IE53438B1

IE53438B1 - A telecommunication system including a wireless telephone set

Info

Publication number: IE53438B1
Application number: IE2170/82A
Authority: IE
Original assignee: Portaphone Ag
Priority date: 1981-09-08
Filing date: 1982-09-07
Publication date: 1988-11-09
Also published as: AU8781282A; NO823048L; JPS5854743A; FI823070L; FR2512621A1; ES515536A0; ES8306429A1; FI75716C; DK396082A; DK159355C; FI75716B; EP0074940A1; CH647631A5; IT8223128A0; EP0074940B1; AU555241B2; IE822170L; AT391234B; DK159355B; FR2512621B1

Abstract

1. Cordless telephone set as well as fixed relay station which exclusively corresponds with the telephone set or with telephone sets of one subscriber station connected in parallel, for telephone calls using the public network, said telephone set as well as said relay station, which is directly connected to the telephone network, each comprising a radio transmitter and a radio receiver, and the transmitters exchanging identification signals for avoiding wrong connections, characterized in that the transmitters (4, 14) are provided with a coding device (7, 17) for generating the identification signal or -signals in order to transmit a code on a channel identified as being idle by the corresponding receiver (3, 13), the code of the transmitters (4, 14) being invariable and independent from the dialled telephone number of a subscriber station, that the receivers (3, 14) are provided with a scanner for distinguishing between free and busy channels from a plurality of present communication channels or pairs of communication channels respectively, and that a comparator (9, 19) is located after the receivers (3, 13) which seizes the transmitted invariable code and compares it with the receiver-side code, as well as a circuit to establish the cordless connection especially in duplex operation between the telephone set and the telephone relay station in case of code identity detected in the comparator (9, 19) circuit.

Description

The present invention relates to a telephone or other telecommunication system wherein a multiplicity of subscribers are served by one or more central offices through a wired network terminating at fixed relay stations which communicate by radio links with associated subscriber stations.

In such a system the subscriber stations will be movable relatively to their relay stations within a range whose radius is determined by the power of the transmitters at each end of the respective links. In order to insure the necessary privacy, each subscriber station conventionally operates on a radio frequency individually assigned thereto. In practice, an operating range with a radius of about 50 meters will allow telephone calls to be received and transmitted anywhere in a subscriber's house.

With only a limited number of radio frequencies available, the same frequency will have to be allocated to stations spaced sufficiently far apart to prevent possible cross-talk or listening-in; in rural areas, for example, 40 communication channels may suffice to cover a region of about 3 km in radius. In multidwelling urban housing, however, such a channel allocation would be inadequate. - 2 53438 An object of the present invention is to provide a telecomnunication — especially telephone — system in which the problem of channel allocation to radio links between mobile subscriber stations and fixed relay stations 1n densely populated areas is solved.

According to the invention, there Is provided a telecomnunication system including a wireless telephone set and a fixed relay station for telephone calls using the public telephone network, the relay station being exclusively connectable to a telephone set of one subscriber station and being directly connectable to the telephone network, the telephone set and the relay station being each provided with a radio transmitter and a radio receiver, said transmitters being for the exchange of identification signals for the avoidance of false connections, each transmitter having coding means for generating an identification signal or signals and transmitting a code on a channel found to be idle by its associated receiver, the code of each transmitter being fixed and independent from the selected call number of a subscriber station, each receiver having transmission scanning means for distinguishing between free and busy channels among a plurality of available communication channels or pairs of communication channels, a conparlson circuit being connected to each of the receivers for detecting the transmitted fixed code and conparing It with a receiver-side code, and a circuit being provided for establishing wireless connection when identity of the transmitted code and the receiver-side code is detected by the conparison circuit.

Said wireless communication Is suitably established by duplex operation between the telephone set and the relay station. In a favoured embodiment of the invention, the transmitter code of the relay station corresponds to the receiver-side code of the telephone set on the one hand, and, on the other hand, the transmitter code of the telephone set corresponds to the receiver-side code of the relay station, the codes for the relay station transmitter and the telephone set receiver differing from those for the telephone set transmitter and the relay station receiver.

