Proceedings of the 2005 IEEE International Conference on Information Acquisition June 27 - July 3, 2005, Hong Kong and Macau, China
Physiological Information Acquisition through Wireless Biomedical Sensor Networks Hongliang Ren, Max Q.-H. Meng, Xijun Chen Department of Electronic Engineering The Chinese University of Hong Kong Hong Kong, China { hlren, max & xjchen}@ee.cuhk.edu.hk rate via sensors and sending the information to a computer for later analysis. For emergency medical care[2], WBSN can be used to capture real-time vital signs from patients in a moving ambulance, relay the data to handheld computers carried by physicians for pre-hospital diagnosis. Skeleton diagram of these scenarios are shown in Fig.1.
Abstract - The monitoring and acquisition of patients’ physiological information are quite crucial for the further treatment. Wireless network of biomedical sensors shows great potential to significantly enhance the biometrics performance. Meanwhile, it poses prominent characteristics and challenges to medicos and engineers for its particular medical application compare to other application of wireless sensor networks (WSN). This paper first investigates the exciting conceivable application of WSN in medical area. Then the unique challenges and requirements are analyzed in detail. Along with the further study of the logical architecture and network protocols, preliminary solutions are proposed for the WBSN in a hospital environment.
II. PHYSIOLOGICAL SENSORS, WIRELESS, AND NETWORKS A. Biomedical Sensors The biomedical sensor devices play a key role in the networks and they can be either external or embedded, either non-invasive or invasive. The development of biosensors compatible with conventional CMOS integrated circuit and non-invasive detection technologies enable the WBSN more feasible. These sensors, very small and inexpensive, lead to many applications in pharmaceuticals and medical care. Typically, there are following several kinds of physiological sensors that would augment WBSN applications: 1) The swallowed pills with wireless transceiver containing sensors that can detect enzymes, nucleic acids, intestinal acidity, pressure, contractions of intestinal muscle and other parameters, allow WBSN involved in the gastrointestinal diseases monitoring in a non-invasive manner. The schematic graph is shown as the congestive system in Fig.1. 2) Wired sensors plus local wireless device —For instance, wireless ECG [3], as shown in Fig.1, with several wired electrodes put on the chest to measure the potential drop, is able to continuously radio the patient’s electrocardiogram to a sink node located on the arm or a waist-belt. 3) Portable sensors mounted on the surface of human body—Such as, non-invasive glucose monitoring sensor[4] based on impedance spectroscopy is also available to help diabetes to trace their status. The sensor detects changes in the glucose concentrations by varying the frequency in the radio band to measure the impact of glucose on the impedance pattern. There are also some other general health monitors such as ambulatory cuffless blood pressure monitor, ringshaped sensor worn on the finger to monitor heart rate[5], etc. 4) Implantable physiological sensors—For example, human-embedded glucose level [6] monitor could be implanted in the patient once with more accurate results in a
Index Terms - WBSN, Information Acquisition, Network Protocol
I. INTRODUCTION Wireless Biomedical Sensor Networks (WBSN, for short), the convergence of biosensors, wireless communication and networks technologies, consists of a collective of wireless networked low-power biosensor devices ("motes" or "nodes”), which integrate an embedded microprocessor, radio and a limited amount of storage. WBSN can wirelessly monitor patients’ physiological signals (EEG, ECG, GSR, blood pressure, blood flow, pulseoxymeter, glucose level, etc.) by individual node or pill that is worn, carried or swallowed by the patients. It then alerts the healthcare professionals with the abnormal changes of patients’ physiological condition, while delivering the data to a database system for long-term storage. The data, such as regular blood sugar monitoring for a diabetic, gathered by the sensor network can give important clues to a person’s state of health. WBSN, unlike wired monitoring system, can be used for long-term and continuous monitoring even when people move. In addition, higher level medical tasks can be operated because of the coordination of networked nodes. The typical application scenarios of WBSN are various. Smart home health monitoring [1], for example, expects a WSN to help older people or the patients with chronic disorders to live on their own longer. Smart ward in the hospital reduce the time of routine checkup and its real-time monitoring also allows emergency situation to be handled immediately. Moreover personal contacts can be avoided for the nurses to reduce the possibility of infection in a contagion ward. WBSN also can be utilized for Athletic performance monitoring, for example, tracking one’s pulse and respiration
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Embedded Sensors
Insulin Injector
VENTILATION Gas Sensors
Remote Access
Free Court Sink
Workstation ECG Sensor
Ambulance
Circulatory System Sink
Wireless ECG Sink Smart Home
Swallowed Sensors
Congestive System The elder
Contagion Ward Fig. 1 Typical Scenarios of WBSN Applications.
