Wireless Networks manuscript No.(will be inserted by the editor)A Survey on Wireless Body Area NetworksBenoˆ Latr´ · Bart Braem · Ingrid Moerman · Chris Blondia · ıt ePiet DemeesterReceived: date / Accepted: dateAbstract The increasing use of wireless networks and 1 Introductionthe constant miniaturization of electrical devices hasempowered the development of Wireless Body Area Net- The aging population in many developed countries andworks (WBANs). In these networks various sensors are the rising costs of health care have triggered the in-attached on clothing or on the body or even implanted troduction of novel technology-driven enhancements tounder the skin. The wireless nature of the network and current health care practices. For example, recent ad-the wide variety of sensors oﬀer numerous new, practi- vances in electronics have enabled the development ofcal and innovative applications to improve health care small and intelligent (bio-) medical sensors which canand the Quality of Life. The sensors of a WBAN mea- be worn on or implanted in the human body. Thesesure for example the heartbeat, the body temperature sensors need to send their data to an external medicalor record a prolonged electrocardiogram. Using a WBAN, server where it can be analyzed and stored. Using athe patient experiences a greater physical mobility and wired connection for this purpose turns out to be toois no longer compelled to stay in the hospital. This pa- cumbersome and involves a high cost for deploymentper oﬀers a survey of the concept of Wireless Body Area and maintenance. However, the use of a wireless in-Networks. First, we focus on some applications with terface enables an easier application and is more costspecial interest in patient monitoring. Then the com- eﬃcient . The patient experiences a greater physicalmunication in a WBAN and its positioning between mobility and is no longer compelled to stay in a hospi-the diﬀerent technologies is discussed. An overview of tal. This process can be considered as the next step inthe current research on the physical layer, existing MAC enhancing the personal health care and in coping withand network protocols is given. Further, cross layer and the costs of the health care system. Where eHealth isquality of service is discussed. As WBANs are placed deﬁned as the health care practice supported by elec-on the human body and often transport private data, tronic processes and communication, the health caresecurity is also considered. An overview of current and is now going a step further by becoming mobile. Thispast projects is given. Finally, the open research issues is referred to as mHealth . In order to fully exploitand challenges are pointed out. the beneﬁts of wireless technologies in telemedicine and mHealth, a new type of wireless network emerges: a wireless on-body network or a Wireless Body Area Net-Benoˆ Latr´, Ingrid Moerman, Piet Demeester ıt e work (WBAN). This term was ﬁrst coined by Van DamDepartment of Information Technology, Ghent University / et al. in 2001  and received the interest of severalIBBT, Gaston Crommenlaan 8 box 201, B-9050 Gent, Belgium, researchers [4–8].Tel.: +32-45-678910Fax: +132-45-678910 A Wireless Body Area Network consists of small, in-E-mail: email@example.com telligent devices attached on or implanted in the bodyBart Braem, Chris Blondia which are capable of establishing a wireless commu-Department of Mathematics and Computer Science, University nication link. These devices provide continuous healthof Antwerp / IBBT, monitoring and real-time feedback to the user or med-Middelheimlaan 1, B-2020, Antwerp, Belgium ical personnel. Furthermore, the measurements can be
2recorded over a longer period of time, improving the – And ﬁnally the devices are often very heterogeneous,quality of the measured data . may have very diﬀerent demands or may require Generally speaking, two types of devices can be dis- diﬀerent resources of the network in terms of datatinguished: sensors and actuators. The sensors are used rates, power consumption and reliability.to measure certain parameters of the human body, ei- When referring to a WBAN where each node com-ther externally or internally. Examples include mea- prises a biosensor or a medical device with sensing unit,suring the heartbeat, body temperature or recording some researchers use the name Body Area Sensor Net-a prolonged electrocardiogram (ECG). The actuators work (BASN) or in short Body Sensor Network (BSN)(or actors) on the other hand take some speciﬁc ac- instead of WBAN . These networks are very similartions according to the data they receive from the sensors to each other and share the same challenges and prop-or through interaction with the user. E.g., an actuator erties. In the following, we will use the term WBANequipped with a built-in reservoir and pump adminis- which is also the one used by the IEEE .ters the correct dose of insulin to give to diabetics based In this article we present a survey of the state ofon the glucose level measurements. Interaction with the the art in Wireless Body Area Networks. Our aim is touser or other persons is usually handled by a personal provide a better understanding of the current researchdevice, e.g. a PDA or a smart phone which acts as a issues in this emerging ﬁeld. The remainder of this pa-sink for data of the wireless devices. per is organized as follows. First, the patient monitoring In order to realize communication between these de- application is discussed in Section 2. Next, the char-vices, techniques from Wireless Sensor Networks (WSNs) acteristics of the communication and the positioningand ad hoc networks could be used. However, because of WBANs amongst other wireless technologies is dis-of the typical properties of a WBAN, current proto- cussed in Section 4. Section 5 gives an overview of thecols designed for these networks are not always well properties of the physical layer and the issues of com-suited to support a WBAN. The following illustrates municating near or in the body. Existing protocols forthe diﬀerences between a Wireless Sensor Network and the MAC-layer and network layer are discussed in Sec-a Wireless Body Area Network: tion 6 and Section 7 respectively. Section 8 deals with cross-layer protocols available for WBANs. The Quality – The devices used have limited energy resources avail- of Service and possible security mechanisms are treated able as they have a very small form factor (often less in Section 9 and 10. An overview of existing projects than 1 cm3 ). Furthermore, for most devices it is given in Section 11. Finally, the open research issues is impossible to recharge or change the batteries al- are discussed in Section 12 and Section 13 concludes though a long lifetime of the device is wanted (up the paper. to several years or even decades for implanted devi- ces). Hence, the energy resources and consequently the computational power and available memory of 2 Patient Monitoring such devices will be limited; – All devices are equally important and devices are The main cause of death in the world is CardioVascular only added when they are needed for an application Disease (CVD), representing 30% of all global deaths. (i.e. no redundant devices are available); According to the World Health Organization, world- – An extremely low transmit power per node is needed wide about 17.5 million people die of heart attacks or to minimize interference and to cope with health strokes each year; in 2015, almost 20 million people will concerns ; die from CVD. These deaths can often be prevented – The propagation of the waves takes place in or on a with proper health care . Worldwide, more than 246 (very) lossy medium, the human body. As a result, million people suﬀer from diabetes, a number that is the waves are attenuated considerably before they expected to rise to 380 million by 2025 . Frequent reach the receiver; monitoring enables proper dosing and reduces the risk – The devices are located on the human body that can of fainting and in later life blindness, loss of circulation be in motion. WBANs should therefore be robust and other complications . against frequent changes in the network topology; These two examples already illustrate the need for – The data mostly consists of medical information. continuous monitoring and the usefulness of WBANs. Hence, high reliability and low delay is required; Numerous other examples of diseases would beneﬁt from – Stringent security mechanisms are required in order continuous or prolonged monitoring, such as hyperten- to ensure the strictly private and conﬁdential char- sion, asthma, Alzheimer’s disease, Parkinson’s disease, acter of the medical data; renal failure, post-operative monitoring, stress-monitoring,
