5 Group receive mode

03.223GPPFunctions related to Mobile Station (MS) in idle mode and group receive modeRelease 1999TS

5.1 General description

The purpose of the group receive mode process is for a MS to determine the most suitable cell while listening to a group or broadcast call.

There are various requirements that a cell must satisfy before an MS can perform group receive mode on it:

i) The radio path loss between MS and BTS must be below a threshold set by the PLMN operator. This is estimated as shown in subclause 3.6. That means that the cell is suitable for transmission and reception.

ii) It should not be "barred" (see subclause 3.5.1);

iii) The NCC of the PLMN should be permitted;

The choice of a cell is based on the C2 criterion, as defined in GSM 05.08. . When in group receive mode on a cell the MS regularly looks to see if there is a better cell in terms of the cell reselection criterion C2, as defined in GSM 05.08. C2 is determined by the MS from reception measurements on the BCCH frequency on the cell and reselection parameters relative to the cell. The reselection parameters are the same as those used in idle mode.

If there is a better cell which is suitable, the better cell is selected. (This is called "cell reselection in group receive mode").

The switching from idle or dedicated or group transmit mode to group receive mode is controlled by the Radio Interface Layer 3 protocols, see GSM 04.08.

If no suitable cell can be found the MS immediately returns to idle mode by choosing a cell like releasing a dedicated channel (see 05.08 subclause 6.7.1).

5.2 Requirements and technical solutions

5.2.1 Network provisions

For group and broadcast calls, the network provides a BA list in the SI TYPE 5, 5bis or 5ter message on the SACCH of the channel used for the group or broadcast call. The BA list indicates the BCCH carriers of the surrounding cells that MSs engaged in the call should monitor. The list shall include the cell on which it is sent. The cells in the list may belong to different PLMNs.

The network also provides the SI TYPE 6 message including an NCC permitted information element on the SACCH of the channel used for the group or broadcast call.

5.2.2 Group receive mode cell monitoring

A MS in group receive mode shall perform synchronization, identification and receive signal strength measurements of the BCCH carriers in the BA(SACCH) list of BCCH frequencies which was provided by the network in the serving cell. The regularity for performing synchronization and identification of the BCCH carriers shall be performed with a regularity as specified in GSM 05.08 for a MS in dedicated mode. The regularity of performing receive signal strength measurements of the BCCH carriers shall be as specified in GSM 05.08 for a MS in idle mode.

As long as no BA(SACCH) list is available on the SACCH, the last received BA(BCCH) list of the current cell shall be used instead if available.

The MS does not have to monitor cells for with NCC not permitted. The NCC permitted information is provided on the SACCH of the current cell. As long as no NCC permitted information is available on the SACCH, the last received NCC permitted information on the BCCH of the current cell shall be used instead if available.

The MS must also read the parameters affecting cell reselection of the surrounding cells from their respective BCCHs. These parameters are:

– MS txpwr max cch;

– rxlev access min;

– cell reselect offset;

– temporary offset;

– penalty time;

– location identity.

The MS shall attempt to decode this information within 15s after there are less than six stronger suitable cells and the MS is allowed to discard up to 6 speech frames during this period. The MS shall attempt to update the information at least every five minutes. Before selecting a cell in group receive mode the MS must have read the information which is presented in SI TYPE 1 and 3.

On this basis, the MS determines which of the cells in the list are suitable and computes their reselection criterion, and decides on its own whether one of the surrounding cells should be selected instead of the current one, as described in the next subclause.

The procedures above do not consider whether the cell supports the call or not.

To perform cell change, the MS shall at least have performed measurements on cells of the BA‑list received and read the corresponding BSIC.

5.2.3 Group receive mode cell change

Either of the following events trigger a cell change by a MS in group receive mode:

(i) The pass loss parameter C1 indicates that the path loss to the serving cell has become too high.

(ii) There is a radio link time‑out.

(iii) There is a better cell (in terms of the criterion C2) in the same LA, or a much better cell in another LA (using CRH parameter, subclause 3.4) and with NCC permitted.

After cell change, the MS either

‑ leaves the group receive mode and goes to idle mode in order to establish a dedicated connection with the network to perform a location update if the cell belongs to a new LA;

‑ detects that the selected cell does not belong to an equivalent PLMN and therefore leaves the group receive mode and goes to idle mode;

‑ detects that no NCH is present and therefore leaves the group receive mode and goes to idle mode;

‑ reads the NCH but does not receive any notification message for the current group or broadcast call and therefore leaves the group receive mode and goes to idle mode;

‑ reads the NCH and receives a notification message for the current group or broadcast call with the related channel position. If the channel is found, the MS changes to it and stays in group receive mode;

‑ reads the NCH and receives a notification message for the current group or broadcast call without information on the related channel position. The MS leaves the group receive mode and goes to idle mode in order to establish a dedicated connection with the network to become informed on the related channel position.

During these procedures, the MS may not be able to listen to the group or broadcast call, in which case it should mute the downlink.

For all procedures, which require that the MS leaves group receive mode, the MS goes to idle mode by choosing a cell like releasing a dedicated channel (see 05.08 subclause 6.7.1).

5.2.4 Uplink access in group calls

A MS assigned the uplink for the user to talk switches to group transmit mode.

It thus performs synchronisation, identification and measurements of the BCCH carriers according to the BA(SACCH) list as well as radio link measurements and reporting and is subject to hand over control from the network as for dedicated mode.