Interchannel Interference in FM Systems Produced by Continuous Random Mode Conversion in Circular Electric Waveguide

Abstract

Long-distance, high-capacity transmission via the TE 01 mode in circular waveguide is an attractive goal because the theoretical attenuation caused by heat loss decreases monotonically as the frequency of operation increases. However, continuous random mode conversion caused by the manufacturing or the laying of the waveguide causes severe random fluctuations with frequency in the transfer function of present-day waveguide, a fact which apparently eliminates all but the “toughest” modulation schemes, such as pulse code modulation and angle modulation In this paper, we present the derivation of a technique for analyzing the effects of continuous random mode conversion on an angle-modulated wave. The random coupling coefficient is assumed to have a Gaussian probability density. The modulating signal is assumed to be a single-sideband frequency division multiplex which may be simulated by a band of random Gaussian noise. The quantity of interest is the expected value of the interchannel interference noise which appears at baseband. A self-contained section on principal results and examples is presented. The examples use typical state-of-the-art data and are significant in that they demonstrate that FM appears to be an attractive modulation scheme. They are, however, not meant to be an exhaustive study of the problem. Copper waveguide (with and without mode filters) and helix waveguide are considered. The Fourier series, multiple-echo representation of a stochastic transfer function presented in this paper should prove useful in handling other modulation schemes as well as other random channels