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.

Date of Award	1971
Original language	English