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Communication Principle of LoRa FHSS (Frequency-Hopping Spread Spectrum)

Views: 846     Author: James Wu     Publish Time: 2020-10-14      Origin: Site

By using Frequency-Hopping Spread Spectrum (FHSS) technology, wireless module receives a specific type of narrow-band carrier to transmit signals at both ends (transmitter & receiver) synchronously and simultaneously. For an unspecified receiver, the jitter signal generated by FHSS is only impulse noise.

FHSS technology is used when a single packet time may exceed the maximum channel dwell time allowed by relevant regulations. Turn on hopping mode in LoRa by operating RegHopPeriod and FreqHoppingPeriod registers with non-zero values.

1. Principle

The FHSS (Frequency Hopping Spread Spectrum) Scheme works by switching part of each LoRa packet to the next channel in the predefined list of frequency hopping to continue sending when the desired frequency of "hopping" is sent (according to the frequency query table) through a frequency hopping channel managed by the MCU, and then receive the next part of the packet. The residence time in any channel is determined by the FreqHoppingPeriod, which is an integer multiple of the symbol length.

The process of sending and receiving through frequency hopping starts with channel 0. Therefore, the preamble and header parts are first sent on channel 0. Each time a packet is sent, the channel counter FhssPresentChannel (located in RegHopChannel) increases in reading and produces an interrupt signal FhssChangeChannel for frequency hopping. A new frequency must be set during the frequency hopping cycle to ensure that the new frequency will be overwritten the next time the frequency is hopped.

FHSS reception usually starts from channel 0. After Check completes the preamble, the receiver will start the above frequency hopping process. At this time, if the CRC of Header is incorrect, the receiver will automatically request channel 0 (packet retransmit mechanism) and restart the Check preamble.

2. Channel Refresh Time

When it goes to a new frequency, an FhssChangeChannel interrupt occurs, which alters the channel interrupt. The following figure shows the interrupt signal when the frequency hopping is successful

LoRa FHSS Principle

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