Introduction

The ability to effectively communicate underwater has numerous applications for researchers, marine commercial operators and defense organizations. As electromagnetic waves cannot propagate over long distances in seawater, acoustics provides the most obvious choice of channel to enable underwater communications.

Although acoustics has been used effectively for point-to-point communications in vertical deep water channels, acoustics has had limited success in warm shallow water. Effects such as rapid time-varying multi-path propagation and non-Gaussian noise are two of the major factors that limit acoustic communications in warm shallow water. Multi-path propagation increases the inter-symbol interference (ISI) and causes frequency dependent fading, thus limiting the communication data rates. Rapid time variation in the multi-path structure limit the use of traditional equalization techniques. Most wireless communication techniques are designed for additive white Gaussian noise (AWGN). The application of such techniques in shallow waters, where the noise characteristics are significantly different due to the impulsive noise from snapping shrimp, may not be optimal.

Research Areas

Near-optimal Detection & Decoding in non-Gaussian Noise

Snapping shrimp are the primary contributors of ambient noise at high frequencies in warm shallow waters. Due to the impulsive nature of the noise, standard communication algorithms that make AWGN assumptions do not work well in warm shallow waters. We develop alternate detectors and decoders (such as those based on a p-norm metric) for use in these waters.

Coded differential OFDM

To combat a rapid time-varying channel, we have adopted a differential OFDM based communication scheme. The cyclic prefix in the OFDM helps make it robust to multi-path while the use of differential modulation removes the need to explicitly track the rapidly varying channel. By applying the appropriate coding & interleaving scheme, the time-frequency diversity of the channel can be exploited to give good performance. We are now investigating techniques to reduce the length of the cyclic prefix through partial equalization or passive phase conjugation. We are also developing techniques for Doppler compensation in case of moving platforms, impulse noise cancellation, peak-to-average power ratio reduction and exploiting inferred error correlations for forward error correction.

Dynamic Adaptive Communications

In order to communicate effectively through rapidly varying channel conditions, the modems need to adapt their communication schemes. We explore protocols to dynamically control modulation schemes, frequency bands, scheme parameters (such as number of carriers, prefix length, etc in OFDM) and forward error correction codes as the channel changes.

Underwater Networking

Acoustic bandwidth is a scarce resource underwater. In order to use it efficiently, appropriate media access control (MAC) schemes need to be in place. Our networking research focuses on this with a special interest in MAC schemes that utilize multiple modems (capable of communication in different frequency bands, modulation schemes, etc).

To extend the reach of a network, routing is key. We have demonstrated a small network capable of routing packets underwater. Future research in this area will improve performance of routing schemes and target specific applications for optimal routing.

End-to-end reliability is necessary in many applications. Most protocols used some form of acknowledgments to achieve reliability. Due to the long propagation delays involved in underwater networks, acknowledgment-based protocols perform poorly. We are investigating the use of rate-less codes to reduce or remove the need for explicit acknowledgments while providing end-to-end reliability.

Standardization

Currently there is no standardization in the area of underwater communications. We believe that some degree of standardization would benefit the research community as well as the industry immensely. Along with MIT and WHOI, we have proposed a basic architecture for underwater networking (UNA). Adherence to UNA will allow researchers to easily test various combinations of protocols from different research groups. We are also working with the JANUS initiative to develop a standard that will allow modems to co-exist, advertise their presence and potentially interoperate.

References

Several publications related to the above research are downloadable from the publications page.