Mobile Wireless Sensor Networks: A Cognitive Approach

Transcription

1 Chapter 17 Mobile Wireless Sensor Networks: A Cognitive Approach M. Sujeethnanda, Sumit Kumar, and G. Ramamurthy IIIT Hyderabad Contents 17.1 Introduction Types and Applications of WSNs Environmental and Health Monitoring Military Applications Habitat Monitoring Other Applications MWSNs Challenges and Design Issues Resource Constraints Dynamic Topologies Quality of Service Variable-Link Capacity Ad Hoc Nature Design Goals Resource-Efficient Design Adaptive Network Operation Secure Design Self-Configuring and Self-Organizing Networks Design Issues Scalable Architectures Hardware and Software Design Itinerary Planning

2 492 Wireless Sensor Networks 17.4 QoS in MWSN Cognitive Radio CR-Based WSN CWSN Architecture Spectrum-Sensing Schemes for CWSN Learning in CWSN Challenges in Designing a CWSN Lifetime Maximization or Energy Efficiency PU Detection and Localization Fusion Routing Resource Allocation Problems Power Allocation Optimization of the Radio Module Spectrum Sensing Representation of Network Architecture of CWSN Futuristic Concepts Conclusion and Summary References Introduction Recent advances in wireless communications and digital electronics have enabled the development of low-cost, low-power, sensor nodes that are small in size and can communicate over a radio link in short distances. These tiny sensor nodes, which consist of sensing, data processing, and communicating components, are potentially low-cost solutions to a variety of real-world challenges [1]. Sensor networks are deployed in one of the following two ways [2]: Sensors can be deployed far away from the sensing location, that is, something known by sense perception. In this approach, large sensors that use some complex techniques to distinguish the targets from environmental noise are required. The sensing element of sensor nodes can be deployed in the sensing location. The positions of the sensors and the network topology are designed carefully. They transmit the time series of sensed variables to the central nodes in which data fusion and aggregation can be performed. A wireless sensor network (WSN) is composed of a large number of sensor nodes that can communicate over radio link, which are then deployed either in the actual site of measurement or very close to it. The deployment of sensor nodes does not need to be predetermined. These sensor nodes are randomly deployed in inaccessible terrains. This means that sensor network protocols and algorithms should have self-organizing capabilities. The unique feature of sensor networks is the cooperative effort of the sensor nodes. These nodes have small onboard processing elements. They send only the required and partially processed data to the fusion nodes or higher capability nodes. The above described features ensure a wide range of applications for sensor networks. Some of the application areas are health, military, and security. For example, physiological data about a

3 Mobile Wireless Sensor Networks 493 patient can be monitored remotely by a doctor. Although this is more convenient for the patient, it also allows the doctor to better understand the patient s current condition. We can say that, soon, WSNs will be an integral part of our lives, more so than the present-day personal computers. Realization of these sensor network applications requires wireless ad hoc networking techniques. However, the protocols and algorithms that have been proposed for traditional wireless ad hoc networks are not well-suited for the unique application requirements of sensor networks. To illustrate this point, the differences between sensor networks and ad hoc networks [3] are outlined below: The number of sensor nodes in a sensor network can be several orders of magnitude higher than the nodes in an ad hoc network. Sensor nodes are densely deployed. Sensor nodes are prone to failure. The topology of a sensor network changes very frequently. Sensor nodes mainly use broadcast communication paradigms whereas most ad hoc networks are based on point-to-point communications. Sensor nodes are limited in power, computational capacities, and memory. Sensor nodes may not have global identification (ID) because of the large amount of overhead and the large number of sensors. Because large numbers of sensor nodes are densely deployed, neighbor nodes may be very close to each other. Hence, multihop communication in sensor networks is expected to consume less power than the traditional single-hop communication. Multihop communication can also effectively overcome some of the signal propagation effects experienced in long-distance wireless communication. One of the most important constraints on sensor nodes is the low power consumption requirement. In many applications, sensor nodes carry limited, irreplaceable, power sources. Therefore, whereas traditional networks aim to achieve high quality of service (QoS) provisions, sensor network protocols must focus primarily on power conservation. They must have trade-off mechanisms that give the end user the option to choose network lifetime, lower throughput, or higher transmission delay Types and Applications of WSNs Sensor networks may consist of many different types of sensors such as seismic, low sampling rate magnetic, thermal, visual, infrared, acoustic, and radar, which are able to monitor a wide variety of ambient conditions [4]. Sensor networks can be used for continuous sensing, event detection, location sensing, and actuating some control action. The microsensing elements and wireless communication capabilities of these nodes have many application areas. The applications can be of military, environment, health, home, security, and other commercial areas. Sensor networks consist of various different sensors that measure ambient conditions. These sensors are temperature, humidity, vehicular movement, lighting conditions, pressure, soil makeup, noise levels, object detection, strain and stress gauge, tachometers to measure speed, etc. The architecture of the sensor node is illustrated in Figure The general WSN is illustrated in Figure 17.2.

