Acoustic Modem

Functional Description  Underwater acoustic communication is relatively slow when compared to radio communication. This has to do largely with the speed of sound in water which is roughly 1500 meters/second. The result is a relatively low baud rate (typically 9600 baud). 

Not only is the medium slow but there are complications with the transmission due to signal absorption, geometric spreading losses, boundary effects, and multipath to name a few. Manufacturers have several techniques they employ to handle these challenges. The techniques come in the form of signal processing, data packaging, and coding schemes. These techniques, which are not the same for all manufacturers, help ensure reliable communication and possibly identify bit loss and/or repair these lost portions of data at the receiver end.

There are several methods of transmitting data acoustically (i.e. modulation), but the most common method is the use of spread spectrum. Briefly, this is a method of sending data at several different frequencies (Multi-Frequency ­Shifted Key, MFSK) in order to increase data throughput. Another modulation scheme is the Phase Shifted Key, or PSK; this modulation scheme permits higher baud rates but is more susceptible to error sources.

The data are packed to ensure that a few errors will not corrupt the entire data message. This means that large amounts of data are sent as a series of these data packages. A typical data package is approximately 4 kb. A package contains the data plus additional bytes of data for identifying the package boundaries, modem identity, checksum, and error correction codes.

Some modems allow for a configuration where a retransmission request is sent from the receiver if errors are detected in a data package. The implication of lost data is that it must be retransmitted. This affects the effective baud rate if a modem is operating at a high acoustic baud rate.

Apart from the modulation schemes and packaging techniques there are also techniques to minimize the effects of multipath. Multipath is the reception of the same signal several times, yet slightly delayed from one another. Since the signal is the same frequency and arrives at more or less the same time, it is challenging to separate the original signal from time delayed versions overlapping each other.

As the name suggests, multipath is the source of these “different” signals that are reflections of the original signal from boundaries that lie between the transmitter and receiver. Multipath is most prominent over long ranges and shallow water, whereby the original signal can bounce between the surface and bottom before arriving at the receiver. There are a few tricks in use to reduce the effects of multipath. These are convolutional coding, multipath guard period, and data redundancy.

• Convolutional coding is data in a following frame that is capable of correcting up to one bit errors in the data frame previously sent.
• Multipath guard is a time delay inserted between data frames. Increasing the delay between frames reduces the interference from multipath.
• Data redundancy is simply the process by which data is retransmitted in the same data frame.

All of these methods improve the reliability of a transmission, however they also reduce the data transmission rate. This means there is a trade off between reliability and data rate.

Limitations  Now that we have provided the functional description, it is worth taking a look at some of the more practical aspects of acoustic communication and the associated limitations. In short, when and where does the performance of acoustic modems become unreliable?

1. Range and Depth Communication over short distances (approximately 200 meters) is quite dependable. This is particularly true for vertical communication in deep waters with few boundaries. Horizontal communication in shallow waters is increasingly more challenging as the depth/range aspect ratio becomes smaller. An example of a challenging scenario is 5 meter depth over a range of 3000 meters.
2. Clear Line of Sight  If you do not have a clear line of sight between the modems, it is very unlikely that there will be communication between them. It is also unlikely that an acoustically rigid boundary can be used to reflect energy in order to achieve an indirect transmission path.
3. In-band Noise  Sources operating at the same frequency as the modems will likely present a problem for communication. It is advisable to identify and remove all sources that could harm communications. Typical sources of noise are echo sounders from ships. Acoustic communication is surprisingly resilient to out-of-band noise from sources such as shipping traffic.
4. Shadow Zones  A shadow zone is defined as a region with no direct path of acoustic energy, and only reflected energy may enter this zone. An acoustic shadow zone will occur if the speed of sound profile is not uniform; this will lead to bending of the transmission path or “rays”. A conceptual example is presented in the figure below. The trouble with shadow zones is that they can often exist with a mildly non-uniform speed of sound profile, and that they are usually non-stationary over time. This means that a location may have variable reception.
5. Air Bubbles  From an underwater acoustics standpoint, air-water interfaces are more or less impassable. Clouds of air bubbles can largely inhibit acoustic transmission. This is usually a greater problem at the surface where breaking waves create clouds of air bubbles. Bubbles near the receiver are more problematic. For buoy mounted acoustic modems we advise mounting the modem away from the surface where the buoy itself contributes to bubble formation.

Nortek Acoustic Modem
The Nortek Acoustic Modem utilizes the Benthos acoustic modem OEM kit. The modem is delivered in a Delrin pressure housing, much like the Aquadopp class of instruments, and is rated for a depth of 400 meters. The diameter is slightly larger at 9.5 cm, and the length is 40 cm.

The modem is specifically designed to function with Nortek instruments and power supplies. All modems are delivered with a special cable that transfers data and provides power to the modem from the Nortek Instrument. The connector is an eight pin inline connector and the required cable is delivered with the modem for your particular Nortek instrument. The acoustic modem has an internal DC-DC converter that provides the required power for the modem.

The modems are available in three different transmit frequencies: 9-14 kHz, 16-21 kHz, and 25-30 kHz. They are also available as omni-directional transducers or as directional transducers (150 degree beam width).

Power Consumption  As a rough rule of thumb, the modem uses about 25% of the power consumption of the AWAC. This assumes a standard configuration of current profiles and wave measurements. Power consumption can be greatly reduced by selecting a suitable baud rate and power level; i.e. full power is not necessary in all installations.