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Chinese Optics Letters, 2019, 17 (10): 100012, Published Online: Oct. 15, 2019
Copy Citation Text  本文已被引 2 次  Follow This Article


HIGH-SPEED UNDERWATER WIRELESS OPTICAL COMMUNICATIONS: FROM A PERSPECTIVE OF
ADVANCED MODULATION FORMATS [INVITED] DOWNLOAD： 1029次

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Chao Fei 1,2Xiaojian Hong 1,2Ji Du 1,2Guowu Zhang 1,2Yuan Wang 1,2Xiaoman
Shen 1,2Yuefeng Lu 1,2Yang Guo 1,2Sailing He 1,2,*
Author Affiliations
1 Centre for Optical and Electromagnetic Research, National Engineering Research
Center for Optical Instruments, Zhejiang University, Hangzhou 310058, China
2 Ningbo Research Institute, Zhejiang University, Ningbo 315100, China
060.2605 Free-space optical communication 060.4080 Modulation 140.7300 Visible
lasers 010.4450 Oceanic optics 
Abstract
In this paper, recent advances in underwater wireless optical communication
(UWOC) are reviewed for both LED- and LD-based systems, mainly from a
perspective of advanced modulation formats. Volterra series-based nonlinear
equalizers, which can effectively counteract the nonlinear impairments induced
by the UWOC system components, are discussed and experimentally demonstrated.
Both the effectiveness and robustness of the proposed Volterra nonlinear
equalizer in UWOC systems under different water turbidities are validated. To
further approach the Shannon capacity limit of the UWOC system, the
probabilistic constellation shaping technique is introduced, which can overcome
the inherent gap between a conventional regular quadrature amplitude modulation
(QAM) format and the Shannon capacity of the channel. The experimental results
have shown a significant system capacity improvement compared to the cases using
a regular QAM.

As increase in the ongoing expansion of human activities in ocean environments,
such as oceanographic research, offshore oil exploration, tactical surveillance,
pollution monitoring, and underwater salvage, high-bandwidth data transfer, like
the exchange of large volumes of data files between fixed sensor nodes and
autonomous underwater vehicles (AUVs) or remotely operated vehicles (ROVs),
shuttling real-time video from untethered vehicles for inspection and
identification, is indispensable. The mature technology of acoustic
communication could support transmission distances of tens of kilometers[1], but
its drawbacks include low bandwidth, low data rate (∼kbps), severe multipath
latency, high energy consumption, and lack of stealthiness[2]. Radio frequency
(RF) communication, while effective in terrestrial applications, is severely
restricted in the ocean due to the exponential decay at radio frequencies in
seawater, e.g., the attenuation in the ocean is about 169 dB/m for the 2.4 GHz
band[3]. Thus, RF communication can only work at ultralow frequencies,
supporting near-field communication, and consequently paradoxically limits its
bandwidth. Moreover, RF-based communication requires huge antennas and high
power, which poses more challenges to cost and energy efficient oceanic
applications. Tethered fiber-optic cables can offer robust and high-bandwidth
communication, but they face sophisticated engineering and maintenance issues
and are not appropriate for underwater moving platforms[4]. Moreover, the deep
ocean environment is challenging, which makes optical connectors very expensive
and short-lived. Underwater wireless optical communication (UWOC) turns out to
be an appropriate solution for communication over short and medium ranges
(within hundreds of meters) due to its high bandwidth, low latency,
cost-effectiveness, and low energy consumption.

