GB2281676A

GB2281676A - System for broadcast and group communications in a communications system

Info

Publication number: GB2281676A
Application number: GB9318522A
Authority: GB
Inventors: Ian Neville Bendle; Paul Crichton
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1993-09-07
Filing date: 1993-09-07
Publication date: 1995-03-08
Also published as: FR2709908A1; FI944095A0; GB9318522D0; FI944095A; CA2130012A1; DE9413749U1

Abstract

A communications system comprises base station (18, 19, 20, 21) and remote units (22 - 26) arrangeable into at least one group and communicating with the base station over a radio link. A group service centre (GSC 12) comprises means for receiving traffic from an inbound channel from the radio link and copying it into an outbound channel to the radio link. A remote unit communicates its identification to the GSC and establishes a link with it. The GSC comprises look-up means for correlating the identification of the remote unit with identifications of other remote units in a common group and means for individually establishing links with said other remote units using their identifications. Control information is periodically transmitted to each remote unit in the group while the group is established and upon input of a transmission requirement the remote unit transmits on the reserved traffic channel with transmissions adjusted according to the control information. <IMAGE>

Description

A Method and Svstem for Broadcast and Group Communications in a Communications Svstem Field of the Invention This invention relates to a method and system for broadcast and group communications in a communications system comprising at least one base station and a plurality of remote units, where the remote units have individual identifications, as is the case in, for example, point-to-point communications such as GSM cellular radio.

Background to the Invention It is proposed to use GSM (Groupe Speciale Mobile) cellular radio technology in private mobile radio (PMR) dispatch-type systems for use by railway operators where there is a requirement for point-to-point communications and for group communications.

In the cellular radio field, it has been proposed to provide a conference service centre for setting up conference calls in which the a number of point-topoint calls are set up simultaneously on an ad hoc basis and each inbound channel time slot is bridged across to each outbound time slot at the conference service centre.

It as been suggested in the paper "Group Calls in GSM" ETSI SMG-UIC Task Force 15 February 1993, that a conference centre of this type could be used for managing group calls in a PMR system.

The problem remains of how to manage such an arrangement in a spectrally efficient manner and a manageable manner without undue delays at each step of the procedure such that the system will find acceptance in the marketplace.

Summary of the Invention According to a first aspect of the present invention, a communications system is provided comprising at least one base station and a plurality of remote units. Each remote unit has an identification. The remote units are arrangeable into at least one group and are arranged to communicate with the base station over a radio link. A group service centre (GSC) is provided comprising means for receiving traffic from an inbound channel from the radio link and copying it into an outbound channel to the radio link. At least one remote unit is provided comprising means for communicating its identification to the GSC and establishing a communication link with the GSC.The invention is characterized in that: the GSC comprises look-up means for correlating the identification of said remote unit with identifications of other remote units in a common group and means for individually establishing communication links with said other remote units using the identifications of those remote units.

Thus, by adding intelligence to the GSC, the GSC becomes more than a mere conference bridge and actively sets up the group calls. The GSC can set up the group call based on prior knowledge of the group to which given remote unit belongs or, where a remote unit belongs to more than one group, the GSC need only be informed of the identification of the group to which the remote unit requests connection.

According to a second aspect of the invention, a communications system is provided comprising at least one base station and a plurality of remote units, each remote unit having an identification, the remote units being arrangeable into at least one group and being arranged to communicate with the base station over a time divided multiple access (TDMA) radio link having inbound time slots and outbound time slots, means for establishing a group of remote units, allocating a control channel to each remote unit in the group, and reserving a traffic channel for the group, means for periodically transmitting remote unit control information to each remote unit in the group while the group is established, means at a remote unit for inputting a transmission requirement and means at the remote unit for providing transmission access by the remote unit in the group to the reserved traffic channel upon input of a transmission requirement to that remote unit and for transmitting on the reserved traffic channel with transmissions adjusted according to the remote unit control information upon providing said transmission access.

In this manner, transmission parameters, such as timing advance and power level, are established already and upon pressing of the PTT, the channel can immediately be accessed.

Brief Description of the Drawings Fig. 1 is a block diagram of a PMR system in accordance with the invention.

Fig. 2 is a block diagram of the group service centre of Fig. 1.

Fig. 3 is a flow diagram showing the steps involved in setting up a group call in the system of Fig. 1.

Fig. 4 is a flow diagram showing the operation of the system following the steps of Fig. 3.

Glossarv of Terms BS Base Station BSC Base Station Controller BTS Base Transceiver Station GSC Group Service Centre GSC Group Service Controller GSM Groupe Speciale Mobile MS Mobile Station MSC Mobile Switching Centre PSTN Public Switched Telephone Network PTT Push-to-talk SDCCH Stand-alone Dedicated Control Channel TDMA Time Division Multiple Access Detailed Description of the Preferred Embodiment Fig. 1 shows the relationship between the GSC and the network. In a GSM network, mobiles 22-26 are connected by radio communications channels to BTSs 18-21. Each BTS 18-21 contains the radio components of the system and modules that handle the interface to a BSC 15-17. A BTS 18-21 is connected using terrestrial links to its BSC 15-17.A BSC (for example BSC 15) manages a group of BTSs (for example BTSs 18,19) and acts as the digital processing interface between them and the MSCs 13-14 and performs GSM defined call processing. A BSC 15-17 is connected using terrestrial links to an MSC 13-14.

The MSCs 13-14 are telephone switching centres. Each MSC 13-14 provides service to mobiles located within its coverage area. It also provides interfaces to a PSTN network 10 or ISDN network 11 and to terrestrial circuits from the BSCs 15-17.

