SENSOR NODES IN WIRELESS BODY NETWORKS Vincas Benevicius1, Vytautas Ostasevicius2, Gintaras Rimsa3 1
Kaunas University of Technology, Institute of High-tech Development, Studentu g. 65, Kaunas LT-51369, Lithuania,
[email protected] 2 Kaunas University of Technology, Institute of High-tech Development, Studentu g. 65, Kaunas LT-51369, Lithuania,
[email protected] 3 Baltec CNC Technologies, Raudondvario pl. 148, Kaunas LT-47175, Lithuania,
[email protected]
Abstract: Paper describes applications and equipment for various biomechanical parameters aggregation and processing. Aggregation of acceleration data from acceleration sensors, their possible complex applications in sports and health care is presented. Initial processing on micro controller unit and their real time forwarding to other devices via radio transceiver is discussed. Various radio link characteristics with specific prototype are analyzed using Texas Instruments CC2500 radio transceiver with PCB folded dipole antenna using 2.4GHz radio frequency. Keywords: Accelerometer, CC2500, biomechanics, radio, body wireless network.
1
Introduction
Movement is a key factor of any life. Biomechanics of the human body is the widest part of general biomechanics. Human’s movement, research on support mechanism’s operation, heart’s mechanical operation and blood vessels modeling belongs to it. One of the most important indices when analyzing human body movement are dynamic characteristics: speed, acceleration and angular speed. With quickly developing MEMS [1] technology, the measurements of this kind of data becomes easy, nonintrusive and cheap. Today attempts to find more and more application areas of such small sensors are widely made in health care, both normal and elderly, rehabilitation. Some of the most used sensors are accelerometers [2] and gyroscopes [3]. Together with different wireless transmission technologies, computer development achievements these sensors allow of creating small, battery operated systems for biomechanical data aggregation and processing [4]. Such include creating wireless body networks that utilize few sensors to collect measurements and central device to do the aggregation and processing/forwarding.
2
Real time system for data aggregation and processing
One of the most widely analyzed parameter of biomechanics is acceleration. Acceleration data can be used in wide range of applications such as: a) Energy expenditure calculation using accelerometer data [5][6]. b) Human position evaluation (standing, laying, etc) [7][8]. c) Activity recognition (walking, running, sit-ups, etc) [9]. d) Human movement analysis. This is an insight of how wide acceleration data can be used, though joining few areas to achieve more complex results using wireless body networks gives some new challenges. Such attempt is made in a project “VitaActiv”. The project is funded by Lithuanian State Science and Studies Foundation, is still ongoing (started in 2007, ends in the last quarter of 2009), and intended to develop innovative high-tech products: personal wireless heart rate and biomechanical parameters analyzer for evaluation of human functional status and selection of corresponding pulse rate and physical exercising parameters. Scheme of such system which utilizes wireless body network is presented below. Algorithm implementation and data evaluation methodologies are out of this paper’s scope. As the good wireless connection between network modules and main module is the key factor for the system to work reliably, a more comprehensive study of radio signal strength (RSS) in such environment will be carried out in further sections to investigate environmental effects that alter radio signal in different ways.
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Figure 1. System of VitaActiv project prototype utilizing wireless body network
3
Experimental setup
An important issue influencing RSS is the surrounding objects, causing the reflection of the transmitting RF signal both on the surrounding objects and ground. Other consideration is RSS variability which is influenced by hardware factors, such as RF frequency, antenna orientation and transmission power, and environmental factors such as obstructions. The experiments were conducted inside, having free line of sight between transceivers and with obstacles – human body to be specific. Measurements were performed using a prototype made in VitaActiv project with Texas Instrument CC2500 radio chip and PCB antenna which configuration is presented in figure below. Both receiver and transceiver were placed at 1.5 m height. Distance varies between 1 and 4 meters, as this is more than enough for wireless body network. Transmission power varies from 0bBm (1mW) to -10dBm. For all experiments the same transceiver pair is used to avoid influence of transmitters and receivers inequality on the results. The RSSI is measured every 1 m for some test configurations. The process of measuring RSSI consists of following steps: The transmitter sends a packet of 1 byte to the receiver repeatedly. Receiver gets a packet from transmitter and sends an acknowledge packet with RSSI value of the received. Transmitter catches acknowledge packet and stops sending forward packets. The RSSI of the packet is displayed in receiver, the RSSI of the acknowledge packet is displayed in transmitter (here words “transmitter” and “receiver” are used to describe transmission initiator and responder, not the function of the radio, as both ends have transmitting and receiving capabilities).
Figure 2. CC2500 Folded Dipole used in the prototype
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4
Results
4.1
Antenna orientation According to antenna specification, it does not have omnidirectional radiation pattern [10]. This means that the position of the modules influence RSSI value considerably. To study the impact of antenna orientation, two test sets were performed: one with vertical, and one with horizontal orientation as shown in figure below.
