GB2291313A

GB2291313A - Cordless telecommunications handover

Info

Publication number: GB2291313A
Application number: GB9513239A
Authority: GB
Inventors: Peter Frank Rowlatt; Rick Dingle
Original assignee: GPT Ltd; Plessey Telecommunications Ltd
Current assignee: GPT Ltd; Plessey Telecommunications Ltd
Priority date: 1994-07-07
Filing date: 1995-06-29
Publication date: 1996-01-17
Also published as: GB9513239D0

Abstract

A method of transferring a call from a cordless handset 6 between two of a plurality of frame synchronised base-stations 4 connected to a Private Automatic Branch Exchange PABX 2, comprises the steps of; (i) the first base-station monitoring the quality of the first radio link, and upon deterioration of the link sending a first message to the PABX, (ii) the PABX upon receipt of the first message initiating monitoring of the quality of radio transmissions from the cordless handset by the base-stations, (iii) a new base-station detecting a radio link from the handset whose quality exceeds a predetermined value sending a second message to the PABX, and (iv) the PABX connecting the call from the handset to the new base-station in parallel with the first base-station, and then transferring transmission between the first and second radio links. <IMAGE>

Description

CORDLESS TELECOMMUNICATIONS HANDOVER The present invention relates to a cordless telecommunications system in particular to cordless telephone apparatus comprising a cordless handset and base-station conforming to the European Telecommunications Standards Institute (ETSI) I-ETS 300 131 R1. This standard, which is incorporated herein by reference, is for radio equipment and systems and provides a common air interface specification to be used for the interworking between cordless telephone apparatus in the frequency band 864.1 MHz to 868.1 MHz, including public access services.

In a cordless telecommunications system a network of base-stations are provided which communicate, by means of a radio link, with the cordless handset. For a given call the base-station, usually the one geographically closest, will communicate with the cordless handset. The strength of the radio link with a given base-station will depend upon the location of the cordless handset which being mobile may change during a call. It is therefore desirable during a call to switch, or handover, the call between base- stations to ensure the radio link with the handset is of an acceptably high standard.It is an object of the present invention to provide a method for hanover between a cordless handset and a burst synchronized base-station conforming to I-ETS 300 131 R1 in which neither a change of radio channel nor burst structure is necessary.

Accordingly the present invention provides in a cordless telecommunications system a method of transferring a call carried by a first radio link between a first base-station and a cordless handset, to one carried by a second radio link between a new base-station which is one of a plurality of frame synchronised base-stations and the cordless handset, the basestations being connected to a Private Automatic Branch Exchange PABX, comprising the steps of;; (i) the first base-station monitoring the quality of the first radio link, and upon deterioration of the link sending a first message to the PABX, (ii) the PABX upon receipt of the first message initiating monitoring of the quality of radio transmissions from the cordless handset by the base-stations, (iii) upon a new base-station detecting a radio link whose quality exceeds a predetermined value sending a second message to the PABX, and (iv) the PABX connecting the call from the handset to the new base-station in parallel with the first base-station, and transferring transmission between the first and second radio links.

Preferably the cordless telecommunications system operates according to I-ETSI 300 131 R1 and the transfer of transmission between the first and second radio links includes the first base-station transmitting a PAR~REQ to the cordless handset, in response the cordless handset +;ansmitting a PAR~RES, the first base-station ceasing transmission and the new base-station transmitting after a pre-selected delay.

A method of performing the invention will now be described by way of example only with reference to the accompanying drawings in which : Figure 1 is a schematic representation of a cordless telecommunications system and Figure 2 is a flow chart of the hanover method in accordance with the present invention.

Figure 1 shows a schematic representation of a cordless telecommunications system, which comprises a telecommunications network 1 which is typically a Public Switched Telephone Network (PSTN), although in the future an Integrated Services Digital Network (ISDN) may be more common. The telecommunications network 1 is connected to Private Automatic Branch Exchanges (PABX's) 2 which themselves control a plurality of CT2 base-stations 4 (CT2 - Cordless telephones of the second generation conforming to I-ETS 300 131 R1) of a cordless telephone apparatus. Each PABX 2 is connected to the telecommunications network 1 by a network link, which will typically include some or all of a wired link, an optical fibre link or a long distance radio link.