4 494 Wireless Sensor Networks Various sensors and actuators Power source Processing unit Communication module (RF) Memory Figure 17.1 Architecture of sensor node. User Internet Sink node Figure 17.2 General structure of WSN. Sensor node Wireless sensor network Target Current deployment of wireless sensor nodes can be classified mainly in to following types: (1) terrestrial WSN; (2) underground WSN; (3) underwater WSN; (4) multimedia WSN; and (5) mobile WSN. Terrestrial WSNs consists of hundreds wireless sensor nodes deployed for sensing a phenomenon, either in ad hoc or preplanned manner. For these kinds of networks, reliable communication in dense environment and power conservation are very important. For terrestrial WSNs, energy can be conserved by optimal routing, short transmission range, reduced data redundancy, and minimizing delays [5 11]. Underground and underwater WSNs consist of a number of sensor nodes deployed underground or underwater. These kinds of networks are expensive compared with terrestrial networks in terms of equipment, deployment, and maintenance. They are expensive due to the extra cost incurred in choosing the appropriate parts for reliable communication through soil, rocks, water, and other minerals encountered between the sensor node and the sink node. Replacement of energy sources is also a difficult task in these kinds of networks. Once deployed, it is almost impossible to replace them. Multimedia WSN have come into existence to monitor and track events in the multimedia format. The data in these networks include audio, video, and images. These networks require high bandwidth, high energy consumption, QoS provisioning, high data rates, and high data

5 Mobile Wireless Sensor Networks 495 processing and compression techniques. Video transmissions require high bandwidth to transfer data from the source node to the sink node, hence higher data rates lead to higher energy consumption. QoS provisioning is a challenging task in multimedia WSNs. It is important that QoS is required for reliable content delivery. The detailed descriptions of QoS and the factors affecting it are discussed in later sections of this chapter. Mobile WSNs (MWSNs) consist of sensor nodes that can move on their own and collect the sensed phenomenon, and then communicate this sensed phenomenon. The key difference between static nodes and mobile nodes is that mobile nodes have the capability of repositioning and selforganizing themselves in the network. In static WSN, data can be transferred using fixed routing mechanisms whereas data transmission must be done using dynamic routing in MWSN. MWSNs pose more challenges in terms of deployment, localization, navigation, control, coverage, energy, maintenance, data processing, self-organizing, etc. MWSN applications include real-time monitoring of hazardous minerals, disaster areas, military surveillance, and tracking. Design issues and applications of MWSNs are explained in the latter sections of this chapter. WSNs are classified as shown in Figure 17.3 based on the applications requirements Environmental and Health Monitoring Environmental monitoring can be further classified into two types: 1. Indoor environment monitoring 2. Outdoor monitoring The deployment of sensor nodes in an indoor environment can be used optimally to monitor light, temperature, air streams, and indoor air pollution. For example, UC Berkeley deployed 50 Smart Dust motes throughout the Departments of Electrical and Computer Science to monitor light and temperature [13]. Moreover, a lot of energy is wasted in buildings due to unnecessary heating or cooling. WSNs can be used for a healthier environment and a greater level of comfort for residents. Other indoor applications may be for fire detection and earthquake damage sites Environmental and health Military Wireless sensor networks Habitat Others Environment monitoring and patent monitoring Enemy tracking and security detection Animal tracking and animal monitoring Human tracking, vehicle tracking, structural monitoring, chemical monitoring, inventory monitoring Figure 17.3 Classification of WSN.

6 496 Wireless Sensor Networks [14]. In addition to the systems described above, civil engineering research has shown that it is possible to monitor the health of structures based on vibrations [15]. Outdoor monitoring is a vast area for WSN applications. The most popular example for outdoor monitoring networks is the Great Duck Island project [16]. This network was used for habitat monitoring. Other applications pertaining to outdoor monitoring have been implemented for environmental observations and forecasting weather phenomenon. An early implementation of sensor networks was the Automated Local Evaluation in Real-Time (ALERT) system developed by the National Weather Service for monitoring rainfall and predicting floods in California in the 1970s [17]. Other outdoor applications include agricultural applications such as precision farming and irrigation management. These applications enable an efficient and economic way to use water resources by monitoring soil, air humidity, etc., and also helps in frost detection and as a warning for pesticide application and disease detection. Precision agriculture is mainly used for lowering costs and increasing yield quality [18]. Health care applications are used for patients health monitoring systems. Systems such as FireLine, Heart@Home, and LISTSENse are some of the examples of applications of WSNs in health care. The main objective of these applications is to provide better service and increase the personal health of the patients Military Applications Military applications are closely related to WSNs. Regarding military applications, the areas of interest are enemy tracking and battlefield surveillance [19,20]. For example, a number of motes are deployed in the battlefield to detect the intrusion of hostile units. Then, the defense system is able to respond with preventive measures [21]. The Ohio State University has also developed a similar application called A Line in the Sand, which refers to the 90 nodes that are deployed and capable of detecting metallic objects. The ultimate objective of this application is to detect objects with metallic content such as vehicles, armed soldiers, etc. [22]. The above examples mostly refer to wartime scenarios, their results can also be used in applications such as homeland security and property protection. These activities perhaps may be the future of WSNs Habitat Monitoring Habitat monitoring applications include animal tracking and animal monitoring. The famous ZebraNet [23] application is deployed at the Sweetwater Game Reserve in Kenya to monitor and collect the movement data from zebras. wildcense [24] is another application used for monitoring the movement of animals Other Applications Applications such as inventory management, asset tracking, fleet management, etc., come under this category. Delivery and distribution systems are another area of applications for WSNs [25,26] MWSNs Challenges and Design Issues WSNs have gained much attention in the last few years. One deployment and multiple applications is the motto of WSNs nowadays. Therefore, special capabilities are required for handling

7 Mobile Wireless Sensor Networks 497 multiple applications. However, it is not possible to run all the programs on these embedded nodes, which have tight memory constraints. The use of mobile agents seems to be an effective idea to overcome this situation. The basic MWSN is shown in the figure below. These mobile agents are special nodes that migrate between nodes and perform the task allotted to them. These mobile agents are very useful in efficient data fusion and dissemination in WSNs [27 29]. The benefits of mobile agents in WSNs are that they reduce bandwidth consumption by moving the data processing elements to the location of the sensed phenomenon, further reducing the energy consumption of the nodes. They also ensure uniform energy consumption among the sensor nodes, that is, they remove the battery drainage problem in sensor nodes near the sink node because they have to transfer more data compared with the nodes that are far away from the sink node. Thus, with the introduction of mobile agents in WSN, a new paradigm called MWSNs came into existence. The architecture of MWSNs is shown in Figure Furthermore, MWSNs can have three paradigms 1. Mobile sensor nodes, static sink node 2. Static sensor nodes, mobile sink node 3. Mobile sensor nodes, mobile sink node The first paradigm can be explained as where the source nodes, which sense the sensing phenomenon, are moving around in the entire sensor field region and reporting the sensed phenomenon to the sink node in a periodic manner. The second paradigm is where the source nodes, which sense the phenomenon, are static and the sink node moves around the entire region to collect the sensed phenomenon. The third paradigm is where both the source nodes as well as sink nodes will be moving in the sensor field and, when both come into range, where they exchange the sensed phenomenon between them. The potential applications of MWSNs are mainly where we need to track, monitor, or collect data from real-time environments. Thus, the applications include hospitals, agricultural fields, military, inventory, and asset tracking. MWSNs can be used in hospitals where doctors carry handheld embedded devices or PDAs and do their rounds attending patients. Each patient is equipped with sensor nodes to collect information; data regarding the patient s vital parameters can be transferred to the doctor s handheld device for further examination as well as for recordkeeping. Health status monitoring applications such as MobiHealth [30], was one of the early studies proposing the mobility aspects that enabled personalized health care systems. AlarmNet [31] is a combination of MWSN and IP networks. MWSN and mobile sensors enable the measurement of vital parameters of the patient and IP networks are responsible for passing the information to the health care professional. Mobile agents Sensors Figure 17.4 Mobile WSN.

8 498 Wireless Sensor Networks MWSNs can be used in precision agriculture. The data collected by the sensors deployed in an agricultural field is very useful for the future. Based on past data, the farmers can opt for different crops for different seasons. Thus, collecting data from farms and storing them on an embedded device is not a feasible option. With the help of mobile sink nodes, which can travel through the entire farm periodically in different seasons, information is collected and furthermore that information can be sent to an expert s location. There, the data can be preserved for longer periods for further use. Other applications include habitat monitoring, in which sensor nodes are attached to animals to monitor their movements and to track them in different seasons. These kinds of applications are very useful for knowing the migration strategies of animals with respect to various seasons of the year. Inventory and fleet management for the bigger companies is very complex and very tiresome work. These can be automated with the help of MWSNs. The network can keep track of the inventory and fleet easily with the help of these nodes. The widest area in which MWSNs could be feasible is in military applications. There, WSNs are often used for detecting enemy intrusion, tanker movements, and soldier s movements. All these areas need MWSN platforms as a base to perform the tasks in efficient manner. The challenges in MWSNs are as follows Resource Constraints The design and implementation of any WSN application suffers from resource constraints such as energy, memory, and processing. The MWSN will also have the same constraints as far as resources are concerned Dynamic Topologies In MWSN, the constant change in the positions of various sensor nodes, due to mobility, changes the network topology very frequently. Furthermore, sensor nodes may also be subject to environmental changes like dust, vibrations, wind, and other conditions that cause change in the network topology. Also, harsh conditions may cause malfunctions in some sensor nodes Quality of Service The QoS in MWSN refers to how data reaches the mobile agents and how accurate the data is, as well as the latency of the data reaching the mobile sink node. The most difficult challenge in MWSN is QoS provisioning. These problems are due to constant changes in network topology and how fast the network is self-organizing and transferring the data to the mobile agents Variable-Link Capacity Compared with wired and static WSN, the wireless link depends on the interference level when deployed in harsh environments. In addition, they exhibit abnormal characteristics over time and space due to obstructions. Because of the presence of mobile agents, links are varying continuously, making QoS challenging Ad Hoc Nature MWSN contains a large number of sensor nodes deployed randomly over the field. The lack of predetermined infrastructure and mobility makes maintaining network connectivity a major

