The invention relates to a random access method in a radio system comprising a plurality of radio stations and at least one base station serving said radio stations, in which method the base station transmits an invoking message to the radio stations in a first time slot to grant the radio stations the right to send a random access message to the base station, one of the radio stations transmits a random access message containing a random identifier to the base station in a second time slot subsequent to the first time slot, the base station transmits a response message containing the random identifier to said radio station in a third time slot subsequent to the second time slot.
It is generally known to use a random access method in the implementation of control channel operation, in connection with the registration of a radio telephone in a network, and in call establishment. By means of the random access method a radio telephone, typically a mobile radio, connected via a radio path to a base station and thus to a radio telephone network, informs the base station that it wishes to use the signalling capacity of the system. A radio telephone needs signalling capacity in order to be able to use a speech channel or to send for instance messages, such as short messages.
A typical random access method currently used is the so-called slotted Aloha method, which is described in Computer Networks by A. S. Tanenbaum, 1981, Prentice Hall, Englewood Cliffs, pp. 249-323, particularly pp. 253-257. The use of the slotted Aloha method in connection with modern radio systems is described more closely in the standard MPT 1327, 1991, "A Signalling Standard for Trunked Land Mobile Radio Systems", Radiocommunications Agency, London, Chapter 1.3.3 and particularly Chapter 7. The slotted Aloha method is a random access method in which a radio telephone can attempt access to the control channel only within accurately defined time slots. The control channel is a radio channel or a time divided part of a radio channel used by a base station for call establishment in radio communication between it and mobile stations, i.e. radio telephones and other radio equipment. Typically, a base station transmits repeatedly--although at intervals it has defined, and irregularly if necessary--call attempt invocations, i.e. invoking messages, to radio telephones within the coverage area of its own transmitter. By means of the invoking message it has sent, the base station defines one or typically several time slots for all or a given part of the radios for random access, i.e. for the transmission of the first communication message. The radio which attempts access, i.e. which tries to establish a radio connection with the base station, selects at least one time slot for its random access message according to a rule, and transmits a random access message in that time slot, i.e. random access time slot. The random access message contains at least data for the identification of the radio which attempts access or its user. This identification data may be an address which uniquely identifies either the radio equipment or its user.
Modern radio systems must, however, be prepared for a large number of radio telephones within the area of one base station, and thereby for an extremely high load in the system. The aim is therefore to increase the random access signalling capacity between the base station and the radio equipment. The latest advance in this respect is to shorten the random access message by deleting the address which uniquely identifies the user. Even in this case, however, it is advantageous to distinguish the radio telephones attempting access in some way from each other. The address uniquely identifying the user has therefore been replaced by a random number, a random audio frequency AF, or by sending a random access message on a discrete frequency other than the nominal radio frequency. In cases like this, the (real) address data uniquely identifying the radio telephone is not sent to the base station until during subsequent signalling.
It is also known to use a short, call-specific identifier on a radio path during signalling traffic used for establishing call connection and during call connection. The base station gives the identifier to the radio telephone for the establishment of a certain call and for the identification of the signalling traffic relating to the call.
If the address uniquely identifying the user, i.e. the radio telephone, is replaced in a random access message by a random number, a random frequency or by sending the random access message on a discrete frequency other than the nominal radio frequency, it may cause the following problem. As the identification of radio telephones is effected by means of a random identifier, there is always The risk that two or more radios use the same random identifier, random number or some other corresponding signal in their access attempts, and the base station does not observe this; the call-specific identifier is then allocated to more than one radio telephones. This results at least in collisions between messages sent by the radios, and the transmitted messages are thus destroyed, especially if digital messages are concerned. The signalling traffic is disturbed and becomes more complicated; it is even possible that more than one radio telephone starts to operate on the same voice channel, whereby calls are cross-connected. One method of solving the problem described above is to lengthen the random word or increase the number of audio frequencies used, but this means that the benefits of the shortness of random access messages are mostly lost.