Said telephone set may alternatively be one of several parallel-connected telephone sets of said subscriber station. - 3 53438 Thus, in a system incorporating the present invention, each relay station connected to a branch of a wired network is uniquely identified by an individual call number assigned to an associated subscriber station coupled therewith through a radio link, the combination of a relay station and a subscriber station being referred to hereinafter as a post; it should be understood, however, that the subscriber station of any post may encompass several extensions reached for example through a common switchboard. The subscriber station of each post comprises a first radio transmitter and a first radio receiver which are tunable to any one of a plurality of communication channels available to a group of such posts, the first radio receiver being provided with first scanning means for cyclically exploring those channels of the subscriber station in an idle state to determine their free or busy condition. The subscriber station further comprises first coding means and first comparison means, the latter being connected to the first radio receiver and to the first coding means for detecting an identity between a locally generated reference code and a code received on any of the channels in order to cause the first radio transmitter to send out a first channel upon detection of such identity. The subscriber station additionally includes selector means for initiating an outgoing call by triggering the first radio transmitter to send out a locally generated identification code, on a channel found idle by the first radio receiver, preparatory to the emission of a call signal to the wired network in response to reception of a second verification signal on the channel found idle. In an analogous manner, the associated relay station comprises a second radio transmitter and a second radio receiver tunable to the same channel the second radio receiver being provided with second scanning means for cyclically exploring those channels of the relay station in an idle state to determine their free or busy condition. The relay station further comprises second coding means and second comparison means, the latter being connected to the second radio receiver and to the second coding means for detecting an identity between a locally generated reference code and a code received on one of the channels and for causing the second radio transmitter to send out the aforementioned second verification signal upon detection of such identity, the second radio transmitter being triggerable by an incoming call signal from the network to send out a locally generated identification code on a channel found idle by the second radio receiver preparatorily to seizing the idle channel in response to reception of the aforementioned first verification signal over that channel for enabling an extension of the incoming call to the associated subscriber station.

In the field of use specifically contemplated, and as particularly described hereinafter, the selector means of the subscriber station is part of a telephone set. - 5 53438 Such a system will not only insure the necessary privacy but also prevent a subscriber from channeling an outgoing call through a relay station of an adjacent post whose subscriber would then be falsely charged for that call.

The reference code generated by the first coding means matches the identification code generated hy the second coding means hut need not he identical with the identification code generated by the first coding means or the reference code generated by the second coding means. A diversity of the two reference codes used hy the two stations of a post will in fact provide additional safeguards against Invasion of privacy or unauthorized channeling of outgoing calls through unrelated relay stations.

In telephone systems with mobile stations on automotive vehicles it is known to use address codes for discriminating among different satellite stations aboard vehicles within range of a central relay station. In such a system, however, the address codes are the equivalent of call numbers individually assigned to the various mobile satellite stations. This contrasts with the present system in which a code (e.g. a 15-bit word) is used strictly for verification since no relay station can communicate with any subscriber station other than the one forming part of the same post.

I The invention will now be described in detail with reference to the accompanying drawing the sole FIGURE of which shows a post adapted to communicate through a branch of a wired telephone network with a central office. - 6 53438 Ihe post shown in the drawing comprises a mobile subscriber station 1 and a fixed relay station 2 communicating with each other through a radio link. Subscriber station 1 includes a receiver 3, having means for cyclically scanning (in an idle state of the station) a multiplicity of radio-frequency channels allocated to a group of such posts, and a transmitter 4. Receiver 3 has two output terminals 5 and 8, the latter serving to deliver a 15-bit code word encountered on any channel during the scanning thereof. Such a code word may appear, upon demodulation, in a low-frequency band reserved for voice transmission and separated from a narrow frequency band of the same channel on which a pilot signal appears when the channel is busy. When a channel is found to be idle and carries no code word, receiver 3 energizes its output 5 to unblock a gate 10 for enabling the initiation of an outgoing call by a subscriber with the aid of a keyboard 6; the selection signals from the keyboard then reach the transmitter 4.

