IE55042B1 - Radio system - Google Patents

Abstract

1. Radio system which enlarges a public telephone network by mobile receivers (RAa, RB'o) and by radio call exchanges (Z), which are each provided for a respective call region (A), wherein call orders, which come indirectly or directly from a telephone network (F), are with the aid of a radio call exchange transferable as call telegrams (fig. 3) into a radio network in order there to initiate the emission of a radio call, and wherein the same transmission frequency, called national frequency (Fo), is used for the emission of radio calls for several transmission areas which are directly adjacent and overlapping one the other, a decoupling in the mutual overlap ranges of the transmission areas however being provided thereby, that different cyclically recurring time slots (t1) are provided in the individual transmission areas for the emission of this national frequency, and wherein furthermore belonging to the different transmission frequencies (Fo, Fa; f1, f2, f3), which are each emissible in a respective transmission area, there are transmission frequency channels which are occupiable by radio calls and namely each by at least one (Fa; f1, f2, fe) also outside a single time slot (tl), characterised by the following features : - a code signal within a call telegram (Fig. 3) serves for the selection of two different transmission frequencies, which are each emissible in a respective transmission area and of which the one, namely the named national frequency (Fo), is provided for emission in only that time slot which is allocated to the respective transmission area (call region A), whilst the other is provided in case of need also outside this time slot for emission as transmission frequency, named homeland frequency (Fa), especially allocated to the transmission area (call region A) and namely for the emission of radio calls for such receivers (RAa), which are constantly disposed in only this transmission area, and - the homeland frequencies (Fa, Fb) of mutually adjacent transmission areas (call region A, call region B) differ each from the other and from the national frequency (Fo). [EP0111164A1]

Description

550 42 This invention relates to a broadcasting or radio system which enlarges a public telephone network by means of mobile receivers and by means of radio call centres or exchanges, each provided for a respective call district, in which, with the aid of a radio call centre, call instructions 5 coming directly or indirectly from a telephone network are connectible or transferable as call telegrams into a radio network, there to effect or initiate the broadcasting or transmission of a radio call, in which the same transmission frequency, designated national frequency, is used for transmitting radio calls for several mutually directly adjacent and 10 overlapping transmission regions, but in which decoupling is provided in the mutually overlapping areas of the transmission regions in that different cyclically recurring time segments or slots are provided in the individual transmission regions for transmitting this national frequency, and in which furthermore transmission frequency channels which can be occupied by radio 15 calls are associated with different transmission frequencies, each transmissible in a respective transmission region, and moreover, in each case at least one of said frequencies being also transmissible outside a single time segment.

Such a system is known from German Patent Specification No.

DE-OS 1441 130. In it, the same carrier frequency is used for several adjacent transmission regions (e.g. broadcast zones), but at different times, i.e., in different so-called time segments.

The invention starts from a system of this type with time segments and is suitable not only for a nationwide personal call broadcast 25 installation, but also for the organising channel of a radio-telephone network. The following description uses the conventional vocabulary for - la - 55042 or nationwide personal call radio installations for countrywide/ca 1 1 networks, although in the earlier passages,the designations commonly used for a radiotelephone network are given in parentheses. or radio A personal call broadcast/instal1 at ion of this type permits a telephone subscriber to implement the input of radio calls into separate call districts (radio service areas), or indeed nationwide .of a maximum of 10 or 12 ,., (up to 15.digits for diallingfrom.abroad) . . digits of information,/which are made visible in an optical display and stored in a call receiver. For this purpose a call instruction must be given over the telephone network by dialling a number, a so-called code word, to a radio call exchange, (connecting apparatus), which transforms the call instruction into a radio call. The transmission of a time-limited speech communication is also possible. An installation of this kind can be integrated into the overall public telephone network without making significant inroads into existing telephone technology, and can be used from every telephone terminal. The possibility exists of transmitting a call-back number as information to a call receiver subscriber not located near a telephone, by radio means. The call-back can then take place from any given telephone in the telephone network.

