Ricardo Barrios

Exponentiated Weibull distribution family under aperture averaging for Gaussian beam waves

Nowadays, the search for a distribution capable of modeling the probability density function (PDF) of irradiance data under all conditions of atmospheric turbulence in the presence of aperture averaging still continues. Here, a family of PDFs alternative to the widely accepted Log-Normal and Gamma-Gamma distributions is proposed to model the PDF of the received optical power in free-space optical communications, namely, the Weibull and the exponentiated Weibull (EW) distribution. Particularly, it is shown how the proposed EW distribution offers an excellent fit to simulation and experimental data under all aperture averaging conditions, under weak and moderate turbulence conditions, as well as for point-like apertures. Another very attractive property of these distributions is the simple closed form expression of their respective PDF and cumulative distribution function.

Probability of fade and BER performance of FSO links over the exponentiated Weibull fading channel under aperture averaging

Recently a new proposal to model the fading channel in free-space optical links, namely, the exponentiated Weibull (EW) distribution, has been made. It has been suggested that the EW distribution can model the probability density function (PDF) of the irradiance under weak-to-strong conditions in the presence of aperture averaging. Here, we carry out an analysis of probability of fade and bit error-rate (BER) performance using simulation results and experimental data. The BER analysis assumes intensity modulation/direct detection with on-off keying, and new expressions are derived. Data is modeled following the statistics of the EW fading channel model, and compared with the Gamma-Gamma and Lognormal distributions, as the most widely accepted models nowadays. It is shown how the proposed EW model is valid in all the tested conditions, and even outperforms the GG and LN distributions, that are only valid under certain scenarios.

Digital modulation and coding for satellite optical feeder links

In this paper, a digital transmission scheme protected by a packet-level forward error correction (FEC) coding technique is proposed for optical feeder links in a satellite communication system. The architectures of the gateway and the satellite are defined, including the building blocks of the interface between the radio frequency (RF) front-end and the optical front-end, as well as the digital signal processor. The system is designed to cater for Terabit/s high-throughput satellite (HTS) applications. In addition, the turbulent atmospheric optical channel is modeled for different elevation setups and optical ground station (OGS) altitudes in untracked and tracked beam scenarios. The performance of the digital transmission scheme is evaluated in the forward and return link channels. It is shown that fade mitigation techniques such as packet-level FEC coding in the forward link, as well as beam tracking, and large-aperture OGS telescope in the return link are essential to close the link budget of a Terabit/s satellite transmission link.

Exponentiated Weibull fading model for free-space optical links with partially coherent beams under aperture averaging

Abstract. The recently proposed exponentiated Weibull (EW) fading channel model is assessed with partially coherent beams (PCBs) using the Gaussian Schell-model (GSM) theory. The use of a PCB in free-space optical links has been suggested as an atmospheric turbulence mitigation technique, called transmitter aperture averaging. A wave optics code based on the GSM theory is used to generate a PCB propagating in atmospheric turbulence, and the probability distribution function (PDF) is obtained from the simulated irradiance data. This study on the PDF includes the new proposed EW model in the presence of aperture averaging, in weak-to-strong turbulence regime. Point receiver conditions are also analyzed. For completeness of the study, the Lognormal and the Gamma-Gamma models have also been included in the analysis, allowing for a direct comparison to the EW distribution. Using a least-squares curve fitting algorithm, the EW model always has the best fit to simulated PDF data. Thus, the EW fading channel model proves to be valid for PCBs.

Performance of FSO links under exponentiated Weibull turbulence fading with misalignment errors

The exponentiated Weibull (EW) distribution has been recently proposed for the modelling of free-space optical (FSO) links in the presence of finite sized receiver aperture. In this paper, the performance of FSO communication systems over EW is studied. Specifically, the probability density function (PDF) and cumulative distribution function (CDF) of the instantaneous signal-to-noise ratio (SNR), over EW turbulence fading, are studied. The derived statistics of the SNR is utilized to analyse the performance of an FSO communication system over a generalized communication environment with turbulence induced fading, misalignment errors and path loss. New expression for the outage probability is obtained, and exact expressions for the average bit error rate (BER) are derived for various binary modulation schemes. Finally, the obtained analytical results are verified via Monte Carlo simulations.

