What is LoRa?
LoRa is a type of chirp spread spectrum transmission that has been implemented by Semtech for use in low cost Industrial Scientific and Medical (ISM) band radio devices. The SX127x devices became available in 2014\2015. LoRa is short for Long Range.
As a general guide a LoRa device will have up to 10 times (or maybe more) the range \distance of other typical radio modules, when using the same frequencies, data rates, antennas and transmit powers.
The first LoRa devices were for the UHF bands and modules for 169Mhz, 434Mhz, 868Mhz and 915Mhz are available to match the ISM band requirements of a particular global location.
LoRa devices are programmed over an SPI interface. The older SX127x range of devices are the ones in common use and are register centric, they are programmed by changing the contents of 128 seperate single byte registers. Newer LoRa devices such as SX126x (UHF) and SX128x (2.4Ghz) are programmed through a command interface, you send a command byte followed by a series of parameter bytes across the SPI bus.
LoRa devices are packet based, you load a first in first out (FIFO) buffer with bytes of data and tell the device to transmit that packet of data. The packets can be up to 256 bytes long and can have an integral cyclic redundancy check (CRC) automatically added. When a packet is received it is loaded into the FIFO from where the bytes of data can be read out.
There are now several software libraries that allow LoRa devices to be used with a range of Arduino devices. Note that a library specifically for the SX1276 and SX1278 devices will not correctly drive an SX1272.
Comparison to FSK devices
With frequency shift keying (FSK) style transmitters and receivers such as the Hope RFM22B or RFM69 for reception to work the receiver needs to see a signal that is strong enough to be above the local radio frequency (RF) noise level. Noise levels in the ISM bands are a performance limiter and each device in use adds to the local RF noise level. The total noise a receiver sees is a combination of local RF noise and noise generated by the electronics inside the receiver.
A typical FSK receiver needs to see a signal that is at least 5dB above the receiver’s noise level in order to decode it. In comparison LoRa technology allows for signals to be received that are as much as 20dB below noise level, thus the LoRa signal could be up to 25dB (5dB + 20dB) weaker than the FSK one and still be received correctly. If LoRa can decode a signal that is 25dB weaker, then the transmitter can be further away than in the FSK example.
So due in the main to the below noise level performance, LoRa devices are able to receive signals that are very much weaker and thus much further away than with previous technology ISM band modules.
Configuring a LoRa module
LoRa modules are straight forward to drive and flexible, the internal firmware takes care of internal adjustments and optimisations, although there is an extra optimisation flag to set for long range slow to transmit packets.
The parameters that need to be configured for LoRa are, Spreading Factor, Coding Rate, Bandwidth, frequency and transmit power. These parameters can be easily changed so that you can set-up for low data rate long range or high data rate shorter range.
The LoRa device itself is a single surface mount integrated circuit, these are built onto small, typically 15mm x 15mm, modules that contain the crystal (or oscillator) and antenna matching components the modules need. Examples of LoRa modules are the Hope RFM98 and Dorji DRF1278F.
LoRa is a two way technology, the transmitter device is a receiver too, so if you can pick-up signals from your transmitter you can send it commands or exchange data with it also.
The LoRa bandwidth is the amount of frequency spread the signal occupies. The bandwidths LoRa can be set to are;
7.8kHz, 10.4kHz, 15.6kHz, 20.8kHz, 31.25kHz, 41.7kHz, 62.5kHz, 125kHz, 250kHz, 500kHz.
A low bandwidth signal, such as 7.8kHz will give the best (longest distance) performance, but with the low cost components used in the Hope and Dorji LoRa modules a transmitter and receiver may not talk to each other at this bandwidth. The receiver has a frequency capture range, this is the percentage difference allowable in relation to the bandwidth that the transmitter and receiver can be apart in frequency in order for the receiver to pick up the transmitters signal. This percentage is typically 25% of the bandwidth.
Lets assume the bandwidth in use is 10kHz, the allowable variation is 25% or 2.5kHz and the transmitter is on 434.000Mhz. For the link to work the receiver needs to be on or between 433.9975Mhz and 434.0025Mhz. This is an exacting requirement for a low cost crystal, even if transmitter and receiver are at the same temperature.
Low bandwidths of 20.8kHz have been used for long range trackers and they do give better performance, but care is needed as devices used at this bandwidth may not talk to each other unless a frequency calibration factor is measured and applied. Once communication is established at low bandwidth a form of automatic frequency control can be used to adjust the receivers base frequency to keep it within the capture range.
A bandwidth of 62.5kHz has been found to be reliable and allows for differences in manufacturing tolerance and temperature between transmitter and receiver devices. Applications such as The Things Network (see later) use a bandwidth of 125kHz.
Newer LoRa devices such as the SX1262 are designed to use temperature compensated crystal oscillators (TCXO). The TCXO is very stable in frequency and can allow LoRa devices to be used at the lowest bandwidth (7.8kHz) even when there are significant temperature differences between transmitting and receiving LoRa devices.
This controls the amount of signal processing gain. The spreading factor can be set between 6 and 12. A setting of 6 can only be used with fixed length packets, so is not often used in applications.
The spreading factor defines the below noise performance. SF6 gives a -7.5dB below noise performance and SF12 a -20dB below noise performance.
The larger spreading factors therefore give more range, as it allows the reception of weaker signals, the trade off is that higher spreading factor packets take longer to transmit.
Additional data can be added to the packet to allow for error correction. This can result in small signal gains at the limit of reception. The coding rate can be varied between 4:5 and 4:8, the higher 4:8 rate will result in longer packets.
A fuller description of LoRa will be found in this document;
And a good video on the basics of radio propagation, antennas and LoRa can be fould here, LoRa modulation itself is described from 34:00;
Next - Part 2