B&B Electronics APXN-Q5428 - User Manual - Manual (Page 20)

B&B Electronics, Inc. AirborneM2M™ User Manual
20 1/9/2014
 Locate the antenna where there is a minimum of obstruction between the
antenna and the location of the Access Points. Access Points are typically
located in the ceiling or high on walls.
 Keep the main antenna’s polarization vertical, or in-line with the antenna of
the Access Points. 802.11 systems utilize vertical polarization and aligning
both transmit and receive antenna maximizes the link quality.
No connection will ever be perfect. Experiment with the various possibilities until
you get the best connection permitted by the circumstances.
7.3 Performance
Performance will vary according to the application and the circumstances. In
most cases your primary concern will be the link quality, which is a function of the
bandwidth available between two devices. In general, as the link rate drops the
radio’s Transmit Power, Receive Sensitivity and link quality improve.
Measurement of link quality can be made in several ways. Bit Error Rate (BER),
Signal to Noise (SNR) ratio and Signal Strength are all very useful. The link
quality is used by the radio to determine the link rate. When the link quality for a
given link rate falls below a predefined limit, the radio will drop to the next lowest
link rate and try to communicate using that one.
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 be maintained at the
higher rate.
So consider your application ’s actual bandwidth requirements and tailor your link
rate to optimize the link quality. For example, the link quality at 6Mb/s is likely to
be 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 the 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 Link Margin, an overall measurement of link quality.
Link Margin provides a measurement 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)
To learn more about Link Margin, visit B&B Electronics ’ online technical library.
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