If such an apparatus were to be set up in the Federal Republic of Germany, for example, the latter would have to be divided into call districts which the calling telephone subscriber can reach singly or all together (nationwide) by dialling a code number. There thus arise three different call levels with different traffic densities, in which one call district, for example, can be expanded to a central processing point: 55042 First Cali Level : Traffic only within one call district, i.e. the calling telephone subscriber and the call receiver subscriber/ addressee are located in the same call district. This is 05 called the home district in this case. It is expected that about 80S of the total traffic takes place within home districts.

Second Call Level: This contains the radio call traffic initiated by a 10 telephone subscriber in one call district and received by a call receiver subscriber in another call district, and might be called the district cross-border traffic. This is expected to account for about 15% of the total traffic.

Third Call Level: 15 This relates to radio calls which are to be broadcast nationwide. This is estimated at about 5% of the traffic.

Now if, as shown in Fig 1, the same carrier frequency is used for three neighbouring call districts A, B, C for broadcasting radio calls, but a time segment procedure is 20 used , then,for example,within one minute,radio calls on this frequency will be sent out only within call district A in the first 20 seconds (first time segment), within call district B in the next 20 seconds,and within call only district C/in the last 20 seconds etc. In call districts 25 A', B* etc,the same carrier frequency can be used,if care is taken that different time segments are used in adjoining call districts. This is illustrated in Fig 2 for the carrier frequency Fo which is later to be differently designated, plotted against the time t in seconds. It is 30 clear that this carrier frequency is broadcWffft^iifi** the first 20 seconds in the call districts A and A', within - 4 - 55042 the second 70 seconds in the call districts Ii and H‘ etc, so that disturbing interferences cannot arise on the borders between neighbouring call districts.

A disadvantage of such a method of approach is that only 05 relatively short time segments are available for each call district. This limitation of the transmission time is felt to be a nuisance, all the more since it results from the demand that the broadcasts ^n* nae?gli1>Souring call districts are not mutually disturbing. But actually>this care for 10 one call district rather than the neighbouring ones seems nonsensical when one considers that on the above-mentioned first call level ,80* of the total traffic takes place anyway only within home districts. This traffic inside one and the same call district is designated home traffic. From 15 the viewpoint of one call district which perhaps has in fact a heavy home traffic, while the home traffic in the neighbouring districts is perhaps very small, it would be more appropriate if an enlarged time segment were placed at the disposal of this home district. Such a method of 20 approach is actually used in Great Britain, where a longer time segment is available for large cities than for rural areas where the traffic is smaller (European Patent Application 40 954, file reference 81 302 244). Often, however, this method cannot be applied to other countries, 25 if,for example, large cities with high density traffic are located not only in call districts B and C,but also in call district A.

German Patent Specification No.

For circumventing these difficulties,/DE-PS 26 59 570 describes a telephone and data network, in which different 30 transmission frequencies are used for decoupling mutually overlapping call districts; having reqard to a high home traffic density in an area of high subscriber density,this area (embracing several call districts) is assigned its own home frequency; this is, so to speak, overlaid on the 35 different transmission frequencies of the call districts of - 5 - 55043 the area as a supplementary transmission frequency. But this in turn has the disadvantage that receivers working on the different transmission frequencies of the call districts must be switched over when changing to another call district, and this either by hand (which is often forgotten) or automatically 5 (which demands a high technical outlay).

The object of the invention is, firstly, to avoid an inappropriately large restriction of transmission time within one call district as a result of the distribution of time segments, and secondly, to circumvent the difficulties which would arise in changing a call receiver from one call 10 district to another, if, instead of one carrier frequency with time segments, several carrier frequencies with respective time segments assigned to them were used.

This problem is solved by the radio system of the invention.

According to the invention, there is provided a radio system which 15 enlarges a public telephone network by means of mobile receivers and by means of radio call exchanges, each provided for a respective call district in which, with the aid of a radio call exchange, call orders coming directly or indirectly from a telephone network are transferable as call telegrams into a radio network, there to initiate the transmission of a radio call, in which 20 the same transmission frequency, designated national frequency, is used for transmitting radio calls for several directly adjacent and overlapping transmission regions, but in which decoupling is provided in the mutually overlapping areas of the transmission regions in that different cyclically recurring time slots are provided in the individual transmission regions for 25 transmitting this national frequency, and in which furthermore transmission frequency channels which can be occupied by radio calls are associated with different transmission frequencies, each transmissible in a respective transmission region, and moreover, in each case, at least one of said transmission frequency channels being also occupiable outside a single time 30 slot, wherein a code signal within a call telegram serves for selection between two different transmission frequencies, each transmissible in a respective transmission region, of which one, namely the said national frequency, is provided for transmission only in that time slot which is assigned to the respective —--------- - 6 - 550 42 transmission region or call district, while the other is provided in case of need for transmission outside this time slot also, as a transmission frequency, called the home frequency, specially assigned to this transmission region or call district for the transmission of radio calls for such receivers as are always located in this transmission region only, and the home frequencies of mutually adjacent transmission regions or call districts differ from each other and from the national frequency.

