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Spread Spectrum Based Energy Efficient Wireless Sensor Networks


Published on

The Wireless Sensor Networks (WSN) is
considered to be one of the most promising emerging
technologies. However one of the main constraints which
is holding back its wide range of applications is the
battery life of the sensor node and thus effecting the
network life. A new approach to this problem has been
presented in this paper. The proposed method is suitable
for event driven applications where the event occurrence
is very rare. The system uses spread spectrum as a means
of communication.

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Spread Spectrum Based Energy Efficient Wireless Sensor Networks

  1. 1. ACEEE International Journal on Network Security, Vol 1, No. 2, July 2010 Spread Spectrum Based Energy Efficient Wireless Sensor Networks Wg Cdr Varghese Thattil (Retd)1, Dr. N Vasantha2, T. Esther Rani3 1 CVR College of Engineering Electronics and Communication, Hyderabad, India 2 Vasavi College of Engineering, Hyderabad, India Email: 3 CVR College of Engineering Electronics and Communication, Hyderabad, India Email: esterlawrence@rediffmail.comAbstract—The Wireless Sensor Networks (WSN) is energy saving is one of the key issues in the design ofconsidered to be one of the most promising emerging systems based on WSNs.technologies. However one of the main constraints which It has been experimented and seen that the datais holding back its wide range of applications is the communication of sensor nodes consumes more energybattery life of the sensor node and thus effecting thenetwork life. A new approach to this problem has been than the data processing and therefore there is a need topresented in this paper. The proposed method is suitable reduce the communication while achieving the desiredfor event driven applications where the event occurrence network operation [1]. The energy cost of transmittingis very rare. The system uses spread spectrum as a means one single bit of information is approximately same asof communication. that needed for processing a thousand operations in a typical sensor node [2]. However, since the sensorIndex Terms—Rare event, Wake-up receiver, Single hop, network is very dense; composed of nodes with lowSpread spectrum duty-cycles the medium access decision is a hard problem. Therefore it is important to know the peculiar I. INTRODUCTION features of sensor networks including reasons of Wireless Sensor Networks (WSNs) is considered as potential energy wastes at medium accessa new technological vision because of the rapid communication.development in miniaturization; low power wireless A. Reasons of Energy Wastagecommunication, microsensor, and microprocessorhardware. WSNs may, in the near future, will provide Energy wastage occurs when there are collided“Ambient Intelligence” where many different devices packets, which occurs when a receiver node receiveswill gather and process information from many more than one packet at the same time or when they aredifferent sources to both control physical processes and partially overlapped. All packets that cause theto interact with human users. Potential applications of collision have to be discarded and the re-transmissionsWSNs include environmental monitoring, industrial of these packets are required which increase the energycontrol, battlefield surveillance and reconnaissance, consumption. Although some packets could behome automation and security, health monitoring, and recovered by a capture effect, a number ofasset tracking. requirements have to be met to achieve a successful Improvements in hardware technology have resulted capturing. The second reason of energy waste isin low-cost tiny sensor nodes which are composed of overhearing, meaning that a node receives packets thatthree basic components: a sensing subsystem for data are destined to other nodes. The third energy wasteacquisition from the surrounding environment, a occurs as a result of control packet overhead. Minimalprocessing subsystem for local data processing and number of control packets should be used to make astorage and a wireless communication sub system for data transmission. One of the major sources of energydata transmission to a sink node or access point. In waste is idle listening, i.e., listening to an idle channeladdition to this; the energy required to carry out the to receive possible traffic. The last reason for energyprogrammed task is supplied from battery pack with a waste is overemitting, which is caused by thevery limited energy budget. In most of the applications transmission of a message when the destination node isit has been observed that the energy source can not be not ready. Given the above facts, an efficientlyreplaced due to hostile environment or other practical designed MAC protocol should reduce these energydifficulties. However the sensor networks should be wastes to a large to sustain till such time the indented task is B. Communication Patternscompleted. Though energy scavenging from external Kulkarni et al. defines three types of communicationenvironment can be a possible solution, the technology patterns in wireless sensor networks [3]: broadcast,is not yet developed into a reliable source. Therefore convergecast, and local gossip. Broadcast type of 1© 2010 ACEEEDOI: 01.ijns.01.02.01
