Coding and Modulation - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 181

UnipolarRZcode A binary 1 is represented by a 'high' signal during the first half
bit period, a binary 0 is represented by a 'low' signal lasting for the entire duration of
the bit.
DBP code A binary 0 is coded by a transition of either type in the half bit period,
a binary 1 is coded by the lack of a transition. Furthermore, the level is inverted at the
start of every bit period, so that the bit pulse can be more easily reconstructed in the
receiver (if necessary).
Miller code A binary 1 is represented by a transition of either type in the half bit
period, a binary 0 is represented by the continuance of the 1 level over the next bit
period. A sequence of zeros creates a transition at the start of a bit period, so that the
bit pulse can be more easily reconstructed in the receiver (if necessary).
ModifiedMillercode In this variant of the Miller code each transition is replaced
by a 'negative' pulse. The modified Miller code is highly suitable for use in inductively
coupled RFID systems for data transfer from the reader to the transponder.
Due to the very short pulse durations ( t pulse T bit ) it is possible to ensure a con-
tinuous power supply to the transponder from the HF field of the reader even during
data transfer.
Differentialcoding In 'differential coding' every binary 1 to be transmitted causes
a change (toggle) in the signal level, whereas the signal level remains unchanged for
a binary zero. Differential coding can be generated very simply from an NRZ signal
by using an XOR gate and a D flip-flop. Figure 6.3 shows a circuit to achieve this.
Pulse-pause coding In pulse-pause coding (PPC) a binary 1 is represented by
a pause of duration t before the next pulse; a binary 0 is represented by a pause of
duration 2 t before the next pulse (Figure 6.4). This coding procedure is popular in
inductively coupled RFID systems for data transfer from the reader to the transponder.
Due to the very short pulse durations ( t pulse T bit ) it is possible to ensure a contin-
uous power supply to the transponder from the HF field of the reader even during
data transfer.

Data in
(NRZ)

Data out
(differential)

XOR

DQ

Clock

Figure 6.3 Generating differential coding from NRZ coding