7. PIE is used for the data transmission
and FM0 is used for backscattering the response, through absorption or re?¬‚ection of the
transmitted RF energy.
The physical layer characteristics of PIE are shown in Figure 3.7a. Encodings for both
1 and 0 are based on an active high pulse followed by a ?¬?xed width space called PW. The
length of the pulse determines whether a 1 or 0 is encoded. Thus, the period T is different
for each value. Unlike previous encodings, there is a large amount of ?¬‚exibility in the pulse
lengths to make a valid PIE-encoded value.
For PIE encoding, in the textual representation shown in Figure 3.8 we introduce the
error keyword, which allows a transition to take place within a range of times. For
example, 7.5 us error 1.6 us means that the transition could occur anywhere from
5.9 to 9.1 ms. This is different from the A which corresponds to jitter associated with the RF
transmission. For the PIE encoding described in the ISO 18000 Part 6C standard, there are
three possible periods for encoding the values, each described in a separate statement in
Bit 0
Bit 1
50%
50%
T = 36 ?µs
50%
50%
T = 36 ?µs
(a)
Bit 0
50%
T = 36 ?µs
50%
Bit 1
50%
T = 36 ?µs
50%
or
or
(b)
FIGURE 3.4
Continuous waveform for bits 0 and 1 of NRZ encodings. (a) Manchester encoding. (b) Differential Manchester
encoding.
??™0??™: Sig????™1??™ after 0 us & ??™0??™ after 18 us;
T??36 us; A??12.
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