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Abstract
In this paper, we proposed and experimentally demonstrated a long-distance high-speed underwater optical wireless communication (UOWC) system in a laboratory environment by using a low-cost green laser diode (LD) and power-efficient non-return-to-zero on-off keying (NRZ-OOK) modulation. The system successfully achieved a data rate of 500 Mbps through a 100 m tap-water channel by using a pigtailed single-mode fiber 520 nm green LD. The tap water was measured to have an attenuation coefficient comparable to pure seawater. The measured system bit error rate (BER) value of 2.5 × 10−3 was below the forward error correction (FEC) limit of 3.8 × 10−3 with 7% overhead. The distance can be extended if the received optical power is allowed to reduce to the minimum power to meet the data rate requirement. Based on the measured minimum required power and the power decay model in the water channel, the transmission performance was predicted to be 146 m/500 Mbps and 174 m/100 Mbps.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
A reliable long-distance and high-speed underwater communication link plays an important role in the undersea exploration and data collection. At present, the most widely used underwater communication method is still dominated by underwater acoustic communication [1]. Although it can reach the transmission distance of several kilometers and more, the data rate is very low, typically on the order of tens of kilo-bits per second. As another option, the use of radio frequency (RF) communication is limited by the skin effect of seawater, and the transmission distance is generally below 10 m [2].
Optical waves can serve as new information carriers in underwater communication. The seawater has a wavelength absorption window of blue-green light [3–5], which provides the feasibility for underwater optical wireless communication (UOWC). Research on various underwater optical channel characteristics and models has made breakthroughs in recent years [6–8]. In addition, the rapid development of the semiconductor technology offers a possibility of system implementation. Light-emitting diodes (LEDs) and laser diodes (LDs) have been used as light sources for building UOWC systems [3,9–12]. The large divergence angle of the commercial LED results in severe geometric losses in the water, so the transmission distance is limited. Moreover, its limited bandwidth of the 10 MHz level makes it difficult to achieve a higher data rate [13]. A collimated laser beam has very small divergence on the order of milli-radians, and is able to propagate along a long-distance underwater channel. Meanwhile, the modulation bandwidth of an LD is relatively high, which is the key to realization of a high-speed UOWC system.
Recently much attention has been paid to the long-distance or high-speed UOWC based on blue or green LDs and LEDs. Key performances of the UOWC systems with data rate at least 1.5 Gbps or distance longer than 100 m reported in the literature are summarized and compared in Table 1. In 2016, a 1.5 Gbps UOWC link over 20 m underwater channel using a 450 nm LD and non-return-to-zero on-off keying (NRZ-OOK) modulation was demonstrated [14]. In the same year, an innovative UOWC system based on a two-stage injection-locked 405 nm LD transmitter with 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) signal was proposed and demonstrated, achieving up to 8 m/9.6 Gbps [15]. In 2017, a Japanese team designed an underwater optical communication detector called “Kaiko”, which used avalanche photodiode (APD) and photomultiplier (PMT) as receiver to successfully realize 120 m and 20 Mbps data transmission at a depth of 700 m [16]. By using NRZ-OOK modulation, a 2.2 Gbps UOWC link at the underwater transmission distance of 12 m was reported [17]. Data rate of 16 Gbps at 10 m transmission distance was achieved by using four-level pulse amplitude modulation (4-PAM), in which the light injection and optoelectronic feedback techniques were utilized to enlarge the modulation bandwidth [18]. In addition, a UOWC system with the maximum rate of 5.6 Gbps and a distance of 10.2 m was reported [19]. The modulation method used was 16-QAM-OFDM. A high-speed communication system of 5.5 Gbps operated through a water-air channel with 21 m water sub-channel and 5 m air sub-channel was realized [20]. The modulation format of 32-QAM-OFDM was adopted. By using NRZ-OOK modulation, 2.7 Gbps UOWC communication at the underwater transmission distance of 34.5 m was demonstrated [21]. In 2018, 3.32 bits per photon were delivered by an UOWC system using 256-pulse-position modulation (PPM) and different rate Reed-Solomon (RS) and Low-density Parity-check (LDPC) codes [22]. In 2019, a commercial underwater optical communication product called BlueComm-200 was released. The combination of a 450 nm LED and a PMT yielded the maximum reach of 150 m/10 Mbps [23]. Recently, a team also implemented a 60 m UOWC system with the maximum data rate of 2.5 Gbps based on NRZ-OOK and a 450 nm LD [24].
