Higher order nonlinear combination tones applied to underwater digital communications

Nonlinear 'parametric' sonar is distinguished by highly predictable in-water formations of identifiable von Helmholtz spectral energies produced directly as a result of two or more preselected 'primary' harmonics simultaneously contained in a transmit waveform. In the nearly half-century of scientific endeavors within the field of parametric sonar, the methodical investigation into formulation techniques and practical applications using higher-order combination tones has been noticeably lagging the attention received by their more commonly recognized kin of second-order sum and difference frequencies. Generalized mathematical and graphical viewing techniques are presented for elucidating the abundance of cross-band complexities and facilitating preliminary design efforts specifically employing any of these higher-order parametric frequency components on operational systems. Recent sonar experiments implementing pulsed parametric transmit waveforms intended to fully exploit their intrinsic broadband nonlinear energy have demonstrated the potential for improved underwater target detection and classification in acoustically harsh environments. However, research efforts could benefit from more efficient and universal tools for predetermining all of the desired in-water spectral-temporal characteristics. New developments utilizing this methodology have led to unique approaches for constructing coding schemes resistant to jamming and frequency band fading for undersea acoustic digital communications in the horizontal shallow-water channel.