IEEE Consumer Electronics Magazine - March/April 2020 - 111

Table 2. Summary of main challenges toward implementable IoUT as a humanitarian technology.
Task

Description

Cost

Maintenance requirements

Performance
Quality of
sensing
coverage and
network
connectivity.

UWSN
deployment

Ship missions happen for the deployment
of underwater sensor nodes, which can be
deployed randomly or at specific
locations.

Monetary

Periodic when nodes move
involuntarily, as a consequence of
water currents, to outside the area
of interest.

Data
routing

Protocols for cooperative multihop data
transmission for the delivery of collected
data to surface sonobuoys.

Energy

Periodic route determination and
establishment.

Data delivery
rate and end-toend delay.

Dutycycling

Each node periodically alternates its state
between awake and sleep modes.

Energy

Periodic duty-cycling schedule
exchange among nodes.

Network lifetime
and data
delivery rate.

Data
aggregation

Protocols for overlapping data routes at
some nodes, which will fuse the received
packets to reduce the amount of data to be
transmitted.

Energy

Frequent battery recharge or
replacement of aggregate nodes
due to overuse.

Network lifetime
and quality of
information.

sensor nodes are subject to involuntary mobility
due to ocean currents. When acoustic links are
used for underwater wireless communication,
the network connectivity will be impaired by frequent changes in link quality, multipath propagation, and shadowing zones.
When optical links are used for underwater
wireless communication, the connectivity will
be affected by the limited communication range
(10-100 m) of this technology, misalignment
between the optical transceivers, and water turbidity. Moreover, the communication between
two nodes will happen only if the receiver is
within a certain beam scanning angle around the
transmission trajectory of the sender.
Saeed et al.12 developed a stochastic model
to analyze the network connectivity of a UWSN
using optical links. They used the scaled sector
graphs to model the communication links
between nodes when the receiver is within the
beamwidth of the transmitter. The results
showed that the number of nodes, communication range, and width of the scanning angles are
key parameters that affect the UWSN connectivity when optical links are considered.
In acoustic-based IoUT, topology control techniques can be employed for improving network
connectivity.6 For instance, mobility assisted
topology control algorithms can move underwater nodes located in shadow zones to new depths,
or determine the optimal trajectory of AUVs
aimed to restore the network connectivity.

March/April 2020

Network Deployment
The low-density deployments of underwater
sensor nodes and their uncontrolled mobility
due to the ocean's current can affect the performance and lifetime of a UWSN. This fact demands
the proposal of redeployment strategies to control the UWSN density and the design of distributed algorithms for the nodes' organization.
The controlled depth adjustment of underwater sensor nodes or the use of AUVs with controlled trajectories can be used to guarantee
network connectivity and a suitable coverage in
the area of interest. Otherwise, the prompt alert
of earthquakes and critical events from other
UWSN-based humanitarian applications might
not be detected, and catastrophic consequences
might occur due to the lack of early information
about the event.
Data Routing
Data routing in underwater sensor networks
continues to be a challenge. Overall, geographic
routing is presented as the most suitable paradigm for routing in IoUT, as the only overhead
for this approach is the broadcast of beacon
messages for the discovery of one-hop neighboring nodes. Moreover, since it can be done prior
to the data routing, there will be no additional
delay for the application. However, it does not
deal with the low reliability of the acoustic links.
In addition, it suffers from the communication
void region problem, which happens whenever a

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IEEE Consumer Electronics Magazine - March/April 2020

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