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Our earth the only planet where water could be found and covered more than seventy percent with it. Monitoring different phenomenal activities in an underwater environment, such as environmental impact surveillance, marine life, oil and gas exploration is essential in underwater. In this regard, underwater wireless communication (UWC) has become a significant field. Optical, acoustic and electromagnetic waves have been widely used for data transmission in UWC. Investigation of possible UWC techniques has an influential impact on wireless communications. Nowadays, UWC is being used for experimental observation, oceanographic data collection and analysis, underwater navigation, disaster prevention and early detection warning of a tsunami. This work presents an overview, main initiatives and up-to-date contributions of the most widely used UWC techniques, i.e, underwater wireless optical, acoustic and electromagnetic communications. In addition, we summarize emerging technologies in the UWC, future research directions and recommendations using fifth generation (5G) communication techniques.
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Underwater Communications: Recent Advances
Mohammad Furqan Ali, Dushantha Nalin K. Jayakody∗†, Tharindu D. Ponnimbaduge Perera,
Kathiravan Srinivasan, Abhishek Sharma§and Ioannis Krikidis
School of Computer Science and Robotics, National Research Tomsk Polytechnic University, RUSSIA
School of Postgraduate Studies, Sri Lanka Technological Campus, Sri Lanka
School of Information Technology and Engineering, Vellore Institute of Technology, INDIA
§Department of Electronics Communication Engineering, The LNM Institute of Information Technology, INDIA
Department of Electrical and Computer Engineering, University of Cyprus, Cyprus
Email:[ali89,ponnimbaduage,nalin]@tpu.ru, kathiravan.srinivasan@vit.ac.in, abhisheksharma@lnmiit.ac.in.
Abstract—Our earth the only planet where water could be
found and covered more than seventy percent with it. Monitoring
different phenomenal activities in an underwater environment,
such as environmental impact surveillance, marine life, oil and
gas exploration is essential in underwater. In this regard, under-
water wireless communication (UWC) has become a significant
field. Optical, acoustic and electromagnetic waves have been
widely used for data transmission in UWC. Investigation of
possible UWC techniques has an influential impact on wireless
communications. Nowadays, UWC is being used for experi-
mental observation, oceanographic data collection and analysis,
underwater navigation, disaster prevention and early detection
warning of a tsunami. This work presents an overview, main
initiatives and up-to-date contributions of the most widely used
UWC techniques, i.e, underwater wireless optical, acoustic and
electromagnetic communications. In addition, we summarize
emerging technologies in the UWC, future research directions
and recommendations using fifth generation (5G) communication
techniques.
Index Terms—Underwater wireless acoustic communication,
Underwater electromagnetic communication , Underwater optical
communication, 5G wireless communication.
I. INTRODUCTION
Global warming has become an issue for several decades.
In rising of global warming, the polar ice caps melt gradually
cause of rising sea level. Hence, the importance of observing
ocean environmental activities such as oceanographic data
collection, water sampling, etc., has gradually increased with
time variation. Underwater Wireless Communication (UWC)
supports surveillance of coastal securities, especially for mil-
itary purposes and commercially for investigation of natural
resources in underwater environment. Moreover, it also helps
for mapping and discovering the unknown regions of under-
water. Nowadays, UWC is being used for experimental obser-
vation, data collection, and analysis, underwater navigation,
disaster prevention and early detection warning of tsunami [1].
Optical, acoustic and electromagnetic (EM) wireless carriers
are considered to envisage UWC in underwater applications.
Deploying UWC techniques in an unexplored water medium
are highly challenging as compared to terrestrial wireless
communication [2]. However, the quality and reliability of data
transmission in shallow and deep water are dependent on the
physical characteristics of the water channel [3]. The Quality
of Service (QoS) of UWC, depends on the physicochemical
properties of water medium and physical characteristic of
optical, acoustic and EM waves. UWC plays a significant role
in deployed underwater applications, which has an influential
impact on the wireless network. The deployment of commu-
nication network setup in underwater systems consist of fixed
and anchored sensor nodes with the seabed, floating unmanned
underwater vehicle nodes (UUVs) or autonomous underwater
vehicle (AUVs), signal receiver processing towers, floating
devices (buoy), submarines, ships and onshore base stations
[4].
