A Benchmark for the Number of Independent Line of Sight Links on a Given Volume Platform

The number of independent links that can be hosted by an antenna platform for line-of-sight (LoS) communications is limited by its physical size and the interference between the beams associated with different users. For large-size platforms, the interference can be reduced by compromising the aperture efficiency, and this tradeoff is the metric to quantify the effective use of the platform. This metric fails for antenna platforms that are not electrically large, for which the aperture efficiency is no longer a useful parameter. Here we resort to the concept of the observable field, related to the maximum theoretical directivity, to estimate the potential number of independent links supported by moderate-size platforms. This allows the introduction of coupling coefficients between the beams associated with the observable portion of the incident field and the beams associated with the receiving antennas. These coefficients are bounded to unity for any platform dimension, unlike the aperture efficiency, and they are maximized when the antenna pattern is equal to the pattern predicted by the observable field. Accordingly, selecting beams dictated by the observable field constitutes a benchmark for the effective use of the volume. Any antenna design can be compared to this benchmark to assess its merits.