Testbed Design for Wireless Biomedical Sensor Network (WBSN) Application

Mohd. Rozaini Abd. Rahim1, Rozeha A. Rashid2, Sharifah Hafizah Syed Ariffin3, Norsheila Fisal4, Mohd Adib Sarijari5, Abdul Hadi Fikri Abdul hamid6

Faculty of Electrical Engineering Universiti Teknologi Malaysia

Skudai Johor 1mohd_rozaini@yahoo.com, 2rozeha@fke.utm.my, 3sharifah@fke.utm.my, 4Sheila@fke.utm.my, 5 adib_sairi@fke.utm.my,

6hadifikri@gmail.com

Abstract—In wireless sensor network (WSN), a collection of nodes interacts with each other to gather information from the surveillance area. Some of the applications that WSN are currently being used are habitat monitoring, health care system and building automation, to name a few. Wireless Biomedical Sensor Network (WBSN) allows capturing of physiological signals, continuous monitoring and updating of a patient’s medical status remotely. This paper focuses on the development of a wireless sensor node testbed for WBSN application which complies with IEEE 802.15.4 standard and operates in 2.4 GHz ISM (industrial, scientific and medical) band. The initial state of WBSN development is the design of the wireless sensor node called TelG. The main features of TelG include low power consumption, wearable, flexible and small size. It is then embedded with a self-built operating system called WiseOS to support customized operations. A pulse oximeter is then integrated with TelG for the purpose of experimentation. The testbed performance is analyzed in terms of packet reception rate (PRR) and end-to-end delay. The results exhibit a decrease in PRR as distance increases and increasing network delay with increasing number of hops. It is also observed that received signal using the testbed is satisfactory for distances less than 10 meters per hop.

Keywords-component; Wireless Sensor Network, Wireless Biomedical Sensor Network, Wireless Sensor Node Platform

I. INTRODUCTION

The recent development of high performance microprocessors and novel processing materials has stimulated great interest in the development of wireless sensor nodes for Wireless Biomedical Sensor Network (WBSN) application [1]. It allows physiological signals such as electroencephalography (EEG), electrocardiogram (ECG), blood pressure, glucose to be easily monitored wirelessly and attached to the patient’s body. The wireless sensor nodes in WBSN application can be classified into several types, which are the swallowed capsule pill sensor, wired sensor with the wireless sensor node, portable sensors mounted on the surface of human body, implantable physiological sensor and nano-physiological sensors [2]. In this work, the wired sensor with the wireless sensor node will be used.

Some of the generic wireless sensor node platforms available have not been designed specifically for WBSN application but they are more on network research or environmental monitoring [3]. However, among these generic wireless sensor nodes, the Mica[4] and Telos[5] series have been used into WBSN application at Harvard University as their first prototype for CodeBlue project [6-8]. In the CodeBlue project, this generic wireless sensor node has been connected to the pulse oximeter, electrocardiogram (EKG) and electromyography (EMG) sensor board to provide continuous monitoring. Another type is the Tmote Sky platform [9], where it is used to design a wireless ECG monitoring system [10].

There are a number of wireless sensor node platform purposely designed for WBSN application such as ECO[11], Body Sensor Network (BSN)[12], MASN[13], TELEMON[14], Wearable Patch ECG[15], zEEG[16], Ultra Low Power Node[17], Pluto[18], WBSN Node[19] and SHIMMER(Sensing Health with Intelligence, Modularity, Mobility, and Experimental Reusability)[20]. Most of these sensor nodes have been used in the ECG signal monitoring. Most of the existing specialized wireless sensor node platform for WBSN used ISM band as the frequency for transmission.

This paper focuses on the development of our own wireless sensor node for WBSN application named TelG mote. The rest of the paper is organized as follows. Section II will describe briefly on the design of the TelG mote based on the requirement for biomedical application. The architecture of the TelG mote will be explained in section III while section IV features the WiseOS operating system. Section V will discuss the results. Conclusion is drawn in Section VI.

II. WBSN MOTEDESIGN REQUIREMENT

Most of the present WSN platforms in the market are designed for all-purpose application and network research [21,22]. However, platform design and requirements of WBSN application are different from usual WSN application platform, while some subset of these requirements may be shared. TelG mote has been

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general, a microcontroller would support several common features such as timer, analog to digital converter (ADC), pulse width modulator, two-wire serial interface, serial USART and serial peripheral interface. These common features are important for the microcontroller to interact with its environment and for communication purposes. Atmega644pv in particular has two serial USART interfaces. Atmel family microcontrollers have several programming methods to choose from in order to program its flash memory. TelG sensor node platform is designed to be programmed using in-system programming (ISP) mode for easy usage.

