Use of Synthesized System Models and Frequency-Distance Criteria for a Parameter Sensitive Interference Prediction Model Developed for Tactical Radio Environment

Abstract

The mathematical interference predic­ tion model based on synthesised transmitter and receiver response has been developed to evaluate the Frequency-Distance (F—D ) separation criteria for electromagnetic compatibility for tactical radio 130-1000MHz) environment. The synthesised modulation envelope at the fundamental tuned frequency of culprit emitter is also transferred to harmonically/sub harmonically related frequencies of culprit emitter to construct the composite emission spectra. The slope for fall of power per decade for harmonics/ sub harmonics is synthesised. The value of spurious emission level taken from technical literature is modified to take care of worst deviation. In an exactly similar fashion the victim receptor response spectra is synthesised. The culprit emission spectra and victim response spectra are graphically convolved to find the off-frequency rejec­ tion potential of the victim receptor to unwanted signal. For many emitter/receptor combination the F-D templates are super­ imposed for predicting compatibility. The model is parameter sensitive. The synthesi­ sed slopes when replaced with measured slopes will reduce the statistical deviations. The model utilises the statistical descriptions of the individual contributors of interference, Peculiarities of Co-Site Situation And Mobile Tactical Environments The fluid situations in mobile battle­ field scenario coupled with probabilistic nature of various parameters of potential sources of Electromagnetic Interference make the analysis and prediction task very cumbersome and complex. The complexity is increased further by the very fact that it is difficult to pin-point exact location of emitters/receptors, antenna heights, antenna beamings and power output in case of variable output facility provided to counter EGM. Hence the analysis and prediction by dynamic changes as a result of contingency planning when superimposed on planned connectivity will have to be based on worst case analysis. The worst co-site problem implies that antennas may be in each others fresnel region or induction field. The high power environments (like radars) complicate the situation by making receptors and emitters to operate in non-linear mode. The worst case co-site situation and high power environments result in interactions like Intermodulation, Desehsitization, Cross Modulation and Spurious emissions/responses. All these interactions contribute to inter­ rering power levels which vary considerably, necessitating statistical approach. These interactions can occur simultaneously thus complicating the Analysis problem. The front-end-burn-outs, case penetrations and radiations from local oscillator are not considered. The above mentioned problems were catalytic in evolving a composite spectra approach for analysing complex pro­ blems and predicting frequency-distance criteria for Electromagnetic Compatibility. Basic Concepts of F-D Model Fundamental-Emission Modulation Envelope, The transmitter fundamental output is not confined to a single frequency but it is distributed over a range of frequencies around the fundamental. The power distri­ bution around the fundamental is determined by base-band modulation characteristics of the transmitter. The modulation envelope model can be represented as :-