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RFM95/96/97/98(W)

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WIRELESS & SENSING

PRELIMINARY

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 59 Range of Spreading Factors

SpreadingFactor

(RegModulationCfg)

Spreading Factor

(Chips / symbol)

LoRa Demodulator

SNR

-5 dB

128

-7.5 dB

256

-10 dB

512

-12.5 dB

1024

-15 dB

2048

-17.5 dB

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 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 60 Cyclic Coding Overhead

CodingRate

(RegTxCfg1)

Cyclic Coding

Rate

Overhead Ratio

4/5

1.25

4/6

1.5

4/7

1.75

4/8

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