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Abstract
For some industrial underwater wireless optical communication (UWOC) applications, the transmission distance matters more than the communication rate. Attenuation length (AL) is an important distance indicator of UWOC system. In this paper, to the best of our knowledge, the spread spectrum (SS) technology is firstly applied in a UWOC system and the capability to extend transmission distance or AL is demonstrated. A 42-m UWOC is experimentally demonstrated with 6.68 ALs. Compared with the conventional not-return-to-zero on-off-keying (NRZ-OOK) modulation scheme, the proposed SS scheme with a spread spectrum gain (SSG) of 5 achieves an AL extension by 0.51 and 0.81, respectively, with the same data rate and bandwidth. And the minimum required signal-to-noise ratio (SNR) is reduced by 9 dB to as low as −0.8 dB. Besides, the feature of the SS scheme that could work in a bandwidth-limited long-reach underwater channel without the equalization process is experimentally demonstrated.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the past few years, with the increasing interest in monitoring and exploring the oceans, long-distance and high-speed wireless transmission links are highly desirable. However, conventional underwater acoustic communication suffers from low bandwidth, severe multi-path fading, large time latency and bulky antennas [1]. On the other hand, radio-frequency communication suffers severe attenuation in seawater [2]. Therefore, with the feature of high bandwidth, low latency, low power consumption and flexible deployment, underwater wireless optical communication (UWOC) is more preferable in bandwidth-intensive applications, and becomes a promising alternative or complementary solution to underwater broadband data, images and video transmissions [3–5]. Besides, the green and blue laser diodes (LDs), with high modulation bandwidth, can generate light with relatively low attenuation in UWOC transmission. Many research groups have demonstrated theoretically and experimentally the feasibility of UWOC in visible light region with high data rate (e.g. several Gbps) and medium transmission distance (e.g. more than tens of meters) by employing highly sensitive optical detectors such as the avalanche photodiode (APD) [6–9].
For some industrial applications, the transmission distance of a UWOC system matters more than the communication rate. A data rate of hundreds of Mbps could meet a vast majority of underwater communication demands (e.g. the video transmissions). On the other hand, the data rate achieved by many researchers has gone beyond several Gbps. To achieve such a high communication rate, high-level modulation formats are usually used to improve the spectral efficiency, such as quadrature amplitude modulation (QAM) [7–11], pulse amplitude modulation (PAM) [12] and orthogonal frequency division multiplexing (OFDM) [7,9–11], which generally require a relatively high signal-to-noise ratio (SNR). However, a longer communication distance always means a lower received optical power (ROP) and SNR, which is hard to achieve and further extended. Some long-distance UWOC systems based on single-photon receivers, such as single photon avalanche diode (SPAD) and multi-pixel photon counter (MPPC), were usually experimentally demonstrated at relatively low data rates due to the dead time or limited bandwidth [13–15]. Therefore, it is attractive to find out a new dimension to extend the transmission distance based on commercially available devices while maintaining the date rate of no less than hundreds of Mbps.
In this work, we adopt the spread spectrum (SS) technology to further improve the transmission distance based on limited device parameters (e.g. output optical power of optical sources, the sensitivity of optical photodiodes, and system bandwidth). According to Shannon’s theorem, the channel capacity is related to the bandwidth and the SNR, which can be approximated as 1.44*Bandwidth*SNR when the SNR is low enough. Given a fixed channel capacity, we can reduce the required SNR of the received signals via deliberately encoding the signal into a larger bandwidth, which is the basic principle of spread spectrum techniques. While the additional bandwidth requirement reduces the system bandwidth efficiency, the spread spectrum gain (SSG) brings substantial advantages, such as increased resistance to natural and intentional interference, higher security, and reduced power flux density [16–18]. With the rapid development of manufacturing technology, the bandwidth of optical devices in visible band or blue-green light region have been greatly improved. In [19], the 3-dB bandwidth of the adopted positive-intrinsic-negative (PIN) photodiode is 7 GHz and that of the 680-nm vertical cavity surface emitting laser (VCSEL) even reaches 26 GHz in [20]. Therefore, certain bandwidth could be traded via spread spectrum technology to reduce the required SNR or optical power of received signals, and then extend the transmission distance.
