A mobile network generally comprises base stations which are arranged in an approximately hexagonal pattern. The pattern results from radio cells. The special extension of each radio cell is formed by the covered range of the base stations which are in contact with a mobile station. For this purpose each base station supplies three radio cells by means of three antennas having a relative angle of 120.degree. Each of the three antennas of a base station thereby emits a “sending lobe” of about 120.degree. Such an arrangement of the radio cells of a mobile network system is called “clover model”. For the wireless data transmission various resources are available for the operator of a mobile network, such as, for example, frequency bands which are divided into several physical transmission channels. Different transmission resources, for example, frequency bands, in two adjacent radio cells of a mobile network are attributed to the mobile stations. The transmission resources are adjusted in such a way that there is no superposition at all at the borders.
In order to simultaneously support as many connections as possible transmission channels are divided by frequency slots within a frequency bands, time slots within the transmission frame, codes with UMTS. In a GSM-mobile network (=Global System for Mobile communication), for example, a physical channel is formed by a frequency slot and a time slot within the transmission frame of eight subsequent time slots.
The operator of a network must, therefore, re-use the transmission resources as often as possible within the network. Generally, this is achieved when the entire amount of transmission resources is divided into orthogonal subgroups which are then attributed to the respective radio cells. A careful planning of the resources is required for this purpose. Only resources from the subgroup attributed to one particular radio cell are used for the communication in this particular radio cell. Thereby, the distance between radio cells is increase which use the same resources and which are exposed to mobile network signals interfering with each other. A measure for the influence of a transmission by interference is the ratio of a wanted signal intensity and the interference intensity, abbreviated by SIR (Signal to Interference Ratio). Only from a certain SIR-value an acceptable communication can be achieved. This threshold depends on additional thermal noise in the components and the requirements of the individual application, i.e. of quality parameters QoS (Quality of Service).
A further important measure with the planning of the resources, such as, for example, the planning of the frequencies, is the reuse factor. This reuse factor of the individual resource describes, for example, the frequency reuse factor, i.e. the amount of orthogonal subgroups. A reuse factor of one corresponds to the case where each radio cell uses the same resources because there is one group only. A high reuse factor reduces the interference intensity in a mobile network considerably, but limits the spectral efficiency of the network, because in each radio cell only a small amount of the entire resources of the operator of the mobile network is used.
Very often reuse factors “3” and “7” are used in mobile networks. In order to achieve a SIR-value required for a certain service in the entire cell area the system is designed based on the worst case. Therefore, the edge areas of a radio cell where higher interferences occur due to users in adjacent radio cells sending nearby determine the reuse factor of the planned resource. Thereby regions are generated which have an unnecessarily high SIR-value in the central region around the base station thereby reducing the spectral efficiency of the mobile network system.
In order to achieve a special distribution of the SIR in a radio cell which is as homogenous as possible overlay concept systems are described in the literature which have overlapping resource planning. For example the resource can be attributed with a low reuse factor in the inner region of the radio cell and with a high reuse factor in the edge regions. It is a disadvantage of this solution that further reduction of the interfering intensity due to interference suppressing methods is not possible because there are no strong and thereby detectable interferences occur due to the resource planning.