WO1987001897A1

WO1987001897A1 - Cellular mobile telephone system and method of controlling a cellular mobile telephone system

Info

Publication number: WO1987001897A1
Application number: PCT/SE1986/000403
Authority: WO
Inventors: Ha^okan SÖDERHOLM; Sven Ekholm; Kenneth Mattsen; Dag Levinsson; Per Jundin
Original assignee: Comvik Ab
Priority date: 1985-09-13
Filing date: 1986-09-11
Publication date: 1987-03-26
Also published as: SE8604011D0; SE455250B; GR862339B; SE455251B; SE8604011L; PT83369A; SE8504251L; PT83369B; EP0273913A1; SE8504251D0

Abstract

A cellular mobile telephone system having a central exchange (4) for receiving incoming calls from an external telephone net work (5), and a plurality of base stations (1, 2, 3) having connections (8, 9, 10) to the central exchange and having a plurality of radio channels AC, BC, HC for communication with intelligent mobile telephone units (6) in the associated cell area. The radio channel communication (33) of a mobile telephone unit (6) is transferred between various base stations dependent on the movement of the mobile telephone unit within various cell areas and the signal strength of the respective communication link to the base stations involved. A call transfer is commanded by the mobile telephone unit (6), the unit itself making the necessary signal strength measurements (35, 36) during short interruptions of the call in progress. The switch of the call in the mobile telephone unit is not made until the new call path has been established. Each base station (1, 2, 3) is connected to the central exchange (4) via one signle or a few lines (8, 9, 10), primarily intended for incoming calls. Outgoing calls (33) will be set up directly to the public telephone net work (5) via a local exchange (7) assigned to the base station.

Description

TITLE OF INVENTION

Cellular mobile telephone system and method of controlling a cellular mobile telephone system.

TECHNICAL FIELD This invention relates to a cellular mobile telephone system comprising a central exchange or switching station arranged for communication with an external or public telephone net work, a plurality of base stations and a number of mobile telephone stations or units, each base station having a corresponding cell area and each base station being arranged for communication with said central exchange and also having a plurality of radio channels for radio communication with mobile telephone units existing in or passing through the respective corresponding cell area. The invention also relates to a method of controlling such a mobile telephone system.

TECHNICAL BACKGROUND

As is well known a so called cellular mobile telephone system is characterized by the use of one and the same radio frequence severals times, that is, in connection with several base stations, within a rather limited area. This means that the output power of the radio transmitters of the base stations is restricted, such that the range of a radio transmitter of a radio channel will be small, typically only two to five kilometres. One and the same radio channel may then be used for instance both within a rather small cell area on one side of a greater city and within a rather small cell area on an opposite side of said city. This would have been impossible in connection with a mobile telephone system made in a conventional way. In the area of a big city in partricular, owing to a cellular configuration of the mobile telephone system it is possible to provide a great number of radio channels despite a limited allocation of radio channel frequences, such that the number of subscribers to the mobile telephone system can be comparatively high. However, a cellular mobile telephone system also means that certain specific demands must be met. Thus, the mobile telephone unit must be able to decrease and increase its transmitter power and to adapt to the transmitter power of the base station involved. For instance, if a mobile telephone unit is out in the country where it is possible to have powerful base station transmitters and great cell areas, then the mobile telephone unit must also have good output power so as to have a corresponding good range. On the contrary, if the mobile telephone unit is in a big city region and is in communication with a base station having a transmitter covering a very small cell area, then the mobile telephone unit must adapt thereto and use a correspondingly lower transmitting power in order not to interfere with mobile telephone units using the same frequence within other adjacent cell areas.

Furthermore, the mobile telephone system and a mobile telephone unit involved in a call transmission must be able to transfer the connected call between various base stations while the call is still going on, as the mobile telephone unit moves between various cell areas. The resason for this is, of course, that the range of a base station having a low transmitting power is very limited. If for instance the mobile telephone unit is provided in a car moving while the call continues, then soon the unit will be so far from the base station involved that the signals strength will be very poor with a corresponding reduction of the quality of the call. In this case, the mobile telephone station and the mobile telephone unit then must be able to transfer the call, without stopping the same, to another radio channel belonging to the new cell area towards which the car is moving, thereby securing a good quality of the call during the entire course thereof. This transfer of the call is often called hand-off. Previously known cellular mobile telephone systems are based upon the use of a very powerful central exchange which controls the. entire complicated procedure of continuously checking the call quality of each mobile telephone unit involved in a call communication, and when needed transferring a connected mobile telephone unit to another better radio channel belonging to another base station. The amount of information to be handled and further prosecuted by said central exchange is very substantial, for what reason the central exchange must include big and powerful computers. Also, known cellular systems must have intelligent base stations which on behalf of the central exchange can make all the measurements necessary in order to decide whether a connected mobile telephone unit has an acceptable call quality and to detect which other base station should be used when hand-off is needed. Therefore, previously known cellular mobile telephone systems are complicated and necessitate high investment costs. Furthermore, in the known cellular mobile telephone systems each base station is connected to the central exchange via permanently or fixedly connected telephone lines, the number of the lines being equal to the number of radio channels of the respective base station. As will be understood, this is an expensive arrangement.