The basic considerations are as follows: In a personal call radio installation for nationwide calls, or in a radio-telephone network in which decoupling of mutually adjacent call districts is achieved by the assignment of different time slots, extension of the available transmission time within one call district beyond the duration of one time slot can be taken care of, inasmuch as the constant accessibility of a high percentage of the receivers receiving within one call district is not reduced by the assignment of a further transmission frequency (home frequency), because these receivers are anyway in continuous use only within the same district, so that difficulties cannot arise in changing to another call district with another home frequency. For this category of receivers, therefore, a home frequency can be made available, which may be transmitted both inside and outside the time slot provided for this call district, and, for decoupling mutually adjacent call districts, different respective home frequencies are to be provided in the latter. These different home frequencies do not disturb the operation, because the receivers assigned to them do not leave their respective home districts.

Advantageous further embodiments are described below.

In addition, and preferably, in a cycle region, different transmission frequency channels, each assigned to one transmission region in accordance with Claim 6 herein, are assigned in timewise fashion only, and the temporal assignment is exchanged or transposed cyclically for the duration of a cycle and in the rhythm of the time slot sequence, and each of the transmission frequency channels exchanging its assignment is a transmission frequency channel loaded with the same radio calls in multiple fashion successively, and in the rhythm of the time slot sequence in accordance with - 7 - 55042 Claim 8 herein.

Thus, in addition, the provision of a plurality of time slot systems permits circumvention of the permanent or lasting limitation of transmission time which arises in the cyclic relay switching of one time slot of a time 5 slot system over several mutually adjacent call districts; these time slot systems are temporally offset against one another, from the point of view of one call district, and can entirely fill a cycle; moreover, the different time slot systems have different transmisison frequencies assigned to them.

Thus, one of the data blocks normally transmitted in one time slot within a 10 cycle occupies a particular transmission frequency channel, while another data block occupies another transmission frequency channel; thus the same data block is broadcast within the cycle in several mutually overlapping transmission areas (e.g. call districts) as often as there are time slots contained in the cycle, and every data block is always transmitted in only one 15 of the transmission areas for any particular point in time.

Preferred embodiments of the invention are described with reference to the drawings, in which: Figure 1 shows various call districts to be read as a map of a country, 20 Figure 2 shows the mutual arrangement of call districts, time, and the different carrier frequencies, for a first embodiment, Figure 3 shows schematically a part of a radio telegram which is a component of a radio call.

Figure 4 is a block circuit diagram, 25 Figure 5 shows a section from the network structure of the broadcasting or radio system according to Figure 1, Figure 6 shows a frequency and time slot plan for a second embodiment, - 8 - 55042 Figure 7 shows a section of a block diagram for the second embodiment.

Figure 8 shows one of the cycles in which transmission takes place in both embodiments, and Figure 9 shows the transfer function in performance units plotted against the frequency, for the transmission amplifier of the transmitters of the second embodiment.

In Figure 1, a radio call exchange Z (connecting apparatus) is located in each call district A, B, C, A1, B'. The various radio call exchanges are connected with one another by means of data telecommunications lines DF. A part of the public telephone network can, however, take the place of the latter. The radio call exchanges Z serve as input gates for call instructions which come from the public telephone network. Each radio call exchange Z serves sub-exchanges US1, US2, ..., UAn distributed over the relevant call district, e.g. A, and each of these in turn serves at least one double transmitter, viz. SAol/SAal, SAo2/SAa2, .... SAon/SAan. In these designations, valid for the first embodiment with the features of patent Claim 1, S stands for transmitter, A for the call district A, o for the national frequency Fo transmitted by this transmitter as carrier frequency, a for the home frequency Fa transmitted by the relevant transmitter, and the digits 1, 2, 3 ... n for the number of the transmitter within a call district. Accordingly, for example, the double transmitter SB'o2/SB'b2 is set up in the call district B1 as a second double transmitter, and it consists of two transmitters, namely a regional transmitter SB'o2, which works on the national frequency Fo, and a home transmitter SB'b2 which works on the home frequency Fb.

