VOLUME 3, JULY 2010
UNDERWATER ACOUSTIC COMMUNICATION: ITS CHALLENGES AND RESEARCH OPPORTUNITIES
By: Mohd Faizal Abd Rahman
In recent years, underwater communication has become an active research area as there is still a big gap between the communication technology for terrestrial and underwater application [1-4]. Researchers and scientists have continually put a massive effort exploring the underwater world. The advancement in underwater technology helps human to understand better about a place that has totally different environment in terms of its nature, creatures, composition and physics. The growing research for underwater application has drawn interests of many sectors and industries around the world; government- based or private sector. Among the sectors that benefited much from the advancement of this technology are military, oil and gas industries, fisheries, underwater instrumentation companies, research agency etc. Works such as seismic monitoring, underwater robot operation, underwater surveillance and detection, sea exploration, ocean mapping and research data collection are also getting easier due to this advancement [1-4].
Communication is the most important process in underwater technology. The process enables the data transfer between two or more groups/entities. These data are used for navigation, tactical strategies, monitoring, identification etc. Communication can be established either by wired or wireless connection. Both methods have their own advantages and disadvantages, depending on the application. Current trend has opted wireless communication as the preferred way especially when it comes to deal with the depth that wired connection is not practical or impossible.
SOURCES OF PROBLEMS
As for terrestrial application, the underwater wireless communication is not a straight forward process. When considering the underwater communication process, the primary concern that researchers always consider are the channel model(underwater), attenuation, transmission distance, power consumption, SNR ratio, bit error, symbol interference, error coding, modulation strategies, instrumentation and underwater interferences. Dealing with interferences for underwater research is a complex task due to dynamic nature of water. Interferences are mainly caused by three major factors:
1 Characteristics of signal carrier
In underwater world, there are 3 types of carrier wave that are most commonly used in wireless communication [1-4].
i. Electromagnetic wave
Using electromagnetic wave, the communication can be established at higher frequency and bandwidth. The limitation is due to high absorption/attenuation that has significant effect on the transmitted signal. Big antenna also needed for this type of communication, thus affects the design complexity and cost.
ii. Optical wave
Optical wave also offers high data rate transmission. Nevertheless, the signal is rapidly absorbed in water and suffers from scattering effect [1-4]. This will affect the data transmission accuracy.
iii. Acoustic wave
Acoustic is the most preferred signal used as carrier by many application, owing to its low absorption characteristic for underwater communication. Even though the data transmission is slower compared to other carrier signal, the low absorption characteristic enables the carrier to travel at longer range as less absorption faced by the carrier.
2 Environment/Propagation Medium
Unlike the communication in terrestrial application, for underwater wave propagation, the challenges are quite different. Water itself has become the main source for the signal interference. The type of water (freshwater/sea water), depth pressure, dissolved impurities, water composition and temperature affect the sound propagation. Common terrestrial phenomena like scattering, reflection, refraction also occurs underwater communication.
3 Instrumentation System Devices.
In ensuring the effective underwater communication, the communication system design plays a vital role. Factors such as transducer parameters (sensitivity, power consumption, noise immunity, transduction mechanism, directivity, resolution and properly matched impedance must be taken into account during the design process. One of the important areas that worth focusing on is the receiver (sensor) design. Nowadays, with the advancement in electronic technology, the transducer design (especially receiver) can adopt MEMS technology to overcome several sensor issues that proves to have several advantages compared to the conventional approach . It is found to have many advantages compared to the conventional design.
RECENT PROGRESS: INSTRUMENTATION ASPECTS
Although the complete system for underwater communication has been around throughout various applications, nevertheless, research in this field is still going on due to the diversity of its research area. Research in this field can be said as an ongoing research as technology advancement in instrumentation element such as sensor, transceiver, signal processing unit and communication modem has always changed rapidly. Researchers are competing in developing a better system with better performance.
One example is an effort carried out by a group of researchers from Micro and Nanotechnology lab, University of Illinois. They have come out with a miniature acoustic communication system prototype with the suitability to be used underwater. Its operation has been tested in two different applications to verify its functionality .