9 Mobile Wireless Sensor Networks 499 issue. To overcome the challenges present in constructing MWSNs, the following design goals need to be followed Design Goals Resource-Efficient Design In every type of WSN, energy efficiency is important to increase the lifetime of the network. This can be accomplished by implementing energy-aware routing and energy-saving modes of the sensor nodes Adaptive Network Operation Adaptability in the MWSN is very crucial. It enables networks to adapt to dynamic or varying wireless channel conditions in mobile environments. It is also important for gaining the new connection requirements needed for the network to maintain connectivity issues Secure Design During the design of the security issues for MWSN, issues such as robustness of the communication with varying topologies, secure routing, resilience to node capture, secure group management, and intrusion detection should be addressed. The overhead associated with security issues should be balanced with the energy and QoS requirements of the networks Self-Configuring and Self-Organizing Networks The dynamic topology caused by node mobility and failure needs self-organizing and self-configurable architectures. It should be noted that with the help of self-configurable and self-organizing WSN, the addition and deletion of nodes is possible without affecting the objective of the application. It is very challenging to meet all the design goals simultaneously. Different applications have different requirements on design objectives. Therefore, there should be a balance between the different parameters when designing the MWSN networks Design Issues To design a MWSN that meets the above design goals, we consider some important design issues Scalable Architectures The MWSN supports various heterogeneous network applications with various requirements. It is very much necessary to develop flexible and suitable architectures for various applications. The flexible and hierarchical networks enhance robustness and reliability in the network. The newly developed architectures must support or must be interoperable with existing network topologies such as Internet-based networks, cellular networks, etc.

10 500 Wireless Sensor Networks Hardware and Software Design Hardware design is the most challenging issue in the sensor network. The hardware architecture of mobile sensor nodes is composed of four basic components: 1. Sensor 2. Processing unit 3. Transceiver unit 4. Mobile unit Sensors and actuators are the physical devices that respond to the change in the sensing phenomenon and convert them into the appropriate signals. These signals are supplied to the processing unit for further processing. The processing unit is responsible for processing the data, storing the data, and issuing the control commands to the actuators. As a whole, the processing unit controls the functionality of entire components in the sensor network. The transceiver is the component responsible for connecting the sensor nodes to the network and is also responsible for maintaining network connectivity. The mobile agents in the network are responsible for covering the entire region and collecting the data from the entire sensor field. The operations carried out by all the components in the node or network consume power. Therefore, the selection of low-power and low-cost equipment is necessary. Power can be conserved by enabling sleep modes for the sensor nodes and enabling idle and sleep cycles for the transceiver. The power source for mobile agents can be replaced or recharged as they will be moving and are easily accessible compared with the other static sensor nodes. The application software of sensor networks must be designed to be very user-friendly as well as being easily assessable. The application programming interface (API) does this job. The API enables rapid development and deployment of the networks. Furthermore, operating systems for sensor networks are available. TinyOS is one of the earliest and most popular operating systems for sensor networks [32]. A recent development in these operating systems is the introduction of mobility patterns as well as support for various network protocols and multitasking. Simulators like NS2 and NS3 will help in the prototyping stage of the network before actual deployment by simulating the entire network [33,34]. These simulators are mainly used ad hoc and sensor networks. Recent developments include WSNs and cognitive networks Itinerary Planning Itinerary is defined as the route followed by mobile agents to collect data from the sensor field. Itinerary planning helps mobile agents to select the set of nodes to be visited by the mobile agent and determines the path in an energy-efficient manner. The order in which the mobile agents cover the sensor region will have a considerable effect on the energy consumption of the network. Finding the best possible path is a challenging task. For a better understanding of itinerary planning, we should know how the sensor fields are represented, divided, and how data is collected. The basic tasks are leveling, sectoring, and clustering. These terms are explained briefly to better understand itinerary planning. For detailed descriptions, please refer to the study by Hajela et al. [35].

11 Mobile Wireless Sensor Networks 501 Leveling. The entire sensor field is divided into a number of levels. Leveling can be done by using the number of hop counts required to transmit data from a source to the base station (BS) or the various power levels to which that BS can transmit. Sectoring. A BS using a directional antenna will transmit the signals with maximum power and divide the sensor field into equiangular sectors with an angle of θ (let θ = 45 ). So that each node knows its level ID and sector ID. Clustering. Clusters can be formed based on the signal strength and the cluster head will be decided by using a round-robin technique [36]. The entire sensor field is divided into levels and sectors as shown in Figure Itinerary planning can be categorized as static planning, adaptive planning, or hybrid planning. Static planning is when the path followed by the mobile agents is predetermined and the mobile agents will follow that path to cover the entire sensor field to collect data. Adaptive planning is when the path followed by the mobile agents is determined dynamically based on the current network status. Hybrid planning is the combination of both static planning and adaptive planning. The path followed by the mobile agent is predetermined but, based on the network status, the path can be dynamically decided. S 1 Low interest region S SCH C CH C CH S + 1 SCH BS L 3 L 2 L 1 L High interest region S + 2 Figure 17.5 Sensor field after leveling sectoring and clustering.

