1. Introduction
Free space optical (FSO) communications is a significant application of the laser technology initially developed in 1960 [
1]. Since then, much research has been conducted into FSO communications and different applications have been demonstrated, including terrestrial, maritime, space and deep space applications [
2]. Despite the initial uncertainties about its potential, the ongoing development of optoelectronic devices and its proven success in military applications provided the required boost to continue investments in the field [
2]. FSO technology offers significant advantages over its RF counterpart and benefits applications in platforms with increased weight and space limitations [
3]. The principle advantages of FSO communications include increased bandwidth, greater security and immunity, lower cost of installation and, finally, no license restrictions [
2]. However, FSO communications are susceptible to various atmospheric effects and phenomena, including molecular and aerosol absorption and scattering as well as atmospheric turbulence. The performance of a laser communication link is highly affected by these phenomena that can ultimately cause temporary link interruption [
4]. The evaluation of the impact of these effects on the performance of a real laser communications link over maritime environment is the primary objective of this paper. To the best of the authors’ knowledge, it is the first time the performance of a real FSO link over maritime environment is evaluated using accurate local meteorological parameters.
Atmospheric turbulence can be a major degradation factor for an FSO link and, therefore, extensive theoretical and experimental research work has been devoted to quantify its effects on atmospheric laser propagation. For example, the Naval Research Laboratory have been operating a laser communications test facility where a 32-km retro-reflected link has been demonstrated at data rates up to 2.5 Gbps [
5,
6]. Along with these measurements, atmospheric characterization of the optical path over the maritime environment has also been investigated. In [
7], a novel analogue FM ship-to-shore communications system has been utilized to successfully demonstrate bidirectional video and audio transmission along a 3-km link. In [
8], a measurement campaign over a 15-km range has been set up and all propagation effects have been investigated quantitatively. In [
9], a very promising experiment took place during a US Navy sea trial exercise where the capabilities of a short ship-to-ship FSO link to transfer data obtained during a maritime interdiction operation have been investigated, proving that laser communication systems can complement their RF counterparts in the near future. In [
10,
11], the effects of humidity and temperature on the performance of an FSO link operating in a coastal environment have been investigated. In those works, two mathematical models that linked the FSO attenuation coefficient to the humidity and the air temperature or dew point, respectively, have been proposed. In [
12], Michael et al. exploited scintillation measurements from a 5-km horizontal path optical link to compile ensemble probability distributions and compare those with standard channel models such as the lognormal and gamma–gamma distributions. In [
13,
14], the bit rate of a commercial FSO link over maritime environment has been measured under weak to moderate turbulence conditions. The observed data probability distributions have been compared to theoretical Lognormal and Gamma distributions and demonstrated a very good fit. In [
15,
16], Tunick used optical scintillometer data collected from a near-horizontal path to explain the physical relationships between refractive index structure parameter and microclimate fluctuations. By using regression analysis, it has been shown that there is high correlation in 8 out of 21 cases studied. Still, others have shown a correlation between the RSSI—a metric of the link performance—and local macroscopic meteorological parameters [
17,
18,
19].
The purpose of this paper is to explore the performance of a commercial FSO link in a maritime environment. Since measuring directly an optical link over sea is rather difficult, it is very helpful to construct simple models for optical link performance quantification based upon routinely single point-measured environmental parameters. To this end, a second-order polynomial model is proposed to predict the RSSI of the system based upon local macroscopic parameter measurements. The collected data spanned over a period of approximately 40 days, within which the fluctuations of these parameters were quite intense. The model was validated twice against observed data in later periods and proved to be very accurate, i.e., a correlation >0.8. The model includes basic meteorological parameters, including wind speed, air temperature, humidity, air pressure, solar radiation, dew point, and rainfall rate. By utilizing well known models available in the open technical literature for the refractive index structure parameter,
, we estimated its value for the same periods and correlated these values with the modelled RSSI values. Finally, the probability density function of the RSSI data has been compared against standard channel models, i.e., Gamma, Lognormal, Weibull, and the best fit is estimated using the Kullback-Leibler (KL) divergence. The rest of the paper is organized as follows.
Section 2, provides the background of atmospheric turbulence and presents the literature models for
estimates.
Section 3, describes the whole experimental setup, which was located across the entrance of Piraeus port.
Section 4, presents and analyzes the findings of the measurements whereas
Section 5 concludes the paper.
2. Atmospheric Turbulence
Optical propagation through the atmosphere experiences disturbances due to small spatial and temporal fluctuations of the refractive index. These fluctuations comprise the so-called optical turbulence and range in size from a few mm to a few meters [
20]. These random changes of refractive index cause various deleterious effects to the optical wave, including irradiance fluctuations, i.e., scintillation, beam wander and beam spread.
Due to the nonlinear nature of turbulent motion, it is rather difficult to predict the strength of this phenomenon in a specific point around space. Instead, a statistical description is used to characterize the strength of the refractive index variations. For mathematical simplification purposes, the axiom of statistical homogeneity and isotropy is utilized for this statistical description [
1]. Small-scale temperature fluctuations lead to the definition of the temperature structure function,
DT(R), as derived from the Kolmogorov theory which follows the two-thirds power law [
20]:
where
T1 and
T2 are the ambient temperatures at two different points in space, separated by the distance
R,
l0 and
L0 the inner and outer scale of turbulence while
stands for the temperature structure parameter. These temperature fluctuations result in atmospheric index of refraction fluctuations. The refractive index,
n(R), assuming that time variations are omitted, can be mathematically evaluated in a point
R as [
20]:
where the unity represents the mean value of the index of refraction and
n1its random deviation from the mean value.
