Parallel RFID system using CDMA
Various embodiments of the invention may use code division multiple access (CDMA) technology in communications from radio frequency identification (RFID) tags to RFID readers, so that multiple RFID tags may respond to the same RFID reader at the same time on the same frequency and still be reliably decoded by the RFID reader. In some embodiments, orthogonal CDMA PN sequences for the RFID tags to use in creating their responses may first be transmitted to those RFID tags, possibly using a non-CDMA technique.
1. A radio frequency identification (RFID) reader device:
select a CDMA group of two or more RFID tags based on addresses of the RFID tags;
transmit a non-CDMA enabling signal to the RFID tags of the selected CDMA group by including within the enabling signal an ID number of each RFID tag of the selected CDMA group and a different orthogonal PN sequence associated with each ID number;
concurrently receive response signals from the RFID tags of the selected CDMA group, each response signal comprising a preamble for synchronization and data encoded with one of the PN sequences; and
synchronize with and decode the concurrently received response signals to reproduce the data transmitted by each of the RFID tags of the selected CDMA group,
wherein RFID tags receiving the enabling signal are configured to determine from the ID numbers within the enabling signal whether or not to respond,
wherein based on the ID numbers within the enabling signal, the RFID tags within the selected CDMA group receiving the enabling signal are configured to store the PN code associated with their ID number and concurrently respond to the enabling signal by transmitting a response signal that include the preamble for synchronization and data encoded with the PN sequence associated with their ID number,
wherein based on the ID numbers within the enabling signal, RFID tags not within the selected CDMA group receiving the enabling signal are configured to refrain from storing a PN code and refrain from responding to the enabling signal, and
wherein the enabling signal is configured to be of sufficient strength to power circuitry of an RFID tag to:
receive the enabling signal,
determine from the ID numbers within the enabling signal whether or not to respond;
to store the PN code when determined to part of the CDMA group; and
transmit the response signal when determined to part of the CDMA group.
2. The RFID tag reader of claim 1 , further including a dipole antenna to receive the response signals.