In a typical transponder system, a reader radiates an energy field, which powers up transponders within the reading zone. The transponders send back their identity information to the reader either using the same frequency that is supplied by the energising signal, or a seperate communicating frequency. As the messages sent back from the transponder are very weak, other transponders in the field cannot hear those communicating, resulting in them not knowing if a transponder is already communicating on the communications channel and hence whether their communication is being interfered with. As a result, in many situations where there are more than one transponder in a reading field at the same time, the communications received by the reader for some of the transmissions are garbled due to simultaneous transmission from two sources.
To overcome the situation where transponders are interfering with each others transmissions, for the case where the transponder only broadcasts its fixed identity and where the system can accept that messages can be discarded as they will be rebroadcast; is to arrange for the transmissions to occur with a varying quiet interval between the transmissions from each transponder. If the pattern of variation of interval is different for each transponder (say based on an independently seeded random number algorithm), then at times interfering transmissions will "unlock" from each other and both will be heard. This method is limited in the number of transponders that can be handled, due to length of transmission, clock rates and error rates of the system (transponder's whose identity is always garbled for the reader for the duration of the reading cycle).
A system with increased capacity requires two way communication between the reader and the transponders. Here the transponders need to send their identity with varying inter message spacing, and the reader needs to acknowledge receipt so that the transponder will become quiet once it has been identified to provide capacity for those transponders that have yet to be read. This method is generally achieved by providing all transponders with their own receivers and communicating on an acknowledge communications channel the identity of the transponder just received. The transponder receives this specifically identified acknowledgement message and switches off its random message broadcast.
This patent provides a simple protocol for providing the benefits of the
two way communications system without the additional costs.
The patent introduces time as a critical parameter. The patent claims a protocol whereby a transponder broadcasts its identity on a common communications channel when energised in a reading field. A fixed time after completing the broadcasting of the identity, the transponder checks a common receiving channel (which can even be the energising field) for a common acknowledgement event that is addressed to all transponders, and should such an acknowledgement be received then it disables further communications by that specific transponder for the remainder of its duration in the energising beam.
This system operates on the principle that the data broadcast has properties such as datalength, headers and a checksum that can be used to verify that a received transmission is not being contaminated by a second simultaneous transmission from another transponder, and that if the reader does send the acknowledgement event, then only one transponder could have been broadcasting to pass all the criterea, and that transponder must have been the transponder that is listening for the acknowledgement. Tranponders only listen for the acknowledgement a fixed time after their transmission which means that despite the acknowledgement being a common signal that all transponders could receive, only the one transponder is in the correct phase of its cycle to interpret the acknowledgement.
This protocol could be explained by means of a practical example. A system could use the energising field for the acknowledge communications channel. The reader energises the transponders in its energy field. The transponders broadcast their identities using their onboard clock for timing purposes, at randomly spaced times. After each broadcast they monitor for a variation of the energising field between 3 and 5 clock cycles after their broadcast ended. If the energising field is removed three cycles after the end of their transmission, and returns on the fifth cycle, they understand that their transmission was received and they cease to broadcast further messages until they have been reenergised by entering another reader energising field. The transponders operate on an internal capacitor for the short period when the energising field has been removed for acknowledge purposes.
The features resulting from these developments are:Synchronised electronic identification system