LoRa 915MHz Transceiver Packet Test


LoRa ParameterRegion/Country Specific
900MHz ISM Freq Band
North America (902-928MHz)
Europe (863-870MHz)
R Data RateUS(902-928): 980bps – 21900bps
EU(863-870): 250bps – 11000bps
BandwidthUS(902-928): 125KHz, 500KHz
EU(863-870): 125KHz, 250KHz
Tx PowerUS(902-928): 2dBm – 28dBm
EU(863-870): 2dBm – 16dBm EIRP
SF Spreading Factor7,8,…..12
Number Symbols 2^SF128(2^7),……4096(2*12)
Fig.1 LoRa Specifications (LoRa Alliance)

LoRa (Long Range) is a very popular protocol for long range communication between low powered battery operated devices. It uses Chirp Spread Spectrum. In the last post I examined the protocol by building the LoRa Chirp Symbols in Scicos (Ref.1). Spread Spectrum comes in various formats, such as DS_SS & FH_SS (Ref.2). In a previous post I examined DS_SS as used in GPS (Ref.3). I always find it intriguing to study telecom systems that are simple, low powered and can communicate over long distances. Apparently the record for LoRa is 832Km used in a helium balloon (Ref.4)! Figure 1 reviews some of the basic LoRa parameters.

LoRa 915MHz Packet Test

Fig.2 Rpi3B LoRa 915MHz Transceiver (Ref.5)
Fig.3 Packet Transmission Test Layout
Fig.4 PiTop RPi3B Waiting for Packet
Fig.5 Rpi3B Waiting for Packet
Fig.6 Packet “Button A!” Received
Fig.7 Gqrx Chirp Bart Head

Figure 2 shows the LoRa 915MHz transceiver that I purchased (Ref.5). It fits nicely on the Raspberry Pi3B/4B GPIO. It comes with Python Libraries. These libraries work on top of Circuit Python. So setup involves updating the RPi OS to the latest version, installing Circuit Python (Blinka) and then installing the Python libraries required to use the module. The instructions are very thorough. Figure 4 shows successful installation on a PiTop RPi3B and Figure 5 shows successful installation on a RPi3B.

Python file ‘radio_rfm9x.py‘ sends a packet between the units. Pushing ButtonA sends “ButtonA!” packet, pushing ButtonB sends “ButtonB!” and finally ButtonC sends packet “ButtonC!”. The results print out on the opposite end OLED display. Figure 6 shows this operation. Figure 7 shows the flat top Chirp Spread Spectrum.

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#1. – “LoRa-Scicos Simulation_a”

#2. – “Spread Spectrum Systems”, R.C. Dixon,
John Wiley & Sons, 1976, ISBN 0-471-21629-1

#3. – “RTL-SDR for Satellite GPS”

#4. – “LoRaWAN”, The Things Network

#5. – “LoRa 915MHz Transceiver”

By Jeremy Clark

Jeremy Clark is a Senior Telecommunications Engineer and Advanced Amateur Radio Operator VE3PKC. He is the author of E-Books on Telecommunications, Navigation & Electronics.