Effect of Propagation Fading and Antenna Fluctuations on Communication Systems in a Jamming Environment

Abstract

The effect of different degrees of both slow propagation fading and antenna fluctuations on the performance of communication systems in the presence of interference was analyzed. The performance of the system was evaluated from the aspect of the probability of error which in turn is related to the required system margin. The results are plotted for a large variety of fading conditions for both the desired and undesired signals and antenna fluctuations. Some typical results are given below: Propagation Fading with No Antenna Fluctuations [1] For greater than 85 percentage satisfactory operation, the fading of the additive interfering signal has negligible effect on the probability of error when the desired signal experiences "strong" or Rayleigh distributed type of fading. The system margin for 99 per cent of satisfactory operation when the desired signal is Rayleigh fading and whether or not the undesired signal is fading is 20 db. [2] When the fading in the desired path is normally distributed, the effect of a normal fading interference signal is to increase the system margin compared to a nonfading interfering signal by 1 db and 1.4 db for 90 and 99 per cent satisfactory operation, respectively. The system margin for 99 per cent satisfactory operation is 5.2 db and 6.6 db when the undesired signal is not fading and is normally fading, respectively. [3] When there is no fading in the desired signal path and the undesired signal is fading, the system margin for 99 per cent satisfactory operation is 5 db for normal fading and 6.8 db for Rayleigh fading. [4] The system margin for 99 per cent of satisfactory operation when the desired signal is normal fading and the undesired signal is Rayleigh fading is 15 db. The system margin must be increased to 20 db when the desired signal is Rayleigh fading and the undesired signal is normal fading. Propagation Fading and Antenna Fluctuations [1] For a good UHF airborne antenna, the approximate variance of 0.1 is calculated from the vertical and horizontal patterns. The system margin for 99 per cent satisfactory operation is 6.4 db for no propagation fading in the desired or undesired paths. When there is normal fading in both the transmission paths, the system margin for 99 per cent satisfactory operation increases to 8 db or only an increase of 1.6 db over the case for no propagation fading. [2] Increasing the antenna variance to 0.3 causes an increase in the system margin for 99 per cent satisfactory operation and normally distributed fading in both paths to 9.6 db or an increase of 1.6 db due to the increased antenna variance. [3] The percentage of satisfactory operation as a function of system margin for antenna variances equal to 0 and 0.1 for a Rayleigh fading desired signal and a normally fading undesired signal shows that above a 10-db system margin, the curves are indistinguishable. A 20-db system margin is needed for 99 per cent of satisfactory operation. [4] The effect of the antenna variance is very significant when the desired signal is normal fading and the undesired signal is Rayleigh fading. For 99 per cent of satisfactory operation, the system margin must be increased from 15 db to 26 db when the antenna variance is increased from 0 to 0.10.