3prevention of sudden infant death syndrome etc [9, 16, EEG17]. These applications can be considered as an indica- Hearing Aid Cochlear Implanttor for the size of the market for WBANs. The numberof people suﬀering from diabetics or CVD and the per- Positioning Motion sensorcentage of people in the population age 60 years and Blood pumpolder will grow in the future. Even without any further ECGincrease in world population by 2025 this would mean a Insulin Injectionvery large number of potential customers. WBAN tech- Blood oxygennology could provide the connectivity to support the Glucoseelderly in managing their daily life and medical condi-tions . A WBAN allows continuous monitoring of Personal device Lactic Acidthe physiological parameters. Whether the patient is inthe hospital, at home or on the move, the patient will Artificial Artificialno longer need to stay in bed, but will be able to move Knee Kneearound freely. Furthermore, the data obtained during alarge time interval in the patient’s natural environmentoﬀers a clearer view to the doctors than data obtained Pressure sensorduring short stays at the hospital . An example of a medical WBAN used for patientmonitoring is shown in Figure 1. Several sensors are Fig. 1 Example of patient monitoring in a Wireless Body Area Network.placed in clothes, directly on the body or under the skinof a person and measure the temperature, blood pres-sure, heart rate, ECG, EEG, respiration rate, SpO2 - The WBAN monitors for example the level of toxicslevels etc. Next to sensing devices, the patient has actu- in the air and warns the ﬁreﬁghters or soldiers if aators which act as drug delivery systems. The medicine life threatening level is detected. The introduction ofcan be delivered on predetermined moments, triggered a WBAN further enables to tune more eﬀectively theby an external source (i.e. a doctor who analyzes the training schedules of professional athletes.data) or immediately when a sensor notices a problem.One example is the monitoring of the glucose level in Next to purely medical applications, a WBAN canthe blood of diabetics. If the sensor monitors a sudden include appliances such as an MP3-player, head-mounteddrop of glucose, a signal can be sent to the actuator (computer) displays, a microphone, a camera, advancedin order to start the injection of insulin. Consequently, human-computer interfaces such as a neural interfacethe patient will experience fewer nuisances from his dis- etc . As such, the WBAN can also be used for gam-ease. Another example of an actuator is a spinal cord ing purposes and in virtual reality.stimulator implanted in the body for long-term pain This small overview already shows the myriad ofrelief . possibilities where WBANs are useful. The main char- A WBAN can also be used to oﬀer assistance to the acteristic of all these applications is that WBANs im-disabled. For example, a paraplegic can be equipped prove the user’s Quality of Life.with sensors determining the position of the legs orwith sensors attached to the nerves . In addition,actuators positioned on the legs can stimulate the mus-cles. Interaction between the data from the sensors andthe actuators makes it possible to restore the ability to 3 Taxonomy and Requirementsmove. Another example is aid for the visually impaired.An artiﬁcial retina, consisting of a matrix of micro sen- The applications described in the previous section in-sors, can be implanted into the eye beneath the surface dicate that a WBAN consists of several heterogeneousof the retina. The artiﬁcial retina translates the elec- devices. In this section an overview of the diﬀerent typestrical impulses into neurological signals. The input can of devices used in a WBAN will be given. Further the re-be obtained locally from light sensitive sensors or by an quirements and challenges are discussed. These includeexternal camera mounted on a pair of glasses . the wide variability of data rates, the restricted energy Another area of application can be found in the do- consumption, the need for quality of service and relia-main of public safety where the WBAN can be used by bility, ease-of-use by medical professionals and securityﬁreﬁghters, policemen or in a military environment . and privacy issues.
43.1 Types of Devices Table 1 Examples of medical WBAN applications [21, 25–27](Wireless) Sensor node: Application Data Rate Bandwidth Accuracy A device that responds to and gathers data on phys- ECG (12 leads) 288 kbps 100-1000 Hz 12 bits ical stimuli, processes the data if necessary and re- ECG (6 leads) 71 kbps 100-500 Hz 12 bits ports this information wirelessly. It consists of sev- EMG 320 kbps 0-10,000 Hz 16 bits eral components: sensor hardware, a power unit, a EEG (12 leads) 43.2 kbps 0-150 Hz 12 bits processor, memory and a transmitter or transceiver . Blood saturation 16 bps 0-1 Hz 8 bits(Wireless) Actuator node: Glucose monitoring 1600 bps 0-50 Hz 16 bits A device that acts according to data received from Temperature 120 bps 0-1 Hz 8 bits the sensors or through interaction with the user. The components of an actuator are similar to the Motion sensor 35 kbps 0-500 Hz 12 bits sensor’s: actuator hardware (e.g. hardware for medi- Cochlear implant 100 kbps – – cine administration, including a reservoir to hold the Artiﬁcial retina 50-700 kbps – – medicine), a power unit, a processor, memory and Audio 1 Mbps – – a receiver or transceiver. Voice 50-100 kbps – –(Wireless) Personal Device (PD): A device that gathers all the information acquired by the sensors and actuators and informs the user is a higher than the raw bit rate of most existing low (i.e. the patient, a nurse, a GP etc.) via an exter- power radios. nal gateway, an actuator or a display/LEDS on the The reliability of the data transmission is provided device. The components are a power unit, a (large) in terms of the necessary bit error rate (BER) which is processor, memory and a transceiver. This device is used as a measure for the number of lost packets. For a also called a Body Control Unit (BCU) , body- medical device, the reliability depends on the data rate. gateway or a sink. In some implementations, a Per- Low data rate devices can cope with a high BER (e.g. sonal Digital Assistant (PDA) or smart phone is 10−4 ), while devices with a higher data rate require used. a lower BER (e.g. 10−10 ). The required BER is also dependent on the criticalness of the data. Many diﬀerent types of sensors and actuators areused in a WBAN. The main use of all these devices isto be found in the area of health applications. In thefollowing, the term nodes refers to both the sensor as 3.3 Energyactuator nodes. Energy consumption can be divided into three domains: The number of nodes in a WBAN is limited by na- sensing, (wireless) communication and data process-ture of the network. It is expected that the number of ing . The wireless communication is likely to benodes will be in the range of 20–50 [6, 24]. the most power consuming. The power available