12 502 Wireless Sensor Networks 17.4 QoS in MWSN MWSNs pose more challenges compared with the regular WSNs, wireless local area networks (WLAN), and mobile ad hoc networks (MANET). The following are the challenges posed by MWSN [1,37]: A sensor node suffers from very limited power source. A sensor network topology faces frequent changes due to external forces such as animals, humans, power, or software failure. A sensor node does not have a global ID, which makes most of the current network protocols inapplicable to WSN. Sensor networks mainly operate without any human intervention and they should be selfconfigurable and self-organizing. Sensor nodes are densely deployed increasing redundancy and collisions. Sensor nodes normally use the broadcast communication model, whereas traditional networks use point-to-point communication. For all the above reasons, implementing QoS in WSNs differs from regular QoS implementations in other types of networks. The next section is a discussion of QoS in MWSNs in general followed by some challenges in deploying normal QoS mechanisms in MWSNs. QoS is the ability to provide different performance levels to different applications and users, or to guarantee certain performance measures for latency, jitter, throughput, bit error, error rate, packet dropping, etc. Delay is the time that the network spends to deliver a data packet from the source to the destination. Jitter, in turn, is the delay between two consecutive packets in the same frame. Packet dropping determines the maximum amount of data packets that can be lost in the frame to provide good QoS. QoS metrics are important if the network limitation is limited resources. QoS is not only dependent on the abovementioned performance measures but also depends on some nonfunctional parameters. The nonfunctional parameters include energy or power, reliability, robustness, security, scalability, mobility, cost-effectiveness, delay, etc. The main factors affecting QoS parameters can be represented in the form of Figure Power. This considers the most critical limitation. Therefore, almost every protocol proposed considers the energy problem. The main power consumer, as discussed previously, is communications. Therefore, implementing a data aggregation and local data fusion technique for achieving a better service (QoS) is always the price of energy [38]. Bandwidth. Bandwidth is one of the factors affecting QoS parameters; thus, the lack of bandwidth presents more difficulties in achieving QoS in WSN. Techniques for utilizing bandwidth efficiently based on the nature of the data stream are necessary to overcome the scarcity of bandwidth. Memory size. The limitation of memory size affects most applications to enhance WSN networking capabilities. In some cases, local memory is not enough to load the whole QoS technique to implement QoS measures. Lifetime. The nature of WSN s lifetime is limited because most nodes operate on unchangeable power sources such as a battery. Another reason is the ease of node damage. Attempts to recharge the battery using solar or wind power have been proposed.

13 Mobile Wireless Sensor Networks 503 Quality of service Functional parameters Nonfunctional parameters Jitter Throughput Bit error and error rate Bit rate and packet dropping Figure 17.6 QoS and factors affecting QoS. Mobility Energy Scalability Security Delay Robustness Redundancy. It may help achieve reliability but it may add overhead and consume power to aggregate traffic to the sink. Also, it may add some sort of latency and complexity to QoS design [39]. Application diversity. WSNs are considered to be application-specific rather than general purpose. They carry only the hardware and software actually needed for an application. The vast number of applications in WSN offers different QoS requirements. The major challenges in the implementation of QoS in WSNs are resources, that is, energy, bandwidth, and transmission capacity of sensor nodes. These are some extreme resource constraints; among these constraints, efficient energy utilization is a considerable factor because installed batteries replacement and recharging is highly expensive and difficult. Usually, QoS are affected by the mobility of sensor nodes, that is, a change in network topology due to this link failure and node failures occur frequently. Adaptability and self-organizing properties will help in avoiding the failures that are caused by mobility; however, implementation of the properties in WSNs that influence QoS is another challenge. In WSNs, air acts as a transmission medium. It is less reliable and has high security risks. For instance, if a sensor network is deployed in a hostile and hazardous environment, and in coexistence with other WSNs, the other networks significantly cause interference in the newly deployed network. Thus, the data received may be faulty or unpredictable. To avoid these instances, in the process of building a sensor network, care should be taken to consider factors such as robustness, reliability, and security. Another challenging part is QoS providing multiple BSs deployed according to the nature of applications. Currently, most of our sensor networks have only one BS or sink node. Soon, WSNs will be a part of our everyday life. These networks have to coexist with huge numbers of WSNs, cellular networks, vehicular networks, etc. There is a lot of demand for spectrum. Nowadays, spectrum is becoming very scarce. Soon, we will need to share and utilize the