  2. 2. ACEEE International Journal on Network Security, Vol 1, No. 2, July 2010communication pattern is generally used by a base Additional components like as location finding systemstation (sink) to transmit some information to all sensor to determine their position, a mobilizer to change theirnodes of the network. Broadcasted information may location or configuration etc. may be added dependinginclude query-processing information, program updates upon the applications where they are used.for sensor nodes, control packets for the whole system Out of the above subsystems, the radio componentetc. In some scenarios, the sensors that detect an account for maximum power consumption. Thereforeintruder communicate with each other locally. This many protocols have been designed with the idea ofkind of communication pattern is called local gossip, minimizing the radio communication this in most of thewhere a sensor sends a message to its neighboring cases has been achieved by increased data processing atnodes within a range. The sensors that detect the the nodes.intruder, then, need to send what they perceive to theinformation center. That communication pattern iscalled convergecast, where a group of sensorscommunicate to a specific sensor. The destination nodecould be a clusterhead, data fusion center, base station.C. Properties of a Well-defined MAC Protocol To design a good MAC protocol for the wirelesssensor networks, the following attributes must beconsidered [4]. The first attribute is the energyefficiency. We have to define energy efficient protocolsin order to prolong the network lifetime. Otherimportant attributes are scalability and adaptability to Figure 1 Wireless Sensor Node Architecturechanges. Some of the reasons behind these networkproperty changes are limited node lifetime, addition of The communication distances in WSNs are typicallynew nodes to the network and varying interference small; a few meters in most of the applications and itwhich may alter the connectivity and hence the has been found that power consumed for reception isnetwork topology. almost same or even more as compared to transmission. Other typical important attributes such as latency, One of the most power efficient designs for WSN couldthroughput and bandwidth utilization may be secondary be the one where in the radio, which consumesin sensor networks. Contrary to other wireless maximum power could be kept in sleep mode. Thenetworks, fairness among sensor nodes is not usually a radio should be switched on only when there is adesign goal, since all sensor nodes share a common specific need to transmit or receive data. Therefore thetask. radio should alternate between active and sleep mode There are mainly two approaches to energy keeping the duty cycle as low as possible.conservation: In-network processing and through duty However since the WSN is a cooperative system thecycle controlling. In–network processing is a method sleep/wakeup schedule normally follows a distributivewherein the amount of data being transmitted is being algorithm. An efficient distributive algorithm shouldreduced; either by compression or aggregation ensure that the neighboring nodes in the radio rangetechniques. It typically exploits the temporal or spatial should be able to receive whenever a node iscorrelation among data acquired by sensor nodes. On transmitting keeping the duty cycle as low as possible.the other hand, duty cycling schemes define Many WSN applications are event based wherein acoordinated sleep/wakeup schedules among nodes in communication of data is to be essentially carried outthe network. only on detection of an event. Massively deployed sensor nodes simultaneously detect the event and hence II. ENERGY CONSUMPTION REDUCTION can cause duplication of information. Therefore application layer energy efficiency can be achieved byA. Possible Energy Saving Approaches in-network processing or data aggregation. The energy To tackle the problem of energy consumption it is efficiency processing can be either through data qualityessential to know how much power each node or avoiding unwanted data redundancy [7].component dissipates during normal operating It is important to note at this stage that whileconditions, i.e., which are the power dissipation improving the energy efficiency in a single layer itcharacteristics of sensor noses [6]. In a typical wireless should not effect the overall energy consumption in thesensor node there are four main components: Sensing node as a whole. In many applications, a cross layersubsystem having sensors for data acquisition and approach may be more suitable to reduce the overallanalog to digital converter, processing subsystem energy consumption in a particular node. The topologyconsisting of micro-controller and memory for local control [8] and power management are the twoprocessing, radio subsystem for wireless data approaches to energy efficiency. In the case ofcommunication and a power supply unit Figure 1. topology control, the network redundancy is exploited 2© 2010 ACEEEDOI: 01.ijns.01.02.01
