SX1276IMLTRT View Datasheet(PDF) - Semtech Corporation

Part Name

Description

Manufacturer

SX1276IMLTRT

137 MHz to 1020 MHz Low Power Long Range Transceiver

Semtech Corporation 

SX1276/77/78/79

WIRELESS, SENSING & TIMING

DATASHEET

4.1.1.8. Frequency Hopping with LoRaTM

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 LoRaTM 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 LoRaTM 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 = 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.

Timing of Channel Updates

The interrupt requesting the channel change, FhssChangeChannel, is generated upon transition to the new frequency. The

frequency hopping process is illustrated in the diagram below:

Rev. 4 - March 2015

©2015 Semtech Corporation

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