Reply to comment on “The exponentiated Weibull distribution family under aperture averaging for Gaussian beam waves”

In response to a comment by H. T. Yura and T. S. Rose on our recent work about the exponentiated Weibull distribution [Opt. Express 20, 13055–13064 (2012)], we present here a defense of the proposed model from their criticism.

Wireless Optical Communications Through the Turbulent Atmosphere: A Review

In the past decades a renewed interest has been seen around optical wireless communications, commonly known as free-space optics (FSO), because of the ever growing demand for high-data-rate data transmission as to a large extent current applications, such as the high-definition (HD) contents and cloud computing, require great amount of data to be transmitted, hence, demanding more transmission bandwidth. Nowadays, the last mile problem continuous to be the bottle neck in the global communication network. While the fiber-optic infrastructure, commonly called network backbone, is capable of coping with current demand, the end user accesses the network data stream through copper based connection and radio-frequency (RF) wireless services, that are inherently slower technologies. As the number of user increases, the radio-frequency spectrum is getting so crowded that there is virtually no room for new wireless services within the RF band, with the added inconvenient of limited bandwidth restrictionwhen using a RF band and the license fees that have to be paid in order to use such a band. Regarding cooper-based technologies and the lower-speed connections, compared with the backbone, that are offered such as DSl (digital line subscriber), cable modems, or T1’s (transmission system 1), they are alternatives that makes the service provider to incur in extra installation costs for deploying the wired network through the city.

Relayed FSO communication with aperture averaging receivers and misalignment errors

In this paper, the performance of decode-and-forward relay-assisted free-space-optical (FSO) communication systems under atmospheric turbulence induced fading and misalignment errors is investigated. To mitigate the adverse effects of the atmospheric turbulence, the aperture averaging receivers are considered both at the relay and destination sides. The atmospheric turbulence induced fading is modeled via the exponentiated-Weibull distribution, which has recently been proposed to characterize an FSO link in the presence of finite-sized receiver aperture. The expression for the moment generating function (MGF) of the instantaneous signal-to-noise ratio is derived. Further, new closed form expression for the outage probability is obtained. Moreover, the new expression for the average symbol error rate of the subcarrier intensity modulated M-ary phase shift keying is obtained using the MGF-based approach. Finally, numerical examples are discussed and all the derived analytical results are corroborated by Monte-Carlo simulations. Relayed FSO Communication with Aperture Averaging Receivers and Misalignment Errors. Available from: https://www.researchgate.net/publication/307931308_Relayed_FSO_Communication_with_Aperture_Averaging_Receivers_and_Misalignment_Errors [accessed Nov 10, 2016].

Optical feeder links for high throughput satellites

Optical feeders for geostationary High Throughput Satellites (HTS) systems based on 1.55μm wavelength technology are expected to enable to transmit up to several terabits over one active link. A desirable option of transmission architecture is an optical feeder link transparent with respect to the user air interface. This can be implemented using either a digital or an analog modulation of the optical carrier. The digital option increases the optical bandwidth to be transmitted, however it benefits from error correcting codes, interleaving and framing which are efficient against atmospheric turbulence impairments. The analog option is more efficient concerning the optical bandwidth; however the atmospheric turbulence impairments can only be mitigated by a more complex optical ground terminal. Both analog and digital options could be feasible in the 2025-2030 time-frames but the digital option is more mature with respect to the atmosphere impairments mitigation techniques.

Aperture averaging in a laser Gaussian beam: simulations and experiments

In terrestrial free-space laser communication, aside from pointing issues, the major problem that have to be dealt with is the turbulent atmosphere that produces irradiance fluctuations in the received signal, greatly reducing the link performance. Aperture averaging is the standard method used to mitigate these irradiance fluctuations consisting in increasing the area of the detector, or effectively increasing it by using a collecting lens with a diameter as large as possible. Prediction of the aperture averaging factor for Gaussian beam with currently available theory is compared with data collected experimentally and simulations based in the beam propagation method, where the atmospheric turbulence is represented by linearly spaced random phase screens. Experiments were carried out using a collecting lens with two simultaneous detectors, one of them with a small aperture to emulate an effective point detector, while the other one was mounted with interchangeable diaphragms, hence measurements for different aperture diameters could be made. The testbed for the experiments consists of a nearly horizontal path of 1.2 km with the transmitter and receiver on either side of the optical link. The analysis of the experimental data is used to characterize the aperture averaging factor when different values of laser divergence are selected.