Figure 2: High base station antennas lead to stronger propagation paths
between base stations than between a user set and the its base stations
In the cellular mobile environment base station antennas are raised considerably to increase the coverage area. Antennas mounted on towers and rooftops are a common sight and antenna heights of 50 meters above ground are no exceptions. Besides increasing the coverage area, this has the additional effect that frequently there exists a better propagation path between two base station antennas than between a mobile and the base station (see Figure 2).
Assuming that FDMA is used in conjunction with TDD as motivated above, then base stations and mobiles transmit on the same frequency. Now, unless there is tight synchronization between all base stations, signals from other base stations will interfere with the reception of signals from portables at the base station. To keep the interference at acceptable levels it is necessary to increase the re-use distance, i.e., the distance between cells using the same frequencies. In other words, sufficient insulation in the spatial domain must be provided to facilitate the separation of signals. Notice, that these comments apply equally to co-channel and adjacent channel interference.
This problem does not arise in cordless applications. Base station antennas are generally of the same height as user sets. Hence, interference created by base stations is subject to the same propagation conditions as signals from user sets. Furthermore, in cordless telephone applications there are frequently attenuating obstacles, like walls, between base stations which reduce intra-cell interference further. Notice that this reduction is vital for the proper functioning of cordless telephones as there is typically no network planning associated with installing a cordless telephone. As a safety feature, to overcome intra-cell interference, adaptive channel management strategies, based on sensing interference levels, can be employed.
