Feature ArticleWideband Acoustic Communication For Short Ranges, Deep Waters, High Speeds
By Hiroshi Ochi
Japan Agency for Marine-Earth Science and Technology
At the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), researchers have been studying underwater acoustic communications over the past two decades, during which two practical image transmission systems have been developed and are now operational. One is the manned submersible SHINKAI 6500's communication system, which transmits color still images from the submersible to a surface mother ship. This one-way transmission system's range is 7,000 meters maximum, with a transmission rate of 16 kilobits per second. The other is a full-duplex system for the autonomous underwater vehicle (AUV) URASHIMA, which transmits color still images from the AUV to a surface ship. Its transmission range is 3,700 meters maximum with a transmission rate of 32 kilobits per second.
In the past several years, JAMSTEC scientists have been studying short-range, high-speed acoustic communications as a solution for large-capacity data communications such as the acoustic remote control of underwater vehicles or a bottom-deployed seismometer. It is assumed that the range is 500 meters to 1,000 meters, and the bit rate is more than 80 kilobits per seconds.
In January 2010, JAMSTEC carried out an ocean experiment at near-bottom in about 1,600 meters' depth using quadrature phase-shift keying (QPSK) and the eight-phase shift keying (8PSK) modulation method. By using these modulation methods, a transmission rate between 80 kilobits and 100 kilobits per second was achieved.
The four-channel adaptive decision feedback equalizer (DFE) with phase compensator is applied to the receiver. The feed forward block is driven in every TS per two seconds, where TS is the symbol interval (1/4,000 seconds). The feedback block is driven in every TS seconds. A least mean square errors algorithm is applied for an adaptive algorithm.
The four-hydrophone array on the receiver is spaced at 20l of the carrier frequency.
Transducer and Hydrophone
A tilted toroidal beam wideband transducer and hydrophones were used in the experiment, with the main beam of the transducer and the hydrophone facing each other. The main beam is approximately 30° wide.
By controlling the signal phase, the toroidal beam is tilted approximately 20° above the horizontal. Transmitted voltage responses of 60, 80 and 100 kilohertz are approximately 118, 121 and 124 decibels, respectively, at the main lobe direction.
JAMSTEC also developed a hydrophone with open-circuit voltage sensitivities of 60, 80 and 100 kilohertz, at -175.5, -175.5 and -172.1 decibels, respectively, for the main lobe direction. This hydrophone is suspended from the surface ship so that its main lobe is directed to approximately 20° below the horizontal. The receiver is made up of four hydrophones in a square array that measures 375 square millimeters, which is 20λ of the 80-kilohertz carrier frequency that was used.
Transmitter and Receiver
In this experiment, the carrier frequency was 80 kilohertz, and the bandwidth was 40 kilohertz. The transmitter and the receiver were driven by a lithium-ion battery and triggered by a trigger pulse generated by a timer unit. The data being transmitted were from two fixed-color 320-by-240-pixel JPEG images that were 60,080 and 46,600 bits.
To train the adaptive filter, preamble data were transmitted before image data, which is composed of 13 digits of Barker code and 1,024 digits of random code. The length of preamble code was approximately 26 milliseconds. Transmitted data were preinstalled in the hard disk drive on the transmitter as modulated binary data.
The arrangement of the experiment.
At the receiver, each channel of the acoustic signals was preamplified and band-pass filtered, followed by being digitized by an analog-to-digital converter at 800 kilohertz sampling frequency. This was then recorded to the hard disk drive in each channel. The demodulation was processed after recovering the receiver on the vessel by the software, which is composed of the four-channel adaptive DFE.
The experiment was carried out on January 19, 2010, at Suruga Bay in Japan, which is approximately 120 kilometers southwest of Tokyo. The transmitter was moored near the bottom at a height of approximately eight meters, and the receiver was suspended from a surface vessel by an armored coaxial cable. The angle between the transmitter and the receiver was maintained at around 20°, and the distance was between 560 meters to 940 meters.
To acquire the position of the receiver, JAMSTEC used an acoustic navigation system (acoustic transponder system) and an ATD-HR conductivity, temperature, depth sensor made by JFE Advantech Co. Ltd. (Kobe, Japan). The relative receiver position was moved along the 20' transducer beam angle, meaning directivity of the transducer and hydrophones was not necessary to consider. Channel responses were measured by chirp pulse. Bottom-reflected pulses were observed at 1.5 milliseconds delayed after direct pulses.
Symbol error rate versus slant range characteristics. Lines are weighted averages. .
Using QPSK-modulated signals, error-free transmissions were carried out up to slant range of 840 meters. Average input signal-to-noise ratio (SNR) was 9.5 decibels at 840 meters and 10.9 decibels at 780 meters. In the case of a slant range of 780 meters, the output SNR was 15.1 decibels with error-free transmission. In the case of 940 meters of transmission distance, the output SNR was 12.6 decibels and two symbols error occurred. The average input SNR was 8.6 decibels. Using QPSK modulation, one color image can be transmitted in approximately 800 milliseconds.
Using 8PSK modulated signals, error-free transmissions were carried out up to a slant range of 680 meters. The average input SNR of four hydrophones was 16.4 decibels, and the output SNR was 19.9 decibels. At the distance of 780 meters, average input SNR went down to less than 12 decibels and more than 10 symbol errors occurred. Using 8PSK modulation, one color image can be transmitted in approximately 550 milliseconds.
To conclude the demodulation results, using QPSK at 940 meters with 27 packets caused a single error in four packets and two symbol errors in two packets. In 8PSK, error-free transmission was achieved at less than 620 meters of slant range. At 650, 680 and 710 meters, a single error occurred in only a small number of packets. But at longer than 730 meters of slant range, errors increased greatly, with more than 10 symbol errors occurring in a number of packets.
A high-speed short-range underwater acoustic transmission experiment was carried out at the depth of approximately 1,600 meters using 80 kilohertz as the carrier frequency. QPSK and 8PSK were applied for modulation method. The bit rates of each modulation were 80 kilobits and 120 kilobits per second.
Error-free transmission was carried out at the distance of 840 meters using QPSK and 620 meters using 8PSK. In this experiment, transmission between a moored (almost fixed) transmitter and a receiver suspended from a surface ship were carried out. Doppler effects, caused by ship heaving and drifting, were well compensated.
The next step in research is to estimate the Doppler compensation performance in case a larger Doppler shift occurred.
Hiroshi Ochi is a group leader of the underwater acoustic technology group at the Japan Agency for Marine-Earth Science and Technology. He received a Ph.D. from the University of Electro-Communications in 2009. His research interest is high-speed communication with autonomous underwater vehicles or sensors.
Takuya Shimura joined the Japan Agency for Marine-Earth Science and Technology in 1995, and he received a Ph.D. in ocean engineering from the University of Tokyo in 2009. He has been researching the application of time reversal to long-range communication with autonomous underwater vehicles.
Yoshitaka Watanabe is an engineering researcher in the underwater acoustic technology group at Japan Agency for Marine-Earth Science and Technology Center. He received a Ph.D. from the Tokyo University of Marine Science and Technology in 2009. His current research covers underwater acoustic positioning, communication and navigation of underwater vehicles.