GB2120049A

GB2120049A - Radio calling system

Info

Publication number: GB2120049A
Application number: GB08310068A
Authority: GB
Inventors: Harry Koch; Jochen Drake
Original assignee: AEG Telefunken Nachrichtentechnik GmbH
Current assignee: Bosch Telecom GmbH
Priority date: 1982-04-20
Filing date: 1983-04-14
Publication date: 1983-11-23
Also published as: PT76560A; NL8301371A; DK171483A; FI77553C; PT76560B; NO831378L; DK160031B; DE3215292A1; GB2120049B; NO160957C; ES521575A0; DE3215292C2; FI77553B; ES8402487A1; DK171483D0; FI831316A0; DK160031C; NO160957B; FI831316L; GB8310068D0

Abstract

A radio calling system comprises a plurality of service areas (6) each with local exchange (1,2, 3, 4 or 5) and respective transmitters (e.g.7 to 13) synchronised by signals from local exchange (2) and working in synchronous common carrier frequency broadcast operation. To ensure maintenance of high frequency synchronism in the entire region composed of these service areas in conjunction with minimum costs, a respective highly stable frequency standard is provided in each of the local exchanges. Synchronization between the local exchanges and a main exchange may take place daily, weekly, or monthly. The main exchange supplies modulation signals to the local exchanges. <IMAGE>

Description

SPECIFICATION Radio calling system The present invention relates to a radio calling system.

A radio calling system is disclosed in "Technischen Mitteilungen AEG-TELEFUNKEN 68(1978)6/7, pages 287 to 296 and serves particularly for a town area, an entire region or even several regions.

In the servicing of such a large service area by synchronous common frequency broadcast operation, it is disadvantageous that high costs are involved in synchronising signal connections, for example line costs, orforfrequency separating filters.

Although these costs would disappear if a quasisynchronous common frequency broadcast operation, which is described in the Journal "Elektrizitats- wirtschaft, year80 (1981), volume 6, pages 187 to 198, were to be used in place of synchronous common frequency broadcast operation, the frequency of each and every transmitter in this mode of operation is prepared by a highly stable oscillator (frequency standard). This oscillator is present as a self-sufficient component group in each local transmitter and the carrierfrequency is thereby independent of external reference frequency sources and their availability, for example atmospherically disturbed standard frequency reception.However, in the case of a large service area and thus a plurality oftransmitters, the financial expenditure forthe equivalent plurality of frequency standards is considerable.

There is accordingly a need for a radio calling system which is suitable for a region largerthan a service area and in which the expenditure for synchronisation connections orfrequency separating filters is reduced on the one hand yet without requiring a highly stable frequency standard for each of the transmitters within the region.

According to the present invention there is provided a radio calling system comprising a plurality of local exchanges each associated with a respective one of a corresponding plurality of service areas which each overlap at least one neighbouring one ofthe areas, a respective plurality of local transmitters which are disposed at fixed locations distributed within each service area and which are arranged to receive synchronising and modulation signals from the associated local exchange to enable synch ronous common frequency broadcast operation with simultaneoustransmission of identical call signals for reception by mobile receiving means in that service area, and a main exchange for supplying the modulation signals to the local exchanges, the local exchanges having a highly stable frequency standard to enable quasi-synchronous common frequency broadcast operation ofthetransmitter pluralities in respectively overlapping service areas.

In a preferred embodiment, the radio calling system comprises a plurality of local exchanges forthe emission of radio calls, through several locally fixed transmitters associated with each exchange and distributed over a respective service area, to mobile radio call receivers. The transmitters of each service area are provided for working in synchronous common frequency broadcast operation and for simultaneouslytransmitting call signals, which are alike, by reason of modulation signals which come from the local exchange and are compensated with respectto the transit time, synchronising signal connections being provided to the transmitters to respective synchronising equipment.Each two neighbouring service areas overlap at their edges and a respective highly stable frequency standard is provided in each ofthe local exchanges so thatthe service areas neighbouring each other work in the quasi-synchronous common frequency broadcast operation. The local exchanges themselves receive the modulation signals from a main exchange for largely simul taneousemission bythetransmitters.