EM waves in radio frequency (RF), 3Hz to 3 kHz frequency
range is capable for high data acquisition and transmission
in shallow water over short distances and usually attenu-
ated easily by seawater [4]. On the other side, acoustic
waves are affected by different propagation factors due to
ambient noise, external interference, water-surface geomet-
rical expansion, attenuation, multi-path effects, and Doppler
spreading. Optical waves have high bandwidth, but affected
by absorption, scattering and different level of temperature
in underwater. Underwater wireless optical communication
(UWOC) has less explored and somewhat challenging to de-
ploy than acoustic propagation in underwater [5]. The existing
Underwater wireless acoustic communication (UWAC) has the
limited performance of low bandwidth, latency and multi-path
propagation in an underwater medium. The maximum data
acquisition in UWAC is roughly 100 kbps for short distances
while 10 kbps over long distances. The possible bandwidth
with respect to propagation distance in UWAC are listed down
in Table I.
The main purpose of this survey is to understand the main
characteristics and existing features of UWC. This work has
an overview of possible UWC techniques and up-to date
literature. The remaining structure of this paper as follows:
In section II, we discuss the main deployable techniques
of UWC towards the next generation of wireless network.
Underwater wireless RF communication (UWRFC) and related
issues are described in section III. Underwater wireless optical
communication (UWOC) has been widely discussed in section
IV. In section V, underwater wireless acoustic communication
(UWAC) and it’s issues are discussed. The paper contributes
emerging communication techniques proposed by recent re-
search in section VI. Finally, we conclude the paper in section
VII.
Classified Prop-
agation distance
Possible range
(km)
Maximum
bandwidth
(Hz)
Possible
data rate
(kbps)
Very Sort Less than 0.1 More than
100
500
short 0.1 to 1 20 to 50 30
Medium 1 to 10 Up to 10 10
Long 10 to 100 2 to 5 5
Very long more than 100 Less than 1 600
TABLE I
THE P OSS IB LE BA ND WID TH F OR D IFF ERE NT AC OU ST IC P ROPAG ATIO N
DISTANCES [6]
II. THE INTEGRATION OF 5G IN U NDERWATE R
COMMUNICATION
5G wireless network will be the future networking technique
in wireless communications with extremely low latency and
high data rate [9] [10]. A high range acoustic communication
through orthogonal frequency-division multiplexing (OFDM)
techniques has been discussed in [11]. The authors [3] dis-
cussed UWC techniques and its’ related issues along with the
UWC emerging technologies. Filter bank multicarrier (FBMC)
and generalized frequency division multiplexing (GFDM) are
the latest promising techniques for 5G applications in un-
derwater environment. GFDM is a multicarrier scheme that
based on time and frequency, which is derived from filter bank
approach [12]. FBMC also addresses both time and frequency
dispersions that should be constructed by using prototype filter.
The parametric constraints should matches channel properties
[13]. An experimental work on GFDM technique towards
underwater 5G communication systems based on multi-carrier
filter bank has been discussed in [12]. The concept of adopting
MIMO-OFDM for underwater acoustic channel and using
FBMC modulation technique towards 5G wireless network
investigated in [14].
A. Types of water in UWC
The seawater has been categorized into three different cate-
gories in UWC, i.e, clearest, intermediate and murkiest water
[15]. The clearest water is the most transparent water, which
could be found in Atlantic and Mid-Pacific ocean. Secondly,
intermediate water can be found in North Pacific ocean. The
murkiest water can be found typically in the Northern Sea and
Eastern Atlantic ocean. Due to optical propagation, seawater
has been divided into four categories due to optical inherent
properties (IOP) [6] i.e. pure seawater, clear ocean water,
coastal ocean water, turbid harbor and estuary water. Typical
values of absorption and scattering coefficients in different
waters are listed-down in Table II.
