Fsk Modem - STMicroelectronics STM32WL5 Series Reference Manual

RM0453
Implicit header mode
In certain operation modes where the payload coding rate and CRC presence are fixed or
known in advance, it can be advantageous to reduce transmission time by invoking implicit
header mode. In this mode, the header is not present in the packet frame. The payload
length, forward error correction coding rate and presence of the payload CRC must be
configured on both sides of the sub-GHz radio link.
LoRa time-on-air
The total transmission time of packet can be calculated as follows:
TotalTimeOnAir = (PreambleSymbols + NbSymbolPayload + 4.25 + 8) x (2
where PreambleSymbols is the number of preamble symbols programmed in
Set_PacketParams(PbLength).
The number of payload symbols can be calculated as follows:
NbSymbolPayload = CEILING(NbSymbolPayloadFrac,4 + CR)
where CEILING is the function that rounds up to the integer multiple of (4 + CR) immediately
superior to the fractional first parameter.
The number of payload fractional symbols in Explicit mode can be calculated as follows:
NbSymbolPayloadFrac = ((PL x 8 + CRC x 16 - 4 x (SF - 7)) x (4 + CR)) / (4 x SF)
The number of payload fractional symbols in Implicit mode can be calculated as follows:
NbSymbolPayloadFrac = ((PL x 8 + CRC x 16 - 4 x (SF - 2)) x (4 + CR)) / (4 x SF)
where:
•
CRC = 0 (no CRC) or 1 (16-bit CRC)
•
CR = 0 to 4
•
PL = 1 to 255, user data payload length in number of bytes
•
SF = 7 to 12 spreading factor
LoRa data rates "raw data rate" and "real data rate" are defined as:
RawBitRate = ((SF x BW) / 2
RealBitRate = (PayloadLength x 8) / TotalTimeOnAir
Channel activity detection (CAD)
The channel activity detection is used to detect the presence of a LoRa signal, by detecting
a LoRa preamble or data symbols.
Once in channel activity detection mode, the band is scanned for a determined duration as
set in Set_CadParams() command. If LoRa symbols are detected during this period, the
channel activity detected IRQ is set.
The time needed to perform the channel activity detection depend on the LoRa modulation
settings. For a given SF / BW, the typical CAD detection time can be selected to be 1, 2, 4,
8, or 16 symbols. The CAD duration time is further more extended with half a symbol.
5.5.3

FSK modem

The FSK modem provides a 2-FSK modulation over a range of data rates from 0.6 Kbit/s up
to 300 Kbit/s. In receive mode, the bandwidth is automatically adjusted to match the
selected data rate. In transmit mode, the frequency deviation is selected via the modulation
SF
) x (4 / (4 + CR))
RM0453 Rev 5
Sub-GHz radio (SUBGHZ)
SF
/ BW)
165/1450
227