Thus, when a region is sub-divided into several service areas, within which transmitters each synchronised by a local exchange work in synchronous common frequency broadcast operation, then the local exchangesthemseivesdo nothaveto be synchronised by the main exchange and thereby become dependent in respect of the synchronisation on its operational capability. Instead, the local exchanges can be provided with frequency standards with little cost by virtue of their relatively low number and in such a manner that service areas, which neighbour one another and intentionally overlap at their edges, work in quasi-synchronous common frequency broadcast operation. A practical compromise is thus provided between the required operational reliability and the cost of achieving this reliability.

A preferred embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawing, which is a diagram of the layout of a radio calling system embodying the invention.

Referring now to the drawing, there is shown a radio calling system comprising a plurality of exchanges 1 to 5 each associated with a respective service area, for example 6, disposed within a region to be serviced.

Several transmitters, for example 7 to 13, are arranged in each service area for emission of radio calls to mobile radio call receivers. Within each service area 6, synchronous commonfrequency broadcast operation takes place. In that case, modulation signals and synchronising signals are delivered to the transmitters 7 to 13 from the exchange 2 byway oftwo-wire or four-wire connections 7a to 1 3a.After equalisation of any transit time differences within the service area, it is ensured thatthese transmitters emit the call signals simultaneously by reason ofthe modulation signals from the exchange 2 and thatthetransmitters are exactly synch ronised with each other in respect of the highfrequencytransmitting carrierfrequency by reason of the synchronising signal connections. In that case, such techniques can be used as are known in connection with synchronous common frequency broadcast operation.

One main exchange 14 is associated with several exchanges 1 to 5. This main exchange does not, however, serve for synchronisation of the exchanges 1 to 5 in terms of high frequency, butmerelydelivers the modulation signals through connections 15to 19, for example two-wire connections or directional radio linkstothe exchanges 1 to 5 and thustothe transmitters 7 to 13 forthe largely simultaneous and phase-rigid emission. The transit time differences within the region and also within several regions are to be equalised in that case.A synchronisation ofthe exchanges 1 to 5 in terms of high frequency from the main exchange 14 either does nottake place or at most occasionally, for example daily, weekly or monthly, wheneverfollow-upsynchronisation may be necessary, because the exchanges 1 to 5 comprise long term highly stable oscillators as frequency standards in orderto achieve a quasi-synchronous common frequency broadcast operation. In that case, known techniques can again be resorted to, for example as described in the journal "Elektrizitatswirtschaft", year 80(1981), volume 6, pages 187 et seq.On the basis of the recommendations in this journal the frequency standards in the exchanges 1 to 5 can be arranged to beso stable that the frequency deviations between the individual service areas amount to at most 20 Hertz for the carrierfrequency used. Fortlris ,purpose, the relative frequency deviation of each frequency stan- dard should be smaller than 2.1 of8 per day in a temperature range of - 20" to +65 centigrade.

Access from a public telephone networktothe main exchange 14can be effected bywayofatransfer exchange 20 so that a user of a public telephone can reach a mobile radio call receiver by way of the radio calling system.

Several such radio calling systems can be associ ated with a telephone network so thattransferfrom the telephone network to one of several main ex changes can take place, in particularto that onewhere the desired mobile radio call receiver happens to be, ortoseveral main exchanges when the whereabouts of the receiver is not known.

Claims

1. A radio calling system comprising a plurality of local exchanges each associated with a respective one of a corresponding plurality of service areas which each overlap at least one neighbouring one of the areas, a respective plurality of local transmitters which are disposed at fixed locations distributed within each service area and which are arranged to receive synochronising and modulation signals from the associated local exchange to enable synchronous common frequency broadcast operation with simultaneoustransmission of identical call signals for reception by mobile receiving means in that service area, and a main exchangeforsupplyingthe modulationsignalstothelocal exchanges,the local exchanges having a highly stable frequency standard to enable quasi-synchronous common frequency broadcast operation of the transmitter pluralities in respec tivelyoverlapping service areas.

2. Asystem as claimed in claim 1, wherin the local exchanges are arranged to receive synchronising signals from the main exchange at desired time intervals.

3. A system as claimed in either claim 1 or claim 2 in combination with a telephone network, the main exchange of the system being accessable byway of the telephone network.

4. A system as claimed in any one of the preceding claims in combination with at least one further such system and with a telephone network, the main exchange of each system being accessable by way of the telephone network.

5. A radio calling system substantially as hereinbefore described with reference to the accompanying drawing.