14 504 Wireless Sensor Networks spectrum very efficiently. This spectrum sharing and utilization lead to a new concept called cognitive networks. Cognitive networks are designed to use the spectrum more efficiently and in an opportunistic way. The primary user (PU) and secondary user (SU) share the spectrum for better utilization of the spectrum. The ability to sense the environment and adapting to the environment for achieving optimal performance is the design motto of cognitive networks. A detailed description of cognitive networks can be found in the following section Cognitive Radio Currently, we are living in an era of conventional fixed spectrum assignment policy, in which regulatory authorities (government bodies) assign the spectrum chunks to service providers on a long-term basis for large geographical regions. Once these spectrum chunks are leased, they can be used only by the licensed users. Even though they are vacant for some time or for some geographical region, they cannot be used by unlicensed users. However, it has been found that those licensed spectrum chunks are underutilized as they remain unused 90% of the time temporally and spatially [40]. Considering this scenario, the federal communications commission (FCC) and the research community is focusing on dynamic spectrum allocation [41]. The goal of dynamic spectrum allocation is to remove regulatory barriers and facilitate the development of secondary markets in spectrum usage rights among the wireless radio services. Dynamic spectrum allocation technology senses open channels and allows devices to communicate in unused parts of the spectrum. These unused parts of the spectrum, also called spectrum holes/white space, can be visualized in Figure Dynamic spectrum allocation is implicitly required and can be achieved with the use of cognitive radios (CR) to improve spectral efficiency. CRs help in the efficient use of the existing spectrum through opportunistic access to licensed bands without interfering with the PUs. CR is an intelligent wireless communication system that is aware of its surrounding environment. It works on the idea of intelligent signal processing and decision making to enable the radio to not just utilize the spectrum efficiently but also to manage/adapt the other wireless parameters as required. CR is able to dynamically reconfigure its center frequency, waveform design, time diversity, and spatial diversity options to achieve efficient communication without interfering with PUs (Figure 17.7). A simple model of a cognitive radio in which the cognitive engine interacts with the radio is shown in Figure The cognitive engine is the heart of the cognitive radio and is responsible for all intelligence, adaptation, and controls. Time Spectrum holes Occupied band Frequency Figure 17.7 Spectrum holes in between the occupied frequency bands.

15 Mobile Wireless Sensor Networks 505 Radio Tx Spectrum statistics Cognitive engine Radio parameters Radio Rx Figure 17.8 Simplified model of cognitive radio CR-Based WSN In both CR and WSN, sensing tasks are performed to collect information from the operating environment about spectrum occupancy and environmental parameters, respectively, and then appropriate actions are taken accordingly. CR-based WSN (CWSN) speaks about the application of CR in WSN and not only in the PHY and MAC layers but also in all the layers and thus employs a cross-layered approach. It is promising as well as challenging for WSNs to adopt CR technology. It enables the WSN to sense spectrum holes and utilize vacant frequencies to improve spectrum utilization. It is also capable of increasing its own QoS and throughput by adaptively and cognitively changing various transmission and reception parameters such as transmitted power, operating frequency, modulation, pulse shape, symbol rate, coding technique, and constellation size. A wide variety of data rates and QoS can be achieved, improving power consumption and network lifetime in a CWSN. CR technology in WSNs can provide access not only to new spectrum bands but also to spectrum bands with better propagation characteristics. Generally, the lower frequencies have better propagation characteristics than the higher ones (refer to the free space propagation formula). The operation of WSNs at lower frequency bands allows range extension and higher energy efficiency. This helps in getting simpler topology as well as fewer sensor nodes to cover a given area because, with lower frequencies, the transmission range of the same node with the same transmission power is increased. Higher transmission range improves several important factors in WSNs including network connectivity, lifetime, and end-to-end delay. Some of the advantages of using low frequency for transmission are: Higher transmission range Fewer sensor nodes required to cover a specific area Lower energy consumption Smaller number of hops to the destination Lowered end-to-end delay Another advantage of using CR technology in WSNs is that data from various sensor nodes, which are not spatially correlated or which have low redundancy, can be transmitted to the sink

16 506 Wireless Sensor Networks simultaneously in different channels (noncontiguous transmission). This reduces the delay and enables the sink to monitor a large number of nonspatially correlated information in a real-time manner CWSN Architecture CWSN is also like WSN in the sense that it consists of several tiny sensor nodes with all the constraints that a normal WSN has, especially the limited battery energy. They differ in their transceiver hardware architecture and states. In a CWSN, the hardware also consists of a cognitive module that is responsible for spectrum sensing and adaptively changing the transmission parameters in a reconfigurable transceiver (the reconfigurable transceiver is another advantage of the CWSN). Apart from these, there is also a cognitive engine in the cognitive module, which is responsible for learning and achieving cognition to make the changes automatically without human intervention. This cognitive engine is also responsible for controlling the changes that should take place in the transmitter and receiver. Cognitive engine works on the concept of cognition cycle (CC) [42]. States of a CC are shown in Figure The cognitive engine is composed of six main states: observe, orient, act, decide, plan, and learn. It enables the nodes to achieve context-awareness and intelligence so that it can be aware of its operating environment to sense for the white spaces, and use them in an intelligent and efficient manner. With regard to WSN, this cognitive engine may also assist in intelligent localization, routing, and scheduling in WSN (Figure 17.9). In contrast to WSN, the CWSN nodes have an additional state called sensing state where they keep sensing their environmental parameters. Spectrum sensing state consumes a lot of energy Infer on context hierarchy Receive a message Read buttons Preprocess Parse Observe Prior states Orient Establish priority New states Immediate Learn Normal Urgent Save global states Plan Generate alternatives (Program generation) Decide Evaluate alternatives Register to current time Allocate resources Outside world Send a message Act Set display Initiate process(es) (Isochronism is key) Figure 17.9 Cognitive cycle. (Redrawn from I. Howitt and J. Gutierrez. IEEE low ratewireless personal area network coexistence issues. In Proceedings of IEEE WNCN 2003.)