In recent years, UWOC has attracted considerable interest from academic,
industrial, and military circles, and is deemed as a revolutionary and
competitive technology to its acoustic and RF counterparts, particularly when
applied in broadband communications between diverse underwater vehicles,
underwater sensor nodes, and underwater base stations[5]. Blue-green wavelengths
of the visible light spectrum are found to be in the low absorption window of
seawater[6,7]. Light-emitting diodes (LEDs) or laser diodes (LDs) are widely
employed as transmitters of UWOC systems. An LED features low-cost and
high-energy efficiency, and the broad-angle beam profile of an LED can relax the
alignment requirement of the transmitter and receiver. However, the broad-angle
beam profile makes an LED more susceptible to scattering in natural waters. The
emitted optical power from an LED can only support a relatively short
transmission distance. The product BlueComm 200 from Sonardyne[8] can sustain a
maximum distance of 150 m using a highly sensitive receiver photomultiplier tube
(PMT). Furthermore, the bandwidths of LEDs are limited to the order of MHz
(e.g., the 3 dB bandwidth of LED was ∼20 MHz in Ref. [9], 160 MHz in Ref. [10],
and 752 MHz in Ref. [11]). On the contrary, the bandwidths of LDs are much
larger (e.g., the 3 dB bandwidth of an LD was ∼1.4 GHz in Ref. [12], 2.6 GHz in
Ref. [13], and 5.3 GHz in Ref. [14]). Meanwhile, LDs can be collimated into very
narrow beam profiles that will significantly increase the optical power
delivered to a remote undersea terminal. Meanwhile, an extremely narrow laser
beam can reduce the temporal spread of the signal and enable enhanced spatial
and spectral filtering options to reduce the background light noise[15]. The
significant hurdle for a reliable LD-based UWOC is the link alignment, which may
require the pointing, acquisition, and tracking (PAT) system.

With the increased attention in UWOC research, several surveys have been
published to review this emerging subject. In Ref. [16], Arnon evaluated the
link performance of a typical UWOC system and analyzed several UWOC link models,
including the line-of-sight (LOS) link, the modulating retroreflector link, and
the reflective link. Hemani et al. discussed various propagation phenomena that
impacted the performance of a UWOC system and reviewed recent research progress
in UWOC, including channel characterization, modulation schemes, coding
techniques, and various sources of noise that are specific to UWOC[17]. In
Ref. [18], Zeng et al. provided an exhaustive survey of the state-of-the-art
UWOC research in three aspects: channel characterization, modulation, and coding
techniques, together with the practical implementations of UWOC. Nasir et al.
presented a comprehensive survey on the challenges, advances, and prospects of
underwater optical wireless networks from a layer by layer perspective, and
pointed out the prospective directions for UWOC, networking, and localization
studies[19]. In Ref. [20], the authors provided a detailed comparison of
applying three advanced modulation formats, including carrierless amplitude and
phase (CAP) modulation, orthogonal frequency division multiplexing (OFDM), and
discrete Fourier transform spread orthogonal frequency division multiplexing
(DFT-S OFDM) in LED-based UWOC systems. In Ref. [21], the authors gave a review
on the recent advances and highlights in LD-based UWOC systems from a device
perspective, mainly focused on LDs. In the present paper, we review recent
research progress in the UWOC subject, mainly from a perspective of advanced
modulation formats, focusing on both LED- and LD-based UWOC systems. In
addition, some studies of channel modeling used for simulation and other
characteristics (such as the turbulence) that may remarkably affect the UWOC
performance are mentioned briefly.

It is important to understand aquatic optical channel characterization before
conducting a UWOC experiment. Monte Carlo simulation is a popular numerical
method achieved by sending and tracking the trajectories of a large number of
emitted photons[22,23]. In Ref. [24], the authors designed a Monte Carlo
simulator and the simulation results showed that the UWOC distance could be
extended to 500 m in pure seawater when a single photon avalanche diode (SPAD)
was employed as the receiver. Gabriel et al. used the two-term Henyey–Greenstein
(HG) channel model to quantify the time dispersion of the UWOC channel for
different water types, link distances, and transmitter/receiver characteristics.
The performance of a typical UWOC system in terms of bit error rate (BER) using
on–off keying (OOK) modulation was studied[25]. By taking into account three
types of propagating photons including non-scattering, single, and multiple
scattering, the authors presented a general stochastic channel model that fits
well with Monte Carlo simulations in coastal and turbid water channels[26]. In
Ref. [27], Tang et al. proposed a closed-form double-gamma function to model the
channel impulse response of a UWOC system. This impulse response is able to
evaluate the BER and channel bandwidth for various link ranges of a UWOC system.
Zhang et al. introduced a weighted gamma function polynomial (WGFP) to model the
impulse response of general M×N multiple-input multiple-output (MIMO) UWOC
links. Numerical results of Monte Carlo simulations validated the proposed WGFP
model in a turbid water environment[28].

For a UWOC lab experiment, one principal target is to improve the capacity and
extend the transmission distance of a UWOC system. Since the first experimental
demonstration of high speed UWOC on the order of Gb/s[29], a lot of works have
explored higher data rate and longer transmission distance in LED- or LD-based
UWOC systems. Xu et al. have demonstrated a blue LED-based UWOC system using
16-QAM-OFDM, achieving a net bit rate of 161.36 Mb/s at a BER of 2.5×10−3[30].
Tian et al. have reported a data rate of 800 Mb/s over a 0.6 m UWOC system using
a micro-LED[10]. Kong et al. have proposed a UWOC system using an arrayed
transmitter/receiver and optical superimposition-based pulse amplitude
modulation with 4 levels (PAM-4), which enhanced tolerance to the modulation
nonlinearities of LEDs[31]. Wang et al. have realized an OFDM-based UWOC system
using a multipixel photon counter (MPPC) as the receiver, which has a high
sensitivity and supports a relatively long underwater transmission distance[32].
In Ref. [33], the authors have proposed a UWOC system utilizing QAM-OFDM and
multi-PIN reception for maximum ratio combination (MRC) receiving, acquiring a
data rate of 2.175 Gb/s. Chi et al. have demonstrated a novel Gaussian
kernel-aided deep neural network (GK-DNN) equalizer that can effectively
mitigate the high nonlinear distortion of PAM8 LED-based UWOC systems[34]. Oubei
et al. have conducted a lab experiment obtaining 4.8 Gb/s over 5.4 m underwater
laser transmission using 16-QAM-OFDM[35]. Chen et al. have demonstrated 26 m /
5.5 Gb/s air-water channel transmission employing 32-QAM-OFDM[36]. Injection
locking has been utilized to enhance the frequency response of LDs, and the
authors have experimentally demonstrated a 16 Gb/s over 10 m[37] and a 25 Gb/s
over 5 m[14] UWOC systems. Huang et al. have reported 14.8 Gb/s over 1.7 m
seawater transmission using 16-QAM-OFDM with a modulation bandwidth of
3.7 GHz[38]. Our previous works have probed post nonlinear equalization and
adaptive bit-power loading discrete multitone (DMT)-based UWOC systems under
different water turbidities[39], and demonstrated 16.6 Gb/s over 5 m and
6.6 Gb/s over 55 m transmission[40]. We have also introduced multiband DFT-S to
reduce the peak-to-average-power ratio (PAPR) of a DMT-based UWOC transmitter
and improved the BER performance of a long reach UWOC system[41]. We have probed
probabilistic constellation shaping (PCS) in a DMT-based UWOC system to approach
the underwater channel capacity limit, and the net data rate has been pushed to
18.09 Gb/s over a 5 m water channel with a single LD[42]. Hu et al. have
demonstrated a 120 m distance in Jerlov II water for Reed–Solomon (RS) (255,127)
code with 256 pulse-position modulation (PPM) at a bandwidth of 13.7 MHz[43].
100 m/500 Mb/s LD-based UWOC has been experimentally demonstrated using OOK
modulation and nonlinear equalization[44]. Al-Halafi et al. have probed
real-time video streaming underwater optical wireless transmission using
software defined platforms[45,46].