in the nodes is often restricted. The size of the battery used to store the needed energy is in most cases the largest3.2 Data Rates contributor to the sensor device in terms of both di- mensions and weight. Batteries are, as a consequence,Due to the strong heterogeneity of the applications, kept small and energy consumption of the devices needsdata rates will vary strongly, ranging from simple data to be reduced. In some applications, a WBAN’s sensor-at a few kbit/s to video streams of several Mbit/s. Data /actuator node should operate while supporting a bat-can also be sent in bursts, which means that it is sent tery life time of months or even years without interven-at higher rate during the bursts. tion. For example, a pacemaker or a glucose monitor The data rates for the diﬀerent applications are given would require a lifetime lasting more than 5 years. Es-in in Table 1 and are calculated by means of the sam- pecially for implanted devices, the lifetime is crucial.pling rate, the range and the desired accuracy of the The need for replacement or recharging induces a costmeasurements [25, 26]. Overall, it can be seen that the and convenience penalty which is undesirable not onlyapplication data rates are not high. However, if one has for implanted devices, but also for larger ones.a WBAN with several of these devices (i.e. a dozen mo- The lifetime of a node for a given battery capacitytion sensors, ECG, EMG, glucose monitoring etc.) the can be enhanced by scavenging energy during the op-aggregated data rate easily reaches a few Mbps, which eration of the system. If the scavenged energy is larger
5than the average consumed energy, such systems could intervention. The self-organizing aspect also includesrun eternally. However, energy scavenging will only de- the problem of addressing the nodes. An address canliver small amounts of energy [5, 28]. A combination be conﬁgured at manufacturing time (e.g. the MAC-of lower energy consumption and energy scavenging is address) or at setup time by the network itself. Fur-the optimal solution for achieving autonomous Wireless ther, the network should be quickly reconﬁgurable, forBody Area Networks. For a WBAN, energy scavenging adding new services. When a route fails, a back up pathfrom on-body sources such as body heat and body vi- should be set up.bration seems very well suited. In the former, a thermo- The devices may be scattered over and in the wholeelectric generator (TEG) is used to transform the tem- body. The exact location of a device will depend on theperature diﬀerence between the environment and the application, e.g. a heart sensor obviously must be placedhuman body into electrical energy . The latter uses in the neighborhood of the heart, a temperature sen-for example the human gait as energy source . sor can be placed almost anywhere. Researchers seem During communication the devices produce heat which to disagree on the ideal body location for some sensoris absorbed by the surrounding tissue and increases the nodes, i.e. motion sensors, as the interpretation of thetemperature of the body. In order to limit this temper- measured data is not always the same . The net-ature rise and in addition to save the battery resources, work should not be regarded as a static one. The bodythe energy consumption should be restricted to a min- may be in motion (e.g. walking, running, twisting etc.)imum. The amount of power absorbed by the tissue is which induces channel fading and shadowing eﬀects.expressed by the speciﬁc absorption rate (SAR). Since The nodes should have a small form factor consis-the device may be in close proximity to, or inside, a tent with wearable and implanted applications. Thishuman body, the localized SAR could be quite large. will make WBANs invisible and unobtrusive.The localized SAR into the body must be minimizedand needs to comply with international and local SARregulations. The regulation for transmitting near the 3.6 Security and Privacyhuman body is similar to the one for mobile phones,with strict transmit power requirements [11, 30] The communication of health related information be- tween sensors in a WBAN and over the Internet to servers is strictly private and conﬁdential  and should3.4 Quality of Service and Reliability be encrypted to protect the patient’s privacy. The med- ical staﬀ collecting the data needs to be conﬁdent thatProper quality of service (QoS) handling is an impor- the data is not tampered with and indeed originatestant part in the framework of risk management of med- from that patient. Further, it can not be expected thatical applications. A crucial issue is the reliability of the an average person or the medical staﬀ is capable of set-transmission in order to guarantee that the monitored ting up and managing authentication and authorizationdata is received correctly by the health care profession- processes. Moreover the network should be accessibleals. The reliability can be considered either end-to-end when the user is not capable of giving the password (e.g.or on a per link base. Examples of reliability include to guarantee accessibility by paramedics in trauma sit-the guaranteed delivery of data (i.e. packet delivery ra- uations). Security and privacy protection mechanismstio), in-order-delivery, . . . Moreover, messages should use a signiﬁcant part of the available energy and shouldbe delivered in reasonable time. The reliability of the therefor be energy eﬃcient and lightweight.network directly aﬀects the quality of patient monitor-ing and in a worst case scenario it can be fatal when alife threatening event has gone undetected . 4 Positioning WBANs3.5 Usability The development and research in the domain of WBANs is only at an early stage. As a consequence, the termi-In most cases, a WBAN will be set up in a hospital nology is not always clearly deﬁned. In literature, pro-by medical staﬀ, not by ICT-engineers. Consequently, tocols developed for WBANs can span from communi-the network should be capable of conﬁguring and main- cation between the sensors on the body to communica-taining itself automatically, i.e. self-organization an self- tion from a body node to a data center connected tomaintenance should be supported. Whenever a node is the Internet. In order to have clear understanding, weput on the body and turned on, it should be able to join propose the following deﬁnitions: intra-body communi-the network and set up routes without any external cation and extra-body communication. An example is
6 Intra-body communication WAN WMAN WLAN WPAN AN WB WBAN Medical Server BAN MAN Sensor Internet Sensor Emergency Personal Device Personal Device Extra-body communication Communication Distance Physician Wireless communication link Fig. 2 ExampleWBAN of intra-body and extra-body communication in Fig. 3 Positioning of a Wireless Body Area Network in the realm a WBAN.a BAN communicatie of wireless networks. shown on Figure 2. The former controls the informa- values around 1-2 meters. While a WBAN is devoted tion handling on the body between the sensors or actu- to interconnection of one person’s wearable devices, a Medische Server ators and the personal device [34–37], the latter ensures WPAN is a network in the environment around the Sensor communication between the personal device and an ex- person. The communication range can reach up to 10 ternal network [32, 38–40]. Doing so, the medical data meters for high data rate applications and up to sev- from the patient at home can be consulted by a Spoed physi- eral dozens of meters for low data rate applications. A Internet cian or stored in a medical database. This segmentation WLAN has a typical communication range up to hun- is similar to the one deﬁned in  where a multi-tiered dreds of meters. Each type of network has its typical telemedicine system is presented. Tier 1 encompasses enabling technology, deﬁned