11th Annual Conference of the International FES Society September 2006 – Zao, Japan
A wearable high throughput wireless data transmitter for medical monitoring applications Benoit Gosselin, Laurent Faniel, Mohamad Sawan Polystim Neurotechnologies Laboratory, Electrical Engineering Dept., Ecole Polytechnique de Montreal, P.O.Box 6079, Station Centre-Ville, Montreal, (Quebec), Canada, H3C 3A7
[email protected], www.polystim.polymtl.ca
Abstract This paper presents a high throughput wireless data transmission system dedicated to biomedical monitoring purposes. Its main component is a low-end wireless IEEE 802.11g compliant LAN router, which makes it cost effective and reliable. In addition, it is portable/wearable, it requires a very short development time, and it presents a flexible user I/O interface. The router runs OpenWrt, a free Open Source Linux distribution for wireless routers. As a proof of concept, the proposed architecture has been prototyped with a Asus wireless router and achieves more than 6.1 Mbps of sustained throughput over TCP/IP.
1. INTRODUCTION With the increasing needs for improved health care and safety, there is a great demand for wearable/implantable systems that can monitor patients metabolism or continuously record any vital signs with reliable performances. As a matter of fact, these devices are often intended for concurrent operation, in networks topologies, and in hospital environments. Such systems include neural recording devices (e.g. EMG, ENG), body/brain imaging devices (e.g. Near infrared spectroscopy, ultrasound ecography), as well as various body sensors for health monitoring (e.g. pH, temperature). Therefore, high speed and reliable data interfaces that avoid cumbersome connecting cables would be of great help to collect the data streams and wirelessly transmit them to a remote host for storage and post processing. However, there is presently a lack of such commercially available systems, especially when looking for general user I/O interface that could connect to custom designs. Hence, a few researchers have put efforts to implement their own transmission devices based on off the shelf components. Such systems preferably provide a flexible user I/O interface, allow a short
development time, present wearable size and enable a high data throughput. Obeid et al. used a single board 486 computer running DOS to control an IEEE 802.11b Ethernet card [1]. This system was intended for multi-channel neural recording and achieves a data rate of 4.5 Mbps within 9m of range. But, since UDP was used as transport protocol, additional control flags had to be added to the payload for data losses detection. More recently, Bluetooth was used to implement a multisensor interface [2] and a low-power sensor network access point [3], both achieving data rates below 250kbps. This paper presents a high throughput wireless interface which allows a continuous data transfer rate of 6.1Mbps over TCP/IP. The system is of wearable size and is suitable to interface most monitoring/recording devices or sensors networks. The remaining sections of the paper present the system design and the experimental measurements from an implemented prototype.
2. METHODS 2.1 Wireless technology and protocol Compared to other common wireless technology and standards, such as Bluetooth and Zigbee, IEEE 802.11g is much faster and therefore better suited for this type of applications. Although it consumes more power, Wi-Fi is also the only well spread existing technology that can practically bear high throughputs with TCP/IP. TCP ensures data integrity and packets ordering, and simplifies the data transfer. Consequently it is well suited for medical applications and it is used for this system. 2.2 System Design The proposed system is mainly composed of a versatile user I/O interface, including data buffers, a USB 2.0 link and a Wi-Fi router which implements the TCP/IP stack, packetizes
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11th Annual Conference of the International FES Society September 2006 – Zao, Japan the data and transfers them over IEEE 802.11g. Figure 1 shows a block diagram of the proposed design. At first, the samples are collected at the user I/O interface, then, buffered and grouped into 512 bytes USB 2.0 packets to be delivered to the router, which sends them over the network. Using a low-end wireless router as the main component for Wi-Fi compliance has been found to be a very effective solution and it features the following advantages: It already includes all the hardware needed and implements the TCP/IP stack of protocols; It is relatively small and suited for wearable applications; It is inexpensive, compared to a single board computer or fabricating a custom ASIC. The router, which is in fact a small computer, is easily customized by replacing its original firmware by OpenWrt, a free Linux distribution for wireless routers, available since 2004. OpenWrt supports a large set of routers (http://openwrt.org) and, as an operating system, it is provided with the TCP/IP stack. In this design, the router interfaces with sensors devices in a simple, reliable and flexible fashion through the router USB 2.0 port and an adapter. USB is fast, reliable and easy to interface through programmable logic devices or microcontrollers.