A code word appearing on terminal 8 is fed to a comparator 9 which receives a locally generated reference code from a coder 7 connected thereto. Coder 7 also delivers a 15-bit identification code, which may be different from the aforementioned reference code, to transmitter 4 whenever that transmitter is triggered to seize an idle channel upon a lifting of a handset 21 by the subscriber wishing to initiate an outgoing call. Ihe transmitter 4 then sends out the locally generated identification code by way of the radio link, using the heretofore idle channel, to relay station. where that code arrives at a receiver 13 which also cyclically scans the available communication channels. Receiver 13 has two - 7 53438 output terminals 15 and 18, similar to terminals 5 and 8 of receiver 3, the first of which serves to unblock a gate 20 upon detection of an idle channel while the second one delivers an incoming identification code to a comparator 19 also receiving from a coder 17 a locally generated reference code. A transmitter 14 in relay station ' 2 = > receives from coder 17 a locally generated 15-hit identification code, identical with the reference code emitted by coder 7 of subscriber station;1, for emission via the radio link whenever an incoming call addressed to post 1, 2 arrives over a line 22 by which station 2 is connected to the otherwise nonillustrated telephone network. The use of 15-bit words enables discrimination among niore than 30,000 posts to which the same set of, say, 40 communication channels is available. Naturally, the scanner of each receiver 3, 13 remains connected long enough to any channel found to be idle, i.e. one that lacks the pilot signal serving to indicate its busy condition, to receive the entire 15-bit code word recurrently sent out by transmitter 14 or 4 upon the arrival of an incoming call or the initiation of an outgoing call, respectively.

When comparator 9 detects an identity between an identification code received from station 2 and the locally generated reference code, it enables -- via handset 21 in the illustrated embodiment — the transmitter 4 to send out the identification code generated by coder 7 which in turn is relayed by receiver 13 via terminal 18 to comparator 19, the latter thereupon checking its identity with the reference code generated by coder 17. When that check has a positive outcome, transmitter 14 is enabled to - 8 53438 send a handshake signal to receiver 3 and to extend an incoming call waiting on line 22 to subscriber station 1 which in turn can now transmit messages to relay station 2 for establishment of full-duplex communication between line 22 and station 1.

Comparator 18, similarly, enables the transmitter 14 (through the intermediary of line 22 and the central office connected thereto) to send the locally generated identification code to subscriber station 1 whose receiver 3 feeds it to comparator 9 for checking against the reference code generated by coder 7. When identity is found to exist, a handshake signal is sent to relay station 2; the subscriber then receives dial ί tone and can now proceed to select the call number of the remote station by means of keyboard 6.

The 40 or so available channels may be modulated upon a carrier of 900 MHz, for example, with sidebands spread over 45 MHz. A transmitting power of 10 mW will allow radio communication between subscriber station 1 and relay station 2 over a radius of about 50 meters. The two stations may each comprise a crystal-controlled master oscillator and a voltage-controlled oscillator slaved thereto by a phase-*locking loop (PLL) stabilizing its operating frequency at, say, 70 MHz. That frequency is stepped down in a mixer working into an adjustable frequency divider which, under the control of a programmed sequencer, successively emits the several channel frequencies to a scanner within receiver 3 or 13. The stabilized VCO signal of 70 MHz is fed via respective buffer amplifiers to a receiving-side multiplier and to a transmitting-side mixer. The receiving-side multiplier has an output - 9 53438 frequency of about 850 MHz fed via a multi pole filter, which removes undesired spectral components, to a receiving-side mixer. The quartz-controlled transmitting oscillator operates with approximately 80 MHz. Its output signal is phase-modulated with the suitably amplified message signals from handset 21 or from line 22. The modulation product is additively combined in the transmission-side mixer with the VCO signal, the result being stepped up in a transmission-side multiplier to the carrier frequency of about 900 MHz. Upon amplification in a final stage a power of +10 dBm is available for transmission. Band-pass filters at the antenna output and in the output of the transmission-side multiplier are necessary for proper suppression of undesirable frequencies. With the high operating frequencies utilized, only so-called helix filters will be suitable for this purpose in order to provide the necessary circuit quality with compact dimensions, low series attenuation and high selectivity. Despite the relatively high cost of such filters, they insure an optimum suppression of interference from harmonics or image frequencies.

