![](bg3.png)
For pure seawater type on upside graph of Figure
5, the absorption is dominated almost by the
attenuation of seawater molecule, the region of ideal
wavelength with lowest attenuation for visible-light
propagation is within the blue-green band between
400nm and 500nm. In case of high-turbidity bay on
bottom of Figure5, the total absorption in visible-
light band is dominated by the combination of
organic and inorganic particles, the ideal
transmission wavelength is shifted from blue-green
wave band towards green-yellow band around 550-
600nm.
On the other hand, the spatiotemporal change of
seawater color (Figure 6(a)) is also an intrinsic noise
which can affect light propagation. And the marine
snow (Figure 6(b)) in deep sea is an external noise
which able disrupts the optical link because it is a
visually observable enough large particulate organic
materials.
(a) (b)
Figure 6: Noises in underwater optical channel: (a)
spatiotemporal change of seawater color, it is an intrinsic
noise (picture by Johnson, The Univ. of Warwick) and (b)
marine snow in Sagami bay of Japan (picture by Kitamura,
2006).
2.2 Underwater Link Configuration
Underwater link configuration between sensor nodes,
and what is the difference between underwater links
and terrestrial links in a VLC system also should be
considered. Typical underwater diffuseness link
configurations between a transmitter and a receiver
are tabulated in Table 1 (Johnson et al., 2014).
Table 1: Typical diffuseness links for UOWC.
LOS diffuseness
retroreflector
Line of sight (LOS) refers to the ability to see the
transmitter from the receiver. The LOS link is the
simplest type which a direct link between the
transmitter (Tx) and receiver (Rx). Non-LOS link
uses reflections from the sea surface to overcome
underwater obstacles. LOS retroreflector link is
useful when bidirectional communication is
required, but the receiver is too low power to
support a full transceiver for underwater sensor
nodes. Theoretically, all these configurations have
enough good bit error rate (BER) and are viable for
short-range of under 15m underwater data
transmission (Arnon, 2010).
For turbid nature seawater, the attenuation of
light propagation is dominated by scattering, a wide
field-of-view (FOV) angle of transmitter can
compensate for the increased attenuation. Incoherent
visible-light LED (Light Emitting Diode) is most
suitable for this diffuseness-link application.
2.3 Constructing an Adaptive UOWSN
Such as a WSN in terrestrial environment, a WSN in
marine environment also has different types to favor
different applications. Figure 7 shows link
topologies of different UWSN types.
Figure 7: The link topologies of different UWSN type.
In order to construct a UOWSN which is viable
to work in a seafloor observatory (see Figure 2), the
mesh-type WSN topology is adopted to multipoint
simultaneously measure and transmit data from
different detectors and share these data each other
without human operations. The wavelength-
adaptation control technique (Lin, 2017) is used to
help overcome the “spectrum-intensity attenuation”
which shown in Figure 5 in different seawater types.
The links between sensor nodes are “LOS
diffuseness” type configuration because it is easier
to implement and most energy efficient for UOWC.
The UOWSN with mesh type that is proposed in
this work is shown in Figure 8. It consists of one
gateway sensor node (i.e. main node) and multiple
sensor nodes (i.e. sub nodes). Each node is
connected to other nodes to make a mesh; such,
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