CPU, including 32 MB of RAM, 4 MB of flash ROM, and two USB 2.0 ports. The EZ-USB FX2-LP USB 2.0 integrated microcontroller (Cypress Semiconductor) has been used as an adapter to implement the USB 2.0 link towards the router. Among other capabilities, it can easily be configured as a FIFO-to-USB 2.0 bridge. It implements an asynchronous 8-bits FIFO with a depth of 3u512 bytes (3 time the size of a USB frame) for this design. A Spartan 3 FPGA (Xilinx, Inc.) is used to implement the user I/O interface and to write the input data to the USB 2.0 adapter. It also implements an additional FIFO with a larger depth, which is needed to hold data when the router is busy (this can happen when TCP/IP packets need to be retransmitted). This additional FIFO reduces data losses and allows continuous data transfer, even in noisy environments. Development boards have been used, for the EZ-USB chip as well as for the FPGA interface, to fasten the implementation. 2.4 Embedded software C software applications are used to customize the router. A first application implements a daemon which is launched when the router boots up. Then, the daemon starts or stops a server application, which implements the TCP/IP stack and enable data transfer. The daemon establishes a socket connection towards a remote client and reads incoming configuration messages. The server reads the input data from the USB interface. The USB 2.0 adapter uses a bulk data transfer type, and the USB 2.0 packets are 512 bytes long. After, being read by the router from the USB adapter, the data are written to a memory buffer before encapsulation and transmission. The length of this buffer must be carefully chosen in order to avoid data losses and maximize the throughput. When this buffer gets full, its content is written to a TCP/IP socket and sent to the remote client by the server. Therefore, its length is set to a multiple of a USB packet size. 2.5 Remote host software
Figure 1: Block diagram of the proposed wireless interface.
2.3 System prototyping As a proof of concept, the Asus WL-500G Deluxe wireless router has been used for prototyping the proposed architecture. This router is fully supported by OpenWrt, and is provided with a Broadcom, 200 MHz MIPS
The remote client software can initiate data transfers or system configuration through a custom graphical user interface. It allows to remotely start and stop the server, initiates data transfers and save transferred data in files. The source codes of the embedded server and remote application are available online on our website (www.polystim.ca/soft/).
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11th Annual Conference of the International FES Society September 2006 – Zao, Japan
3. RESULTS 3.1 Testing environment The presented Wi-Fi interface was fed with known sequences of input data at constant rate. It then transmits these data to a laptop computer running the remote software and equipped with 802.11g wireless card and an Ethernet adapter. Idle flags placed between the useful samples were used to control the throughput. When detected, the idle flags were ignored, and the requested throughput was modified by changing the number of idle flags inserted. The received data were verified upon reception at the remote host software. Finally, the tests were carried out in a noisy environment, with several surrounding wireless LANs to validate the behaviour of the system in realistic operating conditions. 3.2 Throughput, transmitting range, and current consumption Tests show that the prototype provides up to 6.1 Mbps of sustained throughput within a range of 30 feet. It has been however found that the actual transmitting range highly depends on the level of surrounding interference and noise. Therefore higher ranges could be expected in areas with less LAN traffic. The prototype presents a maximum current consumption of 1.2A with a 5V power supply. Table 1 summarises its measured performances. Also, extensive testing shows that the best value for the data buffer length (see section 2.4) is located between 10 and 15 times the size of a USB packet. Figure 2 shows the maximum throughput obtained according to the data buffer length.
4. DISCUSSION AND CONCLUSIONS The presented Wi-Fi interface provides a high throughput, without data loss, within several tens of feets. Moreover, it uses TCP as transport protocol which ensures that data are delivered integrally and in the right order. On the other hand, using UDP, which presents less overhead than TCP/IP, can increase the throughput of a data network. According to [4], it is expected that UDP should provide a 200% bulk throughput enhancement for this application. However, this would obviously come at the cost of data losses, reduced transmitting range and extra data payload for re-ordering packets. Nevertheless, UDP would clearly be a good way to enhance the throughput of the presented interface when used in data losses
Figure 2: Throughput according to the router data buffer length. Table 1: Summary of proposed system performances Parameter Throughput Range Transport protocol Dimensions Weight Supply voltage Current consumption
Value 6.1 Mbps 30 feet TCP 185x205x36 mm 500g 5V 1.2A
tolerant applications. Besides, another way to increase the throughputs or decrease the power consumption would certainly be to use a faster or a more power efficient router. Presently, the number of routers provided with a USB 2.0 controller and supported by OpenWrt is increasing. Smaller and faster routers can be chosen within a wide range of possibilities for increased performances. Then, the presented embedded software would simply have to be recompiled for this specific hardware. In a next step, a custom printed circuit board (PCB) including a FPGA, which will implement the FIFOs as well as the USB 2.0 driver, will be designed. This PCB will then be mounted with the router board to implement a wearable size system powered by batteries.
References [1] I. Obeid, M. A.L. Nicolelis, P. D. Wolf. “A multichannel telemetry system for single unit neural recordings”. Journal of Neuroscience Methods. Vol. 133, pp. 33-38. 2004. [2] T. Désilet and M. Sawan, “Wireless esophageal catheter dedicated to respiratory diseases diagnostic”. IEEE ISCAS. May 2006. [3] D. G. Park, S. W. Kang. “Development of Reusable and Expandable Communication Platform for Wearable Medical Sensor Network”. IEEE EMBS. Vol. 7, pp. 5380-5383. Sept. 2004. [4] S. Bansal, R. Shorey, A. Kherani. “Performance of TCP and UDP Protocols in Multi-Hop Multi-Rate Wireless Networks”. IEEE WCNC, Vol. 1, pp. 231-6. Mar. 2004.
Acknowledgements The authors acknowledge the financial support from NSERC and CRCSMD and the design and testing tools from CMC Microsystems.
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