For the first embodiment, Figure 2 shows, plotted against the time t, the particular points of time at which the national frequency Fo and the home frequencies Fa, Fb and Fc may be transmitted. Thus the national frequency in the various_ - There is thus provided a decoupling of mutually adjacent call districts A, 8, C by assignment of different time slots tl, t2, t3 for a transmission frequency (national frequency or time segment frequency Fo) which is common to the call districts, transmitted by wireless means,and sends calls. An extension of the transmission time one available within one district beyond the duration of/ time slot is achieved in that although a further transmission frequency, the so-called home frequency Fa, is made available, it may also be transmitted outside the time slot tl provided for the district A in question; thus the decoupling of mutually adjacent call districts A, B, C is achieved by different home frequencies Fa, Fb, Fc. Furthermore ,the home frequencies are only made available to receivers which anyway are always used only inside the same district A.

In order to permit a call receiver subscriber who has received a radio call sent by a telephone subscriber to call back, it is necessary to transmit, with the call to the call receiver subscriber, the exchange code which the call receiver subscriber should dial for his telephonic call back. Apart from the exchange code, the remainder of the telephone number of the person to be called by the call receiver subscriber must be transmitted as well. The complete call number will be designated information in what follows. The information capacity must suffice for at least ten digits (fifteen for incoming foreign calls); it is assumed that the 0 of the exchange code can be dispensed with.' The totality of the numbers to be dialled by the telephone subscriber as a call instruction (beginning with the call number of the call receiver subscriber who is to be called) will be designated “call telegram" in what - 10 - 5 5 0 4 2 follows, although in practice it is supplemented at the beginning by a preamble and at the end by a redundancy signal for code security.

A call telegram of this kind begins as shown in Fig 3, with four digits, namely the call sign of the radio call exchange (connecting apparatus) followed by two so-called code digits; then follow six digits, namely the number of the call receiver subscriber who is to be called (radio call address), and finally a further ten digits (information), namely the telephone number of the telephone subscriber who is to be called by the call receiver subscriber, which latter are displayed on the display appliance of the receiver of the call receiver subscriber.

The choice of the two code digits, which were also provided in conventional personal call radio installations for nationwide call networks, allows the following to be achieved: by means of the first code digit, the calling telephone subscriber can inform the radio call exchange(connecting apparatus) in which call district (radio service area) (maximum 9) the call is to be made. In the case of a nationwide radio call (code digit 0),the radio call exchange addressed will transmit the call to all other connected radio call exchanges by means of a data communication link (e.g. the standing line "Datex Netz"). If the code digit 1 is dialled, that is, when it is known that the call receiver subscriber who is to be called is located in the call district A, the radio cal 1 exchanged!n the call district A is dialled. Dialling the code digit 2 selects the radio call district B etc.

The second code digit in the known personal call radio installations for nationwide call networks determines the type of call. In all there are ten possible call types, which can also be associated with different tariffs - 11 - - 11 - 5S042 (different tariffs can also be imposed for a call instruction for call districts located at different distances). Examples of call types are group call, spoken message, urgent call etc. 05 Now it is new that by means of the second code digit there can also be determined whether the radio call is to take place through a national transmitter or a home transmitter, i.e. using the national frequency Fo which is limited to time slots or using the home frequency Fa assigned to 10 the relevant call district. The telephone subscriber who is doing the calling must know which of two types of receivers the call receiver subscriber has at his disposal, namely either a receiver which works on the national frequency,or one which works on the home frequency. In 15 this connection,receivers which work only on the home frequency are provided for call receiver subscribers who wish to be accessible always only in the same call district and are willing to do without accessibility in other call 20 districts.