by the IEEE. A WPAN Arts the intra-body communication, tier 2 the extra-body uses IEEE 802.15.1 (Bluetooth) or IEEE 802.15.4 (Zig- communication between the personal device and the Bee), a WLAN uses IEEE 802.11 (WiFi) and a WMAN Internet and tier 3 represents the extra-body commu- IEEE 802.16 (WiMax). The communication in a WAN nication from the Internet to the medical server. The can be established via satellite links. combination of intra-body and extra-body communica- In several papers, Wireless Body Area Networks tion can be seen as an enabler for ubiquitous health are considered as a special type of a Wireless Sensor care service provisioning. An example can be found Network or a Wireless Sensor and Actuator Network in  where Utility Grid Computing is combined with (WSAN) with its own requirements1 . However, tradi- a WBAN. Doing so, the data extracted from the WBAN tional sensor networks do not tackle the speciﬁc chal- is sent to the grid that provides access to appropriate lenges associated with human body monitoring. The computational services with high bandwidth and to a human body consists of a complicated internal envi- large collection of distributed time-varying resources. ronment that responds to and interacts with its exter- To date, development has been mainly focused on nal surroundings, but is in a way separate and self- building the system architecture and service platform contained. The human body environment not only has for extra-body communication. Much of these imple- a smaller scale, but also requires a diﬀerent type and mentations focus on the repackaging of traditional sen- frequency of monitoring, with diﬀerent challenges than sors (e.g. ECG, heart rate) with existing wireless de- those faced by WSNs. The monitoring of medical data vices. They consider a very limited WBAN consist- results in an increased demand for reliability. The ease ing of only a few sensors that are directly and wire- of use of sensors placed on the body leads to a small lessly connected to a personal device. Further they use form factor that includes the battery and antenna part, transceivers with a large form factor and large antennas resulting in a higher need for energy eﬃciency. Sensor that are not adapted for use on a body. nodes can move with regard to each other, for example In Figure 3, a WBAN is compared with other types a sensor node placed on the wrist moves in relation to a of wireless networks, such as Wireless Personal (WPAN), sensor node attached to the hip. This requires mobility Wireless Local (WLAN), Wireless Metropolitan (WMAN) support. In brief, although challenges faced by WBANs and Wide Area Networks (WAN) . A WBAN is op- erated close to the human body and its communication 1In the following, we will not make a distinction between a range will be restricted to a few meters, with typical WSAN and a WSN although they have signiﬁcant diﬀerences .
7 Coexistence/ of them came to the conclusion that the radio signals Non invasive Heterogeneous experience great losses. Generally in wireless networks, Reliability nodes it is known that the transmitted power drops oﬀ with dη where d represents the distance between the sender and the receiver and η the coeﬃcient of the path loss Simple WSN nodes Multi-hop (aka propagation coeﬃcient) . In free space, η has a value of 2. Other kinds of losses include fading of signals due to multi-path propagation. The propagation can be Energy Node density classiﬁed according to where it takes place: inside the efficiency Cost body or along the body. structure WBAN WLAN 5.1.1 In the BodyFig. 4 Characteristics of a Wireless Body Area Network com- The propagation of electromagnetic (EM) waves in thepared with Wireless Sensor Networks (WSN) and Wireless Local human body has been investigated in [49,50]. The bodyArea Network (WLAN). Based on . acts as a communication channel where losses are mainly due to absorption of power in the tissue, which is dissi- pated as heat. As the tissue is lossy and mostly consistsare in many ways similar to WSNs, there are intrin- of water, the EM-waves are attenuated considerably be-sic diﬀerences between the two, requiring special atten- fore they reach the receiver. In order to determine thetion. An overview of some of these diﬀerences is given amount of power lost due to heat dissipation, a stan-in Table 2. A schematic overview of the challenges in a dard measure of how much power is absorbed in tissue isWBAN and a comparison with WSNs and WLANs is used: the speciﬁc absorption rate (SAR). It is concludedgiven in Figure 4. that the path loss is very high and that, compared to the free space propagation, an additional 30-35 dB at small distances is noticed. A simpliﬁed temperature in-5 Physical layer crease prediction scheme based on SAR is presentedThe characteristics of the physical layer are diﬀerent in . It is argued that considering energy consump-for a WBAN compared to a regular sensor network tion is not enough and that the tissue is sensitive toor an ad-hoc network due to the proximity of the hu- temperature increase. The inﬂuence of a patient’s bodyman body. Tests with TelosB motes (using the CC2420 shape and position on the radiation pattern from antransceiver) showed lack of communications between implanted radio transmitter has been studied in . Itnodes located on the chest and nodes located on the is concluded that the diﬀerence between body shapesback of the patient . This was accentuated when (i.e. male, female and child) are at least as large as thethe transmit power was set to a minimum for energy impact of a patient’s arm movements.savings reasons. Similar conclusions where drawn witha CC2420 transceiver in : when a person was sitting 5.1.2 Along the Bodyon a sofa, no communication was possible between thechest and the ankle. Better results were obtained when Most of the devices used in a WBAN however are at-the antenna was placed 1 cm above the body. As the tached on the body. The propagation along the humandevices get smaller and more ubiquitous, a direct con- body can be divided into line of sight (LOS) and non-nection to the personal device will no longer be possible line of sight (NLOS) situations. In the former, the cur-and more complex network topologies will be needed. vature eﬀects of the body are not taken into account asIn this section, we will discuss the characteristics of the simulations are performed on a ﬂat phantom or exper-propagation of radio waves in a WBAN and other types iments are done at one side of the body. In the latter,of communication. the eﬀect of propagation from the front of the body to the side or back are evaluated. The channel model for line of sight (LOS) propaga-5.1 RF communication tion along the human body was studied in [24, 52–55], both by simulations and experiments. The studies wereSeveral researchers have been investigating the path done for both narrowband and UWB signals. However,loss along and inside the human body either using nar- the results can be compared as the studies for UWBrowband radio signals or Ultra Wide Band (UWB). All signals were performed in a band between 3 to 6 GHz