These refractive index fluctuations are related to the temperature and pressure fluctuations as [
20]:
where
P is the atmospheric pressure in millibars and
T the local temperature in Kelvin. The strength of these fluctuations are characterized by the refractive index structure parameter, related to temperature structure function as [
20]:
From Equations (1) and (4), it can be deduced that simultaneous temperature measurements of two points with a known distance between them allows for the direct calculation of
and, consequently,
. Assuming that the turbulence is well described by Kolmogorov theory, then
should (on average) be independent of the separation distance between the two points as long as
l0 <
R <
L0. This method to estimate turbulence strength from simple point temperature measurements has been used before, e.g., see [
21].
The prediction of
has been a topic of extensive research. Most of experimental research works have applied different methods to measure
and validated them by path-averaged scintillometer measurements. However, there exist a few mathematical models that have demonstrated very good fit to observed measurements and are based upon macroscopic meteorological models [
22]. Some of the more prominent models in the open technical literature is the Huffnagel-Valley model, the Huffnagel and Stanley model and the Submarine Laser Communications (SLC) Day and SLC night models [
22].
On the other hand, Sabot and Kopeika, have also proposed two simple mathematical models to predict
strength based upon macroscopic meteorological parameters which can be easily obtained from a local weather station [
23]. The first model can be mathematically expressed as [
23]
where
W(t) is a weight function,
T is the air temperature in Kelvin,
RH the relative humidity in hPa and
WS the wind speed in m/s.
The second model, apart from wind speed and relative humidity, takes into account the solar flux in Cal/(cm
2·min) and the total cross-sectional area of particles in cm
2/m
3, namely [
23],
Both empirical models are valid under specific limits of the macroscale parameters, i.e.,
The
strength is strongly height dependent. The highest values are observed at almost zero altitude, whereas at higher altitude decrease rapidly [
24]. The above models have used a height of 15 m, therefore all subsequent users need to scale them in the desired height [
21]. A typical diurnal profile of
is characterized by higher values during the day, with a peak around midday and lower ones during night. The lowest values appear around sunrise and sunset. In order to emphasize this profile, both models include a weight function, calculated on the basis of the temporal hour that relates the actual time to the times of sunrise and sunset [
21]:
where
HT is the temporal hour,
Hactual is the actual time,
Hsunrise is the sunrise time and
Hsunset the sunset time.
Then the weight factor can be assigned based upon
Table 1.
Both models were utilized for
estimation and correlation with RSSI of the FSO receiver. The resulted
values have also been compared with those obtained from the Huffnagel-Valley model. The value of
over a maritime environment can differ significantly comparing to a terrestrial one. In general, the atmospheric turbulence strength and scintillation diurnal variation over a maritime environment is less than over land [
25].
3. Experimental Setup
The experimental instrumentation was located on the roof of the Hellenic Naval Academy (HNA), i.e., primary terminal, and the lighthouse of Psitalia island, i.e., remote terminal. The horizontal optical link is located 35 m above the sea and crosses the entrance of the Piraeus port; nearly the entire path is over the water and, thus, clearly a maritime environment.
Figure 1 shows the exact spots of both terminals in the map, as well as the ambient environment that the 2958-m-long optical link operates.
This link will be disrupted whenever a vessel taller than 35 m crosses the path; therefore, to minimize these disruptions, the experiment was carried out during the winter when fewer cruise ships visit. The FSO system used in the experiment was an MRV TS5000/155 model. The setup consisted of two terminals with operational characteristics available in
Table 2. The system’s scheme used is intensity modulation/direct detection (IM/DD) and it operates in a data rate of 155 Mbps.
Both terminals utilized stand-alone PCs in order to send and receive/store data. The interface between them is achieved through an SFP multimode fiber cable, operating at 1310 nm, which drives the optical signal from the detector through an O–E converter directly to the PC. The RSSI data is then stored and is available to export for further analysis. The terminal over Psittalia island (
Figure 2) can be remotely operated from the HNA through the optical link.
Additionally, an Ambient Weather (WS-2000) weather station is co-located with the HNA FSO terminal (
Figure 3) to provide real time measurements of macroscopic meteorological parameters that include wind speed, wind direction, air temperature, relative humidity, air pressure, dew point, solar radiation and rainfall rate. These measurements are then stored and readily available to export, analyze and study.
5. Conclusions
In this paper, we proposed a new mathematical model to predict the received signal strength of an FSO optical link. The model has the form of a second-order polynomial with seven macroscopic meteorological parameters as the independent variables. An optical communications link over a maritime environment and a weather station provided the required data. The predicted RSSI values fitted the observed values quite well, yielding an R-squared value of 68.2%. The correlation of all seven parameters to the RSSI has been calculated to deduce the weight of each one’s effect. Emphasis has been given to the rain effect, where 32% of the RSSI variance was explained by the rainfall rate variance (R-squared = 0.32). The proposed model has been validated against real data in two separate periods and the R-squared and correlation coefficient between the observed and modeled RSSI values has been computed to check how good the fit was. Both periods exhibited high R-squared and correlation coefficient, namely 69% and 0.8327, respectively. Two empirical models have been utilized to estimate the parameter for the same periods and its relationship with RSSI has been explored. Finally, the KL divergence was used to compare the goodness of fit for different probability distributions, i.e., gamma, lognormal and Weibull to the RSSI data probability distribution, and the gamma distribution yielded the best fit. Experimenting with a laser link in the open sea for extended periods of time is not trivial, therefore we utilized an established link between two fixed points on land that crosses a maritime environment and allows for adequate experimental data to be obtained in order to build our model. Therefore, our model allows the prediction of a maritime optical link with the utilization of a single point measurement system for atmospheric parameters.