DK141190B

DK141190B - Radiocommunications Systems.

Info

Publication number: DK141190B
Application number: DK100374A
Authority: DK
Inventors: Peter L Son Lunden
Original assignee: Philips Nv
Priority date: 1973-02-28
Filing date: 1974-02-25
Publication date: 1980-01-28
Also published as: FR2219595B1; DE2408587C3; FR2219595A1; NL7402506A; IT1008329B; GB1423972A; DK141190C; BE811603A; SE370841B; DE2408587A1; JPS5026417A; DE2408587B2; CH574697A5

Description

(li) FREMLÆGGELSESSKRIFT 141190 (S) v Rfty DANMARK (bi) int.ci.3 h % a 7/02 «(21) Ansøgning nr. 100^/7^- (22) Indleveret den 25· feb. 197^ (23) Løbedag 25, feb. 197^ (44) Ansøgningen fremtogt og 0n 1nfin fremlaggelseeetcriftet affentHggjort den d.O. jan. lyou(li) PUBLICATION 141190 (S) v Rfty DENMARK (bi) int.ci.3 h% a 7/02 '(21) Application No. 100 ^ / 7 ^ - (22) Filed on 25 · Feb. 197 ^ (23) Race Day 25, Feb. 197 (44) The application was forwarded and presented in the submission letter dated the day before. Jan. Lyou

DIREKTORATET FORDIRECTORATE OF

PATENT-OG VAREMÆRKEVÆSENET (30) Prioritet begesret fra denPATENT AND TRADE MARKET (30) Priority requested from it

28. feb. 1975, 7502805, SEFeb 28 1975, 7502805, SE

(71) N.V. PHILIPS' GLCEILAMPENFABRIEKEN, Emnesingel 29, Eindhoven, KL.(71) N.V. PHILIPS GLCEILAMPENFABRIEKEN, Emnesingel 29, Eindhoven, KL.

(72) Opfinder: Peter L.son Lunden, Fack 175 20 Jaerfaella 1, SE.(72) Inventor: Peter L.son Lunden, compartment 175 20 Jaerfaella 1, SE.

(74) Fuldmægtig under sagens behandling:(74) Plenipotentiary in the proceedings:

Internationalt Patent-Bureau.International Patent Office.

(54) Radiokommunikationsanlæg.(54) Radio communication systems.

Opfindelsen angår et radiokonzmunikationsanlaeg med et antal stationer, som hver består af en sendesektion til overføring af et informationssignal, som er moduleret på en bærebølgefrekvens, der er udvalgt ud af et antal bærebølgefrekvenser eller kanaler, samt en modtagesektion til modtagelse og demodulering af et bærebølgesignal, som udgår fra en anden station, i hvilket anlæg sende- og modtage-sektionerne samvirker med en kanalvælger, som automatisk udvælger en af de ikke-optagne og interferensfri kanaler, når der skal etableres en forbindelse med en eller flere andre stationer, hvorhos der endvidere findes et skanderingsorgan, som for hver skanderingscyklus successivt afstemmer modtagesektioneme på hver af de nævnte kanal frekvens er i hver station, som ikke er optaget af en forbindelse, og detekterer et muligt indkommende opkald for en af stationerne i form af et særligt identifikationssignal, som er tilknyttet den relevante station. Opfindel- 141190 2 sen vedrører navnlig et apparat til automatisk udvælgelse af en passende kanal i en station, som ønsker at etablere en forbindelse med en eller flere andre stationer, hvilken udvælgelse dels skal ske på en sådan måde, at indbrydning i en optaget kanal ikke kan udføres, dels på en sådan måde, at anlægget er maksimalt ufølsomt over for forstyrrelser eller blokeringer.BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a radio communication system having a plurality of stations, each consisting of a transmitting section for transmitting an information signal modulated on a carrier frequency selected from a plurality of carrier frequencies or channels, and a receiving section for receiving and demodulating a carrier signal. starting from another station in which the broadcast and receiving sections interact with a channel selector, which automatically selects one of the non-recorded and interference-free channels when establishing a connection with one or more other stations, further comprising there is a scanning means which, for each scanning cycle, successively matching the receiving sections of each of said channel frequency is in each station not occupied by a connection and detects a possible incoming call for one of the stations in the form of a special identification signal which is associated with the relevant station. In particular, the invention relates to an apparatus for automatically selecting an appropriate channel in a station which wishes to establish a connection with one or more other stations, which selection must be made in such a way that intrusion into a recorded channel does not occur. can be performed, in part in such a way that the system is at maximum insensitive to interference or blockage.

