LORA
LORA¶
- Long Range of up to 13.6 miles, 1300km
- Uses 902-928 MHz in US
- Uses 863-870 MHz in Europe
- Uses 433 MHz in Asia
- Between 0.3 kbps to 5.5 kbps.
LoRa: Physical Layer
LoRaWAN: MAC, Network, and app built on LoRa.
Building a private LoRa network
GNU Radio implementation of a LoRa transceiver
RF¶
Uses Modulation in the form of Chirp Spread Spectrum. This means that the frequency will either grow or decrease at a constant rate with a wrap around.
- Upchirp is for increasing frequency
- Downchirp is for decreasing frequency
Resilient against multi-path and Doppler effects. This is also used in radar.
Demodulation¶
- 2.25 Downchirps seems to be the Start of Frame Delimiter (SFD)
- Then the data is encoded using modulation using Frequency modulation
Bandwidth: The Spectrum occupied by the chirp.
- uses 125 kHz, 250 kHz, or 500 kHz
Spreading Factor: Number of bits encoded in each state
- Uses 7-12 bits per symbol
Chirp Rate: The rate at which it changes in frequency because of the chirp
- is bandwidth/(2^spreadingFactor)
Dechirping¶
When adding and upchirp and downchirp together you get a constant frequency if there is no data.
When there is data using a FFT with the number of bins for the spreading factor allows the highest bin to be the correct state.
Finding the Preamble¶
After dechirping you have two signals the upchirped and downchirped. Finding the constant bin for some time in the upchirp. then finding the same for the downchirp.
Encoding¶
- Adds Error tolerance
- Induces randomness
- Scrambles bits with in the frame
- Adds correcting parity bits
Whitening:
- XOR pseudo random sequence
- Make it better for synchronization
- Interleaves the first 2 bits¶
Packet structure¶
Implicit mode: knows the length of the data being transmitted and does not include the header
Explicit mode: has a header that has information about the length of the packet.
There are different whitening on each of the modes
- First 8 symbols
- Spreading factor is two less then the actual packet spreading factor.
- Code rate is 4/8
LORAWan¶
Internet protocols over LORA
Meshtastic¶
An open source, off-grid, decentralized, mesh network built to run on affordable, low-power devices