A New Scheme for Cooperative Communication in LEACH based Wireless Sensor Network

Neetesh Purohit1, Himanshu Agrawal2, Ankit Jain3 Indian Institute of Information Technology

Jhalwa, Allahabad, India 1np@iiita.ac.in

2himanshuvds@gmail.com 3iwc2010020@iiita.ac.in

Abstract — Wireless Sensor Networks need energy efficient protocol for carrying out various operations. Cooperation among nodes may save significant energy by avoiding retransmission from distant nodes. In this work, a new scheme has been suggested for LEACH based WSN by introducing cooperation among various cluster heads. Significant improvement in BER has been observed for comparable overall lifetime of ordinary WSN. Cooperative communication with punctured convolutional channel coding is used in operation phase where as point to point transmission technique is used in clustering phase. Rayleigh fading channel and additional energy consumption due to retransmissions by relay nodes has been accounted in simulation. Bit error rate (BER) and overall life time of WSN is found to be dependent on the number of cooperative nodes, sensor nodes density in WSN, etc factors. Keywords— WSN, LEACH, Cooperative Communication, Rayleigh fading

I. INTRODUCTION Many application areas require random deployment of

WSN over the intended area and it need to perform unattended operation due to several practical constraints [1, 2]. This unique characteristic requires additional features in WSNs like ability to withstand harsh environmental conditions, ability to cope with node failures, ability to manage dynamic network topology etc [3, 4]. The limited power they can harvest or store requires energy efficient ways for implementing above and several related jobs. The sleep-awake cycles, clustering etc techniques have been proposed for reducing the energy consumption but still a big chunk of energy is consumed in data transmission from sensor nodes to cluster head or sink, which can be checked by improving the transmission quality over wireless channel. Several receiver centric diversity techniques have been used in various wireless systems for aforesaid purpose. The transmitter centric and Cooperative communication technique, are recent additions to this field [5]. The performance improvement in cooperation is proportional to total number of cooperating nodes used at transmitter side. In such schemes two or more nodes cooperate for communicating with the base station. Overhearing based cooperation has been widely studied in which the transmission from a node of a cooperative group to the base station is overheard by other nodes, called relay nodes, then in next slot these nodes transmit the amplified overheard symbol or a new

symbol derived from overheard symbol by following some distributed coding and cooperative scheme [6]. Cover and Gamal [7] give the concept of relay network in 1979 which improves the capacity of the network and it further enhance to the concept of cooperative communication. For implementing the concept of cooperative communications, it is essential to design proper protocols and signal processing techniques. There are so many relaying protocols and channel coding techniques have been evolved in the last decade. There is a small difference between cooperative networks and relay networks. In relay network relay node is a special node. It forwards the incoming signal to the destination. Relay nodes do not have their own data to transmit [8]. While in cooperative networks each node can behaves as a source and relay. So that each node transmits its own information as well as cooperate with other nodes to transmit. However the signal processing at the relay is identical in both cases. The basic theory behind a relay network is as follows. The total frame time is divided into two time slots. In the first time slot source node transmit its information to the destination. Relay overhear the transmitted signal, after processing the received signal forward it to the destination. There are various processing techniques that can be used by the relay to process the signal. For example it can amplify and forward the incoming signals. It can also decode the received signal before forwarding it to the destination. To enhance the system performance relay can execute various operations like coded cooperation [9] in which they use the concept of incremental redundancy. In the second time slot relay transmit to the destination. Both source node and relay can transmit on orthogonal or non orthogonal channels. Since signals received at the destination arrive via different paths from the source and various relays, spatial diversity can be achieved at the destination. Amplify and forward, decode and forward, Incremental redundancy etc cooperative schemes have been suggested for the same [8]. The author of [10] has proposed a scheme of cooperation in clustered wireless sensor network, where members of the cluster cooperate with the cluster head to communicate with the cluster head of neighbouring cluster. In [11] author has presented a cooperation scheme that is based on feedback from the destination node. Feedback is used to decide whether cooperation is necessary or not. If it is necessary than destination select a node from available nodes for cooperation. Destination selects that particular node as

2011 International Conference on Computational Intelligence and Communication Systems

978-0-7695-4587-5/11 $26.00 © 2011 IEEE

DOI 10.1109/CICN.2011.10

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2011 International Conference on Computational Intelligence and Communication Systems

978-0-7695-4587-5/11 $26.00 © 2011 IEEE

DOI 10.1109/CICN.2011.10

47

2011 International Conference on Computational Intelligence and Communication Systems

978-0-7695-4587-5/11 $26.00 © 2011 IEEE

DOI 10.1109/CICN.2011.10

47

cooperative node that can provide maximum instantaneous mutual information. In any case the forwarded messages by cooperative users follow different paths thus obtain the spatial diversity. The retransmission of overheard symbol by cooperative nodes improves the communication performance but it can be further enhanced if appropriate nodes are selected as relay nodes. The cluster head choose another cluster head as its relay on the basis of the best source-relay channel and if relay can decode the message of source node correctly. Choosing an optimal relay that can give satisfactory BER performance and keep the overall lifetime comparable to ordinary WSN is an important issue. In this work a new scheme has been suggested for addressing the same. In cooperative communication relay node regenerate the overheard message and then transmit it to the base station. There are some issues like errors in decoding of the codeword at relays, variations in the channel condition and allocation of the data rate and power at source and relays must be taken care of.

