Semtech LR1110 - Channel Activity Detection

LR1110
User Manual Rev.1.0
UM.LR1110.W.APP March 2020
54 of 130
Semtech
www.semtech.com
8.2.2.3 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 in the register settings in implicit mode. An optional CRC may be appended.
8.2.2.4 LoRa® Packet Time On Air
The Time On Air of the LoRa® packet is given by:
with:
• SF: Spreading Factor (5 to 12)
• BWL: 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:
• 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
The ceil function indicates that the portion of the equation in square brackets should be rounded up to the next integer
value.
8.2.3 Channel Activity Detection
Used only in LoRa® packet type, the Channel Activity Detection (CAD) is a LoRa® specific mode of operation where the
device searches for the presence of a LoRa® preamble signal. After the search has completed, the device returns in
STDBY_RC mode. The length of the search is configured via the command SetCadParams( ). At the end of the search period,
the device triggers the IRQ CADdone. If a valid signal has been detected it also generates the IRQ CadDetected. A minimum
of 2 symbols is recommended to perform CAD.
ToA
2
SF
BWL
------------
N
symbol
=
N
symbol
N
symbol
preamble
6.25 8 ceil
max 8 N
byte
payload
N
bit
CRC
4 SF 8 N
symbol
header
0,+ +–+ 
4 SF
-----------------------------------------------------------------------------------------------------------------------------------------------------------
 
 
CR 4+ + + +=
N
symbol
N
symbol_preamble
4.25 8 ceil
max 8 N
byte_payload
N
bit_CRC
4 SF 8 N
symbol_header
0,+ +–+ 
4 SF
--------------------------------------------------------
-------------------------------------------------------------------------------------------------------
 
 
CR 4+ + + +=
N
symbol
N
symbol_preamble
4.25 8 ceil
max 8 N
byte_payload
N
bit_CRC
4 SF 8 N
symbol_header
0,+ +–+ 
4 SF 2– 
---------------------------------------------------------------------------------------------------------------------------------------------------------------
 
 
CR 4+ + + +=