FHSS, frequency hopping spread spectrum(FHSS) technology, under the condition of synchronization and simultaneous,receives signals transmitted by specific types of narrow-band carriers at bothends. For a non-specific receiver, the hopping signal generated by FHSS is onlypulse noise.
When a single packet time may exceed themaximum channel dwell time allowed by relevant regulations, FHSS technologywill be used. To turn on the frequency hopping mode in Lora, you need to setthe reghopperiod and freqhoppingperiod registers to non-zero values.
The principle of FHSS frequency hoppingspread spectrum scheme is as follows: part of the contents of each Lora packetare transmitted through the frequency hopping channel set by MCU management,i.e. the required "hopping" frequency (according to the frequencyquery table). After the scheduled frequency hopping cycle is completed, thetransmitter and receiver switch to the next channel of the predefined list offrequency hopping so as to continue transmission And accept the next part ofthe package. The dwell time in any channel is determined by freqhoppingperiod,which is an integral multiple of the symbol length.
The process of transmitting and receivingby frequency hopping starts from channel 0.. So the preamble and header will besent in channel 0 first.. Each time a packet is sent, the reading of thechannel counter fhsspresentchannel (located in reghopchannel) will increase,and an interrupt signal fhsschangechannel will be generated to realizefrequency hopping. The new frequency must be set in the frequency hopping cycleto ensure that the new frequency will be covered in the next frequency hopping.
FHSS usually starts from channel 0. Afterchecking the preamble, the receiver will start the above frequency hoppingprocess. At this time, if the CRC of the header is not correct, the receiverwill automatically request channel 0 (packet retransmission mechanism) andrestart the check preamble.
2. Channel update time
After switching to the new frequency, fhsschangechannelinterrupt will be generated, that is, the interrupt of changing channel. Asshown in the figure below, the interrupt signal generated when the frequencyhopping is successful: