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Introduction
Significance of the Study
One of the most influential and large impact factors in wireless body area networks is the efficiency of energy consumption. This essay seeks to present an extensive study of energy consumption technologies in WBANs. This is achieved through, concerted focus on power-efficient models and battery-driven media access control layer protocols.
Research Gap
In a wireless Body Area Network, the implanted and wearable sensors/actuator devices are powered by batteries that have only limited energy capacity. The advances in battery technologies have been much slower than the recent advances achieved in the wireless communication and networks. Su and Zhang (2009) asserts that the battery capacity has further been limited by the WBAN nature constraints in size, weight and (for some implanted devices) accessibility aspects.
At the same time, due to life threatening situations, the reliability and timely delivery of vital body parameters is a critical requirement in patient monitoring (p. 424). Hence, both efficient and effective designs to prolong the lifetime of the sensor devices while guaranteeing the reliability and timely message delivery is the key design challenge in Wireless Body Area Networks.
In WBAN, it is well known that the radio transceiver requires more energy compared to all other components of the WBAN. Hence, designing energy efficient MAC layer protocols remains the main challenge in Wireless Body Area monitoring Networks (Su and Zhang, 2009, p. 424).The main constraints of WBANs are the power consumption since power supply is very limited.
Introduction to WBAN
A study conducted by the US Bureau of Census statistics revealed that, the population of elderly people is predicted to double to 70 million people by the year 2025 (in the US). A direct application of these statistics shows that, the world elderly population is expected to double itself to 761 million by year 2025 from the 375 million estimated in 1990. Further statistics by the US Bureau reveal that, healthcare expenditure in the US will triple by year 2020 to a projected $5.4 trillion from the $1.8 trillion spent in year 2004 (Ullah et al., 2010, p. 2). The overall healthcare system is indeed in an impending crisis. According to Ullah et al. (2010) one possible direction is Wireless Body Area Networks (WBANs), aimed towards providing such solutions. This makes WBANs the pinnacle of providing economical solutions to the challenges that healthcare systems face (p. 2).
The healthcare sector is increasingly investing in WBANs technology for more efficient and effective healthcare service delivery. WBANs wireless vital signs monitoring is one of the main areas experiencing the quickest growth in modern healthcare. This is as a result of its potential for slowing down unsustainable increase in healthcare spending, caused by a high number of people living with chronic conditions that require constant clinical management.
Latre, Braem, Moerman, Blondia and Demeester (2010) assert that WBANs consist of a set of tiny portable communication devices, two types of which can be distinguished: sensors and actuators (p. 2). The sensors, e.g. as pulse oximeter or accelerometer sensor, are either external or internal. The internal sensors are called implantable medical devices (IMDs). These have been partially or totally produced into the human body, either surgically or medically. The latter have proven to be remarkably successful in the treatment of many diseases. Wireless Body Area Networks are used to monitor IMDs, providing services such as management of chronic disease, and medical diagnostics.
These allow, automatic collection of data from the patient, integration of the data into the patients medical records, processing of the data and issuing a recommendation when necessary. As opposed to the sensors, which are collecting data, the actuators (or actors) take specific actions according to the data received from either the sensors or through interaction with the user. This could be combined with glucose level sensors and a Smartphone, serving as a sink for the sensors data. The actuator would then administer the correct dose of insulin and inject it to the diabetic patient (Latre et al., 2010, p.2)
The whole system collects relevant data, takes the appropriate actions and updates the patients medical records to the main server all in real time. Continuous monitoring of patients life signs, and analyzing signal patterns that enable early detection of dangerous medical conditions result in a more effective treatment and minimization of hospital stays. In addition, the long-term life sign data could improve the patients diagnosis when ill.
Much of the worldwide deaths are attributed to cardiovascular disease, making it a serious cause of death globally. Every day many people die out of an arrhythmogenic event, which is generally preceded by irregular heartbeat, accompanied later on by a heart attack. WBANs can monitor irregular heartbeats before they turn into deadly heart attacks. Rather than staying at home or hospital for observation, WBAN enables long term monitoring of patients health parameters as the patient engages in normal daily life activities (Ullah et al., 2010, p. 3).