In the first embodiment,if call types such as group call, priority call, spoken message, etc are also to remain possible in the system according to the Invention, two code digits must be available for each of these call types, 25 namely, one for a radio call on the national frequency,and a further one for a radio call on the home frequency of the relevant call district.

Using the designations of the previous figures, Fig 4 reproduces a block circuit diagram of the first embodiment 30 of a radio system. Radio call exchanges Z are accessible from telephones T by way of the public telephone network F, the exchanges Z being mutually interconnected by data telecommunications lines OF. The radio call exchanges Z control sub-exchanges UA1, ..., UAn, in the call district A, 35 and/or UB1, .... UBn in the call district B etc, which - I? - »5042 sub-exchanges in turn control double transmitters for broadcasting radio calls. The radio calls are received by receivers RAa (in the call district A, working on the home frequency a), RB'o (arriving from the call district B' into 05 the call district A and working on the national frequency Fo), RAo (coming from the call district A into the call district B and working on the national frequency Fo)a,|?1lBb (remaining in its home district B and working on the associated home frequency Fb). The receivers RB'o and RAo 10 are accessed either by a purpose-made radio call with the first code digit 1 (for the call district A) or with the first code digit 2 (for the call district B), accessed, that is,by the synchronously and simultaneously working transmitters (SAol, ..., SAon) within the call district A or by the corresponding transmitters SBol..... SBon of the call district B, or - when it is not known in which call district the receivers are presently located - they are accessed as a result of dialling the first code digit 0 based on a nationwide radio call through any one of all 20 the transmitters Sxoy transmitting in the respective permitted time slot, whereby x stands for A, B, C, A', ... and y stands for 1, 2, ---- n.

The receivers RAa and RBb on the other hand are only accessible within their home district and only through the 25 transmitters SAal, ..., SAan, or SBbl, .,., SBbn on their respective home frequency Fa or Fb as the case may be.

The network structure shown in Fig 5 is a section from Fig 1 for the overlap area between the call districts A ahd B. Since this structure, however, is also to serve for the 30 description of the second embodiment, the designations selected are somewhat more general than in Fig 1. The telephone network, and with it,in particular ,the central processing point ZVST, for example,of call district A, can be reached from a telephone T. A call processor RP is 35 assigned to the central processing point. The two together - 13 - - 13 - 55042 form a call exchange Z. Several call processors are connected together by data lines. And each of them has to take over the processing for its call district from the telephone network. Each call processor RP imparts call 05 telegrams, combined into data blocks, through a fixed line network with intermediately connected sub-exchange UA to the individual transmitters SA1, SA2, ____ of the call district A, or SB1, SB2, ..., of the call district B etc.

Transmission frequencies in the VHF or the UHF region are 10 used for preference. Having regard to the resulting small range of the transmitters, less than 30 kilometers as a rule, several transmitters in the area of one call district are to be operated synchronously and simultaneously in shared channel broadcast mode, or in quasi-synchronous 15 shared channel broadcast mode, in which the respective broadcast areas should overlap geographically, in order to obtain an exhaustive radio coverage. Apart from the high frequency stability of the transmitter oscillators, J ,. . width modulators with high sweep/constancy are required. In order 20 that the reception signal can be demodulated in phase in overlap areas between radio service areas of transmitters within one district, the transmitters must be modulated with equal phases, which requires a travel time equilibrium for the modulation carriers. In.larger 25 nationwide broadcasting systems, on account of the greater distances, low frequency cables or pupinized lines can no longer be used as modulation carriers, but use is made of transmission pathways with carrier frequency sections and also pulse code modulation systems. However, a travel time 30 equilibrium for the call telegrams transmitted on the modulation carriers in these systems is very difficult to effect. The aim is therefore to use shared channel broadcast systems as far as possible ortly within one call district, where it is still possible to bridge the desired set of distances with low frequency cables or coil-loaded circuits. - 14 - *5042 The second embodiment will now be explained with reference to Figs 1, 5 and 6. To simplify the explanation the concept of "cycle region" will first be introduced. This refers to a region within a broadcasting or radio system embracing a group of mutually immediately adjacent call districts which overlap each other. In Fig 1, for example, the call districts A, B and C are mutually overlapping. In order to avoid interference disturbances, it is known, first to provide, in the overlap areas,different transmission frequency channels for the call districts, and secondly,to provide a common transmission frequency channel which nevertheless is successively occupied by the same call telegram or data block in multiple fashion in a cycle subdivided into time slots, each of the time slots ^"Sssigned to only one call district.

