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  • Tim Kannegieter

    LoRaWAN is an open global standard for low power WAN connectivity. LoRa is a registered trademark of Semtech corporation, which is a chip company using chirp spread spectrum modulation. LoRa is  the term used to describe the physical layout and LoRaWAN is the name given to the MAC layer protocol.

    LoRA operates in unlicensed spectrum. The exact frequencies vary by region, and the channel plan standard for Australia was released in 2016 by the LoRa Alliance following the release of standards for the EU, USA, and China. 

    One of the more unique features of LoRa is that it can adjust the data rate depending on the path or distance between the base station and the node. This is important because it allows optimization of energy usage. Being a spread spectrum technology the data rate is adjusted by means of modifying the spreading factor of the modulation. 

    The way spread spectrum works is that each bit or symbol is transmitted as a sequence of chirps (or chips), which spreads the signal over a wide channel bandwidth. The spreading factor determines how many chirps are sent for each symbol, which is why the spreading factor affects power.

    The more chirps that are sent, the more the coding gain, which helps the receiver recover the signal even from well below the noise floor. LoRa is capable of reconstructing a signal 19.5 db below the noise floor, which is why it can transmit at such long a range with low power. The chirps are actually burst of signal with rising or falling frequency (analogous to a bird chirping). Chirp spread spectrum is claimed to be resistant against broadband and narrow-band disturbances and multipath fading, which are important.

    LoRa networks have been demonstrated to work in a coastal environment across a 15km range using a 100 milliwatt transmitter with only directional transmit receiver antennas. In an urban environment the range is more typically 4.5km.

    The spreading factor affects time on air and the time the transmitter is turned on to send a packet, so the energy used depends on which channel bandwidth and spreading factor is used. The transmission energy that's in, say, an 11 byte packet, can vary from a few millijoules to a couple hundred millijoules per packet transmitted. Once this is known it is possible to work out how many transmissions you can get out of a battery, taking into account other electronics on the device. Typically this would be in on the order of 10 to 20 thousand transmissions out of a typical cellular phone-type battery.  

    The maximum packet length for LoRAWAN depends on a number of factors and can be up to 256 bytes. However, communications can be broken up into multiple packets which can then be hopped across different frequencies that allow them to comply with dwell time requirements in different regions e.g. using HCMA or FCCs rules which are is slightly different. Typical applications would only transmit approximately 11 bytes which is usually enough to transmit the kind of information being generated by IoT devices.

    LoRaWAN is symetrical and configurable meaning it can have transmit only, or transmit with an acknowledgement, or you can have bidirectional communication. There are three classes of protocol - A, B, and C. Class A allows asynchronous at any time. Class B transmits on a schedule. Class C is always listening. The listening windows in class A and B are open depending on what the transmitter decides it needs to do.

    There's a possibility of building different classes of servers. You can also choose to send an acknowledge packet or not, and then you can choose to do what you want to do depending on whether you get that acknowledge. For example, you might transmit, wait for an acknowledge for a period of time. If you don't get it, you transmit again, and you can do that any number of times. You can choose to configure the server to respond in ways that are appropriate for the application.

    Sources: The information on this paged is sourced primarily from the following:

    Links

    The Lora Alliance is at https://www.lora-alliance.org/

    Because LoRaWAN is open there are several providers of network services in Australia and some large government departments and private companies are implementing their own networks. LoRaWAN network providers include:

     

    Edited by Tim Kannegieter




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