Moreover, in Latre et al (2010) study, the authors articulate how other implants can be used in other areas, for example to restore control over paralyzed limbs, bowel muscle control, maintain regular heart rhythm, and many other functions. These implants improve the quality of life for many patients. Emergence of wireless on-body network or a Wireless Body Area Network would enable us to fully exploit m-Health (p. 1).
The body area channel is very different from other wireless channels, in the sense that the antenna-human body interaction is an integral part of the channel. Propagation in WBAN is indeed confined within a limited range around the human body where transmitters and receivers are almost co located to each other. Body area networks address a technology segment not currently covered by existing connectivity standards, and that is for two reasons.
First reason is that each of the miniature sensors and actuators nodes of a WBAN should have its own energy supply for autonomous operations. Therefore, in order to provide longer monitoring time frame, the energy consumption of these miniature sensors is optimized so that the battery does not need to be changed often. The second reason is that the nodes should be intelligent enough to perform their tasks and communicate with each other. Consequently, WBANs main objectives can be formulated as following:
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To provide an infrastructure for intelligent, miniaturized, low power, invasive and non-invasive sensor nodes, allowing them to monitor body function and the surrounding environment.
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To supply a communication channel between noninvasive/wearable devices and a base station.
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To ensure a broad range of data rates at lower power consumption rate.
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To guarantee that wireless communication Quality of Service (QoS) can be quantitatively pre-defined, while explicitly assuming that the range to be covered is limited to the immediate environment around and within a persons body.
In this essay, we deal mainly with the issue of energy consumption in Wireless Body Area Networks. The essay analyzes energy consumption in WBANs in three major aspects, each of which is assigned a solitary chapter. Firstly, we are going to look into Power models in the WBANS, while focusing on Beacon-mode and Carrier sense multiple accesses with collision avoidance (CSMA/CA) mode access schemes (Cheng and Huang, 2008, p. 2).
Secondly, well examine a device introduced by Yan, Zhong and Jha (2007) known as the wireless device driver which is an energy-efficient computing model for low-duty cycle peripherals (p. 1). Finally, we will observe power saving Media Access Control (MAC) layer protocols. Four different protocols of different techniques are introduced, while considering various aspects of their design and implementation, such as electrochemical battery properties, time-varying wireless fading channels, and packet queuing characteristics, different types of data, communication reliability, and throughput (Su and Zhang, 2009, p.1).
In analysis of power consumption in the model for beacon-mode and CSMA/CA-mode access schemes used in WBANs, we take into account two types of network nodes collisions. The first type is a conventional one collisions from intra-network nodes. The other, not so obvious type, relates to collisions caused by nodes from different moving WBAN groups. The analysis of these types in the relevant chapter is followed by discussion of a hybrid-mode access scheme with two multi-WBAN groups advantages: over 60% power consumption in beacon-mode, and double node capacity in CSMA/CA-mode at a manageable power consumption level (Cheng and Huang, 2008, p. 1).
When covering the wireless device driver an energy-efficient computing model of a WBAN network topology, well see that its method of operation is such that, many low-duty peripheral nodes communicate with a more powerful central device. In our essay, the model will be compared to two WBAN technologies, Bluetooth and ZigBee, in terms of cost of design, performance/effectiveness, and energy efficiency.
The comparison is continued by an in-depth review of techniques that ensure efficiency in energy consumption while meeting connection latency requirements. Well begin by low-level inspection of the impact on energy consumption of several tunable parameters of the wireless device driver. Then continue by higher level inspection of the connection latency VS energy consumption trade-off impact on address dynamic resource management. Finally, we end the chapter by proposing an upper level energy-efficient power down policy and adaptive connection latency management technique, which was recently invented (Yan et al., 2007, p. 1).
As stated earlier, an analysis, of energy consumption in low-power MAC layer protocols, designed and implemented in WBANs is carried out in the essay.The essay will contemplate and compare four different techniques.
The first is oriented towards beacon-enabled ZigBee, Preamble Based Time Division Multiple Access (PB-TDMA), and Sensor-MAC (S-MAC) controlled in-body and on-body sensors. This technique introduced by Ullah, An and Kwak (2009) uses traffic-based wakeup mechanism for WBANs and achieves power efficient and reliable communication via exploiting the traffic patterns of the body sensor nodes (BSNs) to accommodate the entire BSN traffic. Logical connection between different BSNs is enabled through working on different frequency bands by a method called bridging function. The latter integrates all BSNs working on different bands into a complete BSN (p. 336).
