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Abstract
In addition to requirements on increasing transmission distance and bitrate, the study of underwater wireless optical communication (UWOC) is also facing limitations and challenges, such as interference induced by background noise, demand of higher receiver sensitivity, and communication security issues. In this paper, we experimentally demonstrate a physical layer secure and noise-resistant UWOC system based on spectrum spread and encrypted orthogonal frequency division multiplexing (SSE-OFDM) modulation, transmission through a 14.2 m sediment circulating water tank. Firstly, experimental results show that the required optical power ratio of signal and noise light (OPR) for QPSK signal under BER threshold of 3.8×10−3 is around −5.77 dB for a spectrum spread factor (N) of 100, with a signal-to-noise ratio (SNR) improvement of 19.06 dB. Secondly, without the background noise interference, the receiver sensitivity is also improved from −50 dBm to −62.4 dBm by using the SSE-OFDM modulation, achieving a maximum attenuation length (AL) of 19.67. Thirdly, physical layer security of UWOC can also be realized, which suppresses the SNR of eavesdropper to −3.72 dB while improving SNR of the authorized receiver to 17.56 dB under the condition of no leakage of keys. Additionally, analytical expressions for SSE-OFDM based UWOC performance are also derived, which agree well with the experimental results. Based on the analytical expressions, the maximum secrecy capacity Cs for SSE-OFDM based UWOC system under eavesdropping can be obtained by optimizing the intentionally inserted artificial noise power ratio and the spectrum spread factor N.
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1. Introduction
Compared with underwater acoustic communication, underwater wireless optical communication (UWOC) is featured by lower power consumption, higher bandwidth, lower latency [1–3] and is attracting great interests for short-range underwater wireless communication applications such as offshore exploration, environmental monitoring, ocean engineering [1–3]. Recently, extensive studies and significant progresses on UWOC have been achieved, focusing on increasing the system bitrate and transmission distance [4–17]. H. M. Oubei et al. demonstrate a 2.3 Gbit/s UWOC link over 7 m distance with directly modulated 520 nm laser diode using on-off keying non-return-to-zero (OOK-NRZ) modulation scheme [4]. C. Fei et al. demonstrate a 7.33 Gbit/s UWOC with discrete multitone transmission using post nonlinear equalization over 15 m tap water [7]. C. Tu et al. experimentally realize 1 Gbit/s UWOC link over 130 m distance with pulse amplitude modulation 4-level (PAM4) and digital equalization [11]. J. Wang et al. demonstrate 500 Mbit/s underwater transmission over 100 m distance using green laser diode [12]. Y. Dai, et al. conduct experiments in 50 m swimming pool achieving a data rate of 500 Mbit/s through 200 m underwater transmission utilizing a sparse nonlinear equalizer with a variable step size generalized orthogonal matching pursuit [14].
In addition to increasing the transmission bitrate and distance, the UWOC system is also facing challenges such as the resistance of noise light interferences, requiring of higher receiver sensitivity and communication security issues. Various methods have been proposed, aiming to solve one part of these challenges. On the aspect of background noise interference, T. Hamza et al. propose a numerical model for UWOC system under the impact of solar background noise, which shows the UWOC performance is very sensitive to solar radiations for APD and PMT-based receivers if the working depth is above 200 m [18]. For deep water UWOC with large aperture PMT detector, it observed that the performance can be interfered by bioluminescence and Cerenkov Radiation, even below 1000 meters seawater, which depth is sufficient to attenuate solar irradiance [19]. Moreover, in another open sea trial conducted in shallow depth during the night, it is observed that even the ambient light from moon and the city (few miles away) can inevitably limit the attainable range for both PMT and SiPM detectors [20]. To suppress the background noise caused by solar radiation, natural bioluminescence, and other artificial illumination, dedicated design of UWOC system with spectrum matched laser or LED sources and narrow filters have been investigated. For instance, F. M. Levinton et al. develops a wavelength-tunable, wide field-of-view Lyot type birefringence interference filter with a passband as narrow as 0.075 nm for UWOC applications [21]. J. Sticklus et al. investigate the effects and constraints of different thin-film bandpass filters in LED-based underwater communications [22]. N.E. Farr et al. from Woods Hole Oceanographic Institution demonstrate daylight performance enhanced UWOC system using near-ultraviolet LEDs and short-wavelength pass filters to block ambient noise in turbid and shallow water [23]. By employing a pair of frequency accurately matched Faraday laser and filter, a background noise resistant UWOC system is also experimentally demonstrated [24]. Although ensential performance improvement can be achieved by using various filters for UWOC system after filtering the out-band background light, the noise light within passband of filters still remains and there is a lack of studies for further improving the UWOC performance under noise interference within the communication passband.
Extensive studies have been performed to increase the receiver sensitivity, by using high sensitive detectors [25–31], arrayed receivers [28–30] and also power-efficient modulation formats [31–33]. For instance, J. Shen et al. demonstrate a single LED-based UWOC system with 64-PPM over 46 m distance in a tube filled with water [31]. A photon-counting UWOC system with high order pulse-position-modulation (PPM) is demonstrated, with a record result of 35.88 attenuation lengths and 3.32 bits/photon detection efficiency [32]. Y. Guo et al. validate the robustness of PPM-based mobile UWOC system, realizing a diffused-line-of-sight communication for mobile and fixed underwater nodes [33]. Spectrum spread modulation technology is also used to extend the transmission distance or the BER performance of UWOC systems. For instance, a 42 m UWOC is experimentally demonstrated by using tamed spectrum spread technology (TSS), achieving an attenuation length (AL) of 6.68 [34].
As is usually regarded to be a line-of-sight (LOS) communication method, UWOC system is often considered to be more secure compared with other underwater wireless communication manners. However, the scattering characteristic of underwater channel will lead to the deflection of light from its original direction, which provides opportunities for eavesdropping. For instance, M. Kong et al. prove the security weaknesses of UWOC by Monte Carol simulation and demonstrate the possibility of tapping an UWOC link by placing MPPC aside the light beam [35]. D. Shaboy et al. demonstrate the hacking of UWOC system by using a diffraction grating [36]. To enhance the communication security of UWOC system, Y. Zhou et al. propose a spectral scrambling scheme to enhance the physical layer security [37]. Moreover, J. Du et al. propose a low-complexity two-level chaotic encryption scheme to improve the physical layer security of a 450 nm laser underwater UWOC system, where 55 m 4.5 Gbit/s and 50 m 5 Gbit/s underwater transmissions are successfully demonstrated [38]. In addition to UWOC applications, physical-layer security has also been implemented for fiber optical communications, such as quantum noise stream cipher (Y-00) [39], combining constellation encryption with probabilistic shaping [40], integrating chaos encryption inside Polar code [41], and applying Y-00 scheme with DFTs-OFDM modulation [42], and coherent stealthy and encrypted transmission for data center interconnection [43].
As the current studies are focusing on solving one or part of these challenges, there is lack of study to resist the background noise influence, improve the receiver sensitivity and enhance the communication security simultaneously, using only one method.
In this paper, we propose and demonstrate a spectrum spread and encrypted orthogonal frequency division multiplexing (SSE-OFDM) based UWOC system, which can mitigate both the in-band and out-band background noise interference, improve the receiver sensitivity, and realize physical layer security at the same time. The experiment is performed over a 14.2 m UWOC link within a sediment circulating water tank which is used to study the emulating a practical underwater environment. Intentionally inserted in-band noise light is obtained by modulating the amplified spontaneous emission (ASE) noise of an Erbium-doped fiber amplifier (EDFA) after opto-electrical conversion to emulate the true-random noise light.
Firstly, the experimental results show that, with a spectrum spread factor (N) of 100, the SNR improvement reach 19.06 dB, and the optical power ratio between signal light and noise light (OPR) can be as low as -5.77 dB for SSE-OFDM based QPSK signal, under the assumption of bit-error-ratio (BER) threshold of 3.8×10−3. Secondly, the receiver sensitivity is improved from -50 dBm to -62.4 dBm by using the SSE-OFDM modulation with spectrum spread factor (N) of 72, achieving a maximum attenuation length (AL) of 19.67 over a fluid underwater channel with sediment. Thirdly, physical layer security enhancement of UWOC system is achieved through the SSE-OFDM modulation, which can realize conditional information-theoretic security and is resistant of known-plaintext attack (KPA). Experimental result show that the SNR of authorized receiver and eavesdropper is 17.56 dB and -3.72 dB respectively, with spectrum spread factor (N) of 100 and key leakage percentage of 0%. In addition, analytical expressions for SSE-OFDM based UWOC performance are also derived, and substantiated by the experimental results. Using the analytical expressions, the maximum secrecy capacity (Cs) for SSE-OFDM based UWOC under eavesdropping can be obtained by optimizing the intentionally inserted artificial noise power and the spectrum spread factor (N).
2. Principle
The principle of SSE-OFDM is shown in Fig. 1. Comparing with conventional orthogonal frequency division multiplexing (OFDM) scheme, there are two more step added at transmitter and receiver respectively. At the transmitter, information subcarriers data are first replicated over the communication frequency band, which improves the resistance to noise as other spectrum spread methods, at the cost of reducing available information bitrate. The additional step is to apply phase masks over the entire OFDM subcarriers, achieving a physical layer encryption effect. After transmission along the UWOC link with intentionally inserted or natural background noise interference, the spread subcarriers are covered by noises, as shown in Fig. 1. For the receiver, the corresponding operation is to remove the phase mask for subcarrier decryption and superpose the assigned replications in frequency band, after which the SNR of the information subcarriers will be improved.
At the costs of reducing the available information data rates, the SSE-OFDM based UWOC method possesses three advantages as shown in Fig. 2. Firstly, the SNR of SSE-OFDM under in-band or out-band optical noise can be improved considerably according to the spectrum spread factor (N), achieving negative OPR with the SSE-OFDM spectrum fully covered under the background noise. Secondly, for UWOC links with weak received optical power, the scheme could improve the sensitivity and extend the transmission distance. At last, by employing SSE-OFDM modulation, the system’s physical layer security is enhanced and possessing the ability of resisting known-plaintext attack (KPA) under the background noise interference, similar to Y-00 scheme [39].
