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TECHNICAL PAPER PRESENTATION
TOPIC : WIRELESS COMMUNICATION
SUB : UNDERWATER WIRELESS COMMUNICATION
Underwater wireless communication is a flourishing research area in the field of wireless communications. This paper presents the overall framework of the necessity of underwater wireless systems, characteristics of an acoustic channel, hardware and working of acoustic modems, sensor networks and different communication architectures involved in the sensor networks. Applications till date, like oceanographic data collection,AUVs(autonomous underwater vehicles),underwater radio etc.., future challenges like effective transmission of video and audio signals by real time monitoring have been emphasized with a view to overcome the present limitations.
Wireless is a term used to describe the telecommunication in which the electromagnetic waves carry the signal over part or all of the communication path.
The signals that are used to carry digital information through an underwater channel are not radio signals as electromagnetic waves propogate over short distances. Instead acoustic waves are used which can propogate over long distances. Over the past decades, cabled submersibles were used to discover the remains of titanic and hydrothermal vents due to the burden and cost of heavy cables that must be used to establish a high-speed communication between the remote end and the surface. To overcome such impediments, underwater wireless communication has come into existence.
NECESSITY OF UNDERWATER WIRELESS SYSTEM:
Wired underwater is not feasible in all situations as shown below.
· Temporary experiments
· Breaking of wires
· Significant cost for deployment
· Experiment over long distances
UNDERWATER ACOUSTIC CHANNEL
· Severe multipath- 1 to 10msec. for shallow water at upto 1 km range
· Doppler shifts
· Long latencies-speed of sound underwater is approximately 1500m/sec
The modems employ advanced modulation scheme and channel equalization to combat multipaths for improved signal to noise ratio. A high performance error detection and correction coding scheme is employed that reduces the bit error rate to less than 10-7
PARTS OF AN ACOUSTIC MODEM:
· DSP BOARD(Digital Signal Processing Board):
It serves both as signal processing module and a microcontroller.
· AFE BOARD(Analog Front End Board):
It performs signal filtering and amplification functions.
· DC/DC CONVERTER:
It converts a wide range of input voltage to the operating voltage of the system and the transducer.
When no data is being transmitted, the modem stays in sleep mode thereby it periodically wakes
Up to receive possible data being transmitted by far end modem. This results in low power consumption.
DATA TRANSMISSION IN MODEM:
Suppose the bottom modem tries to send data to surface modem, it receives data from its link while it is in sleep mode and then it switches to the transmit mode and begins to transmit. As the surface modem wakes up and detects data from bottom modem, it switches from sleep mode to receive mode.
ADVANCED MODEMS AVAILABLE:
UWM1000 UWM2000 UWM3000 UWM4000
DEPTH(in m) 200 1000 - 6000 RANGE(in m) 300 1500 3000 4000 POWER CON-
1 4 20 -
UNDERWATER ACOUSTIC SENSOR NETWORKS:
Underwater networks consist of a variable number of sensors and vehicles that are deployed to perform collaborative monitoring tasks over a given area. Traditional approach for ocean bottom monitoring is to deploy underwater sensors that record data and then recover the instruments. By this method real time monitoring is not possible, failures occur. This can be overcome by connecting underwater instruments by means of wireless links.
UNDERWATER ACOUSTIC SENSOR NETWORKS COMMUNICATION ARCHITECTURE:
· Sensor nodes are anchored to the bottom of the ocean with deep ocean anchors.
· By means of wireless acoustic links, underwater sensor nodes are interconnected to one or more underwater sinks (UW-sinks).
· UW-sinks are equipped with two acoustic transceivers, horizontal and vertical transceiver. The first is used by the UW-sinks to communicate with the sensor nodes, while the second is used by the UW-sinks to relay data to a surface station.
· Vertical transceivers must be long range transceivers for deep water applications. The surface station is equipped with multiple acoustic transceivers, one for each UW-sink deployed.
· It is also endowed with a long range RF or satellite transmitter to communicate with the onshore sink (OS-sink) or to a surface sink (s-sink).
· Sensors can be connected to sinks by means of direct links or through multi-hop paths. In case of multi-hop paths, as in terrestrial sensor networks , data produced by a sensor is relayed by intermediate sensors until it reaches the UW-sink
· Sensor nodes float at different depths in order to observe a given phenomenon.
· The possible solution to achieve different depths would be to attach each UW-sensor node to a surface buoy, by means of wires.
· Multiple floating buoys may obstruct ships navigating on the surface.
They may also be easily detected and deactivated by enemies in military settings.
Due to the above reasons, each sensor is anchored to the ocean bottom and equipped with a floating buoy that can be inflated by a pump. The buoy pushes the sensor towards the ocean surface. The depth of the sensor can then be regulated by adjusting the length of the wire that connects the sensor to the anchor, by means of an electronically controlled engine that resides on the sensor.
· Acoustic sensor technology has paved way for oceanographic data collection, pollution monitoring, offshore exploration, disaster prevention and tactical surveillance applications.
· Vehicular Applications:
Ø ROV(Remotely Operated Vehicle):
Ø ROV for the inspection of underwater structure.
Ø Low cost AUV for coastal oceanography.
Ø Low cost sensor modules for environmental data collection.
Ø Acoustic navigation technology for multiple AUVs.
· Solar powered AUV:
It is designed to provide better observation and monitoring of complex aquatic system
· Deep Sea Observatory:
The figure above shows the concept of the deep sea observatory. At the core of the system is an underwater cable that hosts a multitude of sensors and instruments and provides high-speed connection to the surface.
· New Solar Underwater Robot Technology:
New solar underwater robot technology has been developed for undersea observation and water monitoring.
Underwater sound propagation is highly affected by environmental and natural factors such as heterogeneities of the water column, variations of sound velocity versus depth, temperature and salinity, multiple and random sea reflections and significant scattering by fish, bubble clouds and plankton.
This paper gives the overall view of the necessity of underwater wireless communication and its applications. Despite much development in this area of the underwater wireless communication, there is still an immense scope so more research as major part of the ocean bottom yet remains unexploded. Advanced versions of the existing applications and innovative inventions have become a must in this field. Therefore, the main objective is to imbibe knowledge about this emerging field and thereby encourage research and implementation of advanced technology to overcome the present limitations such as the environmental effects on the noise performance of acoustic systems as mentioned in this paper.
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