2009 IEEE International Advance Computing Conference (IACC 2009)Patiala, India, 6-7 March 2009

Reducing MAI in Cluster Based Sensor NetworkUsing FDMA-CDMA Technique

'Sankar Mukherjee, 2Jyoti Prakash Singh'Durgapur Institute of Advanced Tech. & Mgt., Durgapur, West Bengal, India

2Academy of Technology, Aedconagar, Hooghly, Indiasankar_mukherjee2000(yahoo.co.in, jyotip.singh@gmail.com

Abstract-The performance of Code Division Multiple Accesses k

in sensor networks is limited by Multiple Access Interference E 2 Pi(MAI). In this article, we propose a frequency division technique =N ( 3L+= ( )to reduce the MAI in a DS-CDMA sensor network. Our proposal Nej 3LPO o

also reduces the energy consumption of the network. In themodel, first a new clustering technique is used over several Where L is the processing gain, P0 is the average receivednumbers of randomly deployed sensor nodes to form different power of the desired signal and P1, P2, ... Pk are receivedclusters and then use FDMA-CDMA technique in different powers of k interferers. The probability of bit error Pe with aclusters. Simulation is done for the proposed system andcompared it with other systems, which do not use frequency givn ofit.division. The study found that, by using few number of frequency funcIon ofce.channels, the MAI can be reduced significantly. The system also In a cellular DS-CDMA network there is a central basehas less channel contention, and lower energy consumption. station. This station controls the MAI by controlling the

transmission power of the active nodes. The received powerI. INTRODUCTION from all active nodes at the base station is the same. But in the

case of sensor network there is no central base station. So it isA wireless sensor network consists of a number of really difficult to control the MAI. Consider the situation insensor/actuator devices that can sense the environment, Fig 1, where sensors are randomly deployed. Here R,calculate/aggregate data and transmit the aggregated data to represents the communication range. Each node has a numbernodes within its transmission range. Every node has hardware of neighbors situated at different distances. For example, Afor sensing, microprocessors for computation and low-power has neighbors B, E, F, D, and G, with each having differentcommunication radios for transmission. The sensor nodes are distance to A. Assume that each node uses the minimumnormally battery operated. Hence, they have very limited required power to communicate with each other. When A isenergy. They also have limited memory to buffer data packets. transmitting to a neighbor, the interference power caused byThe contention based medium access (MAC) protocols are this transmission at other neighbors can have different values.obviously not a good choice for this network. The RTS/CTS Considering two simultaneous transmissions from A to B andcontrol packets usually employed in contention based protocol C to D, where distancelAB IAD, the interference power at Dproduces significant overhead. According to Woo and Culler[2] this overhead is up to 40% in small packet size sensornetwork. Although IEEE 802.11 standard specifies thatRTS/CTS can be avoided with small data packet transmission,this may not be a suitable choice for sensor networks. TheIEEE 802.11 network has a data rate of 2 MB/s where assensor network usually have data rate of around 20 KB/s. A H Fpacket will take much longer time in sensor network comparedto IEEE 802.11 networks. Hence the probability of collision isquite high in sensor network. Hence the proposal made in JOIEEE 802.11 network that for small packet RTS-CTS packetcan be avoided is not suitable in sensor network. CDMA may X

GX

come very handy in sensor network application. The majorproblem in using CDMA is multiple access interference(MAI). In this article, we propose an efficient way of reducingthe MAI (both primary and secondary) in by clustering thesensor network and then applying frequency division multipleaccess. Our algorithm is applicable in one hop clustered sensor Fig. 1 Randomly deployed sensor networknetwork. For a DSSS/BPSK (Direct Sequence SpreadSpectrum/Binary Phase Shift Keying) system, the effective bitenergy-to-noise ratio at the detector is as discussed in [1, 5, 6]

