Patent GB-1472212-A (GB)
Title: Multiple zone communications system and method
IPC: H04W36/08
Filed: 1974-05-13
Abstract: 1472212 Mobile telephone systems; telephone sets MARTIN MARIETTA CORP 13 May 1974 [15 May 1973] 21115/74 Headings H4K and H4J In a mobile telephone system of the type in which a control channel is used for allocating speech channels for use between mobiles and a fixed station, the control channel can be allocated as a speech channel if all the speech channels are in use. Fig. 1 shows a complete mobile system wherein a plurality of overlapping service areas (Fig. 7) are provided each having a base station connected by digital signalling paths and speech path 55 to a S.P.C. central control terminal 59 having acess to the public telephone system 52. The C.C. terminal 50 may have duplicated processers. Duplex speech and control signal channels are allocated on a non- interfering basis to the base station permanently, dynamically or a mixture of both types of allocation. General operation.-Each base station transmits a binary signal or the OUT leg frequency of its control channel which gives the identity of the base station and a channel free or busy indication. When a mobile subscriber enables his unit, the receiver is caused to scan all the control channel frequencies of the system. In a first arrangement the receiver locks on to the first control channel which exceeds a predetermined threshold level. Alternatively an initial threshold level is set and all the channels are scanned. Each time a channel exceeding the threshold is detected, the level is increased to the value of the detected channel and the channel identity is stored, thereby resulting in the receiver locking on to the highest level control channel. If the scan reveals no channel above the initial threshold level, the level is lowered and the scan repeated.  Having locked on to a control channel an indication is given to the subscriber who can then depress a key to cause an "in service" signal to be transmitted to the base station either immediately or when the control channel if busy becomes free, this action causing a location store in the central control terminal to be updated. If at any time the level of the channel at the mobile drops below a minimum threshold level, a search for a new control channel is initiated. Mobile initiated call.-The required number is keyed, visually displayed for checking and then a "send" key is depressed. When the control channel is next determined as free, a binary coded signal is transmitted to the base station and thence via a signalling path 55 to the C.C. terminal 50 (detailed in Fig. 10). At the C.C. terminal the received signal is examined for the identity of the calling mobile and if the identity is not recognized, e.g. the mobile belongs to another system, the call is processed as if it were to an operator. Irrespective of the type of call, a speech channel allocation procedure is entered upon as follows:- If any speech channel is free at the base station at which the call was received or at a base station of a geographically adjoining service area, one such channel is allocated. If not, any speech channels available for dynamic allocation are searched and if none are available the control channel of the base station which received the call is allocated for use as the speech channel. If the call is to a public telephone subscriber the trunk 53 (Fig. 10) is seized and the number of the called subsriber is sent. A speech connection is established between the base station which will handle the call and the C.C. terminal and a speech channel allocation message is transmitted over the control channel to the calling mobile which then locks on to the allocated speech channel, transmits an acknowledge signal on the speech channel and awaits a 'ring-back' tone. If the allocated channel is the control channel the base station ceases to transmit its code on that channel so as to prevent any mobiles from locking on to that control channel. If the call is to another mobile subscriber the location store is checked for the address of the base station via which the mobile can be contacted, and a call up signal is transmitted on that base station control channel. If no acknowledgement is received, the call up signal is transmitted from the base stations of neighbouring areas and an acknowledgement awaited on any of the respective control channels. Receipt of the acknowledge signal leads to a speech channel allocation as for the calling mobile station. Completion of call rinqing.-Ringing or ring- back tone is supplied to the mobiles over the respective speech channels and the tones are supplied to either a speaker 310 or the handset 312 (Fig. 18). Ringing current is supplied to a called public telephone subscriber in the normal manner. Change of service area during a call.-A base station continuously monitors the signal levels of the incoming legs of the speech connections being handled by it and when the level of a speech connection falls below a minimum level, the C.C. terminal is notified and all neighbouring base stations are instructed to monitor the particular speech channel. The C.C. terminal then causes the mobile unit to emit a test signal which is monitored at all the base stations and a new speech channel is allocated at a base station having free channels and at which the monitored test signal has the highest level. If no other speech channel can be allocated a flasher is caused to operate in the mobile unit to indicate that the call will be cut off if the mobile carries on moving. Change of speech channel within the same area. If a control channel is allocated as the speech channel, a channel change procedure is put into operation as soon as a speech channel of the particular base station becomes free. Details of the central control terminal (Fig. 10). The central control terminal operates in the general manner of a telephone exchange and in addition exercises control over the base stations using signalling paths separate from the speech paths. An operator position is provided as is a voice announcement unit for transmitting certain messages when a connection cannot be established for various reasons. A teletypewriter 132 and tape deck 136 are provided for filing purposes and for providing statistical information on the system usage. Programs are provided for automatically testing the operation of the system. The mobile unit (Fig. 18) comprises a keyboard 306 and buttons 342 for frequently wanted numbers. Keyed numbers are displayed at 303 and a "Horn" button is provided which when depressed causes the car horn to sound in response to an incoming call. Both a loudspeaker 310 fed from an amplifier and a handset 312 are provided, the lifting of the handset causing the speaker to be muted. The unit circuitry (Fig. 16, not shown), includes means to prevent two mobiles from repeatedly seizing a control channel simultaneously. It includes a random number generator which causes a random delay to be inserted between a first and second attempt to seize the control channel. Use of the system by other types of mobile unit.- The system may be used by mobile units of the I.M.T.S. type which do not have remote channel assignment capabilities. For such mobiles the control channel has to be allocated for a speech connection. The system may also provide for both despatch and mobile telephone type traffic.