LLCC68 datasheet(36/106 Pages) SEMTECH

36 / 106 page

LLCC68

Data Sheet

Rev. 1.0

DS.LLCC68.W.APP

July 2019

36 of 106

Semtech

www.semtech.com

6.1.3.2 Implicit Header Mode

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.

6.1.4 LoRa® Time-on-Air

The packet format for the LoRa® modem is detailed in Figure 6-3: Fixed-Length Packet Format and Figure 6-4:

Variable-Length Packet Format. The equation to obtain Time On Air (ToA) is:

with:

•

SF: Spreading Factor (5 to 11)

•

BW: Bandwidth (in kHz)

•

ToA: the Time on Air in ms

•

Nsymbol: number of symbols

The computation of the number of symbols differs depending on the parameters of the modulation.

For SF5 and SF6:

FFor CR being legacy coding rate (ie. Not Long Interleaving):

For all other SF:

For all other SF with Low Data Rate Optimization activated:

With:

•

N_bit_CRC = 16 if CRC activated, 0 if not

•

N_symbol_header = 20 with explicit header, 0 with implicit header

•

CR is 1, 2, 3 or 4 for respective coding rates 4/5, 4/6, 4/7 or 4/8

6.1.5 LoRa® Channel Activity Detection (CAD)

The use of a spread spectrum modulation technique presents challenges in determining whether the channel is already in

use by a signal that may be below the noise floor of the receiver. The use of the RSSI in this situation would clearly be

impracticable. To this end the channel activity detector is used to detect the presence of other LoRa® signals.

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