Absorption and scattering of signals play a major role in
deciding QoS in UWC. In optical signal propagation, the
scattering losses arise due to the high particle concentration
in clear ocean water while the absorption and other major
Description of water
for UWC
a(λ) b(λ)bb(λ) c(λ)
Pure sea water 0.053 0.003 0.0006 0.056
Clear Ocean water 0.069 0.08 0.001 0.15
Coastal Ocean water 0.088 0.216 0.0014 0.305
Turbid Harbor water 0.295 1.875 0.0076 2.17
TABLE II
TYPICAL VALUES OF ABSORPTION AND SCATTERING COEFFICIENTS IN
DI FFER EN T WATER ME DI UM S [5]
Fig. 1. Acoustic and RF signal propagation between underwater sensor nodes
and terrestrial offshore station
losses occur in pure seawater. The higher concentration of
suspended particles in ocean water support to scattering and
absorption phenomena and affect to signal propagation. The
highest concentration of particles could be found in turbid
harbor and estuary water than pure and clear ocean waters.
III. UND ERWATER RF COMMUNICATION
EM waves used for signal transmission between underwater
and terrestrial communication platforms [16]. EM waves cov-
ers frequency ranges from few kHz to 1 GHz [17]. EM waves
propagation setup deploy for shallow water over tens of meter
called by RF buoyant underwater communication [6]. The
possibility of RF signal propagation is higher in shallow water
than deep ocean water. The oceanic water offers high con-
ductance which could be seriously affected to electromagnetic
wave propagation [3]. Thus, very less possibility is available
to establish communication over long distance in the under-
water environment, with ultra-high frequency and very high-
frequency ranges (VHF and UHF), or even in high-frequencies
(HF). The electromagnetic waves attenuation considered to
be lower enough to achieve expected communications over
several kilometers in underwater environment [4]. Thus, the
multiple-path propagation could be an additional benefit for
underwater signal transmission in inland water reservoirs, i,e
lakes, rivers, etc. from submerged communication nodes to
Types of Tech-
nology
Distance Propagation
Speed(m/s)
Frequency Bandwidth Data
rate
Attenuation Affecting factors deter-
mine channel quality
Reference
RF Very short dis-
tance (Up to 10
m)
2.255x10830-300 Hz MHz Mbps Frequency and
conductivity
dependent (3.5-5
dB/m)
Conductivity and permi-
tivity of channel
[2] [4]
[7] [6]
[8]
Optical waves Short distances
(Up to 100 m)
Almost
same as RF
(2.255x108)
1012
-1015Hz
MHz Gbps 0.39 dB/m (Ocean)
and 11 dB/m (Turbid
water)
Absorption, scattering,
turbidity, suspended and
organic matter of channel
link
[2] [4]
[7] [6]
[8]
Acoustic Long distance
(Up to 20 km)
1500 10-1 kHz Hz kbps Distance and
frequency dependent
(0.1-4 dB/m)
Absorption, scattering,
pressure, temperature and
salinity of water medium
[2] [4]
[7] [6]
[8]
TABLE III
COMPARISON AMONG DEPLOYED UNDERWATER WIRELESS COMMUNICATION TECHNOLOGIES
onshore base stations [18]. In UWRFC, signals experience
high attenuation than in terrestrial communication.
In UWRFC, EM wave propagation depends on underwater
environment. The physical properties of the water, i.e., salinity,
conductivity, and temperature affect electromagnetic propa-
gation in underwater [19]. In seawater, the average value of
conductivity approximately 4 mhos/m, which is doubled in the
magnitude of conductivity in freshwater [6]. The absorption
coefficient in seawater can be expressed as [19].
α(f) = pπσµ0f, (1)
where radio frequency denotes by fand σrepresents the
conductivity of the water. The vacuum permeability describes
by µ04π107H/m. The value of µ0is almost equal in fresh
and seawater [4]. A typical channel model transfer function for
different channel parameters is widely discussed in [19].