3. Analytical performance expressions for SSE-OFDM based UWOC system
3.1 SNR under background noise interference
There are several steps to obtain the theoretical expression of SNR with SSE-OFDM based UWOC. At the receiver, after the photoelectric conversion, the received electrical signal can be denoted by:
where ${I_s}$ is the photo-current generated by signal light in photo detector at receiver, G is the average gain of photo detector. $\eta$ is the quantum efficiency, $e$ is the electron charge, $h$ is the Planck’s constant, and $v$ is the frequency of the light wave. ${P_{ROS}}$ is the received optical signal power which can be measured by optical power meter. For a UWOC system with biased OFDM modulation, the signal power is mainly alternating current (AC) rather than direct current (DC). Assuming that the optical power measured by optical meter is DC, and the ratio between DC and AC component is defined by $\gamma$, the available OFDM signal power could be denoted by $\gamma {P_{ROS}}$. Here ${I_s}$ is directly correlated to the signal power used in the SNR calculation, and the electrical signal power ${P_{Sig}}$ at receiver can be further expressed by: where R is the resistor at receiver, and k is a dimensioned constant parameter.Assuming the number of subcarrier replications is denoted by spectrum spread factor N, the enhanced electrical signal power ${P_{Sig\_SS}}$ after the superposition of N times replications could be expressed by:
Similarly, the electrical noise power at detector caused by external intentionally inserted interference noise light or other natural background noise light can be expressed by:
where ${P_{RON}}$ is the received optical power of external noise source or background noise measured by optical meter at receiver.Assuming all the noise at receiver attributes to the white Gaussian noise (WGN), the SNR with spread spectrum can be expressed by:
3.2 Receiver sensitivity improvement
When there is no spectrum spread applied in OFDM modulation, the $SN{R_{origin}}$ can be expressed by:
where $SN{R_{origin}}$ is proportional to the square of $P_{ROS}^{}$, as shown in the following expression [37]:Thus, the improvement of the receiver sensitivity $\Delta {P_{ROS\_dB}}$ is half of the value of spectrum spread factor $\Delta {N_{dB}}$:
3.3 Information secrecy capacity
Here we focus on deriving the information secrecy capacity of SSE-OFDM based UWOC system, where the information subcarriers are replicated over the OFDM spectrum and encrypted with spectrum phase mask (SPM). The series of these replications can be represented as $ {\{{r[n]} } \}_{n = 0}^{N - 1}$, where all the replications are equal to the same value: $r[n] = a + bi$, $(n = 1,2,3\ldots N - 1)$. The SPM $ {\{{\psi [n]} } \}_{n = 0}^{N - 1}$ is stated by:
For the eavesdroppers, there are two decryption method to analysis, for a given percentage of key leakage. Assuming there are m elements of key leaked from all the N elements, the first decryption method for eavesdropper is to decrypt and superpose all the subcarriers where m elements in all N key elements are correct and other $N - m$ element of keys are generated randomly or incorrect. The first method is named as all subcarriers superposition decryption (ASSD) in this paper. For this case, the SNR of eavesdroppers can be expressed by following:
For the ASSD scheme, since the other incorrect N-m keys are all added into the decryption and superposed over the subcarriers, these N-m subcarriers with incorrect decryption would cause degradations for the overall SNR performance. The second method is to use only the correct m keys for decryption and superpose over only the subcarriers which are correctly decrypted, which is named as correct subcarriers superposition decryption (CSSD) in this paper. The SNR for this case can be expressed as following:
Assuming the system bandwidth for all the N times of replicated subcarriers is B, the secrecy capacity ${C_S}$ that can be obtained for the information bandwidth can be expressed by following according to [44]:
Since the information subcarriers are replicated and encrypted over all the system spectrum, which are further buried under the intentionally inserted or natural generated background noise light, the SSE-OFDM based UWOC is also able to resist the known-plaintext attack (KPA) [39], especially when the optical power ratio between signal light and noise light (OPR) is negative.
Through the derivation above, conditional information-theoretic security can be realized for UWOC system employing SSE-OFDM, depending on the conditions of OPR, N, and key leakage ratio, assuming the underwater channels are the same for authorized receiving end and eavesdroppers.
4. Experimental setup
The schematic diagram of this experiment is depicted in Fig. 3. At the transmitter, the SSE-OFDM digital signal is converted to electrical signal through an AWG (Agilent m8190a, 12 GSa/s, 14 bit). The electrical signal is amplified and biased through an amplifier (Mini-Circuits, ZHL-6A-S+) and a bias-tee (Mini-Circuits, ZFBT-4R2GW-FT+). After that the electrical signal could drive the 450 nm laser to generate signal light. The signal light is collimated by a lens, mixed with noise light and transmitted from a side of the multifunctional simulation water sink. The head and tail of the water sink is not sealed and the water in the tank is always flowing with a certain amount of sediment, simulating the real water environment. A pair of mirrors is applied to adjust the light path in the water and the maximum transmission distance has reached 14.2 m.
Fig. 3. The experiment setup of SSE-OFDM based UWOC system. The following abbreviations are used: AMP-amplifier, AWG-arbitrary waveform generator, PD-photon detector, EDFA- Erbium-Doped Fiber Amplifier, OAT-optical attenuator, LC-liquid crystal, MCU- microcontroller unit, OPM-optical power meter, PMT- photomultiplier tube. TIA-transimpedance amplifier. DSO-digital storage oscilloscope.
