LR1110
User Manual
Rev.1.0
UM.LR1110.W.APP
March 2020
53 of 130
Semtech
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 types of packet formats: explicit and implicit. The explicit packet includes a short header
that contains information about the number of bytes, coding rate and whether a CRC is used in the packet.
Figure 8-3: LoRa® Packet Format
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.
8.2.2.2 Header
The header provides information on the payload:
•
The payload length in bytes
•
The forward error correction coding rate
•
The presence of an optional 16-bit CRC for the payload
The header is transmitted with maximum error correction code (4/8). It also has its own CRC to allow the receiver to discard
invalid headers.
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.