- It is the only way to deliver medical data in cases of emergency or disaster in remote or isolated areas. C. Networks The need of an interconnected network is because - The system should enable automatic establishment and configuration of the sensor network; - The system should support reliable and automatic network reconfiguration in case of new or removal of the existing sensors [7]; - Sensor network access methods have to provide a consistent interface for access to sensors regardless of their type, measured phenomena; - The networked sensors can provide higher level medical performance by coordination. One of the envisioned scenarios is several nodes can cooperate to inject insulin automatically: the node carrying insulin asks for relevant parameters from other monitoring nodes, and then decide when and how much to inject. - Multi-hop routing is necessary for large area monitoring.
less invasive way. As the blood circulatory system shows in Fig.1, all these sensors can be wireless networked to perform high level monitoring such as insulin injection. Obviously wireless transmission is a necessary way to relay the data for the embedded sensors that operate within the human body. 5) Nano-physiological sensors with wireless communication —the futuristic and exciting concepts, nanomachines that are biodegradable are able to run through the bloodstream for monitoring physiological changes. B. Wireless communication Why do we need wireless transmission and networks for the biomedical sensor devices? Actually, the WBSN described in this paper is developed with the following set of requirements and functionalities in mind. Wireless tools play a vital role in creating and maintaining the electronic medical record (EMR), because - With wireless systems, patients can move freely around the hospital or home while they are monitoring; - Long term monitoring doesn’t discomfort the patient for their freedom; - Wireless transmission is almost the only way to communicate with embedded, implanted or swallowed biosensors; - Compare to wire sensors, the wireless ones are simple to deploy;
III. REQUIREMENT AND CONSTRAINTS WBSN pose many unique challenges when related to medical care. No one can afford the risk that critical health data – such as the vital signs of a heart patient will be
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The IEEE and WHO have developed Radio Frequency exposure recommendations specifying near-field restrictions referred to as SAR between 10 MHz and 10 GHz. High power devices need to be tested and it’s desired to implement ultra-low power transmission herein. - Interference: as wireless tools proliferate, the likelihood increases that they will interfere with each other, potentially putting the patient’s health at risk. Techniques against interference should be developed to meet the safety requirement. Network protocols should be able to identify the messages are from which patient. In summary, the challenge of WBSN is the need for low power consumption to enable long term monitoring, no unwanted interference with other wireless systems, efficient protocol due to the limited computation, extremely reliability and security as it's related to the health and life, and troublefree when network topology changes.
maliciously modified or be lost in transmission, even for just a few seconds. Therefore, the special requirement and constraints should be analyzed here. The general challenges in terms of sensor networks are well known as low power constrains, limited computation, robustness and fault tolerance, scalability, densely deployment, security and latency, many of which still exist in WBSN. However, the major requirement and concerns of WBSN in terms of medical care are: - Reliability: the first key challenge to make sure that information reliably reaches its destination. The reliability of WBSN is a systemic design which relies on many aspects such as reliable wireless communication between nodes, efficient computation in each sensor node, stable software programming, and some of the following issues to be addressed. - Biocompatibility: the shape, size and materials are restricted for the sensors that directly act on the human body. One of the solutions is to package the sensor nodes in biocompatible materials[6]. - Portability: the size of the smart medical sensor should be small and lightweight for swallow or carry. - Privacy and security [8]: the areas of concern are eavesdropping, identity spoofing or the redirection of data to unauthorized individuals. Security can be preserved by using the data encryption. In terms of individuals’ privacy, it is essential that the data is protected from improper access or modification. Consented acquisition of data, proper storage of data, secured transmission, and integrity of data and authorized access of data are vital areas for development of hardware or software solutions. - Light weight protocols for WBSN ranging from selforganization, to network maintenance, to security, to data collection and fusion, to routing, among many others. - Retrievability: retrieving patient health histories and other information from the EMR by way of a mobile device. For instance, a patient’s medical record can be retrieved from a tiny database by a physician. - Energy aware communication: For the purpose of long term monitoring and minimum interference, it is desirable for nodes to minimize their transmit power to achieve acceptable connectivity. Energy aware protocol would allow nodes to negotiate their transmission power to a minimum. - Energy efficiency maybe not a major concern since the batteries of external biosensors can be recharged, but for embedded internal biosensors, it’s also a challenge to be concerned. Power efficiency becomes a challenge when the networks are entirely self-organizing and operate with limited energy and computational resources. - Prioritized traffic[2]: content-based prioritized transmission of critical data, such as patient stop breathing or loss of network connection is critical for medical applications. The system should offer such kind a service other than best effort service provided by existing wireless networks. - RF radiation safety[9]: the electromagnetic radiation must be strictly limited to meet the standards of patient safety.