At the noise emitter, an Erbium-doped fiber amplifier (EDFA) is employed as a noise source. The EDFA (Erbium-Doped Fiber Amplifier) could generate noise light with a broad band spectrum of amplified spontaneous emission (ASE). The noise light is converted to electrical noise via a photo detector (PD). ASE noise has been employed as noise source for true random number generator (TRNG) and physical layer encryption due to its random characteristic [45–48]. Similar to the setup at transmitter, the electrical noise is amplified and biased before driving the 450 nm laser. The amplifier, bias-tee and laser are the same as those used at transmitter.
At the receiving end, a liquid crystal (LC) is applied to adjust the optical power arriving at the detector, and the LC is controlled by a microcontroller unit (MCU) [49]. We use a photomultiplier tube (PMT) to collect the light, and an optical power meter (OPM) is used to measure the optical power behind the LC. The electrical output of PMT is amplified by a transimpedance amplifier (TIA), and then the output data is captured by a digital storage oscilloscope (DSO).
The UWOC link in the experiment is shown in Fig. 4(a). The received optical power (ROP) is measured at different distance, and the measured attenuation coefficient of the water channel is -0.387 m-1 in the experiment, as shown in Fig. 4(b).
Fig. 4. (a) The experimental photographs of transmitter, receiver and fluid water channel. (b) The result of fluid water channel attenuation coefficient measurement.
The digital signal processing (DSP) procedure diagram of SSE-OFDM encryption is shown in Fig. 1. At the transmitter (TX), the original data is serial pseudorandom bit sequence (PRBS) and it becomes parallel binary streams via serial-to-parallel (S/P) conversion. Then the parallel binary streams are mapped to M-array quadrature amplitude modulation (M-QAM) symbols. After that, the procedure of spectrum replication is executed, the data is copied into N replications. Then a series of random phase masks, which is also the key of encryption, will be applied on the replications in the procedure of phase encryption. A cyclic prefix (CP) is applied before each OFDM symbol to mitigate the deterioration caused by inter-symbol interference (ISI). The CP accounts for 1/16 of the OFDM symbol length. In front of each frame, 6400 samples are used as synchronous header. After the digital to analog conversion (D/A) in AWG, the signal is eventually output by the laser. The electrical bandwidth of the signal is set to 10 MHz, which is also the system bandwidth in the analytical model.
After passing through the UWOC channel, the laser is attenuated and deteriorated, also contaminated by noise light. In practice, the intentionally inserted artificial noise light should be mixed with signal light at the transmitter side for covering the signal spectrum, which is moved to the receiver side in the experiment for power measurement convenience purpose.
The signal light arriving at receiver is detected by PMT, and the following processing is carried out in the reverse order of TX data process. The processes of equalization and frequency offset compensation are executed to optimize the performance of bit error rate (BER), Q factor, and error vector magnitude (EVM).
5. Results and analysis
5.1 Performance improvement under background noise interference
Firstly, under the interference of noise light, we measure the transmission performance of SSE-OFDM with fixed symbol rate. Three kinds of digital modulations are used in the experiment, including QPSK, 16QAM and 64QAM.
After the signal laser passing through the 14.2 m fluid underwater channel, the received signal power is fixed at 0.53 µW. For different modulation and spectrum spread factors (N), the relationship between SNR and noise power is shown in Fig. 5, where the SNR for QPSK, 16QAM and 64QAM are 8.53 dB, 15.19 dB, and 19.34 dB at the FEC limit assuming BER threshold of ×10−3. For QPSK modulation with N of 25, the maximum noise power under FEC limit is 1.3 µW, corresponding to the optical power ratio of signal and noise light (OPR) of -3.9 dB. With N of 100, the maximum noise power under FEC limit is 2 µW, corresponding to the OPR of -5.77 dB, achieving a data rate of 0.1 Mbit/s, and a SNR improvement of 19.06 dB. For 16QAM and 64QAM modulation, when N is 25, the corresponding noise power is 0.5 µW and 0.2 µW respectively. And when the N is 100, the corresponding noise power is 0.5 µW and 0.2 µW respectively.
Fig. 5. SNR versus noise power for different spectrum spread factors (N) and digital modulation. The colored dash lines represent the FEC limit for different modulation. For QPSK, 16QAM and 64QAM modulation, the SNR corresponding to the FEC limit are 8.53 dB, 15.19 dB, and 19.34 dB, respectively.
The relation between spectrum spread factor N and SNR is shown in Fig. 6 for QPSK modulation and Fig. 7 for 16QAM modulation. As shown in Fig. 6 and Fig. 7, the curves are basically linear when the noise power exceeds 0.5 µW, meaning that SNR increases linearly with spectrum spread factor N. For instance, with N varying from 1 to 100, SNR increases about 20 dB when the noise power is 1 µW.
Fig. 6. (a) N versus SNR under different noise power interference with QPSK modulation. (b) The constellation diagrams under the noise power of 0.65 µW and N of 1, 10, 25, 100, respectively.
Fig. 7. (a) N versus SNR under different noise power interference with 16QAM modulation. (b) The constellation diagrams under the noise power of 0.65 µW and N of 1, 10, 25, 100, respectively.
