Communication Scheme for a Highly Collision-Resistive RFID System

A highly collision-resistive RFID system multiplexes communications between thousands of tags and a single reader in combination with time-domain multiplexing code division multiple access (TDCDMA), CRC error detection, and re-transmission for error recovery. The collision probability due to a random selection of CDMA codes and TDMA channels bounds the number of IDs successfully transmitted to a reader during a limited time frame. However, theoretical analysis showed that the re-transmission greatly reduced the collision probability and that an ID error rate of 2.5 × 10−9 could be achieved when 1, 000 ID tags responded within a time frame of 400 msec in ideal communication channels. The proposed collision-resistive communication scheme for a thousand multiplexed channels was modeled on a discrete-time digital expression and an FPGA-based emulator was built to evaluate a practical ID error rate under the presence of background noise in communication channels. To achieve simple anti-noise communication in a multiple-response RFID system, as well as unurged re-transmission of ID data, adjusting of correlator thresholds provides a significant improvement to the error rate. Thus, the proposed scheme does not require a reader to request ID transmission to erroneously responding tags. A reader also can lower noise influence by using correlator thresholds, since the scheme multiplexes IDs by CDMA-based communication. The effectiveness of the re-transmission was confirmed experimentally even in noisy channels, and the ID error rate derived from the emulation was 1.9 × 10−5. The emulation was useful for deriving an optimum set of RFID system parameters to be used in the design of mixed analog and digital integrated circuits for RFID communication. key words: RFID, anti-collision, TD-CDMA, low-power, impulse modulation, inductive coupling, hardware emulation