17 Mobile Wireless Sensor Networks 507 because it is directly related to transmission and reception, which is the most energy consuming activity of a CWSN/WSN node [44] (Figure 17.10). Spectrum sensing can be done in distributed as well as centralized fashion as shown in Figure In the distributed scheme, all the nodes keep sensing the spectrum environment on their own and compete with other sensor nodes to grab the unoccupied spectrum. However, the problem in this architecture is that all the nodes essentially need a spectrum-sensing module, which may not be economically feasible. In the centralized scheme, there is one network coordinator that has the responsibility of spectrum sensing as well as spectrum scheduling, that is, allotting the free channels to the needy nodes according to their need in preferential order. However, in centralized control there has to be a separate control channel, which could become a problem. In this control channel, there is a broadcast channel switch command according to which the sensor nodes change their Tx/Rx frequencies. The control channel could be from a licensed or ISM band. But there is always a fear that the control channel could somehow get faded, and in case this occurs, the entire network will be in chaos. In a common WSN scenario in which there are a large number of sensor nodes, it may not be feasible to have a spectrum-sensing module on each node. The feasible architecture will be to make only a few specialized nodes capable of performing the spectrum sensing. The network co-coordinator or the cluster head can do this task in realistic deployment. If we take a deterministic deployment scenario like a home or office, then easily a few specialized nodes capable of spectrum sensing can be deployed especially for spectrum sensing [45]. Because spectrum sensing is a repetitive process, which would consume extra energy from battery-powered sensors, implementing spectrum sensing in all nodes in a WSN may not be efficient in terms of energy consumption (Figure 17.11). Sensing the target frequency/frequencies can be fixed or vary depending on the changing environmental and mobility condition of the nodes. Here, the cognitive engine can play an important role. After collecting spectrum sensing data over a long period, the cognitive engine will be able to know at what time, which channel, or which specific band has to be sensed, instead of sensing all the channels every time. The channel occupancy pattern also varies spatially. Hence, the cognitive engine can assist the spectrum-sensing nodes to decide which channel/channels to sense at a particular time and in a particular geographical region. Sensing duration depends on the accuracy of sensing and also the required probability of detection. The longer the sensing duration, the more complex the algorithm for it, and thus the WSN CWSN Sending Sending Receiving Receiving Idle Idle Spectrum sensing Figure Environmental parameter sensing in CWSN. (Redrawn from J. Mitola III and G.Q. Maguire. Cognitive radio: making software radios more personal. IEEE Personal Communications, vol. 6, pp , 1999.)

18 508 Wireless Sensor Networks Primary transmitter Channel sensing Spectrum hole information Centralized sensing Distributed sensing Primary transmitter Spectrum coordinator Figure Centralized and distributed sensing. (Redrawn from D. Cavalcanti, S. Das, J. Wang et al. Cognitive radio based wireless sensor networks. In Proceedings of the 17th International Conference on Computer Communications and Network. IEEE Press. pp. 1 6, 2008.) more energy it will consume. There has to be a trade-off between sensing duration and sensing accuracy. During sensing, there has to be network-wide quiet period, that is, nodes have to suspend their transmission following some schedule, which would have been prebroadcast well ahead of time by the coordinator to avoid any overlap; otherwise, false alarms may occur, indicating that some channel is occupied because of its own transmission or by some of the member sensor nodes. Spectrum sensing has to be done only during network-wide quiet periods. This is usually done to detect incumbents at low incumbent detection threshold (IDT) values and avoid false alarms (event when a system detects a nonexistent PU). All nodes are given a fixed schedule of quiet periods that they have to follow essentially. These quiet periods can be scheduled well ahead of time by a broadcast through the network coordinator so that all nodes can adjust their transmission to avoid overlaps with scheduled quiet periods [46]. This will require a very tight and proper coordination among the nodes in WSN quiet period (QPs) [47]. QPs may be a critical issue for high-throughput networks, but in WSN, the traffic load is typically much lower and it won t be that much of an overhead because most of the time, the sensors would be in a stand by or sleep mode [48]. Now, we should be acquainted with some terms related to spectrum sensing, which may be helpful for further readings. There have been some limits set by the FCC for IDT, probability of detection (PD), probability of false alarm (PFA), maximum probability of false alarm, channel move time (CMT), and channel closing transmission time (CCTT) for CR networks. The same set of limits would be applicable to a CWSN. However, some parameter values can be relaxed for