For the conditions where noise power is lower than 0.5 µW and spectrum spread factor is very large, the SNR exceeds 23 dB and the curves become nonlinear as the increase N. This is because the SNR performance is approaching its performance ceiling for the UWOC system, and the spread spectrum technology gradually becoming ineffective to further improve SNR performance.
In Fig. 6 and Fig. 7, the constellation diagrams under the condition of noise power of 0.65 µW are also presented, from which it is observed that the performance is improved obviously with the increase of N.
As is shown in Fig. 8, the performance improvement in terms of SNR gain is analyzed using both experimental and analytical results. For different noise power, the SNR gain using SSE-OFDM performance is shown in Fig. 8(a), where the color indicates the value of the gain and the color mapping legend is shown in the left side. It is observed that all the color maps under different noise power are all similar, meaning that the gain is consistent for different noise power conditions. Experimental results with noise power of 2 µW and 2.5 µW, and the theoretical result with noise power of 2.5 µW are shown in Fig. 8(b), where the analytical and experimental results matches quite well.
Fig. 8. (a) SNR Gain versus noise power and N with QPSK modulation. The color indicates the value of the gain. (b) The gain comparation between experimental results of 2 µW, 2.5 µW and theoretical results of 2.5 µW. The theoretical gain expression of 2.5 µW is also shown. The signal power is set to 0.53 µW.
5.2 Results of sensitivity improvement
The receiver sensitivity without adding the artificial optical noise generated by EDFA is shown in Fig. 9. The red triangle points are the minimum N required to meet the FEC limit at the current receiver optical power. The black dash line is the theoretical result between N and ROP. From Fig. 9, it is observed that when N is larger than 4, the trend of the theoretical results and experimental results matches well. When the spectrum spread factor N equals 1, the minimum ROP is -50 dBm, corresponding to attenuation length (AL) of 16.12. And when N is 6, the minimum ROP is -57 dBm, corresponding to AL of 17.73. The maximum measured AL is 19.67 when N is 72 and the ROP is -62.4 dBm. Compared with N of 1, there is a receiver sensitivity gain of 12.4 dB by adopting SSE-OFDM modulation with N of 72. As N increases from 4 to 72, the receiver sensitivity improvement is 7 dB and 6.28 dB for experimental result and theoretical result, respectively, showing good consistence.
Fig. 9. SNR versus N and received optical power (ROP). The color bar indicates the value of the SNR. The red triangle line shows the minimum N within FEC limit. The black dash line is the theoretical result between N and ROP.
5.3 Information secrecy capacity
Based on the measured data in the experiment above, we analysis the physical layer security performance of UWOC system based on SSE-OFDM modulation, as shown in Fig. 10, where N is set to 100, the optical signal power is fixed at 0.53 µW and optical noise power is 0.65 µW. The experimental and theoretical SNR results for eavesdroppers using both the decryption method are plotted on the left blue axis in Fig. 10. For ASSD, as the keys which are not leaked to eavesdropper are randomly generated and also involved in the decryption process, this will lead to random discrepancy between theoretical results and experimental results, as shown in Fig. 10. When there is no leakage of keys, the SNR for eavesdropper is -3.72 dB while the authorized receiving end is 17.56 dB for ASSD. Moreover, the derived secrecy capacity Cs for ASSD and CSSD is plotted on the right red axis in Fig. 10, showing that positive Cs can still be obtained for UWOC system using SSE-OFDM scheme, even if parts of keys are leaked to eavesdropper.
Fig. 10. The eavesdropper’s SNR and authorized receiver’s Cs versus percentage of keys leaked. The suffix ‘1’ in the legend represents eavesdroppers adopting all subcarriers superposition decryption (ASSD), and ‘2’ represents eavesdroppers adopting correct subcarriers superposition decryption (CSSD). The suffix ‘exp’ represents the experimental results, ‘theo’ represents the theoretical results. The optical signal power is set to 0.53 µW and optical noise power is set to 0.65 µW.
5.4 Further discussions
Based on the analytical expressions previously obtained, we investigate the relationship between N, OPR, and secrecy capacity (Cs) as shown in Fig. 11 and Fig. 12, assuming there is one key leakage.
Fig. 11. The secrecy capacity (Cs) versus N and OPR for ASSD based decryption. The color mapping legend indicates the value of the Cs. The relationship between Cs and N is plotted when optical signal to noise power ratio is 4.23 dB, -0.89 dB and -5.77 dB. The Tx power is set to 0.53 µW.
Fig. 12. The secrecy capacity (Cs) versus N and OPR for CSSD based decryption. The color mapping legend indicates the value of the Cs. The relationship between Cs and N is plotted when optical signal to noise power ratio is 4.23 dB, -0.89 dB and -5.77 dB. The Tx power is set to 0.53 µW.
As is shown in Fig. 11 and Fig. 12, there will be a maximum Cs and optimum spectrum spread factor (N) for different OPR values. If the OPR is 4.23 dB, the maximum Cs is 12.64 Mbit/s for ASSD and 4.86 Mbit/s for CSSD, with optimum N of 2 and 3, respectively. If the OPR is -0.89 dB, the maximum Cs is 3.44 Mbit/s for ASSD and 2.83 Mbit/s for CSSD, with optimum N of 3 and 4, respectively. If the OPR is -5.77 dB, the maximum Cs is both 0.68 for ASSD and CSSD, with optimum N of 7 and 8, respectively.
