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performance.
<p>
The second part involves developing novel decoders and
demodulators that achieve favorable tradeoffs of complexity
versus performance. Within this area a number of current
topics are being investigated, such as the design of minimal
trellises for block codes, fundamental limits for decoders
with a reduced number of states, decoding algorithms for
time-varying channels, and so on.
<hr>
<br>
<DT><B>
Spread-Spectrum in Faded Channels
<p>
Graduate Students:  D. Goeckel and V. Chang
<br>
Professor:  W. Stark
</B>
<DD>
In this project we are examining the performance of
spread-spectrum systems in a faded channel.  The type of
fading is such that in one spread-spectrum system with a
(relatively) small bandwidth the fading appears to be
nonselective in frequency.  In another spread-spectrum
system the fading is frequency selective.  The goal is to
examine the performance of a direct-sequence spread-spectrum
system operating in the presence of multipath fading with
error control coding.  The important issues are the channels
memory, the selectivity of the channel, the synchronization
algorithm, and the decoding approach.  These are issues that
are not very well understood by system designers presently.
Our preliminary results indicate that for a nonselective
channel larger spreading improves performance in spite of
the fact that more of the received energy is treated as
interference rather than part of a faded signal.  In other
words, as the bandwidth increases the multipath channel
becomes more resolveable and signals that were unresolved
before can be resolved and rejected by the processing gain
of the spread-spectrum system.
<hr>
<br>
<DT><B>
Communicating Over Power Lines
<p>
Graduate Student:  Y-P. Wang
<br>
Professor:  W. Stark
</B>
<DD>
The goal of this research is to investigate different
alternatives for transmitting data over a power line.  The
power line suffers from distortion because of the nonideal
characteristics of the media.  This comes in two forms.  The
first is due to the attenuation varying as a function of
frequency.  The second form is multipath due to reflections
off of mismatched lines. Transmitting data over power lines
using spread-spectrum techniques can mitigate the distortion
present in the channel.  We are investigating different
modulation and coding schemes with spread-spectrum for
transmitting data over the power line.
<hr>
<br>
<DT><B>
Optical Communications and Very Noisy Channels
<p>
Graduate Student:  S. Lee
<br>
Professor:  K. A. Winick
</B>
<DD>
A very noisy channel is a channel whose capacity is close to
zero.  Very noisy channels (VNCs) can be used to model many
physical channels operating at low signal-to-noise ratios.
More importantly, a large class of physical channels,
operating at arbitrary signal-to-noise ratios, can be
modeled as repeated uses of a VNC.  In particular, this is
true for the infinite bandwidth additive white Gaussian
noise channel and the direct detection optical Poisson
channel.  The error exponent indicates the best achievable
performance of  any block codes used over a communications
channel.  A code which achieves this best performance is
said to be exponentially optimum.   For most channels, the
error exponent is not known and can only be bounded.  In
this research, the error exponent is computed exactly for a
large class of VNCs, and exponentially optimum codes are
explicitly constructed for these channels.  These ideas are
applied to derive both the error exponent and exponentially
optimum codes for the direct detection,
polarization-switched, optical channel.
<hr>
<br>
<DT><B>
Distance Bounds for Runlength-Constrained Codes
<p>
Graduate Student:  S-H. Yang
<br>
Professor:  K.  A. Winick
</B>
<DD>
One of the most basic problems in coding theory is to find
the largest code of a given length and minimum distance.
There are several known upper and lower bounds when the
codewords are unconstrained.  In many digital transmission
and recording systems, considerations such as spectral
shaping, self-clocking, and reduction of intersymbol
interference require that the recorded sequences satisfy
special run-length constraints.  In this research distance
bounds and the construction of runlength-constrained
error-correcting codes are investigated.  Upper bounds are
derived for the minimum achievable distance of
runlength-constrained sequences, and lower bounds are also
derived which include cost constraints. 
<hr>
<br>
<DT><B>
Runlength-Constrained Write-Once Memories
<p>
Graduate Student:  S-H. Yang
<br>
Professor:  K. A. Winick
<br>
Sponsors:  Office of Naval Research; Office of Naval
Technology
<br>
</B>
<DD>
A write-once memory (WOM) is a storage medium where the
value in each bit location can only be changed from the
virgin 0-state to the permanent 1-state irreversibly.  Data
can be recorded by marking blank (i.e., 0-state)  bits.
Those marked locations are stuck in the 1-state and hence
limit to some degree further use of the memory.  Examples of
WOMs in the electronic and computer industry are punch
cards, paper tapes, PROMs and optical disks.  Current
laser-optics technology produces the "write-once" CD-ROMs
that are especially sutiable for storing archival data.
Usually this data must be periodically updated after it has
been initially recorded.  If we can re-use the write-once
disk by implementing an efficient coding scheme, then the
expense of replacing the whole disk may be saved.  In this
research, the ultimate capacity of runlength-constrained
write-once memories is investigated using techniques from
information theory.
<hr>
<br>
<DT><B>
Corrugated Waveguide Filters
<p>
Graduate Students:  C. Brooks and G. Vossler
<br>
Professor:  K.  A. Winick
<br>
Sponsor:  National Science Foundation 
<br>
</B>
<DD>
Corrugated thin film waveguides play a major role in
lightwave devices.  Applications include distributed
feedback lasing, bistable switching, phase matching in
nonlinear materials, pulse compression, grating coupling,
and optical filtering.  In many of these applications, the
corrugation is periodic.  In an aperiodically corrugated
thin film waveguide, however, the frequency dependent
coupling between waveguide modes can be used to produce a
filter which has a specified spectral response.  Inverse
scattering techniques have been developed for designing such
filters, and  efforts are currently underway to fabricate
these devices.  Several new fabrication techniques are being
pursued.  These include an optical direct write method,
based on photobleaching gamma ray-induced defect centers in
ion-exchangeable glasses, and a Rbent waveguideS approach.
The first filter to be demonstrated will compensate for
dispersion-induced pulse spreading in optical fibers.
<hr>
<br>
<DT><B>
Rare Earth-Doped Waveguide Lasers
<p>
Graduate Students:  G. Vossler and C. Brooks
<br>
Professor:  K.  A. Winick
<br>
Sponsors:  National Science Foundation;
NSF Center for Ultrafast Optical Science;
Smith Industries;
IMRA America, Inc.
<br>
</B>
<DD>
Recently, the development of rare earth-doped fiber lasers
has received considerable attention.  These fiber lasers
exhibit a host of desirable properties.  First, they permit
wide tuning ranges and short pulse generation because of
their broad emission lines.  Second the pump powers required
for lasing are low, since the pump beam is strongly confined
to a small volume.  Finally, rare earth-doped lasers offer
better frequency stability, longer lifetimes, and less
temperature sensitivity than semiconductor devices.  These
traits make them promising devices for telecommunications,
sensing, and spectroscopic applications.  Glass waveguide
lasers on planar substrates are a natural extension of the
fiber technology.  As opposed to a fiber, it should be
possible to integrate monolithically multiple components
onto a single glass substrate.  These components could
include distributed feedback laser mirrors, grating
couplers, mode-lockers, and nonlinear elements.  We have
fabricated neodymium-doped, channel, waveguide lasers in
special glass melts and have demonstrated the first glass
integrated optic distributed Bragg reflector laser.  Efforts
are currently under way to passively mode-lock these lasers
and  to extend theses results to rare earth-doped lithium
niobate hosts.  Novel sensors, based on this technology, are
also under development.
<hr>
<br>
<DT><B>
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