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Reading Tags in Space
Texas State University engineering students are designing an RFID system that could read multiple tags simultaneously.
The team considered frequency division multiplexing, an RF coding scheme in which numerous signals are combined for transmission on a single communications line, but quickly found the system's narrow bandwidth would limit the number of RFID tags that could be read. "The clear option was code division multiple access," Doany says.
A CDMA-based system would not be better in every application, but it was ideal to meet NASA's needs because it requires less power and works well in noisy environments. Passive ultrahigh-frequency readers use time division multiplexing, which requires sending signals at specific intervals.
"Since a standard passive UHF reader doesn't know how many tags it is going to read, once it signals for all the tags in the area to generate a response delay, it transmits a signal and waits for a tag to respond at each possible delay time slot, whether there is a tag at that slot or not," Doany says. "If the reader is transmitting without any response, this is a huge waste of information and power. Additionally, if there are collisions, this process may repeat several times. In a CDMA-based system, each beckoning transmission will yield information from the tags within range of the reader, even in the presence of high amounts of noise."
While the researchers test the simulation with larger numbers of tags and improve the design, they are looking for an RFID manufacturer to assist them in obtaining the necessary tools to alter the tags, so they can test CDMA-based RFID in the real world. "There have been discussions about future teams expanding this project to design a versatile RFID system that can switch between current and CDMA protocols, and even work as a tag-locating system," Doany says. "We hope to involve future senior design teams and/or graduate students on the expansion of this project."
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