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Abstract
In this study, a quasi-omnidirectional underwater wireless optical communication (UWOC) system is implemented with a prismatic array consisting of three uniformly distributed high-power LED modules as the transmitter. Over a 10-m underwater channel in a 50-m standard swimming pool, a data rate of 22 Mbps is achieved without adopting any digital signal processing algorithm. With zero forcing (ZF) based frequency domain equalization (FDE) and a maximum ratio combining (MRC) algorithm, the maximum net data rates achieved are 69.65 Mbps, 39.8 Mbps and 29.85 Mbps over 10-m, 30-m, and 40-m underwater channels, respectively. In the proposed UWOC system, the receiver could successfully capture optical signals at different directions from the transmitter and the bit error rates (BERs) measured in different directions show small fluctuations. The proposed system could meet the demands of high-speed data transmission among units in a swarm-robot system and last meter user access in an underwater optical cellular network system.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
On account of the shortage of terrestrial resource, the exploration and exploitation of the ocean are increasingly intensified. A swarm-robot system can effectively execute more diversified tasks at a lower cost than a single complex robot. An efficient underwater communication method is of vital importance when a swarm robotics system is on a mission. Acoustic waves are the only known wave that can transmit over a long distance in water, and are the only wave that can be employed to establish long-range underwater communication links. However, underwater acoustic communication (UAC) cannot satisfy the demands of high-speed and real-time underwater wireless communication due to its limited bandwidth and high latency [1,2]. Underwater wireless optical communication (UWOC) is an ideal alternative owing to its high speed, low latency, power efficiency and high security [3].
Since S.Q Duntley discovered the blue-green window, which shows the possibility of underwater wireless communications with visible light [4], significant progress has been made. Till now, researchers have mainly focused on achieving higher data rates or enhancing transmission distances [5–9]. In order to achieve such goals, a laser diode (LD)-based system is an ideal choice thanks to its high bandwidth and small divergence angle. In 2019, a 500-Mbps UWOC link through a 100-m tap-water channel with nonlinear equalization was established using a green LD [7]. In the same year, a 60-m UWOC system with a maximum data rate of 2.5 Gbps with a non-return-to-zero (NRZ) on-off keying (OOK) modulated blue-light LD was demonstrated [8]. Moreover, with a frequency-domain equalizer and a time-domain feedback noise predictor, a 32-quadrature amplitude modulation (QAM) single-carrier UWOC system with a data rate of 3.31 Gbps over a 56-m tap-water channel was achieved [9]. This significant progress shows the great potential of the UWOC technology. Generally, the communication distances and data rates of UWOC systems with light emitting diodes (LEDs) are markedly smaller than those of systems with LDs. However, some studies have shown that UWOC systems with LEDs can achieve high-speed and long-distance links as well [10,11]. With a 600 μm × 600 μm green-emitting silicon substrate LED, a 2.175-Gbps underwater link was achieved by F. Wang in 2018 using a pre-equalizer and maximum ratio combining (MRC) algorithm [10]. It is an effective way to extend the communication distance of UWOC systems by using ultra-sensitive detectors like multi-pixel photon counter (MPPC) and energy-efficient modulation formats like pulse position modulation (PPM). With an MPPC and a PPM-modulated LED, a 46-m underwater link was achieved by J. Shen in 2019 [11], with less than 100 incident photons in every pulse slot.
Nevertheless, in most reported UWOC systems, link alignment between the transmitter and the receiver is indispensable. What is worse, some high-speed systems require high-precision alignment, which is almost impossible in practical underwater dynamic environments especially when the size, weight and power consumption of the system are restricted. Recently, some studies on alignment tolerance in UWOC were conducted. The alignment requirement can be relaxed by using high-sensitivity arrayed receivers [12]. With such receivers, the UWOC link can be maintained if transmitters and receivers are shaking because of due to the water flow. Moreover, by employing precoding schemes, the packet loss rate caused by the relative movement of the transmitter and the receiver can be reduced, and the robustness of the UWOC system can be enhanced [13].
