Progress in underwater acoustic telemetry since 1982 is reviewed within a framework of six current research areas: (1) underwater channel physics, channel simulations, and measurements; (2) receiver structures; (3) diversity exploitation; (4) error control coding; (5) networked systems; and (6) alternative modulation strategies. Advances in each of these areas as well as perspectives on the future challenges facing them are presented. A primary thesis of this paper is that increased integration of high-fidelity channel models into ongoing underwater telemetry research is needed if the performance envelope (defined in terms of range, rate, and channel complexity) of underwater modems is to expand.

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The state of the art in underwater acoustic telemetry

The need for underwater wireless communications exists in off-shore oil industry, environmental monitoring, speech transmission between divers, control of autonomous underwater vehicles, and mapping of the ocean floor for detection of objects and discovery of new resources. Wireless underwater communications can be established by transmission of acoustic waves. However, the communication channels have limited bandwidth, and often cause severe multipath dispersion and time-variability. Despite these limitations, research efforts of the 90's have culminated in the development of underwater acoustic modems that are capable of transmitting information at rates on the order of several kilobits per second systems over varying distances. The emerging scenario is that of an underwater communication network consisting of both stationary and mobile nodes. Current research is focusing on the design of efficient modulation and coding schemes, adaptive signal processing algorithms for equalization and diversity combining, multiple-access communication methods and network protocols suited for low bandwidth, long propagation delays and strict power requirements encountered in the underwater environment.


Keywords:

underwater communications;
acoustic communications;
bandwidt-efficient modulation;
adaptive equalization;
diversity combining;
reduced-complexity receivers;
phase-coherent detection;
channel estimation;
multipath propagation;
multiuser detection;
underwater networks

Acoustic (Underwater) Communications

Underwater wireless communications over distances in excess of about 100 m are established using acoustic signals. Acoustic signals propagate as pressure waves, whose energy absorption limits the available bandwidth. As a result, existing technology provides bit rates on the order of several kilobits per second for transmission over distances on the order of several kilometers. Additional challenges are presented by multipath propagation that causes frequency selectivity, random time variation, and Doppler effects that occur due to low speed of sound (1500 m/s). This article overviews the development of acoustic modem technology, which evolved over the past several decades from noncoherent modulation/detection techniques to bandwidth-efficient phase-coherent modulation/detection. A survey of techniques for channel equalization in single-carrier systems as well as recent advances in multicarrier acoustic communications is also presented.


Keywords:

Acoustic;
communications;
coherent;
equalization;
channel estimation;
phase synchronizations;
multipath;
Doppler;
sparse channels;
diversity combining;
beamforming;
multiuser detection;
interference suppression;
time reversal;
multi-input–multi-output (MIMO) processing;
multicarrier modulation;
OFDM;
adaptive modulation;
underwater networks

Underwater Acoustic Communication

An underwater apparatus for transmitting and receiving high rate data and voice communication including a transmitter, a receiver, and a Doppler frequency shift compensator. The transmitter includes a data source comprising digital data to be transmitted by the apparatus through the water, a serial-to-parallel data processor for splitting the serial digital data into n parallel data channels, a n-channel modulator for receiving the n parallel data channels and for modulating those channels with n pairs of ultrasonic carriers to produce a modulated signal. The transmitter also includes a hydrophone for receiving and transmitting the modulated signal. The receiver includes a hydrophone for receiving a modulated signal. The receiver also includes RF circuitry for amplifying and shaping the received modulated signal, a serial-to-parallel data processor for splitting the amplified and shaped data into n parallel channels, a n-channel demodulator for demodulating the shaped signal and for outputting n channels of digital data, and a parallel-to-serial data processor for receiving n parallel channels of data from the demodulator and for combining those channels into serial data. The Doppler compensator measures the frequency of at least one of two unmodulated signals transmitted as part of the modulated signal and compares the measured frequency with a predetermined frequency. Operation of the demodulator is adjusted to compensate for any deviation between the measured and predetermined frequency.

Method and apparatus for carrying out high data rate and voice underwater communication

Underwater acoustic communications are a rapidly growing field of research and engineering, driven by the expansion of applications which require underwater data transmission without wired connections. In this chapter, we explore the problems of underwater acoustic communications in three parts. The first part presents an overview of modern applications in underwater data transmission and today’s achievements in this area. System requirements are reviewed, and propagation characteristics of underwater acoustic channels are given. It is shown that the majority of underwater acoustic channels are severely band-limited, with signal distortions depending on the link configuration, and ranging from benign to extreme ones caused by time-varying multipath propagation and signal phase variations. Examples of existing systems are given, with emphasis on the methods used for intersymbol interference mitigation. Most of these systems use noncoherent or a differentially coherent signal modulation and detection methods. Phase-coherent detection, which offers better efficiency in bandwidth utilization, is the subject of the second part of this chapter. In this part, the design of high-speed digital communication systems, which rely on powerful equalization and multiple sensor signal processing methods is treated.

High-Speed Underwater Acoustic Communications

Parametric transduction offers valuable advantages for underwater acoustic communications. Perhaps the most significant benefit is the fact that high directivity is achieved by means of a physically small transmit transducer. This feature may, ultimately, be employed to permit long-range, low-frequency communication using a compact source. The high directivity is desirable to combat multipath propagation and to achieve data communications in water which is shallow by comparison with range. A real-time, high data-rate "model" differential phase shift keying (DPSK) communication system has been constructed and demonstrated. This system uses parametric transduction, with a 300-kHz primary frequency and a 50-kHz secondary frequency. Experimental results show that the system can be employed to combat multipath propagation in shallow water and can achieve high data-rate text and color image transmission at 10 and 20 kb s/sup -1/ for 2-DPSK and 4-DPSK, respectively, through a transmission bandwidth of 10 kHz. The "model" system was developed to confirm performance predictions for a future, operational long-range link employing a 50-kHz primary frequency and a 5-kHz secondary frequency.

Liansheng Wang

Papers

"BASS 300 PARACOM": a "model" underwater parametric communication system