LLCC68 datasheet(34/106 Pages) SEMTECH

34 / 106 page

LLCC68

Data Sheet

Rev. 1.0

DS.LLCC68.W.APP

July 2019

34 of 106

Semtech

www.semtech.com

permissible occupied bandwidth. The LoRa® modem bandwidth always refers to the double side band (DSB). The range of

LoRa® signal bandwidths available is given in the table below:

For BW_L up to 250 kHz, the receiver performs a double conversion. A first down conversion to low- IF is performed inside

the RF chain, a second conversion to baseband is performed digitally inside the baseband modem. When the 500 kHz

bandwidth is used, a single down-conversion to zero-IF is performed in the RF part.

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 36) 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 16.38 ms.

Table 6-2: Signal Bandwidth Setting in LoRa® Mode

Signal Bandwidth

BW_L [kHz] 0

1250

250 1

1. For RF frequencies below 400 MHz, there is a scaling

between the frequency and supported BW, some BW

may not be available below 400 MHz

500 1

Table 6-3: Coding Rate Overhead

Coding Rate

Cyclic Coding Rate CR

[in raw bits / total bits]

Overhead Ratio

4/5

1.25

24/6

1.5

4/7

1.75

44/8

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