Table 1. Comparisons of UOWC system performances
The significant findings by the active groups have advanced UOWC research and technologies. Except the air-water links considered in [20], underwater links were primarily considered in all other works and the background light interference could be disregarded in most cases. If APD is adopted as a receiver, its noise floor limits the received signal to noise ratio (SNR), or communication distance for a given bit error rate (BER) requirement. Its limited bandwidth also imposes challenges to high data rate communication. In this paper, we experimentally demonstrated a 500 Mbps UOWC link over a 100 m water channel based on light reflections in a 10 m water tank in the laboratory. We adopted a 520 nm LD at a power of 7.25 mW, NRZ-OOK modulation, and APD detection. The attenuation coefficient of the tap water was comparable to pure seawater, thus it might simulate a deep-ocean communication environment from the air quality perspective. The measured BER level of 2.5 × 10−3 was below the FEC limit of 3.8 × 10−3 with 7% overhead.
2. Experimental setup and details
The block diagram of the experimental long-distance UOWC system based on green LD and NRZ-OOK modulation scheme is shown in Fig. 1. At the transmitter, a pseudo-random binary sequence (PRBS) of length 216-1 is subject to a 4-times pulse-shaping filter to reduce the impact of the band-limited system and the roll-off factor is set to 0.5. Then an arbitrary waveform generator (AWG) generates the electrical signals, which are combined with a direct current (DC) via a bias-tee (THORLABS, LDM9LP) and injected into the LD source. In such a way, the electrical signals are converted to optical signals for underwater transmission. The water tank is of dimension 10 m × 0.75 m × 1 m and filled with tap water whose attenuation coefficient is measured to be 0.05162 m−1 (at wavelength 520 nm). In the experiment, the maximum communication distance of 100 m (10 m × 10) is achieved by using 9 pieces of reflective mirrors (THORLABS, BBSQ2-E02) on both sides of the tank.
Fig. 1 Experimental setup of the proposed long-distance LD-based UOWC system. PRBS: pseudo-random binary sequence; AWG: arbitrary waveform generator; DC: direct current; APD: avalanche photodiode.
At the receiver on the same side of the transmitter, a condenser lens focuses light transmitted through the underwater channel. After concentrating, an APD module is used to convert the received optical signals into electrical signals and then sent to the oscilloscope (OSC) for data acquisition and further signal processing. We employ a high-sensitivity receiver in the experiment for a longer distance: APD C12702-11 from Hamamatsu with 100 MHz 3 dB bandwidth. The offline digital signal processing (DSP) operations contain digital low-pass filtering, synchronization, equalization, decision making and BER calculation.
The key components of the experimental setup are shown in Fig. 2. A picture of transmitter and receiver on the same side is provided in Fig. 2(a). The underwater channel used to transmit the light is built in a 10 m water tank in Fig. 2(b). The black material on the surface is used to minimize the influence of ambient light during the experiment and protect the water from dust contamination. In order to reduce the attenuation of light during the reflection process, the mirrors on the opposite side of the transceiver are attached to the inner wall of the tank while others are externally attached. The reflection of each 10 m light path is realized by finely adjusting the angle of the light-beam as shown in Fig. 2(c). A pigtailed single-mode fiber green-light LD (THORLABS, LP520) with emission peak wavelength of 520 nm is driven by a LD controller (THORLABS, LDC205C) and employed for long distance transmission as shown in Fig. 2(d). Figure 2(e) shows the light reflections through water after 9 times respectively, which accumulate the total underwater transmission distance of 100 m (10 m × 10).
Fig. 2 Components of the experimental UOWC system: (a) transmitter and receiver, (b) water tank, (c) mirrors, (d) 520nm pigtailed laser, (e) green light reflection paths.
3. Results and discussions
The performance of an UOWC system depends on the optical source, detector, optical channel and communication techniques to deal with system imperfectness. We will study those factors and communication performance in detail next.
3.1 Operation conditions of the LD
We first characterize the optical devices used in our system and find their best possible operation parameters. For this purpose, the channel is assumed ideal in the absence of water attenuation. The characteristics of the employed 520 nm LD are illustrated in Fig. 3. Figure 3(a) shows the output optical power versus driving current (P-I) curve and the forward voltage versus driving current (V-I) curve. The P-I curve is measured by an optical power meter (THORLABS, PM100D). The functional threshold current is around 26 mA. As the current increases, the P-I curve enters the linear region, but the V-I curve clearly shows nonlinearity. Considering the received signal is weak due to severe channel attenuation after long-distance propagation, an APD module with high sensitivity is selected as the receiver in order to achieve better system performance at low SNR. According to the measurement in the absence of light injection, the APD noise level in terms of voltage peak-to-peak (Vpp) is about 5 mV, which can meet our sensitivity requirement. Figure 3(b) shows the frequency response of the system front-end when the detector is APD module C12702-11. The 3 dB bandwidth of the whole system is about 50 MHz.