In the second embodiment of the present invention,supplementary measures are adopted, as illustrated in Fig 6.

Thus, different transmission frequency channels with the national frequencies fl, f2, f3 are provided, in addition to home frequencies (which are not mentioned in Fig 6); and they are at the same time used as transmission frequency channels common to the call districts A, B, C, inasmuch as a different one of the call telegrams or data blocks is transmitted successively and repeatedly in each transmission frequency channel for the duration of one cycle, but in a different call district from time slot to time slot corresponding to the mutual assignment of time slots to broadcast areas,which is unique to each transmission channel. In Fig 6,the time segments are designated tl, t2, t3,and their number within one cycle is governed by the number of the call districts A, B, C which are present in one cycle region as defined above. In Fig 1 there are three call districts which mutually overlap, but there could be more. A larger number of time segments 15 would have to be chosen for one cycle corresponding to the largest number of mutually overlapping broadcast regions in a broadcasting system.

There are three transmitter types in the broadcasting system described here, apart from receivers which work only on a respective home frequency. One type works on the transmission frequency fl, the second on f2, and the third on f3. They can all be accessed in every call district, because the same radio calls are transmitted successively on all three transmission frequencies in every call district. For this reason also.no difficulties arise in changing a receiver from one call district to another. Nevertheless,the receivers can be single-channel appliances and be fixedly tuned to one transmission frequency. This is a particularly economical solution. In order that this economy extends to the transmission side, the transmitters should be so designed that each transmitter can work on the three different transmission frequencies, so that one and the same transmitter needs only to be switched over from one transmission frequency to another in the time slot procedure.

Transmission frequency switchover is preferably undertaken with the aid of a coded switch-on signal.

The block circuit of Fig 7 partly corresponds to Fig 5. In the call processor RP, the arriving call telegrams are prepared as data blocks, which are conveyed, via modulation carriers and the sub-exchanges UA,to a transmitter SA1, and which, in principle, are of the same construction for all transmitters. The data blocks each fill up a time slot tl, t2, t3 and contain, as shown in Fig 8, a preamble for the time segment synchronization in the receiver E and the following call telegrams, as illustrated in Fig 3. A coded switch-on signal for switching over the transmission frequency in the transmitters is located in front of each - 16 - - 16 -55043 preamble as a prelude VI, V2 or V3. Each transmitter has an address decoder D with frequency recognition, so that the respective transmission frequency to be broadcast in each cycle can be switched in; for this purpose, the address decoder D controls a quartz oscillator 0 or a synthesizer. The transmission frequency delivered by the quartz oscillator 0 is modulated in a modulator A with the preamble and the following call telegram, and the modulated broadcast oscillation is fed to a transmission amplifier SV and finally to an antenna AT.

The broad-band radio amplifier SV has a switch bandwidth (performance bandwidth) which embraces the transmission channels of the transmission frequencies fl, f2, f3 to be cyclically broadcast from a transmitter, so that (in each case) only one respective transmitter and one common carrier line from the sub-exchange is necessary. Fig 9 Indicates the size of the bandwidth of the transmission amplifier. In it, the conversion function UL is plotted in performance units against the frequency f. It can be seen that the frequency spectra FS1, FS2, FS3 of the transmission channels which pertain to the transmission frequencies fl, f2, f3, lie within the performance bandwidth BB.

In practice,the transmitters in continuous operation work alternately on the three different transmission frequencies, and the switchover timepoints between the time slots are centrally controlled in synchronous fashion by the call processors for all connected transmitters of all districts. or radio This broadcasting/system is flexibly applicable to areas of greater or lesser size, because the size of a call district can be varied by setting up there either simply one transmitter or several synchronized shared channel or quasi-shared channel transmitters, and these trans- 65043 - 17 - mitters are centrally controlled by a call processor via a sub-exchange. Several call processors can in turn be connected together by data lines and thereby receive a common time pulse for the synchronously cycling time 05 segments of a nationwide radio call system. Thus a timed synchronous switchover of the transmission frequencies is possible for all transmitters.

