GB2227143A

GB2227143A - Mobile radio-telephone system with mobile units and base stations

Info

Publication number: GB2227143A
Application number: GB8926766A
Authority: GB
Inventors: Kenneth David Mccann; Wilfred Brent Harding
Original assignee: INDEP BROADCASTING AUTHORITY; Independent Broadcasting Authority
Current assignee: INDEP BROADCASTING AUTHORITY; Independent Broadcasting Authority
Priority date: 1988-11-25
Filing date: 1989-11-27
Publication date: 1990-07-18
Anticipated expiration: 2009-11-27
Also published as: GB8827554D0; GB2227143B; HK1005329A1; GB8926766D0

Abstract

Each mobile radio-telephone 4 has a unique identification code and responds to reception of a signal broadcast on a unidirectional link which contains the identification code thereof by transmitting a signal on a bidirectional communications link. If the mobile radio-telephone is within range of base station 5, then the latter responds on the bidirectional link establishes a communications channel between the mobile unit and the caller. If the mobile radio- telephone is not within range of a base station it will not receive a response and, after a predetermined time with no response, it may store information on the call-in attempt, or indicate to the user that an unsuccessful attempt has been made. This avoids the need to continuously track mobile radio-telephones. The unidirectional link may be a television or radio broadcast network, or a communications satellite. <IMAGE>

Description

Mobile Communication System and Mobile Unit Therefor The present invention relates to a mobile communication system and to methods and apparatus for use in such a system. In particular, the invention relates to a system in which mobile communications devices can both send and receive messages.

In the last few years there have been several developments in the field of mobile communications (often referred to as mobile radio or mobile telephony).

These developments have resulted in the production of cordless telephones, cellular radio, and, most recently, second generation cordless telephones (CT2, or "telepoint" services).

The cordless telephone operates by means of a pair of low power, short range, transceivers, one located in the telephone and one physically connected to a particular extension line of the public switched telephone network (PSTN or PSN). The cordless telephone is limited in that it is restricted to operation within a small area around a specific geographical location (ie around the transceiver /PSN interface).

Second generation cordless telephones can operate in a rather larger geographical area than their forerunners. The transceiver in the CT2 telephone is not restricted to communication only with one particular base transceiver but rather can communicate with any one of a number of geographically spaced base transceivers which connect with the PSN. When a CT2 telephone is within range of a given base transceiver then that telephone may be used to make a telephone call, with the eventual telephone call being carried between the telephone transceiver and the base transceiver on any one of the number of radio frequency channels available to the system.

Although operable in a greater variety of locations than their predecessors (it has been proposed that the eventual "telepoint" service could use base transceivers on most street corners) second generation cordless telephones have the disadvantage that they cannot receive telephone calls, they can only make telephone calls.

Cellular radio offers, dt least on a theoretical level, a mobile communications system with full geographical coverage. In a cellular system a geographical area, e.g the United Kingdom, is divided up into contiguous areas, called cells, each having its own base transceiver or transceivers connected to the PSN via a mobile switching centre. A cellular radiotelephone within a cell can make a telephone call, with the eventual conversation being carried between the base transceiver and the telephone on one of a range of channels which have been allocated to that cell.

Neighbouring cells use different ranges of radio frequency channels, whereas geographically seperated cells can use the same ranges of frequencies, so as to ensure optimum use of the available bandwidth with the minimum of interference.

Cellular radio-telephones can both make and- receive telephone calls. However in order to enable cellular radiotelephones to receive telephone calls the overall system has to continuosly monitor the geographical location of each radio-telephone so that a base transceiver in the appropriate cell can make contact with the radio-telephone being called.

In summary, of the three mobile communications systems discussed above one provides radio-telephones of only limited mobility and the other two provide radiotelephones which either cannot receive incoming telephone calls, or can do so only at the expense of increased complexity of the overall system.

The present invention provides a mobile commur,ications system in which mobile radio-telephones can both make and receive telephone calls. The advantage of this invention is that the geographical position of each mobile telephone does not have to be closely monitored to determine which cell to use for calls to the mobile telephone.

This means that the system is simpler and of lower cost than those used heretofore whilst still providing in going and out going facilities. The system uses two distinct communications links, one being unidirectional and the other being bidirectional, and each operative at one of a plurality of frequencies. The unidirectional link is used to broadcast signals informing individual mobile radio-telephones that a message is present in the system for them.