B. Wireless Transceiver

The wireless transceiver is the most important part on a WBSN platform since it is the primary energy consumer. Today, there are many types of wireless transceivers with various features that exist in the market. In WBSN application, the selection criteria are low data rate, low power, short range and low complexity. IEEE802.15.4 wireless standard has been chosen for WBSN application after evaluating the characteristics of existing wireless standard [24].

XBee module from Digi is used on the wireless device. XBee module is an IEEE 802.15.4 compliant radio device based on CSMA-CA (channel sense multiple access) which will provide point to point, point to multipoint and also peer to peer communication. It is designed for low-latency and predictable communication timing applications [25].

Radio Frequency (RF) data rates for the XBee module can go up to 250Kbps and operates at 2.4GHz ISM (Industrial Scientific and Medical) frequency band. XBee module has a small form factor (2.438cm x 3.294cm). It has the power output of 1mW (+0dB) and capable of transmitting up to 30m indoor and 100m outdoor with the receiver sensitivity of -92dBm [25].

The XBee module has sixteen 5MHz channels ranging from 2.405 to 2.480 GHz, with 65,000 addressable network addresses for each channel. Since it employs IEEE802.15.4 standards, the data transmitted is in the form of packets where the maximum transmission unit (MTU) for each packet is 127 bytes. Each packet is acknowledged at the link layer in unicast mode providing best-effort-delivery except for broadcast mode. It is interesting to note that, the link layer standard required a coordinator in the network but XBee is designed to work even without a coordinator.

XBee uses serial USART as its interface. The host can be interfaced with XBee module with rates of up to 115.4Kbps. Since both the USART interface and operates at 3.3V and ATmega644PV (host) also operates at the same voltage level, XBee module can be connected directly without any voltage level circuit. One of the most appealing features of XBee module for application developers is its API Operation. The API provides an easy way for developers to issue a command to the module.

C. Medical Sensor Medical sensors are used to measure physiological

signals from the body. There are many existing medical sensors in the market such as ECG, EEG and SpO2 (oxygen saturation)sensors. Most of these medical sensor modules can be easily integrated with TelG sensor node platform. For proof of implementation, Medlab’s EG00352 pulse oximeter board is integrated with TelG mote in this paper.

The EG00352 medical sensor board has the capability to provide SpO2 value, the pulse rate, the plethymsmographic curve and status indicators for quality of signal management. This board operates using 3.3V battery, similar to TelG mote power supply. Hence, it can directly integrate with TelG mote without using extra circuit. Table II shows the features of EG00352 pulse oximeter medical sensor board.

TABLE II FEATURES OF EG00352 PULSE OXIMETER MEDICAL SENSOR

[28]

Specification Data Operating Voltage 3.3 Volt DC, +200mV, - 100mV

15-20 mA Power consumption

50mW to 65mW, depending on light absorbance at measurement site

Environmental Temperature Storage -30°C to 90°C Operation -10°C to 50°C Humidity Storage 0-95%, non condensing Operation 5-90%, non condensing

SpO2 measuring range

30% -100% of SpO2

SpO2 Accuracy 90% -100% : 1% , +/- 1 Digit 80% -89% : 2% , +/- 1 Digit 70% -79% : 3% , +/- 1 Digit below 70% : not specified

SpO2 Averaging fixed to 8 seconds Pulse Rate measuring range

25-248 bpm

Pulse Rate Accuracy

+/- 1%, +/- 1 Digit

Pulse Rate Averaging

fixed to 8 beats

Interface asynchronous, serial interface with CMOS levels 9600 baud, 8N1

Protocol simple unidirectional standard protocol custom protocol versions available on request

Power

The heart of the wireless sensor node is the power supply. Power supply design (battery or solar) will give an impact to the size and the lifetime of the sensor node platform. Most of the sensor node depends on the design of the size of the power supply itself, regardless whether it is using dry battery or solar. Large battery size will generate higher capacity current and at the same times extends the life time of the sensor node.

To balance the tradeoff between size and life time, TelG wireless sensor node uses three pieces of AA batteries as its

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WLEDGMENT

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