With conventional direct-sequence spread spectrum (DSSS) technology, the message signal is modulated by a bit sequence known as a pseudo noise (PN) code to spread the pieces of data, thereby resulting in a bandwidth nearly identical to that of the PN sequence. With the tamed spread spectrum (TSS) technology, which is similar to DSSS, every k bits data being transmitted will be spread by a cluster of orthogonal sequences with a length of n bits, leading to a better anti-interference ability. Then the spread spectrum gain (n/k) could be adjusted more flexibly. Another technology, namely channel coding technology, which also decreases the bandwidth efficiency but improves the system reliability via detecting and correcting the errors, is widely adopted to the UWOC system [13,21–23]. However, the performance of channel coding will degrade when the errors burst too much or the SNR is low, while the SS technology could still work well at a low SNR.
In this work, we experimentally demonstrate the feasibility of spread spectrum technology in the UWOC system to extend the transmission distance. A maximum transmission distance of 42 meters, corresponding to 6.68 attenuation lengths (ALs), is achieved in the experimental demonstration. The calculated maximum transmission distance and AL of our system could reach 56.9 m and 9.05, respectively. Table 1 summarizes the representative configurations and performance of UWOC system in recent years. Since water types may vary largely among different reports, here we use AL instead of transmission distance as an indicator of system performance. We can find that the link AL realized in this work is superior to that realized in other PIN or APD based UWOC systems (with at least a 30% improvement over all other reported ALs), especially under a relatively low injection optical power of 10 mW. While the date rate is not remarkably high but could meet a vast majority of underwater communication demands. Besides, compared with the scheme employing the conventional NRZ-OOK modulation, an improvement of 8.4 dB in the SNR and 2.2 dB in the receiver sensitivity, could be observed for the TSS scheme with an SSG of 5, respectively, at the same data rate of 200 Mb/s. In this preliminary experiment, the improvement means a 3.2-m extension in transmission distance or 0.51 in AL. For the SS schemes with higher spread spectrum gains, the extended distance will be longer. Besides, the feature of SS scheme that could work in a bandwidth-limited channel without the equalization process is experimentally demonstrated.
Table 1. Comparison of UWOC systems configurations and performance
2. Spread spectrum technology in UWOC system
In an additive-white-Gaussian-noise (AWGN) channel, spread spectrum technology can obtain an improved SNR by collecting the power of spread signal. And the improvement of SNR (dB) equals to the value of SSG (dB). However, in a direct-detection optical system such as the UWOC system, the channel is not seriously an AWGN channel due to the existence of shot noise. Besides, ROP or receiver sensitivity is more commonly adopted than SNR to evaluate the system performance. Thus, it is necessary to analyze the relationship between the SSG and the ROP in a spread spectrum based UWOC system. If the input bits ${b_k}$ are uniform on {0,1} and the bit ‘0’ is biased below the turn-on voltage of the laser diode, the ROP for each bit is:
where $RO{P_{AVE}}$ is the measured average ROP. After the photoelectric conversion, the theoretical SNR could be expressed as:3. Experimental setup
The block diagram of experimental setup is shown in Fig. 1. In the experiment, the information flow was generated via the pseudo-random binary sequence (PRBS) with a length of 216−1. For n-DSSS scheme, the information flow was spread by a PN sequence with a length of n. For (n, k) coded TSS scheme, we cyclically shifted a single sequence with a length of n and obtained a matrix space with a size of n×2k. Then the information flow was spread to n bits in the coding matrix space for every k bits. The bit sequences were sent to an arbitrary waveform generator (AWG, Tektronix AWG70002A) to generate the electrical signals. Then the voltage of the output signal was properly adjusted via an electric amplifier (AMP) and a variable electrical attenuator (ATT) before modulating a 450-nm blue laser diode (LD) (Thorlabs, LP450-SF15). The P-I curve of the LD was measured and shown in Fig. 2(a). After being collimated via a collimating lens (CL), the modulated blue light was then launched into a 7-m water tank filled with tap water. The light was reflected 5 times between high-reflectivity mirrors close to sidewalls of the tank. The total transmission distance was 42 m. Then the blue light was focused by a focusing lens (FL) and incident on an avalanche photodiode (Thorlabs, APD210). The detected signal was amplified and then captured by a mixed signal oscilloscope (MSO, Tektronix, 71254) before being sent to a computer for offline processing. At the receiving end, after synchronization, the DSSS signals were recovered and decided via correlating with the coding sequence. The TSS signals will be firstly correlated with each orthogonal sequence in the coding matrix space and then the most matched one will be mapped to the information bits. Finally, the BERs were calculated for each scheme. Besides, the system frequency response was measured by a network analyzer (Hewlett Packard, 8753D), which is shown in Fig. 2(b). The 10-dB bandwidth was around 1000 MHz.
