B&B Electronics WLNN-EK-DP551 - Product Specification - Manual (Page 28)

B&B Electronics, Inc. Airborne WLNN DP550 Family Databook
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can be defined as the available bandwidth between the two devices. The lower
the link quality, the less likely it will be that the devices can communicate.
Measurement of link quality can be made in several ways; Bit Error Rate (BER),
Signal to Noise (SNR) ratio, Signal Strength and distortion. The link quality is
used by the radio to determine the link rate. In general, when the link quality for
a given link rate drops below a predefined limit the radio will drop to the next
lowest link rate and try to communicate at that rate.
The reverse 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 still be sustained.
Note that for a given position the link quality improves as the link rate is reduced.
As the link rate drops, the radios
’ Transmit power and Receive sensitivity
improve.
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.
But be careful not to over specify the link r
ate. Consider your application’s
bandwidth requirements and tailor your link rate to optimize the link quality. For
example, if the link quality for a location at 6Mb/s is better than it would be for
54Mb/s, and the application only needs 2Mb/s of data throughput, the 6Mb/s rate
would provide better link quality.
Radio performance is only one of the things that contribute to the link quality.
Other factors include the items discussed earlier and choices made regarding
overall antenna gain. The antenna gain contributes to the Equivalent Isotropically
Radiated Power (EIRP) of the system. This is called link margin, and it is part of
the overall measurement of the link quality.
Link Margin provides a measure of all the parts of the RF path that impact the
communications between two systems. 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 we won’t try to cover it here. B&B’s technical
experts can help you if you need to explore it in more detail. But you should
understand that optimizing link quality involves a combination of hardware
selection, design choices and radio configuration.