Risikoen for indbrydning i en optaget kanal kan være afhængig af de forskellige stationers geografiske beliggenheder. En første station, som er i færd med at udvælge en kanal, kan være beliggende uden for området for en anden station, som har en forbindelse med en tredje station, hvilken anden station på detpågældende tidspunkt overfører et signal til denne tredje station. Ved valg af den kanalfrekvens, på hvilken den anden station overfører, vil modtagelsen i den nævnte tredje station blive forstyrret på trods af, at den første station ligger uden for den anden stations område.The risk of breaking into a recorded channel may depend on the geographical locations of the various stations. A first station which is in the process of selecting a channel may be located outside the range of a second station having a connection with a third station, which at that time transmits a signal to that third station. In selecting the channel frequency at which the second station transmits, the reception in said third station will be disturbed despite the fact that the first station is outside the area of the second station.

Med henblik på at reducere denne risiko for indbrydning i en bestående forbindelse til et minimum har det været foreslået sammen med informationen at medoverføre et særligt optaget-signal, som har et større modtagelsesområde end et koblesignal, der overføres sammen med kombinationen af informationssignalet og optaget-signalet. Med henblik på at realisere dette på en effektiv måde er det nødvendigt, at optaget-signalet har et større modtagelsesområde end koblesignalet, f.eks. et område der er dobbelt så stort, men dette er vanskeligt at realisere.In order to reduce this risk of intrusion into an existing connection to a minimum, it has been proposed, together with the information, to transmit a particular busy signal which has a larger reception area than a switch signal transmitted together with the combination of the information signal and the busy signal. signal. In order to realize this effectively, it is necessary that the busy signal has a larger reception area than the switch signal, e.g. an area that is twice the size, but this is difficult to realize.

Opfindelsen tager sigte på at reducere risikoen for indbrydning i en bestående forbindelse til et minimum uden medoverføring af et særligt optaget-signal, som har et større modtagelsesområde.The invention aims to reduce the risk of breaking into an existing connection to a minimum without co-transmitting a particular busy signal which has a larger reception area.

Hvis kommunikationsanlægget forstyrres eller blokeres med overlæg, udføres dette almindeligvis ved en enkelt frekvens eller samtidigt ved nogle få frekvenser. Hvis anlægget har et stort antal kanaler til rådighed, blokeres disse kanaler successivt, og blokeringstidsintervallet pr. kanal vil være forholdsvis kort, således at en sådan blokerings indvirkning på en bestående forbindelse ikke er særlig alvorlig. Under etableringen af en forbindelse er anlægget imidlertid meget følsomt overfor denne art af tilsigtet blokering.If the communication system is interrupted or blocked with overlay, this is usually done at a single frequency or simultaneously at a few frequencies. If the system has a large number of channels available, these channels are blocked successively and the blocking time interval per channel will be relatively short, so that the effect of such a block on an existing connection is not very serious. However, during the establishment of a connection, the plant is very sensitive to this kind of intended blocking.

Opfindelsen tager også sigte på at realisere en kanaludvælgelse, ved hvilken risikoen for, at en sådan hændelig eller tilsigtet blokering hindrer etableringen af en forbindelse, er minimal. I tilfælde af hændelige eller tilsigtede periodisk forekommende blokeringer må man naturligvis under udvælgelsen foretrække en kanal, som er fri for blokering, frem for en blokeret kanal.The invention also aims to realize a channel selection in which the risk that such an accidental or intentional blockage prevents the establishment of a connection is minimal. Of course, in the case of accidental or intentional periodic blockages, a channel that is free of blocking must be preferred over a blocked channel during selection.

3 1411903 141190

Basisaspektet for opfindelsen er, at hver station, sot ikke er optaget i en forbindelse, kontinuerligt skanderer alle kanaler ved successiv periodisk afstemning af modtagesektionen på de forskellige kanalfrekvenser med henblik på at detektere et muligt opkald i form af et særligt identifikationssignal, som er tilknyttet den pågældende station,og at etablere forbindelsen,når dette særlige i-dentifikationssignal modtages på efi af kanalfrekvenserne.The basic aspect of the invention is that each station, soot not recorded in a connection, continuously scans all channels by successively periodic tuning of the receiving section at the various channel frequencies in order to detect a possible call in the form of a special identification signal associated with it. and to establish the connection when this particular identifier signal is received on the efi of the channel frequencies.