The second technique introduced by Omeni, Wong, Burdett and Toumazou (2008) fits WBANs with an architecture, where the by the body-worn node (slave) periodically sends sensor readings to a central node (master). The nodes in this network are not deployed in an ad hoc fashion, but rather join the network in a centrally controllable way. Communications of such networks are usually single-hop. The sensor nodes are asleep until the centrally assigned time slot arrives. No collision of a node within a cluster can occur. A clear channel assessment algorithm is applied to manage collisions with nearby transmitters. Time slot overlaps are handled by wakeup fallback time. This technique has significant energy reductions when compared to more general purpose network MAC protocols such as Bluetooth or Zigbee (p. 252).
The third technique introduced by Marinkovic, Popovic, Spagnol, Faul and Marnane (2009) is best for devices measuring physiological signals such as EEG and ECG over fixed topology WBANs.Power consumption and battery life are estimated by duty cycle calculations and validated through measurements. The results show that the protocol is energy efficient for both streaming communication and short transmissions of data bursts (p. 915).
The fourth technique is a cross-layer based and battery-aware MAC layer set of protocols for WBANs. Here we consider the battery internal properties, time-varying and wireless fading channels and packet queuing characteristics. As a result we achieve prolonged the battery lifespan of the wireless sensor nodes, while guaranteeing reliable and timely message delivery, which is critically important for WBANs. The obtained results, both analytical and simulation show that the proposed schemes can reduce energy consumption significantly in wireless body-area monitoring networks (Su and Zhang, 2009, p. 424).
Literature Review
According to the discussed above, energy efficiency is a major challenge in WBANs design. Long- duration networks are essential in medical applications. For example, an endoscopy capsule needs approximately 12 hours to transmit images from the internal organs of the inner body, while a heart patient may need daily or monthly ECG monitoring (Cheng and Huang, 2008, p. 1). Many factors, such as collision, overheating, over-emitting, packet overhead, etc. can cause inefficient energy consumption in WBANs. These factors can be classified into categories, such as network topology, communication protocols, and power schemes and so on.
Various studies of different aspects of WBANs have been carried out in numerous literatures to eradicate, or at least minimize, these shortcomings towards increasing the lifetime duration of a WBAN. Several of these literatures have been chosen for this essay. Each of the chosen papers and a summarized indication of their relevancy to the essay are as mentioned.
WBANs SURVEY
In this survey by Ullah et al (2010), the authors discuss the fundamental mechanisms of WBANs including architecture and topology, wireless implant communication, low-power Medium Access Control (MAC) and routing protocols. A comprehensive study of the proposed technologies for WBAN at Physical, MAC, and Network layers is presented and many useful solutions are discussed for each layer (p. 1).
This study is considered relevant to the essay due to the fact that in the system architecture of the PHY layer different methodologies of wireless communication are reviewed, along with a conclusion. The study proposes low-power MAC protocol for WBAN followed by important suggestions. For the Network layer, the possible network topologies for WBAN are discussed, taking into account the required energy efficiency and reliability. A classification of existing routing strategies is given for future research directions.
Finally, numerous WBAN applications are highlighted. These applications include in-body applications and on-body applications. Examples of in-body applications are monitoring and program changes for pacemakers and implantable cardiac defibrillators, control of bladder function and restoration of limb movement. On-body medical applications include monitoring ECG, blood pressure, temperature, and respiration (Ullah et al., 2010, p. 25).
A survey on wireless body area networks
According to a survey on WBANs carried out by Latre et al. (2010), the network consists of various sensors attached on patients clothes, body or implanted under the skin. The survey focuses on patient monitoring applications with special interest (p. 3).
This study focuses on three main heterogeneous devices used within WBANs, these are; Wireless sensor nodes, wireless actuators nodes and wireless personal devices (PD). The survey considers three domains of energy consumption: sensing, wireless communication, and data processing by the specific absorption rate (SAR) (Latre et al., 2010, p. 4). This survey is most relevant to this essay due to its analysis of the three energy consumption sources stated
Moreover, Latre et al (2010) considers factors in WBANs that cannot be compromised even for energy efficiency; these factors are quality of service, reliability, usability, security and privacy (p. 4). The survey discusses open research issues in current and past research on WBAN on main challenges like, the restricted energy consumption in wireless body area networks.
