Communication system for communicating with tags

A communication system for efficiently identifying large numbers of items through transmission and reception of signals in a communication region, where the number of items is large and may exceed one hundred in the region, and where the location of the items in the region may not be known, said communication system comprising:

• a plurality of tags, one for each of said items,
• an interrogator for communicating with said tags,
• said tags and said interrogator communicating over a common communication channel with a communication protocol, said communication protocol including a sequence common to all tags, said sequence including a plurality P of collection periods CP(1), CP(2), . . . , CP(p), . . . , CP(P), each collection period for identifying different ones of the tags whereby all tags can be identified during said sequence, where one number of said tags are identified during one of said collection periods and where different numbers, less than said one number, of said tags are identified during different ones of said collection periods, each collection period having,
• a synchronization period during which said interrogator can communicate with said plurality of tags whereby respective synchronization periods SP(1), SP(2), . . . , SP(p), . . . , SP(P) are provided,
• a listen period during which each of said plurality of tags can communicate a tag identifying signal to said interrogator over said communication channel whereby respective listen periods LP(1), LP(2), . . . , LP(p), . . . , LP(P) are provided, and wherein each of said listen periods has a plurality of time slots such that,
• said listen period LP(1) has D1 time slots TS(1,1), TS(1,2), . . . , TS(1,d1), . . . , TS(1,D1),
• said listen period LP(2) has D2 time slots TS(2,1), TS(2,2), . . . , TS(2,d2), . . . , TS(2,D2),
• and so on, and
• said listen period LP(p) has Dp time slots TS(p,1), TS(p,2), . . . , TS(p,dp), . . . , TS(p,Dp),
• and so on, and
• said listen period LP(P) has DP time slots TS(P,1), TS(P,2), . . . , TS(P,dP), . . . , TS(P,DP),
• an acknowledge period during which said interrogator can communicate acknowledge signals to said tags over said communication channel whereby respective acknowledge periods AP(1), AP(2), . . . , AP(p), . . . , AP(P) are provided,
• said interrogator including,
• interrogator memory for storing interrogator code including one or more broadcast commands, one or more directed commands and duration control code for controlling the duration of said collection periods such that one or more of the listen periods LP(2), . . . , LP(p), . . . , LP(P) has fewer time slots than the listen period LP(1) whereby said one or more listen periods LP(2), . . . , LP(p), . . . , LP(P) are shorter than said listen period LP(1),
• interrogator processor means connected for accessing said interrogator memory and for processing said interrogator code and for sequencing said broadcast commands and said directed commands,
• signal sending means, operable for each of said collection periods, for sending during said synchronization period one of said broadcast commands over the common communication channel to the tags to signal the commencement of the listen period,
• receiver means, operable for each of said collection periods, for receiving tag identifying signals over the common communication channel from said tags during said listen period,
• acknowledge sending means, operable for each of said collection periods, for sending acknowledge signals as directed commands over the common communication channel to said tags during said acknowledge period,
• each of said tags including,
• signal receiving means, operable for one or more of said collection periods, for receiving said one of said broadcast commands from the signal sending means during said synchronization period so as to identify the start of the listen period,
• identification sending means, operable for one or more of said collection periods, for sending a tag identifying signal during only one of said time slots during the listen period,
• acknowledge receiving means, operable for one of said collection periods, for receiving an acknowledge signal from said interrogator during the acknowledge period,
• control means, operable for said one or more of said collection periods, for controlling said identification sending means,
• said control means operative, before said acknowledge receiving means receives said acknowledge signal and during each of said one or more of said collection periods, for selecting one of said time slots for sending a tag identifying signal whereby two or more of said plurality of tags may each send a respective tag identifying signal over said common communication channel during the same time slot such that during some time slots of listen periods collisions of tag identifying signals may occur and such that during other time slots of listen periods no collision of tag identifying signals occurs,
• said control means operative, after said acknowledge means receives said acknowledge signal, for inhibiting said identification sending means from sending said tag identifying signal.