Andrew Puryear

Reciprocity-enhanced optical communication through atmospheric turbulence — Part I: Reciprocity proofs and far-field power transfer optimization

Deep (>;10 dB) long-duration (>;1 ms) scintillation fades, caused by propagation through refractive-index turbulence, are the principal impairment that must be overcome to realize Gbps-class laser communication over line-of-sight atmospheric paths in clear-weather conditions. Spatial diversity reception can ameliorate such fades, to a degree, but current systems typically rely on forward error-correction and interleaving to achieve reliable communication over the atmospheric channel. This paper, together with its companion [A. L. Puryear, J. H. Shapiro, and R. R. Parenti, “Reciprocity-enhanced optical communication through atmospheric turbulence - Part II: Communication architectures and performance,” to be submitted to J. Opt. Commun. Netw.], comprise a two-part study that introduces and analyzes an alternative approach, in which atmospheric reciprocity is exploited to eliminate the need for interleaving and minimize the amount of forward error-correction required. The present work (Part I) first describes the problem setting and then presents proofs for reciprocity principles - with and without phase compensation - that apply under rather general conditions. By specializing to the far-field regime, the optimum (power-transfer maximizing) phase compensation is identified. These results underlie the communication architectures and performance analysis that will be reported in the Part II paper.

Using Spatial Diversity to Improve the Confidentiality of Atmospheric Free Space Optical Communication

Free space optical communication through the atmosphere has the potential to provide secure, low-cost, rapidly deployable, dynamic, data transmission at very high rates. Despite the ability for free space optical technology to transmit a narrow beam of light to a specific destination, it is still possible for an eavesdropper to gather light and decode information intended for another user. This paper addresses the use of spatial diversity to improve confidentiality, thereby impeding potential eavesdroppers. We find that spatial diversity with coherent detection and wavefront predistortion based on turbulence state feedback is an effectively technique to improve the confidentiality of free space optical communication.

Reciprocity-enhanced optical communication through atmospheric turbulence — Part II: Communication architectures and performance

Free-space optical (FSO) communication provides rapidly deployable, dynamic communication links that are capable of very high data rates compared with those of radio-frequency systems. As such, FSO 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 FSO communication links are primarily limited by fading caused by microscale atmospheric temperature variations that create parts-permillion 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 tolerance is driven lower. This paper, along with its companion [J. Opt. Commun. Netw. 4, 947 (2012)], present communication systems and techniques that exploit atmospheric reciprocity to overcome turbulence that are viable for high data rate and low delay tolerance systems. Part I proves that reciprocity is exhibited under rather general conditions and derives the optimal power-transfer phase compensation for far-field operation. Part II presents capacity-achieving architectures that exploit reciprocity to overcome the complexity and delay issues that limit state-of-the-art FSO communications.

Experimental analysis of the time dynamics of coherent communication through turbulence: Markovianity and channel prediction

Clear air atmospheric turbulence causes significant fading for terrestrial-terrestrial and terrestrial-satellite free space optical communication systems. Typically extra link margin is used to assure link availability and reliability, however this extra margin is an inefficient and expensive use of resources. In this paper, we analyze data collected by an experimental system with a single laser transmitter located 250 meters from two coherent receivers. We first use the data to validate the use of a two-state continuous time Markov process to model outage statistics of the diversity system. In the two-state channel model, symbols received during an outage are assumed to be lost, and symbols received during a non-outage are assumed to be received correctly. This channel model can be used to analyze the performance of the transport layer. Next, we use statistical and spectral analysis techniques to create a linear prediction model for signal attenuation for both the single-receiver and diversity systems. The prediction model is an optimal estimator that predicts signal attenuation 1 ms into the future to 1.5 dB accuracy for the single-receiver cases and to 1 dB accuracy for the diversity case. The maximum amount of time the estimator can predict into the future with some confidence is about 5-10 ms. This channel prediction and adaptation can be used to greatly improve the efficiency of free-space optical communication systems in the atmosphere.

