Noise modulated covert communications

In recent years, the Ultra-wideband (UWB) random noise waveforms have been getting increased attention in secure communications because they exhibit unpredictable behavior which can not be detected by conventional receivers and they are jam-resistance. This thesis describes a novel spread spectrum technique that can be used for covert communications. This noise modulated covert communication system (NMCC) is based on the use of heterodyne correlation techniques to inject coherence into a random noise signal. The BPSK-modulated signal to be transmitted containing the carrier is mixed with band-limited UWB random noise signal. The frequency range of the UWB noise signal is appropriately chosen so that the lower sideband of the mixing process falls over the same frequency range. Thus, the dispersive effects caused by the atmosphere and other factors are significantly reduced since both polarization channels operate over the same frequency band.;In military communications, the communication system will suffer interference from ambient noise, jamming from hostile force and multi-path. To fully understand this prototype system, the system behaviors in a Gaussian channel, various jamming environments and multi-path channels are properly modeled. The theoretical performance is compared with MATLAB simulation results and the results show the deviation between theory and simulation is smaller than 1 dB.;The system has also been built and tested in the open field. All the baseband processing is implemented in software defined radio architecture and allows processing of the real time audio and video experiment in the field. The audio and video can be successfully retrieved and the result clearly validates that the system concept will work. The empirical mode decomposition (EMD) is applied into the baseband signal processing to alleviate the noise interference and allow the system to operate in a noisy or jamming channel. By using noise pilot and noise radar concept, the channel estimation in a multipath channel can be accomplished while no code needs to be inserted in baseband signal processing and the system structure can be simple.