Nowadays, passive RFID tag is used everywhere. However, in order to work, it either requires a high power reader to provide power to the tag or requires a short distance of communication. So there comes active RFID tag which contains a small power source and some active circuitry to extend the range or provide extra functionality. Although active tags would have less longevity than their passive counterparts, yet they can achieve high sensitivity and larger communication ranges which release reader from delivering large power. Besides, the increased functionality can enable some new application. One possible example is a remote sensor that can be queried, not by a dedicated reader, but instead with a general-purpose wireless device like a mobile phone.
Our low power active RFID tag is based on the idea of asynchronous wake-up by adding an auxiliary receiver called a wake-up receiver (WuRx) to every tag. WuRx takes the place of timer in traditional duty-cycle based methods, continuously monitoring the channel for communication requests or wake-up signals. The use of WuRx breaks the trade-off between latency and average power assumption. Our WuRx consume less than 100uW and it makes tag respond immediately to requests, thus latency is effectively eliminated. The baseband processing is mainly based on ISO Active RFID protocol, using packet format, random-number based collision arbitration and following the entire command format. For clock recovery, digital synchronization method is used to simplify the analog circuitry. ADC oversamples the signal at 8 times to meet the resolution requirement. Also additional preamble is added every packet for amplitude and timing estimation.
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