Fig. 1. The system schematic of 42-m UWOC experiments. S/P: Serial to Parallel conversion; P/S: Parallel to Serial conversion
In the preliminary experiments, a 200-MHz NRZ-OOK signal was adopted to measure the system performance. The bias current of LD was adjusted from 57.5 mA to 70 mA by a step of 2.5 mA, while the voltage of the electrical signal generated from the AWG was set as 350 mV, 400 mW, and 450 mV, respectively. Since the system runs error free for all cases, a variable optical attenuator (VOA) was used to lower the received optical power. Then the BER performances are shown as Fig. 3, from which we could observe that the optimal signal voltage was 350 mV. To achieve a longer transmission distance, the bias current was set as 62.5 mA and the output optical power of LD was measured to be 10.0 mW. The ROP after the 42-m transmission was 9.8 µW before the VOA. The transmittance of the tank windows and focus lens is measured to be 0.801 and 0.981, respectively, and the reflectivity of the mirrors is 0.999. Since the spot diameter after the 42-m transmission is smaller than the diameter of collimating lens, Beer-Lambert law is adopted to estimate the attenuation coefficient c. The value of c is calculated to be 0.159/m (equivalent to 0.691 dB/m), which is a bit larger than the Jerlov IB type of seawater with the c of 0.144/m. The achieved AL of our system is 6.682.
4. Experimental results and discussion
After setting the experimental parameters, NRZ-OOK signals, TSS signals, and DSSS signals were transmitted to measure the maximum communication capacity at the fixed transmission distance of 42 meters. We gradually increased the sampling rates or signal bandwidths (BW) for each modulation scheme. The SSGs of (8, 2), (8, 3), (10, 3), (12, 3) and (15, 3) coded TSS signals are 4, 8/3, 10/3, 4 and 5, respectively. And the SSGs of 4-DSSS and 5-DSSS are 4 and 5, respectively. The BER performances with different bandwidths were obtained and shown as Fig. 4(a). SS schemes with higher SSGs could work at higher bandwidth while keeping the BER below the forward-error-correction (FEC) limit of 3.8 × 10−3. The corresponding data rate R could be expressed by R = BW/SSG bps. Considering the SSGs for different schemes, we replotted the curves in Fig. 4(a) with the horizontal axis of data rate, which are shown as Fig. 4(b). The maximum data rate of the 42-m SS-based UWOC link changes with different spread spectrum gains between 700 Mbps and 1 Gbps, which could meet a vast majority of underwater communication demands. For the fixed 42-m link length and 9.8-mW received optical power, the maximum data rate of the OOK scheme is 1.9 Gbps.
Then the receiver sensitivity for each scheme at the same signal bandwidth was measured and the feasibility of SS technology to extend the transmission distance was experimentally demonstrated. The bandwidths of the NRZ-OOK signals, TSS signals, and DSSS signals were all set to 200 MHz firstly. After the offline processing, the BER and SNR were calculated, and the results are shown as Fig. 5. Note that a conventional Least-Mean-Square (LMS) equalization technology was adopted to the OOK scheme, while it was generally unnecessary for TSS and DSSS schemes. An obvious correlation could be observed between the SSG and system performance, i.e. a higher SSG brings a higher receiver sensitivity that implies a longer transmission distance, at the expense of a reduced data rate. Generally speaking, the TSS scheme have a better performance than the DSSS scheme with the same SSG. Comparing the ROP under the FEC limit, the (15, 3) coded TSS scheme has a maximum budget gain of around 3.5 dB with regard to the OOK scheme without equalization. That is to say that the receiver sensitivity under the FEC limit is improved by 3.5 dB to −30.5dBm, which means an extended transmission distance of 5.1 meters or 0.81 ALs. Meanwhile, a decrease of around 9 dB in required SNR is observed, verifying the feasibility of TSS technology in improving the system performance. The required SNR for the (15, 3) coded TSS scheme is even reduced to −0.8 dB. The inserts of Fig. 5(a) are the eye diagrams plotted for the OOK scheme and (12, 3) coded TSS scheme below the FEC threshold, respectively, which also indicates that the SS schemes could work under a low SNR as well as with a higher physical layer security. Note that, since the SNR is calculated offline via the received signal, it also includes the interference arising from the limited system bandwidth. Strictly speaking, the SNR mentioned in this section should be described as signal-to-interference-plus-noise-ratio (SINR).
