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8.6 LR-FHSS Modulation
LR-FHSS is a frequency hopping spread spectrum modulation, perfectly suited for situations where long-range
communication is required for a larger number of devices in the same area. LR-FHSS can nearly equate the link-budget of
SF12 BW125kHz of LoRa, allowing many devices to coexist on the same spectral allocation. LR-FHSS implementation in the
LR1121 is transmit only. The LR1121 fully controls the following functionality:
•
frame construction including preamble, sync headers, payload, integrity check, whitening, interleaving and forward
error correction calculations
•
calculation of hopping sequences
•
handling of the whole frequency hopping mechanism
8.6.1 LR-FHSS Modulation Principle
The LR-FHSS is a low data rate modulation derived from a GMSK modulation, with intra-packet frequency hopping
sequences configurable for each device. Only 488.28215Hz data rate is currently supported. The payload uses convolution
encoding to perform forward error detection and correction, implying a transmission overhead.
Spectrally speaking, the LR-FHSS packet is split into several frames, sent over pseudo-randomly distributed frequencies
spaced by a configurable step size (Grid). The total spectral occupation of the LR-FHSS packet corresponds to the
Bandwidth parameter. LR-FHSS modulation is represented in
Each payload frame, except the last frame, contains exactly 50 encoded bits, and are then of equal duration in time. The last
frame may contain less than 50 bits. The number of frames varies therefore with the payload size and the Coding Rate.
The Payload frames are preceded with one to four synchronization sequences (Headers) to allow robust frequency and
timing synchronization. Even if receiving a single Header is sufficient to be able to decode the payload, a minimum of 2
Headers improves immunity against radio interference. If multiple headers are used, they are sent on different frequencies.
Each header contains exactly 114 encoded bits, and are then of equal duration in time.
Additional information is provided in AN1200.58 Long Range FHSS Demo and AN1200.64 Long Range FHSS System
Performance.
LR1121
User Manual
Rev 1.1
UM.LR1121.W.APP
Mar 2023
Figure 8-6: LR-FHSS Spectral Plot
Figure 8-6: LR-FHSS Spectral Plot Example
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Example.
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