1. Field of the Invention
The invention relates to a method of selecting a control channel by a mobile station in a cellular radio system.
2. Description of the Related Art
Prior art radio transmission systems include base radio stations which in a spatial arrangement form a cellular system with overlapping radio zones. The radius of such a radio zone is between 2 and 30 kilometres, depending on the height of the aerials of base stations and the transmission power. Each radio zone is supplied by a base radio station. This station can convey conversations via what are commonly known as radio exchanges from and to the public telephone network. A plurality of adjacent radio zones can be combined into a so-called call area. At the base station end, the whereabouts of all the mobile radio stations are collected and stored in what is commonly known as an address book. If a mobile radio station moves into another call area, then the address book is updated.
Depending on the volume of traffic, a number of speech channels and always at least one control channel are assigned to the base radio stations of each radio zone of the radio transmission system. In geographically adjoining radio zones different frequencies, spread codes or time channels can be used for different control channels. To distinguish between control channels and speech channels each of these channels is characterized by a special code. If a control channel fails or is subject to interference, any speech channel can take over the functions of the control channel by a change in code.
The search of a mobile station for a control channel can take a very long time, when the radio transmission system has a plurality of transmission channels (control channels and speech channels).
The published European patent application EP-A No. 2-0 111 972 discloses a method of selecting a control channel by a mobile radio station in which long search tuning times are avoided for at any optional arrangement of control channels in the frequency band of the transmission channels. In accordance with this known method, the base radio station transmits at variable time intervals references to the control channels of adjacent base radio stations, references to a substitute channel assigned to the base radio station and references to the control channel itself. If several control channels are assigned to the base radio station, then the base radio station also transmits references to these channels, it being possible that al the transmitted references contain in addition to the control channel number also additional information, such as, for example, country code and/or a subset code and/or a call area code.
In addition, the DE-OS No. 32 00 965, which corresponds to U.S. Pat. No. 4,527,284, describes a neighbouring radio cell-control channel-reference system, in which, when the control channels have free transmission capacities, references to control channels of adjoining radio cells are transmitted. The content of the references is the control channel frequency used, and the mobile radio station performs level comparison tests on the indicated control channel frequency at different time intervals.
An exchange of references between the base radio stations is necessary for monitoring the state of the network. If a given base radio station has, for example, five adjoining base radio stations, then each of these adjacent base radio stations must be informed of the failure of a control channel of the given station and of a switch to a substitute control channel assigned to such station.
In an integrated services radio transmission system there are few services which must usually be available everywhere. The total overall bandwidth available in the integrated services radio transmission system is always divided as regards time and areas (geographical frequency repetition) on the basis of the actual requirements of that moment. The base stations comprise only those arrangements for different services which for the relevant radio zones the subscribers are in need of. The cost of an integrated services radio transmission system is basically determined by the functions which are at least required for accessing the system and the arrangements required for performing the different services in correspondence with the different subscribers sets. These various subscribers sets may, for example, consist of:
a simple radio receiver with alphanumerical display;
a "pocket telephone" with a receiving and a transmitting frequency for use in a limited geographical area and with limited service quality;
a "dispatch device" (in the "dispatch service" a common radio transmission channel is used in the direction from the base station to the vehicles of a group of vehicles, whilst in the return direction each vehicle can acces its own radio transmission channel) with a receiving and a small number of transmission frequencies for half-duplex operation;
a "portable" with several receiving and transmission frequencies for duplex operation and country-wide supply, which is also suitable for use in vehicles,
a mobile data terminal with half-duplex transmission in a packet-switching method having a low data rate;
a "mobile office" (for example for building sites, banks etc.) with transparent duplex transmission of 64 kbit/s, which is only operated in the stationary state etc..
If in the radio transmission system different types of subscriber sets and different services are performed and if also different control channels are provided for the different services, then the references for all the different control channels must be exchanged between the base radio stations. If, for example, four different services are provided in the radio transmission system--for example a speech service, a low-bit rate data service, a data service with a high bit rate etc.--then the above example requires 20 control channel references to be transmitted between the neighbouring radiostations.