The increase of N stands for the increased number of spectrum replications, and reduction of available information bandwidth, while the decrease of N stands for the reduction of SNR gain and also the increase of key leakage percentage for SSE-OFDM. Moreover, it can be inferred that with the increase of OPR, the optimum N for obtaining the maximum Cs is becoming larger. This is because larger spectrum spread factor (N) is needed to resist the noise light interference and enhance physical layer security as the OPR become lower.
Thus, based on the analytical expressions, the optimum spectrum spread factor can be calculated to achieve maximum Cs under different UWOC conditions, showing potential of background noise resistance, receiver sensitivity improvement and physical layer security enhancement.
6. Conclusion
The SSE-OFDM modulation based UWOC system is proposed and demonstrated over a 14.2 m sediment circulating underwater channel, where three features including the resistance of background noise interference, the receiver sensitivity improvement, and physical layer security enhancement are realized simultaneously. The experimental results show that by using the proposed SSE-OFDM scheme, the SNR of QPSK modulation can be improved by 19.06 dB, with spectrum spread factor N of 100, achieving a negative optical power ratio of signal and noise light (OPR) of -5.77 dB. Compared with the conventional OFDM modulation, the receiver sensitivity is also improved by 12.4 dB, achieving maximum measured AL of 19.67. Moreover, physical layer security of UWOC is also be realized, which suppresses the SNR of eavesdropper to -3.72 dB while improving SNR of the authorized receiver to 17.56 dB. Analytical performance expressions for all these features are also derived, which match with the experimental results quite well. Based on the expressions, the relationship between spectrum spread factor (N), optical power ratio of signal and noise light (OPR), and secrecy capacity (Cs) is obtained, where the optimum spectrum spread factor N can be calculated to achieve maximum Cs under different UWOC conditions.
Funding
State Key Laboratory of Information Photonics and Optical CommunicationsFunds (IPOC2020ZZ02); Fundamental Research Funds for the Central Universities (2019XD-A15-2); National Natural Science Foundation of China (U1831110).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. Z. Zeng, S. Fu, H. Zhang, Y. Dong, and J. Cheng, “A survey of underwater optical wireless communications,” IEEE Commun. Surv. Tutorials 19(1), 204–238 (2017). [CrossRef]
2. H. Kaushal and G. Kaddoum, “Underwater optical wireless communication,” IEEE access 4, 1518–1547 (2016). [CrossRef]
3. N Saeed, A Celik, TY Al-Naffouri, and MS Alouini, “Underwater optical wireless communications, networking, and localization: A survey,” Ad Hoc Networks 94, 101935 (2019). [CrossRef]
4. H. M. Oubei, C. Li, K.-H. Park, T. K. Ng, M.-S. Alouini, and B. S. Ooi, “2.3 Gbit/s underwater wireless optical communications using directly modulated 520 nm laser diode,” Opt. Express 23(16), 20743–20748 (2015). [CrossRef]
5. H. M. Oubei, J. R. Duran, B. Janjua, H.-Y. Wang, C.-T. Tsai, Y.-C. Chi, T. K. Ng, H.-C. Kuo, J.-H. He, M.-S. Alouini, G.-R. Lin, and B. S. Ooi, “4.8 Gbit/s 16-QAM-OFDM transmission based on compact 450-nm laser for underwater wireless optical communication,” Opt. Express 23(18), 23302–23309 (2015). [CrossRef]
6. M. Kong, W. Lv, T. Ali, R. Sarwar, C. Yu, Y. Qiu, F. Qu, Z. Xu, J. Han, and J. Xu, “10-m 9.51-Gb/s RGB laser diodes-based WDM underwater wireless optical communication,” Opt. Express 25(17), 20829–20834 (2017). [CrossRef]
7. C. Fei, J. W. Zhang, G. W. Zhang, Y. J. Wu, X. Z. Hong, and S. He, “Demonstration of 15-M 7.33-Gb/s 450-nm underwater wireless optical discrete multitone transmission using post nonlinear equalization,” J. Lightwave Technol. 36(3), 728–734 (2018). [CrossRef]
8. Y. Chen, M. Kong, T. Ali, J. Wang, R. Sarwar, J. Han, C. Guo, B. Sun, N. Deng, and J. Xu, “26 m/5.5 Gbps air-water optical wireless communication based on an OFDM-modulated 520-nm laser diode,” Opt. Express 25(13), 14760–14765 (2017). [CrossRef]
9. M. Chen, P. Zou, L. Zhang, and N. Chi, “Demonstration of a 2.34 Gbit/s real-time single silicon-substrate blue LED-based underwater VLC system,” IEEE Photonics J. 12(1), 1–11 (2020). [CrossRef]
10. Z. Chen, X. Tang, C. Sun, Z. Li, W. Shi, H. Wang, L. Zhang, and A. Zhang, “Experimental Demonstration of Over 14 AL Underwater Wireless Optical Communication,” IEEE Photonics Technol. Lett. 33(4), 173–176 (2021). [CrossRef]
11. C. Tu, W. Liu, W. Jiang, and Z. Xu, “First Demonstration of 1Gb/s PAM4 Signal Transmission Over A 130 m Underwater Optical Wireless Communication Channel with Digital Equalization,” 2021 IEEE/CIC International Conference on Communications in China (ICCC). IEEE, 2021.