IV. PROPOSED SYSTEM LOGICAL ARCHITECTURE The following sections first examine existing network architectures and models in WSN, and then propose our solution to WBSN in a hospital application. In order to make the notation clear, we will briefly categorize some network and communication models in terms of medical care. - Leaf nodes are referred as the nodes directly sensing the physiological data in the bottom layer of the network. Branch nodes or patient nodes are the cluster heads of patient body area network and may be located on the waist belt or in the pocket. Root nodes are the ultimate data sinks at the central control station. - Sensor network composition model can be classified as homogeneous(the same types of sensor nodes) or heterogeneous( different types); - Intercommunication model: cooperative and noncooperative. Cooperative sensors are able to directly communicate with any other sensors in the network so that smart sensing and multi-hop capabilities can be offered. However, a non-cooperative model only allows a node to connect with its parent node which requires much simpler communication protocols. - Data delivery model: continuous, event driven, system initiated and mixture. For instance, heart rate data should be continuously delivered when monitoring a heart disease patient. Event driven data occur when the predefined events are triggered such as the heart beat stopping. System initiated model is a central control way with the data transmission at the request of upper level node. For this reason, the proposed data delivery model is a mixture model in WBSN. - Communication model: unicast, broadcast, and multicast. The downlink stream from root nodes to leaf nodes maybe broadcasted in a polling style protocol. The uplink stream of physiological data can be delivered in a unicast model. Whereas the branch node can poll a group of leaf nodes by multicast when necessary. - Network dynamics model: static or dynamic. Obviously, the ambulatory patients form a dynamic network topology,
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V. PROPOSED NETWORK PROTOCOL ARCHITECTURE
which require more communication protocol overhead. However, when a patient stays in the bed, only several of the leaf nodes are moved around the patient, forming a relative static network. So a dynamic model is necessary, which not only allows patients to be mobile but also allows physical sensors to be dynamic relative to the patients. - Network logical organization model: ad-hoc (nonhierarchical), hierarchical or flat tree (mixture) as shown in the Fig.2. Ad-hoc model is well suited for environments that aren’t complex but that change often. However, hierarchical model allow complex processing to be done on data from multiple sensors. In a hospital environment, a very likely scenario would be many patients that carry multiple physical sensors, as shown in Fig.1. Therefore, a flat tree model (Fig.2) is desirable since each patient branch node only collects data from its group of physical sensors and avoids interference with other patient body area networks. Moreover, all the patient nodes are fully interconnected to allow the patient roaming by relaying messages to each other. -
Ad-hoc Model
Wireless network protocol stacks are the realization of many functionalities of WBSN mentioned in the former sections, such as authentication, prioritized traffic, security, data delivery, cooperative sensing, power-aware link, and routing etc. Therefore, network protocols are of great importance to meet specific design goals, particularly for the unique requirements and challenges of WBSN. We begin this section with a discussion of the similarities and differences among traditional networks, typical sensor network and WBSN, and next specify the important issues that cross all layers of the protocol stack. WBSN share some common features with WSN, but also distinguish itself from the WSN in many ways, as shown in the following table1. For instance, typical sensor network allow error rate to some extent, but for WBSN it’s prohibited, because high error rate will lead to latency and congestion, even loss of vital signals. Moreover, many typical WSN involve hundreds of thousands of sensors that are spread out over a large area. In contrast, a patient body area network (PBAN) will generally involve no more than a hundred sensors, and a ward accommodates no more than ten patients. Because of this small area of patient body area network, all leaves may be within communication range with the branches, leading to serious interference and traffic congestion. TABLE I COMPARISON OF THE FEATURES OF WLAN, WSN AND WBSN Traditional Typical WSN WBSN networks Instance WLAN Smart Dust Smart Ward Coverage LAN@50m PAN@10m PBAN@1m+PAN Density Sparsely Densely Densely Data-centric Address-centric Data-centric Data-centric Large scale N Y Y Workloads Unpredictable Unpredictable Partly predictable Error rates Medium High Must Very Low Energy constraint No Yes for embedded node Hops Single Multi-hop Optional Infrastructure Y N N: Self-organizing Node Failure N Y Prohibited Deployment Random Random Planned
