HMM-Based Methods
In the prediction-based methods discussed above, the narrowband
interference environment is assumed to be stationary or, at worst, slowly
varying. In some applications, however, the interference environment is dynamic
in that narrowband interferers enter and leave the channel at random and at
arbitrary frequencies within the spread bandwidth. An example of such an
application arises in the littoral sonobuoy arrays mentioned in Section 7.1, in which
shore-based commercial VHF traffic, such as dispatch traffic, appears throughout
the spread bandwidth in a very bursty fashion. A similar phenomenon arises when
the direct-sequence system coexists with a frequency-hopping system, which
happens, for example, when wireless LANs and Bluetooth systems operate in the
same location. A difficulty with use of adaptive prediction filters of the type
noted above is that when an interferer suddenly drops out of the channel, the
"notch" that the adaptation algorithm created to suppress it will persist for
some time after the signal leaves the channel. This is because, while the energy
of the narrowband source drives the adaptation algorithms to suppress an
interferer when it enters the channel, there is no counterbalancing energy to
drive the adaptation algorithm back to normalcy when an interferer exits the
channel. That is, there is an asymmetry between what happens when an interferer
enters the channel and what happens when an interferer exits the channel. If
interferers enter and exit randomly across a wide band, this asymmetry will
cause the appearance of notches across a large fraction of the spread bandwidth,
which will result in a significantly degraded signal of interest. Thus, a more
sophisticated approach is needed for such cases. One such approach, described in
[67], is based on a
hidden-Markov model (HMM) for the process controlling the exit and entry of NBIs
in the channel. An HMM filter is then used to detect the subchannels that are
hit by interferers, and a suppression filter is placed in each such subchannel
as it is hit. When an exit is detected in a subchannel, the suppression filter
is removed from that subchannel. Related ideas for interference suppression
based on HMMs and other "hidden data" models have been explored in [219, 238, 355, 375].
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