Power Model for Wireless Body Area Network
An analytical study on power consumption in WBANs was carried out by Cheng and Huang (2008). According to the authors, WBAN consists of a central processing node (CPN) and wireless sensor nodes (WSNs). Each WSN constantly transmits monitored signals from body to CPN while the CPN forwards these signals to the hospital. During the long-term monitoring and transmitting data, low power WSNs becomes a crucial issue in WBANs. On the contrary, the power efficiency of CPN is less important than that of WSNs. CPN in WBAN can be implemented in a smart phone or notebook with plug-in power or much larger battery than WSNs. Thus, the major issue in WBANs is how to save the power of WSNs (p. 1).
This Cheng and Huangs study is relevant to this essay especially due to the fact that in their study they identified the sources of power consumption in a conventional sensor network, to include idle listening, control messages, collision, packet forwarding and overhearing. In their study, however, Cheng and Huang reasonably decide to ignore some of this energy inefficiency sources in WBANs. According to their paper, low power MAC layer for ad-hoc routing networks is not necessary, since WBANs can be formed by a single-hop star network. As a result, packet forwarding is ignored. The study considers Low Rate Wireless Personal Area Network (LR-WPAN), which is a candidate of WBAN with a transmission range of 3-10 meters, to cover the humans body via single hop star topology.
Over hearing and idle listening are ignored for the imbalanced traffic load of WBANs. Imbalanced traffic load in WBANs occurs where WSNs continuously transmits vital signals to CPN, while CPN transmits only a few controlled packets to WSNs. Since overhearing and idle listening are associated to receivers only, whereas WSN is designed as a transmitter, idle listening and overhearing are ignored in low power WSN design. Also, energy consumed on control messages can be ignored when the data volume is larger than the control message. Their study considers collision as the only major source of energy efficiency to be investigated. Thus, the study focuses on beacon-mode and CSMA/CA mode access schemes to achieve energy efficiency (Cheng and Huang, 2008, p. 2).
A study carried out by Yan et al (2007) investigates an energy-efficient computing model called wireless device driver, for communication between low-duty peripherals, sensors and other I/O devices employed in a WBAN, and a more powerful central device. They present an extensive comparative study of two popular WBAN technologies, Bluetooth and ZigBee in terms of design cost, performance, and energy efficiency. The study discusses the impact of tunable parameters of the wireless device driver on connection latency and energy consumption for both Bluetooth and ZigBee.
Dynamic resource management in higher-level protocols is addressed by investigating the trade-off between connection latency and energy consumption. An energy-efficient power down policy that utilizes the interval between consecutive connection requests for energy reduction is proposed.
The study analyses an adaptive connection latency management technique. The technique adjusts various tunable parameters dynamically to achieve minimum connection latency without changing the energy consumption level. This study was especially considered and applied in this essay due to the exceptional measurements and experimental results of the proposed techniques, which appeared to be very effective in reducing energy consumption, while meeting connection latency requirements (Yan et al., 2007, p. 1).
A study by Ullah et al. (2009) analyzes the behavior of several power-efficient MAC protocols including a beacon-enabled protocol for on-body sensor networks. It classifies the entire traffic in BSN as normal, on-demand, and emergency.
Traffic based Wakeup Mechanism is proposed for BSNs which exploits the traffic patterns of the BNs to accommodate the entire traffic classification. A Bridging function that integrated all the body area network (BAN) nodes working on different Physical layers (PHYs) into a complete BSN is introduced.
The study is relevant in this essay since it proposes a wakeup mechanism, backed by the Bridging function, providing a complete solution towards power-efficient and reliable communication in a BSN (Ullah et al., 2009, p. 343).
According to this study by An et al. (2010), MAC layer is the most suitable level to address the energy efficiency. The fundamental task in MAC protocol is to maximize energy efficiency through avoiding collisions and preventing simultaneous transmissions, while preserving maximum throughput, minimum latency and communication reliability in WBANs (An et al., 2010, p. 739).