978-1-4244-1888-6/08/f$25.00 Q 2008 IEEE 740

caused by the closer neighbor A is much higher than that of fashion by using the algorithm CLUSTER (N). Every clusterthe desired power from C and this makes the desired signal consists of (i) one clusterhead, (ii) one or more Gatewaydifficult to be recovered. However, if instead of from A to B, nodes, (iii) one or more intermediate nodes and (iv) severalthe transmission is from A to E the interference caused to D's normal member nodes. Each and every node is a member of atreception is negligible. The problem caused due to least one cluster. Every normal member node communicatesinterference signal(s) makes desired signal go down at a through their clusterheads. Intra-cluster communication of thereceiver is an effect of MAI. So it is found that MAI can't be sensor nodes are totally handled by the clusterheads. Inter-reduced by using the power control in a CDMA based sensor cluster communications involves two clusters and so are donenetwork. The MAI may cause significant degradation in through the gateway nodes. Every clusterhead aggregates thenetwork throughput and is considered the main problem data received from his cluster members and send it to the sinkprohibiting the usage ofCDMA in sensor networks. node through different clusterheads. So to reach the sink node,

In this paper FDMA and clustering are used both for conventional shortest path routing algorithm throughreducing the MAI in CDMA based sensor network. clusterheads and gateways is chosen. A gateway is a node thatThe rest of the paper is organized as follows. Section II directly connects to more than one clusterheads. All otheroutlines proposals made by researchers for controlling MAI in gateway nodes covering the same clusters are marked asCDMA-based sensor network. In section III, we describe our Intermediate nodes. Every clusterhead will assign differentsystem model and describe a technique for reducing the MAI orthogonal codes to its members and itself. These codes can bein the system. reused in other clusters. In the cluster based CDMA system

Section IV describes the proposed Algorithms. Section V two types ofMAI can occur.provides Experimental results and finally we sum up and Primary MAI: Inside clusters MAI caused at theconclude in section VI with some discussions on future clusterhead by the simultaneous transmission of itsprospects. cluster members.

Secondary MAI: MAI caused at the clusterhead,II. RELATED WORKS gateway node and the intermediate node due to

simultaneous transmission of other nodes belongingDow, Lin, and Fan [7] used DS-CDMA over cluster-based to other neighbor clusters.

wireless networks to avoid the hidden node problem. It is well Primary MAI can be solved by the clusterhead itself byknown that energy consumption is the crucial factor in sensor synchronizing all its member nodes. It is quite easy to do allnetwork design. This may lead to sensor network MAC member nodes are in directly connected to clusterhead. Toprotocols which prioritize energy savings over network mitigate the problem of secondary MAI, we have proposed athroughput and packet latency. The multi-user, multiple access new algorithm. Because MAI is caused by the non-perfectinterference (MAI) environment of DS-CDMA introduces orthogonality ofCDMA codes, the rationale of the design is tosignificant challenges on how interference can be properly orthogonalize the reception in the vicinity of a sensor node bycontrolled. Code assignment to the nodes in each cluster is using frequency division. As most sensor network applicationsdone by the clusterhead in two phases. normally operate with low data rate, it is possible to use aMuqattash and Krunz [1] proposed a CDMA-based MAC narrow band CDMA system. Let's assume that data rate of the

protocol for wireless ad hoc networks where out-of-band application is 20Kbps, and we use 50 chip/bit PseudorandomRTS/CTS are used to dynamically bind the transmission codes (PN) to spread the baseband signal. The resultingpower of a node in the vicinity of a receiver. Both RTS and bandwidth requirement is 1MHz. With 2.4GHz ISM bandCTS are enlarged to accommodate MAI related information. (2400-2483.5MHz) we can have more than 80 similarHowever, our design goal is to reduce MAI by using FDMA frequency channels. So if different frequency band is assignedand clustering technique. to each cluster then there will not be any MAI due to

Liu, Chou, Lipman, Jha [6] proposed using frequency transmission of other nodes belonging to its neighbor clusters.division to reduce the MAI in a DS-CDMA sensor network. According to the clusters formed and the gateway are chosen,They provide theoretical characterization of the mean MAI at only 7 frequency bands are sufficient. So the reuse factor isa given node and show that a small number of frequency 7.So if 80 MHz ISM band is divided into 7 equal sub bandschannels can reduce the MAI significantly. Each pair of nodes (fl, f2, f3, f4, f5, f6, f7) then these can be assigned to theare assigned different frequency channel so that no clusters.interference occurs in the neighboring nodes. There is noclustering concept is used in this paper. Theorem 1: The reuse factor is 7. It means 7 frequency

band is sufficient to assign the clusters so that there will not beIII. SYSTEM MODEL any secondary MAI.

