AirborneM2M™ User Guide
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The reciprocal is also true. If the radio observes good link quality at one rate, it will try to move up to the
next rate to see if communication can be sustained using it. It is important to note that, for a given position,
the link quality improves as the link rate is reduced. This is because, as the link rate drops, the radio’s
Transmit Power and Receive Sensitivity improve.
From this it can be seen that looking at the link rate is an indirect way of assessing the quality of the link
between the device and an access point. You should strive to make the communication quality as good as
possible in order to support the best link rate. However, be careful not to over specify the link rate. Consider
your application’s bandwidth requirements and tailor your link rate to optimize the link quality. For example,
the link quality for a location at 6Mb/s is better than it would be for 54Mb/s. If the application only needs
2Mb/s of data throughput, the 6Mb/s rate would provide a better link quality.
Aside from the radio performance, there are a number of other things that contribute to the link quality.
These include items discussed earlier and choices made when looking at the overall antenna gain. The
antenna gain contributes to the Equivalent Isotropically Radiated Power (EIRP) of the system. This is part of
an overall measurement of the link quality called “link margin”.
Link Margin provides a measure of all the parts of the RF path that impact the ability of two systems to
communicate. The basic equation looks like this:
EIRP (dB) = TxP + TxA – TxC
Link Margin (dB) = EIRP – FPL + (RxS + RxA – RxC)
Where: TxP = Transmitter output power (dBm)
TxA = Transmitter antenna gain (dBi)
TxC = Transmitter to Antenna coax cable loss (dB)
FPL = Free Path Loss (dB)
RxS = Receiver receive sensitivity (dBm)
RxA = Receiver antenna gain (dBi)
RxC = Receiver to Antenna coax cable loss (dB)
This is a complex subject and requires more information than is presented here. B+B SmartWorx
recommends reviewing the subject and evaluating any system at a basic level.
It is then possible, with a combination of the above items and an understanding of the application demands,
to achieve a link quality optimized for the application and host design. It is important to note that this is
established with a combination of hardware selection, design choices, and configuration of the radio.
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