Ifølge opfindelsen omfatter kanalvælgeren et tilknyttet lager for hver af kanalerne, midler til ændring af en i hvert lager oplagret signalværdi med et givet beløb i den samme retning, hver gang den relevante kanal skanderes og intet ' - r- , signal modtages deri, således at størrelsen af den totale oplagrede værdi udgør en indikation af den tid, i hvilken den relevante kanal ikke har modtaget et signal, og midler til tilbagestilling af den oplagrede signalværdi til en begyndelsesværdi, når den relevante kanal skanderes og et signal modtages deri, hvilken kanalvælger endvidere er forsynet med et styreorgan, sot er forbundet med lagrene og sot reaktion på en anmodning om forbindelsesetablering og på de nævnte oplagrede signalværdier udvælger en kanal, hvis oplagrede signalværdi har en størrelse, der er repræsentativ for det øvre område af tidsindikeringen.According to the invention, the channel selector comprises an associated memory for each of the channels, means for changing a signal value stored in each memory with a given amount in the same direction each time the relevant channel is scanned and no signal is received therein, so that the size of the total stored value is an indication of the time during which the relevant channel has not received a signal, and means for resetting the stored signal value to an initial value when the relevant channel is scanned and a signal is received therein, which channel selector further is provided with a control means, soot is associated with the bearings, and soot response to a connection establishment request and on said stored signal values selects a channel whose stored signal value has a size representative of the upper range of the time indication.

Ved anvendelse af disse trådt ifølge opfindelsen vil størrelsen af de signalværdier, som er oplagret i de respektive lagre, indikere den tid, sot er forløbet siden den pågældende kanal blev optaget eller blokeret. Kanaludvælgelsen kan da udføres på basis af de målte tider. Som udvælgelseskriteriim kan man benytte det forhold, at en signalværdi, sot indikerer et langt tidsinterval, indikerer en stor sandsynlighed for, at den pågældende kanal er ledig, og at udvælgelsen af en sådan kanal må foretrækkes frem for udvælgelse af en kanal, hvis lagers signalværdi indikerer et kortere tidsinterval.Using these strands according to the invention, the magnitude of the signal values stored in the respective stores will indicate the time that soot has elapsed since the particular channel was recorded or blocked. The channel selection can then be performed on the basis of the measured times. As a selection criterion, one can use the fact that a signal value, soot indicates a long time interval, indicates a high probability that the channel in question is free and that the selection of such a channel must be preferred over the selection of a channel whose storage signal value indicates a shorter time interval.

Af specielle årsager kan det imidlertid være ønskeligt ikke at udvælge den kanal, hvis lager indikerer det længste tidsinterval, men i stedet at udvælge en kanal, som f.eks. giver den tredjelængste tidsindikering. Når anlægget blokeres ved hjælp af et blokeringssignal, hvis frekvens varieres på en sådan måde, at de forskellige kanalfrekvenser blokeres successivt, gældet det i virkeligheden for en kanal, hvis lager indikerer det længste tidsinterval, at denne kanal har været fri for blokering i den længste periode, og at det derfor er meget sandsynligt, at denne kanal snart vil blive blokeret. Ved udvælgelse af den kanal, hvis lager giver den tredjelængste tidsindikering, er der større sandsynlighed for, at forbindelsen kan etableres inden kanalen blokeres.However, for special reasons, it may be desirable not to select the channel whose storage indicates the longest time interval, but instead to select a channel, such as provides the third longest time indication. When the system is blocked by a blocking signal, the frequency of which is varied in such a way that the different channel frequencies are blocked successively, in effect for a channel whose storage indicates the longest time interval that this channel has been free from blocking for the longest time. period, and therefore it is very likely that this channel will soon be blocked. When selecting the channel whose storage provides the third longest time indication, the connection is more likely to be established before the channel is blocked.

