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4.2.6.2.10 Cell Distance Parameters (AU only)
The higher the distance of an SU from the AU that is serving it, the higher the
time it takes for messages sent by one of them to reach the other. To ensure
appropriate services to all SUs regardless of their distance from the AU while
maintaining a high overall performance level, two parameters should be adapted
to the distances of SUs from the serving AU:
The time that a unit waits for a response message before retransmission (ACK
timeout) should take into account the round trip propagation delay between
the AU and the SU (The one-way propagation delay at 5 GHz is 3.3
microseconds per km/5 microseconds per mile.). The higher the distance from
the AU of the SU served by it, the higher the ACK timeout should be.
The ACK timeout in microseconds is: 20+Distance (km)*2*3.3 or
20+Distance (miles)*2*5.
To ensure fairness in the contention back-off algorithm between SUs located
at different distances from the AU, the size of the time slot should also take
into account the one-way propagation delay. The size of the time slot of all
units in the cell should be proportional to the distance from the AU of the
farthest SU served by it.
The Cell Distance Mode parameter in the AU defines the method of computing
distances. When set to Manual, the Maximum Cell Distance parameter should be
configured with the estimated distance of the farthest SU served by the AU. When
set to Automatic, the AU uses a special algorithm to estimate its distance from
each of the SUs it serves, determine which SU is located the farthest and use the
estimated distance of the farthest SU as the maximum cell distance. The value of
the maximum cell distance parameter (either computed or configured manually)
is transmitted in the beacon messages to all SUs served by the AU, and is used
by all units to calculate the size of the time slot, that must be the same for all
units in the same sector. When the Per SU Distance Learning option is enabled,
the AU uses the re-association message to send to each SU its estimated distance
from the AU. The per-SU distance is used to calculate the ACK timeout to be used
by the SU. When the Per SU Distance Learning option is disabled (or if it cannot
be used because the SU uses a previous SW version that does not support this
feature), the SU will use the maximum cell distance to calculate the ACK timeout.
The AU always uses the maximum cell distance to calculate the ACK timeout.
It should be noted that if the size of the time slot used by all units is adapted to
the distance of the farthest unit, then no unit will have an advantage when
competing for services. However, this reduces the overall achievable throughput
of the cell. In certain situations, the operator may decide to improve the overall
throughput by reducing the slot size below the value required for full fairness
(using the Fairness Factor parameter). This means that when there is competition