GSM mobile radio systems allow communications connections to be set up to mobile subscribers by transmitting information by means of electromagnetic waves via a radio interface between a transmitting and a receiving radio station. In GSM mobile radio systems, electromagnetic waves are emitted with carrier frequencies in the 900 MHz, 1800 MHz and 1900 MHz ranges. To distinguish between different signal sources at the location of the relevant receiver, a combination of frequency and time division multiplex methods is used. An important function is performed by base stations, which maintain the radio links within one or more cells with one or more subscriber stations. Connection-oriented and packet-oriented concepts can be used in order to transmit data. In connection-oriented data transmission, physical resources must be made available between the participants in the communication throughout the entire period. In contrast to this, in packet-oriented data transmission, physical resources need to be made available only during the actual transmission times. This method is based on the concept whereby the data are transmitted in short data packets, between which longer pauses may occur. The physical resources are available to other logical connections in the pauses between the data packets.
GSM mobile radio systems were originally designed to transmit voice, whereby one channel is reserved for continuous information transmission between the subscriber stations and the base station. However, in packet-oriented data transmission, one common channel is used for a plurality of subscriber stations for packet data transmission.
In some countries, for example the USA, numerous frequency bands are already used by GSM mobile radio systems for connection-oriented services and are therefore not available for packet-oriented data services, the implementation of which requires greater bandwidth. When setting up overlaid cellular radio networks for packet-oriented data transmission, which are known by the name of EDGE (Enhanced Data Rates for GSM Evolution), certain carrier frequencies previously used by connection-oriented services are released for the overlaid radio networks. However, despite such measures, relatively few carrier frequencies are available to the overlaid radio networks. This results in a narrow repetition pattern of the carrier frequencies for the overlaid cellular radio networks, creating the risk of increased common channel interference.