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# A Mirror-based Jamming Scheme against Eavesdropping in Underwater Optical Communication

Andrea Petroni, Muhammad Shoaib, Valeria Loscri, Mauro Biagi

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Andrea Petroni, Muhammad Shoaib, Valeria Loscri, Mauro Biagi. A Mirror-based Jamming Scheme against Eavesdropping in Underwater Optical Communication. OCEANS 2025 Conference and Exposition, Jun 2025, Brest, France. hal-05113095

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# A Mirror-based Jamming Scheme against Eavesdropping in Underwater Optical Communication

Andrea Petroni*, Muhammad Shoaib Khan†, Valeria Loscri‡, and Mauro Biagi†,

*Fondazione Ugo Bordoni, Rome, Italy

†Dept. of Information Engineering, Electronics and Telecommunications, Sapienza University of Rome, Rome, Italy

‡FUN Team, INRIA Lille-Nord Europe, Lille, France

***Abstract***—Providing secure optical wireless communication is a crucial challenge also in underwater scenarios, even though it has not been extensively investigated in the literature yet. In this paper, we propose a novel physical layer optical jamming scheme for underwater communications that, by leveraging signal reflection by mirror surfaces integrated with the reference transmitter (Alice), allows a reliable signal detection for a legitimate node while denying the transmission interception to potential eavesdroppers. Preliminary results demonstrate the proposed solution to effectively deny a reliable signal detection for an eavesdropper, while preserving the legitimate link integrity.

***Index Terms***—UWOC, security, friendly jamming.

## I. INTRODUCTION

Wireless sensor networks have become a widespread support to data sharing and communication in many kinds of environments. The design and implementation of such infrastructures have become particularly attractive when dealing with challenging domains like the underwater one. In this regard, the Internet of Underwater Things (IoUT) has risen as a promising paradigm, leveraging underwater acoustic communication (UWAC) as well as underwater optical communication (UWOC) to realize wireless connectivity among nodes.

UWAC is a well-established technology providing high performance in terms of coverage, but showing also large propagation delay and poor bandwidth that limit the achievable data rate. On the other hand, UWOC exhibits superior performance in terms of rate and latency, even though constrained on a shorter link distance since the optical signal is highly vulnerable to propagation impairments such as absorption and scattering [1]. Despite underwater networks based on UWAC and UWOC show many differences in terms of provided quality of service, they both share common challenges related to security. As the communication infrastructure becomes more complex and extended, guaranteeing robustness to malicious attack and preserve data integrity represents a crucial aspect to be necessarily and efficiently addressed [2].

While security in UWAC has been already largely discussed in the literature [3], the investigation of such problem in the UWOC context is still at an early stage [4]. Overall, underwater communication performance strictly depends on

physical layer issues, which have a straight impact on the whole protocol stack, including for instance medium access control and routing protocols. Similarly, the identification of potential threats covers different levels of the link, with several solutions being preliminary proposed for indoor wireless optical communications. In UWOC, one of the widespread research directions considers security to be conveniently addressed at the physical layer [5]. In this regard, eavesdropping represents a severe risk, as adversaries can stealthily intercept the communication by positioning themselves between known transmitting nodes. Typically, UWOC leverages the intensity-modulation direct-detection (IM/DD) approach to encode the information onto optically modulated optical signals emitted by laser diode (LD) or light emitting diode (LED) sources. From a security point of view, the use of LDs is preferable since sources are characterized by a narrow emission, and an eavesdropper should be positioned exactly along the line of sight (LOS) between transmitter and receiver to intercept the signal. Anyway, this may lead to an obstruction of the legitimate link, so the eavesdropper could be easily identified. However, it is worth noting that the emission features of LDs guarantee limited power losses due to beam divergence, but the system is more prone to pointing errors due to nodes misalignment. So, the quality of service to be provided requires complex mechanisms to be implemented. On the other hand, the use of LEDs with wider beam emission allows signal detection at the receiver to be more reliable with respect to misalignment, even though signal divergence losses may have a non-negligible impact. Furthermore, in this case, an eavesdropper may exploit the large emission angle of the LED to intercept the signal while being located far from the LOS path between transmitter and receiver, hence reducing the possibility of identification.

Given the distinct properties of the underwater optical channel, such as strong attenuation and scattering, traditional terrestrial anti-eavesdropping methods are not directly applicable. A promising approach to mitigate such vulnerability in UWOC considers friendly jamming designed to generate interference signals that degrade malicious nodes detection while preserving data quality for legitimate receivers. Jamming
<figure><img src="image_3.png" /><figcaption>Fig. 1. 2-D representation of the considered reference link.</figcaption></figure>

strategies have shown to be an effective solution to preserve the communication security in challenging circumstances [6]. The optimization of such mechanisms mainly relies on the search for an appropriate balance between useful signal and jammer emission, allowing the legitimate link performance to satisfy the target constraints in terms of reliability.

