The present invention relates to radio frequency identification (RFID) systems, and in particular, to RFID systems that operate in a constrained environment.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Detection (that is, detecting the presence or absence of an RFID tag) is an important issue with RFID systems. Strategies for improving detection include increasing tag power (e.g. with batteries), selecting frequencies that minimize “shadows”, increasing reader power, and increasing the number of readers. All these strategies are directed toward providing overlapping coverage areas to increase the likelihood that the RFID tag is detected.
Discrimination (that is, discriminating the location of the RFID tag) is another important issue. However, discrimination is somewhat in conflict with detection: For detection, overlapping coverage areas are good, but overlapping coverage areas make it harder to determine exactly which reader is closest to the tag. One strategy for improving discrimination is to increase the distance (separation) between readers.
The conflict between detection and discrimination is keenly felt in constrained environments where a number of readers are in close proximity, and it is desired to determine which reader is reading a particular tag. One example of such a constrained environment is a casino gaming environment in which the gaming tokens include RFID tags. As a specific example, a gaming table may have multiple locations that gaming tokens may be placed, and it is desirable both to detect (the ID of the token) and to discriminate (that the token is placed in a particular functional area of the gaming table).
The technical challenge is to insure that only tags inside a betting zone are read. An ideal method is that only tags inside a betting zone receive sufficient energy to respond and tags outside remain unpowered. A second method to differentiate signals coming from tokens inside a betting zone versus those tokens outside a betting zone is using a signal strength threshold. Signals above the threshold are believed to be inside the betting zone, while signals below the threshold are believed to be outside the betting zone. A third method, comparing the signal strength of an individual token signal as measured by two adjacent (or nearby) readers, can also be used. Cross-talk occurs when one either does not read a tag inside the betting zone or the betting zone erroneously reads a tag that is outside the betting zone. It remains desirable that the signal strength from tokens at the top of a stack inside the betting zone be greater than the signal strength of any tokens outside the betting zone.
FIG. 12 shows three views of a typical loop antenna 1204. The top view shows a three-dimensional view of the flux lines 1200 and the loop antenna 1204. The middle view shows a cut away side view along Section A-A of the top view. The bottom view shows a graph of the field strength 1202 as one traverses across Section A-A of the loop antenna 1204. Two features are noteworthy: the “null” 1210 and the “tail” 1212. The “null” 1210 is a region of low sensitivity arising from horizontal flux lines 1200 in the h-field that do not intersect the horizontal antenna loop 1204. Ideally, the null 1210 is outside the defined borders of the betting zone. If the “null” falls inside the betting zone, there will be “dead spots” in the betting zone that may result in read errors. The “tail” 1212 is a region of undesirable sensitivity outside the defined betting zone. If the “tail” is sufficient to energize tokens outside the betting zone, the result may be cross-talk errors.
Thus, there is a need for RFID systems that operate in a constrained environment.