Our system consists of several identical sensor nodes Proof: A cluster is the neighbor one of another cluster ifdeployed in a region along with a sink node that accumulates these two clusters are connected by either gateway node orthe sensed data. The sink node is a powerful laptop or personalcomputer. The sensor nodes are organized in a hierarchical

2009 IEEE Internlationlal Advance Computing Conference (IACC 2009) '14:1

Proper choice of intermediate nodes is also very important.fi . In Fig.2 cluster Cl is connected to cluster C5 by an

S 0f2S =< intermediate node. This node should be chosen in such a wayNA / 1 f sG w \ that, it should be not directly linked with the nodes of other

C J ibclusters having frequency band same as Cl .Otherwise a/SE \r2f; \ < \ i ! secondary MAI will occur at the intermediate node due to its' il X 4 f one hop neighbor nodes belonging to different clusters which

AC IF \m C3 = have same frequency as Cl.And it is assumed that this type ofintermediate nodes which satisfy this requirement, areavailable. Neighbor clusters which are connected through theintermediate nodes are chosen only for making the whole

fl C4network connected. From Fig.2, for the cluster Cl, the clusterC5 is a neighbor one and it is connected through theintermediate node. Now if C5 is reached through other clusters

* CbI5terkiad A Ga±ewayn&de from C1, then there is no need of direct connection to C5

le mde Inueia±e m through the intermediate nodes. It may be that the path to thatcluster will be long enough but this restriction can help tomake the reuse factor bound to 7. And to find a path to reach aspecified cluster, routing algorithm is used. Gateway node

Fig. 2 Sensor networks with one hop clusters selection also an important factor. Gateway should be chosenin such a way that it does not get overloaded. So gatewayintermediate nodes. To avoid secondary MAI, neighbor noearths,cnonctnltwcutr.

clusters must have different frequency bands. Every cluster After assigning frequency band to different clustersmay have maximum twelve neighbor clusters. In Fig.2, cluster rcseration ofteqmembership todfgeway nodersaC4 may have maximum 6 neighbor clusters like C2 and C3 si desare neces suppos ga2, noteomnand maximum 6 more neighbors like Cl. Clusters like Cl and part between ClusterCe and C2, there may be a gateway nodeC4 have a common area BEDF (Fig. 2) and some sensor nodes and some simple nodes who are the members of Cl.lf amay exist there. Now in the zone BEDF if any sensor node and some betwees C2are C4 is of to a

exists and if it is treated as the gateway node of this four gateway node between cluster C2 and C4 is connected to anyclusters,organdeifaiti threaed clusts

the htewa cter ort isf one of these, both should change their membership to C2.clusters, or gateway of three clusters where cluster Cl or C4 15 Otherwise secondary MAI will occur at the gateway node dueone of them or gateway of two clusters Cl and C4, or to transmission of the other connected gateway node or simplememghber ofany onedamong cln'tuster Csand C4,hencyC

.wlB node. Because they are using the same frequency and may use

neighbor of C4 .And they can't use same frequency band. But same code also. If these nodes are members of same clusters,if those nodes belong to BEDF are allowed to be members of then this problem can be avoided.only any one of C2 and C3 and don't choose it as a gateway Though the frequency band used by each cluster is reducednode then cluster C1 and C4 can use same frequency. In that to 1/7th of the whole ISM band still it is sufficient for low datacase Cl and C4 is no longer neighbor of each other. To do this rate sensor node communication and the system will be totallyfollowing techniques are proposed collision free. On the other hand total energy consumption will

If any node belongs to the Zone like BEDF, must get be reduced a lot. If all clusters use the same frequency thensignal from all four clusterheads. But it must sense due to MAI, in a cluster, data transmission will collide withstronger signal from either the clusterhead of C2 or transmission from nodes of other neighboring clusters. MoreC3 compare to the clusterheads of Cl and collision and hence more retransmission means more energyC4.Because from the geometry it is obvious that the consumption. Even by simulation it can be shown that if alllength BE or DE is smaller than AE. So according to clusters use same frequency of 80 MHz then as the loadthe received maximum signal strength, this node will increases the successful transmission will decrease a lot and itbe a member of either C2 or C3 cluster. In case of is worse than the proposed model even at medium load.any tie between C2 and C3, randomly one will bechosen.

