The invention concerns cellular radio systems, such as mobile phone systems and PMR systems (PMR=private mobile radio), i.e. cellular radio networks in which the area covered by the network is divided into radio cells. The subscribers roaming in the area of these radio cells communicate with the system when they are within the coverage area of the base stations located in the radio cells.
In a system like this, a subscriber station, such as a mobile phones, located in a radio cell listens or tunes the receiver unit of its radio to the frequency of a signalling channel of a base station located in the radio cell. A signalling channel is used for exchanging messages concerning the connection establishment between a subscriber station and a base station. On the basis of connection establishment, the system controller of the cellular radio network assigns the subscriber stations to actual traffic channels, in which the actual transfer of information, i.e. speech or data, takes place. Traffic between a base station and subscriber stations takes place on radio channels that are implemented such that downlink traffic (from a base station to a subscriber station) takes place at a first frequency, and uplink traffic (from a subscriber station to a base station), at a second frequency. In FDMA systems (FDMA=Frequency Division Multiplexing Access) a channel thus means a pair of channels used for one conversation. In a TDMA system (TDMA=Time Division Multiplexing Access) a channel means a carrier and its time slot.
It is typical of channels provided between base stations and subscriber stations that there is a limited number of channels available since there are few radio frequencies and/or time slots. It is thus typical of cellular radio systems that one--possibly the only--signalling channel of a base station can be converted to a traffic channel, e.g. a speech or data channel, when a traffic channel is needed for a call between a subscriber station and a base station and the actual traffic channels of the base station are busy. Typically, this kind of procedure is followed in areas where the traffic and subscriber density is low and where the number of radio channels is small; the use of a signalling channel as a traffic channel significantly improves the transmission capacity of the system. When a signalling channel that can be converted to a traffic channel (i.e. a non-dedicated control channel) is used as a traffic channel, signalling messages cannot be transmitted on said channel unless the channel has an inband signalling channel, such as an associated signalling channel, whereby it is possible to transmit short signalling messages in the middle of speech or data to the parties listening to the channel (base station and subscriber station). However, it is not possible to set up new calls on the associated signalling channel nor to transmit data messages on it since the transmission capacity of said channel is very limited. Typically all the channels of a base station can function as signalling channels or traffic channels when necessary and that if all the channels of the base station function as traffic channels at the same time, any channel that is released from use as a traffic channel can be converted to a new signalling channel for traffic between the base station and the subscriber stations roaming within its coverage area. This is optimal in respect of formation of a new signalling channel: a new signalling channel is formed without delay. When a channel like this is used as a signalling channel, signalling messages concerning all the services of a mobile phone system can be transmitted thereon. It should be noted that a radio system may simultaneously comprise base stations with dedicated control channels only and base stations with non-dedicated control channels.
It is typical of cellular radio systems, particularly of PMR systems, that the radio cells or the coverage areas of the base stations intersect or overlap, whereby a subscriber station can select--by measuring the signals transmitted on the signalling channels by the base stations--the base station whose signalling channel (control channel) it will start to listen to. The coverage areas of the base stations may overlap heavily. The coverage areas overlap especially in low-lying and flat areas when great transmission power is used, e.g. in PMR systems.
Due to the overlapping of the coverage areas of the base stations in the prior art, when all the channels of a base station are converted to traffic channels, the subscriber stations which have listened to a base station channel that has functioned as a signalling channel but has been converted to a traffic channel move to a signalling channel of an adjacent base station. Since in normal mobile phone traffic, tele-communication links, such as speech or data links, are established from and to subscriber stations and since the adjacent base stations have few free traffic channels or none at all when the traffic load is normal/optimal, all the traffic channels and non-dedicated control channels of the adjacent base stations are quickly allocated for use as traffic channels. Those subscriber stations that do not find a signalling channel to which they could be tuned or a traffic channel on which they could communicate with the base station start to tune their radio units to the frequencies of the different base stations, attempting to find a free signalling channel and a traffic channel if necessary and simultaneously attempting to register in the network and send the parameters for establishing a telecommunication link with several base stations in succession yet always failing since the other base stations do not have free channels either. The subscriber stations thus roam between the coverage areas of different base stations, causing instability and unnecessary signalling load in the network. Further, the fact that the subscriber stations roam to listen to the signalling channels of base stations that are not their original base stations may lead to a situation where not a single subscriber station is listening to the signalling channel of the original base station when a signalling channel or even a traffic channel of said original base station becomes available, whereby the network has a free signalling channel and traffic channel although at the same time the subscriber stations roaming in the area have too few traffic channels in their use. The network capacity is thus divided unevenly and resources are wasted. The subscriber stations detect this as a decrease in the quality of service: it is difficult for them to find a signalling channel at a base station and to establish telecommunication links, and their call set-up attempts are interrupted since often the signalling channels on which a subscriber station attempts to set up a call, i.e. to send messages necessary in call set-up, is allocated to another subscriber station for use as a traffic channel.