Three-dimensional interference investigations for LMDS networks using an urban database

Local to multipoint distribution systems (LMDS) operate in the frequency range of 26 and 43 GHz with a large bandwidth of 0.5-2 GHz but a very limited area coverage of a few kilometers only (1-5 km) due to line of sight constraints. As suggested in the Cellular Radio Access for Broadband Services (CRABS) report, the maximal spectral efficiency can be obtained with a dual frequency and polarization reuse plan. This frequency and polarization reuse leads to interference. In this paper we report the interference calculations for urban scenarios at 28 GHz. In a real life scenario, the interference is governed by some or all of the following reasons: half-power beamwidth in azimuth and in elevation as well as blocking due to tall buildings. These three effects have been further investigated with regards to the influence of the half-power beamwidth, the cell size and the effect of varying the base-station heights. In this context interference calculations have been done using the urban three-dimensional database of Munich in order to show the cochannel interference situation in a real urban scenario. It has been found that the spatial-filtering effect of the elevation pattern has a significant impact only for small cells. In a dense city like Munich, coverage is a major problem with a maximum coverage of 70% in a 1 km cell even for a very high base station. Further, to reduce interference, all base stations should be located at nearly the same height. If this is the case, about 3%-7% of the covered customers would not be able to use higher modulation schemes due to cochannel interference. To reduce these values, the receiving antennas should be located as low as possible, but with a line of sight to the desired base station.