OBJECT OF THE INVENTION

The object of the present invention is to provide a novel design of a mobile telephone system, as well as a novel method of controlling such a system, in particular with regard to hand-off, whereby investment and operational costs will be substantially reduced, while at the same time giving a less complicated system and a very efficient utilization of the resources of the system.

SUMMARY OF THE INVENTION

The above-mentioned object is achieved by means of a mobile telephone system and a method of controlling such a system having the features defined in the appended claims. Thus, the main idea of a system according to this invention is the use of decentralized intelligence, that is decentralized to the mobile telephone units, instead of the more centralized intelligence used in the prior art systems. Each individual mobile telephone unit thus has to provide for its own intelligence, so that it can make all the evaluations and measurements necessary in order to make it possible for the mobile telephone unit itself to control its demands (in particular with regard to hand-off) by means of simple command signals i.a. to a central exchange that consequently can be made in a much more simple way and typically can be a conventionel so called company exchange (PABX). The system according to the present invention use simple base stations which in pricipal merely pass on the radio signals transmitted by the mobile telephone units and a simplified central exchange, respectively. This is in a pronounced contrast to the intelligent stations of the prior art systems which are provided with equipment for measuring and evaluating the situation of the respective mobile telephone units involved.

According to a preferred feature of the present invention, at least most of the base stations are provided with a local or miniexchange with a call up function. This gives essential advantages as will be made more clear later on.

A further aspect of the present invention is to give a good operational economy. When using conventional cellular mobile telephone systems a great line net work has to be provided, because as previously mentioned each base station has to be connected to the central exchange via a great number of telephone lines, namely one line for each radio channel of the base station. Usually, these telephone lines are hired. On the contrary, a system according to this invention has a very limited number of fixed or permanent telephone lines (one or a few lines from the central exchange to each base station). In addition thereto, the mobile telephone system according to the present invention is a dynamic system, because a base station will connect further telephone lines to the central exchange (by means of its local exchange) only when needed, as will be described later on. A particularly essential feature of the system according to the invention is based upon the recognition that a major part of all telephone calls passing through the system is from a mobile telephone unit to a subscriber of the external or public telephone net work. According to the present invention, in conjunction with an outgoing call from a mobile telephone unit the base station need not be connected to or provide connection with the central exchange via special or fixed telephone lines. The reason for this is that the telephone call passes from the mobile telephone unit to the base station involved and the local exchange in said base station. The local exchange having a subscription of its own directly calls up the wanted subscriber number while using the public telephone net work, thereby connecting the mobile telephone unit to the wanted subscriber. As should be realized, this latter subscriber could be a mobile telephone unit of the system as well, in which case the local exchange will call up the central exchange via the public telephone net work in order to receive a further connection in the same way as when a subscriber of the public telephone net work calls up the central exchange in order to establish contact with a mobile telephone unit of the system.

As previously mentioned, a significant function or operation of a cellular mobile telephone system is the handoff function. In the system according the present invention this function is based upon the feature that the mobile telephone unit i t se l f measures the s igna l s t rength o f the rad i o c ommunication with the base station involved, that is the base station to which the mobile telephone unit is connected. When the signal strength becomes too low the mobile telephone unit detects the signal strength of the radio channels of the base stations of the adjacent cell areas. According to the invention preferably this detection is made on a selective bases which means that certain suitable radio channels only of the surrounding cell areas are detected with regard to the signal strength. The mobile telephone unit receives information as to which radio channels should be detected or examined by means of specific signalling from the base station involved in the call communication in progress. The actual detection of the respective signal strength of the appropriate surrounding radio channels is made during very short interruptions of the call communication in progress. Such short interruptions (having a typical duration of up to 200 milliseconds) have proved not to disturb the call in question. If the detection or examination of the signal strength shows that another radio channel will give better signal strength, then the mobile telephone unit knowβ that its call suitably should be transferred or switched to the base station having a better radio channel. The mobile telephone unit now sends (also during a short interruption of the call in progress) a request for hand-off or in other words a call transfer to the new base station. After necessary preparations have been made, the actual transfer of the call is made.

It should be realized that according to the invention the mobile telephone unit itself handles the operations which are to be handled by the central exchange and the base stations of the conventional cellular mobile telephone systems, namely measuring or monitoring the situation or conditions of the mobile telephone unit itself, detecting which other base station is a station suitable for hand-off, and sending a request for hand-off accordingly.