In instrumentation system point of view, power consumption becomes one of the main concerns when developing a complete system. A system with low power consumption is admirable and leads to a cost and energy efficient system. Several important parameters should be considered in order to minimise the power consumption. Nejah et al  proposed the optimal parameter to overcome this situation. Right selection of carrier frequency is one of the approaches that can be utilised to optimize the power consumption. The paper also discussed the relationship between several important parameters such as attenuation, distance, frequencies and transmission loss.
In today’s world, many electronic devices are designed to be small. In most cases, small devices offer several advantages over the conventional approach. In electronic industries, the manufacturers are competing to produce a device with better performance and in smaller size. This scenario has alleviated the IC and MEMS technology to grow faster in electronic industries. Until now, most of the application that utilize this approach mainly focuses in imaging industries owing to the fact that this type of sensor can offer a high bandwidth and sensitivity .
However, the realisation of MEMS in underwater communication especially in sensor design could be a worth effort to bridge the gap between the terrestrial and underwater communication system. The significance of this effort has been reviewed in .
Reduction in size has offered a lot of advantages in terms of it s power consumption, portability, production and cost. Even though the exploration of this approach in underwater communication is still new, the possibilities of this new approach to be implemented is technically possible. The main contribution is due to its ability to overcome the problem caused by the size and power consumption. In Autonomous Underwater Vehicles (AUV) for example, the utilisation of MEMS device with smaller battery will reduce the overall weight of the AUV, thus reducing the power needed to drive the vehicles. Many researchers in underwater communication system are mainly concentrating on other aspect such as overall system development, communication protocol, signal processing and conventional transducers. Therefore MEMS based sensor for underwater communication provide a new platform for researchers to explore more of what this technology can offer and it can be thought of as a new research area that require an extensive studies and could contribute to many novel outcome.
Until now, most of the application that utilize this approach mainly focuses in imaging industries owing to the fact that this type of sensor can offer a high bandwidth and sensitivity .
Aside from imaging, MEMS based sensor is also thought be worthwhile if it can also be implemented for communication purposes, especifically for underwater.
 M.Stojanovic.,”Acoustic (underwater) communications,” in Encyclopedia of Telecommunications, Ed. John Wiley and Sons, 2003
 Liu,L et al. (2008). Prospects and Problems of Wireless Communication for Underwater Sensor Networks, Invited Paper Wiley WCMC, pp 977-994
 I.F.Akyildiz,D.Pompili,T.Melodia (2005). Underwater Acoustic Sensor Networks:Research Challenges. Ad Hoc Networks Vol 3, 257-279,
 J. Heidemann, W. Ye, J. Wills, A. Syed, Y. Li, Research challenges and applications for underwater sensor networking, in: Proceedings of the IEEE Wireless Communications and Networking Conference, IEEE, Las Vegas, NV, USA, Vol 1,2006, pp. 228-235
 Nasri,N.,Kachouri,A,L.M.Samet,(2008) Design Considerations For Wireless Underwater Communication Transceiver, ICSCS08,1-5
 Pandya, S., Engel, J., Chen, J., Fan, Z., and Liu, C. CORAL: Miniature acoustic communication subsystem architecture for underwater wireless sensor networks. 4th IEEE Conference on Sensors (Irvine, CA, Oct. 2005). Proceedings of the 4th IEEE Conference on Sensors (2005).
 Arshad,M.A.,(2009) ,Recent Advancement in Sensor Technology for Underwater Application. Vol 38,IJMS09. January 2009, 267-273.
 O.Oralken et all.,Underwater Acoustic Imaging using Capacitive Micromachined Ultrasonic Transducer Arrays. OCEANS02, Vol.4,2354-2360, 2002
 J.Chen et al.,A Capacitive Micromachined Ultrasonic Transducer Array for Minimally Invasive Medical Diagnosis, Journal of MEMS Systems, Vol.17,2008