Datasheet

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SX1272/73
WIRELESS, SENSING & TIMING DATASHEET
Rev. 2 - July 2014
©2014 Semtech Corporation
Table 37 RssiSmoothing Options
The RSSI is calibrated when the image and RSSI calibration process is launched. Please see Section Ta b le 4.2.3.8 for
details.
5.5.5. RSSI and SNR in LoRa
TM
Mode
The RSSI values reported by the LoRa
TM
modem differ from those expressed by the FSK/OOK modem. The following
formula shows the method used to interpret the LoRa
TM
RSSI values:
RSSI (dBm) = -139 + Rssi, (when LnaBoost=1)
The same formula can be re-used to evaluate the signal strength of the received packet:
Packet Strength (dBm) = -139 + PacketRssi, (when LnaBoost=1 and SNR >= 0)
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) = -139 + PacketRssi + PacketSnr (when LnaBoost=1 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, -139, may vary with the front-end setup of the SX1272 (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 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. Rssi is updated every 1/BW
(i.e. 8us in 125kHz mode, 4us in 250kHz, etc…)
- When SNR>=0, the standard formula can be adjusted to correct the slope:
RSSI = -139+16/15 * PacketRssi
RssiSmoothing Number of Samples Estimated Accuracy Response Time
‘000’ 2 ± 6 dB
‘001’ 4 ± 5 dB
‘010’ 8 ± 4 dB
‘011’ 16 ± 3 dB
‘100’ 32 ± 2 dB
‘101’ 64 ± 1.5 dB
‘110’ 128 ± 1.2 dB
‘111’ 256 ± 1.1 dB
()
[]
[]
ms
kHzRxBw
ingRssiSmooth
⋅
+
4
2
1