RFM95 Datasheet, PDF (39/121 Pages) List of Unclassifed Manufacturers – Low Power Long Range Transceiver Module

WIRELESS & SENSING
RFM95/96/97/98(W)
PRELIMINARY
DATASHEET
Principle of Operation
The channel activity detection mode is designed to detect a LoRa preamble on the radio channel with the best possible
power efficiency. Once in CAD mode, the RFM95/96/97/98(W) will perform a very quick scan of the band to detect a LoRa
packet preamble.
During a CAD the following operations take place:
Š The PLL locks
Š The radio receiver captures LoRa preamble symbol of data from the channel. The radio current consumption during that
phase corresponds to the specified Rx mode current
Š The radio receiver and the PLL turn off, and the modem digital processing starts.
Š The modem searches for a correlation between the radio captured samples and the ideal preamble waveform. This
correlation process takes a little bit less than a symbol period to perform. The radio current consumption during that
phase is greatly reduced.
Š Once the calculation is finished the modem generates the CadDone interrupt. If the correlation was successful,
CadDetected is generated simultaneously.
Š The chip goes back to stand-by mode.
Š If a preamble was detected, clear the interrupt, then initiate the reception by putting the radio in RX single mode or RX
continuous mode.
The time taken for the channel activity detection is dependent upon the LoRa modulation settings used. For a given
configuration the typical CAD detection time is shown in the graph below, expressed as a multiple of the LoRa symbol
period. Of this period the radio is in receiver mode for (2^SF + 32) / BW seconds. For the remainder of the CAD cycle the
radio is in a reduced consumption state.
Figure 11. Channel activity detection (CAD) time as a function of spreading factor
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