A. Main issues in underwater RF communication
EM waves are affected by several factors depend on the
water properties, such as density level, which can vary with
temperature, high permittivity, electrical conductivity, and
salinity. In addition EM waves also affected by turbidity in
underwater and by the various types of noises. In UWRFC,
multi-path propagation is the most influential phenomena that
has a direct impact on RF propagation from water to air. The
refraction angle and losses also have a consideration in RF
signaling to cross the air-water boundary by patching through
an antenna [4]. Electromagnetic waves used for a limited range
of communication in underwater, which could be raise for over
long ranges by implementing a specific design of an antenna.
A large size antenna is required for RF signal propagation
between terrestrial and underwater communication. The mag-
netic coupled loop types of antennas are the most reliable for
practical solutions. An another option is to use an electric
dipole antenna for lateral electromagnetic waves that has been
discussed in [18]. In the deployment of UWRFC propagation
scheme, major factors to be considered are high data rate, the
antenna design and transmitting power strength.
IV. OPT ICAL COMMUNICATION
UWOC has many distinct properties during propagation
at different frequencies over different ranges in dissimilar
water mediums [8]. The light speed might be around four to
five times stronger and higher in magnitude than propagation
speed of acoustic waves in fluids [4]. The sea water offers a
conductive property for RF propagation and dielectric proper-
ties for optical signal propagation [4]. Optical communication
is affected by scattering, dispersion, line of sight (LOS),
fluctuation in temperature and by physiochemical properties
of the water. In dielectric medium, the possibility to achieve
high data rates through UWOC technology as compared to
UWRFC, where the range of propagation limited up to tens of
meters [4]. In addition, the negotiation of Doppler effect can be
obtained in optical communications as compare to competitive
schemes, i.e., EM, acoustic.
According to the environmental conditions, the sea water
has been categorized into two specific categories, inherent
optical properties (IOP) and apparent optical properties (AOP)
in respect with optical propagation. IOP is medium dependent,
while AOP is light source dependent [20]. In optical propa-
gation, photons change their direction due to scattering and
the possibility of scattered photons originated by salt ions in
pure water [20]. In UWOC propagation, the beam attenuation
coefficient is directly related to the intensity and separation
distance of light sources. The light intensity at receiver end
can be expressed as [21].
I=I0expcdλ,(2)
where I0and Iare the light intensities both of transmitter
ends and receiver, ddenotes the distance between transmitter
and receiver.
A. Noises in underwater optical communication
There are different types of noises in UWOC such as
quantum shot noise, optical excess noise, optical background
noise, photo-detector dark current noise and electronic noise
[22].
The Quantum noise: This type of noise occurs due to
receive random variation of photons by optical receiver.
The Optical excess noise: The Optical excess noise
caused due to transmitter imperfection.
The Background noise: In this type of noise the back-
ground consider as a blackbody radiation in underwater
Fig. 2. Illustration of Underwater optical sensor network clusters with
terrestrial RF based station
whose primary source is the refracted sunlight from the
water surface. Due to this phenomena, the solar and
blackbody radiation background noises occur.
The Photo-detector dark current noise:- Due to electrical
current leakage from photo-detector this kind of noise
occurs.
B. Main Issues in optical communication
Absorption and scattering are the two crucial effects that
affect the propagation of optical waves in underwater [7]. We
can understand the simple phenomena of these two factors
by the geometrical model of a water element that has shown
in Fig. 3. If the input beam of light strength Pi(λ), the
small fraction of incident beam Pa(λ) absorbed and fraction
Ps(λ) scattered by water element. The unaffected result Pc(λ)
passing through water element whose volume is δV and
thickness δr respectively. According to energy conservation
balancing, the absorption and scattering phenomena can be
described as [6].
Pi(λ) = Pa(λ) + Ps(λ) + Pc(λ).(3)
The overall attenuation in underwater coefficient c(λ) can
be expressed as [7].
c(λ) = a(λ) + b(λ).(4)
The values of a(λ) and b(λ) in different water medium could
be found from Table II.
