Duty Cycle Impact: LoRaWAN vs Sigfox vs NB-IoT
Compare maximum message frequency for a water meter sending 20-byte readings:
LoRaWAN (EU868, 1% duty cycle on 125 kHz channels): \[
\text{Airtime (SF12)} = 1.318 \text{ s per message}
\] \[
\text{Max airtime per hour} = 3600 \text{ s} \times 0.01 = 36 \text{ s}
\] \[
\text{Max messages per hour} = \frac{36}{1.318} = 27.3 \Rightarrow 27 \text{ messages/hour}
\] \[
\text{Min interval} = \frac{3600}{27} = 133 \text{ seconds} \approx 2.2 \text{ minutes}
\]
Sigfox (140 uplinks/day limit, 12-byte max payload): \[
\text{Messages per hour (spread evenly)} = \frac{140}{24} = 5.83 \text{ messages/hour}
\] \[
\text{Min interval} = \frac{3600}{5.83} = 617 \text{ seconds} \approx 10.3 \text{ minutes}
\]
NB-IoT (no duty cycle limit, licensed spectrum): \[
\text{Messages per hour} = \text{unlimited (constrained by battery/bandwidth only)}
\]
Battery life impact (coin cell CR2032, 220 mAh):
Assuming 50 mA TX current, 5 μA sleep: \[
\text{Energy per TX (LoRa SF12)} = 50 \text{ mA} \times 1.318 \text{ s} = 65.9 \text{ mAs}
\] \[
\text{Monthly TX (1/hour)} = 720 \times 65.9 = 47,448 \text{ mAs} = 13.2 \text{ mAh}
\] \[
\text{Battery life (LoRa)} \approx \frac{220}{13.2} = 16.7 \text{ months}
\]
For Sigfox (6× fewer messages): ~100 months (~8 years) For NB-IoT (1/hour): ~12 months (higher power per TX)
