DESIGN OF GLOBAL SAW RFID TAG DEVICES

The Global SAW Tag [1] is projected to be a large-volume SAW device application. The modulation method and overall tag data structure are reviewed for a family of proposed RFID devices. Current implementations use a basic group structure that encodes 16 information bits using 4 reflectors chosen from 75 reflector slots. Several groups are placed in-line on a single acoustic track to achieve ID tag devices with capacities up to 256 bits. RFID system requirements for anti-collision, low tag loss and high tag accuracy are presented. A design example for a single group shows that data dependent amplitude and phase errors can be eliminated. The tag reflectors use a unique floating-electrode reflector structure in a single acoustic channel. Diffraction compensation of individual reflector taps is used to eliminate loss and numerous phase and amplitude distortion effects. I. Data Group Structure The Global SAW Tag [1] is based on a unique modulation method that combines time overlapped pulse position modulation along with simultaneous phase offsets and with multiple pulses per data group. Figure 1 illustrates the specific data group structure that is used as the core building block for proposed international standards for SAW RFID [7]. Figure 1: Each 16 bit data group has pulses placed in 4 of 75 slots time of ~3.05 nsec. width, with -64° phase shift per slot. A minimum pulse separation of 12 time slots is enforced to allow detection of individual pulses under the Nyquist criteria. Also, 11 unoccupied slots exist between groups for this same purpose. The data group structure of Figure 1 was designed to balance a variety of real world RFID system requirements such as operating in the presence of strong interfering signals, minimizing tag cost, enabling robust anti-collision (simultaneous decoding of multiple tags signals that arrive simultaneously), and successfully operating with the limitation that only an ~40 MHz portion of the 83 MHz bandwidth of the 2.45 GHz ISM band may be accessible. Based on equation 3 of reference 1, the parameters indicated in Figure 1 create a data group with 111,930 unique states. Of these, 65,536 states are chosen to encode 16 bits of data. This excess number of states allows for elimination of undesired pulse patterns (e.g. those that produce strong multi-bounce echoes) and choosing a specific subset that optimizes code orthogonality and data link performance. An encoding/decoding algorithm that maps between a particular 16 bit number and a specific set of 4 pulse positions has been developed [2]. II. Overall Tag Data Structure The overall tag data structure consists of a number of 16 bit data groups that are separated by 11 empty time slots (see Figure 1). Thus 16 bit, 32 bit, 48 bit, 64 bit, and higher sizes can all be implemented. Sizes up to 256 bits appear feasible. Figure 2 illustrates a 128 bit “tag platform” consisting of 8 data groups. Figure 2: 128 bit tag platform consisting of 8 groups with 75 slots per group plus 11 empty slots between groups for a total of 677 slots It should be obvious that many other configurations could be used for encoding data onto a Global SAW Tag. One might, for example, allow occupancy of the empty slots between groups without violating the minimum pulse spacing rule used to satisfy the Nyquist criteria. 0 1 2 3 4 5 6 7