XRAG2

Tag-to-reader protocol

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6

Tag-to-reader protocol

During answer frames, the tag backscatters data in accordance to the encoding format and

data rate chosen by the reader during the Query command starting the inventory round. The

tag backscatters data to the reader by modulating its antenna reflection coefficient.

6.1

Tag-to-reader data encoding

As specified in the EPCglobal Class 1 generation 2 RFID UHF specification, revision 1.0.9,

the tag encodes the backscattered data as either FM0 base band (biphase space) or Miller

modulation of a subcarrier at the data rate requested by the reader.

High values represented on Figure 10, Figure 11, Figure 13, Figure 14, Figure 15, and

Figure 16 correspond to the tag antenna’s reflecting power.

6.1.1

Tag-to-reader FM0 encoding

As specified in the EPCglobal Class 1 generation 2 RFID UHF specification, revision 1.0.9,

Tag-to-reader FM0 modulation is chosen by the reader by setting the Subcarrier Number

parameter (M) to 1 in the query command starting the inventory round.

Figure 10.

FM0 symbols

Tag-to-reader link frequency is defined in Section 5.4: Reader-to-tag communication start

and calibration.

6.1.2

Tag-to-reader FM0 preamble

As defined in the EPCglobal Class 1 generation 2 RFID UHF specification, revision 1.0.9,

the tag can start FM0 backscattering using a 12 0's pilot tone, depending on the value of the

TRext parameter sent during the Query command that initiates the inventory round.

Figure 11 and Figure 12 show the two possible FM0 modulation answer preamble formats

according to the TRext parameter value.

Figure 11.

FM0 answer preamble without pilot tone (TRext=0).

1.

V = violation.

Tpri = 1/LF

data

0

1

data

0

1

Tpri = 1/LF

ai12314

1

ai12315

0

1