1. Introduction
Over the past few years, the need for network capacity has increased significantly due to the introduction of new digital applications and services such as online gaming, high-quality video streaming, and cloud computing [
1]. Therefore, modern communication networks rely heavily on optical communication networks, which serve as their backbone because of the wide bandwidth the fiber provides. Fiber-based networks are the drivers for current 5G and future wireless communication networks, smart cities, the Internet of Things (IoTs), and industry 4 [
2]. They have the capability to provide huge data rates with low latency. However, there are some scenarios where deploying fiber is not feasible or difficult such as over hills, rivers, and lakes. In addition, a fast communication link is necessary to establish during disasters, which is impossible with fiber. Therefore, there is a need to use other technologies such as microwave and millimeter wave links as a bridge to overcome such challenges. However, these techniques have limited bandwidth, which does not match with the fiber bandwidth. The feasible solution is using the free space optic (FSO) technique to avoid fiber discontinuity when facing obstacles [
3]. FSO is similar to the fiber in that it has wide bandwidth that supports current and future high-demand bandwidth applications. Signal transmission over the FSO link can reach several kilometers. However, instead of transmitting the optical signal over fiber, FSO sends the optical signal over free space. This feature makes FSO the best solution when fiber installation is infeasible. Moreover, FSO is a good quick solution for service recovery when the fiber cable is damaged for certain reasons such as disasters and construction work. Also, FSO can be installed as a backup for fiber in availability-sensitive networks [
4,
5]. FSO also offers several other advantages, including cost-effectiveness, rapid deployment, enhanced security compared to RF links, and the use of unlicensed spectrum. However, there are some challenges associated with FSO links. For example, link blockage by obstacles such as construction work may require re-planning of the FSO link. The possibility of unauthorized access to the communication link is another concern, although it is harder than the RF link to gain access because FSO uses a line-of-sight link. FSO link alignment is another challenge, as it requires precise alignment due to its narrow beam. The safety of living creatures such as birds is also a concern, which can be addressed by respecting the recommended maximum transmitted power.
In a traditional intensity modulation/direct detection (IM/DD) FSO system, when the fiber is connected to the FSO system on the transmitter side, the optical signal carried by the fiber should be converted to an electrical signal first to drive the laser. This is achieved using a photodetector (PD), which is an optoelectronic device. The drive circuit modulates the laser light to produce an output-modulated optical signal. Such conversion operation in the transmitter of the traditional hybrid fiber/FSO links from an optical signal to an electrical signal and then to an optical signal increases the system cost and reduces the available bandwidth due to using electronics that have limited bandwidth [
6]. To overcome such a limitation, a hybrid all-optical communication fiber/FSO system is proposed [
6,
7]. In such a system, the optical signal delivered by the fiber in the transmitter side is coupled to the free space using a light collimating lens. Hence, there is no need for optical/electrical/optical conversion. In the FSO receiver side, the light is collimated to the fiber using another collimating lens. Hence, the signal over the entire fiber/FSO link remains optical.
The advantage of a hybrid all-optical fiber/FSO link comes with challenges arising from the channel impairments. In the fiber channel, the optical signal is prone to different types of impairments such as chromatic dispersion (CD) and amplified spontaneous emission (ASE) noise due to signal amplification. In the FSO channel, the signal is prone to turbulence and atmospheric attenuation. Such limitations degrade the optical communication system’s performance. Therefore, it is important to have a feature in the system to monitor the amount and type of distortion in the optical channel. Such a feature could be used in improving the system performance using techniques such as adaptive modulation on the transmitter side or designing appropriate mitigation algorithms on the receiver side based on the type and amount of the distortion. This leads to having a self-aware and self-adaptive communication system.
Various conventional techniques were proposed in the literature for monitoring different channel impairments such as ASE, CD, and polarization mode dispersion (PMD) [
8]. However, such techniques have technical limitations and/or are costly. Recently, machine learning (ML) techniques are exploited in many applications [
8,
9,
10,
11]. One application is to monitor the optical network performance. The use of ML techniques holds great potential for enhancing the intelligence of optical network nodes. By utilizing ML, network nodes can gather insights from network conditions, which can then be applied to optimize network resources in the future.
