In free-space optical communication links using intensity modulation and direct detection (IM/DD), atmospheric turbulence-induced intensity fluctuations can significantly impair link performance. Communication techniques can be applied to mitigate turbulence-induced intensity fluctuations (i.e., signal fading) in the regime in which the receiver aperture D/sub 0/ is smaller than the fading correlation length d/sub 0/ and the observation interval T/sub 0/ is smaller than the fading correlation time /spl tau//sub 0/. If the receiver has knowledge of the joint temporal statistics of the fading, maximum-likelihood sequence detection (MLSD) can be employed, but at the cost of high computational complexity. We introduce a single-step Markov chain (SMC) model for the fading correlation and use it to derive two low-complexity, suboptimal MLSD algorithms based on per-survivor processing (PSP). Simulations are presented to verify the SMC model and the performance improvement achieved using these suboptimal per-survivor processing (PSP) algorithms.

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Markov chain model in maximum-likelihood sequence detection for free-space optical communication through atmospheric turbulence channels

P-i-N (PIN) diodes and avalanche photo-diodes (APD) are the most commonly used photo-detectors in terrestrial FSO systems. In this paper, we review the photo-detection process for the cases of PIN- and APD-based receivers and provide a comprehensive study of different noise sources that affect signal detection in an FSO system. We present a complete and precise model for the receiver noise by taking different receiver parts into account. In particular, we study the impact of thermal, shot, background, and transmitter noises on the receiver performance by considering practical and realistic case studies. We bring clearance on the impact of the interaction of signal and background noise due to non-linear characteristic of the photo-detector, and on the role of the trans-impedance load resistance. We discuss the dominant noise components for the cases of using a PIN or an APD, and compare their performances at the presence or not of background radiations.

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Impact of different noise sources on the performance of PIN- and APD-based FSO receivers

This work analyzes laser communication between a cluster of nanosatellites, which is a concentrated formation of small lightweight satellites and a ground station. The scenario under consideration is a cluster of nanosatellites communicating by means of a laser beam with a detector array receiver that is located on the earth's surface and equipped with a common optical system for all incoming beams. The beams are concentrated to spots over the detector plane by the receiver's optics. The detector array enables the ground station to communicate with a tight concentration of the nanosatellites, which reduces system complexity and cost. A critical parameter that determines the successful receipt and subsequent decoding of a transmitted signal for a given configuration is the angular separation between the satellites within the cluster. This separation must be retained to prevent critical overlapping of the spots on the detector's surface. The maximum allowable overlapping is calculated in terms of given bit-error rate. The spatial spreading of the beams, caused by scattering from aerosols in different layers of the atmosphere, is calculated for the case of single scattering. A stratified model of the atmosphere is used. Turbulence influences the beam width, especially for the case of short exposure, and is primarily caused by temperature changes, which result in fluctuations in the refractive index. In this research, a new approach is adopted for analyzing communication network performance through the atmosphere by applying optical-transfer function (OTF) concepts used in imaging and remote sensing. We evaluate the effectiveness of this new approach in applications where spatial spread between the users is very important.

Analyzing the performance of a nanosatellite cluster-detector array receiver for laser communication

Photon counting detector arrays are commonly used for free-space optical communications in deep space. Such detector arrays—by virtue of their size—help in the collection of the optical signal even when there is some misalignment between the transmitter and receiver systems. In this paper, we argue that, for the common Gaussian beam profile, a detector array receiver is more useful for minimizing the probability of error than a single detector receiver of the same dimensions. Furthermore, the improvement in the error probability is more pronounced for low signal-to-noise ratio conditions, and the probability of error decreases monotonically as a function of the number of detectors in the array. However, communication with detector arrays results in a larger computational and hardware complexity at the receiver.

Free-Space Optical Communications With Detector Arrays: A Mathematical Analysis

Absorption, scattering and turbulence are the three main characteristics for underwater optical wireless communications (UOWC). Among these three factors, absorption and scattering, respectively, characterize the energy loss and direction change when photons propagate through underwater wireless channels interacting with water molecules or suspended particles. In recent years, several analytical methods originated from free space optical communications are used to model various underwater channels, while only statistical models are employed to consider the effects of absorption and scattering containing all the scattering components. To facilitate in-depth theoretical analysis, we propose an expression to model the spatial probability density function of optical intensity for all scattering components in UOWC links in this paper. After that, we model the photodiode receiving process and give a clear explanation on the related parameters. Numerical results indicate that the bit-error rate performance deteriorates as the turbulence gets stronger, and larger multiple-input multiple-output array can alleviate the negative effect caused by fading. Besides, the effect of turbulence on BER is more important than that of the link geometry, and the increase in transmitted power will weaken the diversity gain from MIMO configuration.

On Integrated Stochastic Channel Model for Underwater Optical Wireless Communications

An avalanche photodiode (APD) array for ground-based optical communications receivers is investigated for the reception of optical signals through the turbulent atmosphere. Kolmogorov phase screen simulations are used to generate realistic spatial distributions of the received optical fleld. It is shown that use of an APD array for pulse-position modulation detection can improve performance by up to 4 dB over single APD detection in the presence of turbulence, but that photoncounting detector arrays yield even greater gains. I. Introduction Ground-based reception of optical signals from space sufiers from degradation of the optical phase front caused by atmospheric turbulence, leading to a reduction in the efiective diameter of the receiving telescope and to random ∞uctuations of the point-spread function in the focal plane. A proportional increase in the receiver’s fleld of view in order to collect all of the signal also causes a corresponding increase in the amount of interfering background radiation, resulting in degraded communications performance. These problems may be mitigated through the use of an optical detector array assembly in the focal plane that can adaptively select areas of higher signal density while ignoring areas predominated by background noise. In [1], the performance of a detector array composed of photon-counting detectors was evaluated and found to yield up to a 5 dB improvement over a conventional single-detector photoncounting receiver. However, the current baseline receiver design for deep-space optical communication utilizes readily available avalanche photodiode detectors (APDs) rather than photon-counting detector arrays, which are still in the development stage at the wavelengths of interest for optical communications. In this article, we extend some of the results obtained in [1] to the APD array case.

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V. Vilnrotter

Papers

Avalanche Photodiode Arrays for Optical Communications Receivers