Reader and Antenna Primer

Readers have different air interfaces depending on the tag and licensing requirement. Gen 2 readers modulate an RF carrier using double-sideband amplitude shift keying (DSB-ASK), single-sideband amplitude shift keying (SSB-ASK) or phase-reversal amplitude shift keying (PR-ASK) using a pulse-interval encoding (PIE) format. Tags receive their operating energy from this same modulated RF carrier. A reader receives information from a tag by transmitting an unmodulated RF carrier and listening for a backscattered reply.

Because many tags may be in the presence of a reader, they must be able to receive and manage many replies at once, potentially hundreds per second. Tag population management features are used to allow tags to be sorted and individually selected. A reader can tell some tags to wake up and others to go to sleep to suppress chatter. Once a tag is selected, the reader is able to perform a number of operations, such as reading the identification number, and writing information to the tag in some cases. The reader then proceeds through the list to gather information from all the tagsReaders serve as gateways between the physical world of tags on packages and the on-line world. Some readers appear as SNMP (simple network management protocol) devices or as internet web servers, with databases that respond to standard SQL (structured query language) commands like other data sources on a network. Multi-protocol or agile readers can identify virtually all classes of tags, or be set to recognize only certain types. Readers that conform to the Gen 2 specification will have variable read speed, and single, multi and dense modes to accommodate various operating environments.

For most applications, readers will operate in one of two ways, either autonomously or as directed/interactive devices. The air interface protocol is the same. They transmit signals in half-duplex, using frequency hopping across approximately 60 bands between 902-928 MHz (USA). Frequency hopping is an FCC requirement to minimize interference with other RF devices. Anti-collision algorithms, combined with a sequence of scroll, quiet and talk commands, are used to read and sort multiple simultaneous incoming tag signals.

Autonomous mode - A reader can be set to continuously operate, accumulating lists of tags in its memory. Tag lists represent a dynamic picture of the current tag population in its read window. See Figure 2.13. As tags respond to reader broadcasts, they are put on the list. If they don't respond they are dropped from the most recent list stored in memory list. A persist time is set to determine the duration between the time a tag was last read and when it is removed from the list. A host system on the network can receive a list of tags from the reader whenever it chooses to listen. The information available to the host would include the reader location, time read, the size of the tag list, and the IDs of the tags on the list.

Directed/interactive mode - Readers in this mode will respond to commands from the network host. The host can instruct the reader to gather a list of tags within its read window, or look for a specific tag. In both cases the reader starts by gathering a list. Once it completes the host command, the reader waits until it receives another.