I decided to do some initial RSSI testing at various locations inside my house and outside in the backyard to see how much loss I was getting due to the structure of my house. This is to get a baseline for further outdoor testing.
There are two components that determine the quality of the received radio signal, RSSI and SNR. The LoRa Gateway measures these values for each transmission that it receives.
The Received Signal Strength Indication (RSSI) is the received signal power in milliwatts and is measured in dBm. LoRa typically operates with RSSI values between -30 dBm and -120 dBm. RSSI = -30 dBm is a very strong signal and -120 dBm is a very weak signal.
The Signal-to-Noise Ratio (SNR) is the ratio (difference) between the received signal power and the noise floor power level. Typical LoRa SNR values are between -20 dB and +10 dB. A value closer to +10 dB means that the received signal is less corrupted. LoRa can actually demodulate signals that are -7.5 dB to -20 dB below the noise floor.
RF Noise Sources in my house
- WiFi access points and WiFi and Bluetooth devices (2.4 GHz and 5 GHz)
- Z-Wave hub and a couple of Z-Wave devices and the Electric Smart Meter used by the local power Utility (~900 MHz)
- X-10 RF links, garage door opener remotes, and car remotes (~300 MHz)
- Cordless phones (1.9 GHz, DECT 6)
- Cellular (1.9 GHz - AT&T)
- Random stuff like the microwave oven and the LED lights (wide spectrum EMI)
Amazing that anything wireless can work in all that noise! Of course, my neighbors have similar stuff. Their WiFi access points have reasonable signal strength at my location. One day I should do a survey of the noise magnitudes at different frequencies. It would be interesting to know the power level of the devices that are close to the 915 MHz LoRa band.
Gateway Data from TTN Console
Here is a sample data point:
The data of interest:
- Frequency: 905.3 MHz which corresponds to Channel 7; the gateway continuously switches channels.
- Data rate: To increase range and data reliability the gateway can trade off data rate and bandwidth (see spec below). Spreading factor relates to the data rate. The higher the spreading factor, the lower the data rate but the required RSSI is lower so the relative range is increased. SF7BW125 indicates Spreading Factor of 7 and Bandwidth of 125 KHz.
- RSSI: -45 dBm
- SNR: 9 dB
Here is a table that illustrates the effect of spreading factor:
You can see that a higher spreading factor can tolerate a larger negative SNR at the cost of bitrate. You can also get range improvement by lowering the bandwidth, but for the Murata LoRa module used on the MKR 1300 - the lowest bandwidth is 125 KHz with spreading factors from 6-12.
Murata module LoRa specification:
I currently planning to use adaptive data rate (ADR) which allows the gateway to adjust the spreading factor to increase the range as necessary. For my test measurements I am going to use SF7BW125.
RSSI and SNR readings
Here is my test data: The gateway is on the second floor at the front of the house. The signal loss doesn't correlate directly to distance because of the intervening structure of the house. The data point are averaged across different frequencies (channels).