In the framework of satellite-to-ground laser downlinks, an analytical model describing the variations of the instantaneous coupled flux into a single-mode fiber after correction of the incoming wavefront by partial adaptive optics (AO) is presented. Expressions for the probability density function and the cumulative distribution function as well as for the average fading duration and fading duration distribution of the corrected coupled flux are given. These results are of prime interest for the computation of metrics related to coded transmissions over correlated channels, and they are confronted by end-to-end wave-optics simulations in the case of a geosynchronous satellite (GEO)-to-ground and a low earth orbit satellite (LEO)-to-ground scenario. Eventually, the impact of different AO performances on the aforementioned fading duration distribution is analytically investigated for both scenarios.

/pdf/statistical-properties-of-single-mode-fiber-coupling-of-25ebq54ndg.pdf

Statistical properties of single-mode fiber coupling of satellite-to-ground laser links partially corrected by adaptive optics.

The impact of atmospheric turbulence is one of the most challenging issues for the widespread deployment of free-space optical (FSO) communication systems. To enhance the systems' performance, adaptive-rate (AR) transmission and automatic repeat request (ARQ) have been separately considered at the physical and data link control layers. This paper introduces a framework of cross-layer design, analysis, and optimization for FSO communication systems, in which ARQ and AR transmission are jointly integrated to further improve the overall system performance over atmospheric turbulence channels. Two cross-layer designs are considered: 1) AR and standard ARQ and 2) AR and ARQ with frame combining. In addition, we newly develop a Markov chain model-based cross-layer analysis to evaluate system performance. System performance metrics, including spectral efficiency, maximum expected number of transmissions, and outage probability, are analytically studied under the presence of atmospheric turbulence. In numerical results, how the cross-layer designs significantly outperform conventional ones is quantitatively shown. Furthermore, we discuss a cross-layer optimization of selecting the ARQ persistence level for the tradeoff between the spectral efficiency and the number of transmissions.

/pdf/cross-layer-designs-and-analysis-of-adaptive-rate-nfsl3l98wj.pdf

Cross-Layer Designs and Analysis of Adaptive-Rate Transmission and ARQ for Free-Space Optical Communications

This paper proposes the concept of adaptive multi-rate (AMR), which jointly employs switching between two links and adaptive rate on each link, for hybrid freespace optical/radio-frequency (FSO/RF) systems. Moreover, we present the cross-layer design of AMR switching, which is based on both the physical and link layers with an automatic-repeat request (ARQ) scheme. We develop an analytical framework based on a Markov chain model for system performance analysis. System performance metrics, including frame-error rate, goodput and link switching probability, are analytically studied over fading channels. Numerical results quantitatively show how the proposal significantly outperforms conventional ones with physical layer-based design and/or fixed-rate switching operation. key words: Hybrid FSO/RF, Turbulence Channel, AutomaticRepeat reQuest (ARQ), Adaptive Multi-Rate (AMR).

/pdf/adaptive-multi-rate-designs-and-analysis-for-hybrid-fso-rf-3qr8j0l8sl.pdf

Adaptive Multi-Rate Designs and Analysis for Hybrid FSO/RF Systems over Fading Channels

In integrated free-space optical/passive optical networks (FSO/PON), providing efficient media access control (MAC) protocol and ensuring high reliable transmission are two major challenging issues. This paper comprehensively addresses these issues through a protocol stack design, which jointly considers the dynamic bandwidth allocation (DBA), adaptive rate (AR) transmission, and automatic repeat request (ARQ) with different retransmission strategies. A 2-D Markov chain model is developed to qualify quality-of-service (QoS) performance metrics in terms of frame loss probability, throughput and delay. Selected numerical results show that the proposed protocol stack design can offer considerable performance improvement over conventional ones.

Integrated FSO/PON for Broadband Access Networks: A Comprehensive Protocol Stack Design and Analysis

The paper deals with a parameterized model for simulation of packet transmission on duplex atmospheric Free-Space Optical (FSO) links. Under certain conditions the optical propagation through atmospheric turbulence is reciprocal, i.e. the power fluctuation in forward and backward channels is highly correlated. The paper deals with a model for generating correlated and colored lognormal time series for time-domain simulation of packet transmission. The probability of occurrence of errors in each packet can be computed analytically based on noise performance of the receiver and one sample of randomly generated received power. The model is demonstrated in a parametric study of performance of FSO terminals whose link layer includes transmitter blocking during deep fades.

Simulation model of correlated FSO channels

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.

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

A high-precision human metabolic measurement system is designed. The system uses STM32F103 as the main control chip to acquire oxygen, carbon dioxide and flow signals to calculate four quantitative indicators: oxygen consumption(VO2), carbon dioxide production(VCO2), respiratory entropy(RQ) and resting energy metabolism(REE), and finally uses an upper computer to display the calculation results.In this paper, the signal acquisition circuit design was carried out for the oxygen sensor, carbon dioxide sensor and flow sensor, and the validity of the device was verified with the American machine MGCDiagnositcs using Bland-Altman analysis method, and the results showed that the four parameters of VO2,VCO2, RQ and REE of both devices fell in the agreement interval of more than 95%. The device thus provides accurate metabolic measurements and offers an effective tool for the field of general health and clinical nutrition support in China.

[Design and Verification of Human Metabolic Measurement System Based on STM32].

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

[Design and Verification of Human Metabolic Measurement System Based on STM32].