  3. 3. ACEEE International Journal on Network Security, Vol 1, No. 2, July 2010to improve the energy efficiency whereas in power power management service based on wake-up/sleepmanagement sleep/wakeup timings are controlled to scheduling.achieve minimum duty cycle. The study of the energy consumption throughout the lifespan of a typical surveillance systems network nodeB. Power management shows that most of the energy is used up in operations Power management schemes need to control when a that do not actively fulfill the system’s purpose. A nodenetwork node should enter a high-power wakeup mode without power management is always turned on, butand when to enter a low-power sleep mode. Under there is no target most of the time. Hence, most of itscertain conditions, the nodes’ hardware components energy is dissipated in a waiting status. Only one permay be shut down to low-power state. The low-power cent of the energy is used in actually tracking targets,to high-power transition is, however, a tricky problem the other 99% of the energy is used in waiting forbecause the network node has its CPU halted and is targets to show up. With a rotation based powerunaware of the external events. Power management management, the energy efficiency is much better.techniques can either be done strictly at MAC layer orusing sleep/wakeup protocols at higher layers (Network D. Low Energy Wakeup Receiveror application layer) which are integrated on top of L. Gu et al. proposed to use passive components toMAC layer. In the case of MAC layer power collect radio energy similar to the case of RFID (Radiomanagement, lower duty cycle is achieved by TDMA frequency Identification) technology. The(Time Division Multiple Access), Contention-based microcontroller is interrupted when the induced RFprocedures or their combinations. In the case of power input is large enough and this in turn wakes upTDMA, the nodes need to turn on their radio only the sensor node for data communication. However theduring the allotted time slots. However this poses disadvantage in such a system is that any strong RFproblem of flexibility and scalability. The scheme also signal can wakeup the node and also the range ofhas to ensure tight synchronization [9]. operation will be limited. A modified system which Energy efficiency in the case of Contention-based was proposed is to have an addressing mechanism withprotocols is achieved by tightly integrating channel the help different RF triggering frequencies. Theaccess functionalities with sleep/wakeup schemes [10]. system will however require additional wakeup hardware for each frequency [13]. In this caseC. Sleep/wakeup protocols minimum required active components are added to the The power management schemes can be broadly passive wake up radio so that messages could be sent todivided into three categories, depending upon the particular nodes to wake up the nodes. The wakeupmethod used to wake up the node. They are on- receiver consists of an impedance matching circuitdemand, schedule based and asynchronous. In the case followed by a voltage multiplier (charge pump). Theof on-demand the node wakes only when there is an charge pump consisting of capacitances along withexternal interrupt from another node [11]. The main schottky diodes, increases the weak input voltage to aproblem associated with on-demand schemes is how to level at which the microcontroller can be triggered. Theinform the sleeping node that some other node is schematic diagram has been shown in Figure 2.willing to communicate with it. To achieve this, Wakeup signal transmitter sends amplitudeadditional wake up radios with low data rate and low modulated carrier signal. The received output is finallypower consumption is used for signaling. In the case of passed through a digital comparator to recover theschedule based approach each node should wake up at envelope. The comparator also helps to have a noisethe same time as its neighbors. However this requires rejection as well as to have square pulse output. In thisrigorous synchronization between the nodes causing architecture digital comparator is the only activeincrease in latency. component which will draw power from the source. The node synchronization between nodes can be Lower distance of wake up radio communication is aavoided by asynchronous power management. In this big disadvantage in this, the nodes can wake up when ever they want andstill be able to communicate with their neighbors. Thisis achieved by having a pattern of wake up wherein atleast two nodes have an overlapping wake up duration.In this approach the nodes are expected to wake upmore often and hence the duty cycle tends to be higheras compared to synchronous protocols [12]. The implementation of the wakeup/sleep schedulingoften involves a timer that wakes up the CPU via aninterrupt. The wake-up/sleep scheduling approach hassome disadvantages. First, the design of a good wake- Figure 2. Wakeup Receiver Block Diagramup/sleep schedule is often application dependent andcomplicated. Hence, it is hard to design a general 3© 2010 ACEEEDOI: 01.ijns.01.02.01