This paper is organized in five sections. The proposed scheme for cooperation in WSN, the selection criteria of relay node etc has been included in section II. The simulation framework has been summarized in section III. Various result followed by discussion has been reported in section IV. Finally conclusion and future prospects are discussed in section V.

II. THE PROPOSED SCHEME FOR COOPERATION The process of cooperation can be divided into following

phases. Clustering is the first step which is followed by relay node discovery then cooperative communication takes place between base station, cluster head and its relay. All these phases require proper allocation of resources. Figure 1 shows the flow chart for the suggested scheme.

LEACH protocol [12] has been used for the purpose of clustering. In this energy is distributed uniformly by choosing cluster head randomly from a group of sensor nodes present inside the network. If cluster head are taken fixed then the energy of the clusters head node dissipate fast and they will die before the other nodes of network. Fixing cluster head is of no use as sensor node doesn’t reply to any query. But leach has many limitations to incorporate cooperation like: (i) It doesn’t ensure that all the cluster head will uniformly distributed in the network, therefore there are some cluster heads that doesn’t find any node (cluster head) for cooperation. (ii) The value of various parameters like number of sensor nodes, probability of being a cluster head etc should be chosen appropriately otherwise it degrade the performance of WSN.

Now for the purpose of relay selection two conditions must be satisfied. First is the location of the relay node must be in between two communicating nodes (i.e. CH and BS) and the total path length should be minimum (i.e. sum of path length between cluster heads and base station).The process of finding the path length is as follows. After performing the clustering in the network, base station broadcast a hello message to all cluster heads, and then on the basis of RSSI all cluster head can calculate their distance from the base station.

After that, Cluster heads broadcast a hello message with very less power to its neighbouring cluster heads. Then neighbouring cluster heads reply back to the hello message and they also send their distance from the base station. Cluster head (source node) may receive the signals from more than one node. Then on the basis of RSSI it calculates the distance of itself from all other nodes. On the basis of all these parameters cluster head (source node) is able to find its most suitable relay node. In this phase it ensures that the node, chosen as a partner must be reliable.

Fig 1: Flow chart of the proposed scheme for cooperation

The selection of the relay is repeated in every round because cluster heads are not static. For the purpose of simplicity a cluster head can be a relay or partner only for one cluster head (source node), and relay should directly connect with base station, cluster head which is very near to base station can only be relay to another cluster head or it should directly connect to sink without having any relay. All the cluster head

no

yes

yes no Node i Cluster head ?

Announce CH status

Wait for CH announcement

Wait for join req msg

Wait for schedule

Send join req msg

Send TDMA schedule

Broad. Hello msg to neighbour CH

CH cal. dis of neighbour CHs

CHs send their dis from BS

Neighbour CH chosen as relay

CH comm with BS

Relay overhear

Decode signal

Steady state operation for t= Tround sec

Relay tx to BS Go for retx

BS cal BER if BER>Thresh

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will run the algorithm to choose the relay but before doing so it has to check whether it has not been selected as a relay for any other node. It is assume that multiple relays are available for the purpose of selection as a partner by the source node.

After finding relay the next phase is sending data to base station. In this phase source node transmit data to base station that is overheard by the relay node. And then it successfully decodes the incoming message. In this paper it is assume that relay is able to faithfully regenerate the code. In the system model there are two nodes in which one node is transmitting to the base station and other node is overhearing the transmission. It is basically the classical relay model in which one node act as a source (cluster head), other as a relay (cluster head) and another one is a destination node (base station). The role of the relay node is to cooperate the source node to communicate with the base station. Following assumptions have been considered in developing the model: (i) The channel between the nodes and each pair of nodes and base station is assumed as a flat Rayleigh fading; (ii) At a time a node can only transmit or receive the signals; (iii) At the destination channel state information and value of the instantaneous SNR is available.

The packet format of hello message is shown in figure 2 [13]. There are three fields: header, information field and trailer. The header field generally contains the identification number of source and destination, packet sequence number, message type, in WSN header is of very few bytes. Data field contains data or control information depending upon the type of message and trailer contains the cyclic redundancy check bits. The control packet contains TDMA schedule of the cluster members just like LEACH, distance between cluster heads and base station and energy level of the node. When cluster head send TDMA schedule to the cluster members, header field contains the node ID of cluster head, packet sequence number and message type is set as control packet. When cluster head send a hello message to its neighbour cluster heads, data field contains distance of cluster head from base station and its energy level and message type is set as control packet. When actual transmission takes place then message type is set as data packet.

