LLCC68 datasheet(36/111 Pages) SEMTECH

36 / 111 page

LLCC68

Data Sheet

Rev. 1.1

DS.LLCC68.W.APP

Dec 2024

36 of 111

Semtech

LLCC68

www.semtech.com

6.1.1.3 FEC Coding Rate

To further improve the robustness of the link the LoRa modem employs cyclic error coding to perform forward error detection and

correction.

Forward Error Correction (FEC) is particularly efficient in improving the reliability of the link in the presence of interference. So that the

coding rate and robustness to interference can be changed in response to channel conditions, the coding rate selected on the transmitter

side is communicated to the receiver through the header (when present).

A higher coding rate provides better noise immunity at the expense of longer transmission time. In normal conditions a factor of 4/5

provides the best trade-off; in the presence of strong interferers a higher coding rate may be used. Error correction code does not have to

be known in advance by the receiver since it is encoded in the header part of the packet.

6.1.1.4 Low Data Rate Optimization

For low data rates (typically for high SF or low BW) and very long payloads which may last several seconds in the air, the low data rate

optimization (LDRO) can be enabled. This reduces the number of bits per symbol to the given SF minus two (see Section 6.1.4 "LoRa

Time-on-Air" on page 37 ) in order to allow the receiver to have a better tracking of the LoRa signal. Depending on the payload size, the low

data rate optimization is usually recommended when a LoRa symbol time is equal or above 16.38ms.

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

Table 6-3: Coding Rate Overhead

Coding Rate

Cyclic Coding Rate CR

[in raw bits / total bits]

Overhead Ratio

4/5

1.25

4/6

1.5

4/7

1.75

4/8

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