IV. ALGORITHMThis type of node can't be allowed to be a gateway Let N= {N1, N2 ... N.f set of n numbers of sensor nodes.node. Di = Degree of node i

So proper choice of gateway nodes and member nodes are NBDi= {Di 1 < j < n} set of degree of the neighboring nodesvery important factors to determine the neighbor clusters and of node i including the node i itself.hence the reuse factor. On implementing the above techniques, MA = Node with maximum degree in the set NBD-and MA N.every cluster like C4 may has maximum 6 neighbor clusters. RJl, = {N, i#j and 1 < j < n} set of nodes from where nodesSo total seven frequency bands are sufficient to allocate get REJ(eet esg rmnd. Intal h Jstidifferent frequency bands to its maximum six neighbor RC1 Reus o lseredfo oeiclusters and to itself.

742 2009 IEEE Internlationlal Advance Computing Conference (IACC 2009)

CMi = request for cluster member from clusterhead i. Node Ni calculate MiACKi=positive acknowledgement from node i. Send a RCHi to node Mi for clusterhead requestLVi=leave cluster request from node i. If (Ni = = Mi)RCi = {Nj i j and 1 X j X n} set of nodes from where nodes Mark itself as clusterhead Ci.gets RCHj message from node j. Initially the RCi set is . Broadcast a message CMi for requestingC = {Ci 1 i X n} ordered set of cluster heads. Priority of its neighbors for Cluster member.Ci > Cj if i<j; endlfGi= {Gj 1 Ej X n} set of gateway nodes in cluster i If (received CMk)IN1= {INj 1 Ej X n} set of Intermediate node in cluster i. Cn++,Si= {Sj 1 X j X n} set of simple nodes which are not a If(Cn==1)gateway node or an intermediate node, in cluster i. Send all node j of set RCi, a REJiCLi= {{Gi}, {INi}, {Sil, Ci} message.FRi = {fij 1 j X 7} frequency pool of cluster i. Send an ACKi to clusterhead k.CDi= set of orthogonal codes. endlf

If (Cn==4)The clustering algorithm CLUSTER (N) is used to form one Send LVi to clusterhead k earlier chosen.

hop clusters. Initially every node calculates their degree of Send an ACKi to newly chosenneighboring nodes including itself. Then broadcast this degree clusterhead n from which it got strongerinformation to every other neighbor. On receiving the degree signals among four clusterhead. For tieinformation from every neighbor, every node calculates the select one randomlynode M, who has maximum degree. And send a clusterhead endlfrequest information to M. A node may get several numbers of endlfclusterhead request information from its different neighbors. If (received RCHj)The node M chooses itself as clusterhead, if it has maximum If (it is not a member of any clusterhead)degree among its neighbors. If it selects itself as clusterhead, it RCi=RCi {j}sends the membership request message to its neighbors. A Elsenode first time get a membership request message from a Send a REJito node j from where itclusterhead, accept it for being the member of the clusterhead got RCHjby sending a positive acknowledgement and for other time it endlfrejects. It also sends reject message to all the nodes from endlfwhere it got the clusterhead request message. On receiving the If (received a REJJ)reject message, nodes will again calculate the maximum Modify the set RJi= RJi {j}degree neighbors excluding the earlier selected nodes, for NBDi=NBDi- RJisending the clusterhead request message. When any Find Mi again and send RCHi to Mi.clusterhead receives ACK message from a node j, selects it as endlfa cluster member. If any node gets membership request from If (received a ACKJ)four clusterhead, must choose its clusterhead, from which it Mark the node j as its cluster member.gets stronger signal. In case of any tie, one is chosen endlfrandomly. Selecting the gateway node and intermediate node enddofollow the logic proposed in the system model. After forming End.the one hop clusters frequency assignment is done by usingthe FRQN (Ci) algorithm. All the clusterheads are in B. Frequency assignment algorithmdescending order in the clusterhead set C. And this set isavailable to all the clusterheads. After the frequency FRQN (C1)assignment, some nodes have to change their membership to Beginavoid secondary MAI. The logic also proposed in the system If ((Ci= = highest_priority_clusterhead)model. Every clusterhead will assign orthogonal codes to its (Get START message from Ci-1))cluster members independently from its code pool CD using For (j=1 to 7)the algorithm CODE_ASSIGN_SENSOR (C1). If (fij is not assigned in frequency pool FR1)

