Multiplex communications – Duplex – Communication over free space
Patent
1995-04-04
1996-12-10
Olms, Douglas W.
Multiplex communications
Duplex
Communication over free space
370350, 370507, 379 61, 379 63, H04L 514, H04J 306
Patent
active
055838545
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to duplex communications systems, and to a method of establishing a digital time-division duplex radio communication link between one of a plurality of portable units and one or more base units forming a cordless telephone system. The invention is directed particularly at the interference problem created by the use of an unsynchronised transmission in the presence of synchronised systems.
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 number of transceivers at least equal to the number of handsets, together with an aerial 5 for transmission and reception of radio signals from the handsets. The handsets may communicate with each other, but only via the base unit. 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 two handsets are shown, this is to be taken as an example of the simplest system and many more handsets, up to the capacity of the system, may be provided.
The present invention is concerned with systems of the type illustrated in FIG. 1, in which the speech and other information to be transmitted between the 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 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 is a need for 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, such a protocol, known as a common air interface (CAI), has been agreed and is described in detail in international patent application WO90/09071. A knowledge of the contents of this application is desirable for a full understanding of the present invention. The present applicant's own air interface, a variant of the common air interface is described in European patent application 0375361.
In the agreed protocol, exchange of signals is by way of three distinct transmitted burst signal patterns or structures exchanged between the fixed and portable parts of the system. These different burst structures are known respectively as MUX3, MUX2 and
MUX1, the acronym "MUX " standing for multiplex. In addition, the agreed protocol defines three sub-channels to be multiplexed within the available data bandwidth: synchronising information.
The structure of the various multiplexes is described in
REFERENCES:
patent: 5103459 (1992-04-01), Gilhousen et al.
patent: 5228026 (1993-07-01), Albrow et al.
AT&T Wireless Communications Products, Ltd.
Olms Douglas W.
Phillips Matthew C.
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