Fig. 3 Characteristics of 520 nm LD, (a) P-V-I curve and (b) frequency response of the whole system. The receiver is C12702-11.
We further measure the BER performance of 250 Mbps NRZ-OOK signal as a function of the driving current IDC as shown in Fig. 4. The peak-to-peak voltage signal Vpp is set at 1 V. The black horizontal dashed line is the FEC limit of 3.8 × 10−3. Initially, as the bias current increases from 30 mA, the BER performance improves and its value quickly falls below the FEC limit. However, as the bias current continues to increase to exceed 50 mA, the BER performance degrades significantly. Based on the above results, the bias current of the 520 nm green LD is set at 50 mA to obtain the optimal performance, which corresponds to the output optical power of 8.60 dBm (7.25 mW).
3.2 Water channel quality assessment
Next, we turn our attention to the water channel and estimate its extinction coefficient. When light is incident on the water at power Po, part of the power is absorbed by substances such as water, chlorophyll and humus, and the absorbed power is Pα. Some power is scattered by suspended particles and dissolved salts, and the scattered power is Pb. The signal power that ultimately projects through the water body at wavelength λ is
As a channel medium of the UOWC system, water plays a particularly important role in determining the degree of attenuation. Beer-Lambert law describes the light attenuation effect in an underwater environment as [25,26]
where Po and Pi are the optical powers before and after propagating distance z, and c(λ) stands for the extinction coefficient. We use a green LD at 520 nm as the light source here.In order to avoid large errors in direct measurement, we used the curve-fitting tool in MATLAB to estimate the extinction coefficient. Since the glass of the water tank also partially attenuates the light during reflection, it is represented by a fixed transmittance coefficient measured as the ratio of the optical powers after and before entering the glass. This transmittance coefficient is found to be K = 0.917. We can thus remove its effect for a long-distance underwater channel. Mirror reflection is 99% and its loss can be neglected. Firstly, we measured the optical power from 0 m to 100 m by the step length of 20 m, and obtained a total of 6 distance points. We then used the exponential model (2) to fit the normalized data. The experimental data was linearly fitted after taking the logarithm, as shown in Fig. 5. The estimated c(λ) at a wavelength of 520 nm is 0.05162 m−1, which is close to the extinction coefficient of pure seawater [27]. The subsequent experiments are based on this water quality.
3.3 Measured communication performance of a long-distance UOWC channel
To investigate the performance of the long-distance UOWC system, we measured the BER performances of 250 Mbps, 400 Mbps and 500 Mbps data rates over 80 m and 100 m. The specific experimental results with hard-decision (HD) detection and nonlinear equalization (NE) detection are shown in Table 2. The spot optical powers after focusing at the receiving end are −12.4 dBm (57 μW) and −20 dBm (10.5 μW) after 80 m and 100 m underwater transmission, respectively. Since the signal is greatly attenuated through transmission, a highly sensitive APD C12702-11 is used. Time domain equalization has been applied in UOWC systems to improve the BER performance when inter-symbol interference (ISI) occurs [28]. In order to eliminate the severe ISI that occurs at higher data rates, the Volterra-based nonlinear equalizers [29] with different orders and memory lengths are applied. Both direct hard-decision and nonlinear equalization are employed and compared in the receiving offline DSP.
Table 2. Measured BER values of different data rates at distances of 80m and 100m
When the data rate is less than 250 Mbps, no severe ISI is observed and hard decision can provide correct demodulation. After the data rate reaches 400 Mbps, severe ISI is introduced due to the limitation of the front-end bandwidth. The BER recovered by the hard-decision is already higher than the FEC limit. In contrast, the nonlinear equalizer improves the BER performance significantly since the ISI is reduced. The maximum transmission distance was extended to 100 m, maintaining data rate of 500 Mbps at BER of 2.5 × 10−3 after the equalization. The performance of the proposed nonlinear equalization can also be verified by the amplitudes of the recovered signal. The signal amplitude distributions with hard-decision and nonlinear equalization for 250 Mbps, 400 Mbps and 500 Mbps are displayed in Fig. 6. At 250 Mbps, since the ISI between symbols is not obvious, two clear distribution lines can be seen without equalization. As the data rate increases, the increased ISI causes the waveforms between “0” and “1” to overlap, creating multiple levels of waveforms and difficulty in demodulation. By introducing the nonlinear equalization, the amplitude distributions are much clear and more compact, which contributes to reduced decision errors. In the experiment, the parameters of the employed nonlinear equalizer for 250 Mbps, 400 Mbps and 500 Mbps data rates are of order 2 and of memory length 8. The signal quality with a nonlinear equalizer can also be reflected by the eye diagrams in Fig. 7. The eye diagram is clear at 250 Mbps, begins to close at 400 Mbps with the increasing ISI, and finally becomes blurred at 500 Mbps.
