Chaotic signals which have a spectrum spread characteristic are useful signals for private communications and multiple communications. In the case of private communication with chaos, the chaotic signal and an information signal are combined by addition, multiplication or convolution. In this study, we aim at the method based on convolution because the modulated signal can be demodulated by the inverse system. However, the property of the generated chaotic signal is changed by the input signal in case of the system which uses convolution, therefore, it is difficult to maintain the chaotic property of the generated signal. Furthermore, the modulation and demodulation system have different structure, respectively. In this study, we first try to unify the structure of the modulation and demodulation system by applying the cascade structure of a digital filter to the chaos generation system. Next, we propose a chaos generation system which does not always depend on the input signal. The proposed system has the structure which uses both a nonlinear digital filter and an oscillator by linear digital filter.

Digital transmission over the empty lower baseband of an existing microwave radio system has the advantages of economical and easy construction. A 1.544 Mbit/s PCM-FDM converter (PF-B1) has been developed. Field test results show that the error-rate per bit is less than 10-8over 840 km with 3 basebands.