LoRaWAN : Doppler,Interference,Multipath fading

Doppler Effect

• It occurs when there is relative motion between transmitter and receiver.
• This causes a frequency shift in the received signal.
• In LoRaWAN, this is relevant for devices that move such as sensors on vehicles and drones etc.

How LoRaWAN withstands against doppler effect: It uses Chirp Spread Spectrum (CSS) modulation; which is inherently tolerant to Doppler shifts, since chirps sweep across frequency ranges. LoRaWAN remains reliable under low to moderate mobility such as walking speed, vehicles with speeds greater than 100 km/h. At very high speeds such as trains; error rates can increase, but it’s resilience is still better than many narrowband systems.

Wireless Interference

• Interference occurs when multiple transmitters operate on the same frequency and overlap in time.
• In unlicensed ISM bands, LoRaWAN must coexist with other wireless devices such as Wi-Fi, Bluetooth etc.

How LoRAWAN withstands against interference :

• It uses orthogonality of Spreading Factors. Different SFs can coexist on the same channel with minimal interference.

• It uses redundancy supported by multiple gateways. Hence if one gateway misses a packet due to interference, another may still capture it.

• LoRaWAN MAC layer supports ALOHA-based access. Hence there is no central scheduling, but random transmission reduces repeated collisions.

• LoRaWAN PHY layer supports Forward error correction (FEC) technique. It includes coding rates of 4/5 to 4/8, which allows recovery from partial interference.

Multipath Fading

Multipath occurs when signals bounce off buildings, terrain or objects, creating multiple delayed copies of the signal at the receiver. These can cause constructive or destructive interference known as fading.

How LoRAWAN withstands against fading :

• Use CSS modulation and its robustness against multipath.
• Can use link budget better than 150-170 dB to overcome fading.
• Can leverage divesity concept by installing multiple gateways.

Conclusion: LoRaWAN’s CSS modulation and network redundancy make it much more resistant to Doppler shifts, interference and multipath fading compared to many traditional narrowband systems.