8Table 2 Schematic overview of diﬀerences between Wireless Sensor Networks and Wireless Body Area Networks, based on . Challenges Wireless Sensor Network Wireless Body Area Network Scale Monitored environment (meters / kilometers) Human body (centimeters / meters) Node Number Many redundant nodes for wide area coverage Fewer, limited in space Result accuracy Through node redundancy Through node accuracy and robustness Node Tasks Node performs a dedicated task Node performs multiple tasks Node Size Small is preferred, but not important Small is essential Network Topology Very likely to be ﬁxed or static More variable due to body movement Data Rates Most often homogeneous Most often heterogeneous Node Replacement Performed easily, nodes even disposable Replacement of implanted nodes diﬃcult Node Lifetime Several years / months Several years / months, smaller battery capac- ity Power Supply Accessible and likely to be replaced more easily Inaccessible and diﬃcult to replaced in an im- and frequently plantable setting Power Demand Likely to be large, energy supply easier Likely to be lower, energy supply more diﬃcult Energy Scavenging Source Most likely solar and wind power Most likely motion (vibration) and thermal (body heat) Biocompatibility Not a consideration in most applications A must for implants and some external sensors Security Level Lower Higher, to protect patient information Impact of Data Loss Likely to be compensated by redundant nodes More signiﬁcant, may require additional mea- sures to ensure QoS and real-time data delivery. Wireless Technology Bluetooth, ZigBee, GPRS, WLAN, . . . Low power technology requiredand the narrowband system around 2.4 GHz is rela-tively close to the band of 3 GHz. It was found thatthe path loss exponent η is between 3 and 4, dependingon the position of the device, e.g. the path loss on thearm is lower than the one on the trunk. It is claimedthat this is probably due to the higher absorption inthe larger volume of the trunk, and because the surfaceof the trunk is less ﬂat than the surface of the stretchedarm. The study in  shows a signiﬁcant impact of theantenna height on the path loss. The closer the antennais to the body, the higher the path loss: a diﬀerence ofmore than 20 dB is found for an antenna placed at 5mm and 5 cm. As the sensors and antennas of a Wire-less Body Area Network will be designed to be as smallas possible, the antenna will be close to the body whichwill result in a higher path loss. In non-line of sight (NLOS) situations, there is no Fig. 5 Measured path loss versus the distance around (NLOS)direct view between the sender and receiver. The EM- and along the torso (LOS). It can be clearly seen that line-of sight communication experiences a lower path loss .waves are diﬀracting around the body rather than hav-ing a direct path through the body. In [16,54,55], a pathloss exponent ranging from 5 to 6 was found. Thus ahigher path loss along the NLOS channel than along theLOS channel was observed, due to diﬀraction aroundthe human body2 and absorption of a larger amount neglected due to reﬂections and the strong attenuationof radiation by the body. In  the dominant prop- of the head.agation mechanism for the the ear-to-ear link, whichcan be regarded as a worst case scenario at the head The results above show that it is not always possibledue to the missing line-of-sight component, was identi- to assume single-hop communication along the body.ﬁed. It was shown that for transmission from one side Figure 5 shows the measured path loss for a LOS andof the head to the opposite direct transmission can be NLOS scenario. Further, it is shown that in terms of en- 2 This is also referred to as creeping waves. A creeping wave ergy eﬃciency, the use of multi-hop communication inis deﬁned as the wave that is diﬀracted around the shadowed a WBAN could lead to a more optimal network topol-surface of a smooth body such as a sphere. ogy [24, 57].
95.2 Movement of the Body sor Networks has some points in common with network- ing in WBANs, it is useful to consider the research inThe movement of the body plays an important role in MAC-protocols designed for WSNs. An overview can bethe strength of the received signal. In  it is shown found in [66, 67]. Two major categories are contention-that arm motions to the front and side of the body can based and schedule-based. For the former, CSMA/CAhave a small impact on the received power. More sig- is a typical example, while TDMA is a typical schemeniﬁcant variations are found when the arms are moved for the latter. The advantages of contention-based ap-so that they block the line of sight between the two proaches are the simplicity, its infrastructure-free adantennas. In  a preliminary system model for gait hoc feature and good adaptability to traﬃc ﬂuctuation,analysis has been proposed. It is concluded that signiﬁ- especially for low load. Schedule-based approaches oncant attenuation can occur (up to 20 dB) when a body the other hand are free of idle listening, overhearing andlimb is moved in between the Tx and Rx antenna. Ac- packet collisions because of the lack of medium com-cording to  the movement of the limbs can induce petition, but require tight time synchronization. Thean attenuation of 30 dB or more. A similar conclusion most commonly used technique for reducing energy con-was found in an actual implementation  where the sumption in contention-based protocols is controllingsensors communicate directly with the personal device the power and duty cycle of the radio.using an RF-radio operating at 868 MHz. Loss rates Some implementations of WBANs use Bluetoothof more than 50% where found when the body was in (IEEE 802.15.1) . This was developed as a cablemotion. replacement and does not support (or only very lim- ited) multi-hop communication. It has a complex pro- tocol stack and a high energy consumption compared5.3 Non-RF Communication to IEEE 802.15.4. It is therefore not suited to be used in a WBAN.Next to the propagation of radio waves, several re- Most current implementations of WBANs use IEEEsearchers have investigated the possibility to transfer 802.15.4  or ZigBee  as enabling technology. Aselectronic data by capacitive and galvanic coupling, also most of the radios used in a WBAN are based on ancalled body-coupled communication (BCC). These ra- IEEE 802.15.4 compliant chip set, some researchers havedios work at low frequencies (ranging from 10 kHz to adapted the IEEE 802.15.4 MAC-protocol to make it10 MHz). Zimmerman  ﬁrst showed the potential more suitable for WBANs. We will therefore ﬁrst dis-of interference-free ultra low power data communica- cuss the usefulness of IEEE 802.15.4 for medical net-tion through the human body. High variations of the working. In a second part, other MAC-protocols fortransmission attenuation have been observed at diﬀer- WBANs will discussed. An overview is given in Table 3.ent locations of the body. Galvanic coupling promises It can be noticed that all proposed MAC-protocols useto be a potential communication technology for sensor slotted communication and assume a star topology byapplication on the thorax and for short distances on the using master-slave communication. However, in the pre-limbs . This technology can also be used to exchange vious section it has been shown that single-hop commu-data from one body to another by for example shaking nication is not always possible.hands . In  OsteoConduct is presented, wherethe human musculoskeletal system is used to trans-mit data and information in a low-power, secure, non-intrusive fashion. Although this research looks promis- 6.1 IEEE 802.15.4ing, only very low data rates can be achieved (5 bits/s). The idea of BCC is further exploited by  for In  the star network conﬁguration of the IEEE 802.15.4bootstrapping WBANs. They argue to equip the nodes standard at 2.4 GHz was considered for a WBAN. Thewith both RF and BCC capabilities. As a BCC is re- analysis considers quite extensively a very low data ratestricted to a person’s body, the BCC can be used to dis- star network with 10 body implanted sensors transmit-cover and identify sensor nodes on the same body and ting data 1 to 40 times per hour. The analysis focusesfor waking up RF radios from low-power sleep mode. on the eﬀect of crystal tolerance, frame size and the usage of IEEE 802.15.4 Guaranteed Time Slots (GTS) on a node lifetime. The main consideration in this work6 MAC layer was the long-term power consumption of devices. The results show that IEEE 802.15.4 provides a limited an-The number of MAC-protocols speciﬁcally developed swer for medical sensor networking when conﬁgured infor WBANs is limited. As networking in Wireless Sen- non-beacon mode with low data rate asymmetric traf-
