Alvarion BreezeAccess II System Manual page 219

Menus and Parameters
2-89
! Interference Avoidance Parameters: These parameters enable to
define the interference avoidance algorithm. The algorithm is
designed to minimize the probability of the modem identifying
interference as a possible desired signal. Such an occurrence should
be avoided, as locking on the interfering signal may disable the
modem from timely identifying the appearance of a desired signal.
Setting a certain level as a threshold below which signal are
considered as interference is not sufficient, since the energy in the
relevant spectrum of interfering signals may temporarily be well
above the threshold level. On the other hand, setting the threshold
at a higher level to overcome the effect of temporary occurrences of
high-level interferences is not a good solution, as it may cause the
unit to reject desired signals. The solution is a dynamic decision
algorithm that will follow temporary high-level interferences and
adjust the decision threshold accordingly.
The algorithm for identifying transmissions of messages initiated by
another unit includes two settable parameters: Noise Floor and
Carrier to Interference Difference Level.
The Noise Floor parameter represents the equivalent noise level in
the neighborhood of the unit, including both the thermal noise and
average level of interference in the relevant spectrum.
Carrier to Interference Difference Level is the minimal difference
between the level that is defined as the Current Noise Level and the
level of a "good" signal (a signal that the unit should treat as a
possibly desired signal).
The modem of the unit continuously estimates the level of the
received energy at the relevant spectrum. The Current Noise Floor
is defined as follows:
If the last estimated energy level was lower than or equal to the
Noise Floor, then the Current Noise Floor is equal to the Noise
Floor.
If the last estimated level was above the Noise Floor, it can be either
interference or a possibly "good" signal. If it is lower than the last
Current Noise Floor, or if the difference from the last Current
Noise Floor is less than the Carrier to Interference Difference
Level, it shall be considered as interference and it becomes the
updated Current Noise Floor.
Carrier to Interference Difference Level is the minimal difference
between the Current Noise Floor and the level of a signal that will
be defined as a "good" one.
This means that the actual threshold to be used for decision on the
existence of a valid signal is the higher of the following two levels:
a. Carrier Sense Level
b. "Current Noise Floor" + Carrier to Interference Difference Level.
This algorithm allows the unit to identify and track temporary high
level interference and to adjust the decision threshold accordingly.
BreezeACCESS Version 4.5 System Manual