Claims (5)

1. A radio system which enlarges a public telephone network by means of mobile receivers and by means of radio call exchanges, each provided for a respective call district in which, with the aid of a radio call exchange, call orders coming directly or indirectly from a telephone network are transferable as call telegrams into a radio network, there to initiate the transmission of a radio call, in which the same transmission frequency, designated national frequency, is used for transmitting radio calls for several directly adjacent and overlapping transmission regions, but in which decoupling is provided in the mutually overlapping areas of the transmission regions in that different cyclically recurring time slots are provided in the individual transmission regions for transmitting this national frequency, and in which furthermore transmission frequency channels which can be occupied by radio calls are associated with different transmission frequencies, each transmissible in a respective transmission region, and moreover, in each case, at least one of said transmission frequency channels being also occupiable outside a single time slot, wherein a code signal within a call telegram serves for selection between two different transmission frequencies, each transmissible in a respective transmission region, of which one, namely the said national frequency, is provided for transmission only in that time slot which is assigned to the respective transmission region or call district, while the other is provided in case of need for transmission outside this time slot also, as a transmission frequency, called the home frequency, specially assigned to this transmission region or call district for the transmission of radio calls for such receivers as are always located in this transmission region only, and the home frequencies of mutually adjacent transmission regions or call districts differ from each other and from the national frequency.

2. A radio system according to Claim 1, wherein the transmission regions are radio traffic areas of a radio-telephone network or are call districts to each of which transfer equipment or a radio call exchange is assigned.

3. A radio system according to Claim 1 or Claim 2, wherein several mutually synchronized home transmitters are provided for transmitting the home frequency in one transmission region or call district, and several mutually - 19 - 55042 synchronized national transmitters are provided for transmitting the national frequency in the transmission regions or call districts, said national transmitters operating during time slots assigned to them regionwise.

4. A radio system according to any of Claims 1 to 3, wherein two types 5 of call receivers are provided, namely one type formed as fixedly tuned single-channel receivers and suitable for receiving only one home frequency, and another type also suitable for reception of the national frequency or adapted as fixedly tuned single-channel receivers exclusively for the reception of the national frequency. 10 5. A radio system according to Claim 4, wherein, in a cycle region, which consists of the largest number of immediately adjacent and mutually overlapping transmission regions, the same transmission frequency channel is occupied, several times in succession, by the same radio calls, within a cycle corresponding to the number of time slots in the cycle, namely once for each 15 of the transmission regions of the cycle region. 6. A radio system according to Claim 5, wherein, within the cycle region and for the duration of one time slot, a different national frequency channel is assigned to each transmission region, and this assignment is transposed for the duration of a cycle cyclically and in the rhythm of the time slot sequence. 20 7. A radio system according to Claim 6, wherein each of the transmitters has a power bandwidth which embraces the transmission channels of the transmission frequencies which are to be transmitted in cyclic fashion by a transmitter. 8. A radio system according to Claim 6 or Claim 7, wherein, for 25 switching the transmitters over to the individual transmission frequencies, each transmitter contains a decoder, which switches a transmitter oscillator or transmitter synthesizer over to the required transmission frequency in the rhythm of the time slots, which rhythm is provided by a call processor by way of the appropriate radio call exchange. 30 9. A radio system according to any of the preceding claims, wherein - 20 - 550 42 control of the time slots which are assigned to individual transmission regions takes place through time clock signals which are conducted to the individual transmitters, especially national transmitters, by modulation lines. 10. A radio system according to any of the preceding claims, wherein call 5 receivers are provided for the reception of radio calls, which are suitable for storing an item of information, transmitted by the radio call for reproduction on a display and thereby to give an alerting signal, and a common transmission frequency channel is provided for transmitting an address contained in a radio call, as well as the information, and, if required, 10 speech transmission, to the call receiver. 11. A radio system substantially as described herein with reference to and as illustrated in the accompanying drawings. Dated this 2nd day of December 1983. BY:- TOMKINS & CO. Applicants' Agents, (Signed)

5. , Dartmouth Road, DUBLIN 6.