Hand-off can be accomplished in various ways. When an incoming call is involved, that is, when the mobile telephone unit has been called up via the central exchange, preferably the mobile telephone unit sends its request for hand-off to the central exchange that makes the necessary switch or transfer of the call to the other base station involved.

However, when an outgoing call from the mobile telephone unit is involved, the call having been linked to the public telephone net work via the local or miniexchange of the base station involved, hand-off suitably is accomplished by a direct connection between the two base stations involved, that is, without involving the central exchange. The above-mentioned direct connection can mean a use of fixed or permanent communication lines between the base stations of contiguous cell areas, the call connection then being accomplished by means of the respective local exchange of the base stations. It should be realized, that such permanent lines may be simple and inexpensive to provide and to keep in operation when small and closely situated cell areas are involved . However, according to the invention said direct connection preferably is established by one of the base stations calling up the other base station via the public telephone net work, that is, while using the respective local exchanges. This kind of direct connection usually will have the nature of an inexpensive local call.

The calling-up suitably is made from the base station receiving the hand-off request from the mobile telephone unit. It may be the base station in call communication with the mobile telephone unit, or the base station that has been found suitable for hand-off when the mobile telephone unit made its examination of the signal strength of the surrounding base stations. In this latter case, said request for hand-off may be transmitted by the mobile telephone unit in conjunction with the mobile telephone unit "listening" on a radio channel of the base station involved in order to detect the signal strength of said station.

According to a preferred feature of the present invention, each base station has radio channels of different types, that is, the radio channels have different roles and they will be allocated these roles in a dynamic way. Incoming radio channels primarily are used for searching mobile telephone units and for distributing incoming calls to the mobile telephone units. Outgoing radio channels primarily are used for distributing outgoing calls from the mobile telephone units. "Carrier" channels primarily are used for the hand-off function. It should be noted that the various types of radio channels may differ merely by different programme controlled signalling. The various types of radio channels will be discussed more closely later on.

According to a preferred feature of the invention, within each cell area to be part of the mobile telephone system, that is, at each base station, there are merely a few incoming radio channels, mostly one single such channel. This incoming radio channel is used for searching mobile telephone units, that is, for sending or setting up a call (usually from a telephone of the public telephone net work) to a mobile telephone unit. Thus, this incoming radio channel is connected to the central exchange providing the call distribution, said connection usually being a permanent hired telephone line. In order not to occupy an incoming radio channel with any other traffic than incoming calls, unless absolutely needed, (occupied incoming radio channels would make it difficult to find a mobile telephone unit when searching therefor), the incoming radio channel has a specific radio signalling that will be recognized by the mobile telephone unit. This means that when someone for instance in a car tries to use his mobile telephone unit, this unit always first tries to occupy another radio channel than the incoming radio channel of the base station involved. The mobile telephone unit will use the incoming radio channel for an outgoing call only if no other radio channel of the base station is free.

If the incoming radio channel or channels if there are more than one, become occupied, the arrangement of a local exchange at each base station means that temporarily a new connection can be set up while calling up the central exchange, this new connection being linked to another free radio channel which thereby can be an incoming radio channel temporarily. Of course, the signalling of this latter channel must be changed accordingly. As previously mentioned, the outgoing radio channels present in a suitable number at each base station, are inten ded for outgooing calls, that is, usually calls from a mobile telephone unit to a telephone or subscriber of the public telephone net work. A mobile telephone unit always choseβ this type of radio channel as its first choice when an outgoing call is to be made from the mobile telephone unit, said radio channel also having a specific signalling that can be recognized by a mobile telephone unit. Such a call automatically will be switched or linked to the public telephone net work at the base station via the local exchange thereof, for what reason it will not require any permanent link or connection to the central exchange. This will give a good telecommunication economy because on one hand there is no need for any hired line to the central exchange to be used for the call in question, and on the other hand the majority of the calls are local calls which means that normally the area code will not have to be changed, thus giving less expensive debit markings .

Carrier channels primarily are to be used for the handoff function. Therefore, it is advantageous to have a specifie carrier channel at each base station. When a mobile telephone unit, as a preparation for hand-off or call transfer, examines the signal strength of the base stations of the adjacent cell areas, the station thus has to measure the respective signal strength of the carrier channels of the base stations involved. Which radio channels are involved (may be predetermined) might be communicated to the mobile telephone unit directly from its base station as a part of the previously mentioned information from said station, but these radio channels may also be identified by having a specific way of signalling.

