IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 17, NO. 4, APRIL 1999 725 Code-Division Multiplexing of a Sensor Channel: A Software Implementation Joshua R. Smith, Christopher Salthouse, and Neil Gershenfeld, Member, IEEE Abstract—This paper demonstrates the use of software radio techniques in the context of sensing, rather than communications. It describes code-division multiplexing (CDMA) and time-division multiplexing (TDMA) of a receiver channel in an electric field sensing system. The only hardware used is a front-end gain stage consisting of two opamps and a microcontroller. The mod- ulation and demodulation operations are implemented entirely in the microcontroller software. Multiple coded waveforms are transmitted simultaneously, and induce a combined signal on a single receive electrode. The combined signal, after passing through a single analog front end terminating in an analog-to- digital converter, is separated into the four original component signals by a software demodulation operation. The signal-to-noise ratio (SNR) achieved by the code-division multiplexed system given a fixed measurement time is compared to the SNR achieved by a time-division multiplexed implemen- tation given the same total measurement time. The paper also compares the scaling of TDMA and CDMA performance with the number of transmitted channels and the number of demodulated channels. Index Terms—CDMA, electric field sensing, sensing, software radio, spread spectrum. I. INTRODUCTION T HIS paper demonstrates the use of software radio tech- niques in the context of sensing, rather than communi- cations. It describes code-division multiplexing (CDMA) and time-division multiplexing (TDMA) of a receiver channel in an electric field sensing system. The demodulation operation is performed entirely in software. Several sensor signals can simultaneously share the same analog front-end hardware, in- cluding ADC, because software performs different processing operations on one set of samples to extract several distinct signals. This means that the number of channels that may simultaneously be received is not rigidly fixed by hardware. Additional receiver channels are extra processing steps on the data already being collected. Additional coded waveforms do appear as noise to the other channels (in the CDMA case), or require additional sensing time (for TDMA), so additional channels are not “free,” but the number of channels is not fixed by the hardware. As long as one is able to pay the price in either signal-to-noise ratio (SNR) or measurement time, and also in processing time, additional channels may Manuscript received September 1997; revised June 19, 1998 and July 31, 1998. J. R. Smith is with the Escher Laboratories, Cambridge, MA 02142 USA. C. Salthouse and N. Gershenfeld are with the Physics and Media Group, M.I.T. Media Laboratory, Cambridge, MA 02139 USA. Publisher Item Identifier S 0733-8716(99)02976-5. Fig. 1. Lumped circuit model of electric field sensing. be added. Furthermore, the software receiver readily changes which particular channels it is demodulating. The work is set in the context of electric field sensing, which is introduced below, but the same principles could be applied in virtually any sensing or measurement system in which the quantity being sensed is modulated by a carrier. For example, these techniques could be applied to systems of ultrasound or infrared emitters and detectors. II. MOTIVATION:ELECTRIC FIELD SENSING The term electric field sensing refers to a family of non- contact methods for measuring the position and orientation of the human body, or parts of the human body such as a hand. Electric field sensing has been used for human–computer interface [1], to make a three-dimensional noncontact mouse [2], for creating new musical instruments (3-D) [3], and in the automotive industry as a solution to the rear-facing infant seat problem. 1 In a typical implementation of electric field sensing, a low- frequency (from 10–100 kHz) voltage is applied to a transmit electrode (labeled in Fig. 1), and the displacement current induced at a receiver is synchronously detected. Fig. 1 shows a lumped circuit model of the electrodes and the body. 1 Because the violent inflation of an airbag can injure infants in rear-facing infant seats, it is desirable to sense the orientation of the child and disable the airbag as appropriate [4]. 0733–8716/99$10.00  1999 IEEE