What is LoRaWAN LR-FHSS modulation?
With the motivation to overcome the connectivity gaps in remote areas, LoRa Alliance has introduced LR-FHSS data rates into the LoRaWAN protocol in the last quarter of 2020. The new data rates exploit the frequency hopping and offer high robustness against the co-channel interference through increasing the number of physical channels with redundant physical headers. Note that the novel LR-FHSS is introduced to reduce the collisions exploiting a large number of channels, frequency hoping, lower coding rate and higher header redundancy.
For the uplink communication in the EU region, an LR-FHSS based end-device initiate the transmission sending N copies of the 0.233 ms long header on different carrier frequencies. Next, it breaks the L bytes physical payload into several fragments not exceeding 50 ms (RP2-1.0.2 LoRaWAN® Regional Parameters) or 102.4 ms (RP2-1.0.3 LoRaWAN® Regional Parameters) and randomly selects a sub-channel for each transmission by using a Pseudorandom number generator (PRNG).
Source: M. Asad Ullah, K. Mikhaylov and H. Alves, “Analysis and Simulation of LoRaWAN LR-FHSS for Direct-to-Satellite Scenario,” in IEEE Wireless Communications Letters, doi: 10.1109/LWC.2021.3135984.
Specifically, LR-FHSS uses the following mechanisms:
- Redundancy and coding: Unlike LoRa, the LR-FHSS device transmits several replicas of headers (N = 1, . . . , 4), where the number of replication is defined by the date rate setting. To successfully decode a packet, a gateway should receive at least one of the N transmitted headers. For the payload data fragments, DR8/DR10 and DR9/DR11 imply coding with the rate equivalent to CR = 1/3 and CR = 2/3 which improves the the gateway’s ability to correctly demodulate the radio packets due to lower coding rate.
- Frequency hopping: For the uplink communication in the EU region, an LR-FHSS based end-device sends each packet fragment (i.e., the header or a data fragment) on another randomly-picked frequency channel. In a single Operating Channel Width (OCW) channel, LR-FHSS specifications define 280 and 688 Occupied Band Width (OBW) physical carries (with a bandwidth of 488 Hz) for DR8/DR9 and DR10/DR11, respectively. On contrary, LoRa modulation carry a complete transmission in a single channel which causes higher co-channel interference probability.
Due to all these promising features, LR-FHSS is a prominent technology enabling mMTC-over-satellite network to fill the connectivity gaps in hard-to-reach areas, which lacks the terrestrial networks infrastructure e.g., offshore wind farms, and vessels monitoring in Arctic.