A carrier channel may be permanently or dynamically allocated. When there are plenty of radio channels, it is possible in a base station to let a certain radio channel permanently be a carrier channel only. When during a signal strength measurement a mobile telephone unit finds a good carrier channel, then the unit may send a request for hand -off directly to the carrier channel in question, said request being a request for call transfer to a radio channel (outgoing radio channel) of the base station involved. The carrier channel involved may then advise the mobile telephone unit of such a free radio channel, and also provide for hand-off accordingly, in the way as previously described. As will be realized, the carrier channel itself will not be occupied. This will give the advantage that a mobile telephone unit always will be able to find a carrier channel at each base station when making signal strength measurements as a preparation for hand-off. However, the fact that one radio channel of each base station will be used only for the handoff function means a disadvantage.

The other possibility is to devide the carrier channel function dynamically between various radio channels, primarily the outgoing radio channels. This means that the radio channel being a carrier channel also is the radio channel that will be "given" to a mobile telephone unit when the latter after having detected good signal strength from the carrier channel in question sends a request for call transfer to the associated base station. When this allocation has been made (and the radio channel signalling has been changed accordingly) and if there is a free channel (primarily an outgoing radio channel) at the base station, then the signalling of this free radio channel will be changed into carrier channel signalling, such that there again will be a carrier channel at this base station. This means that the specific carrier channel function will be allocated only if there is a channel free to be used therefor. A mobile telephone unit will receive the previously mentioned information as to which carrier channels are to be found in the adjacent cell areas and might be used in conjunction with hand-off, preferably in either of the following two ways. If the system has permanent carrier channels, then there is permanent information about these channels stored in the system. When a call is set up via a base station, this permanent information will be sent to the mobile telephone unit in conjunction with the establishment of the call. However, if the system uses dynamic carrier channels which means that there is no predetermined knowledge as to which radio channel of a base station will be carrier channel at a certain time, then the mobile telephone unit will receive updated information when needed. In this case each base station will have a radio listeing on the radio channels of the adjacent cell areas to thereby decide which channels have carrier channel signalling at the moment. The radio also stores relevant information thereon. When a mobile telephone unit needs to start hand-off, first the mobile telephone unit communicates with the base station involved in the call communication, requesting valid updated information as to which carrier channels of the adjacent base stations will transmit at the moment. The mobile telephone unit may then detect the signal strength of the radio channels mentioned in said valid updated information and may request hand-off dependent on the outcome of said detection. As will be clear from the above-mentioned, the present invention means that each base station is characterized by a far-reaching dynamic mode of operation with regard to the various radio channel functions which is extremely advantageous from a telecommunication economy point of view. Thus, with regard to each base station only one of the radio channels need to be permanently connected to the central exchange. This radio channel will then be the incoming radio channel. If the incoming radio channel becomes occupied, then it will give a programme command to a free outgoing radio channel such that the latter both changes its signalling so as to be a new incoming radio channel and calls up the central exchange while using the automatic calling up function of the associated local exchange to thereby establish a stand-by function and to serve as a provisional incoming radio channel while the regular incoming radio channel is occupied. As soon as the regular incoming radio channel beco meβ free again, then programme commands will be given for changing the signalling of the provisional incoming radio channel into outgoing radio channel signalling and for breaking down the provisional connection to the central exchange. This means that almost always there will be an incoming radio channel free for system signalling and searching mobile telephone units within the cell area in question.

A corresponding dynamic mode of operation will be found in connection with the transfer of an outgoing radio channel into a dynamic carrier channel. A carrier channel transmits the specific signal pattern necessary to make a mobile telephone unit know that this is the hand-off channel. Now, if this carrier channel becomes occupied by a call transfer, then the carrier channel sends a programme command to a free outgoing radio channel of the base station such that this latter channel will change its signalling into carrier channel signalling. This means that there will once again be a carrier channel available for the hand-off function. When the radio channel which previously was a carrier channel but now is occupied by a call, becomes free, then it will be a free outgoing radio channel having a signalling adapted thereto.