The receiver operates by the dual-heterodyning principle. With simple conversion it would be necessary to employ a rather high intermediate frequency (100 to 150 MHz) in order to avoid image frequencies as much as possible. The amplification and demodulation of high Intermediate frequencies is, however, difficult on account of limited stability and low electrical efficiency, The first and second intermediate frequencies may be fixed at 75 and 10.7 MHz, respectively. The receiver should be of highly integrated structure. . - 10 83438 After a preselection in a two-stage band-pass filter the level of the useful signal is raised in a preamplifier whose output is fed via a further band-pass filter to the first mixer. The second mixer transposes the 75-MHz signal to 10.7 MHz whereupon the signal is delivered to a limiting amplifier and, after demodulation, is fed to handset 21 or to line 22, Each receiver further comprises a noise harrier whose threshold is adjustable. This insures that a connection is established only when the signal-to-noise ratio exceeds a certain minimum value. If a connection already exists, that connection is maintained for several seconds after a drop below the aforementioned minimum threshold; if the signal does not rise above the threshold during that interval, the connection is terminated.

Claims

1. A telecomnunication system Including a wireless telephone set and a fixed relay station for telephone calls using the public telephone network, the relay station being exclusively connectable to a telephone set of one 5 subscriber station and being directly connectable to the telephone network, the telephone set and the relay station being each provided with a radio transmitter and a radio receiver, said transmitters being for the exchange of identification signals for the avoidance of false connections, each transmitter having coding means for generating an Identification signal or 10 signals and transmitting a code on a channel found to be Idle by its associated receiver, the code of each transmitter being fixed and Independent from the selected call number of a subscriber station, each receiver having transmission scanning means for distinguishing between free and busy channels among a plurality of available cornnunication channels or pairs of 15 communication channels, a conparison circuit being connected to each of the receivers for detecting the transmitted fixed code and comparing it with a receiver-side code, and a circuit being provided for establishing wireless connection when identity of the transmitted code and the receiver-side code is detected by the conparison circuit. 20

2. A telecomnunication system according to Claim 1, wherein said wireless cornnunication is established by duplex operation between the telephone set and the relay station.

3. A telecomnunication system according to Claim 1 or Claim 2, wherein the transmitter code of the relay station corresponds to the receiver-side 25 code of the telephone set on the one hand, and, on the other hand, the transmitter code of the telephone set corresponds to the receiver-side code of the relay station, the codes for the relay station transmitter and the telephone set receiver differing from those for the telephone set transmitter and the relay station receiver. 30

4. A telecommunication system according to any preceding claim, wherein said telephone set is one of several parallel-connected telephone sets of said subscriber station. - 12 83438

5. A telecommunication system substantially as described herein with reference to the acconpanying drawing. Dated this 7th day of September, 1982. BY:- TOMKINS & CO., Applicants' Agents, (Signed) 5, Dartmouth Road, DUBLIN 6. - 13 53438 Abstract of the Disclosure A mobile subscriber station served by a telephone network is coupled by a radio link with an individually associated stationary relay station directly connected to that network. The two stations are able to communicate with each other, over a radius of about 50 meters, on any of 40 channels available to all the posts of subscriber and relay stations in the neighborhood. Each station of a post includes a transmitter and a receiver operating at about 900 MHz; in the idle state of the post, the receivers of both stations continuously scan the available channels to determine their free or busy condition. A person wishing l to make an outgoing call causes the transmitter of the subscriber station to send out a predetermined code word which is picked up by the receiver of the associated relay station in its scanning operation and, upon verification by a code comparator, initiates the emission of a handshake signal to the subscriber- station wtereupon the channel is seized to let the call proceed over the network. Similarly, the arrival of an incoming call at the relay station results in the transmission of the same or another predetermined code word to the subscriber station on a free channel which is seized for reception of the call after the code has been verified. With the code or codes used for outgoing and incoming calls uniquely assigned to a particular team, privacy is assured.