GB2110055A

GB2110055A - Cordless telephone systems

Info

Publication number: GB2110055A
Application number: GB08230394A
Authority: GB
Inventors: John Malcolm Robinson; John Arthur Tritton
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1981-10-26
Filing date: 1982-10-25
Publication date: 1983-06-08
Also published as: GB2110055B

Abstract

In a cordless telephone system in which a plurality of subscriber's sets utilise a common carrier signal frequency in time division multiplex to link their respective base and mobile units, the alternating current waveform of a mains electricity supply to each base unit is used to synchronise timing signals for defining TDM frame periods.

Description

SPECIFICATION Cordless telephone systems The present invention relates to cordless telephone systems.

According to one aspect of the present invention in a cordless telephone system of the kind in which a subscriber's set comprises a mobile unit and a base unit each arranged to transmit to and receive from the other unit information signals modulated on a radio frequency carrier signal, and in which a plurality of such subscriber's sets may utilise the same carrier signal frequency in time-division multiplex with each unit of said plurality of subscriber's sets utilising said carrier signal frequency during a respective time period or slot in a recurrent sequence or frame comprising a predetermined number of such time periods, at least each base unit includes means responsive to the alternating current waveform of a mains electricity supply thereto to generate timing signals substantially synchronised with said alternating current waveform to define one or more frame periods within each cycle of said alternating current waveform.

According to another aspect of the present invention in a cordless telephone system of the kind in which a subscriber's set comprises a mobile unit and a base unit each arranged to transmit to and receive from the other unit information signals modulated on a radio frequency carrier signal, and in which a plurality of such subscriber's sets may utilise the same carrier signal frequency in time-division multiplex with each unit of said plurality of subscriber's sets utilising said carrier signal frequency during a respective time period or slot in a recurrent sequence or frame comprising a predetermined number of such time periods, at least each base unit includes means responsive to the alternating current waveform of a mains electricity supply thereto to generate timing signals substantially synchronised with said alternative current waveform to define one or more frame periods within each cycle of said alternating current waveform, and means responsive to radio frequency carrier signals transmitted by units of other subscriber's sets to identify an available time period or slot within a frame period.

A cordless telephone system in accordance with the present invention will now be described by way of an example with reference to the accompanying drawings, of which: Figure 1 shows schematically an electric circuit forming part of a subscriber's set for the system, Figure 2 shows timing intervals utilised in operation of the system, and Figures 3 and 4 show flow charts illustrating the operation of a subscriber's set for the system.

The system comprises a plurality of subscriber's sets each having a base unit, which is linked to conventional exchange equipment by way of a line-pair, and a mobile unit which is linked by radio to the respective base unit.

The subscriber's sets may all use the same carrier frequency for their respective radio links or may use one of a group of frequencies within an allotted frequency band.

The radio link between a base unit and the respective mobile unit has an effective range of say two hundred metres, and in general radio signals transmitted by the units of one subscriber's set will be detectable by other sets within that range or in some cases well beyond that range. The sets are therefore arranged to operate in burst mode in time division multiplex, the speech signals originating at a mobile unit, for example, being sampled and pulse coded and the coded signals generated during each of a succession of frame periods of 20 milliseconds being stored for subsequent transmission as a group or burst at a high bit rate.

Referring now to the drawing the frame period for each base unit is derived, by means of the circuit arrangement shown in Fig. 1, from the alternating current waveform of the 50 Hz main electricity supply, which supply may also be utilised to energise the circuit arrangement. The waveform is applied by way of a lowpass filter 1 and a preset phase shift circuit 2 to a squaring amplifier 3, the output of which is D.C. restored in a level shift circuit 4 to provide an output pulse-train of positivegoing pulses at a repetition rate of 50 Hz.

An output signal from a voltage controlled oscillator 5 operating at a frequency of 63 KHz is divided in frequency by 1 260 and the resultant 50 Hz signal is compared in phase with the output of the circuit 4 in a phase comparator 6. The lowpass filtered output of the comparator 6 is applied to control the frequency of the oscillator 5, such that this oscillator is phase-locked to the output of the level shift circuit 4. Output signals from the oscillator 5 are used to provide clock signals at 63 KHz for timing the operation of the base unit with respect to the frame periods.

It will be appreciated that the alternating mains supply at any outlet may be derived from any one of three phases and may be connected to the circuit arrangement of Fig. 1 in either sense. In addition any one set of three phases may be displaced from another by 30 degrees depending on the form of three-phase transformer used. Altogether, therefore, any timing maker derived from, say, a positive-going zero crossing in the local mains waveform may have any one of twelve equally spaced positions in a cycle of the grid waveform, depending on the derivation of the local supply from the grid supply.

Referring now to Fig. 2 the 20 milliseconds duration of each frame period, between timing pulses 7, derived from the output of the circuit 4, is divided into twelve equal intervals each of some 1,666 micro-seconds, or 105 clock cycles, duration. It will be appreciated that the particular one of these time intervals shown expanded in Fig. 2 as the seventh interval to one subscriber's base unit may to another subscriber's base unit appear to be the first interval or any other interval in its cycle, depending on the derivation of the respective frame timing pulses.

