Craig R. Benson

A high data-rate, software-defined underwater acoustic modem

Most underwater acoustic modems offer only low data rates. This is largely because they operate at low frequency, which limits the channel bandwidth available, and hence the symbol rate. The low frequency acoustic channel suffers from substantial multipath and doppler effects, which constrain the signal quality at the receiver. As a result only 1 or 2 bits per symbol are achieved, with the effective data rate further reduced by error control coding. High frequency acoustic signals are heavily attenuated in water, severely constraining the range of high frequency links. High frequency signals however offer substantially greater signal bandwidth, and probably improved channel quality which guides our design choice of a high frequency acoustic modem for underwater communication. Contemporary Field Programmable Gate Arrays (FPGAs) can provide good system functionality at low cost and with the flexibility to perform rapid testing and development of communication algorithms. They may also be competitive in production systems. In this paper we describe current progress in development of a high frequency, high data-rate modem which is implemented entirely in FPGA. This differs from most existing modems which are based on DSP processors. Being software defined, the modem is flexible because the parameters can be reconfigured with relative ease, minimising the cost of rework as the design evolves. This modem will not only demonstrate the feasibility of high frequency FPGA based modems, but will also be a valuable tool to provide a better understanding of the high frequency acoustic channel, and demonstrate the utility of absorption to enhance channel re-use rates in underwater acoustic networks. The modulator has been implemented in the FPGA, to produce laboratory and open water tests that conform to modelling. The demodulator has been implemented in Matlab, and recovers the carrier, code synchronisation and data from recordings of both laboratory and open water tests. Coding of the demodulator into the FPGA is currently in progress.

Design of a high frequency FPGA acoustic modem for underwater communication

Contemporary underwater acoustic networks use low frequency modems. While these modems can provide long range communication, their low operating frequencies mean that only low channel bandwidth is available, which results in slow data rates. This motivates our development of a high frequency modem which offers the potential for large channel bandwidth, and hence greater link capacity. There is a range-frequency trade-off because absorption becomes very high at high frequency. Our intended operating frequencies from 100 kHz to 1 MHz would only support link ranges perhaps from 1 km down to under 100m, with communication ranges longer than this requiring forwarding over a network. Reconfigurable computing based Field Programmable Gate Arrays (FPGAs) are used to accelerate product development and support evolution of fielded systems. Given the immaturity of the field of underwater communication, a reconfigurable modem is a valuable tool for development and testing modem techniques. We present a design idea to implement an acoustic modem solely in FPGA, whereas most existing modems are implemented as a combination of FPGA and DSP processors. Aside from simple anti-aliasing filters, which could be incorporated in the preamplifier stage, the modem does all of its processing in the digital domain - maximising flexibility. In this work, we describe the initial design and architecture of our software based acoustic modem that avoids the monetary cost or time investment required to design a commercial modem or custom hardware for many applications. Our demodulator is implemented using a Costas loop which performs both suppressed carrier reconstruction and synchronous data detection within the loop. Results from initial implementation are also reported in this paper.

On the Benefits of High Absorption in Practical Multi-hop Networks

Underwater acoustic communications in their present form do not appear to provide an ability to dramatically increase throughput or range at reasonable energy efficiency. Networks with a long, single-hop between nodes require a significant increase in power to increase throughput or range. Multi-hop networks at current operating frequencies offer improvements in energy efficiency or range, but with severe limits on throughput. We show that the throughput limitations of multi-hop networks with a shared channel can be overcome in a high absorption regime. Underwater acoustic absorption is a well behaved function of frequency, and extremely high absorptions can be accessed at useful frequencies so that network range and capacity may be dramatically improved in an affordable manner.

The high frequency underwater acoustic channel

Underwater acoustic communication systems usually operate at frequencies below 40 kHz, which have low absorption rates that in turn enable long link distances. These links suffer from channel effects including multi-path, fading and doppler. Cellular and multi-hop strategies have been proposed that would use shorter path lengths, so would tolerate much higher absorption rates, and hence could work at higher frequencies. The short-range, high-frequency channel is not well documented. This paper reports on initial measurements of the short-range, high-frequency underwater acoustic channel. Re-examining the basic theory indicates that the short-range high-frequency channel should not suffer from the same multi-path or fading issues as the longer-range lower-frequency channel in common use. Initial collected and processed field data supports this conclusion. The implication is that relatively simple modems should provide adequate performance on the high-frequency channel. Coupled with large available bandwidth, these high frequency modems should offer data communication rates well in excess of those generally available.

High-frequency underwater acoustic communication development system

Underwater acoustic links have low capacity because that operate at low frequencies. These low frequencies are used because they allow long range communication. Networks allow short range links to provide long range communication, eliminating the need for these long link ranges. Therefore high frequencies can be used for underwater acoustic communication. Such high-frequency underwater acoustic communication systems have not been widely studied or developed before. Specialist hardware is required to support such development. This paper explains the development of such specialist hardware.