Various ML techniques have been suggested in the scientific literature for monitoring optical performance in both fiber-based optical links and FSO-based links that exploit IM/DD. Performance monitoring of IM/DD fiber-based optical communication links using an artificial neural network (ANN) model was investigated in [
12,
13,
14,
15]. In [
12], the ANN model was proposed for monitoring three parameters, which are optical signal-to-noise ratio (OSNR), CD, and differential group delay (DGD). The model was trained using features extracted from the eye diagrams of the received signal such as the root mean square and the jitter. However, this technique requires precise timing, which increases the system’s complexity. In [
13], the same parameters are monitored using an ANN model but trained with an asynchronous amplitude histogram (AAH). Using ANN was also proposed in [
14,
15] for monitoring the OSNR, CD, and DGD parameters, using empirical moments and the asynchronous delay-tap sampling (ADTS) technique, respectively. A conventional neural network (CNN) model was proposed in [
16] for monitoring OSNR and CD parameters. In this work, the input data for the CNN model are 2-D images generated using the ADTS technique. However, using images increases the complexity of the system. In [
17], a deep neural network (DNN) model trained on features extracted from the signal eye diagram was proposed for monitoring the OSNR, CD, and DGD parameters.
Monitoring FSO channel parameters was reported in [
18,
19,
20,
21] using a CNN model. In [
18,
19], turbulence impairment monitoring is investigated using orbital angular momentum (OAM) modes as a training feature for the model. Similarly, the OAM modes were used in [
20] to monitor three impairment parameters, which include OSNR, turbulence, and pointing errors. In [
21], the OAM modes are exploited for monitoring the visibility range under dusty weather conditions. The work in [
22] used the support vector machine (SVM) technique to monitor the turbulence and OSNR parameters in addition to the visibility range under fog weather conditions.
Table 1 summarizes the literature work on monitoring the performance of IM/DD optical fiber and FSO systems.
In this work, we consider monitoring three main channel parameters that arise in the hybrid all-optical fiber/FSO channels; these are turbulence, OSNR, and CD. To the best of the authors’ knowledge, this is the first study that addresses monitoring these impairments in a hybrid optical system of this kind. To achieve this goal, Gaussian process regression (GPR) is utilized as an ML model to predict the three parameters. The model’s performance is evaluated using the root mean square error (RMSE) metric and the R-squared metric. Additionally, boxplot graphs are used as a visualization tool to evaluate the model’s performance. The results indicate that the model can predict the various impairments with high accuracy, except under strong ASE noise, where the model demonstrated lower accuracy in predicting light turbulence parameters.
The paper is structured as follows.
Section 2 provides an overview of the system and the ML technique employed. The results’ analysis and discussion are presented in
Section 3. Finally, the paper concludes in
Section 4.
3. Results and Discussions
In this section, we utilize two metrics to evaluate the performance of the proposed model in predicting the various values of the channel parameters. The metrics are the Coefficient of Determination (R-squared) metric and the RMSE metric. The R-squared metric is given mathematically by
The R-squared metric utilizes the actual and estimated data, sample mean (
), and the total number of test samples (
N), with
and
representing the actual and estimated data, respectively. The metric’s range is between 0 and 1, with higher values indicating improved prediction accuracy as
approaches 1. Conversely, the prediction accuracy deteriorates as
approaches 0. On the other hand, the RMSE metric is defined mathematically by
In addition, we use a visualization method, the boxplot method, to examine the proposed model’s performance. It is a graphical tool used to display the distribution of a dataset. It is constructed from five key statistics: the minimum and maximum values, the lower and upper quartiles, and the median. The box in the plot represents the interquartile range (IQR), which is the range between the first (25th percentile) and third (75th percentile) quartiles. The median is represented by a line inside the box. The whiskers extend from the box to the minimum and maximum values that lie within a certain distance from the box, typically 1.5 times the IQR. Any points outside this range are considered outliers and are represented as individual points. Boxplots are used to visualize the spread and skewness of data and to compare multiple datasets side-by-side. The boxplots in
Figure 3 illustrate the prediction accuracy of the model under different channel types. High-accurate predictions are represented by narrow boxes.
