Dual fixed/mobile communications system

A dual fixed/mobile communications system comprises a set of base stations (BS) connected to a local exchange (LE), and a set of wireless terminal stations (WTS) that communicate, by radio, with the base stations (BS) in order to provide telephone services to telephone sets (T) connected, by means of subscriber lines (LS), to the wireless terminal stations (WTS). These wireless terminal stations (WTS) are inhibited when use is made of cordless telephones (WT) which, in an alternative form, communicate directly with the base stations (BS) when they are located within their area of coverage. The wireless terminal stations (WTS) can also include a radio transceiver RT through which a cordless telephone (WT).sup.1 which is outside the coverage of the base stations (BS), can communicate with them.

TECHNICAL FIELD
 This invention relates to a communications system for access to the public
 switched network, through which a conventional fixed telephone set obtains
 access to the switched public network by means of a terminal fixed station
 that is connected, by radio, to a set of base stations of a wireless
 communications system.
 BACKGROUND OF THE INVENTION
 One of the major problems that telecommunications operating companies faced
 with is the need to install infrastructures to give service in certain
 areas of low subscriber density where the economic return is very poor as
 a result of the widespread geographical distribution of the customers.
 This is particularly the case in the access part of the network in
 environments with a certain demand where, in addition, the installation
 and provision of these services have to be implemented rapidly.
 To resolve these problems, in recent years use has been made of what is
 normally termed radio access, in which a set of base stations of wireless
 communications systems (cellular, point to multipoint, etc.) provide radio
 coverage for a certain zone in which are installed a number of "special"
 wireless terminals, which are cordless terminals of the type normally used
 in such communications systems but into which a line interface circuit has
 been incorporated, while personal interfaces (microphone, loudspeaker,
 keypad, display element) have been suppressed. This line interface
 performs the functions of conversion required for both directions of
 transmission, so that, from a conventional telephone set connected to this
 terminal, it is possible to offer the same kind of functions as with a
 fixed telephone connected directly to the public switched network.
 Among these functions are to be found those of four-to-two wire conversion,
 user signal tone generation, dial signal conversion, etc.
 In this way it is possible to have a set of fixed telephones that have
 access to the public switched network by means of a wireless
 communications network. There is no mobility provided by this
 communications network, so that a new wireless communications system would
 be necessary with the consequent duplication of equipment and the possible
 saturation of the radiofrequency spectrum, should the subscribers also
 wish to have cordless telephones to provide a desired mobility within an
 area similar to that covered by the wireless network.
 DISCLOSURE OF INVENTION
 The object of this invention is to develop, on the actual system
 infrastructure described above, functions of mobility within a closed area
 constituted by the same set of base stations as are employed to implement
 the access part of the fixed terminals to the network.
 The system according to the invention comprises a set of base stations that
 are connected to a local telephone exchange, and a set of wireless
 terminal stations which communicate, by radio, with at least one of the
 base stations in order to offer thereby telephone services to telephone
 sets connected, by subscriber lines, to the wireless terminal stations. In
 this system, the wireless terminal stations are inhibited or enabled by
 control means depending on whether or not a cordless telephone is used
 which, in an alternative form to that of the wireless terminal stations,
 also communicates directly with the base stations in the event that these
 cordless telephones are to be found inside the area of coverage of any of
 the system base stations.
 When the cordless telephones are outside the range of any of the system
 base stations, the wireless terminal stations have, in addition, and so as
 to offer a certain local mobility, a radio transceiver that works as a
 repeater and which is activated by the control means when the cordless
 telephone is used to communicate with the base stations through the
 wireless terminal station and the radio transceiver.
 In this way it is possible to provide mobility for fixed terminals that
 make use of wireless access in determined zones of coverage and even
 outside these zones. A given subscriber can have a fixed conventional
 telephone or, if he so prefers, a mobile telephone in the area of coverage
 of a base station system and in an area of coverage of a small repeater
 that allows a degree of local mobility.
 In addition, by the use of radio transceivers of the type dislcosed herein,
 which incorporate directional antennas, it is possible to extend the area
 of coverage economically to any subscriber geographically isolated from
 the base stations, at the extremity of which a wireless terminal station
 is connected and to which a fixed telephone set is connected in a manner
 similar to that described above.
 BRIEF DESCRIPTION OF THE DRAWING
 The system so formed is of special application in the network access part
 where a wireless communications system of the DECT (Digital European
 Cordless Telecommunications) type is used.