In this direction, this paper introduces a jamming framework specifically designed for UWOC to enhance security against eavesdropping. Herein, we discuss the underlying principles, potential system architectures, and strategic considerations for achieving effective jamming, balancing security with communication reliability. In detail, we introduce an UWOC link scenario where mirrors are employed to support a friendly jamming mechanism that denies signal detection from an eavesdropper while keeping the data collection from the legitimate node highly reliable.

The paper is organized as follows. In section II, we present the system model. Section III introduced the proposed friendly jamming aided transmission mechanism. Some numerical results are reported and discussed in Section IV. Finally, Section V draws conclusion.

## II. SYSTEM MODEL

Let us consider a point-to-point link with the transmitting node, namely Alice, equipped with a LED source, while the receiver, referred as Bob, performing signal collection through a photodetector (PD). Furthermore, we assume the presence of an eavesdropper, Eve, able to intercept the message from Alice to Bob by means of its own PD-based hardware.

In order to prevent the signal detection from malicious nodes, the legitimate link between Alice and Bob implements a friendly jamming mechanism where the Bob's transceiver, provided of a additional LED sources, generates interference signals directed to Alice, able to reflect them through a circularly shaped mirror. The jammer is therefore reflected back to Bob, but also towards the eavesdropper Eve. A reference scenario is depicted in Fig. 1. Both the data signal emitted by Alice and the jammer generated by Bob are supposed to be

modulated according to M-Pulse Amplitude Modulation (M-PAM), so as to realize an IM/DD transmission. Specifically, given $M_a$ and $M_b$ as the PAM orders employed by Alice and Bob, we have that the data and jamming signal, referred as $x(t)$ and $j(t)$, respectively, are given as:

$$x_m(t) = A_m g(t) + I_b \quad m = 0, 1, \dots, M_a - 1 \quad (1)$$

$$j_k(t) = A_k g(t) + I_b \quad k = 0, 1, \dots, M_b - 1 \quad (2)$$

where $A_m$ and $A_k$ are the amplitude related to the generic symbols of the PAM constellation, $g(t)$ is the pulse shape, while $I_b$ is a current bias necessary to make $x_m(t)$ and $j_k(t)$ as non-negative optical signal after electro-optical conversion (assumed as ideal without loss of generality).

Signal propagation suffers from both geometrical loss related to the source optical emission, assumed as Lambertian LOS, and path loss induced by medium absorption and scattering. By taking into account such aspects [7], the LOS channel between two nodes can be modeled as the following scalar gain:

$$h = \frac{(k + 1)S}{2\pi} \cos^k(\psi) \cos(\phi) e^{-cD} \quad (3)$$

where $k$ is the Lambertian emission factor, $\phi$ and $\psi$ are the radiance and incidence angle, respectively, $D$ is the link distance and $c$ is the total attenuation coefficient accounting for absorption and scattering based on water type [8]. Finally, $S$ represents the detection area that, related to the scenario in Fig. 1, may refer to either the active PD surface of Bob and Eve, or the signal footprint in the mirror at Alice side. Based on such architecture, we have the received signal model for Bob and Eve, $y_B(t)$ and $y_E(t)$ respectively, as:

$$y_B(t) = \rho(h_{AB}x_m(t) + h_{BA}\Gamma h_{AB}j_k(t)) + w_B(t) \approx \rho(h_{AB}x_m(t) + \Gamma h_{AB}^2 j_k(t)) + w_B(t) \quad (4)$$

$$y_E(t) = \rho(h_{AE}x_m(t) + h_{BA}\Gamma h_{AE}j_k(t)) + w_E(t) \quad (5)$$

detailed as follows. In (4), $h_{AB}$ and $h_{BA}$ are the channel gain related to the propagation from Alice to Bob and vice
versa, $\rho$ is the opto-electrical conversion factor, $\Gamma$ is the reflection coefficient of the mirror, and $w_B(t)$ is the noise term accounting for dark current noise, shot noise and thermal noise. Furthermore, since the mirror operates an ideal reflection of the jammer, it essentially acts as a light source sending back the signal to Bob. Hence, by assuming the channel as reciprocal, that is path loss is the same in both the link direction, we can approximate $h_{BA} = h_{AB}$. Such condition simplifies the model, even though it is not always verified in real scenarios. Anyway, we later address the impact on the accuracy channel estimation. Passing to (5), the propagation of the jammer from Bob to Alice $h_{BA}$ and from Alice to Eve $h_{AE}$ after the reflection on the mirror are different (especially from a geometrical point of view), so no simplification is allowed. Finally, the noise term $w_E(t)$ generally differs from that one in (4).

