A Comparative Study of mioty vs. LoRa-FHSS

LoRa-FHSS (Frequency Hopping Spread Spectrum) is a new member of the LoRa family. Unlike the name implies, the LoRa-FHSS physical layer shares no commonalities with classical LoRa. Instead, it shares many commonalities with mioty. LoRa-FHSS also uses a frequency hopping modulation, but without any pause between the different hops. Due to the frequency hopping, LoRa-FHSS offers a significantly increased capacity and higher robustness in densely used license-exempt frequency bands. However, LoRa-FHSS is not compatible with the existing LoRa gateways.

As LoRa-FHSS is only focusing on the uplink, the downlink still uses the classical LoRa modulation. LoRa-FHSS uses an ultra narrow-band Frequency Shift Keying modulation and a new Forward Error Correction. Hence, new gateways are normally required. Though, according to Semtech, some existing gateways are able to support LoRa-FHSS after a software update. However, to the author’s best knowledge it is not possible to purchase any commercial LoRa-FHSS gateway as of March 2024.

Generally, it offers a significantly higher network capacity compared to classical LoRa. The capacity increase is more than a factor of 100 compared to classical LoRa with maximum robustness (SF12) in a comparable channel bandwidth. Nevertheless, the energy consumption is approximately 40% higher than the power consumption of classical LoRa SF12, and up to six times higher than mioty. At the same time, its sensitivity is 3 dB worse than the sensitivity of classical LoRa SF12 or mioty.

The main issues are the selected modulation and the header. The modulation bases on Gaussian Minimum Shift Keying, which normally offers an operation close to the theoretical limits, if used in addition to coherent demodulation. However, LoRa-FHSS does not use any reference signals or pre-coding, which make the use of coherent demodulation practically impossible. This leads to a loss of approximately 6 dB compared to coherent demodulation, as e.g. used by mioty. LoRa-FHSS compensates this sensitivity loss by means of a very lowsymbol rate, which negatively affects the achievable capacity aswell as the power consumption. This results in a five times longer battery lifetime for mioty, while mioty in addition offers a sensitivity benefit of 3 dB.

The header is a further issue. The receiver has to be able to correctly receive at least one of the transmitted header bursts. Each header burst is 233ms long, which makes it a huge target for interference in license-exempt frequency bands. Further, the header bursts use a relatively weak Forward Error Correction, which makes LoRa-FHSS especially vulnerable against classical LoRa signals. Additionally, LoRa-FHSS transmits – depending on the robustness mode – two or three header bursts. This results in an on-air time of 466ms or 700ms, without the transmission of any payload data, while a complete mioty packet including 10 byte of payload data only requires 340ms.

The long on-air time leads to very few packets per hour in channels with duty cycle limitations, e.g. in Europe. In channels with 0.1% duty cycle and using a typical configuration, only a single packet can be transmitted per hour. In the US, LoRa-FHSS fulfills all frequency regulations according to frequency hopping systems, without any special restrictions on the maximumtransmit power of duty cycle.

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