The invention relates to mobile telephone systems. A subscriber of a mobile telephone system, i.e. a mobile station, such as a radio phone or some other means of communication, may be registered in a radio network or system through system or traffic channels maintained by the base stations of the radio network.
Apart from system channels of mobile telephone systems, direct mode channels can also be used, i.e. direct mode operation can be applied. Mobile stations using direct mode operation do not communicate directly with a radio network or its base stations. Direct mode channels are frequencies at which mobile phones or other means of communication are able to communicate directly with one another without the system.
Direct mode channels are typically used in cases where e.g. a plural number of hand-portable phones communicate with one another at such a distance from the base station that system channels cannot be used.
Direct mode channels are also useful in the addition of capacity when traffic increases rapidly in one part of the service area of the system, e.g. at one point of the radio network.
A direct mode channel is also called a direct or simplex channel, or a simplex connection. A direct mode channel is a channel that is typically not used by the system at all. It may be, for example, a channel with the same channel spacing as the channels of the system, e.g. 12.5 kHz or 25 kHz. Of the radio phones operating on the direct mode channel, the transmitting station has tuned its transmitter to the channel and transmits speech or data information. The other radio phones set to direct mode operation have tuned their receivers to the same channel, whereby they are able to receive the transmission directly.
On a direct mode channel, both analogue modulation and digital modulation can be used. A radio phone transmitting on the channel can also send signalling information, such as information about access rights and priorities or about a group operating on the channel. On a direct mode channel, messages can be encrypted, or speech can be transmitted in clear form.
Hand-portable phones used in mobile radio systems have lower transmission power than vehicle-mounted phones due to their having e.g. small power unit capacity and a weaker antenna. On account of this, hand-portable phones have a shorter coverage range than vehicle-mounted phones in mobile telephone systems. Since radio networks functioning as PMR networks used by authorities are usually dimensioned, at least in the countryside, in accordance with the coverage range of vehicle-mounted phones, hand-portable phones are not able to operate on the periphery of or outside the coverage areas of the base stations of a mobile communication system. If, however, hand-portable phones are also to be used on the periphery of the coverage area of base stations, a repeater station, i.e. a repeater radio, should be used for transmitting calls from a hand-portable phone to the mobile telephone system and vice versa.
A repeater station is a means where two radio units are connected to each other. A repeater station forwards the information messages transmitted on a direct mode channel to desired network elements, such as base stations of a radio network, which forward the messages to mobile exchanges of the radio network.
A repeater station on a direct mode channel may operate either in duplex, semi-duplex or one-frequency simplex mode.
FIG. 1 is a diagram illustrating the operation of a repeater station communicating in duplex mode. The repeater station RS sends a transmission at frequency f1 to a base station BS, which sends its own transmission to the repeater station at frequency f2. The repeater station and base station then use trunking mode operation, i.e. TMO. On a direct mode channel, i.e. in direct mode operation DMO, the repeater station RS receives transmissions from mobile stations MS1 at frequency f3, and sends transmissions to mobile stations MS2 at frequency f4. In other words, when the repeater station operates on a direct mode channel in duplex mode, it receives a signal at one frequency f3 and repeats the received signal at another frequency f1. A mobile station MS3 of the mobile communication system is also connected to the base station BS over a radio path, the mobile station having its own system address, such as a telephone number. The mobile station may also have a group call address.
FIG. 2 is a diagram illustrating the operation of a repeater station RS communicating in one-frequency simplex mode. The repeater station RS sends a transmission at frequency f1 to the base station BS, which sends its own transmission to the repeater station at frequency f2. The repeater station and the base station then use trunking mode operation, i.e. TMO. On a direct mode channel, i.e. in direct mode operation DMO, the repeater station RS receives transmissions from mobile stations MS1, MS2 at frequency f5, and sends transmissions to mobile stations MS1, MS2 at the same frequency f5. The mobile stations MS1, MS2 then also communicate with each other on the same direct mode channel f5. The word `frequency` is used herein either for a frequency or for a channel formed by a frequency and a time slot.
In prior art, a repeater station repeats the calls from the system to a direct mode channel at a predetermined, fixedly programmed direct mode frequency. Likewise, all transmissions from a direct mode channel are repeated to the system, from a predetermined direct mode channel onto a certain system channel. The problem with the repeater station of the prior art is that the repeater station does not make a distinction between different types of calls and between different users. Further, the repeater station of the prior art cannot change the channels and direct mode channels to which calls are transmitted. The problem with the repeater station of the prior art is that it can repeat only fixedly predetermined calls.