Browsing by Author "Gomes, J."
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- Experimental assessment of time-reversed OFDM underwater communicationsPublication . Gomes, J.; Silva, A.; Jesus, S. M.OFDM communication has recently been demonstrated in underwater channels at rates of 10-30 kbps. This is a popular modulation in radio communications due to its spectral exibility and the simplicity of FFT-based transmitters/receivers. Despite widespread interest, experimental data on the performance of underwater OFDM are still scarce. This work aims to contribute to a better understanding of the potential of this technique by examining results from the UAB'07 experiment, which was conducted in Norway, in September 2007. Modulated data were transmitted in a fjord using several formats with bandwidths of 1.5 and 4.5 kHz, and recorded at a range of about 800 m in a 16-hydrophone array. Signi cant multipath was observed over a period of at least 30 ms, which would call for a large OFDM pre x and hence drastically reduce the data rate. Passive time reversal is used here as a computationally inexpensive preprocessing scheme to shorten the e ective channel length to less than 10 ms, so that moderate guard intervals can be used with a conventional OFDM receiver architecture. The same marker signals used for packet synchronization and Doppler compensation are reused as channel probes for passive time reversal, so the latter entails no loss in e ciency.
- Underwater acoustic communication using a time-reversal mirror approachPublication . Silva, A.; Jesus, S. M.; Gomes, J.; Barroso, V.This work presents a technique for reducing the intersymbol interference (ISI) in underwater coherent communications using time-reversal acoustics. The paper introduces a “virtual" time-reversal mirror (TRM) that is implemented electronically at the receiver array and simulates the kind of processing that would be done by an actual TRM during the reciprocal propagation stage. In both cases, a probe pulse sent by the transmitter/receiver located at the (physical or virtual) focal point and received at the array provides a template impulse response for undoing the effects of multipath by straightforward linear filtering. Very simple equalization algorithms may subsequently be used to decode the message. Channel variations between transmission of the probe and the actual message lead to mismatch that can impact the coherence of TRMs, and hence degrade the focusing power of the array. Computer simulations using a normal-mode propagation model in a reallistic shallow water scenario show that, even with high uncertainty in the transmitter and receiver relative positions, the virtual mirror can strongly reduce the effects of multipath. Although a multichannel equalizer attains a lower mean-square error, the "virtual" TRM can provide comparable results under low mismatch with much smaller complexity.