A Scalable, 2.9 Mw, 1 Mb/S E-Textiles Body Area Network Transceiver with Remotely-Powered Nodes and Bi-Directional Data Communication
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A Scalable, 2.9 mW, 1 Mb/s e-Textiles Body Area Network Transceiver with Remotely-Powered Nodes and Bi-Directional Data Communication The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Desai, Nachiket, Jerald Yoo, and Anantha P. Chandrakasan. “A Scalable, 2.9 mW, 1 Mb/s E-Textiles Body Area Network Transceiver With Remotely-Powered Nodes and Bi-Directional Data Communication.” IEEE Journal of Solid-State Circuits 49, no. 9 (September 2014): 1995–2004. As Published http://dx.doi.org/10.1109/JSSC.2014.2328343 Publisher Institute of Electrical and Electronics Engineers (IEEE) Version Author's final manuscript Citable link http://hdl.handle.net/1721.1/98890 Terms of Use Creative Commons Attribution-Noncommercial-Share Alike Detailed Terms http://creativecommons.org/licenses/by-nc-sa/4.0/ 1 A Scalable, 2.9 mW, 1 Mb/s e-Textiles Body Area Network Transceiver with Remotely-Powered Nodes and Bi-Directional Data Communication Nachiket Desai, Student Member, IEEE, Jerald Yoo, Member, IEEE, and Anantha P. Chandrakasan, Fellow, IEEE Abstract—This paper presents transceivers and a wireless sends them on to a healthcare provider using conventional power delivery system for a Body-Area Network (BAN) that data networks. uses an e-textiles-based physical layer (PHY) capable of linking Wireless communication devices forming a BAN typically a diverse set of sensor nodes monitoring vital signs on the user’s body. A central base station in the network controls operate in the unlicensed bands, and have to contend with the power delivery and communication resource allotment for every high losses in those bands in the operating environment [5], node using a general-purpose on-chip Node Network Interface [6]. Another scheme for communication with the local base (NNI). The architecture of the network ensures fault-tolerance, station is the use of body-coupled communication (BCC) [7]– reconfigurability and ease of use through a dual wireless-wireline [9]. BCC transceivers avoid a number of multiple-access prob- topology. The nodes are powered at a peak end-to-end efficiency of 1.2% and can transmit measured data at a peak rate of 1 Mb/s. lems when a large number people operate BANs in close prox- Modulation schemes for communication in both directions have imity. However, the path loss in BCC channels is comparable been chosen and a Medium Access and Control (MAC) protocol to wireless channels. In contrast, textiles with electric routing has been designed and implemented on chip to reduce complexity (e-textiles) printed on top of the cloth or woven in as part of at the power-constrained nodes, and move it to the base station. the yarn offer much lower path loss, and have long been a topic While transferring power to a single node at maximum efficiency, the base station consumes 2.9 mW power and the node recovers of research with the aim of building wearable computers [10]. 34 µW, of which 14 µW is used to power the network interface They allow remote powering of the microwatt sensors and circuits while the rest can be used to power signal acquisition their associated telemetry circuits by a network base station circuitry. Fabricated in 0.18 µm CMOS technology, the base [11]–[13]. Building on these advantages, the proposed BAN 2 2 station and the NNI occupy 2.95 mm and 1.46 mm area improves efficiency by using resonant power transfer between respectively. coupled inductors and combines this with electric routing on Index Terms—Body-area networks, e-textiles, continuous clothing to afford better aesthetics and greater comfort to the health monitoring, wireless power delivery, inductive links, in- user by avoiding wires going all the way to sensors adhered tegrated medium access protocol to the body. This allows for cheap, disposable sensors that do not need an integrated battery or radio and can be easily I. INTRODUCTION attached to and removed from the user’s body depending on his/her specific requirements. The system architecture and the ODY-area networks (BANs) allow continuous monitor- circuits used for resonant power transmission and recovery ing of vital signs by linking small, low-power sensors are presented in Sections II and III respectively. Modulation implantedB in and placed on the surface of the body and schemes for communication to and from the nodes have been transmitting the data they measure to healthcare providers chosen to lower the power requirements at the nodes and push [1]. Current state-of-the-art biomedical front-ends [2]–[4] can complexity to the base station. These modulation schemes and measure signals using only microwatts of power, making the circuits implementing them are presented in Section IV. it possible