This study by Omeni et al (2008) proposes an energy efficient MAC protocol designed specifically for WBANs. The study considers wireless sensor nodes in WBANs, attached to the human body, monitoring vital signs such as body temperature. Unlike in traditional wireless sensor networks, the nodes in this network are not deployed in an ad-hoc fashion.
The network applies a master-slave architecture, whereby the body-worn slave node periodically sends the collected data to a central master node. The network is centrally managed and all communications are in single-hop fashion. To achieve energy efficiency, all sensor nodes remain in standby/sleep mode until the relevant, centrally assigned, time slot arrives. Once a node joins a network, there is no risk of collision since communication is initiated by the central node and addressed uniquely to a slave node. To handle time slot overlaps, the wakeup fallback time concept is introduced.
Application of single-hop communication and centrally controlled sleep/wakeup times results in significant improvement in energy efficiency in this application compared to more flexible network MAC protocols such as Zigbee. As duty cycle is reduced, the overall energy consumption approaches the standby power (Omeni et al., 2008, p. 251).
This study was considered and applied in this essay due to the studys primary design goal which is low power consumption in WBANs. This objective is achieved via a focus on collision avoidance and other primary sources of energy wastage and the use of centrally controlled time slotting for sensor nodes. Through application of this protocol, many of the traditional problems that plague wireless sensor networks have been eliminated or significantly minimized. Idle listening and over-hearing are not an issue in this protocol since traffic is managed centrally. Such factors have immensely improved energy efficiency in WBANs (Omeni et al., 2008, p. 251).
Battery-Dynamics Driven TDMA MAC Protocols for Wireless Body-Area Monitoring Networks in Healthcare Applications
This analytical study on power consumption in WBANs was carried out by Su and Zhang (2009). According to their study, due to advances in system integrations, sensors can be applied in different body locations. However, implanted and wearable devices can cover all needs that include, interacting with the user and communicating. Thus, sensor devices usually communicate with a wearable or near body co coordinator. The coordinator analyses the collected data and transmits it to the hospital network via the internet. Coordinators batteries are not as restricted to size and shape as those in implanted and wearable devices.
However, batteries in implanted and wearable devices have a very limited energy capacity and require practical diminution, as much as possible, of their usage. This study is especially relevant to this essay due to its competent consideration in efficient utilization of battery capacity in sensor nodes to prolong the life time of sensor nodes, along with guaranteeing reliability and timely message delivery as the most crucial goal in WBANs design (Su and Zhang, 2009, p. 424).
A study by Marinkovic et al. (2009) is applied in this essay since it proposes an energy efficient medium access control (MAC) protocol for WBANs. According to their study, advancement in storage and wireless technologies have increased the number of recording devices with the ability of monitoring patients outside a clinical setting. These devices stream data from the patient to the central storage device.
Due to high power consumption of data transmission, the low battery life places a strict limit on many further processes the device could perform.The study suggests solutions to synchronization problems in TDMA. It evaluates the duty cycle of transmitters, consequently predicting battery life. These will reduce transmission, storage and workload on the medical staff and put the sensors in a more suitable position to perform more signal analysis. The protocol proposed by the study capitalizes on the static nature of WBANs to implement TDMA strategy with minimal overhead and idle listening (Marinkovic et al., 2009, p. 915).
Credibility Issues
To ensure authenticity of all data applied in this study; all primary and secondary sources have been derived from or authenticated by Institute of Electrical and Electronics Engineers (IEEE). All studies applied in this essay were vetted and selected after satisfying preset criteria which are:
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Studies considered must be reviewed and approved by the supervisor of this essay Dr. Zeev Weissman, as well as the Departmental Masters degree committee of the Open University of Israel within the relevant field.
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Studies considered must be relevant to the main issues of the essay: energy consumption in wireless body area networks.
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Studies considered must be written in a clear, non-offensive and scientific manner, as the academic ethics demands.
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The referenced studies must be up to date (2008-present). However, other studies applied for either enquiry or comparison purposes are not asserted to these restrictions.
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The studies must contain concrete theoretical or practical experiments to support the proposed or working hypothesis.