The present invention also provides mobile radiotelephones each adapted to respond to an appropriate received signal on the undirectional link by using the bidirectional link so as to receive the message.

The present invention still further provides a method of making communication with a mobile radiotelephone by transmitting a signal to the mobile radiotelephone on a uni-directional communication link at a first predetermined frequency, said signal including a code identifying the mobile radio-telephone; transmitting a signal from the mobile radio-telephone on a bidirectional communications link at a second predetermined frequency different from the first; and responding on the bidirectional link to the signal transmitted by the mobile radio-telephone By providing a system in which it is not necessary to continuously make contact between mobile units and a transceiver network regardless of whether or not there is a call in the system for the mobile unit, in order to track the location of the mobile unit, the present invention achieves a saving in the volume of transmissions needed for control purposes and thus allows a higher density of users to be accommodated within a restricted number of frequencies used for the bidirectional links.

Each radio-telephone has an identification code or a set of identification codes (for example one code for each of a number of communications systems of which that radio-telephone is a part). The identification codes can be changeable overair.

System operation can be simplified if each identification code is unique to a particular radiotelephone but this is not mandatory.

In a preferred embodiment of the invention the unidirectional communication link is provided by the television or radio broadcast network, or by a broadcasting satellite. It is preferable that this unidirectional link should have a large area of coverage so as to enable it to serve the areas of operation of many local bidirectional links.

Features and advantages of the present invention will become clear from the following description of embodiments thereof, given by way of example, with reference to the accompanying drawing which shows diagrammatically a mobile communications system according to one embodiment of the invention.

The mobile communications system shown in Fig.

1 comprises a unidirectional communication link in the form of a broadcast network 1, a broadcast controller 2, a system exchange 3 connected to the broadcast controller and to the public switch telephone network (PSTN), a plurality of mobile radio-telephones 4, a bidirectional communication link provided by a transceiver network 5, and a transceiver network controller 6 connected to the transceiver network 5 and to the PSTN. The broadcast controller 2 prepares signals for transmission by the broadcast network at one of the plurality of predetermined frequencies, F. The transceiver network controller 6 is adapted to selectively connect transceivers in the network 5 to the PSTN or to the system exchange 3. The transceivers of network 5 operate at a plurality of predetermined frequencies, f, different from those used by the broadcast network.

Each of the mobile radio-telephones is adapted to receive signals transmitted at frequencies used by the broadcast network 1 and the transceiver network 5, and to transmit signals at frequencies used by the transceiver network. Each mobile radio-telephone 4 is also adapted to transmit a signal at a frequency used by the transceiver network 5 on reception of an appropriate signal transmitted at a frequency used by the broadcast network 1.

The operation of the mobile communications system of Fig. 1 will now be described.

Each of the mobile radio-telephones 4 has its own identification code. When a radio-telephone 4a is used to make a telephone call it transmits a signal, including its identification code and information identifying the number being called, at a frequency fa. A transceiver 5A within range of the radiotelephone 4A receives the signal and routes the telephone call through to the PSTN. If more than one transceiver is in range of radio-telephone 4A the transceiver network controller 6 allocates the subsequent telephone conversation to one of them. The transceiver network controller 6 also assigns a frequency to the telephone conversation (the radiotelephone retunes if necessary).

It may be seen that in a "calling-out" mode of the mobile radio-telephone 4 the system operates generally similarly to current cellular radio-telephone and telepoint systems.

When a telephone call is made to a radiotelephone 4B by a conventional telephone (fixed or mobile) the call is routed through the PSTN to the system exchange 3. At the system exchange the identification number (or code) for radio-telephone 4B is identified and a signal including the identification number is prepared by the broadcast controller and sent to the broadcast network for transmission at a frequency Fb.

All of the mobile radio-telephones within range of the broadcast network receive the broadcast signal. Radio-telephone 4B recognises its identification number in the received signal and responds by transmitting a signal (including identifying information) at a frequency fb. A transceiver 5B which is within range of radio-telephone 4B receives the signal of fb and isolates the identifying information.

The transceiver 5B then contacts the system exchange (either directly or through the PSTN) with the information and the telephone call for radio-telephone 4B is routed through to transceiver 5B. The transceiver 5B sends a signal to radio-telephone 4B causing it to ring. (Again the transceiver network controller 6 may allocate a particular transceiver and/or frequency to the subsequent conversation). The type and duration of signal transmitted by the broadcast network 1 may be varied as described below.