12. J. Wang, C. Lu, S. Li, and Z. Xu, “100 m/500 Mbps underwater optical wireless communication using an NRZ-OOK modulated 520 nm laser diode,” Opt. Express 27(9), 12171–12181 (2019). [CrossRef]
13. X. Chen, X. Yang, Z. Tong, Y. Dai, X. Li, M. Zhao, Z. Zhang, J. Zhao, and J. Xu, “150 m/500 Mbps underwater wireless optical communication enabled by sensitive detection and the combination of receiver-side partial response shaping and TCM technology,” J. Lightwave Technol. 39(14), 4614–4621 (2021). [CrossRef]
14. Y. Dai, X. Chen, X. Yang, Z. Tong, Z. Du, W. Lyu, C. Zhang, H. Zhang, H. Zou, Y. Cheng, D. Ma, J. Zhao, Z. Zhang, and J. Xu, “200-m/500-Mbps underwater wireless optical communication system utilizing a sparse nonlinear equalizer with a variable step size generalized orthogonal matching pursuit,” Opt. Express 29(20), 32228–32243 (2021). [CrossRef]
15. W. Wang, H. Wang, and G. Lin, “Ultrahigh-speed violet laser diode based free-space optical communication beyond 25 Gbit/s,” Sci. Rep. 8(1), 13142 (2018). [CrossRef]
16. L. Zhang, X. Tang, C. Sun, Z. Chen, Z. Li, H. Wang, R. Jiang, W. Shi, and A. Zhang, “Over 10 attenuation length gigabits per second underwater wireless optical communication using a silicon photomultiplier (SiPM) based receiver,” Opt. Express 28(17), 24968–24980 (2020). [CrossRef]
17. C. Lu, J. Wang, S. Li, and Z. Xu, “60 m/2.5 Gbps underwater optical wireless communication with NRZ-OOK modulation and digital nonlinear equalization,” 2019 Conference on Lasers and Electro-Optics (CLEO). IEEE, 2019.
18. C. Pontbriand, N. Farr, J. Ware, J. Preisig, and H. Popenoe, “Diffuse high-bandwidth optical communications,” OCEANS 2008. IEEE, 2008.
19. P. Leon, F. Roland, L. Brignone, J. Opderbecke, J. Greer, M. A. Khalighi, T. Hamza, S. Bourennane, and M. Bigand, “A new underwater optical modem based on highly sensitive Silicon Photomultipliers,” OCEANS 2017-Aberdeen. IEEE, 2017.
20. F. M. Levinton, “An optical filter for underwater laser communications,” Applications of Lasers for Sensing and Free Space Communications. Optical Society of America, 2011.
21. J. Sticklus, M. Hieronymi, and P. A. Hoeher, “Effects and constraints of optical filtering on ambient light suppression in LED-based underwater communications,” Sensors 18(11), 3710 (2018). [CrossRef]
22. N. E. Farr, C. T. Pontbriand, J. D. Ware, and L. -. A. Pelletier, “Non-visible light underwater optical communications,” 2016 IEEE Third Underwater Communications and Networking Conference (UComms). IEEE, 2016.
23. T. Hamza, M. A. Khalighi, S. Bourennane, P. Léon, and J. Opderbecke, “Investigation of solar noise impact on the performance of underwater wireless optical communication links,” Opt. Express 24(22), 25832–25845 (2016). [CrossRef]
24. J. Zhang, G. Gao, B. Wang, X. Guan, L. Yin, J. Chen, and B. Luo, “Background noise resistant underwater wireless optical communication using Faraday atomic line laser and filter,” J. Lightwave Technol. 40(1), 63–73 (2022). [CrossRef]
25. C Li, Z Liu, D Chen, X Deng, F Yan, S Li, and Z Hu, “Experimental demonstration of high-sensitivity underwater optical wireless communication based on photocounting receiver,” Photonics. Vol. 8. No. 11. Multidisciplinary Digital Publishing Institute, 2021.
26. C. Lu, C. Li, and Z. Xu, “Experimental investigation of underwater weak optical communication using a photomultiplier tube receiver,” Asia Communications and Photonics Conference. Optical Society of America, 2018.