Power Model for Beacon-Mode and Csma/CA-Mode
Introduction
WBAN is a short distance wireless transmission for near or inner body applications which has become an emerging wireless system in recent years. Although conventional cabling monitors can achieve the same objective, one major drawback of cable monitors is limiting patients movements and activities. On the contrary, WBAN can monitor patients and collect appropriate measurements without these limitations.
However, WBANs are not without shortcomings. Due to limited energy supply in WBANs, the key design challenge of WBAN is low power, which is essential for long-duration measurement. Thus, the low power mechanism that involves MAC, baseband chip and wireless front-end designs have become critical in WBANs. Although the WBAN is a subset of wireless sensor network, many features of WBAN make the low power strategy of MAC design different from the conventional sensor network (Cheng and Huang, 2008, p. 1).
Acording to Ullah et al. (2010) Generally MAC protocols are grouped into contention-based and schedule-based MAC protocols. CSMA/CA protocol is categorized as a contention-based MAC layer protocols. CSMA/CA nodes contend for the channel to transmit data. If the channel is busy, a node defers its transmission until it becomes idle. Contention-based protocols are scalable with no strict time synchronization constraint. However, they incur significant protocol overhead. As opposed to contention-based protocols, in schedule-based protocols such as the TDMA protocol, the channel is divided into time slots of fixed or variable duration. These slots are assigned to nodes and each node transmits during its own slot period. These protocols are energy conserving protocols (p. 10).
The most important attribute of a good MAC protocol for WBAN is energy efficiency. In some applications, the device should support a battery lifetime of months or years without interventions, while others may require a battery life of tens of hours due to the nature of the applications. For example, pacemakers have a lifetime of more than 5 years while swallow able camera pills have a lifetime of 12 hrs. Power-efficient and flexible duty cycling techniques are required to solve the idle listening, overhearing and packet collisions problems.
Moreover, low duty cycle nodes should not receive frequent synchronization/control packets (beacon frames) if they have no data to send or receive. The MAC layer protocolsof WBAN should satisfy the MAC transparency requirements, i.e., to operate on multiple physical layerbands, such as Mobile Information and Communication Systems (MICS), Industrial Scientific and Medical (ISM) and Wireless Medical Telemetry Service (WMTS), simultaneously (Ullah et al., 2010, p. 10).
Since most of the traffic in WBAN is correlated, a single physiological fluctuation triggers many sensors at the same time. In this case, a CSMA/CA protocol encounters heavy collisions and extra energy consumption. In CSMA/CA protocol the nodes are required to perform the, so called, Clear Channel Assessment (CCA) before transmission, that is to determine whether the wireless medium is ready and able to receive data, so that the transmitter may start sending it. Using CCA in an ordinary wireless network is, at some point, security vulnerability due to a Denial of Service (DoS) attacks, such as Queensland attack, which makes it appear that the airwaves are busy, stalling the whole system.
Although WBANs are less likely to suffer from such attacks, stillthey have another factor that prevents from the CCA from guaranteed in the MICS band.This is the tissue heating of the human body, which causes the path loss inside the human body to be much higher than in free space. In the Contention-based mechanism, nodes contend for the channel to transmit data regardless of any predefined schedule; making the CSMA/CA protocol the best example of such mechanism (Ullah et al., 2010, p. 11).
In a survey by Ullah et al. (2010), an IEEE 802.15.4 MAC protocol is proposed. This protocol has two operational modes: a beacon-enabled mode and a non-beacon enabled mode. In a beacon-enabled mode, the network is controlled by a coordinator, which regularly transmits beacons for device synchronization and association control. The channel is bounded by a superframe structure which consists of both active and inactive periods.
The active period contains the following three components: a beacon, a Contention Access Period (CAP), and a Contention Free Period (CFP). The coordinator interacts with nodes during the active period and sleeps during inactive period. There are no more than seven Guaranteed Time Slots (GTS) in the CFP period to support time critical traffic. In the beacon-enabled mode, a slotted CSMA/CA protocol is used in the CAP period while in the non-beacon enabled mode, un slotted CSMA/CA protocol is used. Some of the main reasons of selecting IEEE 802.15.4 for WBAN are low power communication and support of low data rate WBAN applications.
The study investigates the performance of the non-beacon IEEE 802.15.4 protocol for low uplo
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