Patent GB-1393614-A (GB)
Title: Control apparatus for allocating a communication channel
IPC: H04W16/10
Filed: 1972-06-20
Abstract: 1393614 Automatic exchange systems WESTERN ELECTRIC CO Inc 20 June 1972 [25 June 1971] 28725/72 Heading H4K A radio telephone system for use with mobile units comprises a number of base stations connected both with a central telephone office and with a control apparatus, e.g. a computer, which controls the allocation of a transmission channel for a particular base station to mobile unit communication according to certain predetermined criteria. The same transmission channel may be allocated to two different base stations for communication with respective mobile units provided that the base stations are a certain minimum distance apart so as to prevent interference between the two calls. No transmission channels are permanently allocated to any of the base stations and separate call start channels are used in setting up connections. General description.-The system described is said to be a modification of known MJ and MK radio systems and most of the apparatus used in the system is known. Fig. 1A shows the general layout of the system in which a number of base stations 103-105 are connected to a central telephone office (not shown), and via an interface unit 102 to a General Electric 635 Computer 101. Fig. 1B shows a block diagram of a base station which essentially contains two MJ (or MK) base stations, one such MJ (or MK) station being constituted by blocks 112, 114 and serving as a call start station and the other being constituted by blocks 113, 115 and serving as a call service station via which the actual telephonic conversation takes place. It is the allocation of transmission channels to the call service station for a particular call, with which the Specification is primarily concerned and this is achieved by suitably programing the computer 101. The program is described in detail with reference to Figs. 3-5 (not shown), but the considerations and decisions made by the computer will be only generally referred to in the following description of the allocation of a transmission channel for telephone connection between a mobile unit and a base station. (a) Call originated at the central office.-The caller's line is connected to the call-start control terminals at every base station via lines 106 and line switches 111 which prevent seizure of the service channel control stations. Thereupon in known manner the call-start controls 112 put a seize tone on the call-start channel of every base station. A logic package added to each mobile unit normally ensures that all units are locked on the call-start channel (the mobile units are otherwise standard MJ or MK units), and hence each mobile unit receives the signal. Every call-start control terminal 112, 114 then transmits a mobile identification code over the call-start channel to all the mobile units in its area. All the mobile units which are not being called receive a mismatch at their control circuitry and are prevented from commencing any channel hunt until they next receive idle tone over the call-start channel. The called mobile unit, if free, transmits an acknowledge tone which is received by all the base stations which are within its transmission range and each base station concerned gives an indication to the computer 101 that it is suitable for use for a particular connection. The computer 101 then chooses a preferred base station according to fixed criteria, e.g. it may choose the base station which appears to be the most desirable from the standpoint of transmission characteristics to maintain the connection with the mobile unit. The transmission channel allocation routine is then followed by the computer 101. Essentially the computer starts a search to determine whether or not each transmission channel is in use by one or more of the base stations. If a channel is found which is not in use by any of the base stations it is allocated to the particular connection in question. However, in the absence of an unused channel, the computer considers the current usage of each channel in turn. If the channel being considered is in use by any base station within a predetermined distance of the base station allocated to the current connection it is immediately rejected. If it is in use by a base station further than the predetermined distance the identity of the channel is stored together with the identity of the base station using it. Subsequent channels are then considered and an optimization criteria is used whereby if a subsequent channel considered is farther from the allocated base station than the predetermined distance but closer to the allocated base station than the previous channel considered and stored, the identity of this channel and the corresponding base station replace the identities previously stored. Hence if a free channel is not available, a channel will be allocated which is in use at a base station which is closest to the allocated base station but outside the predetermined distance. Such an arrangement is said to optimize the usage of channels through the system. It a suitable channel is not found a blocked call procedure is followed and the call-start channel connection with the allocated base station is released. If a suitable channel is available, its identification is transmitted from the computer 101 to the allocated base station. The allocated base station via the service channel control terminal and transmitter 113, 115 places an idle tone on the channel selected. Also, by means of the call-start channel the logic attached to the mobile unit control is energized, thereby releasing the called mobile unit to search for the channel upon which the idle tone is transmitted. After a delay to allow the mobile unit time to find the channel marked idle, the call-start control 112 transfers the connection with the mobile unit to the appropriate service channel 113, 115 by means of the time switches 111. The computer then reactivates the callstart controls of all the base stations. When the call terminates the computer storage is updated to indicate the channel is no longer allocated to the particular base station. (b) Call originated at a mobile station.-Only the initial procedure differs from the above. All mobile units are tuned to the call-start channel. If a mobile unit attempts to initiate a call but finds the call-start channel is in use, i.e. no idle tone on the call-start channel, the unit releases. If the call-start channel is free the mobile unit transmits a guard tone on this channel, which tone is sensed by the base station call-start control terminals within transmission range. Each of these base stations communicates with the computer which removes the idle tone at the call-start terminals of all the base stations. The computer allocates one of the base stations to the call and the transmission channel allocation proceeds as before. Once the mobile unit has locked on to the allocated channel, the idle tone is returned to the call-start channel.