Fig. 5. The BER performance as a function of (a) ROP and (b) SNR under the same bandwidth of 200 MHz. Insets: eye diagrams plotted for the OOK scheme and (12, 3) coded TSS scheme below the FEC threshold.
The equivalent maximum transmission distance as a function of ROP could be expressed as L = (Pin - Llink - ROP)/c, where Llink is the power loss brought from the tank windows, mirrors and focusing lens, and the unit of c is dB/m here. The corresponding equivalent distance of each ROP is labeled at the top of Fig. 5(a). As the ROP after the 42-m transmission and the receiver sensitivity of (15, 3) coded TSS scheme under FEC limit being 9.8 µW (−20.1 dBm) and −30.4 dBm, respectively, a 10.3-dB power margin can be attained. The maximum distance for the (15, 3) coded TSS scheme is expected to reach 56.9 m, corresponding to 9.05 ALs.
To make a fair comparison, we set different signal bandwidths for different SS schemes to keep the same bit rate of 200 Mbps as the plain OOK scheme without SS. The BER performance at different ROPs and SNRs is shown in Fig. 6. We can observe that SS schemes with higher SSGs or coding lengths still perform better in BER results, which is more obvious in Fig. 6(b). A maximum improvement of 2.2 dB and 8.4 dB could be observed for the receiver sensitivity under the FEC limit and required SNR, respectively, which is a bit decrease compared with the former results shown in Fig. 5. The corresponding extended transmission distance is 3.2 meters. A higher SSG requires more bandwidth to keep the same data rate, which increases the interference and lowers the SINR for this bandwidth-limited experimental setup. Comparing the results on the data rate shown in Fig. 4 and the distance shown in Fig. 5 and Fig. 6, we could conclude that the system performance can be flexibly adjusted for certain applications where the data rate or the transmission distance is more eagerly needed. Therefore, considering the application requirements, one can achieve a higher data rate or a longer distance via changing the SSGs of the transmitted signals.
Fig. 6. The BER performances as a function of (a) ROP and (b) SNR at the same data rate of 200 Mbps.
The calculated signal power, noise power and SNR of three typical schemes at different ROPs are shown as Fig. 7. The bandwidth of the OOK signal, (8, 3) coded TSS signal and (15, 3) coded TSS signal is 200 MHz, 540 MHz and 1000 MHz, respectively. We can observe that with the same ROP, there is a maximum 5.5-dB increase in calculated noise (noise and interference) and a 5.5-dB decrease in SNR between the (15, 3) coded TSS scheme and OOK scheme, respectively. That is to say that as the SSG increases, the receiver sensitivity improves but the received signal (or the SINR) degrades in this experiment mainly due to the increased interference at the same time. Therefore, the improvement of receiver sensitivity in Fig. 6 is a bit decreased compared with the former results shown in Fig. 5. But it could be deduced that for the systems with sufficient bandwidths or higher SSGs, the improvement on receiver sensitivity or transmission distance would be larger. Thus, we simulated the receiver sensitivity in optical DSSS system with higher SSGs than in the experiments. The simulation parameters were mainly estimated from the former experimental results, such as the responsivity of the APD, shot noise and thermal noise. Note that we supposed the spectral response was flat and no interferences were introduced. The BER performances as the function of the ROP in the DSSS system with spread-spectrum sequences of different lengths are shown in Fig. 8(a). The sequence length of 1 means the conventional OOK scheme and the SSG of other DSSS schemes is 10 times the logarithm of sequence lengths. We can observe that the DSSS system still performs better under a higher SSG. Then we extract the receiver sensitivity under the FEC limit at each SSG to plot the curve shown in Fig. 8(b). The receiver sensitivity improves approximately linearly as the SSG increases, which verifies that the transmission distance can be extended in the SS based UWOC system by increasing the SSG as long as the bandwidth is sufficient. The improvement on receiver sensitivity is always lower than half of the SSG value in Fig. 8(b), which matches well with the conclusion in part 2. This simulation results show a relatively low gain efficiency of spread spectrum in UWOC system, especially in an SS scheme with a gain larger than 10, where the traded bandwidth grows exponentially faster than the gain value. But in an SS scheme with a gain lower than 10 (e.g. 5 in this experiment), the improvement on receiver sensitivity (3.5 dB in same-bandwidth case and 2.2 dB in same-rate case) is still considerable.
