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This is part 17 of the LoRa/LoRaWAN tutorial.
In this video series different topics will be explained which will help you to understand LoRa/LoRaWAN.
It is recommended to watch each video sequentially as I may refer to certain LoRa/LoRaWAN topics explained earlier.
In this video I will explain what the LoRa packet format is, how the Time on Air is calculated and what Adaptive Data Rate is.
The LoRa packet comprises of three elements:
Preamble, header (optional) and payload.
There are two types of LoRa packet format modes:
- The explicit header mode includes a short header that contains information about the payload length, coding rate and whether a CRC is used in the packet.
CRC stands for Cyclic Redundancy Check and is used to detect errors in digital data.
- The implicit header mode where the payload, coding rate and CRC presence are fixed.
In this mode the header is removed from the packet thus reducing transmission time.
In this case the payload length, error coding rate and presence of the payload CRC must be manually configured on both sides of the radio link.
When a signal is send from a sender it takes a certain amount of time before a receiver receives this signal.
This time is called Time on Air (ToA).
The total air transmission time of a LoRa packet, aka Time on Air (ToA), aka LoRa packet duration (Tpacket) is calculated as follow:
ToA = Tpacket = Tpreamble + Tpayload [2]
Preamble duration (Tpreamble) in sec
Payload duration (Tpayload) in sec
Given the potentially long duration of the packet at high spreading factors the Low Data Rate Optimization option can be set to improve the robustness of the transmission to variations in frequency over the duration of the packet transmission and reception.
When the Low Data Rate Optimization is enabled it increases the robustness of the LoRa link at these low effective data rates.
Its use is mandated when the symbol duration exceeds 16ms.
Note that both the transmitter and the receiver must have the same setting for LowDataRateOptimize.
The LowDataRateOptimize is enabled for bandwidth 125 kHz and Spreading Factor greater or equal than 11
An online tool to calculate the Time on Air see:
www.loratools.nl/#/airtime
A higher spreading factor means a longer Time on Air and a lower data rate as can be seen in the previous graph.
A higher spreading factor also means the sender-receiver range gets larger but it requires more energy (battery power).
The LoRaWAN protocol defines the Adaptive Data Rate (ADR) scheme to control the uplink transmission parameters of LoRa devices:
- Spreading Factor (SF)
- Bandwidth (BW)
- Transmission power
Whether the ADR functionality will be used is requested by the end nodes by setting the ADR flag in the uplink message.
If the ADR flag is set, the network server can control the end node’s transmission parameters.
ADR should only be used in stable Radio Frequency (RF) situations where end nodes do not move.
Mobile end nodes which are stationary for longer times can enable ADR during those times.
What now follows is a simplified explanation how the Adaptive Data Rate works.
The network server collects the 20 most recent uplink transmissions data (such as data rate and Signal-to-Noise Ratio (SNR)) from an end node.
Of the 20 received uplink messages the network server takes the maximum SNR value which is now called the SNRmeasured and the corresponding data rate.
The network server calculates the margin:
margin = SNRmeasured - SNRlimit - margindefault = 5 - (-20) - 10 = 15 dB
The margin is large which means the end node uses unnecessary too much battery power to transmit uplink messages.
The network server does the following calculation with an optimised data rate:
margin = SNRmeasured - SNRlimit - margindefault = 5 - (-7.5) - 10 = 2.5 dB
The margin is changed from 15 dB to 2.5 dB, but it can further be optimised by lowering the transmission power.
This optimised data rate and/or transmission power are the transmission parameters calculated by the network server and send to the end node.
The end node will use these new transmission parameters to transmit its data and thus using less power.
How the ADR actually works is explained in detail in the LoRaWAN specification [4].
Check out all my other LoRa/LoRaWAN tutorial videos:
• LoRa/LoRaWAN tutorials
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/ @mobilefish
The presentation used in this video tutorial can be found at:
www.mobilefish.com/developer/...
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7 окт 2018
