Rassal darbe kalınlığı kiplenimi kullanarak doğrudan dizi yayılı-spektrum tabanlı gizli haberleşme

In this thesis, a novel non-coherent spread-spectrum communication system which uses Random Pulse Width Modulation (RPWM) or equivalently Random Pulse Duration Modulation (RPDM) is proposed. Differing from the conventional spread-spectrum communication schemes where the receiver is assumed to generate the same pseudo-noise (PN) sequence with the transmitter, the receiver determines the transmitted binary message from the statistical properties of the residence times of the binary valued rectangular waveform whose positive and negative states have random lengths according to the prescribed probability density function. Since the residence times for both positive and negative states cannot be smaller than zero, the minimum value of the random variable must be controllable. Therefore, the uniform distribution having bounded interval is a reasonable choice and used to model the random length of the rectangular noise-like signal. At the receiver, there is a moving average process acting as low-pass filter in order to recover these random lengths without distortion under additive channel noise. This method provides to construct a noise sequence which does not exhibit repetitive behaviour for each message bit and instead of conventional PN sequence this proposed binary sequence exhibits more stochastic behaviour to increase security. The randomness of the proposed method is analysed in terms of autocorrelation and triple correlation characteristics according to the literature. This method is observed to provide a certain error performance depending on the selection of noise parameters such as mean and variance at the transmitter and the detector performance can be improved by increasing the number of elements of the vectors achieved by positive and negative residence times of the noise-like signal.