The broadcast network 1 can use one or more of a number of different formats for the signals identifying the called radio-telephones. For example, in a mature system accommodating a large number of mobile units it may be preferably to format the broadcast signal in such a way that each mobile unit need only monitor portions of the broadcast signal to check whether or not it is being called, thus the amount of time during which the radio-telephones need to be powered-up may be reduced. This can be achieved by formatting the broadcast signal as a quasi-periodic signal having a particular portion of each "period" thereof dedicated to identification signals for a particular mobile unit or group of mobile units.Once a mobile unit is synchronised to the broadcast signal it can go into a power down condition until the time when the relevant part of the broadcast signal is due for transmission on the broadcast network.

Furthermore, the broadcast signal can include identification codes therein in a wholly or partially encrypted or scrambled form. Such measures improve the security of the system and act to prevent third parties from intercepting or interfering with calls destined for a particular radio-telephone (since the identity of the radio-telephone being called will not be readily ascertainable from the broadcast signal). Group codes may also be used for transmitting the identification code signals in the broadcast signal. Error correction or protection signals and synchronisation signals may be included in the broadcast signal using conventional techniques.

The signal prepared by the broadcast controller 2, and transmitted by the broadcast network 1, may include information identifying the telephone X initiating the telephone call. In such a case the signal transmitted by radio-telephone 4B also includes that identifying information and the transceiver 5B instructs the special exchange 3 to connect it to the call from telephone X.

Alternatively, the signal prepared by the broadcast controller 2 may contain only the identification code for radio-telephone 4B. In this case the transceiver 5B instructs the special exchange 3 to connect it to the call for radio-telephone 4B, thus, the special exchange needs to keep a short-term record of the identity of the telephone calling a given mobile radio-telephone.

The broadcast controller 2 may arrange for the broadcast network 1 to transmit once or repeatedly the signal identifying a mobile radio-telephone being called. If the signal is to be transmitted repeatedly the number of repetitions may be set arbitrarily (e.g.

so that the signal is broadcast repeatedly for 30 seconds), or may be determined by the response time of the radio-telephone and transceiver network (ie the signal is broadcast repeatedly until the system exchange is contacted by a transceiver requesting connection to the appropriate call).

The broadcast controller 2 may additionally be adapted so that if it is not contacted by a transceiver in communication with a particular called mobile radiotelephone within a preset period following broadcast of a signal identifying that called mobile radio-telephone then the broadcast controller 2 will send an "unobtainable" or other signal back to the caller though the system exchange 3.

In the mobile communication system of the present invention conditions may arise where a called mobile radio-telephone cannot connect up with the system exchange (e.g because the mobile radio-telephone is not within range of a transceiver having available communication frequencies). In order to accommodate this situation each mobile radio-telephone 4 may be provided with a memory in which an entry would be made whenever an incoming call cannot be accessed. The making of an entry in this memory could be triggered by a timing device which outputs a trigger signal if no reply has been received from a transceiver a set time after the radio-telephone has output a signal on the bidirectional communication link at frequency fb.

Alternatively, the system as a whole may be conditioned such that if within a set period of time after an identification code signal has been broadcast by the broadcast network no connection has been established between the relevant radio-telephone and a transceiver then the broadcast network will transmit a signal instructing the mobile unit to make an entry in its memory.

The entry may be made in memory by simply recording the whole or part of the relevant received identification code signals, or by extracting the salient information (such as caller's identity or location) therefrom and recording that information, and the time of the incoming call may be stored.

For some applications it may be preferable for the radio-telephone to make an entry in memory on reception of a relevant identification code signal regardless of whether or not contact is made between the mobile unit and the transceiver network.

Mobile radio-telephones provided with a memory such as that referred to above can also include means for automatically (or on manual instruction) retransmitting a signal on the bidirectional communication link a set period after the first transmission in order to try again to connect up with the caller. An indicator such as a display device can also be provided so as to alert the owner of the mobile radio-telephone that an appropriate signal has been received on the unidirectional communication link but the incoming call could not be accessed. The display can be adapted to show details of the incoming call (e.g. time it was made and telephone number of caller, if known).

Similarly an indicator can be provided on the radio-telephone to show when a relevant identification code signal has been received and some other indication, such as production of a ringing tone, may be given when an acknowledgement signal has been received from, or a connection has been made on, the second communications link.

