Microwave Radio Communications through a Turbulent Flame Using a Fast Correlation Method with Feedback

Abstract

The research had two objectives:- (1) To obtain further insight into the 1/Δf noise which is observed near the signal frequency when the characteristics or parameters of the system or medium through which the signal is transmitted fluctuate in a random manner (Δf is the difference between the signal frequency and the frequency at which measurements are carried out in a bandwidth of equal to or less than Δf), and also (2) To devise and test a means for transmitting signals through such a medium and detecting them in such a manner that the retrieval of the signal is improved. In the first group of experiments the time varying system cui stad of a laboratory flame operatinign an erratic or turbulent manner in a resonator tuned to approximately 9 GHz , ne to a wavelength of about 3 cm. An appreciable degree of attention was given to the development of self stabilising bridges for the separation and measurement of signal and noise powers from the mixture of signal and noise received after the signal was passed through the resonator. The measuring system could also be used to measure the autocorrelation function of the system over a short period of time, i.e. with a short averaging time. It was shown theoretically and demonstrated experimentally that within the coherence bandwidth a convolution of the signal component and the Fourier transform of the system correlation function is a suitable and adequate model to describe the action of physical systems of which the parameters vary fast. Starting from this and the fact that there is no mathematical or’ physical interpretation for an inverse convolution operation, it has been proposed that signals and noise can be unscrambled only in the real time domain and provided the bandwidth is limited to that in which the coherence between various frequency components of the signal and the noise are preserved. It has been demonstrated that such unscrambling can be carried out by a rapidly acting cantzol system operating at the receiver on information brought in by the mixture of signals and noise whereby the amplitude and phase changes due to the fluctuating transmission medium are corrected. A simple form of signal in which there are two orthogonal signals, one of which served as a reference signal, was used. Simultaneously, information was recorded on the initial fluctuations in the form in which they were received and also after they had been corrected by the rapidly acting control system. The experiments covered a range of signal frequencies different from the reference frequency, and the signals were subjected to analysis by the use of computer programs as well as in real time by analog measurements. The second part of the research was therefore concerned with Aiveriment =) ane analysis of the coherence between two signals when rapidly-acting automatic gain control (R.A.A.G.C.) and rapidly-acting automatic phase control (R.A.A.P.C.) are applied. It was found that an improvement in the signal to noise ratio of at least 20 db is possible when both gain and phase controls are used, even with very modest electronic abgeratus: much less comprehensive and precise than that developed in the first part of the investigation. Signals requiring wide frequency bands for transmission should be split into several channels of such a bandwidth that there is coherence over the entire band, and with the simultaneous transmission of a "comb" of reference or pilot signals.