Assign the frequency fij to Ci;A. One hop clustering Algorithm Inform its neighbors to update fij as assigned in

frequency pool FR;CLUSTER (N) Break;Begin endIf

Receive counter Cn=O, endFordo Send a start message to C+1,1

i, 1 to n in parallel Else

2009 IEEE Internlationlal Advance Computing Conference (IACC 2009) 743

Wait (START) more due to MAI for the CDMA system. Simulation is doneEnd for the system using CDMA only, by taking several numbers

of sensor nodes and it is found that network throughput willC. Code assignment algorithm degrade rapidly as the numbers of sensor nodes increases

because the probability of numbers of simultaneousCODE_ASSIGN_SENSOR (C1) transmission will increase. But in the proposed FDMA-Begin CDMA system, no MAI will occur. So all the packets will bedo ( Ni E CL1) successfully transmitted. In figure 6, it is seen that as the range

Forj =1 to n increases, average numbers of nodes in each cluster willIf (code CDij is free in the code pool CDI) increase and without using clustering and FDMA-CDMA

Assign code CDijto it and Mark CDijassigned in technique, MAI also increases. So the throughput will beCODE pool CD i considerably degraded.

endlfendFor

enddoEnd 4

4035-

V. ExPERIMENTAL RiESULTS ~,30This section introduces a simulation environment used in the 25experiments. Simulation has been done on finite random .; 20CDMA based sensor networks with and without using the 15proposed FDMA scheme, by varying the connectivity patterns 10and number of sensor nodes. The nodes are placed on a 600 x 5450 area randomly. The network is fully connected one. The 0transmission ranges of sensor nodes are varied from 50 to 150 50 60 70 80 90 100 110 120 130 140 150units. Simulation is done using OMNet++. In Fig. 3, it is rangeshown that how the number of clusters changes with differenttransmission ranges. From the Fig.3, it is clear that as range Fig. 3 Numbers of clusters formed with transmission range (N=500)increases, numbers of clusters formed decreases. If range ischosen very high then numbers of clusters will be less andaverage numbers of nodes under each cluster will be high. Inthat case bandwidth requirement for each cluster will be highand the proposed frequency band will not be sufficient. On theother hand if range is chosen ver-y low then numbers of 450 -iclusters will be high and average numbers of nodes in each 400cluster will be low. In that case proposed bandwidth will be E 350unutilized. From the simulation it is found that range should -T300-Ebe such that each cluster should have 15 to 20 nodes. 250 ---FDMA-CDMAFrom equation (1), as the interfering power increases, ,ui 200-=CDMA

4-3'decreases, and the bit error probability increases. As an15example [1], consider a CDMA system that uses BPSK ,,0modulation and a convolutional code with rate 1/2, constraint 5length 7, and soft decision Viterbi decoding. Let L = 100. To 0 60 110 160 210 260 310 360 410achieve a bit error probability of 10-6, the required Eb /NOeff is Load5.0 dB. Ignoring the thermal noise and using (1), the totalinterference power must satisfy:

Fig.4 Successful transmission of packets with load (N=500)k

pi_ (2)1=1 < 47.43PO

Using this above equation in the simulation, it is found that,how much data will be successfully transmitted under varyingload applied in the network. From the Fig. 4 and 5 it is foundthat as the load (packets transmitted by different sensor nodes)increases for a particular time duration packet loss will be

744 2009 IEEE Internlationlal Advance Computing Conference (IACC 2009)

sensor network by dividing the available frequencies intosoo several groups and then reusing them. We also proposed an700

- algorithm for clustering the sensor network. From our700 simulation of the proposed system, we found that 15-20600 members in each cluster give the optimal results in terms of500 -_* FDMA-CDMA packet delivery. Our protocol achieves less channel400 CDMA contention and high packet delivery ratio compared to normal

j 300 CDMA-based system. By reducing the channel contention, we200 also achieved energy savings in energy scare sensor network.100 -

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2009 IEEE Internlationlal Advance Computing Conference (IACC 2009) '145