FHSS is a method of transmitting
radio signals by rapidly switching a carrier among many frequency channels, using a
pseudorandom sequence known to both transmitter and receiver. In the context of FCC
Part 15.247 for a frequency hopping system operating in the 902??“928 MHz band: if the 20 dB
bandwidth of the hopping channel is <250 kHz, the system shall use at least 50 hopping
frequencies and the average time of occupancy on any frequency shall not be >0.4 s within
a 20 s period; if the 20 dB bandwidth of the hopping channel is 250 kHz or greater, the
system shall use at least 25 hopping frequencies and the average time of occupancy on any
frequency shall not be >0.4 s within a 10 s period. The maximum allowed 20 dB bandwidth
TABLE 7.1
Transmit and Threshold Power
ERP (W) ERP (dBW) Threshold (dBW)
Up to 0.1 Up to 10 113
0.1 to 0.5 10 to 3 120
0.5 to 2.0 3 to 3 126
Source: From Leong, K.S., Ng, M.L., and Cole, P.H. in IEEE 2005
International Symposium on Microwave, Antenna,
Propagation and EMC Technologies for Wireless
Communications, Beijing, China, 2005, 658??“661, # 2005
by IEEE. With permission.
RFID Reader Synchronization 127
of the hopping channel is 500 kHz. For an RFID system deployed in the United States
following EPCglobal RFID protocol (EPCglobal, 2004), which is discussed in detail in the
next part, a total of 50 channels are available, each with 500 kHz bandwidth.
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