In addition, the alignment requirements can be relaxed by using LEDs as the light source. In 2011, considering its application in underwater mobile carriers, J. Rao used an LED array with a convergent optical system as the transmitter to relax the alignment requirement. They finally achieved 4 Mbps and 1 Mbps over 8.4-m and 22-m underwater channels, respectively. Voice and Morse code communications were achieved within a TX/RX angle of 30° [14]. In 2013, a 38-Kbps UWOC system with a large communication angle was demonstrated in a swimming pool at a distance of 7 m using a single blue LED and a photodiode [15]. In 2018, B. Han designed an optical transmitter with a 150° divergence angle and more than 90% uniformity of radiation intensity by adopting freeform lenses in an LED array, and achieved 19 Mbps over an 8-m underwater channel with an attenuation coefficient of 0.4 m-1. The BER of the proposed system is lower than 10−4 when the direction deviation angle is between −60° and 60° [16]. This design makes it much easier to establish a UWOC link in practical underwater environment especially when the transmitter and the receiver are moving relatively to one another. However, the systems mentioned above still need acquisition and tracking systems to establish and maintain the link. Such systems usually contain moving parts, which makes it hard to miniaturize the transceiver. What’s worse, the moving components may be easily damaged in long-term use due to abrasion.
With an increase in activities under the sea, a variety of devices have been installed, implying that a broadband UWOC network is of vital importance. A hybrid LD- and LED-based UWOC system was proposed in [17,18], in which LDs with high bandwidth were used for high-speed links, and LEDs with large divergence angles were used when the link alignment was difficult to maintain. However, when two or more robots or divers want to connect to a cellular UWOC network via a single node, the positions of the terminals are restricted if using the common design of the LED based UWOC system because the optical signal is not transmitted in different directions. In this case, omnidirectional transmitters can provide efficient accesses for robots or divers to connect to the network [19].
In this study, a prismatic array with three high-power blue LED modules is used to emit optical signals in different directions, realizing a quasi-omnidirectional UWOC system. The half-power emission angle of the single high-power LED module is approximately ±65°. The proposed system enables high-speed and low-latency data transmission among units in a swarm. Moreover, the system can also satisfy the demands of last meter user access in underwater optical cellular network systems. Over a 10-m underwater channel in a 50-m standard swimming pool, a data rate of 22 Mbps is achieved without adopting any digital signal processing algorithm. Over 30-m and 40-m underwater channels, links at data rates of 20 Mbps and 15 Mbps are achieved using two closely installed photomultiplier tubes (PMTs) with an MRC algorithm. Moreover, with MRC and frequency domain equalization (FDE) algorithms, the maximum net data rates achieved are 69.65 Mbps, 39.8 Mbps, and 29.85Mbps over 10-m, 30-m, and 40-m underwater channels, respectively.
The rest of this paper is organized as follows: Section 2 introduces the design principles of the proposed system, including the characteristics of the LED module and the structure of the proposed prismatic array. The experimental setup of the proposed UWOC system is described in Section 3. Section 4 presents the experimental results and their analyses. Finally, we make the conclusion of the paper in Section 5.
2. Principles of the system
2.1 Design principles of the transmitter
A prismatic array consisting of three high-power LED modules is proposed in this study as a quasi-omnidirectional transmitter. Each high-power LED module consists of nine LED chips, which are further divided into three sets. The LED chips in the same set are connected in series and the three sets are connected in parallel to the pins of the package, as shown in Fig. 1.
