Experimental results for multimode interference during dispersive fading

Multimoding is caused by mode conversions and multiple reflections in the horn antenna and the circular waveguide system and leads to an impairment in the performance of digital radio. It is shown that the effect of multimoding is that of an echo with a long delay that cannot be equalized. The multimode energy thus becomes a form of interference. In addition it is shown that the multimode interference increases as a function of angular offset in the elevation of antennas, and is larger for vertical polarization of the signal. It is further shown that present-day 64-QAM (quadrature amplitude modulation) digital radio with angle diversity will perform satisfactorily in the presence of multimode interference. However, for new technology where multimode interference may become a dominant impairment, some form of mode suppression may be required