G)Fsk Modem; G)Fsk Modulation Principle; G)Fsk Packet Engine - Semtech LR1121 User Manual

8.4 (G)FSK Modem

8.4.1 (G)FSK Modulation Principle

The (G)FSK modem can transmit and receive 2-FSK modulated packets over data rates ranging from 0.6kbps to 300kbps.
Both the bit rate (Bitrate) and frequency deviation (Fdev) are directly configured using command
SetModulationParams().
Additionally, in transmission mode, several shaping filters can be applied to the signal in packet mode or in continuous
mode. In reception mode, the user needs to select the best reception bandwidth depending on its conditions. To ensure
correct demodulation, the following limit must be respected for the bandwidth:
(2 x Fdev + BR) < BWF
Where the bandwidth BWF ranges from 4.8kHz to 467kHz.
The bandwidth must be chosen so that:

Bandwidth [DSB] BR + 2 x frequency deviation + frequency error
where the frequency error is twice the frequency error of the crystal oscillator used.

8.4.2 (G)FSK Packet Engine

The LR1121 is designed for packet-based communication. The packet controller block is responsible for assembling
received data bit-stream into packets and storing them in the data buffer. It also performs bit-stream decoding operations
such as de-whitening & CRC-checks on the received bit-stream.
On the transmit side, the packet handler can construct a packet and send it bit by bit to the modulator for transmission. It
can whiten the payload and append the CRC-checksum to the end of the packet. The packet controller only works in
half-duplex mode i.e. either in transmit or receive at a time.
The packet controller is configured using command SetPacketParams(...). This function can be called only after
defining the protocol.
Preamble Detection in Receiver Mode
The LR1121 can gate the reception of a packet if an insufficient number of alternating preamble symbols (usually referred
to 0x55 or 0xAA in hexadecimal form) has been detected. The parameter PreambleDetectorLength in command
SetPacketParams(...) allows the user to select a value ranging from:
• "Preamble detector length off": where the radio performs no gating and locks directly on the following Syncword, to
• "Preamble detector length 32 bits": where the radio expects to receive 32 bits of preamble before the following
Syncword. In this case, if the 32 bits of preamble are not detected, the radio either drops the reception in RxSingle
mode, or restarts its tracking loop in RxContinuous mode.
To achieve best performance, it is recommended to set PreambleDetectorLength to "Preamble detector length 8 bits" or
"Preamble detector length 16 bits" depending of the complete size of preamble which is sent by the transmitter.
Note: In all cases, PreambleDetectorLength must be smaller than the size of the following Syncword to achieve proper
detection of the packets. If the preamble length is greater than the following Syncword length (typically when no Syncword
is used) the user should fill some of the Syncword bytes with 0x55.
LR1121
User Manual Rev 1.1
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