  4. 4. ACEEE International Journal on Network Security, Vol 1, No. 2, July 2010 III. PROPOSED SCHEMEA. Proposed Scheme Motivation There are many applications like habitat monitoringor movement detection etc. of sensor networks whereevent monitoring has to be carried out in a large area.There are applications where the area of interest has tobe continuously monitored for long duration. Thisbecomes very difficult unless the system is madeautomatic. The system should be able to detect the Figure 4. Sector scanning timingsevent and automatically initiate the necessary actions asrequired. These can be setting off alarms, send A detailed timing diagram for the systemmessages switch on cameras/lights in the required implementation is shown in Figure 5. The antennaregion etc. The advantage of the specific application dwelling time in each sector is 10 seconds. Thecan be made use of by having a cross layer protocol so positioning of the antenna in the next sector takes 1that the power energy efficiency can be maximized. second, however this timing will be lesser if electronicB. Proposed Scheme Description scanning is used. Once the antenna has been placed to The proposed scheme is to deploy the sensor nodes the sector; the BS transmitter emits 5 microseconds (µin the entire region of interest. Each sensor node will sec) pulses at an interval of 10 milliseconds (msec) forhave its own unique spread spectrum code to identify duration of 1 second. Immediately after that the BSit. The distance between the nodes have to be decided transmitter shuts off and the receiver is tuned to receiveon the basis of the sensing range of the associated the sensor nodes transmissions.sensors. The Base station (BS) is having four directive C. Wake-up Receiverantennas covering the entire region around it. The All sensor nodes will have wake up receivers whichsensor nodes have low power wake-up radios which are tuned to receive the trigger pulses send from thecan be triggered with the help of Base station directive Base Station (BS). As the pulses received at the inputantennas. The deployment plan has been shown in of the wakeup receiver increases the voltage at theFigure 3 The Base station will sequentially scan the input of the voltage multiplier starts rising; refer Figurefour sectors by sending trigger pulses in the form of 2. When the voltage at the output of voltage multiplierseries of square pulses for few milliseconds and crosses threshold level, the wake up receiver sends ansubsequently the Base Station receivers are switched on interrupt to the microcontroller to initiate theto receive the transmissions from the sensor nodes. transmission of data signals to BS. A timing diagram has been shown in Figure.4. Thesensor devices in the sensor nodes will continuouslyobserve the physical parameters within its range. Thenodes will respond to the wake-up call only incase theobserved parameters crosses a minimum thresholdlevel. This avoids unwanted transmissions and thusimproving the battery life. At the Base station, thecorrelation receivers acquire and track the incomingsignals from the sensor nodes and recover the data. Figure 5. Base Station and node transmit and receive timings D. Sensor Node Architecture The proposed architecture of the sensor node has been shown in the figure 6. The observed analog values from the sensor devices are sequentially taken, converted to digital and stored in the digital storage registers as obs_value. The obs_value from each sensor device is stored till such time it is forwarded to digital comparator. Max_value registers (MVR) shown in the Figure 3. Deployment of Sensor nodes along with Base architecture are used to store the initial minimum Station threshold values. These values are given as input to the digital comparator to compare the obs_value stored in 4© 2010 ACEEEDOI: 01.ijns.01.02.01
  5. 5. ACEEE International Journal on Network Security, Vol 1, No. 2, July 2010the corresponding registers of the digital storage. incoming signal with the known PN sequences of theIncase the obs_value is greater than the Max_value, the sensors and recover the data. Since there is noMVR is appended with the current obs_value stored in synchronization between the nodes and the Base stationdigital storage. The obs_value from each sensor is Asynchronous CDMA communication is required.converted to digital at an interval of 800 msecs. Thedigital comparison is carried out at every 200 msecs. F. Simulation ResultsThe total observation time per sensor is 44 seconds Sensor node architecture has been designed andbefore it being transmitted to BS; Figure 5. implemented using VHDL language in Xilinx 9.2i. The The hostile environment described is such that the simulation has been carried out by ISE simulator andintrusion detection is expected to be a rare event. synthesized by using XST. The digital comparison isTherefore, MVR will not be appended during most of simulated with a timing of 10 nano seconds (equivalentthe sensor observation cycles. The sensor architecture to a real time periodicity of 200 milli seconds of sensorensures that the transmission from the sensor node will multiplexing) for each sensor and sweep timing of 800take place only if one of the sensors obs_value crosses nano seconds has been taken. The timing diagramthe minimum threshold value set in MVR. Minimum shows that the MVR is getting updated only when thethreshold value is selected such that noise and other obs_value crosses the threshold value set in the MVR.interferences will not trigger unwanted transmissions The wake up trigger required for activating thefrom the nodes. This is achieved in the architecture by transmission section will be available only once duringsimply setting a flag when the MVR is appended. The the antenna scan of 44 seconds. On arrival of the wakeoutput from the MVR is taken only if