Figure 3. Transmitter and receiver vertical and horizontal orientation of 45 angles 315 Transmitter angle 270
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Figure 4. RSSI variation dependence on antenna orientation of transmitter and receiver - 127 -
Experiments were performed with transmission power of 0 dBm and range of 1 meter. Device was configured to use 2413.5625 MHz frequency, transmission speed was configured to 250 kbaudps. Figure 4 illustrates the variation of RSSI depending on the orientations of both receiver and transceiver antennas. RSSI maps show a little stronger overall signal when antennas are in vertical planes than in horizontal, but basically signal is strong enough everywhere. There are some bad spots, basically when antennas are perpendicular to each other. At this point RSSI is 5 – 10 dBm lower and such situations should be avoided when possible. 4.2
RF frequency Radio frequency is another factor in wireless transmission that has to be addressed. There are spurious signals at the frequency: 2 where - frequency of crystal oscillator, – integer number [11]. Our application uses 26.45 MHz crystal, so frequencies of 2406.95 13.225 · MHz , where is an integer number, suggested to avoid. 1
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Range, m
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RSSI, dB
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2406.95MHz 2426.7875MHz
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Figure 5. RSSI attenuation over the distance with different transmission frequencies Three tests were conducted to check whether different frequencies lead to different received signal strengths. Output power was set to 0 dBm, height 1.5 m, antennas were placed in the same plane perpendicular to each other as the connection conditions are the worst as stated in section 4.1. Frequency of 2413.5625 MHz showed normal results, but both 2406.95 MHz and 2426.7875 MHz frequencies showed degradation in RSSI in every test as signal was on the lower edge of transceiver sensitivity. Frequency plays a big role in such applications so should be chosen after set of tests are done to find most suitable frequency (here we assume frequency hopping is not going to be implemented). 4.3
Transmission power The following experiments aim at testing the attenuation of the RSS over distance for four different levels of the transmission power. The experiments are conducted inside with distance 1 to 4 meters. Transmission frequency was set to 2413.5625 MHz, antennas oriented perpendicular to each other and in one plane to appear in one of the “bad” orientation setting as stated in section 4.1. The height from the ground was 1.5 meters. 1
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Figure 6. RSSI attenuation over the distance - 128 -
As seen in the given figure, RSSI drops with distance getting bigger. A very low drop with -10dBm transmission power is explained by increased packet loss, where only stronger signals are captured. For such applications as wireless body networks, transmission powers below -6dBm should be avoided to assure reliable transmission. 4.4
Human body impact As human body can be a very disturbing factor in wireless networks [12], a set of test were conducted to see impact of the human body on this specific prototype. Radio was configured to use 2413.5625 MHz frequency, 0 dBm output power.
Figure 7. Tests setup There were six test conducted: 1a. Transmitter was placed in the shoe on the outer side of the left foot, and receiver was held in the left hand. 1b. Transmitter was placed in the shoe on the outer side of the left foot, and receiver was held in the right hand. 2a. Transmitter was placed on the ground in front, receiver was held also in front. 2b. Transmitter was placed on the ground in front, receiver was held in the back. 3a. Transmitter was held in the front at the waist line, receiver was held in the back at the same height. Both devices 0.5 meter form the body. 3b. Transmitter was held in the front at the waist line, receiver was held in the back at the same height. Both devices attached directly to the body (on the belt). 1
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RSSI, dB
-30 -40 -50 -60 -70 -80 -90 Test pair
Figure 8. Tests results Results show the impact on the RSSI the human body invokes. The RSSI drop when human body is in the ways is more than 10dB on the average. Results also show that the closer the transceivers are to the body, the worse results they provide. This has to be taken into well consideration when designing wireless body networks. It is suggested to avoid placements of devices on the different sides of the body, especially when they are placed directly on the surface. If these considerations are taken into account, successful usage of such transceiver (CC2500) is still feasible.
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5
Conclusions
This paper presented various complex applications of accelerations sensors and basic ideas behind them. As the technology develops, there is a high demand of small, nonintrusive and inexpensive mobile systems that can be used in health care, elderly care and sports. Such systems would be cost effective, efficient and reduce human effort. Body wireless networks are unavoidable when creating such systems, unless there is one sensor. One of the key factors for such networks to operate reliably is radio transmission quality. This paper addressed this issue by presenting results of various tests made with a prototype made in VitaActiv project that uses Texas Instruments CC2500 RF transceiver for wireless communications. Folded dipole antenna was used. RSSI signal behavior was analyzed with a set of tests. Antenna orientation is one of the factors that has to be considered when designing such systems. It is suggested that perpendicular orientation of both antennas would be avoided. Antennas placement in vertical planes is advisable as this gives better overall RSSI value. Radio frequency is another important factor as it is been seen that it can greatly impact RSSI value. For best RF selection when no frequency hopping is implemented it is advisable to run a set of tests through all available band to find which frequency yields best results. Transmission power of CC2500 transceiver is advised to be in the range of 0 to -6dBm depending on current consumption and range requirements. Human body is also affecting radio transmission and these effects were analyzed in this paper. As it is seen in test results, human body tends to absorb radio signal giving a low RSSI value when devices are placed direct on the surface and have to do the transmission directly through the body. It is guessed that the transmission is greatly altered by reflections of radio waves than the direct link. It is advisable to arrange body wireless nodes to avoid link directly through the human body. If 2.4GHz wireless radio frequency is to be used in heavy occupied areas, additional measures must be taken to overcome human body effects on the radio signal strength.
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