V. ACOUSTIC COMMUNICATION
UWAC is an alternative communication technique which
can be used for longer distance communication than UWRFC
and UWOC. However it has a limited range of propagation and
is affected by strong attenuation and water turbidity. Acoustic
waves propagation offers low frequency, bandwidth and low
speed around 1500 m/s. The propagation of acoustic waves
is faster in normal water than cold water [3]. Generally, the
speed increase of acoustic waves about 4 m/s, due to rising of
Fig. 3. Optical wave scattering and absorption phenomena in underwater as
given in [6] [8]
1C temperature in water. According to the acoustic waves
propagation range, the water channel has been categorized
very short, short, medium, long and very long propagation
distances [6]. In Table I, mentioned the different bandwidth
ranges according to the propagation range. An acoustic model
for sound speed profile (SSP) has been discussed for under-
water communication environments with 1 km of water depth
[23].
c= 1449.2+4.6T0.055T2+ 0.00029T3
+(1.34 0.01T)(S35) + 0.016z, (5)
where cdenotes acoustic wave speed and Tis channel
temperature, Srepresents the salinity of water and water
depth denotes by z. Thus, the acoustic waves speed is a
function of channel temperature, depth and salinity of water
which can be proportional to water temperature, salinity and
depth of water medium [24]. Due to high absorption and
frequency, wave energy converts into heat energy. Similarly,
longer propagation ranges lead to high absorption losses [25].
The absorption coefficient is the sum of absorptivity and
contribution of chemical reaction by the water medium [26]
[27]. Similarly, scattering loss occurs due to obstacles during
acoustic wave propagation. These obstacles can be consider
by the disturbance at sea surface, existing fixed or movable
objects at the bottom of sea. Spreading and absorption losses
contribute path loss, whose explained by a simplified model
expressed by [25] [28].
10 log A(l, f ) = 10 log (A0) + 10klog (l)
+(l)10 log a(f, S, T , c, p, H, z),(6)
where ldenotes the distance (in meters) or propagation range
between transmitter and receiver, fis the frequency range
(in kHz) and kdenotes the spreading factor. For cylindrical
spreading the value of k= 1, while for spherical spreading
k= 2, the value of k= 1.5can be taken for experimental
spreading [25]. Log A0is called normalizing factor which is
the inverse of transmitted power. The variables are represent-
ing the attenuation coefficient (in dB/meter), depend on envi-
ronmental channel conditions [28]. The variable Srepresents
salinity (in ppt), temperature denotes by T(in degree Celsius),
and cis the speed of acoustic wave propagation (in m/s), z
and Hare showing the depth of water in meters. The path
losses expand when the frequency fand separation distance
dbetween transmitter and receiver increase.
A. Most common Issues in underwater acoustic propagation
The main issues in underwater acoustic propagation that
affect communication links through man-made noises, path
and multi-path losses, Doppler spread, high and variable prop-
agation delay. Above mentioned losses and factors determined
the temporal and spatial variability of the acoustic channel
that cause of limited communication range, frequency and
bandwidth of acoustic communication link. The following
factors are effective issues on acoustic communication.
Losses: The main factors can be taken into an account
while signal propagation energy losses, absorption and
scattering loss respectively.
Man-made and ambient noise: Ambient noise related to
hydrodynamics properties of water such as an underwater
thunderstorm, water movement, water tides, water bub-
bles and fain, wind, rain and biological phenomena. The
ambient noise losses up to 26 dB/km [29]. Man-made
noises produced by machine tools such as pumps, power
plants, submarines and ships etc.
Attenuation: Attenuation occurs by absorption of the
acoustic energy that transforms into heat, scattering,
refraction reverberation phenomenon, and dispersion.
Acoustic waves attenuation is directly proportional to the
frequency of waves and depth of water medium.
Absorption: The energy conversion phenomena of acous-
tic waves in another form of energy by chemical charac-
teristics of the water channel.
Geometric expansion: Geometric expansion is a spreading
experience and function of energy loss in acoustic waves
over the large area. When the acoustic pulse propagates
from source of origin and covers a large water area, the
wave energy per unit area become smaller.
Path and multi-path propagation losses produced by the
degradation of acoustic waves, generates Inter-Symbol
Interference (ISI). The multi-path propagation are the
geometrical constraints and configuration link dependent.
