Free-space optical communication provides rapidly deployable, dynamic communication links that are capable
of very high data rates compared with those of radio-frequency systems. As such, free-space optical
communication is ideal for mobile platforms, for platforms that require the additional security afforded
by the narrow divergence of a laser beam, and for systems that must be deployed in a relatively short
time frame. In clear-weather conditions the data rate and utility of free-space optical communication links
are primarily limited by fading caused by micro-scale atmospheric temperature variations that create
parts-per-million refractive-index fluctuations known as atmospheric turbulence. Typical communication
techniques to overcome turbulence-induced fading, such as interleavers with sophisticated codes, lose viability
as the data rate is driven higher or the delay requirement is driven lower. This paper, along with
its companion [J. H. Shapiro and A. Puryear, “Reciprocity-Enhanced Optical Communication through Atmospheric
Turbulence–Part I: Reciprocity Proofs and Far-Field Power Transfer”], present communication
systems and techniques that exploit atmospheric reciprocity to overcome turbulence which are viable for
high data rate and low delay requirement systems. Part I proves that reciprocity is exhibited under rather
general conditions, and derives the optimal power-transfer phase compensation for far-field operation. The
Part II paper presents capacity-achieving architectures that exploit reciprocity to overcome the complexity
and delay issues that limit state-of-the art free-space optical communications. Further, this paper uses
theoretical turbulence models to determine the performance—delay, throughput, and complexity—of the
proposed architectures.
Turbulence-induced scintillation is the principal impairment to Gbps laser communication over clear-weather
atmospheric paths. This paper, plus its companion [A. Puryear, J. H. Shapiro, and R.R. Parenti, “Reciprocity-
Enhanced Optical Communication through Atmospheric Turbulence—Part II: Communication Architectures
and Performance”], introduce and analyze the exploitation of atmospheric reciprocity for combating turbulence.
Part I presents reciprocity proofs that apply under rather general conditions and underlie the communication
performance analysis in Part II.
Net-centric warfare in todays dynamically changing military environments and the need for low-cost gigabit
intra-city communication present severe challenges for current free-space optical systems. Enabled by high-speed
electronics and advances in wavefront control, we develop an architecture that provides free-space coherent
optical links with information capacity, security, network robustness and power management performance that
exceed the current state-of-the-art, including commercially deployed systems, R&D test-beds, and alternative
theoretical architectures proposed. The deleterious effects of the turbulent atmosphere are mitigated with several
diversity transmitters and receivers. We allow the phase and the amplitude of each transmitter to be controlled
independently and assume, through coherent detection, that the phase and amplitude of the received wave
is measured. Thus we can optimally allocate transmit power into the diffraction modes with the smallest
propagation losses to increase channel capacity and mitigate turbulence-induced outages. Additionally, spatial
mode modulation and rejection provides robust communication in the presence of denial of service via interference
by adversaries with a priori knowledge of the system architecture. Some possible implementations of this system
are described. New results, including asymptotic singular value distribution, expected bit error rate, interference
performance, and performance in the presence of inhomogeneous turbulence, are given. Finally, performance of
this system is compared with the performance of optical diversity systems without wavefront control and optical
systems without diversity, both current state-of-the-art systems.
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