Lora Packet Format; Figure 8-3: Lora Packet Format - Semtech LR1121 User Manual

8.2.1.4 Low Data Rate Optimization (LDRO)
LDRO increases the robustness of the LoRa link at low effective data rates, improving the sensitivity level and increasing the
robustness towards frequency drift and Doppler events. Its use is mandated with spreading factors of 11 and 12 at 125kHz
bandwidth, and SF12 at 250kHz BW.
8.2.1.5 LoRa Symbol Rate (Rs)
With a knowledge of the key parameters that can be controlled by the user we define the LoRa symbol rate as:
BWL
Rs = ------------
2 SF
where BWL is the programmed bandwidth and SF is the spreading factor. The transmitted signal is a constant envelope
signal. Equivalently, one chip is sent per second per Hz of bandwidth.

8.2.2 LoRa Packet Format

The LoRa modem employs two packet formats: explicit and implicit. The explicit packet contains additional information, it
includes a short header that contains information about the number of bytes, coding rate and whether a CRC is used in the
packet.
In certain scenarios, where the payload, coding rate and CRC presence are fixed or known in advance, it may be
advantageous to reduce transmission time by invoking implicit header mode. In this mode the header is removed from the
packet. In this case the payload length, error coding rate and presence of the payload CRC must be manually configured
identically on both sides of the radio link.
8.2.2.1 Preamble
The LoRa packet starts with a preamble sequence, used to synchronize the receiver with the incoming signal. The
transmitted preamble length may vary from 1 to 65535 symbols. This permits the transmission of near arbitrarily long
preamble sequences. In order to optimize the packet reception, it is advised to use a minimum preamble length of 12 with
SF5 and SF6, and of 8 for other SF.
The receiver undertakes a preamble detection process that periodically restarts. For this reason the preamble length should
be configured as identical to the transmitter preamble length. Where the preamble length is not known, or can vary, the
maximum preamble length should be programmed on the receiver side.
LR1121
User Manual Rev 1.1
UM.LR1121.W.APP Mar 2023

Figure 8-3: LoRa Packet Format

www.semtech.com 65 of 130
Semtech
Proprietary & Confidential