I tilfælde af en meget ringe grad af trafik vil størsteparten af lagrene give lange tidsindikeringer. Under disse omstændigheder kan det være ønskeligt at udvælge en vilkårlig kanal. Hvis lagrene har en sådan konstruktion, at den givne tidsindikering ikke kan overstige et bestemt maksimum, kan kriteriet for 1A1190 4 udvælgelse af en kanal være et sådant, at kanaludvælgelsen udføres vilkårligt, hvis i det mindste nogle af lagrene giver den maksimale tidsindikering.In the case of a very small amount of traffic, most of the stocks will give long time indications. In these circumstances, it may be desirable to select any channel. If the bearings are of such a construction that the given time indication cannot exceed a certain maximum, the criterion for selecting a channel may be such that the channel selection is carried out arbitrarily if at least some of the stocks provide the maximum time indication.

Når anlægget sættes i drift er det ligeledes fordelagtigt at udføre kanaludvælgelsen vilkårligt, hvilket kan opnås på enkel måde ved indstilling af alle lagre i et anlæg af den beskrevne art på en sådan måde, at de giver den maksimale tidsindikering, når anlægget sættes i drift.When the system is put into operation, it is also advantageous to carry out the channel selection arbitrarily, which can be achieved in a simple manner by adjusting all the stocks in a plant of the type described in such a way as to give the maximum time indication when the system is put into operation.

I tilfælde af en meget høj grad af trafik vil alle lagre i anlægget give korte tidsindikeringer, og det kan være ønskeligt at udvælge kanalerne i overensstemmelse med et særligt kriterium. Dette kan opnås, hvis de til kanalerne knyttede lagre er indrettet på en sådan måde, at de ikke kun giver de nævnte tidsindikeringer, men også kan oplagre et specielt optaget-signal, hvis den modtagne bærebølge i den pågældende kanal udgår fra en sender, som er tilknyttet anlægget.In the event of a very high level of traffic, all storage facilities in the plant will give short time indications, and it may be desirable to select the channels according to a specific criterion. This can be achieved if the bearings attached to the channels are arranged in such a way that they not only provide the said time indications, but can also store a special busy signal if the received carrier in that channel emits from a transmitter which is associated with the plant.

Kriteriet kan da være et sådant, at den kanal udvælges, som giver den længste tidsindikering, og som ikke har oplagret et optaget-signal i det nævnte lager.The criterion may then be such that the channel which gives the longest time indication and which has not stored a busy signal in said storage is selected.

Opfindelsen er i det følgende forklaret nærmere eksempelvis under henvisning til tegningen, hvor fig. 1 viser en forenklet udførelsesform for en radiostation med en sendesektion og en modtagesektion i anlægget ifølge opfindelsen, og figurerne 2 og 3 viser diagrammer over funktionen dels under skanderingen dels under en automatisk kanaludvælgelse i den i fig. 1 viste radiostation.The invention is explained in greater detail below, for example with reference to the drawing, in which fig. 1 shows a simplified embodiment of a radio station having a transmitting section and a receiving section in the system according to the invention, and figures 2 and 3 show diagrams of the function partly during the scanning and partly during an automatic channel selection in the embodiment of FIG. 1.

Fig.l viser en sender l,en modtager 2 og en frekvenssyntetisator 3.Syntetisato-ren frembringer et antal på f.eks. 32 forskellige frekvenser, som hver bestemmer en kanal til overføring og modtagelse (simplex). Når der er etableret en forbindelse med en anden station, fastlåses syntetisatoren på en udvalgt frekvens. Syntetisatorens udgangsfrekvens er dels tilpasset efter senderen l,hvor den kan frekvensmoduleres med de talesignaler, der skal overføres, dels tilpasset efter modtageren 2, i hvilken den tjener som en lokal oscillatorfrekvens til de-modulering af indkommende signaler. I den givne udførelsesform bliver et specielt optaget-signal, som frembringes ved hjælp af en signalgenerator 4 og er beliggende i et frekvensbånd over talesignalet, overført sammen med informationssignalet.Fig. 1 shows a transmitter 1, a receiver 2 and a frequency synthesizer 3. The synthesizer produces a number of e.g. 32 different frequencies, each of which determines a channel for transmission and reception (simplex). When a connection is established with another station, the synthesizer is locked to a selected frequency. The output frequency of the synthesizer is partly adapted to the transmitter 1, where it can be frequency modulated with the speech signals to be transmitted, and partly adapted to the receiver 2, in which it serves as a local oscillator frequency for demodulation of incoming signals. In the given embodiment, a particular busy signal generated by a signal generator 4 and located in a frequency band above the speech signal is transmitted together with the information signal.