10Table 3 Schematic overview of MAC protocols in a WBAN. MAC-protocol IEEE 802.15.4 TDMA based CSMA based Star topology Time Synchronization available based (master/slave) in the protocol √ √ √ Timmons  √ √ BSN-MAC  mixed √ √ √ Lamprinos  √ √ Omeni  √ √ √ H-MAC ﬁc. Beacon mode can also be used, but with even more at one of the nodes, the node can be assigned an extrasevere restrictions on data rate and crystal tolerance. slot for direct communication. The protocol has been Another adaptation is BSN-MAC . The coordi- evaluated on a Sensium platform. The H-MAC proto-nator controls the communication by varying the super- col  uses the human heartbeat rhythm informationframe structure of IEEE 802.15.4. This divides the time to perform time synchronization for TDMA. The bio-axis in a contention-free and contention-based period. sensors can thus achieve time synchronization withoutThe sensors provide real-time feedback to a BSN co- having to turn on their radio. The algorithm is veri-ordinator with application-speciﬁc and sensor-speciﬁc ﬁed with real world data but assumes a certain buﬀer.information. Hence, based on the feedback the BSN co- The simulations do not show the energy gain and theordinator can make dynamic adjustments for the length protocol is designed for a star-topology WBAN only.of the contention-free and contention-based period toachieve better performance in energy eﬃciency and la-tency. Both  and  come to the conclusion that al- 6.3 IEEE 802.15.6though 802.15.4 can provide QoS, the technology is notscalable in terms of power consumption and can not be Started as a Study Group in 2006 and motivated by theused as a single solution for all WBAN applications. increasing research and industry interest in WBANs, It can be concluded that IEEE 802.15.4 is not the the IEEE Standards Association decided to form thebest solution for supporting communication in WBANs. IEEE 802.15 Task Group 6 in November 2007. It de-Although it can be used for a quick (and easy) imple- scribes itself as follows: The IEEE 802.15 Task Groupmentation, the results are rather poor. IEEE 802.15.4 6 (BAN) is developing a communication standard op-was not designed to support WBANs. Specialized MAC timized for low power devices and operation on, in orprotocols are needed. around the human body (but not limited to humans) to serve a variety of applications including medical, con- sumer electronics / personal entertainment and other6.2 WBAN Speciﬁc Protocols . Project Authorization Request (PAR) 07-0575 presentsOne of the few MAC-protocols for WBANs was pro- an extended description of the task group . It stressesposed by Lamprinos et al. . They use a master-slave the fact that current WPANs do not meet medical com-architecture and, to avoid idle listening, all slaves are munication guidelines, because of the proximity to hu-locked in the Rx-slot of the master and go in standby man tissue. Moreover, WPAN technology is said not toat the same time. The main drawback of this protocol support Quality of Service, low power operation andis that some slaves will have a low duty cycle whereas noninterference, all required to support WBAN appli-the nodes that are serviced later have a higher duty cy- cations. Based on the responses to the Call for Appli-cle. The protocol was implemented nor simulated. An cations , the PAR also outlines a large number ofadaptation of this protocol was used in . This pro- applications that can be served by the proposed stan-tocol divides time into frames in which only one node is dard, going from classical medical usage, e.g. EEG andallowed to transmit. The scheduling order is derived by ECG monitoring, to personal entertainment systems.applying the Earliest Deadline First algorithm. Omeni In 2008, a Call for Proposals on physical layer andet al.  propose a MAC protocol for a star-networked MAC layer protocols was issued . The large num-WBAN that supports TDMA to reduce the probabil- ber of responses, 64 in total, conﬁrmed the industryity of collision and idle listening. Each slave node is as- interest. Currently, the responses are being evaluatedsigned a slot by the central node. When an alarm occurs at monthly meetings, while some proposals are merged.
11 The creation of the IEEE 802.15 Task Group 6 and H Lthe work on an IEEE 802.15.6 standard stresses the High-temperatureimportance of the research with respect to WBANs. node L L H L7 Network layer H HDeveloping eﬃcient routing protocols in WBANs is anontrivial task because of the speciﬁc characteristics D Lof the wireless environment. First of all, the availablebandwidth is limited, shared and can vary due to fad- Low-temperature Destinationing, noise and interference, so the protocol’s amount node Senderof network control information should be limited. Sec-ondly, the nodes that form the network can be very Fig. 6 An example of LTR and ALTR. The white arrows indi-heterogeneous in terms of available energy or comput- cate the LTR-path. The shaded arrows show the adapted pathing power. of ALTR. When the path has three hops, the routing algorithm switches to shortest path routing. Although a lot of research is being done toward en-ergy eﬃcient routing in ad hoc networks and WSNs ,the proposed solutions are inadequate for WBANs. For (hot spots) . Packets are withdrawn from heatedexample, in WSNs maximal throughput and minimal zones and rerouted through alternate paths. TARA suf-routing overhead are considered to be more important fers from low network lifetime, a high ratio of droppedthan minimal energy consumption. Energy eﬃcient ad- packets and does not take reliability into account. Anhoc network protocols only attempt to ﬁnd routes in improvement of TARA is Least Temperature Routingthe network that minimize energy consumption in ter- (LTR) and Adaptive Least Temperature Routing (ALTR)minals with small energy resources, thereby neglecting  that reduces unnecessary hops and loops by main-parameters such as the amount of operations (measure- taining a list in the packet with the recently visitedments, data processing, access to memory) and energy nodes. ALTR switches to shortest hop routing whenrequired to transmit and receive a useful bit over the a predetermined number of hops is reached in orderwireless link. Most protocols for WSNs only consider to lower the energy consumption. An example of LTRnetworks with homogeneous sensors and a many-to-one and ALTR is given in Fig. 6. A smarter combinationcommunication paradigm. In many cases the network of LTR and shortest path routing is Least Total Routeis considered as a static one. In contrast, a WBAN has Temperature (LTRT) . The node temperatures areheterogeneous mobile devices with stringent real-time converted into graph weights and minimum tempera-requirements due to the sensor-actuator communica- ture routes are obtained. A better energy eﬃciency andtion. Specialized protocols for WBANs are therefore a lower temperature rise is obtained, but the protocolneeded. has as main disadvantage that a node needs to know the In the following, an overview of existing routing temperature of all nodes in the network. The overheadstrategies for WBANs is given. They can be subdivided of obtaining this data was not investigated.in two categories: routing based on the temperature ofthe body and cluster based protocols. 7.2 Cluster Based Routing7.1 Temperature Routing “Anybody”  is a data gathering protocol that uses clustering to reduce the number of direct transmissionsWhen considering wireless transmission around and on to the remote base station. It is based on LEACH the body, important issues are radiation absorption and that randomly selects a cluster head at regular time in-heating eﬀects on the human body. To reduce tissue tervals in order to spread the energy dissipation. Theheating the radio’s transmission power can be limited cluster head aggregates all data and sends it to theor traﬃc control algorithms can be used. In  rate base station. LEACH assumes that all nodes are withincontrol is used to reduce the bioeﬀects in a single-hop sending range of the base station. Anybody solves thisnetwork. Another possibility is a protocol that balances problem by changing the cluster head selection andthe communication over the sensor nodes. An exam- constructing a backbone network of the cluster heads.ple is the Thermal Aware Routing Algorithm (TARA) The energy eﬃciency is not thoroughly investigated andthat routes data away from high temperature areas reliability is not considered. Another improvement of