The light radiated from an LED is incoherent so that the illuminous intensity distribution of the LED module can be treated as the intensity superposition of nine LED chips. A single LED chip could be treated as a Lambertian source and the illuminous intensity distribution of the generalized Lambertian LED is given by Eq. (1) [20,21]:
where θ is the divergence angle, and I0 is the illuminous intensity at the zero angle. The constant value m is determined by the divergence angle of the LED chip using Eq. (2) [21,22]: where γ is the full width at half maximum (FWHM) divergence angle of the LED chip. For a common LED chip, assuming that γ=120° [16], Eq. (1) can be simplified to:As shown in Fig. 2(a), the illuminance at a point $({{r_0},\theta ,\varphi } )$ in front of the LED module can be expressed as follows:
Fig. 2. (a) Parameters to calculate the illuminance distribution. (b) Calculated illuminance distribution at different pitch and yaw angles generated by the LED modules.
According to the geometry, Eq. (4) can finally be expressed by Eq. (5):
Figure 2(b) shows the simulation result of the normalized illuminance distribution. Because of the negligible distances between the LED chips in the same module, the LED module can still be treated as a Lambertian source approximately when dealing with the high-power part of the light spot.
To design an omnidirectional transmitter that radiates optical signals in all directions, we first measured the radiation intensity of a single LED module at different angles at a distance of 1 m. The normalized illuminance distribution in $zoy$, the section shown in Fig. 2(a), is shown in Fig. 3. The measured FWHM divergence angle of the LED module is approximately 130°.
Based on the measured illuminance distribution of the LED module, we designed an array in the shape of a triangular prism with three such modules, as shown in Fig. 4(a). The calculated normalized illuminance distribution of such a prismatic array in a horizontal section is shown in Fig. 4(b), which reveals that the signal coverage is similar to that of a base station in mobile communications. The illuminance in the superimposed area is slightly higher than the average value because the FWHM divergence angle is larger than 120°. The calculated results show that the peak value of the illuminance is approximately 1-dB higher than the trough value.
Fig. 4. (a) Structure of the array in 3D-view. (b) Simulated result of the normalized illuminance distribution in the horizontal section.
2.2 Principles of MRC and ZF based FDE
The large divergence angle of the LEDs also leads to a sharply decrease in received optical power as the transmission distance increases. Highly sensitive receivers are used in the proposed system and MRC algorithm is adopted to combine the signals from two PMT detectors linearly and improve the SNR of the signal [23]. The received signal of the $i - th$ detector is:
where ${h_i}$ and ${n_i}$ is the channel and the noise of the $i - th$ detector. According to MRC algorithm, the combined signal is: where N is the number of the detectors, and ${w_i}$ is the weight of the $i - th$ signal determined by its SNR.Besides, in accordance with the common design of the LED based UWOC system, the performance of the proposed UWOC system is restricted by the limited modulation bandwidth of the LED as well. A zero forcing (ZF) based FDE algorithm is used to eliminate the intersymbol interference (ISI) and make full use of the bandwidth and achieve higher data rates. For a UWOC system, the system model in frequency domain can be expressed as [24]:
where ${\mathbf F}$ is the discrete Fourier transform (DFT) matrix and ${\mathbf x} = {[{{x_0},{x_1},{x_2},\ldots ,{x_{N - 1}}} ]^T}$. ${\mathbf Y}$ is the received signal in frequency domain. Matrix ${\mathbf H}$ is a diagonal matrix with its $k - th$ diagonal element ${H_k}$ as the $k - th$ coefficient of channel frequency response.Assuming that the additional noise can be ignored, the estimated channel response $\widehat {\mathbf H}$ can be expressed as:
where ${\widehat H_k}$ is the $k - th$ diagonal element of $\widehat {\mathbf H}$, ${Y_k}$ and ${X_k}$ are the $k - th$ element of frequency-domain signals ${\mathbf Y}$ and ${\mathbf{Fx}}$.The estimated channel frequency response is critical when recovery the signals. In order to minimize the influence caused by the noise [25], a Gaussian filter is used to smooth the channel response curve. The smoothed channel response $\widehat {{\mathbf H}^{\prime}}$ can be expressed as:
where ${\widehat {H^{\prime}}_k}$ is the $k - th$ diagonal element in the matrix $\widehat {{\mathbf H}^{\prime}}$ and $f(u)$ is the normalized Gaussian distribution in 1-D. The recovered received signal $\widehat {\mathbf x}$ is given by:3. Experimental setup
Figure 5 depicts the experimental setup of the proposed quasi-omnidirectional UWOC system. On the transmitter side, an OOK signal was generated offline and loaded into an arbitrary waveform generator (AWG, SDG6012X-E). The signal output from the AWG was amplified by three power amplifiers (AMPs) connected in parallel to the same channel of the AWG. After adjusting the signal amplitudes by three key-press variable electrical attenuators (VEAs), the amplified bipolar signals were biased to 10.6 V by three bias-tees, and the total current of the bias-tees was 1.55 A. Superimposed optical signals were generated by the designed prismatic LED array fixed in a cylindrical watertight cabin made of acrylic tube. After transmitting through the underwater channel, the optical signals were detected by an array of two PMTs that are closely installed in another transparent watertight cabin on the reception side. Then the captured signals were recorded using an oscilloscope (OSC, Rohde & Schwarz RTO2024) and finally processed offline on a computer.