As shown in Figure 1 the cordless telephone apparatus, to which the present invention relates, comprises a plurality of base-stations 4 and a cordless handset 6. The cordless handset will typically be a telephone handset for speech communication but can also comprise a facsimile machine or other data communicating device. The handset 6 communicates with the base-station 4 over a radio link 7. The base-stations 4 are arranged to provide overlapping radio zones ensuring that a radio link 7 will always be available regardless of the geographic location of the cordless handset 6. In this way a user of the handset 6 has access to any other cordless handset, or conventional telephone 3, connected to the telecommunications network 1.

Although Figure 1 shows a single handset 6 communicating with a given basestation 4, a plurality of handsets could be simultaneously communicating with a single basestation using different radio channels. As the user of the handset moves away from the basestation the strength of the radio link within the zone covered by the base-station will weaken. There will come a point where the radio link would be stronger with the basestation in a neighbouring zone. At this point it is desirable to transfer or Handover. the radio link to the new base-station serving this zone to maintain transmission quality.

A method of Handover in accordance with the invention will be described with reference to Figure 2. Figure 2 is a flow chart for the handover operation between a first base-station and a new base-station in a neighbouring zone.

The first base-station which has an established call with the handset continuously monitors the quality of the radio link. When the first base-station detects a trend of deteriorating quality in the radio link it requests Handover of the PABX to which it is connected. It does this by sending a first message, or Handover Request, to the PABX which message includes the Portable Identification Code (PID) or serial number of the handset, the radio channel currently in operation and the multiplex (MUX1.2 or MUXl.4).

All of the terms are defined in I-ETS 300 131 R1.

The PABX in response to the Handover Request sends a message, referred to as a Listen to Handset, to base-stations in neighbouring zones initiating the base-stations to monitor the quality of radio transmissions from the handset. The Listen to Handset also includes the same information as the Handover Request that in PID, radio channel and multiplex. Since the base-stations in the neighbouring zones are frame synchronised they are able to detect and decode transmissions from the handset on the specified radio channel in the appropriate multiplex. If the PID contained in the ID~OK matches the PID in the Listen to Handset the base-station continues monitoring the quality of transmissions from the handset for a short period, typically five seconds. This new base-station then sends a Handset Found message to the PABX if the quality of the transmission link exceeds a preselected threshold. The Handset Found message contains the PID of the handset.

In response the PABX sends a message to the first base-station to continue listening and connects the speech, or data, path to the new base-station in parallel with the first basestation. The PABX sends a Handover Grant message to the first base-station which transmits a PAR~REQ to the handset. Upon receiving a PAR~RES from the handset the first base-station ceases transmitting to the handset and sends a Base-Station Idle message, contain the PID to the PABX. The new base-station which is still monitoring transmissions from the handset upon detecting the PAR~RES pauses a pre-selected period, approximately 5mS, before transmitting data from the speech path to the handset thereby completing the handover sequence to the new base-station.

Claims

1. In a cordless telecommunications system a method of transferring a call carried by a first radio link between a first base-station and a cordless handset, to one carried by a second radio link between a new base-station which is one of a plurality of frame synchronised base-stations and the cordless handset, the base-stations being connected to a Private Automatic Branch Exchange PABX, comprising the steps of;; (i) the first base-station monitoring the quality of the first radio link, and upon deterioration of the link sending a first message to the PABX, (ii) the PABX upon receipt of the first message initiating monitoring of the quality of radio transmissions from the cordless handset by the base-stations, (iii) a new base-station detecting a radio link whose quality exceeds a predetermined value sending a second message to the PABX, and (iv) the PABX connecting the call from the handset to the new base-station in parallel with the first base-station, and transferring transmission between the first and second radio links.

2. A method according to Claim 1 in which the cordless telecommunications system operates according to I-ETSI 300 131 R1 and in which the transfer of transmission between the first and second radio links includes the first base-station transmitting a PAREEQ to the cordless handset, in response the cordless handset transmitting a PAR~RES, the first base-station ceasing transmission and the new base-station transmitting after a pre-selected delay.

3. In a cordless telecommunications system a method of transferring a call carried by a first radio link between a first base-station and a cordless handset, to one carried by a second radio link between a new base-station which is one of a plurality of frame synchronised base-stations and the cordless handset, the base-stations being connected to a Private Automatic Branch Exchange PABX, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.