Fig. 6 Signal amplitude distributions with HD and NE for different data rates, (a) after 80m, (b) after 100m.
3.4 Predicted longest distances at different data rates
The longest communication distance of our system is limited by the current experimental setting such as the length of the tank. It is interesting to further experimentally investigate the minimum required received optical power in order to meet the FEC limit of the BER performance. From that minimum power, the longest communication distance of our system can be predicted according to the exponential power decay model (2). Figure 8 is a block diagram of the system used for the measurement, where the transmitter and receiver are identical to those in Fig. 1. The received optical power is attenuated using a neutral density filter to simulate the channel attenuation process. At the same time, in order to obtain real-time accurate optical power, the beam is evenly divided into two by using a beam splitter before the receiving lens, one of which enters the APD for data processing, and the other beam enters the optical power meter for real-time monitoring.
Figure 9 shows the measured BER vs. received optical power for 4 different data rates to evaluate the minimum required optical power for the UOWC system we proposed. The FEC limit of 3.8 × 10−3 by dashed line is also presented. It is observed that when the data rate is 100 Mbps/250 Mbps, the minimum optical powers of −30.4 dBm/-29.0 dBm is required respectively to achieve the FEC limit by using hard-decision. However, when the data rate reaches 400 Mbps and 500 Mbps, severe ISI makes the system impossible to reduce the BER to the limit by increasing the optical power under only hard decisions. In this case, a nonlinear equalizer is reintroduced to reduce ISI, and effectively lower the BER level near the FEC limit. The experimental results show that the equalizer needs to set memory depth at least 10 at 400 Mbps/500 Mbps, and the second-order nonlinear equalization can bring the BER below the FEC limit. After equalization, the minimum optical powers required for 400 Mbps and 500 Mbps are −26.4 dBm and −24.0 dBm respectively.
Fig. 9 BER vs. received optical power by detector for different data rates at a BER level of 3.8 × 10−3.
With the above optical power results at the FEC limit, it is feasible to reveal the maximum achievable transmission distance at certain data rate. According to Fig. 9, the minimum required received optical power and data rate pairs are found to be (−30.4 dBm, 100 Mbps), (−29.0 dBm, 250 Mbps), (−26.40 dBm, 400 Mbps), (−24 dBm, 500 Mbps). Using the estimated extinction coefficient, Fig. 10 plots the received optical power versus distance according to Eq. (2) and draws horizontal lines of those required power levels. The crossing distances are 174 m, 168 m, 156 m and 146 m. They constitute the predicted maximum communication distances corresponding to four data rates 100 Mbps, 250 Mbps, 400 Mbps, and 500 Mbps respectively.
Fig. 10 Received optical power as a function of transmission distance and the maximum achievable distance for different data rates.
Consider that the laser beam typically has a slight divergence angle which results in a certain size of light spot at a distance. It is worth to check where the receiver can collect the light in our experimental system at the maximum predicted distance of 174 m above. We estimated the beam divergence angle based on measured beam spot sizes at different distances and then estimated the spot size at the maximum predicted distance. The specific measurement diagram is in Fig. 11 and the formula for calculation is as follows
In the actual measurement, two distances are counted from a reference point close to the laser transmitter, d1 = 17.6 m, d2 = 44 m, x1 = 14 mm, x2 = 28 mm. We find the divergence angle θ = 0.5303 mrad (0.0304°) after calculation. According to this angle, the spot diameter at the maximum predicted distance of 174 m can be calculated to be 92 mm, which is typically larger than the APD sensing window. In such a case, a condenser lens of size larger than the beam spot size can be placed in front of the APD to ensure that the total optical power can be collected by the optical receiver.
4. Conclusions
Assisted by a pigtailed single-mode fiber 520 nm LD with small divergence angle, we have successfully established a long-distance high-speed experimental laser communication system in a laboratory environment based on NRZ-OOK modulation for underwater applications. The water channel has an extinction coefficient of 0.05162 m−1 at wavelength 520 nm. The record transmission distance of 100 m at the data rate of 500 Mbps was achieved with BER level of 2.5 × 10−3. According to the measured minimum required optical power and power decay model, the longest transmission distance was predicted to be 146 m for 500 Mbps and 174 m for 100 Mbps by using our proposed UOWC system. These research results are applicable for future long-distance high-speed UOWC systems.
Funding
National Key Basic Research Program of China (2013CB329201); Key Program of National Natural Science Foundation of China (61631018); Key Research Program of Frontier Sciences of CAS (QYZDY-SSW-JSC003); and Strategic Priority Research Program of CAS (XDA22000000).
Acknowledgment
The authors would like to thank Information Science Laboratory Center of USTC for the hardware & software services.
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