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Chapter 4 - Operation and Administration
Multirate Parameters
BreezeMAX employs a multirate algorithm to dynamically adapt the
modulation scheme and Forward Error correction (FEC) coding to actual
link conditions. The algorithm is managed by the AU/µBST taking into
account also information received from the served SUs, and optimal
values are calculated separately for the uplink and downlink for each
SU, taking into account also the applicable QoS requirements. MAP
messages transmitted to the SUs include information on the uplink rate
that should be used by each SU for its next transmission.
The Basic Rate is the minimum rate to be used by the Multirate
algorithm. In the downlink, this is also the rate to be used for
broadcasts and multicasts. Broadcasts and multicasts messages are not
acknowledged, so that the ARQ mechanism cannot be used and there is
no way to guarantee that all intended recipients will receive them
properly. In addition, AU's/µBST's multicasts and broadcasts are sent
to multiple recipients with different link qualities. Therefore, it is
preferred to use a relatively low rate for these transmissions, thus
reducing the probability of errors and increasing the likelihood that all
intended recipient will receive them properly.
In the uplink, this is the rate to be used by SUs for non-scheduled
transmissions such as during the contention period.
The Basic Rate is also the initial rate to be used by the algorithm for
each new SU that joins the cell when the Multirate algorithm is enabled.
When the Multirate algorithm is disabled, communication with
connected SUs will continue using the last uplink and downlink rates
selected by the Multirate algorithm. The Set Rates option in the SU (see
Set Rates
on page 4-58), which becomes available only when the
Multirate algorithm is disabled in the AU/µBST, enables setting the
Uplink Current Rate and the Downlink Current Rate to any of the
values listed in Table 4-3.
The multirate algorithm chooses dynamically between 8 rates. These are
also the rates that can be configured for the Base Rate and Default Rate
parameters.
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BreezeMAX System Manual
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