In Vertical channels develops a little time dispersion but
through water layers, it has a long multi-path spread,
which is depends on water depth.
Doppler frequency spread is an important and noticeable
factor in UWAC, due to a low destruction in the perfor-
mance of digital communication and transmission at high
data rate [8].
VI. EMERGING UN DE RWATER COMMUNICATION
TECHNOLOGIES
A. Energy Harvesting
Energy harvesting (EH) is an approach to capturing and
transformed usefulness energy into usable electric power
where energy requires in terms of heat, vibration and RF
signals. Underwater wireless sensor network (UWSNs) is an
emerging technique to establish reliable data although required
high energy constraints. The wireless power transmission
(WPT) technique is promising EH technique. In WPT the
Fig. 4. Emerging Underwater Wireless Technologies
nodes capable to charge their own batteries sources through
electromagnetic radiation in remote networking area [30].
WPT offers a good performance for short distances while it
depends on application requirement for long distance. The
authors [30] proposed simultaneous information and power
transfer EH technique (SWIPT) that enables to transfer in-
formation and power simultaneously. These investigated tech-
niques support to improve efficiency of system.
B. Massive MIMO adaptive Underwater Communication
Massive multi-input-multi-output (MIMO) supports under-
water acoustic communication through large number of hy-
drophones array. It also support the various types of multime-
dia communication contents in real time activities and audio-
video conferencing [31]. Massive MIMO is an auspicious
solution that support to improve throughput, capacity and
energy efficiency of UWC system in the future.
C. mmWaves enabling Underwater Communication
Due to high demand of improving communication network-
ing capacity, mmWaves are an alternative solution to support
underwater communication. These waves offer likely similar
characteristics to the optical wireless signals. Hence mmWaves
are considering to offer high bandwidth transmission and
efficiency to improve communication performances [32]. The
waves are capable to provide up to 10 gbps data rates which
is the solution of hybrid communication [33].
D. Non-Orthogonal Multiple Access (NOMA) as an emerging
wireless carrier
Non-Orthogonal multiple access (NOMA) offers desirable
communication benefits and a promising multiple access tech-
nique. It allows to connect multiple users simultaneously
and minimize delay. The future acpect of NOMA expected
allocation schemes in Underwater acoustic networks architec-
ture (UWASNs) and equal transmission times (ETT) power
allocation which are capable to prevent the wasteful resources
generation in underwater environment that proposed for the
future perspectives [34].
E. Internet of Underwater Things (IoUTs)
Internet of things is an emerging technology that offers
to connect devices wirelessly. The integration of Internet
of Underwater Things (IoUTs) plays a significant role to
allow data exchange between UWSNs to the base station
in underwater environment. IoUTs provide an opportunity to
observation of marine life and understanding of underwater
habitats [35].
VII. CONCLUSION
UWC technology enables a platform to build up a network
connection between underwater devices with offshore based
station. The main motto of this paper to provide an overview
and challenges posed by the peculiarities of underwater com-
munication technologies with particular reference to monitor-
ing applications in water medium. The different water channels
and communication links have crucially difficult properties
and challenges. The technologies discussed, the possible so-
lutions and grasp to deploy underwater wireless acoustic,
underwater wireless electromagnetic and underwater optical
wireless communication technologies. The main outline of
this paper to encourage research efforts and development of
new advanced communication techniques. This paper has been
contributed and providing a survey of the technical issues
and challenges in underwater networks and communication
of entire technologies towards the next generation wireless
networking system.
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... The attenuation coefficient h F SO l (λ, d) depends on the LOS distance d rd between nodes, distribution of scattering signals and the utilized wavelength (λ). Thus, the underwater attenuation coefficient and terrestrial basis FSO link attenuation coefficients according to Beer Lambert's law, can be expressed by (3) and (4) The typical values of absorption, scattering and extinction coefficients wavelength-dependent at 450nm blue light in different water mediums [20] ...
... Similarly, for the SM2 the average BER can be given as the closed-form expression (20) by replacing the values of (15) and (16) in (18) then we get (20) ...
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