Hierarchical Model Branch
Leaf Cooperative Root Cluster Non-cooperative Three-level Flat Tree Fig. 2 Networks Logical Architecture
There are no dedicated standard protocols for WBSN to date, even for WSN. Novel communication protocols must be developed to support strict services in WBSN. First of all, the metrics we concern to evaluate the performance of protocols include bit error rate, energy consumption, retransmission rate, lifetime, latency, quality, and amount of data disseminated per unit of energy. Energy consumption may be measured as the total consumed energy over a period of time. Latency is the time from the moment a sensor node sends its sensed data out to the destination receives it. The following are the preliminary discussions across physical, data link and network layers. A. Physical Layer The physical layer specifies frequency band, modulation scheme, data rate and acceptable power level, which directly effect on the interference, biocompatibility and RF safety. The US Federal Communications Commission (FCC) established the Wireless Medical Telemetry Service (WMTS)
In a ward, the sensors of one patient don’t have to communicate with sensors on the same patient or another patient for the most cases, except that they have to cooperate to finish a task. As we all know, the more communication links, the more bandwidth, interference and latency will increase. Thereby, the wireless links from leaf to branch are proposed to be non-cooperative to keep the number of links to a minimum. Meanwhile, the links from branch to root have to be cooperative to ensure the mobility of patients. Hence, threelevel flat tree model, the proposed WBSN architecture in a hospital environment, is derived from the analysis above. The model provides the ability of multi-hop routing between patient nodes and single hop delivery between branch and leaf.
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directly used here is mainly because a master node in a star piconet network can only have up to seven slave nodes.
in 2000 to ensure that medical telemetry equipment can operate without interference from other sources. It set aside the 608MHz-614 MHz, 1395MHz-1400MHz and 1429MHz1432MHz frequency bands for wireless medical care, and need license[10]. To avoid having to obtain licenses, currently ISM band (902-928MHz and 2.4-2.4835 GHz) has been supported by many sensor networks. The use of WBSN in these bands is not licensed but susceptible to potential interference from other ISM band users such as WLAN and Bluetooth. So the coexistence problems must be investigated in designing. Considering RF safety problem, human body is composed mostly of water that may be heated by excessive radiation. Hence, the radio power negotiation scheme should be adopted to reduce transmission power. The physical layer specifications of IEEE 802.15.4 [11] are proposed by the author because of many aspects. First, it has been developed as a low data rate (250, 40, and 20 kbps), low power, short range and low complexity solution with up to several-year battery life. Second, it operate in unlicensed ISM band with 16 channels in the 2.4 GHz band, 10 channels in the 915 MHz band, and 1 channel in the 868 MHz band. Third, it offer DSSS (direct sequence spread spectrum) techniques, the wideband less likely to be corrupted by the interference, due to its inherent processing gain. Finally, coexistence with other wireless devices in the ISM frequency band, such as 802.11, is a thorny problem. Study[12] shows that an IEEE 802.15.4 network has little to no impact on the IEEE 802.11b’s performance. Whereas, reliable transmission techniques[13] must be applied in order to run IEEE802.15.4 in parallel with IEEE802.11, especially in the case of frequency overlap and high traffic load of interference.