During the experiment, the cabins were floating uprightly in water by adjusting the position and the amount of additional weight they contained. As shown in Fig. 6, the LED array and the PMT array in the cabins were located 30 cm below the water surface. The positions of the cabins were restricted by some ropes connected to poles at the edge of the swimming pool. Considering the reflection of the pool wall, the cabins were kept away from the wall appropriately, and the system was tested at distances of 10 m, 30 m, and 40 m.
4. Results and discussion
Firstly, we measured the relative radiation intensity distribution of the designed LED array. At a bias voltage of 10.6 V, the optical power of a single LED module was measured to be 1 W, and the total optical power of the array was calculated to be 3 W. We used an optical power meter (THORLABS, PM200) to measure the optical power at different yaw angles at a distance of 1 m. The relative illuminance distribution is shown in Fig. 7, which matches well with the calculated distribution in Fig. 4(b).
Then the frequency responses of the system using either a PIN (Thorlabs PDA10A2) or a PMT as the detector were measured using a network analyzer (Hewlett Packard 8753D). The measured back-to-back results are shown in Fig. 8. The measured frequency response of the LED-PIN system indicates that the −3 dB bandwidth is approximately 7 MHz and the −20 dB bandwidth is approximately 80 MHz. Because the −3 dB bandwidth of the PIN detector is 150 MHz, which is much higher than that of the system, the frequency response of the LED can be regarded as that of the system approximately. In the LED-PMT system, the −20 dB bandwidth is smaller than that of the LED-PIN system because of the smaller bandwidth of the PMT than that of the PIN. An FDE algorithm was adopted to achieve higher data rates.
We measured the spectrum of the LED using a spectrometer (Ocean Optics hr4000CG-UV-NIR) and the result is shown in Fig. 9, which shows that the center wavelength of the LED is approximately 452 nm. Then we conduct the experiment in a standard swimming pool. We use a blue LD, with a wavelength of 450 nm, to measure the attenuation coefficient. We measured received optical power at different distances in air and underwater, the results are shown in Fig. 10. After subtracting the attenuation caused by the expansion of the light spots, the attenuation coefficient was estimated to be about 0.23 dB/m.
We define that the direction perpendicular to one of the LED planes in the array is zero degree and the receiver cabin was well aligned with the transmitter (yaw angle: 0°) at the beginning of the experiment. Then, we rotated the transmitter cabin counter-clockwise around the vertical axis and measured the BER at different yaw angles. The experimental results over a 10-m underwater channel are shown in Fig. 11. Using a single PMT as the receiver, the achieved data rate of the proposed communication system was 22 Mbps with an average BER of 7.59×10−4, which is restricted by the limited bandwidth of the LED. As shown in Fig. 11(a), the system performance can be improved significantly by employing a frequency-domain equalizer. With ZF based FDE, the maximum net data rate was improved to 69.65 Mbps.