the flag for the up trigger the processor will check whether any of thecorresponding sensor is set. MVRs has been updated. The system will transmit only once during the antenna provided MVR is updated. TheE. Spread Spectrum Communication system simulation shows substantial saving in energy Once the wake up signal has been received, the consumption as the MVR updation takes place rarely.Max_value stored in MVR is EXORed with the stored Some random values have been given as input from thePN sequence. The output of the EXOR is BPSK four sensors. The schematic diagram of the sensor nodemodulated with the carrier frequency of 10 GHz. This architecture is shown in Figure 6. The selection timingis further amplified and transmitted to the base station. of obs_value as it crosses the threshold stored MVR isAt the Base station each sensor node is to have a shown in Figure 7.separate CDMA receiver. These receivers correlate the Figure 6. Sensor node architecture Figure 7. Simulation timings at sensor node 5© 2010 ACEEEDOI: 01.ijns.01.02.01
  6. 6. ACEEE International Journal on Network Security, Vol 1, No. 2, July 2010G. Energy Saving At The Nodes [4] C. C. Enz, A. El-Hoiydi, J-D. Decotignie, V. Peiris, “WiseNET: An Ultralow-Power Wireless Sensor Network In most of the WSN applications the detection of Solution”, IEEE Computer, Volume: 37, Issue: 8, Augustan event by a sensor device is a rare event and 2004.therefore the data need to be transmitted only if an [5] V. Rajendran, K. Obraczka, J.J. Garcia-Luna-Aceves,event is detected. In the proposed scheme, the sensor “Energy-Efficient, Collision-Free Medium Access Control for Wireless Sensor Networks”, Proc. ACM SenSys 03,devices continue to observe the physical parameters Pages:181 - 192, Los Angeles, California, 5-7 Novembertill such time the wake up signal is arrived. The 2003.Max_value stored in MVR is appended only when [6] L. Bao and J.J. Garcia-Luna-Aceves, “A New Approach Toobs_value exceeds the minimum threshold value Channel Access Scheduling For Ad Hoc Networks”,stored in it; which means that only when an event Seventh Annual International Conference on Mobileoccurs. Once the wake up signal is received, the Computing and Networking, pp. 210–221, 2001.system verifies whether the data stored in the [7] K. Jamieson, H. Balakrishnan, and Y. C. Tay, “Sift: Acorresponding registers of MVR have been MAC Protocol for Event-Driven Wireless Sensor Networks,” MIT Laboratory for Computer Science, Tech.appended. The appended data is only selected for Rep. 894, May 2003further transmission. This ensures that the data [8] Y.C. Tay, K.Jamieson, H. Balakrishnan, “Collision-communication from the sensor nodes will take place minimizing CSMA and Its Applications to Wireless Sensoronly if an event occurs. Networks”, IEEE Journal on Selected Areas in However, the very low level of communication Communications, Volume: 22, Issue: 6, Pages: 1048 –between the Base station and sensor node can pose a 1057, Aug. 2004. [9] G. Lu, B. Krishnamachari, C.S. Raghavendra, “An adaptiveproblem of identifying dead nodes (dead battery) energy efficient and low-latency MAC for data gathering inover a period of time. The system will have a wireless sensor networks”, Proceedings of 18thmandatory transmission after every ten wakeup International Parallel and Distributed Processingsignal cycles even if there is no event detection. Symposium, Pages: 224, 26-30 April 2004. [10] T.V. Dam and K. Langendoen, “An Adaptive Energy- CONCLUSIONS Efficient MAC Protocol for Wireless Sensor Networks”, The First ACM Conference on Embedded Networked One of the prime criteria that differentiate WSN Sensor Systems (Sensys‘03), Los Angeles, CA, USA,nodes to other wireless nodes is the energy November, 2003.consumption since most of the WSN nodes are [11] P. Lin, C. Qiao, and X. Wang, “Medium access controlsuppose to have minimum human interaction which with a dynamic duty cycle for sensor networks”, IEEEis essential for recharge/replacement of battery. A Wireless Communications and Networking Conference,method has been suggested which can be very Volume: 3, Pages: 1534 - 1539, 21-25 March 2004. [12] A. Safwat, H. Hassanein, H. Mouftah, “ECPS and E2LA:effectively implemented in many event detection new paradigms for energy efficiency in wireless ad hoc andapplications. The full details of the system has not sensor networks”, IEEE Global Telecommunicationsbeen described, however the paper gives sufficient Conference, GLOBECOM’03, Volume: 6, Pages: 3547 -scope for the development of the system. 3552, 1-5 December 2003. [13] S. Cui, R. Madan, A. J. Goldsmith, and S. Lall, “Joint REFERENCES Routing, MAC, and Link Layer Optimization in Sensor Networks with Energy Constraints”, to appear at ICC05,[1] S.S., Kulkarni, “TDMA services for Sensor Korea, May, 2005. Networks”, Proceedingsof 24th International [14] J. Ding, K. Sivalingam, R. Kashyapa, L. J. Chuan, “A Conference on Distributed Computing Systems multi-layered architecture and protocols for large-scale Workshops, Pages: 604 – 609, 23-24 March 2004. wireless sensor networks”, IEEE 58th Vehicular[2] W. Ye, J. Heidemann, D. Estrin, “Medium Access Technology Conference, 2003, VTC 2003-Fall 2003, Control with Coordinated Adaptive Sleeping for Volume: 3, Pages:1443 - 1447, 6-9 Oct. 2003. 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