In terms of practical implementations of UWOC, the Woods Hole Oceanographic
Institution (WHOI) designed an optical modem system based on omnidirectional LED
arrays and hemispherical PMT in 2006, and validated 5 Mb/s over 91 m underwater
transmission[47]. In 2008, they conducted a sea trial and successfully realized
5 Mb/s over 200 m UWOC in 2000 m deep ocean[48]. In 2014, the WHOI completed
bi-directional UWOC between a seafloor test node and the AUV sentry within
20–150 m range at data rates of 5–10 Mb/s[49]. Researchers from Massachusetts
Institute of Technology (MIT) built a UWOC prototype called AquaOptical[50], and
achieved a data rate of 1.2 Mb/s at distances up to 30 m in a pool test.
Afterward, they updated it to AquaOptical II[51] and reported reliable
communication of 2 Mb/s over 50 m in pool water. The Japan Agency for
Marine-Earth Science and Technology (JAMSTEC) has developed a UWOC system of 20
Mb/s over a 120 m communication distance in 700 m deep ocean water[52]. Cossu et
al. have designed a full-fledged 10 Base-T Ethernet UWOC system, which is
developed in the framework of the European Project SUNRISE[53], supporting
10 Mb/s over a 7.5 m distance in a shallow harbor water transmission[54], and
10 Mb/s over 10 m in a highly turbid harbor water transmission[55]. Baiden et
al. have tested an omni-directional UWOC system in lake water[56], and
demonstrated 20 Mb/s over 11 m transmission[57]. Han et al. have proposed a
transmitter with a 150° divergence angle and more than 90% uniformity of
radiation intensity for a quasi-omni-directional UWOC by applying a freeform
lens into an LED array, and achieved 19 Mb/s over 8 m water channel
transmission[58]. In Ref. [59], the authors designed a real-time field
programmable gate array (FPGA)-based LED-UWOC system and demonstrated 25 Mb/s
over a 10 m underwater channel. Shan et al. have built an underwater laser
communication device with high pulse energy and small volume[60]. In
Refs. [61–66" target="_self" style="display: inline;">–66], orbital angular
momentum multiplexing (OAM) has been explored in a UWOC application as an
emerging technique. Xu et al. have put forth a novel concept on an underwater
fiber–wireless (Fi–Wi) communication system with a fully passive wireless front
end, which can save sophisticated, expensive, and short-lived optical connectors
in deep oceans[67].

Sun et al. have demonstrated a non-LOS UWOC link using an ultraviolet (UV)
375 nm laser to circumvent the alignment requirement of an LOS underwater
link[68], and experimentally demonstrated high-speed diffuse LOS optical
wireless communication across a wavy water-air interface[69]. In Refs. [70,71],
the authors have experimentally investigated the impact of turbulence on a UWOC
link. Table 1 summarizes some recent works of UWOC in the literature.

TABLE 1. SUMMARY OF RECENT WORKS IN UWOC

AuthorsTransmitter typeLight output powerPhotodetectorModulation formatsData
rateDistance (m)Distance-data rate product (Gbps·m)Real timeXu et al.[30]Blue
LEDN/APIN16-QAM-OFDM161 Mb/s20.32NTian et al.[10]440 nm
micro-LEDN/APIN/APDOOK800/200 Mb/s0.6/5.41.08NWang et al.[33]521 nm LED160 mW2
PINs64-QAM-DMT, MRC2.175 Gb/s1.22.61NZhou et al.[72]RGBYC LEDPINBit-power
loading DMT15.17 Gb/s1.218.2NWang et al.[59]448 nm
LEDN/AAPDOOK25 Mb/s100.25YWang et al.[32]520 nm
LD15 mWMPPC32-QAM-OFDM312.03 Mb/s216.55NOubei et al.[35]450 nm
LD15 mWAPD16-QAM-OFDM4.8 Gb/s5.425.92NChen et al.[36]520 nm
LD15 mWAPD32-QAM-OFDM5.5 Gb/s5/21115.5NLiu et al.[73]520 nm
LD19.4 mWPIN/APDOOK2.7 Gb/s34.593.15NFei et al.[39]450 nm LD20 mWAPDBit-power
loading DMT, NE7.3 Gb/s15109.95NFei et al.[41]450 nm
LD12.8 mWAPDMB-DFT-S-DMT5.6 Gb/s55308NFei et al.[40]450 nm LD120 mWPINBit-power
loading DMT, NE16.6/6.6 Gb/s5/55462@35 mNLi et al.[37]Two 488 nm
LDs20 mWPINPAM4, injection locking16 Gb/s10160NLi et al.[14]Three 680 nm
LDs3 mWPINInjection locking, OOK25 Gb/s10250NHuang et al.[38]450 nm
LD120 mWPIN/APD16-QAM-OFDM14.8/10.8 Gb/s1.7/10.225.16/110NHong et al.[42]450 nm
LD120 mWPINPCS-DMT18.09/12.6 Gb/s5/35441@35 mNWang et al.[44]520 nm
LD15 mWAPDOOK, NE500 Mb/s10050NHu et al.[43]532 nm LDN/ASPD256-PPM & RS,
LDPC∼MHz120N/ANJAMSTEC[52]450 nm LD>5  WPMTN/A20 Mb/s1202.4Y

