Proc of the 4th Annual IEEE zyxwvutsrqpon Conf on Information Technology Applications in Biomedicine, UK zyxwvut DIRECT SEQUENCE CDMA BASED WIRELESS INTERFACE FOR zy AN INTEGRATED SENSOR MICROSYSTEM N. Aydin’, T. Arslan13’, D. R. S. Cumming3 ‘School of Engineering and Electronics, University of Edinburgh, Edinburgh, UK *Institute For System Level Integration, Livingston, UK 3Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow, UK Abstract-Utilization of wireless miniaturized electronic systems in medical diagnosis, environmental monitoring and other industrial applications has been made possible by emergence zyxwvut of system-on-chip technology. An important task in such a system is to convey information reliably on a multiple access based environment. Communication systems targeting miniaturized sensor networks are also characterized by their restricted power and area constraints. We describe a communication system based on direct sequence code division multiple access method. Keywords zyxwvutsrqpo - Communication, wireless, CDMA I. INTRODUCTION Utilization of wireless miniaturized electronic systems in medical diagnosis, environmental monitoring and other industrial applications is highly desirable. Wireless implantable sensors are also increasingly being used in medical field [I]. Recent developments in system-on-chip (SoC) technologies [2] accelerated these interests and made it feasible to shrink the size of such systems, while increasing their functionality. For example, an ingestible electronic capsule for monitoring some common physiological parameters of Gastro-intestinal tract such as temperature, pH, conductivity, and oxygen concentration has recently been described [3], [4]. It integrates several sensors, amplifiers, analog digital conversion, coding and transmitting circuitry. One of the main problems of such a system is how to retrieve these information from the sensor system in real- time non-intrusively. The most feasible solution is to integrate a wireless communication circuit into the system which constitutes an important part of a SoC implementation. Some of the desirable properties of the communication system are capability of communicating with multiple sensor systems for the purpose of centrally monitoring or controlling more than one medium at the same time and interference rejection for reliable communication in critical applications such as medical diagnosis. A conceptual representation of a desirable system is shown in Fig. 1. Preliminary results of such a system design attempt have been reported in [5] and zyxwvutsrqpo [6]. In this paper, we report the practical results of code division multiple access (CDMA) transmitter implementation, which was fabricated using AMs zyxwvutsrqpo 0.6~ CMOS technology. 11. METHODOLOGY In a multiple access system, a large number of users share a common channel to transmit information to a receiver. The system has to manage resources appropriately in order to cover and support all the users that want to access the system. There are three common technologies used to create an air interface; frequency-division multiple access (FDMA), time-division multiple access (TDMA), code-division multiple access (CDMA) [7]. Fig. 1. Conceptual representation of typical multiple sensor system In FDMA, the available channel bandwidth is subdivided into a number of frequency nonoverlapping channels. These subchannels are assigned to each user upon request by the users. FDMA allocates a single channel to one user at a time. Although technically simple to implement, FDMA is wasteful of bandwidth. TDMA relies upon the fact that the signal has been digitized; that is, divided into a number of packets. It allocates a single frequency channel for a short time and then moves to another channel. The digital samples from a single transmitter occupy different time slots in several bands at the same time. The main drawback is that with narrower bandwidth there is grater distortion. In an environment where the transmission from the various users is bursty and low duty cycle, FDMA and TDMA tend to be inefficient because ,a certain percentage of the available frequency slots or time slots assigned to user do not carry information [SI. An altemative to FDMA and TDMA is to allow more than one user to share a channel by use of direct- sequence spread spectrum (DS-SS) signals [9], [lo]. A. Direct Sequence Spread Spectrum In a DS-SS system, each user is assigned a unique code sequence [ 111 that allows the user to spread the information signal across the assigned frequency band. Signals from the various users are separated at the receiver by cross- correlation of the received signal with each of the possible user code sequences. Possible narrow band interference is also suppressed in this process. By designing these code sequences to have relatively small cross-correlation, the cross-talk inherent in the demodulation of the signals received from multiple transmitters is minimized. This multiple access method is CDMA, which is a form of a DS- SS system [12], [13]. This modulation transforms an information-bearing signal into a transmission signal with a much larger bandwidth. This transformation is achieved by zy 370 0-7803-766740W $17.00 WO03 IEEE