SX1276 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SX1276' PDF : 132 Pages View PDF
SX1276/77/78/79
WIRELESS, SENSING & TIMING
DATASHEET
4.1.1.2. Spreading Factor
The spread spectrum LoRaTM modulation is performed by representing each bit of payload information by multiple chips of
information. The rate at which the spread information is sent is referred to as the symbol rate (Rs), the ratio between the
nominal symbol rate and chip rate is the spreading factor and represents the number of symbols sent per bit of information.
The range of values accessible with the LoRaTM modem are shown in the following table.
Table 13 Range of Spreading Factors
SpreadingFactor
(RegModulationCfg)
6
7
8
9
10
11
12
Spreading Factor
(Chips / symbol)
64
128
256
512
1024
2048
4096
LoRa Demodulator
SNR
-5 dB
-7.5 dB
-10 dB
-12.5 dB
-15 dB
-17.5 dB
-20 dB
Note that the spreading factor, SpreadingFactor, must be known in advance on both transmit and receive sides of the link
as different spreading factors are orthogonal to each other. Note also the resulting signal to noise ratio (SNR) required at
the receiver input. It is the capability to receive signals with negative SNR that increases the sensitivity, so link budget and
range, of the LoRa receiver.
Spreading Factor 6
SF = 6 Is a special use case for the highest data rate transmission possible with the LoRa modem. To this end several
settings must be activated in the SX1276/77/78/79 registers when it is in use. These settings are only valid for SF6 and
should be set back to their default values for other spreading factors:
Set SpreadingFactor = 6 in RegModemConfig2
The header must be set to Implicit mode.
Set the bit field DetectionOptimize of register RegLoRaDetectOptimize to value "0b101".
Write 0x0C in the register RegDetectionThreshold.
4.1.1.3. Coding Rate
To further improve the robustness of the link the LoRaTM modem employs cyclic error coding to perform forward error
detection and correction. Such error coding incurs a transmission overhead - the resultant additional data overhead per
transmission is shown in the table below.
Table 14 Cyclic Coding Overhead
CodingRate
(RegTxCfg1)
1
2
3
4
Cyclic Coding
Rate
4/5
4/6
4/7
4/8
Overhead Ratio
1.25
1.5
1.75
2
Rev. 4 - March 2015
©2015 Semtech Corporation
Page 27
www.semtech.com
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