Data Sheet

Page 24
RFM95/96/97/98(W)
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WIRELESS & SENSING PRELIMINARY DATASHEET
4.1.1.2. Spreading Factor
The spread spectrum LoRa
TM
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 LoRa
TM
modem are shown in the following table.
Table 59 Range of Spreading Factors
SpreadingFactor
(RegModulationCfg)
Spreading Factor
(Chips / symbol)
LoRa Demodulator
SNR
6 64 -5 dB
7 128 -7.5 dB
8 256 -10 dB
9 512 -12.5 dB
10 1024 -15 dB
11 2048 -17.5 dB
12 4096 -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 RFM95/96/97/98(W) registers when it is in use:
 Set SpreadingFactor = 6 in RegModemConfig2
 The header must be set to Implicit mode
 Write bits 2-0 of register address 0x31 to value "0b101"
 Write register address 0x37 to value 0x0C
4.1.1.3. Coding Rate
To further improve the robustness of the link the LoRa
TM
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 60 Cyclic Coding Overhead
CodingRate
(RegTxCfg1)
Cyclic Coding
Rate
Overhead Ratio
1 4/5 1.25
2 4/6 1.5
3 4/7 1.75
4 4/8 2