proposed toovercome these problems. However, the proposed solutions arein general either of high cost or/and complex and they haven’tcompletely eliminated these problems.

In this paper, we propose a new multichannel MAC protocolfor wireless Ad Hoc networks. The proposed protocol is basedon CDMA technique and uses spreading codes to identifydifferent channels. However, the method considered in thisprotocol for channel identification allows us to utilize othermultiple access technique such as Frequency Division MultipleAccess (FDMA) technique where each channel is representedby a frequency band in one radio transceiver systems. It canalso be applied to a multi-radio systems where each radiotransceiver may correspond to one channel.

In the proposed protocol, we consider one common controlchannel on which RTS (Request to send) and CTS (clear tosend) packets are exchanged, and several data channels usedfor data packets transmission. We assume that time is dividedinto frames that are also divided into slots. By inserting asynchronization interval at the beginning of each frame in thecontrol channel, the synchronization among the terminals isestablished. The reservation of a particular data channel forone transmission is realized by the control channel. Obviously,all the terminals have to monitor the common control channelin order to identify and reserve the data channel.

The rest of this paper is organized as follows. In Section II,we discuss the related works with a particular focus on theCDMA-based protocols. Section III describes our proposedmultichannel MAC protocol for wireless Ad Hoc networks.Simulation environments and results analysis are presentedin Section IV showing the improvement of the performancerealized. In terms of simplicity, efficiency and flexibility, adiscussion is established in section V. Finally, we concludethis paper in Section VI.

II. RELATED WORKS

MAC protocols based on CDMA for Ad Hoc networksreturn to [1], in which the authors have proposed two differentprotocols. According to these protocols, the codes are pre as-signed to the terminals. The transmitted packets consist of dataand a header used mainly to identify the destination. Hence,the packet’s data is transmitted on a transmitter-based code, butthe packet’s header is either transmitted on a common code(the first protocol) or transmitted on the receiver code (thesecond protocol). Hence, the number of used spreading codesrelies completely on the network size. In the receiving mode,a terminal monitors either a common code, in the first case, ora receiver-based code in the second case. Subsequently, eventhough the rate of occurred collisions is reduced, the collisionsstill happen for the packets transmitted on the common codein the first protocol or for the ones transmitted on the receivercode in the second protocol.

A pairwise code assignment has been proposed in [2]. Itcentrally assigns codes to each transmitter-receiver pair (link)such that no two adjacent edges have the same code. Accord-ingly, each terminal must maintain many codes to connectwith the neighbored terminals. The overhead, caused by the

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exchange and the broadcast of code assignment signallingmessages, is relatively high.

The number of used codes have been minimized in [3]where the authors have shown that the minimization problemis NP-complete.

Another way of thought is to make use of handshakingcontrol packets to sense the channel. The handshaking pack-ets were first introduced by MACA (Multiple Access withCollision Avoidance) protocol in [4]. The sender and receiverterminals exchange two control packets, namely Ready ToSend (RTS) and Clear To Send (CTS) before sending the datapacket. Two protocols based on common-transmitter based(MACA/C-T) and receiver-transmitter based (MACA/R-T) areproposed in [5]. These protocols can be considered as variantsto the ones presented in [1] except that the new proposedprotocols take into consideration the RTS/CTS packets. Thus,collisions occur only at the level of the control packets (RTSand CTS) exchange.

On the other hand, sending the data with the receiver’s codewas investigated in [6]. However, it is well-known that the bigdisadvantage of this kind of protocols is that it can’t supportmulticast services.

The authors of [7] have proposed a distributed algorithm ofcode assignment in which the number of available codes is atleast d×(d−1)+2 where d is the maximum number of one-hopneighbours any terminal can have. An important characteristicof this protocol is the existence of a common channel that isused to exchange the code assignment information betweenthe terminals.

In [8], the authors have proposed a dual reservation MACprotocol in which N + 2 codes are used (N codes for data,one common code for control messages and one code forbroadcasting). Each terminal has three lists that contain thefollowing information: available codes list, occupied codes listand forbidden codes list. The code used for transmitting datapackets between two terminals is dynamically assigned afternegotiations using the control messages.

A. Muqattash and M. Krunz have proposed a multichannelMAC protocol in [9]. In this proposition, the bandwidth isdivided into two frequency channels. One channel is used toexchange control messages and the other channel is used forthe data transmission. All terminals use the common code onthe control channel; whereas, on the data channel, only onecode is assigned to each terminal for data transmission. Al-though the proposed solution reduces the number of occurredcollisions, the hardware implementation is complex and ofhigh cost.

In [10], two phases coding multichannel MAC protocol wasproposed. They assume that one terminal have to take theresponsibility to assign the codes for the terminals in its cell.In fact, this protocol will be more suitable to wireless LAN802.11 DCF.