to aggressively minimize their volume and the Medium Access and Control (MAC) protocols used in amount of energy stored locally. In order to realize their commercially-deployed standards for wireless BANs, such potential afforded by these advantageous traits, it makes sense as Bluetooth LE and IEEE 802.15.4/Zigbee [14] include to transfer the data using a two step process. Uncompressed significant protocol overhead at the sensor nodes and are not data is first sent to a local base station (like a cellphone) using compatible with microwatt-sensors. Allowing sensors to sleep a link requiring minimal transmission effort by the sensor and requiring them to wake up after a preset amount of time node. The local base station then processes the signals and [15] to check if the channel is unoccupied requires an accurate, Manuscript received August 27, 2013; revised January 16, 2014. This power-hungry clock at the sensor that is not power-gated. A work was funded by the cooperative agreement between the Masdar MAC protocol for e-textile based BANs that relies on Time- Institute of Science and Technology, Abu Dhabi, UAE and the Mas- Division Multiple Access (TDMA) for resource allotment has sachusetts Institute of Technology, Cambridge, MA, USA, Reference No. 196F/002/707/102f/70/9374. been presented in [11]. A custom MAC protocol that moves N. Desai and A. P. Chandrakasan are with the Department of Electrical all decision-making to the base station to reduce power con- Engineering and Computer Science, Massachusetts Institute of Technology, sumption at the sensor nodes has been proposed and integrated Cambridge, MA, 02139 USA e-mail: [email protected] tel: 617-253-0164. J. Yoo is with the Department of Electrical Engineering and Computer on chip. The custom MAC protocol also allows sensors to Science, Masdar Institute of Science and Technology, Abu Dhabi, UAE. be added and removed easily in order to keep the network 2 Inductors forming links with sensor nodes 32 mm Electric routing on textiles Base Station Fig. 1. An e-textiles shirt implemented with screen-printed spiral inductors linking to sensor nodes (worn inside out for demonstration). The base station’s Fig. 2. Cross-section of inductive link between e-textiles and sensor node. size is limited by test and debug structures on the board. inductor and high power consumption when all inductors are reconfigurable and supports TDMA and Contention Access in the same sub-network. Also, by not adhering to a strict (CA) modes for sensors with wide variation in channel access arrangement like the (X,Y) grid in [12] or the linked list and bandwidth requirements. Details of the implemented MAC in [17], the proposed network architecture allows for greater are presented in Section V. freedom in covering sensors around the body based on user- specific sensor locations. An obvious arrangement would be II. SYSTEM ARCHITECTURE to form sub-networks of inductors covering one part of the A. e-Textiles Network body. The only limitations would be the number of sub- Figure 1 shows an example of the proposed network im- networks that can be supported by the base station and the plemented on a shirt with the user holding the local base combined inductance of the parallelly-connected coils in each station, whose size is limited by the test and debug structures sub-network. For example, the shirt in Figure 1 can support on the board. Spiral inductors and their associated routing that 2-channel ECG while a cap made of similar material can connect them to a central base station have been screen-printed function as an EEG helmet. on fabric using the process described in [16]. Restricting the Power is transferred to the sensor nodes by resonant cou- circuits on textiles to screen-printed passives avoids expensive pling between inductors at the base station and the sensor procedures to bond silicon dies to the routing on fabric and nodes. The network has a star topology, with all sensor nodes keeps the costs of the textiles low. These inductors form near- talking directly to the base station. Modulation schemes for field magnetic links with similar inductors on the sensor nodes, data transmitted in both directions have been chosen to push as shown in Figure 2. These nodes are of a band-aid patch form most of the modulation and demodulation effort away from factor and are worn on the body. The local base station can be the sensor nodes towards the base station. integrated in a cellphone linked to the shirt with a snap-button interface or a secondary inductive link (with additional losses), B. Base Station or onto a clip-on bluetooth-like device linked to the user’s cellphone. This hybrid wireless/wired architecture achieves A block diagram of the implemented base station for the better power efficiency compared to fully wireless- or BCC- network-on-shirt of Figure 1 is shown in Figure 4.