EurOCEAN 2000 - Vlaams Instituut voor de Zee

A fundamental limit of this kind of modulation is the selective frequency fading. If the channel
frequency response has a very low amplitude at a given frequency, the corresponding carriers
are highly affected by noise and the data carried is unrecoverable. For this reason, OFDM
modulation requires also error correction coding of data.
Working with this kind of modulation the BER achieved ranges from 0.2% to 3%, with a data
rate of 2048 bit per second (not accounting the overhead due to error correction coding).
SEA TRIALS
The location of both sea trails has been chosen to be the permanent Measuring Platform
‘Noordwijk’ (MPN) in the (Dutch) North-Sea.
In the first sea trial (May 1999) two installations were used:
A fixed installation for the receiver (RX), to be moored on the Meetpost Noordwijk
platform: six receiving transducers are immersed, moored, powered and electrically
connected to the data acquisition hardware; the signals acquired from the transducers
are pre-processed and stored on the platform.
A mobile installation, onboard a support ship, where a transmitter (TX1) and its
driving equipment is installed, deployed underwater at each ship stop for each single
measurement, and recovered after measurement completion.
The receiving array consists of 6 hydrophones, that will be mounted to the MPN. The
hydrophones are used in 3 rows with 2 hydrophones per row. Horizontal spacing is 15 cm, and
vertical spacing equals 4 m. The lowest element is 3 m from the sea bottom. Data have been
gathered at 8 TX locations corresponding to the ranges from 50 to 5000 m.
One Sea Campaign consists of:
One session for ‘ship noise characterisation’: i.e. one data gathering session when
ship is moving from closest ship stop (50 m from MPN) to next ship stop (500 m from
MPN).
Data gathering sessions performed at each ship stop, including different kind of
signals for: Instrumentation check, system calibration, ambient noise characterisation,
and transmissions of useful signals.
Several environmental parameters have also been measured.
The signals transmitted at each ship stop include PSK modulated signals with several different
bit rates and OFDM signals. The frequency bandwidth used ranges from nearly 8 kHz to nearly
12 kHz.
For the second sea trial (planned for May 2000) in addition to the two installations described
above a third installation will be used:
A second mobile installation, onboard of a second support ship, where a second
transmitter (TX2) is installed. The transmitter will be equipped with a projector in the
same band as the TX1 one. This transducer will transmit messages different from
those transmitted at the same time from TX1, but with same modulation and signal
characteristics.
This will allow the direct test of multiple user access to the network (the multiple access has
been simulated with first sea trial data by summing together data gathered in different
sessions).
Moreover the experience of the first sea trial have suggested a change in the receiver array
configuration, which will be made by three receivers on a row, with half wavelength spacing,
plus an additional receiver on the same row at nearly one m distance and two more receiver
above and below these at three m of distance.
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