Abstract
This article describes the implementation of a multiple-input multiple-output acoustic communication link in shallow water conditions to enable a software-defined acoustic modem with a maximum transmission rate of 20 kbps in a 5-kHz bandwidth. The reliability improvement of a low-complexity Alamouti space–time block code is evaluated to improve the diversity in a high-rate transmission mode using single carrier modulation, as well as in a low-rate transmission mode relying on continuous-phase frequency-shift keying. Using measurements in realistic subsea conditions, the effect of the spatial channel correlation is demonstrated. It is found that for the space–time block code/continuous-phase frequency-shift keying, the spatial diversity is significantly degraded due to the high spatial correlation. In contrast, for the high-mode transmission rate, space–time block code with single carrier modulation offers a bit error rate improvement by a factor over hundred, in comparison to a single transmit element, demonstrating that the multiple-input multiple-output optimal code depends on the software-defined acoustic modem transmission mode.
Highlights
Due to the interest in subsea exploration and infrastructure deployment in the oceans for asset monitoring, there is an increasing desire to transmit multimedia information between remote nodes underwater.[1]
Note that during the trials, there was a failure in the transmission of the space–time block code (STBC)/continuous-phase frequencyshift keying (CPFSK), so the received signal is modeled by convolving the transmit signal through the measured multiple-input multiple-output (MIMO) channel that is described and characterized in the previous section
To improve the communication reliability, and improve the probability of outage, a low-complexity Alamouti STBC is implemented in the firmware platform
Summary
Due to the interest in subsea exploration and infrastructure deployment in the oceans for asset monitoring, there is an increasing desire to transmit multimedia information between remote nodes underwater.[1]. Due to the variable underwater channel conditions, software-defined acoustic modems (SDAMs) have been receiving increased interest.[14] most commercial modems describe a high-throughput mode relying on spectrally efficient coherent modulation techniques, as well as an alternative mode relying on a robust low-bit rate FSK modulation. An MIMO system using a lowcomplexity modulation developed is implemented to enable a link with a throughput as high as 20 kbps, and at its core, the system relies on a field-programmable gate array (FPGA) implementation of the signal processing. To achieve high data rate and high quality of service, the analysis of various STBCs is well-documented.[18] Here, the Alamouti STBC is chosen to reduce the probability of outage It is a good choice of code for acoustic transmission, since it does not require channel state information (CSI) at the transmitter. An analytical model of the MIMO system in an acoustic transmission media will be developed; the transmitter hardware architecture will be presented, followed by a description of the trials at sea; and a conclusion will be presented
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