Coherent Optical Communication over the Turbulent Atmosphere with Spatial Diversity and Wavefront Predistortion

Optical communication through the atmosphere has the potential to provide data transmission over a distance of 1 to 100km at very high rates. The deleterious effects of turbulence can severely limit the utility of such a system, however, causing outages of up to 100ms. In this paper, we investigate the use of spatial diversity with wavefront predistortion and coherent detection to overcome these turbulence-induced outages. This system can be realized, especially if the turbulence or the receiver is in the nearfield of the transmitter. New results include closed-form expressions for average bit error rate, outage probability, and power efficiency. Additionally, system performance gains in the presence of a worst-case interferer are presented. These significant results are used to develop design intuition for future system implementation.

On the Time Dynamics of Optical Communication Through Atmospheric Turbulence With Feedback

In the turbulent atmosphere, wavefront predistortion based on receiver-to-transmitter feedback can significantly improve the performance of optical communication systems that employ sparse aperture transmit and receive spatial diversity. The time evolution of the atmosphere, as wind moves turbulent eddies across the propagation path, can limit any improvement realized by wavefront predistortion with feedback. The improvement is especially limited if the latency is large or the feedback rate is small compared to the time it takes for turbulent eddies to move across the link. In this paper, we develop a physics based channel model that describes the time evolution of atmospheric turbulence. Based on that channel model, we derive theoretical expressions relating latencies - such as feedback latency and channel state estimate latency - and feedback rate to optimal performance. Specifically, we find the theoretical optimal average bit error rate as a function of fundamental parameters such as wind speed, atmospheric coherence length, feedback rate, feedback latency, and channel state estimate latency. Further, we describe a feedback strategy to achieve the optimal bit error rate. We find that the sufficient feedback rate scales linearly with the inverse of the atmospheric coherence time and sub-linearly with the number of transmitters. Under typical turbulence conditions, low-rate feedback, of the order of hundreds of bits per second, with associated latencies of less than milliseconds is sufficient to achieve most of the gain possible from wavefront predistortion.

Optical Communication over Atmospheric Turbulence with Limited Channel State Information at the Transmitter

In recent results, a theoretical framework has been developed that shows that wavefront predistortion with transmitter channel state information can effectively mitigate turbulence induced fading for free space communication systems. The theoretical framework, however, relies on the assumption of perfect channel state information available at the transmitter, which is not physically realizable since it would require an infinite rate feedback link. This paper extends the theoretical framework to physically realizable systems with finite rate feedback. Specifically, we provide the performance and optimal feedback strategy given a finite rate feedback link.

Reciprocity-enhanced optical communication through atmospheric turbulence - part I: reciprocity proofs and far-field power transfer optimization

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.

Reciprocity-enhanced optical communication through atmospheric turbulence - part II: communication architectures and performance

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.

Underwater acoustic sparse aperture system performance: Using transmitter channel state information for multipath & interference rejection

Todays situational awareness requirements in the undersea environment present severe challenges for acoustic communication systems. Acoustic propagation through the ocean environment severely limits the capacity of existing underwater communication systems. Specifically, the presence of internal waves coupled with the ocean sound channel creates a stochastic field that introduces deep fades and significant intersymbol interference (ISI) thereby limiting reliable communication to low data rates. In this paper we present a communication architecture that optimally predistorts the acoustic wave via spatial modulation and detects the acoustic wave with optimal spatial recombination to maximize reliable information throughput. This effectively allows the system to allocate its power to the most efficient propagation modes while mitigating ISI. Channel state information is available to the transmitter through low rate feedback. New results include the asymptotic distribution of singular values for a large number of apertures. Further, we present spatial modulation at the transmitter and spatial recombination at the receiver that asymptotically minimize bit error rate (BER). We show that, in many applications, the number of apertures can be made large enough so that asymptotic results approximate finite results well. Additionally, we show that the interference noise power is reduced proportional to the inverse of the number of receive apertures. Finally, we calculate the asymptotic BER for the sparse aperture acoustic system.