RFM92 Datasheet, PDF (29/126 Pages) List of Unclassifed Manufacturers – Low Power Long Range Transceiver Module

RFM92W/93W V3.0
Addition of the preamble and payload durations gives the total packet time on air.
T_packet = T_preamble + T_payload
4.1.1.8. Frequency Hopping with LoRa^TM
Frequency hopping spread spectrum (FHSS) is typically employed when the duration of a single packet could exceed
regulatory requirements relating to the maximum permissible channel dwell time. This is most notably the case in US
operation where the 902 to 928 MHz ISM band which makes provision for frequency hopping operation. To ease the
implementation of FHSS systems the frequency hopping mode of the LoRa^TM modem can be enabled by setting
FreqHoppingPeriod to a non-zero value in register RegHopPeriod.
Principle of Operation
The principle behind the FHSS scheme is that a portion of each LoRa^TM packet is transmitted on each hopping channel
from a look up table of frequencies managed by the host microcontroller. After a predetermined hopping period the
transmitter and receiver change to the next channel in a predefined list of hopping frequencies to continue transmission
and reception of the next portion of the packet. The time which the transmission will dwell in any given channel is
determined by FreqHoppingPeriod which is an integer multiple of symbol periods:
HoppingPeriod[s] = Ts × FreqHoppingPeriod
The frequency hopping transmission and reception process starts at channel 0. The preamble and header are transmitted
first on channel 0. At the beginning of each transmission the channel counter FhssPresentChannel (located in the register
RegHopChannel) is incremented and the interrupt signal FhssChangeChannel is generated. The new frequency must then
be programmed within the hopping period to ensure it is taken into account for the next hop, the interrupt
ChangeChannelFhss is then to be cleared by writing a logical ‘1’.
FHSS Reception always starts on channel 0. The receiver waits for a valid preamble detection before starting the
frequency hopping process as described above. Note that in the eventuality of header CRC corruption, the receiver will
automatically request channel 0 and recommence the valid preamble detection process.
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