When a carrier channel becomes occupied because a call transfer thereto will be made, then a call communication will be established for the channel in question either with the central exchange (incoming call) or with the other base station involved (outgoing call), as has been discussed previously. The call connection to the central exchange may be established in many various ways. The carrier channel now transferred into a call channel, may order the associated local exchange to call up the central exchange using the public telephone net work or vice versa. It is also possible to use a permanent incoming radio channel connection of the base station to the central exchange, in which case, however, the associated radio channel should be disconnected in the local exchange and a provisional incoming radio channel should be established by connecting a free outgoing radio channel to the central exchange by calling up the central exchange via the associated local exchange. In this latter case, at the end of the transferred call the radio channel involved may be caused to adapt to incoming radio channel signalling while keeping the connection to the permanent connection to the central exchange, in which case the provisional incoming radio channel connection can be disconnected and the associated radio channel may again become a free outgoing radio channel. The above mentioned dynamic modes of operation of course also will be present in conjunction with connecting or setting up an outgoing call from a mobile telephone unit. Suppose for instance that the mobile telephone unit tries to call a subscriber of the public telephone net work while utilizing a local exchange of the base station involved. The mobile telephone unit then first tries to find a free outgoing radio channel for connection thereto, because such a radio channel is the natural and dedicated channel for an outgoing call. In case of failure in view -of the fact that no such channel is free, then the mobile telephone unit tries to connect the call using the dynamic carrier channel. If this operation is successful, of course the possibility to make a hand-off will disappear with regard to the cell area in question, but since there is no free channel (possibly with the exception of an incoming radio channel), hand-off will be meaningless. Last of all, that is, should the carrier channel be occupied also, the mobile telephone unit will make use of an incoming radio channel to establish the outgoing call. This dynamic mode and using channels with different priorities means that there will always be the best possibility to find a mobile telephone unit within a cell area when searching based upon an incoming call to the mobile telephone unit in question. Preferably, however, there should always be a possibility to set up an outgoing call on all the radio channels of the base station, because the wish is to have as great as possible the possibility to establish a call, and outgoing calls of the majority of the total number of calls. As previously mentioned the present invention means that the demands to be fulfilled by the central exchange will be essentially lower than in the conventional cellular mobile telephone systems. The decisive reason for this is that a major part of all radio signalling comeβ from and is controlled in the base stationbβ while utilizing the associated programme controlled local miniexchange, as well as the mobile telephone units. Therefore, the central exchange substantially only has to perform conventional switching functions of the kind performed by a conventional so called company exchange. Thus, the exchange has to set up incoming calls. It should be arranged for direct inward dialing and it should also be able to decode which direct inward dialing number has been called for. This function can be found in conventional company exchanges. A required specific function is that the central exchange has to make a group call on all extensions when searching a mobile telephone unit, because in the present case the extensions represent the incoming radio channels of each cell area. As previously mentioned, the mobile telephone units listen on these incoming radio channels. The fact that each extension of the central exchange represent a base station means that the numbers to be stored in a table for occupied numbers will not relate to the normal extensions (which for instance may be about 100 when a conventionel company exchange is used) but those numbers of the mobile telephone system, that is, the mobile telephone units, which are occupied for the moment. When a mobile telephone unit is in a call communication via an incoming radio channel, an outgoing radio channel or a carrier channel, then this fact is signalled to be central exchanged via the established connection between the central exchange and the incoming radio channel of the base station involved, whereby said table will be extended accordingly. This means that in conjunction with an incoming call it will be possible to check whether the called number is occupied directly by examining said table. The conventional call forwarding function which can be found in a company exchange and which means that a call can be switched from one extension to another, may be used in the system according to the present invention in order to provide hand-off in a simple way in conjunction with incoming calls. In this case, when a request for a call transfer is received from a mobile telephone unit via a carrier channel, this means that in the company exchange the only thing needed is a switch from one extension (radio channel) to another (the new radio channel).

In summary, the present invention means that it will be possible to use an inexpensive central exchange, the main function of which merely is to switch calls. In other words, the exchange basically only needs to have the same functions as a conventional company exchange. Furthermore, the invention means low investment costs with regard to the base stations, because in principle each base station is a very simple radio transmitter/receiver having an associated local miniexchange. The decentralized intelligence of the mobile telephone unit that makes this possible does not mean any essential costs, because it requires soft ware only which can be stored in the computer memory that still must be at hand in the mobile telephone unit in order to make it possible to utilize said unit in a mobile telephone system. With regard to the signalling in conjunction with the call set-up, handshaking etc., the present invention does not mean any essential demands in addition to those to be found in conventional cellular mobile telephone systems, for what reason no closer description thereof should be necessary. The invention will now be described in more detail by an exemplifying embodiment while referring to the appended drawings.

BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a schematic block diagram of a mobile telephone system in accordance with the present invention. Figure 2 is a block diagram aβ in figure 1 showing the system configuration in conjunction with an incoming call and when hand-off has to be considered.

Figure 3 is a block diagram according to figures 1 and 2 showing the system configuration after hand-off.

Figure 4 is a block diagram according to figure 1 showing the system configuration in conjunction with an outgoing call and when hand-off is to be considered.

Figure 5 is a block diagram according to figures 1 and 4 showing the system configuration after hand-off.

Figure 6 is a schematic flowchart illustrating the hand-off function.