查看所有表

A schematic diagram of the general UWOC experimental setup in a lab is shown in
Fig. 1. After being amplified and superimposed with DC, the transmitted RF
signal from an arbitrary waveform generator (AWG) drives an LED/LD. The
collimated light beam carrying the electrical signal passes through a water
channel, is then captured by an APD/PIN, and finally seized by a digital serial
analyzer (DSA) for further processing. The DMT modulation is a promising
modulation scheme in UWOC for its intrinsic advantages such as high spectral
efficiency and simple equalization. Moreover, the structure of cyclic prefix
(CP) is inherently tolerant to inter-symbol interference (ISI) caused by
multipath scattering in highly turbid waters. Nevertheless, the main drawback of
DMT modulation is the high PAPR, which may induce severe nonlinear impairments
and inefficient power utilization if advanced modulation formats are further
employed. The nonlinearity widely exists in UWOC systems, which is mainly
introduced by the modulation nonlinearity of an LED[74] or LD[75,76], the
nonlinear amplification of an electrical amplifier[77,78], and the square-law
detection of a photodetector[79]. Nonlinearity is common in single carrier
systems such as multilevel PAM[80] and CAP[81] and multicarrier systems such as
DMT. The nonlinear distortions induced by the LED (or LD), electrical amplifier,
and photodetector may give severe impairments in the received signals, resulting
in a significant signal-to-noise ratio (SNR) degradation. The linear
equalization techniques cannot effectively counteract such nonlinear effects.
Volterra series-based nonlinear equalizers have been intensively studied in
wireless[82], fiber-optic communication[83], and free-space visible light
communication (VLC) systems[80,81,84–86]. Biswas et al. analyzed the
semiconductor LD nonlinearity from the laser rate equations using an
output-to-input approach[75]. Hence, the nonlinearity in both LED- and LD-based
UWOC systems can be modeled by the Volterra series, which is expressed as
y(n)=∑k1=0N1−1w1(k1)x(n−k1)+∑k1=0N2−1∑k2=0N2−1w2(k1,k2)x(n−k1)x(n−k2)+∑k1=0N3−1∑k2=0N3−1∑k3=0N3−1w3(k1,k2,k3)x(n−k1)x(n−k2)x(n−k3)+⋯,(1)Where
y(n) denotes the nth received signal sample, and w1(k1) and wm(k1,k2,…,km)(m≥2)
are the linear coefficient and mth-order nonlinear coefficient, respectively.
Nt (t≥1) is the memory length. x(n−ki) is the (n−ki)th temporal sample of
transmitted signal. The Volterra series-based equalizer can be adopted to
eliminate the nonlinear noise in UWOC systems. In our previous works[33,34], we
proposed simplified Volterra nonlinear equalizers that include only the
2nd-order and partial 3rd-order nonlinear terms. With the simplified Volterra
nonlinear equalizers, the received SNR of the system can be significantly
enhanced by 2–3 dB[39,40]. After the combination with the adaptive bit-power
loading DMT, the average modulation order (bits per symbol) increases from 4.97
to 5.87. With a modulation bandwidth of 1.25 GHz, the system capacities increase
from 6.21 Gb/s to 7.33 Gb/s over a 15 m water channel[39]. In Ref. [40], with a
modulation bandwidth of 2.75 GHz, the system capacities increase from 15.6 Gb/s
to 16.6 Gb/s over a 5 m water channel. The nonlinear equalization can bring a
capacity improvement of about 1 Gb/s in both cases. Figure 2 shows the received
optical power (ROP, red line), SNR with nonlinear equalization (blue line), and
SNR without nonlinear equalization (green line) versus transmission distance
under tap water. The ROP decreases for longer transmission distances due to the
absorption and scattering effects of water and the loss from multiple
reflections by mirrors (used to simulate different water channel lengths). With
the higher modulation depth allowed by the nonlinear equalization, the received
SNR can be enhanced by ∼3 dB compared to the linear equalization case.

FIG. 1. SCHEMATIC DIAGRAM OF THE GENERAL UWOC SETUP IN A LAB EXPERIMENT. AWG:
ARBITRARY WAVEFORM GENERATOR; EA: ELECTRICAL AMPLIFIER; ATT: ADJUSTABLE
ATTENUATOR; DC: DIRECT CURRENT; LD: LASER DIODE; APD: AVALANCHE PHOTODIODE; DSA:
DIGITAL SERIAL ANALYZER; TX-DSP: DIGITAL SIGNAL PROCESSING AT THE TRANSMITTER;
RX-DSP: DIGITAL SIGNAL PROCESSING AT THE RECEIVER.