Performance validation of MAC protocols in underwater acoustic networks

Underwater acoustic networks (UWANs) offer potential in a wide range of applications, but conventional MAC protocols do not work well due to the long propagation delays relative to packet duration. Even for some innovative MAC protocols that show promise, there remains much uncertainty as to their actual performance under anything other than carefully selected scenarios. In some cases the only published results are simulations undertaken by the protocol proposer, resulting in a lack of independent verification. In this paper we detail independent simulation results for several MAC protocols in underwater acoustic networks, compare these to previously published results and accepted theory and analyze the impact of the large propagation delay on different protocols. The limitations of existing published simulations are then addressed, providing a basis for improved simulations in the future. Finally we review progress towards validation of acoustic MAC simulations, and highlight ways in which real-world tests can be improved.

LOARP: A Low Overhead Routing Protocol for Underwater Acoustic Sensor Networks

Underwater wireless communications among underwater sensor nodes enable a large number of scientific, environmental and military applications. For example, autonomous underwater vehicles will enable exploration of deep sea resources and gathering of scientific data for collaborative missions. In order to make underwater applications possible, real-time communication protocols among underwater devices must be enabled. Because of the high attenuation and scattering effect of radio and optical waves, respectively, these underwater devices are based on acoustic wireless technology. The unique characteristics of underwater acoustic channel - such as distance-dependent limited bandwidth and high propagation delays, require new, efficient and reliable communication protocols over multiple hops to network multiple devices which may be either static or mobile. This paper proposes a new low overhead ad hoc routing protocol designed for underwater acoustic sensor network. The protocol performs route discovery when needed in an on-demand manner. It also characterises a route maintenance phase which tries to recover a failed route. Detection of route failure can generate a lot of routing traffic. The proposed protocol tries to minimize this routing traffic by detecting failure in a more intelligent way either by monitoring network data traffic (if present) or generating lazy acknowledgements (if necessary). Reducing routing traffic minimizes the chance of packet collisions which in turn increases data packet delivery ratio. The performance of the proposed protocol is measured in terms of network throughput, packet delivery ratio, average endto-end delay and control overhead. The results are compared to those obtained using similar on-demand routing protocols. Simulation results show that the reduction of routing traffic can improve the performance of the network.

High precision ultrasonic positioning using a robust optimization approach

Typically, indoor positioning systems require higher precision than outdoor positioning systems. Outdoor positioning technologies such as GPS provide poor accuracy in indoor environments due to signal attenuation by the building fabric. In indoor environments, commercial motion-capture systems are proficient at measuring the 3D position of an object precisely. The main disadvantages of this method include its cost and complexity. In this paper, we present a precise 3D ultrasonic positioning system for medical applications, using a robust steepest descent optimization algorithm, to estimate the 3D position of an ultrasonic transmitter. Our experimental results show that the proposed system has the precision required for medical applications and has lower cost and complexity when compared with the alternative traditional optical systems.

High precision ultrasonic positioning using phase correlation

Three dimensional (3D) positioning systems are used in many applications across a wide variety of fields, including entertainment, sports science and medical treatment. Among the medical applications, emerging areas include: gait analysis, rehabilitation, medical robotics and biofeedback systems. Traditionally, these positioning systems use optical motion capture techniques. The main disadvantages of this method include its cost and setup complexity. In this paper we present a new technique for 3D positioning for medical applications using ultrasonic transmitters and receivers and a phase correlation approach for measuring the time-of-flight of these signals in the presence of multipath distortion. Our experimental results show the precision provided by the proposed system is comparable with the alternative optical systems and is acceptable for medical applications which require this high precision. However, the system cost and complexity of the proposed ultrasonic system is expected to be much less than for an equivalent optical system.

Divergent beam shaping for high data-rate underwater communications

High data rate underwater communication systems remain elusive. High frequency propagation combined with a frequency re-use appears to be a viable means to achieve high data rate underwater networks. However the required low-cost, high-frequency, omni-directional transducers with reasonable power handling capabilities do not exist. Small transducers cannot handle power, but large devices become directional at increasing frequency. Small transducers are also likely to be expensive because of manufacturing tolerances. We propose and investigate using a divergent reflector to transform a narrow beam into a donut shaped radiation pattern. The anticipated beamshape from a reflector is modelled using ray tracing techniques. A prototype reflector is then constructed and tested in still fresh water, showing the ability of a reflector to shape the reflected beam in an appropriate pattern. Testing on a spar buoy in salt water confirmed the signal is radiated laterally and provides a consistent signal even between typical floating platforms.