Fig. 7. The calculated signal power (left axis), noise power (left axis) and SNR (right axis) as a function of ROP. Ps: power of signal; Pn: power of noise.
Fig. 8. (a) The simulated receiver sensitivity performance in the DSSS system with different spread-spectrum sequence lengths; (b) The improvement on receiver sensitivity at different SS gains.
However, the system response of UWOC is always not flat even if transceivers with higher bandwidths are adopted. At the receiver, the photons are received at different times due to the scattering effect, leading to temporal dispersion and a bandwidth-limited underwater channel, especially in the extremely turbid water or a long-distance transmission. We have studied the frequency response of light in 8-m harbor water by Monte-Carlo simulation, which shows a 10-dB, 15-dB and 20-dB attenuation at the 1-GHz, 2-GHz and 3-GHz frequency, respectively, for a 520-nm laser beam [29]. For conventional modulation schemes, equalization technology would be necessary to eliminate or mitigate the interference induced by the limited bandwidth. However, for TSS scheme, the orthogonal convolution during demodulation works as an equalization process. Without loss of generality, we compared the BER performances of (12, 3) coded TSS scheme with and without conventional LMS equalization, which is shown as Fig. 9. The two blue solid lines in Figs. 9(a) and 9(b) are the experimental results without equalization shown in Fig. 4 and Fig. 6, respectively, while the red dashed lines with LMS equalization. The BERs are approximately the same at different ROPs under a relative lower bandwidth (800 MHz, corresponding to an around 8-dB attenuation in frequency response), while not much difference could be observed when the bandwidth is lower than 3.3 GHz. Therefore, SS scheme could work well under a bandwidth-limited or long-distance underwater channel without any equalization process, with negligible performance degradation.
Fig. 9. The BER performance comparisons of (12, 3) coded TSS scheme with and without equalization at (a) different ROPs; (b) different bandwidths
The above experimental results demonstrate the feasibility of spread spectrum technology in conventional photodiodes-based UWOC systems. The achieved data rate of spread spectrum schemes reaches 1.1 Gbps at most, and the transmission distance is 42 meters with a receiver sensitivity of −30.5 dBm. Since the sensitivity of APD is limited by the thermal noise in a low-ROP system, single-photon detectors, with lower thermal noise and the sensitivity approaching the quantum limit, may perform better in a longer-distance UWOC. Recent research shows that the data rate of hundreds of Mbps has been achieved in the SPAD-based UWOC with a highest sensitivity of −46 dBm [30–32]. It would be attractive and meaningful if spread spectrum technology could be applied to single-photon detectors to achieve a longer and safer underwater communication.
5. Conclusion
In this work, the TSS and DSSS technology are compared and applied in the UWOC system for the first time and their capabilities of extending transmission distance are experimentally verified. By changing different spread spectrum gains to obtain a higher data rate or a longer distance at certain application scenarios, the system performance can be flexibly adjusted. In the experiment, a maximum transmission distance of 42 meters, corresponding to 6.68 ALs, is successfully demonstrated, which is superior to most reported systems. The achieved data rates of spread spectrum schemes are more than 700 Mbps which could meet a vast majority of underwater communication demands. The maximum transmission distance and AL is expected to be 56.9 meters and 9.05 ALs in this work, respectively. Both the required SNR and the receiver sensitivity for UWOC are significantly reduced. Compared with the conventional OOK scheme, the extension of transmission distance reaches 3.2 meters and 5.1 meters at the same data rate and bandwidth, respectively, by adopting the SS scheme with an SSG of 5. The distance could be longer with a higher SSG or a larger system bandwidth. Besides, the feature of SS scheme that could work in a bandwidth-limited channel without the equalization process is demonstrated, which indicates the capability of further being applied in the turbid harbor water or long-reach UWOC. The spread spectrum technology paves a new dimension to extend the UWOC transmission distance and can be thought as a promising solution to the future long-reach UWOC systems.
Funding
National Natural Science Foundation of China (61671409, 61705190, 61971378); Key Technologies Research and Development Program (2016YFC1401202, 2017YFC0306100, 2017YFC0306601); Strategic Priority Research Program of the Chinese Academy of Sciences (XDA22030208); Fundamental Research Funds for the Central Universities.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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