### III. PROPOSED DETECTION MECHANISM

Based on (4) and (5), it is possible to appreciate that both Bob and Eve experience the reception of two superposed signal, a useful one and a jammer. At Bob side, the jammer is known since self-generated, hence the detection can be operated by prior subtraction of an estimated version of $j(t)$. Such cancellation, as well as the decision on the received signal from Alice requires channel state information as available. So, we can resort to the maximum likelihood (ML) criterion reshaped as follows:

$$ \hat{m}_B = \underset{m=1,...M_a}{\operatorname{argmin}} \left( y_B - \rho(\tilde{h}_{AB}x_m + \tilde{h}_{AB}^2 j_k) \right)^2 \quad (6) $$

to obtain $\hat{m}$ as the index of the symbol transmitted from Alice. Specifically, $j_k$ is the jamming symbol known by Bob, while $\tilde{h}_{AB}$ is the channel estimate.

At Eve side, differently, the presence of a jammer is unknown, so the receiver may be not aware that the collected signal is the summation of two components. At most, Eve may recognize the presence of the jamming signal, but without being able to identify its shape. So, the ML detection would necessarily follow the rule:

$$ \hat{m}_E = \underset{m=1,...M_a}{\operatorname{argmin}} \left( y_E - \rho \tilde{h}_{AE} x_m \right)^2 \quad (7) $$

where $\tilde{h}_{AE}$ is the estimated channel. From (7), we can observe that the absence of the jammer component from the decision process may lead to significantly different and unreliable results with respect to (6). However, it is worth noting that (7) still describes a best case scenario for Eve since assuming the PAM order $M_a$ as known. Actually, an eavesdropper must firstly recognize the transmission scheme employed in the legitimate link before being able to interpret the signal correctly. This task is not easy to be accomplished, especially when there is an ongoing jamming that may also lead to bad modulation order estimation at Eve side.

A final observation regards the accuracy of the available channel knowledge, playing a fundamental role for both Bob and Eve. By focusing on the Alice-Bob link where a reliable

detection must be guaranteed, channel estimation can be performed by resorting to many strategies, for instance by sending pilot signals in a bi-directional fashion. So, Bob can itself estimate the channel $\tilde{h}_{AB}$, while Alice deals with the estimation of $\tilde{h}_{BA}$ and then send the related information back to Bob. Our model assumes the channel as reciprocal, so it would be sufficient the estimation only at Bob side to proceed with the ML detection. Anyway, if the channel reciprocity is not verified, (6) may use inaccurate estimates, finally leading to wrong decision on the received signal.

### IV. NUMERICAL RESULTS

In this section, we evaluate the performance of the proposed scheme by resorting to simulations performed with Matlab software. Specifically, we consider:

* Alice equipped with an LED with transmit power equal to 10 W and with a circular mirror inducing a signal reflection with coefficient $\Gamma = 0.99$;

* Bob provided with a PD with responsivity $\rho = 0.6$ A/W for signal detection, and two LED sources for jamming emission with 10 W transmit power each;

* Eve equipped with a PD with responsivity $\rho = 0.6$ A/W similarly to Bob.

The rest of simulation parameters are reported in Table I. Note that the considered values reflects the typical features of commercially available optoelectronic hardware. Both Alice

<table><caption>TABLE I
SIMULATION PARAMETERS</caption><tr><td>Modulation</td><td>4-PAM</td></tr><tr><td>LED Max Optical Power</td><td>10 W</td></tr><tr><td>LED wavelength</td><td>440 nm</td></tr><tr><td>LED Bandwidth</td><td>10 MHz</td></tr><tr><td>PD responsivity</td><td>0.6 A/W</td></tr><tr><td>PD area</td><td>1 cm<sup>2</sup></td></tr><tr><td>Water type</td><td>coastal</td></tr></table>

main transmission and Bob jamming signaling are based on 4-PAM. Finally, the channel behavior has been simulated following the model in (3), considering coastal water type conditions. It is worth highlighting that we assumed ideal pointing between Alice and Bob. This lead the propagation path loss to be dominant with respect to geometrical one. Please note that such assumption has not impact on the performance of Eve, so it does not simplify the propagation scenario.

