Introduction
| LoRa Parameter | Region/Country Specific |
| 900MHz ISM Freq Band | North America (902-928MHz) Europe (863-870MHz) |
| R Data Rate | US(902-928): 980bps – 21900bps EU(863-870): 250bps – 11000bps |
| Bandwidth | US(902-928): 125KHz, 500KHz EU(863-870): 125KHz, 250KHz |
| Tx Power | US(902-928): 2dBm – 28dBm EU(863-870): 2dBm – 16dBm EIRP |
| SF Spreading Factor | 7,8,…..12 |
| Number Symbols 2^SF | 128(2^7),……4096(2*12) |
LoRa (Long Range) is a very popular protocol for long range communication between low powered battery operated devices (Ref.2). It is part of the new IoT Internet of things and uses Chirp Spread Spectrum. LoRa is a specification for the Physical Layer only (Ref.3), whereas LoRaWAN is a specification for higher layers (Ref.4). Figure 1 shows some of the basic parameters. Figure 2 shows a LoRa 915MHz Transceiver mounted on a RaspberryPi4.
Pulsed Chirp FM Systems
LoRa uses Spread Spectrum Chirp modulation. The number of symbols is equal to 2^SF. So for example SF=3, there are 8 unique spreading symbols representing 3bits (Ref.5). If we consider a VCO being driven over it’s bandwidth by a signal from -1 to 1, then each symbol is staggered over this interval. Figure 3 shows the symbols being created in ScicosLab and read into Scicos using a Structure V. Figure 4 shows the first Symbol S1 starting at -1 and going linearly to +1. The second Symbol starts at -0.75 and continues to 1.0, resets to -1 and continues back to -0.75.
Figure 5 shows the LoRa Transmitter which consists of a VCO being driven over it’s bandwidth by input structure V. Simple units are chosen for convenience, Fo=100Hz, Fmin=50Hz, Fmax=150Hz. Random data symbols are read in. Note the Tx Spectrum=VCO bandwidth of 100Hz. Figure 6 shows a Chirp Up, Figure 7 shows a Chirp Down, Figure 8 shows random data symbols, and Figure 9 shows the Transmit Spectrum = VCO Bandwidth.
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References
#1. – “LoRa Alliance”
https://resources.lora-alliance.org/technical-specifications/rp002-1-0-4-regional-parameters
#2. – “LoRa”
https://en.wikipedia.org/wiki/LoRa
#3. – “AN1200.22 LoRa Modulation Basics”
https://web.archive.org/web/20190718200516/https://www.semtech.com/uploads/documents/an1200.22.pdf
#4. – “The Things Network”
https://www.thethingsnetwork.org/docs/lorawan/
#5. – “How LoRa Modulation Works”, Visual Electric
https://www.youtube.com/watch?v=jHWepP1ZWTk
#6. – “Spread Spectrum Systems”, R.C. Dixon,
John Wiley & Sons, 1976, ISBN 0-471-21629-1