DESCRIPTION OF EMBODIMENTS The mobile telephone system schematically illustrated in figures 1-5 comprises three base stations 1,2,3, a central exchange 4, a public telephone net work 5 and a mobile telephone unit 6 located in a car. Each base station has five radio channels with associated transmitters and receivers. In a basic condition shown in figure 1 these five radio channels comprise one incoming radio channel AC1, AC2 and AC3, respectively, three outgoing radio channels BC11, BC12, BC13, BC21, BC22, BC23 and BC31, BC32 , BC33, respectively, and one dynamic carrier channel HCi, HC2 and HC3, respectively. The signalling of the channels is in accordance therewith. Each channel has been assigned a specific channel number corresponding to a specific channel frequence. In the present case all the channel numbers and thus all the channel frequences are different, because the three cell areas associated with the three base stations are located close to each other. Each base station has an associated local miniexchange 7, the radio channels being connected thereto. Each local exchange 7 consists of a number of programme controlled exchange units or modules 7¹ , 7² , 7³, 7⁴ and 7⁵. In other words, there is one exchange unit for each radio channel, whereby each radio channel with its associated local exchange unit can be made as a module. Consequently, the number of radio channels of a base station may be changed quite simply by changing the number of modules connected in parallel.

The incoming radio channel AC1, AC2 and AC3, respectively, of each base station is permanently connected to the central exchange 4 via its local exchange unit 7¹ and a telephone line 8, 9 and 10, respectively. These telephone lines are in fact established via the public telephone network 5, but in order to make the description more clear the lines have been shown as direct connections between the respective local exchange 7 and the central exchange. Additional corresponding telephone lines 11 and 12 indicate that additional base stations, that is cell areas, may be part of the system. In a manner known per se the base stations use the telephone lines 8, 9, 10, 11, 12 for the necessary signal and information interchange with the central exchange 4. Otherwise, the base stations lack any permanent connection to the central exchange. However, the respective local exchanges have their own telephone subscriptions to the public telephone net work and also have a dialing function, such that when needed a telephone connection to a subscriber of the public telephone net work may be established , said subscribers including the local exchanges being part of the mobile telephone system. Figure 2 shows the situation of the system in conjunction with an incoming call to the mobile unit 6 and when the conditions show that it might be necessary to use the hand-off function. A public telephone net work subscriber 15 having dialed the number of the mobile unit has been connected to the central exchange 4 of the system (as indicated by the corihection 16). After having made a search on the incominbg radio channels of the system, the central exchange has received an answer on channel AC2 and consequently has set up the call on this channel via line 9. The communication with the mobile unit 6 will be via a radio communication connection 17 between the mobile unit 6 and the incoming radio channel AC2. When the incoming radio channel AC2 becomes occupied, it will send (more specifically its programme controlled local exchange unit 7¹) a command to the nearest free outgoing radio channel BC21 such that the latter changes its signalling and becomes a temporary incoming radio channel AC21. By means of its local exchange unit 7² the latter also calls up the central exchange 4 via the public telephone net work 5, as indicated by the lines 18, 19. In view thereof, possible additional incoming calls can be handled by the base station 2.

When the mobile unit 6 observes that the signal strength of communication link 17 is poor, it detects - during short call interruptions - the signal strength of the communication links 21, 22 to the adjacent base stations 1 and 3 by listening on the carrier channels HC1 and HC3, respectively, of these base stations. The mobile unit has received information from base station 2 as to the fact that carrier channels HC1 and HC3 are the channels to be examined with regard to the signal strength. The outcome of the signal strength detection shows that the call should be transferred to base station 3. The mobile unit 6 now sends - also during short call interruptions - to its radio channel AC2 a request for hand-off, that is, a request for call transfer to a radio channel of base station 3. While making use of usual signalling on lines 9 and 10 hand-off will now be prepared and accomplished by a simple switch of the call in the central exchange 4 from line 9 to line 10 and thereby to the free radio channel AC3. Also, sthere will be a transfer of the call to this latter channel in the mobile unit. Of course, the actual call transfer will not be made until the new radio communication 23 (figure 3) has been established in the usual way (including handshaking etc.).

Since the mobile telephone unit now has its call connection to channel AC3 , channel AC2 becomes free again, for what reason channel AG21 (figure 2) will be commanded to become an outgoing radio channel BC21 again. At the same time a temporary incoming radio channel AC31 will be established at base station 3 by means of connections 25, 26, this entirely in correspondence to what took place in conjunction with the temporary incoming radio channel AC21 at base station 2. The situation now obtained is shown in figure 3.

When the call has been ended, incoming radio channel AC3 will become free again. As a result, the temporary incoming radio channel AC31 will be disconnected, such that this channel again will become an outgoing radio channel BC31. Figure 4 shows the situation of the system in conjunction with an outgoing call from the mobile unit 6, hand-off coming into question also in this case. The call to a subscriber 31 of the public telephone net work has been set up by the mobile unit first occupying a free outgoing radio channel BC21 of the associated base station 2 and establishing radio communication 33 therewith. The local exchange unit 7² of channel BC21 calls up the subscriber 31 in the usual way, thereby establishing the connection 34. As should be evident, neither the central exchange 4 or the incoming radio channel AC2 of base station 2 will be involved.

