Collaborative Reader Code Division Multiple Access in the Harmonic RFID System

Reading reliability is a critical aspect in the ultrahigh-frequency radio frequency identification (RFID) system when a large amount of tags are in the reading zone with complex multi-path interferences. The read failure incidences can be caused not only by the random tag position and orientation but also by the RF shadowing of other objects and tags, both of which can misalign the main lobes of the tag and reader antennas. To increase the system coverage and total read yield in practical indoor ambient, a possible solution is to employ multiple readers with overlapping reading zones. However, when the tags are within the reading zones of multiple readers, the conventional tag singulation protocol will suffer serious reader-to-reader collision (R2RC) unless the conflicting readers can agree on proper collaborative time or space division schemes. The uncertain delay of the time division multiple access and the lack of definitive time sequence can further affect the reading process, especially when the RFID system is employed for applications requiring real-time accessing, tracking, and sensing. In this paper, we introduce a collaborative reader code division multiple access (CDMA) protocol in the harmonic RFID system, which provides synchronous tag access for all readers within range. The tag can distinguish the downlink commands from multiple readers simultaneously and then formulate the response accordingly. The proposed reader CDMA is demonstrated in a simple experimental prototype to verify R2RC resolution and read yield improvement. The tworeader collaborative CDMA scheme achieved below 10(-5) in the read failure rate when individual readers had 0.05 - 0.2 failure rates in the complex multi-path scenarios due to rich channel exploitation and anti-correlation in the failure incidences.