12LEACH is Hybrid Indirect Transmissions (HIT) , In  the reliability of CICADA was evaluated andwhich combines clustering with forming chains. Doing additional mechanisms were proposed in order to im-so, the energy eﬃciency is improved. Reliability, how- prove the reliability even further, such as the random-ever, is not considered. ization of schemes and overhearing the control messages This overview clearly shows that routing protocols sent by the siblings.for WBANs is an emerging area of research, the pro- BodyQos  addresses three unique challenges broughttocols described above were only developed in the last by BSN applications. It uses an asymmetric architec-two years. ture where most of the processing is done at the cen- tral device. Second, they have developed a virtual MAC (V-MAC) that can support a wide variety of diﬀerent8 Cross-layer Protocols MACs. Third, an adaptive resource scheduling strategy is used in order to make it possible to provide statisticalCross-layer design is a way to improve the eﬃciency bandwidth guarantees as well as reliable data communi-of and interaction between the protocols in a wireless cation in WBANs. The protocol has been implementednetwork by combining two or more layers from the pro- in nesC on top of TinyOS.tocol stack. This research has gained a lot of interest The desired quality of service will aﬀect the energyin sensor networks [87, 88]. However, little research has consumption. For example, to obtain a lower packetbeen done for WBANs. loss, the transmit power can be increased, which raises Ruzelli et al. propose a cross-layer energy eﬃcient the energy consumption. It is therefore important tomulti-hop protocol built on IEEE 802.15.4 . The achieve the right balance between power consumptionnetwork is divided into time zones where each one takes and the desired reliability of the system.turn in the transmission. The nodes in the farthesttimezone start the transmission. In the next slot, thefarthest but one sends its data and so on until the sink 10 Securityis reached. The protocol almost doubles the lifetimecompared to regular IEEE 802.15.4. The protocol was The communication of health related information be-developed for regular sensor networks, but the authors tween sensors in a WBAN is subject to the follow-claim its usefulness for WBANs. ing security requirements: data conﬁdentiality, data au- CICADA  uses a data gathering tree and con- thenticity, data integrity and data freshness . Datatrols the communication using distributed slot assign- conﬁdentiality means that the transmitted informationment. It has low packet loss and high sleep ratios while is strictly private and can only be accessed by autho-the network ﬂexibility is preserved. It also enables two- rized persons, e.g. the doctor attending the patient. It isway communication. Data-aggregation and the use of usually achieved by encrypting the information beforea duty cycle even further improved the lifetime of the sending it using a secret key and can be both symmet-network. rically and asymmetrically. Data authenticity provides Another approach for cross layering is completely a means for making sure that the information is sent bydiscarding the layered structure and implementing the the claimed sender. For this, a Message Authenticationrequired functionality in diﬀerent modules which inter- Code (MAC 3 ) is calculated using a shared secret key.act and can be changed easily . A ﬁrst attempt for Data integrity makes sure that the received informationWBANs using this method is described in . has not been tampered with. This can be inspected by verifying the MAC. Data freshness guarantees that the received data is recent and not a replayed old message9 Quality of Service to cause disruption. A much used technique is to add a counter which is increased every time a message is sent.The research on QoS solutions is extensive for gen- The security mechanisms employed in WSNs do gen-eral ad hoc networks. However, these QoS solutions erally not oﬀer the best solutions to be used in WBANsare designed for more powerful devices which are of- for the latter have speciﬁc features that should be takenten line-powered. Most of these solutions do not apply into account when designing the security architecture.to WSN or WBAN applications. Several QoS solutions The number of sensors on the human body, and thespeciﬁc for WSNs have been proposed, but these solu- range between the diﬀerent nodes, is typically quitetions mainly focus on one or a few QoS features such limited. Furthermore, the sensors deployed in a WBANas reliability, delay, bandwidth speciﬁcation or reser-vation . For WBANs, researchers have shown little 3 MAC is written in italic in order to avoid confusion with theeﬀort to provide QoS solutions. abbreviation of Medium Access Control
13are under surveillance of the person carrying these de- 11 Existing Projectsvices. This means that it is diﬃcult for an attacker tophysically access the nodes without this being detected. Several research groups and commercial vendors are al-When designing security protocols for WBANs, these ready developing the ﬁrst prototypes of WBANs. How-characteristics should be taken into account in order ever, this research mainly focuses on building a sys-to deﬁne optimized solutions with respect to the avail- tem architecture and service platform and in lesser ex-able resources in this speciﬁc environment. Although tent on developing networking protocols. In this sec-providing adequate security is a crucial factor in the tion, we provide a non-exhaustive overview of projectsacceptance of WBANs, little research has been done in for WBANs.this speciﬁc ﬁeld. One of the most crucial components Otto et al.  and Jovanov et al.  present a sys-to support the security architecture is its key manage- tem architecture which both handles the communica-ment. Further, security and privacy protection mech- tion within the WBAN and between the WBANs andanisms use a signiﬁcant part of the available resources a medical server in a multi-tier telemedicine system.and should therefore be energy eﬃcient and lightweight. The communication between the sensors and the sink A solution for data integrity and freshness was pro- is single-hop, slotted and uses ZigBee or Bluetooth. Theposed in . Their integrity algorithm is based on the slots are synchronized using beacons periodically sentmeasurement of a permissible round trip time threshold by the sink. They use oﬀ-the-shelf wireless sensors toand is computational feasible. Authentication is done design a prototype WBAN such as the Tmote sky plat-by calculating a MAC with a random sequence of num- form from formerly Moteiv , now sentilla .bers. This sequence is determined at the initialization The Tmote sky platform is also used in the CodeBlue-phase. project [102,103] where WBANs are used in rapid disas- ter response scenarios. A wearable computer attached In  a security mechanism was added to CICADA. to the patient’s wrist, i.e. a Tmote Sky mote, formsDoing so, CICADA-S became one of the ﬁrst proto- an ad hoc