BEST MODE FOR CARRYING OUT THE INVENTION
 FIG. 1 shows, as mentioned above, the infrastructure of a switched known
 communications network to provide service for fixed telephone sets. To
 achieve this, the fixed telephone sets T are connected by corresponding
 subscriber lines LS to the access network AN. This connection is made by
 means of line interfaces which, mainly, adapt received signalling, supply
 appropriate levels of direct current and voice signals, etc.
 The access network AN communicates with a local exchange LE over a
 transport network TN such that all the subscribers are connected to the
 local exchange LE in order to be able to make and receive telephone calls
 to or from another subscriber connected to the telecommunications network.
 When the subscribers are distributed over a relatively broad area, the cost
 of installing the access network AN is very high. To reduce this cost it
 has become the practice in recent years to make use of radio access, as
 shown in FIG. 2, in which a number of base stations BS, that provide
 coverage for a certain area, are connected to the local exchange LE by
 means of a base station controller BSC that manages their operation.
 In this way a wireless terminal station WTS is basically a cordless
 telephone of the kind used in this type of network, but powered from the
 electricity mains through its corresponding power supply unit and a set of
 batteries to permit operation in the event of a mains blackout. In
 addition, it is normal practice to use directional antennas (in contrast
 to standard cordless telephones, that use omnidirectional ones) in order
 to attain a range greater than the cell coverage and also minimise any
 interference that could be produced in, or picked up from, other wireless
 networks. This is feasible because the wireless terminal station WTS is
 always fixed in position and does not need to be mobile.
 In order to provide a subscriber line LS with conventional features
 (two-wire communications, DC current, user signalling tones, dialling,
 etc.), the wireless terminal station WTS is equipped with an interface IF
 that performs the functions mentioned above: four-to-two wire conversion,
 connection of a 48 V output from the power supply to the line,
 interpretation of signalling information received from the base stations
 BS in order to activate a tone generator and emit the equivalent voice
 frequency tone of a conventional subscriber, translation of dial pulses
 into the format required for transmission over the wireless network, etc.
 In the implementation described hereunder, the wireless system is of the
 DECT (Digital European Cordless Telecommunications) type, and results in a
 fairly economical system despite its complexity, in the light of the
 anticipated market environment.
 As a wireless terminal station WTS also has a cordless telephone WT that is
 functionally compatible with the latter, it is possible that telephone
 communications could be established either from the telephone terminal T
 by means of the cordless terminal station WTS, or from the cordless
 telephone WT itself.
 The wireless terminal station WTS also includes control means EDC whereby
 it can be enabled or inhibited, depending on the state of a detector on
 top of which the cordless telephone WT should rest; thus, if the cordless
 telephone WT is lifted from its cradle, for example to make a call, the
 latter no longer depresses the detector and the control means EDC inhibits
 the wireless terminal station WTS in order to allow communications to be
 made directly through the cordless telephone WT.
 Logically this is possible when the cordless telephone WT is inside the
 area of coverage of one of the base stations BS of the wireless system.
 This situation is illustrated in FIG. 3.
 When the subscriber is located outside the area of coverage of all the base
 stations BS, its wireless terminal station WTS must obligatorily be
 equipped with a directional antenna as stated above. In these situations,
 the cordless telephone WT cannot contact with any base station,
 consequently communication would not be possible. To resolve this
 predicament, and provide an identical service in the eyes of the
 subscriber, the wireless terminal station WTS also includes a radio
 transceiver RT, as is shown in FIG. 4, so that the unit acts as a two way
 repeater for communications between the base station BS and the cordless
 telephone WT. If the coupling of the electromagnetic field between the two
 antennas, directional in the case of the wireless terminal station WTS and
 omnidirectional in the case of the radio transceiver RT, is sufficiently
 low, this radio transceiver acts simply as a two-way repeater. This is
 shown in FIG. 5 in which the signal coming from the antenna ART of the
 radio transceiver RT, originated in the cordless telephone WT, is filtered
 in a bandpass filter BPF, amplified in a receive-side amplifier RA and
 applied to the transmitter of the wireless terminal station WTS; and the
 signal, also at radiofrequency, coming from the input amplifier of the
 wireless terminal station WTS, is amplified in a power amplifier TA and
 applied to the antenna ART of the radio transceiver RT.
 There is also a switch SW, shown schematically in the above indicated
 figure, that receives a transmit/receive control signal TRC coming from
 the wireless terminal station WTS in order to select the appropriate path
 in each case, transmission in one case and reception in the other.
 When the coupling between the two antennas, that belonging to the wireless
 terminal station WTS and that belonging to the radio transceiver RT, is
 not sufficiently low, as in a cordless DECT system, it would be necessary
 to perform much more complex conversion functions than those required in
 analog systems (generally frequency translation), because the frequencies
 and the channels over which the radio communications are made between base
 station and terminal are not fixed, but are indicated in the actual
 information being transmitted between them.
 Although the invention has been shown and described with respect to a best
 mode embodiment thereof, it should be understood by those skilled in the
 art that the foregoing and various other changes, omissions and additions
 in the form and detail thereof may be made therein without departing from
 the spirit and scope of the invention.