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FIG. 2. RECEIVED OPTICAL POWER (ROP) AND SNR VERSUS TRANSMISSION DISTANCE UNDER
TAP WATER. W/: WITH; W/O: WITHOUT; NLE: NONLINEAR EQUALIZATION[39].

下载图片 查看所有图片

The performance of our proposed scheme under different water turbidity
conditions has been investigated. The received SNR under different water
turbidities after a 1 m transmission distance is shown in Fig. 3(a). The water
turbidity is changed by adding the 0.5% diluted Maalox [Al(OH)3 and Mg(OH)2
powder] suspension in 2 mL steps (a step of 20 mL after 30 mL) into the tap
water (volume of ∼85.7 L). The remarkable SNR gain of >3  dB, regardless of the
water turbidity, shows the robustness of the nonlinear equalization method in
UWOC systems. To quantify the turbidity of water, Fig. 3(b) shows the
corresponding attenuation coefficient of water, which is calculated in
Ref. [87]. The calculated attenuation coefficient ranges from 0.0751  m−1 to
2.078  m−1, which covers the typical values of “clear ocean”, “coastal ocean”,
and “harbor water”[29]. Figures 3(c)–3(f) present the side view of the optical
beam passing through water with different turbidities. The snapshots were taken
one by one under the same settings of exposure time and camera sensitivity. As
can be seen from this figure, the scattered light becomes stronger with a larger
dose of added Maalox suspension.

FIG. 3. (A) RECEIVED SNR VERSUS THE VOLUME OF ADDED MAALOX SUSPENSION AFTER A
1 M UNDERWATER TRANSMISSION. (B) ATTENUATION COEFFICIENT VERSUS VOLUME OF THE
ADDED MAALOX SUSPENSION. (C)–(F) THE SNAPSHOTS OF THE OPTICAL BEAM PASSING
THROUGH WATER OF DIFFERENT TURBIDITIES WHICH REPRESENT (C) “TAP WATER”,
(D) “CLEAR OCEAN”, (E) “COASTAL OCEAN”, AND (F) “HARBOR WATER”[39].

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Apart from enhancing the SNR of the system, increasing system capacity is
another hot topic in UWOC research. Recently, a powerful and flexible channel
capacity limit-approaching technique, called PCS has been extensively studied in
wireless communication[88] and fiber-optic communication[89,90], and introduced
in the VLC area[91]. The PCS technique applied to high-order modulation formats
has become an excellent candidate to overcome the inherent gap between the
conventional regular QAM format and the Shannon capacity of the channel. In our
previous work[42], the PCS scheme with DMT modulation has been experimentally
demonstrated in a UWOC system to approach the channel capacity limit. According
to the pre-estimated SNR of each subcarrier, a fixed QAM format with various
probabilistic distributions is individually allocated for each subcarrier to
achieve maximum capacity. In DMT with the PCS scheme, different probability mass
functions (PMFs) determined by a user-defined distribution matcher, e.g., the
well-known Maxwell-Boltzmann distribution[88,90], are assigned to different
subcarriers. Assuming that the PCS-MQAM constellation values are taken from
χ={x1,x2,…,xM}, the corresponding PMF can be expressed
as[88]P(xi)=e−υ|xi|2/∑x∈χe−υ|x|2,1≤i≤M,(2)where υ is a rate adjustable
parameter. For a given target SNR range, there exists a suitable υ to optimize
the constellation shaping, leading to an achievable maximum channel capacity.
The optimization process can be performed via a look-up table.