The mobile communication system has been described above in relation to its handling of telephone conversations (which term should be read as also including facsimile transmissions and other non-verbal communications). The system can alternatively or additionally handle recorded messages. In this case the broadcast network 1 transmits a signal including the identification code of the radio-telephone for which there is a recorded message and the signal may additionally include information indicating that a recorded message (rather than a call) is present in the system, and/or information on the location of the recorded message. The transceiver in communication with the appropriate radio-telephone accesses the system exchange in different ways depending upon the amount of information present in the signal transmitted by the broadcast network 1.For example, if the broadcast signal includes information on the location of the recorded message then the transceiver simply contacts the appropriate part of the system exchange or the appropriate line of the PSTN.

The broadcast network 1 may be implemented in a large number of different ways. A special unidirectional network could be built dedicated to the mobile radio-telephone system. Alternatively existing unidirectional broadcasting facilities can be used. For example in the UK the national network of television transmitters could be used as the broadcast network in which case the broadcast controller 2 could be adapted to insert its signals into the television signals or teletext signals to be broadcast. The signal may be inserted into the television signal field blanking interval or as a subcarrier.Similarly, in the UK the national network of radio transmitters could be used as the broadcast network, in which case the broadcast controller 2 could be adapted to insert its signals into the radio signals, or radio data system (RDS) signals, or subsidiary communications authorisation (SCA) signals (a radiotext service currently offered in the United States), to be broadcast. Also, although the unidirectional communication link has been described above as a broadcast network, it could equally well be implemented by a single broadcasting satellite the footprint of which covers the geographical area of interest. In such a case the broadcast controller 2 could insert its signals into the satellite signal to be broadcast.In an office environment or the like where a network of transceivers creates zones of the order of 10 yards in diameter (analogous to a microcellular system) the first communications link may be an inductive loop broadcasting the relevant identification code signals.

In some embodiments of the invention information is stored relating to the probable location of the mobile telephones, for example a record may be kept of the last known base station to be in contact with a given mobile telephone, or the pattern of usage of the mobile telephone may be stored. It then becomes possible when a call arises for a radio-telephone to attempt to contact the telephone directly via a single base station or a single one of a plurality of transmitters or satellites making up the unidirectional communications link; only if these attempts fail will the identification code signal corresponding to the called radio-telephone be generally broadcast on the first communications link as described above.

Similarly, when the broadcast network comprises a plurality of transmitters these may be used to call a mobile unit selectively, the most likely possibility first, before generally broadcasting the identification code signal on the whole network.

In embodiments such as those described above it can be useful for the radio-telephones to be overair programmable, for example so that the radio-telephones can be remotely instructed to monitor different channels, frequency ranges and/or modes for transmission. This also allows the frequencies of the first and second communications links to be changeable.

In all cases the broadcast signal could use multiple carrier or spread spectrum techniques to give resistance to information loss due to frequency selective signal strength fading.

The transceiver network 5 and transceiver network controller 6 may be implemented in a new network dedicated to this mobile communications system or existing facilities (e.g. the base stations and multiple switching centres of the existing cellular radio network) could be used.

Existing cellular radio-telephones and second generation cordless telephones can be adapted so as to be able to receive telephone calls by the method of the invention, e.g. a module could be added to the existing telephones, the module including: circuitry to enable the telephone to receive and respond appropriately to the signal broadcast on the unidirectional communications link, a memory for storing details of incoming calls which the telephone did not immediately access, and a display (or other indicator) for showing that an incoming cell has occurred but not been accessed.

Telephones modified in this way, and the method of so modifying the telephone, are encompassed within the invention for which protection is herein sought.

Claims

CLAIMS:

1. A telecommunications system, comprising: a plurality of mobile units each having an identification code and each being adapted to receive signals on a first, unidirectional, channel and to transmit and receive signals on a second, bidirectional channel; broadcast means for broadcasting on the first channel an identification code signal corresponding to a mobile unit for which there is a message in the system, each mobile unit being adapted to respond to a corresponding identification code signal on the first channel by transmitting a requestor signal on the second channel; and a plurality of base stations each adapted to transmit and receive signals on the second channel and to respond to a requestor signal received on the second channel by transmitting an acknowledgement signal on the second channel; wherein each mobile unit comprises means for monitoring the second channel for an acknowledgement signal, the output of the monitoring means being indicative of whether or not an acknowledgement signal is received on the second channel.