MS 22 is shown as having a PTT button 30 which provides tramsmitter key-up. Each other MS has a PTT button.

The GSC is connected to the MSC. The GSC provides management of group calls and broadcast calls using a conference facility or one to many multiplexer. The GSC can reside within the MSC or as a separate piece of equipment connected to the MSC using terrestrial links.

Fig. 2 shows a block diagram of the GSC. It consists of a GSC controller 100, a MSC interface 101, a group database 102 and a conference bridge or one to many multiplexer 103. The GSC controller 100 maintains the group database, manages the interface to the MSC via the MSC interface 101, and controls the conference bridge/multiplexor 103. The group database 102 contains all the information relating to group members and their group/broadcast call privileges. The conference bridge/multiplexor 103 connects two or more audio traffic channels and supports: 1) conferencing these traffic channels (allowing all parties to talk to all other conference members simultaneously); and 2) broadcasting speech from the initiating TCH user to all the group members. The GSC controller 100 indicates to the conference bridge/multiplexor 103 the group members, and the type of call.

The group database 102 is maintained from a control terminal 105.

Alternatively a mobile user can create, modify and delete broadcast and group call groups. Different levels of access may be given to different types of users.

The GSC 12 manages the broadcast or group call and monitors the state of each mobile connection. This allows the service centre to indicate whether a call is successful or not. If all parties are connected to the conference the service centre may inject (or switch off) a warning tone in the speech path. The tone may be sent to the originator or to the whole conference. The service centre controls whether the called parties can respond (group call) or cannot respond (broadcast call). The protocol is optimised for a broadcast call or group call, and management of the call can be very quick (between 1 to 2 standard call set-ups).

This solution offers a very high level of service and call management, comparable to a full conference facility. It is significantly more spectrally efficient than allocating a TCH to each group call member. The SDCCH solution provides a constant signalling connection between the GSC and each member in the group call. Managing the group is therefore easier. Signalling links do not need to be set up every time a mobile changes state, such as handing over to a new cell, entering a new group, and exiting a group. Additionally, during a group call, data can be passed to specific mobiles as well as to an entire group.

Individual mobiles can be taken out of the conference to respond to higher priority calls.

Each SDCCH carries a quarter of the data rate of a TCH. The burden in spectrum terms of permanently using an SDCCH for each mobile in the group may be reduced by use of an eight or sixteenth rate channel - that is to say each SDCCH timeslot has a duration equal to an eight or sixteenth of a traffic channel timeslot.

Figs. 3 and 4 show a flow diagram showing how the SDCCH can be used to control access to a group broadcast channel.

Initially a group call is set-up to Group A (step 200). The GSC controller accesses the group MS IDs from the group database (step 201). The GSC initiates the connection of each group member (step 202). When the first called group member responds, it is allocated a SDCCH. Authentication and cyphering is established (step 203) and the new member is added to the group (step 204) The MSC then reserves a broadcast TCH (step 205). The group call is now established and the system is waiting for the PTT button to be pressed (state 206). If additional group members respond later they are added to the group in a similar fashion (Step 207). Additional broadcast traffic channels are added as necessary since mobiles may be in different cells & paging groups. All mobiles in the group can now listen to the down-link broadcast channel. In the meantime, timing advance and power control are maintained using the SDCCH (step 208).

If the PTT button is pressed (Fig. 4), the PTT command is sent to the MSC (step 257). The MSC allocates the up-link TCH to the MS (step 258). The MS now moves from the SDCCH to the TCH and speech commences (step 259). The speech is duplicated in the GSC and inserted into the outbound TCH (step 260).

The MSC inhibits other mobiles from gaining access to the up-link TCH (step 261). When the MS PTT button is released, the up-link TCH is released, the broadcast is stopped and the mobile is returned to the SDCCH (step 262).

Claims

1. A communications system comprising at least one base station (18-21), a plurality of remote units (22-26) each remote unit having an identification, the remote units being arrangeable into at least one group and being arranged to communicate with the base station over a radio link, and a group service centre (GSC, 12) comprising means for receiving traffic from an inbound channel from the radio link and copying it into an outbound channel to the radio link, at least one remote unit comprising means for communicating its identification to the GSC and establishing a communication link with the GSC, characterized in that:: the GSC comprises look-up means (102) for correlating the identification of said remote unit with identifications of other remote units in a common group and means for individually establishing communication links with said other remote units using the identifications of those remote units.

2. A communications system comprising at least one base station (18-21) and a plurality of remote units (22-26) each remote unit having an identification, the remote units being arrangeable into at least one group and being arranged to communicate with the base station over a time divided multiple access (TDMA) radio link having inbound time slots and outbound time slots, means (12,13, 14) for establishing a group of remote units, allocating a control channel to each remote unit in the group, and reserving a traffic channel for the group;; means (208) for periodically transmitting remote unit control information to each remote unit in the group while the group is established, means (30) at a remote unit for inputting a transmission requirement (PTT) and means (259) at the remote unit for providing transmission access by the remote unit in the group to the reserved traffic channel upon input of a transmission requirement to that remote unit and for transmitting on the reserved traffic channel with transmissions adjusted according to the remote unit control information upon providing said transmission access.

3. A communications system according to claim 2, wherein the control information transmitted is timing advance information and the means at the remote unit are arranged to adjust the timing of the inbound time slots.

4. A communications system according to claim 2, wherein the control information transmitted is output power information and the means at the remote unit are arranged to adjust the output power of the remote unit transmissions.