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How LoRaWAN Works
Date:2017-12-13 14:37:23   Browse:218

Note: A list of common terms and definitions have been included at the end of this post as a Glossary.

           Using the infographic above, sensor connecting to the internet are referred to as end–devices. Whenever the sensor takes a reading the device conditionally sends a signal (data packet) that the gateways to capture the data. Now that data at the gateway uses FSK (Frequency Shift Keying) to transmit that data as efficiently as possible to the server using a process called the Chirp Spread Spectrum (CSS). As the data packet from the end device enters the circuitry of the gateway, it comes in “chirps,” or symbols that represent digital information (like below). The chirp is then parsed down to the frequency domain and then a modulated signal to efficient data transport.

            The LoRa hardware, after converting the input signal to the frequency domain, is searching within the frequency band for other, better frequency channels that can carry the signal. Once the gateway finds one, this whole process modulates the input signal’s frequency to make it more energy efficient, and then “shifts” (hence the “S” in FSK) the signal to that channel for quick data transmission.


           The end-devices and gateways continuously interact with each other so that the data transmission can “hop” to other frequency channels that best suits the system’s power, speed, duty cycle, and range restraints.


           During this frequency modulation, other integrated circuits within the LoRa gateway performs other “improvement” modulations, like filtering out noise, or the jaggedness that you see in a signal.


          Another reason why LoRaWAN is a low-power, long-range network is thanks to a process called ADR (Adaptive Data Rate). Just like how the FSK process “shifts” the input signal frequency to boost efficiency, ADR “talks” to the LoRaWAN network server to boost the data rate. This is how the “talking” is done between device and server:


          The end-devices (nodes) constantly send uplink messages to the network server of LoRaWAN. These uplink messages are comprised of lots of information about the node’s past 20 signals

The network server analyzes the recent history of the node and makes comparisons to see how much “margin” there is to make changes

          The network may observe that there is a “margin” for sacrificing range for something more useful, like a faster data rate. (Notice from the diagram that the trash can is sending its data to more gateways than any of the other devices)

         Instead of sending slower messages to far away gateways, the server would rather have the end device send a quicker message to one gateway nearby.

         Hence, the ADR process takes advantage of opportunities that will boost the data rate. If the sacrifices being made helps the system operate more efficiently, then the sacrifice will be made using ADR.

        After the gateways receive and interprets a data packet using LoRa technology, the gateway forwards the data to the network server via standard IP connections, like Ethernet or 3G. If the network server receives the same data packet from several gateways, it will only process one of them, and disregard the copies. Hence, if the server will receive three of the same data packet, because the trash can is connected to three gateways in our illustration, then only one of these data packets will be processed, making for a highly accurate and very efficient data transfer.


Tradeoffs


As in every engineering application, there are trade-offs in the world of LoRaWAN when it comes to power, speed, and range. This simple diagram below displays the points of consideration.


Increasing time of data bit ——-> reduces data rate ——-> lower speed

Decreasing time of data bit ——-> increases data rate ——-> higher speed

Increasing the range and reducing power ——-> lower speed

Increasing the range and quickening the speed ——–> requires higher power

Increasing the speed and reducing power——-> shorter range


Frequency Bands


LoRaWAN uses lower radio frequencies at a longer range, and the frequency bands differ between countries.


Europe: 863-870 MHz and 433 MHz bands (868 MHz used by The Things Network). Three common 125 kHz channels for the 868 MHz band (868.10, 868.30 and 868.50 MHz) must be supported by all devices and networks.

USA: 902-928 MHz band, divided into 8 sub bands. Each of these sub bands has eight 125 kHz uplink channels, one 500 kHz uplink channel and one 500 kHz downlink channel. As opposed to Europe’s frequency channels, those of USA are classified as uplink and downlink channels

Australia: 915-928 MHz band. Uplink frequencies in Australia are on higher frequencies than in the US band. However, the downlink frequencies are the same as in the US band.

China: 779-787 MHz band, with three common 125 kHz channels (779.5, 779.7 and 779.9 MHz), and also there exists a 470-510 MHz band, with 96 uplink channels and 48 downlink channels


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