Telecommunications – Radiotelephone system – Including private cordless extension system
Patent
1995-09-15
1997-07-22
Bost, Dwayne
Telecommunications
Radiotelephone system
Including private cordless extension system
455566, 455567, 455462, H04M 1100
Patent
active
056510519
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to communications systems, and in particular to the digital time-division duplex radio communication system which is established between one of a plurality of portable units and one or more base units forming a cordless telephone system.
Such a system is shown, in its simplest form, in FIG. 1 of the accompanying drawings to which reference will now be made. The system illustrated comprises a fixed part in the form of a base unit 1, and two portable parts in the form of respective handsets 2,3. Each handset comprises an earpiece, microphone and keypad, this latter being shown diagrammatically under reference 4. In addition, each handset contains a respective radio transmitter/receiver (transceiver) and associated aerial 6,7 by which the handsets may communicate with the base unit by radio, as represented by the dotted lines 8,9. The base unit likewise contains a transceiver by which it communicates with the handsets, together with an aerial 5 for transmission and reception of radio signals from the handsets. The base unit also includes a hard- wired connection 10 to the external telephone system, and contains interface circuitry for interfacing the base unit transceiver to the external telephone line. Although only one base unit and two handsets are shown, this is to be taken as an example of the simplest system.
In such systems, the speech and other information to be transmitted between the or each base unit and the handsets is digitally encoded before transmission, is transmitted as a digital signal, and is decoded after reception to reproduce the original. A limited number (40 in the UK) of radio channels are allocated for the radio links 8,9 and it is clearly therefore preferable to utilise the same channel for both ends of a radio link--i.e. duplex communication. Each transceiver in the system will be able to transmit and receive on a number of these channels, if not all.
In digital second generation (CT2) cordless telephone systems, burst mode duplex is used to provide full duplex speech on a single channel. This essentially means that each transmitter has to compress the encoded speech from a particular time interval (called the burst period) down to just under half that interval (called the burst duration) in order to transmit the encoded speech and have time to receive the returning encoded speech in the other half of the burst period. This action is commonly called ping-pong transmission mode. It should be noted that the encoded speech corresponds to the speech from the entire burst period and on reception is expanded to its normal representation as continuous speech.
There has been established a common protocol for the exchange of signals, primarily control and synchronising signals, between the fixed and portable parts of the system. In the case of CT2, this protocol, known as a common air interface (CAI), is described in detail in international patent application WO90/09071. The present applicant's own air interface, a variant of the common air interface, is described in European patent application 0375361.
When an incoming call is received at the base unit, the base unit's transceiver transmits a callout signal to all of the handsets within range. This means that the handsets have to have their transceivers switched on in order to receive these transmissions which may, of course, come at any time. In practice, and in order to conserve handset battery life, the handsets automatically switch on their receivers at regular intervals, typically every 1.4 seconds. During the on period, the transceiver will scan, in receive mode, across the 40 allocated radio channels in turn, and will attempt to decode any signal being transmitted from the base unit. If no signal is received, the transceiver switches off, and the cycle is then repeated. If, however, a signal is detected, it is decoded to extract the following information: the number of handsets being called; its or their identification number; ring information; and display information. If the handset does not recognise
Bost Dwayne
Ferguson Keith
Lucent Technologies Wireless Limited
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