19 Mobile Wireless Sensor Networks 509 a WSN due to the much lower transmission power used by the WSN transmitters compared with the devices for which these protocols and regulations are intended [46]. Modifying the existing protocol (i.e., ZigBee) to suit the CWSN physical and MAC layer will be a very good approach. The standard defines 16 channels, each at 2 MHz bandwidth, in the 2.4 GHz band [49], among which, only four are nonoverlapping with the channels (at 22 MHz bandwidth) in the same band [46] (Figure 17.12). Whenever an incumbent signal is detected well above IDT, WSN has to switch to a backup channel within the CMT to avoid interference. This requires the coordinator to broadcast the channel switch command. The channel switch command also consists of the scheduled switching times for the nodes. This command indicates the WSN nodes to switch their channels to the free available channel as specified by the coordinator. In case a free channel is not available, they have to utilize the backup channel. This switching is a responsibility held by the coordinator, which decides how to allocate channels, that is, scheduling of free channels among the needy nodes. The coordinator can use its own spectrum-sensing results (centralized spectrum sensing) or it may use the spectrum-sensing reports (distributed spectrum sensing) from other specialized spectrumsensing nodes. In practice, the coordinator has to make a list of backup channels available as there may be a good probability that many backup channels are available at a particular time and geographical region. Generally, in WSN a very fast incumbent recovery mechanism is not required. Once the PUs come into the picture, the nodes have to vacate the channel very fast. However, in some WSNs, in which there is very tight delay requirement, there has to be provision for backup Channel 1 Channel 1 22 MHz Channel 6 Channel 7 Channel MHz 2412 MHz 2437 MHz 2462 MHz MHz 22 MHz Channel MHz 2412 MHz 2442 MHz 2472 MHz MHz 3. 2 MHz Channel MHz MHz Figure Overlap between ZigBee and WLAN channels.

20 510 Wireless Sensor Networks channels. This backup channel can be either from the licensed band shared with the PUs or from the unlicensed band shared with other SUs. Sensing of the backup channel has to be done regularly to make sure that the backup channel is readily available and clean Spectrum-Sensing Schemes for CWSN Energy-efficient spectrum-sensing techniques are required to meet the power constraints of the CWSN. Especially in the case of mobile cognitive wireless sensor networks (MCWSN), the constraint on the energy will be much more than static CWSN. Similar to static cognitive wireless sensor networks (SCWSN) in the MCWSN, a question is raised, how much energy should be spent on channel sensing? There are spectrum-sensing techniques that are quite accurate but have a very high energy budget. However, such accuracy may not be needed all the time. Because, in some cases, the interfering signal may be sporadic or may be perceived with very high power, and thus is easy to detect. Hence, a cognitive spectrum-sensing system is required to tailor the energy budget of spectrum sensing according to the signal strength. A sensing energy budget should also be tailored according to the size of the packet to be transmitted. In WSN, there are packets of several types and sizes, and these packets have different priorities. Sensor packets can be as small as a single to few tens of bytes; therefore, selecting the right amount of energy that has to be devoted to spectrum sensing might significantly improve energy efficiency. Loss of long packets may cause retransmission of the packet, which may be even more costly. Hence, in such cases, high energy budgets can be applied to sense the spectrum. But when small packets need to be transmitted, then low-energy budget sensing algorithms can be applied. In this way, overall lower energy consumption can be achieved [50]. There are several spectrum-sensing techniques available for CR and some of them can also be readily used in the CWSN. A good survey of spectrum-sensing techniques is given in an article by Stabellini and Zander [50]. Interested readers can refer to the same. Spectrum sensing can be done individually or in a cooperative manner to identify the spectrum holes. In general, the cognitive transceiver devices have two important functionalities: spectrum sensing and adaptation. The spectrum-sensing hardware of cognitive transceivers keeps sensing the spectrum over a wide frequency band. This information is then passed to the SUs (in our case, the SUs were WSN nodes). When such a spectrum hole is found, the SUs adapts its transmission power, center frequency, and modulation, etc., to transmit efficiently as well as to minimize interference to the incumbents [51]. An implied assumption here is that all the nodes have reconfigurable hardware. Also, while the transmission is in process, the cluster head or network coordinator, whichever is doing the task of spectrum sensing, should have the ability to detect the appearance of incumbents so that the SUs are able to change or give off the channel when the PUs starts transmitting in that channel. Spectrum sensing for CWSN can be categorized as blind sensing and signal-specific sensing. Blind spectrum-sensing techniques do not rely on any special signal features. Some examples are energy detection [52] and eigenvalue-based sensing [53]. Signal-specific sensing techniques utilize specific signal features for sensing. Some examples are signature sensing for ATSC signal identification [54], FFT-based carrier sensing [55], higher-order statistics sensing (HOSs) [56], PLL-based ATSC pilot sensing [57], and wireless microphone covariance sensing [58]. Also, cooperative spectrum sensing speaks about itself when mobile scenarios come into the picture. In the mobile scenario, the hidden node problem may come often and unexpectedly.