The Shannon capacity limit C=log2(1+SNR)[92] of different underwater
transmission channels is presented in Fig. 4(a). From the figure, PCS-1024QAM is
the optimal option for 5 m and 25 m underwater channels as the maximum Shannon
capacity limit for the two cases is about 10 bits/symbol. Likewise, PCS-256QAM
and PCS-64QAM are suitable for the 35 m and 45 m underwater channel cases,
respectively. For the sake of simplicity, only the cases of 25 m with
PCS-1024QAM-DMT and 35 m with PCS-256QAM-DMT underwater transmission channels
are taken into discussion. For the bit-power loading scheme, different QAM
formats are assigned to different subcarriers in accordance with the
pre-estimated SNR, and the overall BER is below the 7% overhead standard
hard-decision forward error correction (HD-FEC) limit of 3.8×10−3. For the PCS
scheme, a fixed QAM format with different probabilistic constellation
distributions is employed based on the pre-estimated SNR. Figure 4(b) shows the
entropy versus subcarrier index for the bit-power loading and PCS schemes. The
entropy of bit-power loading can only take discrete integer bits while the PCS
can take continuous values (less than log2M for PCS-MQAM scheme). The entropy of
DMT with the PCS scheme of each subcarrier is larger than that with the
bit-power loading scheme, implying that the PCS scheme can carry more
information than the latter. Figure 4(c) exhibits the graphical illustrations of
three regular QAM formats (i.e., 128QAM/64QAM/32QAM) for bit-power loading and
three adopted constellation probability distributions for both PCS-256QAM-DMT
(35 m channel) and PCS-1024QAM-DMT (25 m channel). The distributions become more
centralized as entropy H decreases, and the occurrence probability of the outer
points may tend to zero for stronger shaping resulting from the PCS-QAM.

FIG. 4. (A) SHANNON CAPACITY LIMIT UNDER DIFFERENT UNDERWATER TRANSMISSION
DISTANCES. (B) ENTROPY OF DIFFERENT SUBCARRIERS FOR 25 M AND 35 M UNDERWATER
TRANSMISSION DISTANCES. (C) GRAPHICAL ILLUSTRATIONS FOR BIT-POWER LOADING AND
THE PCS-256/1024QAM-DMT SCHEME OF THREE DIFFERENT ENTROPIES. NOTE THAT THE BARS
DENOTE THE PROBABILITY OF EACH MODULATION SYMBOL[42].

下载图片 查看所有图片

Figures 5(a)–5(c) depict the corresponding received constellation diagrams for
bit-power loading, PCS-256QAM-DMT over 35 m, and PCS-1024QAM-DMT over 25 m
underwater channels, respectively. One can see clearly from Fig. 5 that the
constellation points of the bit-power loading scheme are distributed quite
uniformly, while the constellation points of PCS-QAM resemble Gaussian
distribution. With the PCS-DMT scheme, net data rates of 18.09 Gb/s, 17.21 Gb/s,
and 12.64 Gb/s could be achieved after 5 m, 25 m, and 35 m underwater
transmission with a modulation bandwidth of ∼2.75 GHz, which indicates 32.22%,
30.03%, and 27.55% capacity improvement compared with the widely used bit-power
loading DMT scheme in the current UWOC systems[39,40], respectively. More
details of the generation, encoding, and decoding of the PCS scheme can be found
in Ref. [42].

FIG. 5. RECEIVED CONSTELLATION DIAGRAMS OF (A) BIT-POWER LOADING,
(B) PCS-256QAM-DMT FOR 35 M, AND (C) PCS-1024QAM-DMT FOR 25 M UNDERWATER
TRANSMISSIONS[36].

下载图片 查看所有图片

In this paper, recent research progress has been reviewed for both LED- and
LD-based UWOC systems, mainly from a perspective of advanced modulation formats.
UWOC systems are susceptible to nonlinearity induced by the components (e.g.,
LED, LD), which may yield severe impairments in the received signals. Volterra
series-based nonlinear equalizers can effectively combat such nonlinear
impairments, enhancing the system performance. We have proposed a simplified
Volterra nonlinear equalizer by which the SNR received by the system can be
enhanced by 2–3 dB. The robustness of the nonlinear equalizer in UWOC systems
has also been validated. To further increase the system capacity, the PCS
technique has been introduced to address the inherent gap between the
conventional regular QAM format and the Shannon capacity of the UWOC system.
Together with DMT, a fixed QAM format with different probabilistic distributions
has been individually assigned to each subcarrier to achieve the maximum system
capacity. Our experimental results have shown a net data rate of 18.09 Gb/s over
a 5 m underwater channel, which is the highest data rate ever reported for a
single LD in UWOC[42].

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