Abstract: A data coding method for carrying more energy to the passive tag is presented for the RFID system. The basic idea of the coding is to set adjustable and asymmetric high and low level durations within a symbol cycle, and a longer duration for the high level. The performance analysis results show the energy of baseband data transmitted from reader to tags increases by 84％ at most, compared with that of Manchester coding at typical 40kb／s. The coding achieves the same BER performance of ASK system with non-coherent demodulation as Manchester coding. For the coding, a clock recovery scheme in which an IIR filter is combined with a PLL is proposed. The scheme maintains the accurateness of PLL, and especially for the feedback of the IIR filter, it can recover the clock quickly with long data dropouts. The scheme can withstand data dropouts of 10 baud, which is correlated with the parameters of the filter.

Key words: RFID(Radio Frequency Identify), data coding, timing extraction