Når stationen ikke har nogen forbindelse med en anden station, udføres kanalskanderingen. f*et betyder,at syntetisatorenχ rækkefølge leverer de forskellige frekvenser til modtageren. Til det formål styres syntetisatoren 3 gennem en frek-venskodegenerator 5 fra et kanaludvælgelsesapparat 6. Dette apparat indeholder bl.a. et kanalregister, hvis indhold bestemmer kanalindstillingen, og som hver gang varieres under kommando af et styreorgan 7. For hver kanalindstilling i kanalvælgeren 6 frembringer kodegeneratoren 5 en særlig binærkode, som tilføres til syntetisatoren 3 til indstilling af den relevante kanalfrekvens.When the station has no connection to another station, the channel scan is performed. f * et means that the synthesizerχ orderly supplies the different frequencies to the receiver. For this purpose, the synthesizer 3 is controlled through a frequency code generator 5 from a channel selection apparatus 6. a channel register whose content determines the channel setting and which is each varied under the command of a control means 7. For each channel setting in the channel selector 6, the code generator 5 generates a special binary code which is supplied to the synthesizer 3 for setting the relevant channel frequency.

1Λ1190 51Λ1190 5

Kanalvælgeren 6 indeholder endvidere for hver kanal et lager 6*, som i den relevante .udførelsesform består af en tæller. Denne tæller stilles et trin fremad, hver gang kanalvælgeren 6 indstilles på dan aktuelle kanal, hvilket f.eks. kan udføres med intervaller på ca. 3 sekunder. En tilbagestillingsindgang på den nævnte tæller er forbundet med udgangen på en signaldetektor 8. Denne detektor er forbundet med modtageren 2, og den detekterer, om der fremkommer et signal i den respektive kanal. Så snart der i en kanal modtages et signal, som overstiger en bestemt tærskelværdi, leverer signaldetektoren 8 en impuls til tilbagestillingsindgangen på den tilknyttede tæller og bringer tælleren til nulstillingen.The channel selector 6 further includes for each channel a storage 6 * which in the relevant embodiment consists of a counter. This counter is set one step forward each time the channel selector 6 is set to the current channel, which e.g. can be performed at intervals of approx. 3 seconds. A reset input on said counter is connected to the output of a signal detector 8. This detector is connected to the receiver 2 and it detects whether a signal is appearing in the respective channel. As soon as a signal is received in a channel that exceeds a certain threshold, the signal detector 8 delivers a pulse to the reset input of the associated counter and brings the counter to the reset.

Endelig indeholder kanallageret 6’ for hver kanal en lagerbeliggenhed til oplagring af et optaget-signal, når dette leveres af optaget-signal-detektoren 9.Finally, the channel storage 6 'for each channel contains a storage location for storing a busy signal when supplied by the busy signal detector 9.

Denne detektor 9 er forbundet med modtageren 2, pg den detekterer tilstedeværelsen af det særlige optaget-signal, som udsendes af alle stationer i anlægget, når de optager en kanal. Når der fremkommer et optaget-signal i en kanal, bliver det oplagret på den særlige lagerbeliggenhed for den pågældende kanals optagetsignal, samtidigt med at den nævnte tæller tilbagestilles.This detector 9 is connected to the receiver 2 because it detects the presence of the particular recording signal which is transmitted by all stations in the system when recording a channel. When a busy signal appears in a channel, it is stored on the particular storage location of that channel's recording signal while resetting said counter.

Ved kontinuerlig skandering af alle kanalfrekvenser har kanallageret 6* for hver kanal altid informationer om længden af tidsintervallet udtrykt ved det antal skanderingsintervaller, som er forløbet siden signalet blev modtaget i den pågældende kanal, samt informationer om, hvorvidt det sidst fremkommende signal er udgået eller ikke udgået fra en anlægget tilknyttet station.In continuous scanning of all channel frequencies, the channel memory 6 * for each channel always has information about the length of the time interval expressed by the number of scan intervals elapsed since the signal was received in that channel, as well as information about whether the last emerging signal has expired or not. started from a plant connected to a station.

