LoRa笔记01 sx1276 sx1278信号强度RSSI研究

5.5.5. RSSI and SNR in LoRaTM Mode

The RSSI values reported by the LoRaTM modem differ from those expressed by the FSK/OOK modem. The following
formula shows the method used to interpret the LoRaTM RSSI values:
		RSSI (dBm) = -157 + Rssi, (when using the High Frequency (HF) port)
	or
		RSSI (dBm) = -164 + Rssi, (when using the Low Frequency (LF) port)
		
The same formula can be re-used to evaluate the signal strength of the received packet:
		Packet Strength (dBm) = -157 + Rssi, (when using the High Frequency (HF) port)
	or
		Packet Strength (dBm) = -164 + Rssi, (when using the Low Frequency (LF) port)
		
Due to the nature of the LoRa modulation, it is possible to receive packets below the noise floor. In this situation, the SNR
is used in conjunction of the PacketRssi to compute the signal strength of the received packet:
		Packet Strength (dBm) = -157 + PacketRssi + PacketSnr * 0.25 (when using the HF port and SNR < 0)
	or
		Packet Strength (dBm) = -164 + PacketRssi + PacketSnr * 0.25 (when using the LF port and SNR < 0)

Note:
1. PacketRssi (in RegPktRssiValue), is an averaged version of Rssi (in RegRssiValue). Rssi can be read at any time
(during packet reception or not), and should be averaged to give more precise results.

2. The constants, -157 and -164, may vary with the front-end setup of the SX1276/77/78/79 (LnaBoost =1 or 0,
presence of an external LNA, mismatch at the LNA input…). It is recommended to adjust these values with a single-point
calibration procedure to increase RSSI accuracy.

3. As signal strength increases (RSSI>-100dBm), the linearity of PacketRssi is not guaranteed and results will diverge
from the ideal 1dB/dB ideal curve. When very good RSSI precision is required over the whole dynamic range of the
receiver, two options are proposed:

	- Rssi in RegRssiValue offers better linearity. Rssi can be sampled during the reception of the payload (between
ValidHeader and RxDone IRQ), and used to extract a more high-signal RSSI measurement
	- When SNR>=0, the standard formula can be adjusted to correct the slope:
RSSI = -157+16/15 * PacketRssi (or RSSI = -164+16/15 * PacketRssi)

int16_t SX1276ReadRssi( RadioModems_t modem )
{
	int16_t rssi = 0;

	switch( modem )
	{
	case MODEM_FSK:
		rssi = -( SX1276Read( REG_RSSIVALUE ) >> 1 );
		break;
	case MODEM_LORA:
		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 );
		}
		break;
	default:
		rssi = -1;
		break;
	}
	return rssi;
}

rssi cur: 61,snr: 35 
pkt rssi:113