In [11], Asynchronous Multichannel Coordination Protocol(ACMP) is devised. It is based on 802.11 Distributed Coordi-nation protocol (DCF). It uses a dedicated control channel andmultiple data channels. The terminals contend on the control

channel by exchanging RTS/CTS packets in order to reservea data channel.

III. PROPOSED MULTI-CHANNEL CDMA-BASED MACPROTOCOL

In this work, we propose a new MAC protocol based onCDMA for Ad Hoc networks. We assume N + 1 codes: onecommon code for RTS and CTS packets transmission and Ncodes for Data and ACK packets transmission. Subsequently,there are N + 1 virtual channels. Time is divided into framesand each frame is divided into slots. The number of slots perframe in data channel is equal to that one in control channel.In order to ensure the synchronization among the terminals,each control frame starts by a synchronization interval (SYN).When a terminal wakes up or enters into a geographicalzone of one Ad Hoc network, it has to wait until the nextsynchronization interval in order to ensure the synchronizationwith the other terminals.

Each slot in the control channel is divided into two fields.The first is used by the sender to transmit RTS packet whereasthe second is used by the receiver to respond with CTS packet.We assume that the number of slots in one control frame isequal to the number of data channels (codes). Thereby, theslot’s number during which RTS/CTS packets are exchangedbetween two terminals represents the data channel on whichthe data will be transmitted. The procedure of medium accesscontention is described in the following.

All terminals permanently tune to the common code andmonitor the control channel. When an active terminal has datato transmit, it waits until the next slot in the control channeland then it transmits a RTS packet during the RTS field but ifthe terminal was out of the network coverage or it was inactive,it has to wait firstly until the next synchronization intervaland then it tries to transmit a RTS packet on the first slot inthe frame. If this terminal is the only transmitting terminal inthis field, the destination terminal will receive correctly theRTS packet and will respond with a CTS packet. Otherwise,a collision will occur and the collided terminals will backofffor other slots.

As all terminals tune to the common code, all terminalsoverhear the RTS/CTS packets transmission and they can makebenefits from this exchanged information. In this work, thisis not used because we assume that each connection is madeonly for one frame. However, if we extend this work to ensurethe reservation for multiple frames, the data’s size could beindicated in the RTS and/or in the CTS packets in order toestimate the number of required data frame.

Upon receiving the CTS packet, the sender terminal tunes tothe corresponding data channel. When the RTS/CTS packetsare successfully exchanged and a data frame is reserved, thesender terminal starts transmitting the data at the beginningof the next data channel frame. The destination terminalreceives data and replies with an acknowledgment packeton the same data channel. Thus, each successful RTS/CTSpackets exchange reserves only one frame in the data channel.If the sender has more than one data frame, it has to achieve

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successful RTS/CTS packets exchange for each data frame.The procedure is shown in figure 1.

The figure illustrates an example of different successfuland unsuccessful RTS/CTS exchanges. We notice that whena RTS packet is not transmitted or a RTS packets collisionoccurs (denoted by unsuccessful RTS/CTS packets exchange)during one slot of the common channel, the next frame in thecorresponding data channel of that slot will be idle (slots S1

and SN−1 in frame M ).

When RTS/CTS packets exchange is successful as in slotsS2, S3 and SN , one data frame is reserved on the correspond-ing data channel for each transmission. In this way, the datachannel reservation is simply achieved.

Contrary to many distributed multichannel MAC protocolsfor wireless Ad Hoc networks proposed in the literature, ourproposed protocol is simple since it uses the slot number as animplicit indicator for the data channel. In other words, when aterminal transmits RTS packet in a slot, all terminals includingthe destination terminal know which data channel will be usedfor the transmission of data by the sender. Hence, the terminalsdo not need to transmit any signalling packets to reserve adata channel. Furthermore, the proposed protocol is able to beextended in order to include more complicated functions suchas multiple frame reservation.

The information about the channels (corresponding to dif-ferent codes in CDMA systems) reserved for a particularterminal for data transmission is provided implicitly to thecorresponding terminal.

IV. SIMULATION RESULTS

A. Simulation environments

In order to evaluate the performance of our proposedmultichannel MAC protocol, our computer simulations areachieved using Network Simulator (NS-2). As the latter onlyconsiders the standard protocols for the medium access controllike IEEE 802.11, Carrier Sensing Multiple Access (CSMA),Time Division Multiple Access (TDMA) protocols and othersthat all are based on one channel, we had to modify theprotocol of IEEE 802.11 and TDMA protocols in order toachieve our model implementation.

In our simulations, we assume that the maximum numberof data channels is eight, hence there is up to eight flowscan be simultaneously established during one frame. In otherwords, sixteen terminals can be connected one-to-one. We alsoassume there is no interference form one pair connection onanother connections.

When the number of flows is less than or equal to thenumber of slots, the maximum throughput can be achievedif we consider no RTS packets transmission collision.