Header Information or data Trailer

Fig 2: Packet format of Hello Message

In this system all the members of the cluster send their

information to their cluster head then cluster head aggregate the incoming information and then transmit it to the base station. Before transmitting, it generates the punctured convolutional code of the data. This information is overheard by the chosen relay. Relay decode the incoming information and regenerate the code. Upon receiving, base station calculate the BER of the incoming data, if it is more than a particular threshold value then it asks for retransmission. Then relay node retransmit the decoded information after finding its punctured convolution code. The benefit of this approach is, it increases the reliability of communication and at the same

time it doesn’t degrade the lifetime of network because relay needs less power to transmit as compare to source node.

III. THE SIMULATION FRAMEWORK In this section, various parameters and their default values have been defined. The field of 100x100 dimensions has been taken, number of sensor nodes are 100 and they all are homogenous in characteristics. The nodes are deployed randomly over that field. The horizontal and vertical coordinates of each and every sensor nodes must be saved to calculate the energy dissipation in data transmission. Sink has been placed in centre of the field to neglect the multipath fading error. Each node has 0.5 joule energy. It is just the value and can be assigned any value to nodes but keeping in mind that all sensor nodes have equal energy before any action or event occurs. MATLAB has been used for simulation. Table 1 shows rest of the detail about the energy consumption in various operations [14]. Capacity of the channel is 1 mbps, modulation is BPSK, decoding is Viterbi, size of the data packet is 500 bytes and header is 25 bytes long [14].

TABLE I RADIO CHARACTERISTICS

Operations Energy consumption

Electronics energy (Eelec) 50 nJ/bit

Amplifier energy (efs) (emp)

10 pJ/bit/m2

0.0013 pJ/bit/m4

Energy for data aggregation (EDA)

5 nJ/bit/signal

Fig 3: Random deployment of 100 nodes with 9 cluster heads Figure 3 shows the random deployment of sensor nodes. After deploying the nodes the clustering algorithm LEACH has been run with certain modifications. In this P= 0.04 has been taken i.e. election probability of nodes to become cluster head. Figure 4 shows the communication between the members of the cluster and cluster head.

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Fig 4: Communication between nodes and cluster head After clustering the relay selection algorithm has been executed. In this algorithm the cluster head choose another cluster head as its relay on the basis of the best source-relay channel and if relay can decode the message of source node correctly. The aim is to choose an optimal relay that can give us good BER performance. Figure 5 shows the selection of relay node to communicate with base station.

Fig. 5. Selection of the relay node to communicate with the base station In figure 5 there are two cluster heads, which are successful to find their relay nodes (cluster heads). And it is clear from the figure 5 that the relay node is in between two communicating parties. To simulate the communication many random binary sequences of +1’s and -1’s have been generated, to be used as input data in sensor network. The energy of the node is computed after every transmission of data and check the available energy of the node. So that it is able to further participate in the operation phase of the network. By using convolutional codes symbol transmission is carried out. Now use the channel coefficient H and multiply it with the

transmitted symbol. And in order to find the received symbol add the white Gaussian noise to the final value of multiplication. Now decode the received signal and count the bit errors by comparing transmitted and received bit sequence. If the errors are more than certain threshold value then base station asks for retransmission. The retransmission is done by the relay node. The value of threshold should be chosen carefully otherwise it can degrade the performance of the network. One percent of the length of the message is taken as threshold value. The residual energy of the nodes has been calculated before starting a new round.

IV. RESULTS AND DISCUSSION The LEACH protocol has been used with some

modifications. It assumes that sensor nodes always communicate with the cluster head whenever there is an event sensed by them. The results clearly indicate that by using a relay node for communication, it reduces the BER effectively and at the same time it does not degrade the lifetime of wireless sensor network. Figure 6 shows the different values of BER according the different values of SNR for BPSK modulation and it compares the BER performance for both cases.

Fig 6: Comparison of BER performance in Cooperation and Non-Cooperation case For the low values of SNR there is not much difference between the performances of network. But in case of higher values of SNR huge difference is created. Now the lifetime of the sensor network in both these cases can be compared. Simulation for 2500 rounds in both cases has been executed. The total number of nodes is 100 in beginning of the simulation. In case of non-cooperation first node dies at 1451th round. At 2401th round there are only 12 active nodes. At the end of the simulation only 6 nodes are remain alive. Now in case of cooperation first node die at round number 1412 that is a little bit earlier than non-cooperation case.

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Fig 7: Comparison of Lifetime of sensor network in Cooperation and Non-Cooperation case But at 2401th round there are 12 active nodes that is equivalent to non-cooperation case. And at the end there are 8 nodes remain alive. The comparison of both these cases has been plotted in figure 6.

V. CONCLUSIONS AND FUTURE WORK A new scheme for establishing cooperation among

optimally located cluster heads, designated after clustering phase of LEACH based WSN, has been suggested. The BER performance has been significantly improved at the same time lifetime of the network has not been degraded. The performance of the network largely depends upon the number of relaying nodes, position of the relay nodes, condition of the channel etc. Synchronization among relays, coordination among users, resource utilization etc. issues may play vital role in implementation. Allowing multiple relays, fuzzy logic based relay selection etc may be the future extension of this work.

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