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For underwater acoustic communication, the acoustic signals produce great attenuation in the propagation, and the complex environmental noise leads to the extremely limited bandwidth and capacity of the underwater acoustic channel. In this paper, the bandwidth and capacity of the channel are modeled and simulated under the condition that the transmitted power spectral density is constant. The theoretical analysis and simulation results show that the lower limit of the bandwidth of the underwater acoustic channel is determined by the noise and the upper limit is determined by the attenuation of the channel. Therefore, it is concluded that the narrow bandwidth of underwater acoustic channel is determined by attenuation and noise.

The shooting and bouncing ray (SBR) method is well known for providing more accurate results by including the scattering effect arising from multiple bounces. The fact that the fields tracking in the SBR method is calculated by only a geometrical optics (GO) approximation makes the choice of the exit aperture matter. In this paper, a comprehensive study is made on the exit aperture in the SBR method. Three kinds of exit aperture are compared, i.e., bounding box of the cavity, wavefront of the ray tube and the scatterer itself. Numerical simulations with the exit aperture of different kinds are conducted to verify the theoretical analysis and to draw some conclusions.

Summary form only given, as follows. First, a method to obtain the equivalent circuit of a dipole consisting of frequency independent lumped elements is presented. The input impedance of the equivalent circuit is in accordance with that of simulation of the dipole. Then, based on the equivalent circuit of a single dipole, an equivalent circuit of one row periodic metal rod array is developed. The passbands of periodic metal rod arrays calculated by the present method agree well with the simulation results and measurements.