In the underwater acoustic communication environment, multipath transfer characteristics or Doppler spread due to time and space changes such as seabed, sea level, and water depth affect the performance. The multi-band communication technique is effective in terms of performance and throughput efficiency because it can overcome selective frequency fading by allocating the same data to different frequency bands in the environment of rapidly changing channel transfer characteristic. In addition, the transmission distance can be further extended while overcoming various underwater channel environments. However, the multi-band configuration may have worse performance than the single-band one. It is because the performance degradation in a particular band affects the output from the entire bands, which is input into a decoder, thereby decreasing the overall performance. This problem can be solved through a receiving end that analyzes error rates of each band, sets threshold values, and allocates lower weights to inferior bands. In this paper, we proposed an algorithm to set the threshold value using the preamble error rate, which is known data to be transmitted and received. We have analyzed the efficiency of multi-band transmission scheme by applying 1–4 number of multi-bands using turbo pi code through actual underwater experiment.
In underwater acoustic communication, there has been a lot of interest in detection and acquisition of information about multiple sensors as well as detection of information on a single sensor. However, research on multiple sensors access is lacking. In order to acquire information from each sensor in multiple sensors access underwater communication, we mainly use the spreading method which can obtain information of each sensor individually at the receiving side by multiplying the code with different orthogonal components in the same frequency band. In the case of multiple sensors, successive interference cancellation is used to remove interference from other sensor and apply the RAKE method. However, in a multiple sensor communication environment, if the channel information is not perfect, the performance is reduced. In this paper, we propose a transceiver structure for multiple sensor covert acoustic communication using the spread spectrum method and RAKE method, and propose and effective receiver structure to overcome drawbacks. An actual underwater experiment was conducted to analyze the performance of the covert underwater acoustic communication. The result of experiment indicated that the transmission and receive structure proposed through the actual underwater experiment was well-suited to the communication model for covert multiple accesses.
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