However, when a reader is operating at the maximum radiated power, which is 2 W
effective radiated power (ERP), only 10 sub-bands are available, while the remaining 5 are
used as guard bands or for lower power readers. ETSI 302 208 also introduces the concept
of ??????Listen Before Talk.??™??™ An extract from the ETSI 302 208 best describes the essence of
??????Listen Before Talk.??™??™ It says ??????Prior to each transmission, the receiver in the interrogator
shall ?¬?rst monitor in accordance with the de?¬?ned listen time for the presence of another
signal within its intended sub-band of transmission. The listen time shall comprise a ?¬?xed
period of 5 ms plus a random time of 0 ms to 5 ms in 11 steps. If the sub-band is free the
random time shall be set to 0 ms??™??™ (ETSI, 2006). The threshold to determine the presence of
another signal within the intended sub-band is shown in Table 7.1. The measurement
method is de?¬?ned in the same standard.
Furthermore, once a sub-band has been selected, the RFID reader is permitted to use that
sub-band for up to 4 s. After use, it must free the sub-band for at least 100 ms. A reader can
however, listen to another sub-band for 5 ms and if free use that new sub-band.
7.2.2 FCC
The U.S. FCC Title 47 Part 15.247, with operation within the band 902??“928 MHz, uses the
technique of frequency hopping spread spectrum (FHSS).
Pages:
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255