2. A telecommunications system according to claim 1, wherein each mobile unit comprises means for recording information relating to reception of corresponding identification code signals on the first channel.

3. A telecommunications system according to claim 2, wherein the recording means of each mobile unit is adapted to selectively record information relating to reception of corresponding identification code signals dependent upon the output of the monitoring means.

4. A telecommunications system according to claim 1, 2 or 3, wherein each mobile unit comprises means for indicating reception of a corresponding identification code signal on the first channel.

5. A telecommunications system according to claim 4, wherein the indicating means of each mobile unit is adapted to selectively indicate reception of a corresponding identification code signal dependent upon the output of the monitoring means.

6. A telecommunications system according to any previous claim, and further comprising a system controller for holding information on the location in the system of messages for mobile units and for producing the identification code signals broadcast by the broadcast means.

7. A telecommunications system according to any previous claim, wherein the broadcast means is adapted to include in each identification code signal information on the location in the system of the message for the corresponding mobile unit, each mobile unit is adapted to respond to a received corresponding identification code signal by transmitting a signal including the corresponding message location information on the second channel, and each base station is adapted to respond to a message location signal received on the second channel by establishing a communications channel between the message location and the corresponding mobile unit.

8. A telecommunications system according to claim 6, wherein each mobile unit is adapted to respond to a received corresponding identification code signal by transmitting a signal including the identification code thereof on the second channel, and-each base station is adapted to respond to a unit identification code signal by contacting the system controller and establishing a communications channel between the message location and the corresponding mobile unit.

9. A telecommunications system according to claim 6, wherein the system controller is adapted to store information on one or more previous locations of each mobile unit and to instruct one of the plurality of base stations to transmit a relevant identification code signal on the second channel when there is a a message in the system for a mobile unit, before sending the identification code signal to the broadcast means for broadcast on the first channel.

10. A telecommunications system according to claim 6 or 9, wherein the broadcast means comprises a plurality of geographically separate broadcasting entities, and the system controller is adapted to store information on one or more previous locations of mobile units and to instruct one of the plurality of broadcasting entities to transmit an identification code signal on the first channel before transmission by the other broadcasting entities.

11. A telecommunications system according to any previous claim, wherein the message is a telephone call.

12. A telecommunications system according to any one of claims 1 to 10, wherein the message is a stored message.

13. A telecommunications system according to any previous claim, wherein the broadcast means comprises a television broadcast network.

14. A telecommunications system according to any one of claims 1 to 12, wherein the broadcast means comprises a radio broadcast network.

15. A telecommunications system according to any one of claims 1 to 12, wherein the broadcast means comprises a communications satellite.

16. A telecommunications system according to any previous claim, and further comprising an interface between the plurality of base stations and the public switched telephone network (PSTN).

17. A telecommunications system according to any previous claim, wherein the first channel comprises a plurality of frequency ranges.

18. A telecommunications system according to any previous claim, wherein the second channel comprises a plurality of frequency ranges.

19. A mobile communications unit for use in a telecommunications system, the unit having an identification code and being adapted to respond to an identification code signal received on a first, unidirectional, channel by transmitting a requestor signal on a second, bidirectional, channel; wherein the unit comprises means for monitoring the second channel for an acknowledgement signal, the output of the monitoring means being indicative of whether or not an acknowledgement signal is received on the second channel.

20. A mobile communications unit according to claim 19, and comprising means for recording information relating to reception of identification code signals on the first channel.

21. A mobile communications unit according to claim 20, wherein the recording means is adapted to selectively record information relating to reception of identification code signals dependent upon the output of the monitoring means.

22. A mobile communications unit according to any one of claims 19 to 21, and comprising means for indicating the reception of an identification code signal on the first channel.

23. A mobile communications unit according to claim 22, wherein the indicating means is adapted to selectively indicate reception of an identification code signal dependent upon the output of the monitoring means.

24. A mobile communications unit according to any one of claims 19 to 23, wherein the first channel comprises a plurality of frequency ranges.

25. A mobile communications unit according to any one of claims 19 to 24, wherein the second channel comprises a plurality of frequency ranges.

26. A telecommunication system substantially as hereinbefore described with reference to the accompanying drawing.

27. A mobile communications unit substantially as hereinbefore described with reference to the accompanying drawing.