In the simulations, we consider eight slots per frame,corresponding to eight data channels. We assume that the RTS,CTS and ACK packets have the same structure and the samesize as in 802.11 standards. The maximum size of data packetis up to 800 bytes. We consider 30 terminals in geographicalzone of 500×500m2, in other word, all terminals are in samesensing zone. The traffic generated is Constant Bit Rate CBR.The implementation parameters are depicted in the table I.

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TABLE I

SIMULATION MODEL PARAMETERS

Parameter value

Basic rate 2 Mbps (RTS/CTS)

Data rate 2 Mbps (DATA/ACK)

SYN 144 usec (36 bytes)

Packet size 1000 bytes

RTS/CTS header 20/14 bytes

DATA/ACK header 28/14 bytes

Channel switching delay 0 sec

Scenario 30 terminals

B. Results analysis

In this section, we provide an evaluation of the performanceof our proposed multichannel CDMA-based MAC protocoland we compare it by the performance of IEEE 802.11 basedAd Hoc network with one channel.

Figure 2 depicts the aggregate throughput as a function ofthe number of channels. We notice from figure 2 that ourproposed MAC protocol gives higher throughput in function ofthe number of used channels. In fact, the throughput (definedby the rate of packets correctly transmitted) of 802.11-basedAd Hoc network is almost fixed since only one channel isused. On the other hand, the throughput increases linearlywith the number of channels using our MAC protocol. Whenall channels are used and the number of active terminals(transmitting terminals) is more than the number of channel,the throughput becomes invariant. This behaviour is expectedbecause the saturated case is reached.

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Another profile of results is depicted in figure 3 wherethe throughput versus the packet arrival rate is provided.When the traffic is low, the behaviour of our propositionlikes the behaviours of other propositions including IEEE802.11 protocol. But, when the packet arrival rate increases,the performance of 802.11 gets saturated, even, we notice a

slight decrease in the throughput of 802.11 protocol due tothe increases in the rate of packet collisions. Contrary, thethroughput with our MAC protocol increases when the packetarrival rate increases. However, our proposition performs theaggregate throughput for Ad Hoc networks in comparison withother protocols.

In fact, our proposed protocol does not need any signallingpackets to reserve a data channel, contrary to other protocolsthat need an exchange of signaling packets packets which in-troduces a waste of time and resources in order to reserve onedata channel. In other word, our proposed protocol avoids thePing-Pong phenomena between the communicated terminalsin order to reserve the data channel.

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Fig. 3. Throughput as a function of packet arrival rate per flow

V. DISCUSSION AND FUTUR WORKS

This section presents our future works and provides somediscussion about the flexibility, simplicity and efficiency thatcaracterizes our proposed multi-channel MAC protocol basedon CDMA technique.

We can say that our proposed scheme is efficient, simple andflexible. It is efficient because N connections can be achievedat the same time where N is the number of used spreadingcodes that in turn corresponds to the number of slots in oneframe in the control channel.

The scheme is simple because it uses a simple algorithm ofcode allocation. By the the number of the slot used for controlpackets, the sender terminal implicitly tells the destinationterminal about the data channel on which the data will betransmitted.

Moreover, the proposed scheme is flexible because itsstructure is open for future developments. The protocol canbe applied to systems that use FDMA as access technique.Even, the protocol can be applied to the actual 802.11 systemif we consider that one channel is used as control channeland the rest of channels as data channels. The data channel isreserved during the control channel monitored by all terminals.In this work, a terminal can establish only one connectionbecause it has only one simple transceiver but if this terminal

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has multi-transmitters and multi-users detection receiver, it canreserve up to N connections during one frame under certainconditions, i.e. this terminal can use all the data codes at thesame time.

Our proposed MAC protocol can be improved in order thateach terminal will be able to reserve the necessary framesby using only one RTS/CTS packets exchange. The senderterminal has to indicate the amount of transmitted data in orderto estimate the number of required data frames. A data channelcan be simply reserved by adding busy tone at the beginningof the corresponding slot in the control frame. The busy tonewill prevent the other terminals to contend on this channel.

The impact of Multiple Access Interference MAI on theresources allocation is not studied in this work. Hence, inorder to complete this work, the impact of MAI and the powercontrol must be studied.

VI. CONCLUSION

In this paper, we presented the Multichannel MAC Protocoland its use in wireless ad hoc networks to improve networkperformance. The most significant advantage of this protocol isthat several connections can occur concurrently using multiplechannels.

The channel allocation and identification are simplyachieved by transmitting RTS/CTS packet in one slot in thecontrol channel. The slot’s number represents the number of acorresponding data channel. By this, the hidden and exposedterminals are eliminated. We evaluated the performance usingthe ns-2 simulator in term of throughput when the multi-channel MAC protocol in wireless ad hoc networks is used.The evaluation results show that higher network throughputcan be obtained.

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[7] J. J. Garcia-Luna-Aceves and J. Raju, Distributed assignment of codesfor multihop packet-radio networks, in Proc. MILCOM, pp: 450-454, ,vol. 1, 1997.

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