Et opkald til en anden station udføres ved overføring af en identifikationskode, som er tilknyttet den station, der skal opkaldes. Ved kanalskandering på en bestemt station undersøges hver kanal med henblik på at konstatere, om identifikationskoden svarende til denne station modtages eller ikke modtages. Hvis den modtages i en bestemt kanal fastlåses frekvenssyntetisatoren på den pågældende kanalfrekvens, og der overføres et svarsignal. Efterfølgende informationssignaler kan overføres mellem stationerne.A call to another station is made by transmitting an identification code associated with the station to be called. When scanning a channel at a particular station, each channel is examined to determine whether the identification code corresponding to that station is received or not received. If received in a particular channel, the frequency synthesizer is locked to that channel frequency and a response signal is transmitted. Subsequent information signals can be transmitted between the stations.

Når en station kalder en anden station, skal der først udvælges en ikke-op-taget og interferensfri kanal. Udvælgelsen af en sådkn kanal udføres ved hjælp af styreorganet 7, som er opbygget af logiske kredsløbselementer, og som udvælger en passende kanal på basis af den information, som er oplagret i kanallageret 6' og vedrører de separate kanaler. Kriterierne for kanaludvælgelsen under forskellige driftstilstande vil fremgå af principdiagrammeme i figurerne 2 og 3.When a station calls another station, a non-recorded and interference-free channel must first be selected. The selection of such a channel is carried out by means of the control means 7, which is made up of logic circuit elements and which selects an appropriate channel based on the information stored in the channel memory 6 'and relates to the separate channels. The criteria for the channel selection under different operating conditions will be shown in the principle diagrams in Figures 2 and 3.

Fig. 2 viser et principdiagram for funktionen under aflytning (kanalskandering) i modtageren. Den første blok 10 indikerer funktionen "gå til den næste kanal", og den næste blok 11 indikerer funktionen "lad modtageren lytte i ca.FIG. 2 shows a principle diagram of the function during interception (channel scanning) in the receiver. The first block 10 indicates the function "go to the next channel" and the next block 11 indicates the function "let the receiver listen for approx.

80 ms". For den funktion, som er repræsenteret ved blokken 12, er det bestemmende, om signalet er tilstede eller ikke er tilstede i modtageren. Hvis svaret er80 ms ". For the function represented by the block 12, it determines whether the signal is present or not present in the receiver. If the answer is

Claims

6 U1190 "no", a counting pulse, symbolized by a block 13, is delivered to the counter corresponding to that channel. At the same time, a command is given to proceed to the next channel, which is symbolized by the block 10. If the answer is "yes", a reset pulse, symbolized by a block 14, is delivered to the counter, and furthermore it is examined in the block. 15, whether the identification code corresponding to the station exists or not. If the answer is "yes", an automatic response can be given by transmitting the station code, which depends on the type of the call, block 16. If the answer is "no", it is examined whether the particular busy signal appears or does not appear, block 17. If the answer is "yes", the channel is marked as being stored in the block, block 18. At the same time, a command is given to go one step further to a new channel. If the answer is "no", a command is also given to go one step further to a new channel. The same procedure is then repeated for this channel etc. Fig. 3 shows a similar principle diagram in the case of a call and in the case of an automatic channel selection. The first block 20 of the diagram represents the "Determining a Call to Another Station" feature. The next block 21 indicates the logical function "seek the channel whose corresponding counter has the highest count". The block 22 denotes an examination of whether or not the count found during the function denoted by a block 21 is higher than twenty (corresponding to about one minute). If the answer is "yes", the channel in question will be used for the call (block 23). If the answer is "no," a check is made as to whether a channel that has a non-busy indication exists or not (block 24). If the answer is "no", an indicator light "blocked" (block 25). If the answer is "yes", there is the channel with non-busy indication whose counter provides the longest time indication (block 26) and this channel is selected for the call.

A radio communication system having a plurality of stations, each consisting of a transmitting section for transmitting an information signal modulated on a carrier frequency selected from a plurality of carrier frequencies or channels, and a receiving section for receiving and demodulating a carrier signal, starting from another station in which the transmission and receiving sections interact with a channel selector, which automatically selects one of the non-recorded and interference-free channels when establishing a connection with one or more other stations, in which there is also a scanning means which, for each scan cycle, successively tunes the receiving sections on each of said channel frequencies in each station not occupied by a connection, and detects a possible incoming call for one of the stations in the form of a special identification signal associated with the relevant station , characterized