Subsequently, we present the results of monitoring each parameter, taking into account the prediction of each parameter under different conditions: when there are no other impairments (single impairment: ASE, CD, turbulence), when there is a joint impairment (double impairment: ASE and CD, ASE and turbulence, CD and turbulence), and when all the impairments are present (triple impairment: ASE, CD, turbulence). It is worth noting that we considered the severe impairment conditions in evaluating the model performance defined by 12 dB OSNR, 500 ps/nm CD, and turbulence. Moderate and light conditions will result in much better prediction accuracy.
3.1. OSNR Parameter Prediction
Figure 3a illustrates the prediction accuracy of the OSNR parameter for the five different OSNR values. The narrow width of the boxplot indicates high prediction accuracy. The obtained R-squared and RMSE values were 1 and 0.09 dB, respectively, further confirming the high accuracy of the predictions.
Figure 3b presents the application of the model to predict the OSNR parameter under the influence of 500 ps/nm CD. The results obtained are similar to those in
Figure 3a, indicating that the model can still accurately predict the OSNR parameter with high precision. The achieved R-squared and RMSE values were 1 and 0.13 dB, respectively.
Figure 3c depicts the performance of the model in predicting the OSNR parameter under the influence of OSNR and severe turbulence defined by
. The model continues to accurately predict the OSNR parameter, as evidenced by the achieved R-squared and RMSE values of 1 and 0.16 dB, respectively. Finally, the combined effect of OSNR with 500 ps/nm CD and
turbulence is investigated, with the results presented in
Figure 3d. The model is still able to accurately predict the low values of the OSNR parameter, although the accuracy slightly degrades for high values. The achieved R-squared and RMSE values were 0.96 and 0.54 dB, respectively, indicating high prediction accuracy. Overall, the model demonstrated high accuracy in predicting the various values of the OSNR parameter.
3.2. Chromatic Dispersion Parameter Prediction
The proposed model’s prediction of the CD parameter is visually depicted in
Figure 3e–h.
Figure 3e shows the model’s ability to predict the CD parameter alone without any other impairments, with very narrow boxes indicating high accuracy. The obtained R-squared and RMSE values were 1 and 0 ps/nm, respectively. When the signal was corrupted with severe turbulence, defined by
turbulence, the R-squared value slightly dropped to 0.98, and the RMSE increased slightly to 0.2 ps/nm. Nonetheless, the model was still capable of predicting the CD parameter with high accuracy, as shown in
Figure 3f. Similarly, when the system was corrupted with ASE noise defined by 12 dB OSNR, the R-squared value dropped slightly to 0.96, while the RMSE increased slightly to 0.27 ps/nm. With this slight degradation in the model performance, the model still accurately predicted the CD parameter, as depicted in
Figure 3g. Finally,
Figure 3 shows the model’s performance in predicting the CD parameter when the system was corrupted with both ASE noise defined by 12 dB OSNR and turbulence defined by
. The joint impairments slightly degraded the model’s accuracy, with the R-squared value dropping to 0.93, and the RMSE value increasing to 0.37 ps/nm. In general, the model was able to predict the different CD parameter values with high accuracy.
3.3. Turbulence Parameter Prediction
This subsection discusses the proposed model’s performance in predicting the
parameter, with the results depicted in
Figure 3i–l. Firstly, we examine the model’s performance in predicting the
parameter when no other impairments are present in the system.
Figure 3i illustrates the boxplot of the prediction, indicating high accuracy. The obtained R-squared and RMSE metrics were 1 and 0.1
, respectively. Next, we evaluated the model’s performance in predicting the
parameter when the system was subject to 500 ps/nm CD impairment. The results in
Figure 3j indicate that the prediction performance was similar to that in
Figure 3i when there was no CD impairment. The achieved R-squared and RMSE metrics were 0.03 and 0.1
, respectively.
