Performance characterization of two-way multi-hop
underwater networks in turbulent channels [Invited]
Fangyuan Xing (邢方圆), Hongxi Yin (殷洪玺)*, and Lianyou Jing (景连友)
School of Information and Communication Engineering, Dali an University of Technology, Dalian 116023, China
*Corresponding author: hxyin@dlut.edu.cn
Received August 17, 2019; accepted September 10, 2019; posted online September 29, 2019
This Letter investigates the performance of the two-way multi-hop system for underwater optical wireless com-
munications. With the decode-and-forward (DF) relaying, the two-way multi-hop system is modeled, where the
effects of absorption, scattering, and oceanic turbulence are all taken into account. An exact closed-form ex-
pression for outage probability is derived under the assumption that the oceanic turbulence obeys a log-normal
distribution. Numerical results demonstrate the impacts of various parameters on the outage performance and
indicate that the two-way multi-hop system significantly improves the performance in comparison to both the
one-way multi-hop system and the two-way two-hop system.
OCIS codes: 010.4455, 010.7060, 060.2605.
doi: 10.3788/COL201917.100005.
Underwater optical wireless communication (UOWC) has
attracted widespread attention since it possesses the
advantages of large transmission bandwidth, high data
rate, ultralow propagation delay, and high security
[1–3]
.
The channel and system modeling for UOWC has been
amply studied. The channel loss can be characterized
by the Beer–Lambert law
[3]
and beam spread function
[4]
,
whereas the oceanic turbulence can be typically modeled
as log-normal
[5,6]
, gamma-gamma
[7]
, and exponential gener-
alized gamma distributions
[8]
. However, the UOWC suffers
from several challenges, such as limited communication
range and large path loss. Relay-assisted communication s
can be employed to overcome these shortcomings.
Several notable researches have been explored to analyze
the performance on one-way (unidirection) relay-assisted
systems for the UOWC. The performance of multi-hop
networks over log-normal turbulent channels
[5,6]
and
gamma-gamma turbulent channels
[7]
has been evaluated.
The performance of amplify-and-forward (AF) relaying and
decode-and-forward (DF) relaying for underwater net-
works has been studied and compared
[9]
. The connectivity
of the multi-hop underwater network and its impact on
localization are analyzed
[10]
, where the network is modeled
as randomly scaled sector graphs. These relay-assisted sys-
tems commonly operate in half-duplex mode, since the
severe interference between transmitted signals and incom-
ing weak received signals in full-duplex mode cannot be
omitted and is difficult to be eliminated
[11,12]
. For one-way
half-duplex relay systems, two terminals assisted by a single
relay require four time slots to accomplish information ex-
change, which may lead to the loss of spectral efficiency.
To reduce such loss, two-way relay schemes can be adopted,
where two terminals only require two time slots to exchange
their information and achieve full-rate transmission.
There have been sporadic studies on two-way relay sys-
tems for wireless optical communications. An orthogonalize-
and-forward relaying scheme for two-way multi-user
scenarios is presented
[12]
, which can significantly decrease
the spectral loss. The performance of the two-way system
for a mixed radio frequency/free space optical (RF/FSO)
network is studied
[13]
. For two-way free space optical
(FSO) networks, a closed-form expression of system out-
age and error probability is derived over gamma-gamma
atmospheric turbulence
[14,15]
, whereas an analytic expres-
sion of end-to-end signal-to-noise ratio (SNR) is solved
over Malaga atmospheric turbulence
[16]
. Three algorithms
of multi-user scheduling applied to two-way networks are
proposed
[17]
, including absolute SNR, normalized SNR,
and selective multi-user diversity scheduling algorithms.
The abovementioned studies focus on the two-way relay
systems for FSO networks over atmospheric channels.
These achievements cannot be directly applied in the
UOWC, since the underwater channel suffers from the neg-
ative effects induced by absorption and scattering in addi-
tion to turbulence. Moreover, most existing studies only
consider the two-way system with single relay and two
hops. As the transmission range is extremely limited for
the UOWC, serial relay systems with multiple hops may
be more practical. Consequently, it is necessary to explore
the characteristics of two-way multi-hop systems, where
two terminals assisted by multiple relays exchange informa-
tion concurrently. Such systems can improve the spectral
efficiency and extend the end-to-end transmission distance.
In this Letter, we investigate the performance of the
two-way multi-hop system for UOWC. Specifically, the
two-way multi-hop system based on DF relaying is mod-
eled, with consideration of the effects of absorption, scat-
tering, and oceanic turbulence. A closed-form expression
for outage probability is derived over log-normal oceanic
turbulence channels. Numerical results demonstrate the
effects of various parameters, including the number and
location of relays, the type of ocean water, the type of light
wave (plane or spherical wave), and the power allocation
on the system performance.
COL 17(10), 100005(2019) CHINESE OPTICS LETTERS October 2019
1671-7694/2019/100005(6) 100005-1 © 2019 Chinese Optics Letters