Protocol Frequency Band (MHz) Modulation MAC Raw data rate No. of Channels Users/channel Configuration Range @ TxPower Power Profile Others Features
TABLE II COMPARISON OF THE EXISTING STANDARDS 802.15.1 802.11b IEEE 802.15.4 Bluetooth (Wi-Fi) LR-WPAN 2402-2480 2400-2483 902-928 / 2400-2483.5 GFSK DQPSK /GFSK BPSK/OQPSK Round robin CSMA-CA GTS/CSMA-CA 1Mbps 11 Mbps 20/40/250kbps FHSS:79 DSSS: 11 DSSS:1/10/16 7 active, 127 >256 active piconet/scatternet AP/Ad-hoc Star/peer-to-peer 100m@100mW 100m@100mW 10m@1mW
[email protected] Days Hours Years Authentication, Roaming Link quality voice possible indication
Two MAC schemes are suggested here for WBSN. The first one is IEEE 802.15.4 MAC based scheme, which is the optional contention-free time slot in addition to contention period time slots, as shown in Fig.3. Critical physiological data employ contention-free guaranteed time slots (GTS) to guarantee that must be delivered within a certain period time. For routine monitoring data, it can use contention based CSMA-CA, a fully acknowledged protocol to ensure reliable transmission. The context based prioritization transmission can be preliminarily realized by this way. But the drawbacks are large messaging overhead and link setup delay. Contention-based time slots Contention access period
B. Data Link Layer The data link layer specifies medium access control scheme, authentication and security protocols to ensure reliable and secure connections in a WBSN. Existing MAC protocols can be classified into four categories: scheduling based, collision free, contention based, and hybrid schemes[14]. In scheduling-based MAC protocols, the time slot that a node can transmit is determined by a scheduling algorithm. Most of them are centralized and depend on global network parameters. Collision free scheme requires a router located in the centred area of each cell. The routers are equipped with two transceivers so that they can transmit and receive at the same time using two different frequency channels. This imposes a nontrivial requirement on the hardware of the nodes in a sensor network. Most of the contention based MAC protocols exploit CSMA/CA (carrier sensing multiple access/collision avoidance) mechanisms and employ additional control messages to deal with hidden and exposed node problems. Due to random backoff nature, contentionbased MAC does not guarantee the priority order and latency. Existing issued standards, such as Bluetooth, 802.11b and 802.15.4, are compared in the table. Bluetooth can’t be
Frame beacon
Contention free period
Frame beacon Guaranteed time slots Fig. 3 The superframe structure of IEEE 802.15.4 MAC layer
The other one is scheduling and contention based hybrid. Since the proposed network logical architecture is a threelevel flat tree and the number of leaf nodes connected to its branch is less than 100 generally, scheduling based MAC scheme is suitable for the link from leaf to branch, and CSMA-CA can be used for the link from branch to root. Conflict and delay will be minimized at the leaf level, but latency and congestion control are still problems to be overcome at branch level. However, the MAC design for WBSN needs to accomplish a balance among a number of metrics. In addition to MAC scheme, another three mechanisms should be realized at this layer. Power aware mechanisms are required to reduce the interference between PBANs. Authentication protocol is necessary to automatically add or remove of nodes. For the security schemes, patient node is assigned as coordinator with special capabilities to assist in provisioning link keys to leaf nodes[15]. The off-site central authority (CA) should be used to preload initial authentication
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data to patient branch nodes and leaf node offline in order to keep physically secure. C. Network Layer Network layer is mainly responsible for route discovery and maintenance for WBSN. Actually, it only involves patient branch nodes and root nodes in a hospital environment, which reduce the complexity to some extent. But the requirements such as efficiency, reliability and latency still impose challenges to the protocol design. Researchers have developed a lot of routing algorithms that can be categorized into flat routing and hierarchical routing. In flat routing each node is able to maintain routing table and relay data. Furthermore, flat routing protocols include proactive, reactive and geographic routing. Proactive algorithms maintain routing table aim at all branch nodes, which require periodic exchange of routing update messages. This type of routing protocols is undesirable because of the overhead and potential congestion incurred. Reactive algorithms establish routes only when the nodes are involved communication. This is particularly suitable for the frequently changed topology. The typical ones are the dynamic source routing (DSR) protocol[16] and the ad-hoc on-demand distance-vector (AODV) protocol [17]. The reactive routing implies some ideas to fit the features of WBSN. However, we still need a well-established dedicated protocol to provide reliable end to end guarantees currently. From the above discussion on the three layer protocols, many known algorithms provide us valuable ideas, but no existing protocols can be directly used for WBSN. IEEE802.15.4 may be modified to be the candidate for physical and MAC layer protocol. Power aware and security algorithms can be built on that. Dynamic on-demand routing protocol is suggested here for data delivery. After all, the balance among multiple metrics has to be made with respect to the concerns of WBSN.
This project is supported by RGC Competitive Earmarked Research Grant #CUHK4213/04E of the Hong Kong government, awarded to Max Meng.
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VI. CONCLUSIONS [13]
Wireless biomedical sensor networks exhibit great strength to enhance the medical performance by integrating smart sensors, wireless communication and network technologies. This paper first provides several typical scenarios and then shows the necessity to integrate theses three technologies. Then the unique challenges and requirements are analysed in detail. The investigations of the flat tree logical architecture and network protocols imply our preliminary solutions proposed for the WBSN in a hospital environment. Due to the prominent constraints, researches on dedicated network protocols are the urgent affairs to ensured reliable and secure communication. We are convinced that WBSN will benefit many people in the near future with the quickly developed technologies.
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