Figure 3k reports the
parameter prediction when the channel is subject to ASE noise defined by 12 dB OSNR. In this scenario, we observed that the model demonstrated high accuracy in predicting the harsh
parameter, defined by high
values. However, when the
parameter values were low, the impact of ASE noise on the signal was high, leading to difficulty in predicting the OSNR parameter. The obtained R-squared and RMSE metrics were 0.84 and 0.57
, respectively. Finally, we evaluated the proposed model’s capability in predicting the
parameter when the system was subject to harsh conditions defined by 12 dB OSNR and 500 ps/nm CD. We obtained similar results to those in
Figure 3k because the dominance of ASE noise complicates the model’s capability of predicting the
parameter. In general, the model demonstrated high accuracy in predicting the
parameter when no ASE noise was present or under severely turbulent channels. However, when strong ASE noise was added to the system, the model’s capability decreased significantly in predicting low
values.
Table 2 below summarizes the performance of the proposed model in predicting the various parameters in terms of R-squared and RMSE metrics.
3.4. Performance Comparison with Other ML Models
The present section compares the performance of the GPR model with that of two other commonly used ML models, namely SVM and random forest (RF) models.
Figure 4 illustrates the performance comparison using R-squared and RMSE metrics. For the turbulence parameter monitoring, it was observed that the GPR model outperformed the other models in terms of both RMSE and R-squared metrics. Specifically, the achieved R-squared was 1 under turbulence conditions alone and also under turbulence and 500 ps/nm CD impairment, as indicated in
Figure 4a. Similarly, for these two-channel conditions, the RMSE value was less than 1. However, when the signal was corrupted with turbulence and ASE noise (12 dB OSNR), the R-squared value dropped to 0.84 for the GPR model, which was still slightly better than that of the RF and SVM models. Similarly, the RMSE degraded to 0.55, but still remained better than that of the RF and SVM models as depicted in
Figure 4d. The performance of predicting the turbulence parameter under all harsh conditions, i.e., 12 dB OSNR and 500 ps/nm, remained relatively unchanging, as shown in
Figure 4a,d.
In the following analysis, we compare the performance of the three models in predicting the OSNR parameter under different channel conditions, as shown in
Figure 4b,e. The results indicate an R-squared value of 1 using the GPR model under ASE noise alone, as well as when the channel included 500 ps/nm CD and
turbulence. In comparison, the two other models exhibited slightly lower R-squared values. Furthermore, the GPR model demonstrated significantly better performance in terms of RMSE compared to the two other models. However, when the channel was subjected to both 500 ps/nm CD and
turbulence, the R-squared value declined to 0.96, while the RMSE degraded to 0.55, which was similar to the obtained performance of the two other models.
Finally, we compare the performance of the GPR model with the two other models in predicting the CD parameter, as illustrated in
Figure 4c,f. The results exhibit similar performance in terms of R-squared and RMSE for all three models. Specifically, the achieved R-squared was 1 and the RMSE was 0. However, when a severe turbulence condition, defined by
turbulence value, was added to the channel, the R-squared metric value degraded to 0.98 for all models, and the RMSE increased to 0.2, which was slightly better than that of the SVM model but worse than that of the RF model. When ASE noise was introduced to the channel, defined by 12 dB OSNR, the GPR model’s performance slightly degraded in terms of R-squared and RMSE. It exhibited the same performance as the SVM model and was better than that of the RF model. Finally, when both CD and OSNR severe conditions were considered, the GPR model achieved slightly better performance than the two other models.
It is worth mentioning that the GPR technique is often preferred over supervised learning techniques such as SVM and RF in situations where the dataset is small or noisy, and high prediction accuracy is required. This is because GPR is a non-parametric model that can handle complex input–output relationships without imposing strong assumptions about the underlying data distribution. Moreover, GPR can provide probabilistic estimates of the output variable, making it particularly useful in situations where uncertainty estimates are required. Additionally, GPR can handle high-dimensional input spaces and limited training data more effectively than SVM and RF models.