Fig. 11. Experiment results over a 10-m underwater channel: (a) average BER versus data rate curve, (b) BER versus yaw angle curve.
Over the 30-m and 40-m underwater channels, the BERs were slightly higher than the forward error correction (FEC) threshold of 3×10−3 at data rates of 20 Mbps and 15 Mbps without equalization. With ZF-based FDE, the achieved net data rate was 29.85 Mbps over a 30-m underwater channel using a single PMT as the receiver. Moreover, by employing diversity reception, the system performance was improved significantly. With the MRC algorithm, the BERs at 20 Mbps and 15 Mbps were lower than the FEC threshold over the 30-m and 40-m underwater channels. The net data rate reached 39.8 Mbps and 29.85 Mbps over 30-m and 40-m underwater channels, respectively, with the ZF-based FDE. As shown in Fig. 11(b) and Fig. 12, the BERs measured at different angles show small fluctuations, which is attributed to the dynamic underwater channel induced by water circulating in the swimming pool. The experimental results show that the performances of the links built in different directions in the horizontal plane are almost identical, and the nonuniformity of the illuminance has almost no effect.
Fig. 12. Experimental results over 30-m and 40-m underwater channels: (a) BER versus yaw angle curve with a single PMT, (b) BER versus yaw angle curve with the MRC algorithm.
In this experiment, the reflections from the surface of water and the floor of the pool enhanced the signal power at the receiver. Over a 40-m underwater channel, the received optical power per unit area was measured to be about −45 dBm/cm2. According to our calculation, if there is no reflection from the surface, the received optical power per unit area would be about −52 dBm/cm2 and −57 dBm/cm2 at distances of 30 m and 40 m, respectively. We measured the BERs at different data rates when the received optical power per unit area was −60 dBm/cm2 in a dark room in the laboratory. The experimental results in Fig. 13 show that it is still possible to establish a UWOC link with a data rate higher than 10 Mbps with such a low optical power.
Because of the limited depth of the swimming pool, which is the experimental environment in this study, the links in the vertical directions cannot be tested. The transmitter designed for the current experiment cannot cover the areas right above and under itself because we want to minimize the influence of the reflections from the water surface and the pool floor. However, an array in the shape of a regular tetrahedron with four LEDs would be able to transmit optical signals in all directions, and the 3D-view of the array is shown in Fig. 14. With this array as the transmitter, the intersection angle between the two LEDs is about 70°, which is slightly larger than that of the array used in the experiment. For this reason, the peak value of the normalized illuminance distribution is about 1.7 dB higher than the valley value, which is higher than that generated by the prismatic array. This can be explained by the larger superimposed area of the light beams from two adjacent LEDs in an array with a regular tetrahedral shape. However, by installing attachments such as freeform lenses or mirrors, the optical power distribution could be made more uniform.
5. Conclusion
In this paper, to establish high-speed and real-time wireless communication within an underwater swarm robotics system, we proposed a quasi-omnidirectional UWOC system. There is no lens in the designed system so that the transceiver could be much more compact than the common designs, which makes it more reliable and more suitable for underwater applications. Over a 10-m underwater channel, the highest data rate of 69.65 Mbps is achieved with ZF-based FDE, and the measured BER is lower than the FEC threshold at all directions in the horizontal plane. When the distance between the transmitter and the receiver reaches 30 m and 40 m, the achieved data rates are 39.8 Mbps and 29.85 Mbps, respectively. Compared with some popular UWOC studies aiming at achieving higher data rates and longer distances, this study significantly relaxes the alignment requirement and makes it much easier to build a communicate-on-the-move UWOC system. In the future work, we will further optimize the transmitter and design an omnidirectional receiver.
Funding
National Natural Science Foundation of China (61971378); National Key Research and Development Program of China (2016YFC1401202, 2017YFC0306601, 2017YFC0306100); Strategic Priority Research Program of the Chinese Academy of Sciences (XDA22030208); Zhoushan-Zhejiang University Joint Research Project (2019C81081).
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.
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