Speech signals to be transmitted between the units of a subscriber's set are sampled at a rate of 8 KHz and the magnitude of each sample coded to a four-bit code, so that during a 20 millisecond frame period 640 coded speech bits are generated. These bits, together with groups of bits for timing, alignment, identification and signalling, are assembled to form a group of information signals totalling some 740 bits. At a bit rate of 2 MHz this group can be transmitted as a burst in 370 microseconds, so that each of the twelve time intervals referred to above provides up to four periods or time slots for the transmission of such bursts or, as indicated in Fig. 2, two send and receive exchanges of bursts.The position within a time interval of a burst sent out by a base unit is timed by clock pulses from the oscillator 5, while the timing of the corresponding received burst is at least in part determined by transit time and by the response time of the respective mobile unit. Those portions of a time interval not used for the transmission and reception of bursts of pulses may be utilised as "guard" periods 8 between channels, to accommodate frame phase errors arising from mains phase or frequency variations due for example to different loadings on mains outlets, or from errors in phase detection.

The 100 bits added to the 640 coded speech bits in each burst may be allotted as follows: 32 bits to allow the 2 MHz clock oscillator of a mobile unit to synchronize to the corresponding clock frequency of the respective base unit, 1 2 bits to provide a sequence identifying the start of a channel, 40 bits to provide a unique identification code-word for each base unit and its associated mobile unit, and 1 6 bits for signailing, dialling and status indication.

Provided that the 2 MHz timing source is sufficiently stable this source need not be synchronised with the oscillator 5.

Referring now to Figs. 3 and 4, each unit of a subscriber's set is provided with means to transmit and receive bursts of information signals, modulated onto a radio frequency carrier signal, and the base unit and the mobile unit of each set are provided with respective means to monitor the radio fre quency carrier signals transmitted by the units of any other subscriber's sets within range that are in use. If a subscriber wishes to make a call, or if there is an incoming call to a subscriber's set these monitoring means are used to locate a pair of time slots within the frame period during which there are no other transmissions detectable by the base and mobile units, and which are therefore available for the subscriber's use. Timing signals from the output of the circuit 4 and the oscillator 5 are then combined to locate subsequent recurrences of these available time slots, and if the monitoring means shows them still to be available then the subscriber's set can come into use. Fig. 3 shows a sequence in which an incoming call to the base unit results in the seizing of an availabe pair of time slots, while Fig. 4 shows a sequence in which a subscriber sets up an outgoing call from the mobile unit.

Once an available period has been taken into use by the transmission of a burst the monitoring means of any other set within range which is waiting to come into use will register the activity and will look elsewhere in its frame for an available pair of time slots.

Claims

1. A cordless telephone system of the kind in which a subscriber's set comprises a mobile unit and a base unit each arranged to transmit to and receive from the other unit information signals modulated on a radio frequency carrier signal, and in which a plurality of such subscriber's sets may utilise the same carrier signal frequency in time-division multiplex with each unit of said plurality of subscriber's sets utilising said carrier signal frequency during a respective time period or slot in a recurrent sequence or frame comprising a predetermined number of such time periods, wherein at least each base unit includes means responsive to the alternating current waveform of a mains electricity supply thereto to generate timing signals substantially syn chronised with said alternating current wave form to define one or more frame periods within each cycle of said alternating current waveform.

2. A cordless telephone system of the kind in which a subscriber's set comprises a mobile unit and a base unit each arranged to transmit to and receive from the other unit information signals modulated on a radio frequency carrier signal, and in which a plurality of such sub scriber's sets may utilise the same carrier signal frequency in time-division multiplex with each unit of said plurality of subscriber's sets utilising said carrier signal frequency dur ing a respective time period or slot in a recurrent sequence or frame comprising a predetermined number of such time periods, wherein at least each base unit includes means responsive to the alternating current waveform of a mains electricity supply thereto to generate timing signals substantially synchronised with said alternative current waveform to define one or more frame periods within each cycle of said alternating current waveform, and means responsive to radio frequency carrier signals transmitted by units of other subscriber's sets to identify an available time period or slot within a frame period.

3. A cordless telephone system in accordance with Claim 2 wherein said means responsive to the alternating current waveform of said mains electricity supply comprises a voltage-controlled oscillator the oscillation frequency of which is arranged to be substantially a predetermined multiple of the frequency of said mains supply, output signals from said oscillator being utilised to define said one or more frame periods.

4. A cordless telephone system in accordance with Claim 3 wherein the oscillation frequency of said oscillator is controlled by means of a phase-lock loop, a signal from said oscillator being applied to a frequency divider circuit and an output signal from this circuit being compared in phase with the mains supply to provide a frequency control voltage for said oscillator.

5. A cordless telephone system substantially as hereinbefore described with reference to the accompanying drawings.