Personal Area Networks (PAN): Near-Field Intra-Body Communication by Thomas Guthrie Zimmerman B.S., Humanities and Engineering Massachusetts Institute of Technology February 1980 Submitted to the Program in Media Arts and Sciences, School of Architecture and Planning, in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE IN MEDIA ARTS AND SCIENCES at the Massachusetts Institute of Technology September 1995 (c) Massachusetts Institute of Technology, 1995 All Rights Reserved Signature of Author _______________________________________________________ Program in Media Arts and Sciences June 26, 1995 Certified by _____________________________________________________________ Neil Gershenfeld Assistant Professor of Media Arts and Sciences Program in Media Arts and Sciences Thesis Supervisor Accepted by _____________________________________________________________ Stephen A. Benton Chairperson Departmental Committee on Graduate Students Program in Media Arts and Sciences Personal Area Networks (PAN): Near-Field Intra-Body Communication by Thomas Guthrie Zimmerman Submitted to the Program in Media Arts and Sciences, School of Architecture and Planning, on June 26, 1995 in partial fulfillment of the requirements for the degree of MASTERS OF SCIENCE IN MEDIA ARTS AND SCIENCES Abstract PAN is a wireless communication system that allows electronic devices on and near the human body to exchange digital information through near-field electrostatic coupling. Information is transmitted by modulating electric fields and electrostatically (capactively) coupling picoamp currents into the body. The body conducts the tiny current (e.g., 50 pA) to body mounted receivers. The environment (“room ground”) provides a return path for the transmitted signal. A low frequency carrier (e.g., 330 kHz) is used so no energy is propagated, minimizing remote eavesdropping and interference by neighboring PANs. Digital information is transferred using on-off keying with quadrature detection to reduce stray interference and increase receiver sensitivity. A low cost (<$20) half-duplex modem is implemented using an analog bipolar chopper and integrator as a quadrature detector, and a microcontroller for signal acquisition. The technology used in the PAN can be integrated into a custom CMOS chip for lowest size and cost. Thesis Supervisor: Neil Gershenfeld, Assistant Professor of Media Arts and Sciences This work was supported in part by Hewlett-Packard Corporation and Festo Corporation. 2 Personal Area Networks (PAN): Near-Field Intra-Body Communication by Thomas Guthrie Zimmerman Thesis Readers Thesis Reader____________________________________________________________ Jerome Ysrael Lettvin Professor of Electrical Engineering and Bioengineering and Communications Physiology, Emeritus Thesis Reader____________________________________________________________ Louis Dijour Smullin Professor of Electrophysics, Emeritus 3 Acknowledgments I am indebted to my advisor, Neil Gershenfeld, for reacquainting me with the all- encompassing power of physics through his broad command of the discipline. His class Physics and Media succinctly reviews how important a knowledge of physics is to tackling any engineering problem. I would like to thank all the members of the Physics and Media Group; Josh Smith, Joe Paradiso, Rich Flecher, Barrett, Henry Chong, and David Allport for their camaraderie, team work, stimulating technical discussions, and friendship during a very busy and exciting eighteen months. I would like to particularly thank Joe Paradiso for all the advice, design discussions, and great music. I would like to thank the Media Lab, that big box of very talented people, and those who build it and keep it running. Of particular merit is the work of Linda Peterson, Santina Tonelli and Suzanne McDermott. I came to the Media Lab because it has everything I am interested in under one roof, and my experience here have confirmed my belief. The Media Lab is truly a unique laboratory at a unique institution. The visitors who come to the Media Lab for demonstrations have provided me with instant feedback on the usefulness and value of my research. Without their support and interest, the Media Lab would not exist. I would like to thank Dr. Hermann Klinger and the Festo Didactic Corporation for their enthusiastic support of our research and for the hospitality Dr. Klinger showed my on my visit to Germany. I would like to thank the Hewlett-Packard Corporation for their generous support of our laboratory, and my readers Professor Lettvin and Professor Smullin for their comments. Finally I would like to thank my wife, Patricia Canon Zimmerman, for being the most wonderful, understanding, helpful, loving, and fun person I know. She sacrificed much to come here to see me through this process and now we can go back to California, both Masters, with our wonderful six month old son, Matthew Daniel Zimmerman. 4 Contents 1. INTRODUCTION...............................................................................................................................7 1.1 PREVIOUS WORK ON ELECTRIC FIELD SENSING......................................................................................7 1.2 CURRENT TRENDS IN WIRELESS NETWORKS ..........................................................................................7 1.3 MOTIVATION FOR PERSONAL AREA NETWORKS .....................................................................................8 1.3.1 WAN Þ MAN Þ LAN Þ PAN.....................................................................................................8 1.3.2 Interconnectivity Reduces Redundancy........................................................................................9 1.3.3 Ubiquitous Computing vs. Distributed I/O ...................................................................................9 1.3.4 Connecting to the "Outside" World ............................................................................................10 1.4 COMPARISON OF ELECTROSTATIC TO INFRARED TRANSMISSION ...........................................................10 1.5 COMPARISON OF ELECTROSTATIC TO RF TRANSMISSION......................................................................10 1.5.1 Antenna Size and Efficiency.......................................................................................................11 1.5.2 Interference ...............................................................................................................................11 1.5.3 FCC Field Strength Regulations ................................................................................................12 1.5.4 Eavesdropping...........................................................................................................................13 1.5.5 Power ........................................................................................................................................15 1.6 NETWORKING STANDARDS .................................................................................................................15 1.7 THESIS OVERVIEW..............................................................................................................................16 2. ELECTRICAL MODEL OF PAN SYSTEM ...................................................................................17 2.1 ELECTRICAL IMPEDANCE TOMOGRAPHY (EIT).....................................................................................17 2.2 RESISTIVITY OF MAMMALIAN TISSUE ..................................................................................................18 2.3 HUMAN BODY APPROXIMATES A PERFECT CONDUCTOR .......................................................................18 2.4 BASIC CONCEPT OF PAN COMMUNICATION.........................................................................................18 2.5 LUMPED MODEL OF COMMUNICATION CHANNEL: SYMMETRY BREAKING .............................................20 2.6 PAN DEVICE ELECTRICAL FIELDS.......................................................................................................21 2.7 ELECTRICAL MODEL OF THE COMMUNICATION CHANNEL ....................................................................23 2.7.1 Lumped-Circuit Electrical Model ..............................................................................................23 2.7.2 Body Capacitance Measurement................................................................................................24 2.7.3 Electrode Capacitance Measurements .......................................................................................25 3. PAN DESIGN ISSUES ......................................................................................................................26 3.1 SIZE...................................................................................................................................................26 3.2 POWER CONSUMPTION........................................................................................................................27 3.2.1 Sneaker Power...........................................................................................................................27 3.2.2 Energy Loss Due to Capacitor Charging ...................................................................................28 3.2.3 Resonant Transmitter.................................................................................................................28 3.3 COST .................................................................................................................................................28