LoRa Physical Layer & RF Interface
- the LoRa wireless system has an RF interface or physical layer that enables it to operate very efficiently, enabling low power transmissions to maintain data links over long distances.
The LoRa RF or physical layer encapsulates the all the direct contact with the outside world over the radio interface.
The RF parameters including frequencies, bands, power levels, modulation, and the basic RF protocols are all encapsulated in the LoRa RF or physical layer attributes.
LoRa frequency bands
The LoRa wireless system makes use of the unlicensed frequencies that are available worldwide. The most widely used frequencies / bands are:
- 868 MHz for Europe
- 915 MHz for North America
- 433 MHz band for Asia
Using lower frequencies than those of the 2.4 or 5.8 GHz ISM bands enables much better coverage to be achieved especially when the nodes are within buildings.
Although the sub-1GHz ISM bands are normally used, the technology is essentially frequency agnostic and can be used on most frequencies without fundamental adjustment.
LoRa RF physical layer uses a form of spread spectrum modulation. The LoRa modulation scheme uses wide-band linear frequency modulated pulses. The level of frequency increase or decrease over time is used to encode the data to be transmitted, i.e. a form of chirp modulation.
This form of modulation enables LoRa systems to demodulate signals that are 20dB below the noise floor when the demodulation is combined with forward error correction, FEC. This means that the link budget for a LoRa system can provide an improvement of more than 25dB when compared to a traditional FSK system.
LoRa adaptive link
The fact that only low data rates are used, and low levels of overall data transfer means that low bandwidths are required. A variety of bandwidths are available: 7.8 kHz; 10.4 kHz; 15.6 kHz; 20.8 kHz; 31.2 kHz; 41.7 kHz; 62.5 kHz; 125 kHz; 250 kHz; 500 kHz. The required bandwidth can be selected according to the data requirements as well as the link conditions.
The power level used within LoRa RF physical layer is adaptive. The power level used is dependent upon the data rate needed, link conditions etc. An algorithm is used to determine the required power level - the transmitted power is normally backed off a little from the maximum needed to support fast communications and in this way the battery life is maximised and network capacity maintained.
LoRa data communications
The communication between different end-devices and gateways utilises several different frequency channels and it uses different data rates.
The choice of the data rate is a balance between communication range and message duration, i.e. the rate at which the required data can be sent.
The use of the chirp spread spectrum technology enables communications with different data rates not interfere with each other. In this way a set of "virtual" channels is created which increases the capacity of the gateway.
By Ian Poole
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