27. J. Wang, X. Yang, W. Lv, C. Yu, and J. Xu, “Underwater wireless optical communication based on multi-pixel photon counter and OFDM modulation,” Opt. Commun. 451, 181–185 (2019). [CrossRef]
28. J. Li, D. Ye, K. Fu, L. Wang, J. Piao, C. Li, and Y. Wang, “Photon-counting schemes for MIMO underwater wireless optical communication with arrayed PMTs,” Appl. Opt. 61(2), 403–409 (2022). [CrossRef]
29. J. Li, D. Ye, K. Fu, L. Wang, J. Piao, and Y. Wang, “Single-photon detection for MIMO underwater wireless optical communication enabled by arrayed LEDs and SiPMs,” Opt. Express 29(16), 25922–25944 (2021). [CrossRef]
30. M. Zhao, X. Li, X. Chen, Z. Tong, W. Lyu, Z. Zhang, and J. Xu, “Long-reach underwater wireless optical communication with relaxed link alignment enabled by optical combination and arrayed sensitive receivers,” Opt. Express 28(23), 34450–34460 (2020). [CrossRef]
31. J. Shen, J. Wang, C. Yu, X. Chen, J. Wu, M. Zhao, F. Qu, Z. Xu, J. Han, and J. Xu, “Single LED-based 46-m underwater wireless optical communication enabled by a multi-pixel photon counter with digital output,” Opt. Commun. 438, 78–82 (2019). [CrossRef]
32. S. Hu, L. Mi, T. Zhou, and W. Chen, “35.88 attenuation lengths and 3.32 bits/photon underwater optical wireless communication based on photon-counting receiver with 256-PPM,” Opt. Express 26(17), 21685–21699 (2018). [CrossRef]
33. Y. Guo, M. Kong, O. Alkhazragi, X. Sun, M. Sait, T. K. Ng, and B. S. Ooi, “Diffused-Line-of-Sight Communication for Mobile and Fixed Underwater Nodes,” IEEE Photonics J. 12(6), 1–13 (2020). [CrossRef]
34. W. C. Lyu, M. M. Zhao, X. Chen, X. Q. Yang, Y. Qiu, Z. J. Tong, and J. Xu, “Experimental demonstration of an underwater wireless optical communication employing spread spectrum technology,” Opt. Express 28(7), 10027–10038 (2020). [CrossRef]
35. M. Kong, J. Wang, Y. Chen, T. Ali, R. Sarwar, Y. Qiu, S. Wang, J. Han, and J. Xu, “Security weaknesses of underwater wireless optical communication,” Opt. Express 25(18), 21509–21518 (2017). [CrossRef]
36. D. Shaboy, D. Ronckban, and A. Handelman, “Tapping underwater wireless optical communication in pure water and natural Dead-Sea ultra-high-salinity water by diffraction grating,” Opt. Express 26(23), 29700–29711 (2018). [CrossRef]
37. Y. Zhou, J. Shi, J. Zhang, and N. Chi, “Spectral Scrambling for High-security PAM-8 Underwater Visible Light Communication System,” Asia Communications and Photonics Conference. Optical Society of America, 2018.
38. J. Du, Y. Wang, C. Fei, R. Chen, G. Zhang, X. Hong, and S. He, “Experimental demonstration of 50-m/5-Gbps underwater optical wireless communication with low-complexity chaotic encryption,” Opt. Express 29(2), 783–796 (2021). [CrossRef]
39. G. S. Kanter, D. Reilly, and N. Smith, “Practical physical-layer encryption: The marriage of optical noise with traditional cryptography,” IEEE Commun. Mag. 47(11), 74–81 (2009). [CrossRef]
40. Z. Wang, Y. Xiao, S. Wang, Y. Yan, B. Wang, Y. Chen, Z. Zhou, J. He, and L. Yang, “Probabilistic shaping based constellation encryption for physical layer security in OFDM RoF system,” Opt. Express 29(12), 17890–17901 (2021). [CrossRef]
41. Y. Bai, B. Liu, J. Ren, Y. Mao, Z. Qi, S. Chen, X. Song, S. Zhu, L. Yuan, S. Han, and R. Ullah, “Highly Secure and Reliable 7-Core Fiber Optical OFDM Access System Based on Chaos Encryption Inside Polar Code,” IEEE Photonics J. 14(1), 1–6 (2022). [CrossRef]
42. X. Yang, J. Zhang, Y. Li, Y. Zhao, G. Gao, and H. Zhang, “DFTs-OFDM based quantum noise stream cipher system,” Opt. Fiber Technol. 52, 101939 (2019). [CrossRef]
43. E. Wohlgemuth, Y. Yoffe, T. Yeminy, Z. Zalevsky, and D. Sadot, “Demonstration of coherent stealthy and encrypted transmission for data center interconnection,” Opt. Express 26(6), 7638–7645 (2018). [CrossRef]
44. E. A. Jorswieck, A. Wolf, and S. Gerbracht, “Secrecy on the physical layer in wireless networks,” Trends in Telecommunications Technologies (2010).
45. C. R. S. Williams, J. C. Salevan, X. Li, R. Roy, and T. E. Murphy, “Fast physical random number generator using amplified spontaneous emission,” Opt. Express 18(23), 23584–23597 (2010). [CrossRef]
46. A. Argyris, E. Pikasis, S. Deligiannidis, and D. Syvridis, “Sub-Tb/s physical random bit generators based on direct detection of amplified spontaneous emission signals,” J. Lightwave Technol. 30(9), 1329–1334 (2012). [CrossRef]
47. Y. Guo, Q. Cai, P. Li, Z. Jia, and Y. Wang, “40 Gb/s quantum random number generation based on optically sampled amplified spontaneous emission,” APL Photonics 6(6), 066105 (2021). [CrossRef]
48. J. Yang, F. Fan, J. Liu, Q. Su, and B. Xu, “Randomness quantification for quantum random number generation based on detection of amplified spontaneous emission noise,” Quantum Sci. Technol. 6(1), 015002 (2021). [CrossRef]
49. J. Ning, G. Gao, J. Zhang, H. Peng, and Y. Guo, “Adaptive Receiver Control for Reliable High-speed Underwater Wireless Optical Communication with Photomultiplier Tube Receiver,” IEEE Photonics Journal (2021).