The effectiveness of the proposed scheme has been measured in terms of bit error rate (BER) related to the Alice-Eve link. Performance are shown in Fig. 2 as a function of the distance between Alice and Eve, ranging from 1 to 100 meters. Furthermore, we consider three different cases related to the position of Bob, respectively at 30 meters, 45 meters, and 60 meters from Alice. So, when Eve's distance from Alice exceeds the separation between Alice and Bob, Eve is no longer positioned between them. When Eve's distance from Alice exceeds the separation between Alice and Bob, Eve is no longer positioned between them. Overall, we
<figure><img src="image_4.png" /><figcaption>Fig. 2. BER of Alice-Bob and Alice-Eve links for 4-PAM modulation as a function of distance.</figcaption></figure>

can infer from Fig. 2 how a very high BER is experienced at Eve's side due to the presence of the unknown jammer acting as disturbance. Specifically, the behavior of the curves related to Alice-Eve link can be justified as follows. When the distance is short, jamming is the main responsible for link performance degradation. With the growth of the distance, both the signal sent from Alice and the jamming suffer from stronger attenuation, so the thermal noise becomes the dominant factor leading to an increased BER. Furthermore, when Bob is located far from Alice, the distance traveled by the jammer grows, resulting in a more attenuated signal reaching Eve. Hence, in this case, the impact of the jammer is partially mitigated, even though detection performance remain poor (black dotted curve in Fig 2). For additional comparison, we also provide the BER related to the Alice-Bob link (blue curve), and it is possible to appreciated how the knowledge of the jamming signal shape allows a reliable detection. Of course, performance are good until, a large distance, channel attenuation becomes too strong and noise significantly affects the BER.

By observing Fig.3, it is possible to note how the capacity of the mirror installed on Alice's side, which reflects the jamming signal emitted by Bob, is influenced by the mirror's reflectivity, evaluated through the SINR of the Alice-Eve link.

Indeed, it should be noted that a certain level of reflectivity—let us consider 99%—may decrease over time due to impurities and marine vegetation, thereby reducing its effectiveness. Therefore, the figure could also be interpreted in a "temporal" sense, implying that over time, the system shifts from right to left within the figure.

Overall, the curves depict the behavior in three different propagation scenarios: pure ocean, clear water, and harbor with Alice-Bob distance equating 30 meters and Alice-Eve 10 meters. It is worth noting that the general trend remains fundamentally the same, although in environments with higher

attenuation—such as the harbor—the jamming signal experiences greater attenuation. However, this loss of effectiveness is mainly observed for low reflectivity values. Thus, it is reasonable to assume that for reflectivity values above 50%, the SINR remains low, making the Alice-Eve link perform unacceptably in the context of a passive eavesdropping attack.

<figure><img src="image_5.png" /><figcaption>Fig. 3. SINR for different values of percentage of mirror reflectivity for Alice-Eve link.</figcaption></figure>

Summarizing, the proposed scheme exploiting a jamming signal generated by Bob enables the main communication link (Alice-Bob) to achieve excellent performance while significantly degrading the BER values observed by the eavesdropper (Eve). Such improvement is provided by introducing additional optical power transmitted by Bob, which creates interference that hinders Eve's ability to decode the signal. The use of the reflective mirror at Alice's side is a critical component of this system, as it amplifies the confusion for Eve by directing a significant fraction of the jamming optical power to create additional interference.

V. CONCLUSION

Security is a fundamental challenge in wireless networks, and it recently gained attention also in the field of UWOC. Despite the investigation of such problem is still in a preliminary phase, the proposed solutions are mainly focused on the physical layer of UWOC, aimed to prevent interception of data by working on the single communication link rather than at the network level. This work presented a novel scheme based on friendly jamming to avoid signal detection from eavesdropper while guaranteeing a reliable data transfer between legitimate nodes. The proposed architecture has considered the use of mirrors able to reflect the jamming signal towards both the legitimate receiver and the eavesdropper. By means of a suitable decision mechanism, the legitimate node is able to separate the jammer from the useful signal and operate a reliable detection, while the eavesdropper is not able to intercept the useful information due the presence of the interference caused by the jammer. Numerical results demonstrate promising results
in terms of reliability of the strategy. Future works will be
devoted to a deeper investigation of system, assessing the
performance in more complex scenarios where eavesdroppers
may potentially apply countermeasures to the jamming.

ACKNOWLEDGMENT

This work was partially supported by the European Union
under the Italian National Recovery and Resilience Plan
(NRRP) of NextGenerationEU, partnership on “Telecommu-
nications of the Future” (PE0000001-program “RESTART”).

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