LLCC68 datasheet(35/106 Pages) SEMTECH

35 / 106 page

LLCC68

Data Sheet

Rev. 1.0

DS.LLCC68.W.APP

July 2019

35 of 106

Semtech

www.semtech.com

6.1.2 LoRa® Packet Engine

LoRa® has it own packet engine that supports the LoRa® PHY as described in the following section.

6.1.3 LoRa® Frame

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. The packet

format is shown in the following figure.

Figure 6-2: LoRa® Packet Format

The LoRa® packet starts with a preamble sequence which is used to synchronize the receiver with the incoming signal. By

default the packet is configured with a 12-symbol long sequence. This is a programmable variable so the preamble length

may be extended; for example, in the interest of reducing the receiver duty cycle in receive intensive applications. The

transmitted preamble length may vary from 10 to 65535 symbols, once the fixed overhead of the preamble data is

considered. This permits the transmission of near arbitrarily long preamble sequences.

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.

The preamble is followed by a header which contain information about the following payload. The packet payload is a

variable-length field that contains the actual data coded at the error rate either as specified in the header in explicit mode

or as selected by the user in implicit mode. An optional CRC may be appended.

Depending upon the chosen mode of operation two types of header are available.

6.1.3.1 Explicit Header Mode

This is the default mode of operation. Here the header provides information on the payload, namely:

•

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.

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