When hand-off comes into question mobile unit 6 will examine the carrier channels HC1 and HC3 of interest. As previously, the mobile unit will receive information about these carrier channels when asking its radio channel BC21. Also this time the communication link 35 to channel HC3 has a better signal strength than the communication link 36 to channel HC1. Consequently, the call should be transferred to base station 3.

In this case, however, the mobile unit 6 occupies channel HC3 as soon as the good signal strength has been detected, with a view to command a call transfer to this channel. In doing so, the latter channel cannot be a carrier channel anymore, but will become an outgoing radio channel BC34 (figure 5), while at the same time the former and free outgoing radio channel BC33 (figure 4) will be commanded to become a carrier channel HC31 (figure 5). The mobile unit 6 now commands a call transfer to channel BC34 of base station 3. This can be accomplished in many various ways, but at the present it is preferred to have the transfer controlled by the new channel, that is, channel BC34 in this case. During short interruptions of the call passing via link 33 to channel BC21, channel BC34 will receive a command to establish (while using the call-up or dialling function of its local exchange unit 7⁵) a connection 37, 38 (via the public telephone net work 4) to the local exchange unit 72 of channel BC21. When this connection has been established and the call consequently also passes via connections 35, 37, 38, then communication 33 will be disconnected. The situation according to figure 5 has now been obtained. When the call has come to an end, the dialed connections 34, 37, 38, will be disconnected. However, the channel configuration according to figure 5 will remain, that is, channel HC31 will still be a carrier channel until utilized for a new hand-off, and channel BC34 will remain as an outgoing radio channel until possibly being commanded to become a new carrier channel in conjunction with a later hand-off. As should be evident, all the above-mentioned will happen without the central exchange being involved, possibly with the exception that one might wish to use connections 9, 10 for signalling between the local exchanges of base stations 2 and 3. Finally, in order to make the hand-off function according to the invention still more clear figure 6 shows a schematic flowchart thereover. Numerals 1-10 of the various blocks may then be given the following interpretation:

1: Is the signal strength encounted by the mobile unit good enough for communication?

2: During call interruptions the mobile unit measures the signal strength of known possible carrier channels related to various directions of movement of the mobile unit. 3: Is the signal strength better for anyone of the examined carrier channels? 4: During call interruptions the mobile unit sends a request for base station exchange (change of channel).

The mobile unit continuous the call on the old channel and awaits an assignment or confirmation of the new channel.

6: Has the new channel been confirmed? 7: Transfer of the call to the new channel. 8: The call continuos on the channel involved. 9: Is the call finished? 10: Disconnect the communication path.

Claims

1. Cellular mobile telephone system comprising a central exchange arranged for communication with an external telephone net work, such as a public telephone net work, a plurality of base stations and a number of mobile telephone units, each base station having an associated cell area and each base station on one hand being arranged for communication with the central exchange and on the other hand has a plurality of radio channels for radio communication with mobile telephone units located in the associated cell area, characterized in that it comprises base stations each having a local exchange with a dialing function, the local exchange being arranged for communication with said external telephone net work, such a base station being arranged in response to a request for a call set-up received on a radio channel of the base station radio channels from a mobile telephone unit in the base station cell area, for establishing a call by calling up said external telephone net work via the local exchange.

2. Mobile telephone system according to claim 1, characterized in that respective base station is arranged in conjunction with an outgoing call and in response to a request for transfer to another base station received from a mobile telephone unit within the base station cell area, to establish a call connection to the other base station involved, while using its local exchange, by calling up the other base station on said external telephone net work or alternatively by switching within an internal telephone net work incorporating the local exchangee of the base stations.

3. Mobile telephone system according to claim 1 or

2, characterized in that the respective base station is permanently connected to the central exchange via. at. most a few lines for distributing calls from the central exchange to the base station and preferably also for system signalling between the central exchange and the base station; that said at most a few lines are connected to corresponding incoming radio channels of the base station, said incoming radio channels thus primarily being intended for searching mobile telephone unite and for distributing incoming calls to said units, said incoming radio channels having a corresponding signalling; and that the local exchange of the base station is arranged when said at most a few lines are occupied temporarily to establish at least one additional line for an incoming call by calling up the central exchange using said external telephone net work, said additional line then being connected to a radio channel of the base station, the signalling of which being changed accordingly.

