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Appendix B: Listen Before Talk
airFiber
®
X User Guide
Ubiquiti Networks, Inc.
Opportunities for Other Transmitters
to Operate
No differences are performed between start-up mode
and operational mode. The Master/Slave device goes
into an energy scanning mode where it is allowed to do
one full cycle of energy detection before it is allowed to
transmit. The scan is a mode where the average energy
is accumulated to compare against the programmed
detection threshold. If the energy detected is less than the
detection threshold, the Master/Slave device is allowed to
make a transmission.
In normal operational mode the system does not allow
any transmission once the threshold has been detected.
Once the energy is no longer present will the system
begin to transmit again.
The Master and Slave uses the energy detected prior to
each transmit frame to control the muting of the transmit
function.
At any load level (no load, typical, or overload), the
system transmits data based on the configuration for the
uplink/downlink ratio The same amount of bandwidth is
reserved for channel detection, so performance remains
unaffected.
If there are two AF-3X systems on a co-channel, they
would share the spectrum as follows:
•
Since the system is a synchronized frame-based system,
both systems would operate effectively with each other
because each system can be configured to transmit and
receive at the same point in time. The system uses a
GPS synchronization to time-align the start-of-frame for
all systems deployed.
• If the users configure the number of uplink and
downlink slots and the duty cycle (uplink/downlink
ratio) in an identical manner and each system uses the
same set of uncommitted uplink slots for the detection
sampling interval, then the two systems will co-exist
with no knowledge of the others presence.
• Each system listens prior to transmit and if it detects
activity yields to the other system.
• The system will shut down when the accumulated
energy is above the detection threshold and will
continue to transmit if it is below the detection
threshold. When the co-channel system is clear of the
channel then the target system will go back into regular
operation. Regular operation will consist of a constant
averaging of the energy detected in the uplink slots.
