Background

The FAF-06 sea trial was conducted 16 June to 11 July 2006 as part of NURC Project 4G3. The sea trial involved the research vessels Leonardo & Alliance operating in the Tirrenean & Ligurian Seas. Building upon successful past collaborations in the FAF sea trial series, this year’s experiments validated new, environmentally adaptive methods of increasing the bandwidth content of the underwater communications channel. The success of the experiments was in large part due to the professional capacity and flexibility of the Centre's engineering staff and the crews of Leonardo and Alliance. The sea trial was a collaborative effort between NURC, the Marine Physical Laboratory of Scripps Institution of Oceanography, Massachusetts Institute of Technology, the US Naval Research Laboratory, and the University of Pisa. The objectives of this sea trial, which had a significant component of external participation, are delineated in guidance given in the NURC Director’s memo to ONR, D:SER:ku Ser:197 dtd 7 Oct 2004. One importanat objective of the FAF-06 trial was to conduct an experimental study of environmentally adaptive underwater acoustics to not only develop a sufficient understanding the physics involved but also to take a step toward applications in underwater digital communications, particularly MIMO. This was the eighth Focused Acoustic Field (FAF) experiment. The previous seven experiments have been extensively reported in the literature by the FAF team and have even received coverage in the popular press, including articles in Physics Today and Scientific American. Under support provided by NATO and ONR, the team spearheaded a progression of at-sea experiments that brought the concept of underwater, acoustic time-reversal (TR) focusing in the sea from an unverified hypothesis to reality. When the experiments began, it was completely unknown whether the time-varying nature of the oceanic environment would support sufficient temporal stability for the method to succeed, even at long (> 3 m) acoustic wavelengths. A progression of experiments showed that not only was this possible, but that various applications could be implemented using higher frequency sound (up to 18 kHz), including phase coherent digital communications. Our most recent experiment, FAF-06, investigated multi-user applications for passive TR communications using small transmit and receive arrays. We demonstrated throughput of 10 kbps at several km range in a shallow-water propagation channel where the transfer function had an ISI span of several hundred symbols. The first two FAF experiments (1996 and 1997) used a 450 Hz source receive array (SRA) which was hard-wired to Formiche Island. These experiments were the first to implement and demonstrate the TRM process in the ocean. At 450 Hz, focal distance out to 30 km in approximately 100 m water was demonstrated, the multi-day stability of the focal region was demonstrated, and a new process to shift the focal range was derived and experimentally confirmed. The third and forth experiments, conducted in 1999 and 2000, used a 3500 kHz source-receive array (SRA). The SRA was moored and tethered to a remote, self-contained buoy system with all electronics/computers so that it functioned as a node on a local area network (LAN). However, the probe source was not fixed to the VRA so that field measurements were often not obtained at the precise focal position. Focal ranges out to about 15 km were demonstrated (the maximum LAN range at that time). Data in the Formiche and Elba areas with different bottom types were taken and markedly different dispersion characteristics were observed. The data indicated that for both the 450 and 3500 Hz experiments the diffraction limit on the focal size was achieved. In addition to our investigations at 450 and 3500 Hz, a 29-element source array at 850 Hz also has been deployed in an offboard, radio telemetered configuration where time reversal focusing and reverberation nulling were demonstrated. During these early experiments, sending short communication sequences indicated the potential for communication utility, though in this first communication exploration the quantity of data was insufficient to perform statistical bit error analysis. However, underwater communications schemes tested using active time reversal suggested extremely low bit error rates were possible. The work in FAF-06 built upon these early discoveries to push the technology to new levels. The objective of the FAF-06 trial was to conduct an experimental study in environmentally adaptive underwater acoustics to further develop a sufficient understanding the physics involved to pursue applications in underwater digital communications and cooperative fixed-mobile sensor platform operations. The FAF-06 experiments were intended to lay the groundwork for the design of a prototype system for a 2007 demonstration of an underwater wireless covert communications capability for an undersea, distributed sensor network within the larger context of the Centre’s 4G programme. The emphasis throughout the FAF sea trial series has been on making fundamental discoveries in underwater research that would not be possible at other research institutions. Throughout the historical collaboration on time-reversal acoustics at the Centre, the sophistication and complexity of the experiments have incrementally increased. Success is possible through (a) strong collaboration and (b) seagoing scientific expertise. It has been our experience that NURC is the only organization in the world where these two components seamlessly come together.

Baseline Time Reversal Mirror (TRM) Experiment

The schematic in Fig. 1 illustrates the components of a TRM experiment. A probe source (PS), indicated by one of the rectangles on the vertical receive array (VRA), sends out a pulse that is received at the source-receive array (SRA). The dispersed signal with all its multipath structure is time reversed and retransmitted by the SRA. The resulting signal multipath structure collapses to a spatial and temporal focus (original PS pulse length) at the original PS position that is co-located in range with the VRA. Fig. 1 Configuration for reciprocal focus

FAF-06 Overview

The objective of the FAF-06 trial was to conduct an experimental study in environmentally adaptive underwater acoustics to develop a sufficient understanding the physics involved to pursue applications in underwater digital communications and cooperative fixed-Mobile sensor platform operations in collaboration. The experiment is a joint collaboration of NURC with the Marine Physical Laboratory of Scripps Institution of Oceanography, University of California, San Diego. The experiments were designed to collect high quality data that will lay the groundwork for the future systems of an underwater wireless communications capability for an undersea, distributed sensor network. A new industry standard, IEEE 802.11n, is addressing MIMO applications in radio telemetry through air for consumer applications in high multipath environments like cities and inside buildings. During the sea trial, we investigated the underwater counterpart to this technology that uses digital underwater communications, particularly the MIMO concept. The envisioned transition product is a point-to-point communications demonstration between two undersea nodes. A node could include a modem operating on a moving AUV. This intent stems from: (a) a common recognition for needed improvement in the field of networked, full-duplex, clandestine, high-speed, phase-coherent underwater communications between AUVs and fixed nodes; and (b) an understanding that the sources and subsequent data associated with the above goal can aid in fully utilizing the undersea environment. To this end, the research effort will therefore investigate how transmitted communications sequences can simultaneously characterize the acoustic environment. This experiment was a cooperative effort between the scientific, engineering and ship staff of NURC and scientists and engineers of MPL. From the first experiment in this series 10 years ago, our cooperative efforts have been extremely productive. However, in this experiment, we have now reached a seamless combination that, in the opinion of the senior scientists present, is unprecedented for this complex an operation.