4. Mobile telephone system according to claim 3, characterized in that the respective base station has outgoing radio channels primarily to be used for outgoing calls from mobile telephone units within the base station cell area, the signalling of the outgoing radio channels being in correspondence thereto.

5. Mobile telephone system according to claim 3 or 4, characterized in that the respective base station has a carrier radio channel primarily intended to be the channel to be examined by a mobile telephone unit with regard to signal strength in conjunction with a base station transfer procedure, the signalling of the carrier radio channel being in correspondence thereto.

6. Mobile telephone system according to claims 4 and

5, characterized in that the carrier radio channel is a dynamically allocated channel; that the carrier radio channel is arranged, in conjunction with a transfer of a call to the belonging base station, to be the radio channel receiving the transferred call; that such a transfer of the call to the carrier radio channel means that the channel will be changed into an outgoing radio channel and that a free outgoing radio channel if existing or when established will be changed so as to be a carrier radio channel.

7. Mobile telephone system according to claims 4 and

5 or claim 6, characterized in that when all outgoing radio channels are occupied a mobile telephone station as a first choice will try to establish an outgoing call from the mobile radio unit on said carrier channel if free, and as a second choice on a free incoming radio channel should said carrier channel be occupied.

8. Mobile telephone system according to anyone of claims 1-7, characterized in that in conjunction with an incoming call to a mobile telephone unit and in response to a request for base station transfer from said mobile telephone unit, the call switch will be accomplished in the central exchange.

9. A method of controlling a cellular mobile telephone system, said system comprising a central exchange for receiving incoming calls from an external telephone net work, such as a public telephone net work, and a plurality of base stations each being in communication with the central exchange and each being arranged for radio channel communication with a mobile telephone unit in the associated base station cell area via a radio channel of a plurality of such channels associated with the base station, wherein the radio channel communication of the mobile telephone unit will be transferred between various base stations dependent on the movement of the mobile telephone unit within various cell areas and the signal strength of the respective communication link to the base station involved, characterized by; examining in the mobile telephone unit, while the call communication is established and at least when the signal strength of the radio channel used is lower than a certain level, during short interruptions of the call in progress, whether the signal strength is better for anyone of the radio channels of adjacent base stations whereto a call transfer should be possible; sending from the mobile telephone unit, provided that said examination shows that the signal strength is essentially better for another radio channel of said examined radio channels, during short interruptions of the call in progress, a request for transfer of the radio channel communication of the mobile telephone unit to the base station associated with said better examined radio communication, said request preferably being sent to said better examined base station or alternatively to the base station having the established call communication; providing in response to said request the establishmen of a new call connection in accordance with said request via the central exchange or via a direct connection between the base stations involved by the call transfer, and sending during short interruptions of the call in progress, information as to said new call connection to the mobile telephone unit, and switching in the mobile telephone unit the call to the new radio channel of the better base station associated with the new call connection, whereafter the previous call connection is disconnected where applicable.

10. Method according to claim 9, characterized by providing at base stations, preferably at each base station, a local exchange having a dialing or call-up function, thereby enabling the establishment of the new call connection by establishing a direct connection between the ba.se stations involved, such as by causing one of the base stations to call-up the other base station while utilizing the external telephone net work or alternatively by switching within an internal telephone net work incorporating the local exchanges of the base stations.

11. Method according to claim 10, charact erized in that said direct connection is established when transferring an outgoing call from a mobile telephone unit, while the transfer of an incoming call to a mobile telephone station is accomplished in the central exchange.

12. Method according to anyone of claims 9-11, characterized by sending from a base station to a mobile telephone unit in call communication with said base station information as to which radio channels of adjacent base stations should be examined with regard to signal strength to thereby decide which base station is of interest if a call transfer is to be made.

13. Method according to claim 12, characterized by assigning a specific radio channel to each base station, said specific radio channel being the channel to be examined with regard to signal strength, said information sent from a base station to a mobile telephone unit comprising data aβ to the specific radio channels of the adjacent base stations.

14. Method according to claim 13, characterized by permanently assigning a said specific radio channel to each base station, said information preferably being sent from a base station to a mobile telephone unit in conjunction with the establishment of a radio channel communication between the base station and the mobile telephone unit.

15. Method according to claim 13, characterized by dynamically assigning a said specific radio channel to each base station; causing each base station to listen on the radio channels of the adjacent base stations and based upon the respective transmission of said channels to decide which radio channel is the specific radio channel of the base station in question and to keep its said information up-dated accordingly; providing, in conjunction with a requested mobile telephone unit call transfer from one base station to another base station, a switch to the specific radio channel of said other base station, said specific radio channel being the channel examined with regard to signal strength, the status of said radio channel as a dynamically assigned specific radio channel then being cancelled; and assigning in said other base station another radio channel the status as specific radio channel.