Tag Archives: RSSI

Rfm9xLoRaDevice NetMF SNR and RSSI

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The signal to noise Ratio (SNR) and Received Signal Strength Indication(RSSI) for inbound messages required reading values from three registers
•RegPktSnrValue
•RegPktRssiValue
•RegRssiValue

I had to modify the OnDataRecievedHandler method signature so the values could be returned

 public delegate void OnDataRecievedHandler(float packetSnr, int packetRssi, int rssi, byte[] data);

I was inspired by the RSSI adjustment approach used in the Arduino-LoRa library

// Get the RSSI HF vs. LF port adjustment section 5.5.5 RSSI and SNR in LoRa Mode
float packetSnr = this.Rfm9XLoraModem.ReadByte((byte)Registers.RegPktSnrValue) * 0.25f;

int rssi = this.Rfm9XLoraModem.ReadByte((byte)Registers.RegRssiValue);
if (Frequency > RFMidBandThreshold)
{
  rssi = RssiAdjustmentHF + rssi;
}
else
{
  rssi = RssiAdjustmentLF + rssi;
}

int packetRssi = this.Rfm9XLoraModem.ReadByte((byte)Registers.RegPktRssiValue);
if (Frequency > RFMidBandThreshold)
{
  packetRssi = RssiAdjustmentHF + packetRssi;
}
else
{
  packetRssi = RssiAdjustmentLF + packetRssi;
}

OnDataReceived?.Invoke( packetSnr, packetRssi, rssi, messageBytes);

The values displayed in the Rfm9xLoRaDeviceClient application looked reasonable, but will need further checking

00:06:14-Rfm9X PacketSnr 9.8 Packet RSSI -47dBm RSSI -111dBm = 28 byte message "Hello W10 IoT Core LoRa! 182"
Sending 20 bytes message Hello NetMF LoRa! 38
Transmit-Done
00:06:24-Rfm9X PacketSnr 9.8 Packet RSSI -48dBm RSSI -111dBm = 28 byte message "Hello W10 IoT Core LoRa! 181"
Sending 20 bytes message Hello NetMF LoRa! 39
Transmit-Done
00:06:34-Rfm9X PacketSnr 9.8 Packet RSSI -47dBm RSSI -112dBm = 28 byte message "Hello W10 IoT Core LoRa! 180"
Sending 20 bytes message Hello NetMF LoRa! 40
Transmit-Done
00:06:44-Rfm9X PacketSnr 10.0 Packet RSSI -48dBm RSSI -111dBm = 28 byte message "Hello W10 IoT Core LoRa! 179"

 

Rfm9xLoRaDevice SNR and RSSI revisited

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The magic numbers for the high frequency(HF) vs. low frequency(LF) adjustment bugged me

int rssi = this.RegisterManager.ReadByte((byte)Registers.RegRssiValue);
if (Frequency < 868E6)
	rssi = -164 + rssi; // LF output port
else
	rssi = -157 + rssi; // HF output port

int packetRssi = this.RegisterManager.ReadByte((byte)Registers.RegPktRssiValue);
if (Frequency < 868E6)
	packetRssi = -164 + rssi; // LF output port
else
	packetRssi = -157 + rssi; // HF output port

After some searching I ended up at the Semetch LoRaMac node github repository. In that code they had a number of constants which looked like a better approach.

if( SX1276.Settings.Channel > RF_MID_BAND_THRESH )
{
   rssi = RSSI_OFFSET_HF + SX1276Read( REG_LR_RSSIVALUE );
}
else
{
   rssi = RSSI_OFFSET_LF + SX1276Read( REG_LR_RSSIVALUE );
}

I also fixed a couple of bugs I noticed with the maths and the code now looks like this

int rssi = this.RegisterManager.ReadByte((byte)Registers.RegRssiValue);
if (Frequency > RFMidBandThreshold)
   rssi = RssiAdjustmentHF + rssi;
else
   rssi = RssiAdjustmentLF + rssi;

int packetRssi = this.RegisterManager.ReadByte((byte)Registers.RegPktRssiValue);
if (Frequency > RFMidBandThreshold)
   packetRssi = RssiAdjustmentHF + packetRssi;
else
   packetRssi = RssiAdjustmentLF + packetRssi;

This has also got me thing about how RegLna – LnaBoostLf/LnaBoostHf should be handled as well.

Rfm9xLoRaDevice SNR and RSSI

Posted on by

The signal to noise Ratio (SNR) and Received Signal Strength Indication(RSSI) for inbound messages required reading values from three registers

  • RegPktSnrValue
  • RegPktRssiValue
  • RegRssiValue

I had to modify the EventArgs returned to the customer

public class OnDataReceivedEventArgs : EventArgs
{
	public float PacketSnr { get; set; }
	public int PacketRssi { get; set; }
	public int Rssi { get; set; }
	public byte[] Data { get; set; }
}

I was inspired by the RSSI adjustment approach used in the Arduino-LoRa library

// Get the RSSI HF vs. LF port adjustment section 5.5.5 RSSI and SNR in LoRa Mode
float packetSnr = this.RegisterManager.ReadByte((byte)Registers.RegPktSnrValue) * 0.25f;

int rssi = this.RegisterManager.ReadByte((byte)Registers.RegRssiValue);
if (Frequency < 868E6)
	rssi = -164 - rssi; // LF output port
else
	rssi = -157 + rssi; // HF output port

int packetRssi = this.RegisterManager.ReadByte((byte)Registers.RegPktRssiValue);
if (Frequency < 868E6)
	packetRssi = -164 - rssi; // LF output port
else
	packetRssi = -157 - rssi; // HF output port

OnDataReceivedEventArgs receiveArgs = new OnDataReceivedEventArgs
{
	PacketSnr = packetSnr,
	Rssi = rssi,
	PacketRssi = packetRssi,
	Data = messageBytes
};

The values displayed in the Rfm9xLoRaDeviceClient application looked reasonable, but will need further checking