Optical Communication Through the Turbulent Atmosphere with Transmitter and Receiver Diversity, Wavefront Control, and Coherent Detection
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PuryearThesis.pdf
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Author(s)
Puryear, Andrew Lee
Date Issued
May 2, 2011
Series/Report no.
Technical Report (Massachusetts Institute of Technology, Research Laboratory of Electronics);732
Abstract
Free space optical communication through the atmosphere has the potential to provide
secure, low-cost, rapidly deployable, dynamic, data transmission at very high rates.
However, the deleterious e ects of turbulence can severely limit the utility of such a
system, causing outages of up to 100 ms. For this thesis, we investigate an architecture
that uses multiple transmitters and multiple coherent receivers to overcome these
turbulence-induced outages. By controlling the amplitude and phase of the optical
eld at each transmitter, based on turbulence state information fed back from the
receiver, we show that the system performance is greatly increased by exploiting the
instantaneous structure of the turbulence. This architecture provides a robust highcapacity
free-space optical communication link over multiple spectral bands, from
visible to infrared.
We aim to answer questions germane to the design and implementation of the
diversity optical communication architecture in a turbulent environment. We analyze
several di erent optical eld spatial modulation techniques, each of which is based
on a di erent assumption about the quality of turbulence state information at the
transmitter. For example, we explore a diversity optical system with perfect turbulence
state information at the transmitter and receiver that allocates transmit power
into the spatial modes with the smallest propagation losses in order to decrease bit
errors and mitigate turbulence-induced outages. Another example of a diversity optical
system that we examine is a diversity optical system with only a subset of the
turbulence state information: this system could allocate all power to the transmitter
with the smallest attenuation.
We characterize the system performance for the various spatial modulation techniques
in terms of average bit error rate (BER), outage probability, and power gain
due to diversity. We rst characterize the performance of these techniques in the
idealized case, where the instantaneous channel state is perfectly known at both the
receiver and transmitter. The time evolution of the atmosphere, as wind moves tur-
3
bules across the propagation path, can limit the ability to have perfect turbulence
state knowledge at the transmitter and, thus can limit any improvement realized by
optical eld spatial modulation techniques. The improvement is especially limited if
the latency is large or the feedback rate is short compared to the time it takes for
turbules to move across the link. As a result, we make successive generalizations,
until we describe the optimal system design and communication techniques for sparse
aperture systems for the most general realistic case, one with inhomogeneous turbulence
and imperfect (delayed, noisy, and distorted) knowledge of the atmospheric
state.
Description
Thesis Supervisor: Vincent W. S. Chan
Title: Joan and Irwin M. Jacobs Professor of Electrical Engineering and Computer
Science
MIT Department
Massachusetts Institute of Technology. Research Laboratory of Electronics
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