wireless network with a portable tablet PC.cols where appropriate security mechanisms are incor- They developed a wireless two-lead ECG, a wirelessporated into the communication protocol while address- pulse oximeter sensor and a wireless electromyograming the life-cycle of the sensors. It was shown that the (EMG).integration of key management and secure, privacy pre-serving communication techniques has low impact on Ayushman  is a sensor network based medicalthe power consumption and throughput. monitoring infrastructure that can collect, query and analyze patient health information in real-time. A wire- Another promising solution for key management is less ECG, gait monitoring and environment monitoringthe use of biometrics. Biometrics is a technique com- was developed using oﬀ-the-shelf components with amonly known as the automatic identiﬁcation and ver- Mica2 wireless transceiver. Further, the necessary soft-iﬁcation of an individual by his or her physiological ware for consulting the data at a remote client was de-and/or behavioral characteristics . In  an algo- veloped.rithm based on biometric data is described that can The Human++ project by IMEC-NL  aims “tobe employed to ensure the authenticity, conﬁdentiality achieve highly miniaturized and autonomous sensor sys-and integrity of the data transmission between the per- tems that enable people to carry their personal body areasonal device and all other nodes. Algorithms that use network.”. An ambulatory EEG/ECG system with athe heartbeat to generate a key are proposed in [98,99]. transmitter working on 2.4 GHz was developed. This In  body-coupled communication (BCC) is used system can run for approximately 3 months using 2 AAto associate new sensors in a WBAN. As BCC is limited batteries. In order to obtain a longer autonomy, theto the body, this techniques can be used to authenticate project also investigates energy scavenging with ther-new sensors on the body. moelectric generators (TEG). In 2006, a wireless pulse The developers of WBANs will have to take into oximeter was presented, fully powered by the patient’saccount the privacy issues. After all, a WBAN can be body heat. Further, the project investigates new wire-considered as a potential threat to freedom, if the appli- less technologies such as UWB to make an ultra-lowcations go beyond “secure” medical usage, leading to a power transmitter.Big Brother society. Social acceptance would be the key The European MobiHealth project  provides ato this technology ﬁnding a wider application. There- complete end-to-end mHealth platform for ambulantfore, considerable eﬀort should be put in securing the patient monitoring, deployed over UMTS and GPRScommunication and making sure that only authorized networks. The MobiHealth patient/user is equipped withpersons can access the data. diﬀerent sensors that constantly monitor vital signals,
14e.g. blood pressure, heart rate and electrocardiogram a lot of open research issues. On the data link layer,(ECG). Communication between the sensors and the more WBAN speciﬁc MAC-protocols need to be devel-personal device is Bluetooth or ZigBee based and is oped that take into account the movement of the body,single-hop. The major issues considered are security, i.e. the mobility of the nodes, additional low-power fea-reliability of communication resources and QoS guar- tures such as an adaptive duty cycle for lowering the idleantees. listening and overhearing, the use of the human physi- The French project BANET  aims to provide ology such as heart beat to ensure time synchronizationa framework, models and technologies to design opti- and so on. Concerning the network layer, a promisingmized wireless communication systems targeting the research track is the combination of thermal routingwidest range of WBAN-based applications, in the con- with more energy eﬃcient mechanisms. More eﬃcientsumer electronics, medical and sport domains. They fo- QoS-mechanisms are needed, for example based on thecus on the study of the WBAN propagation channel, BodyQos framework. Other interesting open researchMAC protocols and coexistence of WBANs and other issues are mobility support embedded in the protocol,wireless networks. security, inter operability and so on. In order to deﬁne a The German BASUMA-project (Body Area System globally optimal system, it might be necessary to unitefor Ubiquitous Multimedia Applications)  aims at several of these mechanisms in a cross-layer protocol.developing a full platform for WBANs. As communica- The use energy scavenging was not addressed in de-tion technique, a UWB-frontend is used and a MAC- tail in this paper but is nevertheless important. Withprotocol based on IEEE 802.15.3. This protocol also a smart combination of lower energy protocols and en-uses time frames divided into contention free periods ergy scavenging, the optimal solution for achieving au-(with time slots) and contention access periods (CSMA/CA). tonomous Body Area Networks can be reached. For a A ﬂexible and eﬃcient WBASN solution suitable for WBAN, energy scavenging from on-body sources sucha wide range of applications is developed in . The as body heat and body vibration seems very well suited.focus lies on posture and activity recognition applica- The ultimate goal is to create a small and smart band-tions by means of practical implementation and on-the- aid containing all necessary technology for sensing andﬁeld testing. The sensors are WiMoCA-nodes, where communication with a base station. Very preliminarysensors are represented by tri-axial integrated MEMS examples can be found in the Sensium-platform accelerometers. and the Human++-project . The Flemish IBBT IM3-project (Interactive MobileMedical Monitoring) focuses on the research and im-plementation of a wearable system for health monitor-ing . Patient data is collected using a WBAN andanalyzed at the medical hub worn by the patient. If an 13 Conclusionsevent (e.g. heart rhythm problems) is detected, a signalis sent to a health care practitioner who can view and In this survey, we have reviewed the current researchanalyze the patient data remotely. on Wireless Body Area Networks. In particular, this work presents an overview of the research on the prop-12 Open Research Issues agation in and on the human body, MAC-protocols, routing protocols, Quality of Service and security. ToThe discussions above clearly show that, although a lot conclude, a list of research projects is given and openof research is going on, still a lot of open issues exist. research issues are discussed. Several researchers have already started studying A WBAN is expected to be a very useful technol-the propagation of electromagnetic waves in and on the ogy with potential to oﬀer a wide range of beneﬁts tobody and a few models for the physical layer are pro- patients, medical personnel and society through contin-posed. It should be noticed that none of them take the uous monitoring and early detection of possible prob-movements of the body into account, although move- lems. With the current technological evolution, sensorsments can have severe impact on the received signal and radios will soon be applied as skin patches. Do-strength, as described in Section 5.2. Further, new emerg- ing so, the sensors will seamlessly be integrated in aing technologies such as galvanic coupling and trans- WBAN. Step by step, these evolutions will bring usformation of information via the bones oﬀer promising closer to a fully operational WBAN that acts as an en-results and need to be investigated more thoroughly. abler for improving the Quality of Life. We feel that Although some protocols already exist that take care this review can be considered as a source of inspirationof the data link layer and networking, this area still has for future research directions.
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