6 Radio interface Signalling

3GPP43.129Packet-switched handover for GERAN A/Gb modeRelease 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

6.1 PS Handover Signalling (Um)

6.1.1 General

PS Handover signalling includes the set of all air interface messages (Um signalling for A/Gb mode, Up signalling for GAN mode and Uu signalling for Iu mode) sent to or from the MS in the source and target cells during the PS handover procedure.

6.1.2 Overview of PS Handover messages

6.1.2.1 GERAN A/Gb mode/GAN mode to GERAN A/Gb mode PS Handover

When performing an inter-SGSN PS Handover from GERAN A/Gb mode/GAN mode to GERAN A/Gb mode the following information is sent over the Um and Up interfaces.

– PS Handover Command message (for PS handover from GERAN A/Gb mode) or a GA-PSR Handover Command message including a PS Handover Command message (for PS handover from GAN mode) – sent to the MS in the source cell and includes the following:

– optionally, the NAS container for PS HO

– Indication of the radio resources for uplink and downlink TBFs to be used in the target cell for each PFC receiving PS handover treatment (i.e. the reverse path container created by target BSS/RNC).

– Indication of a PS handover reference number to be used when the MS arrives in the target cell.

– PS Handover Access message – the MS sends 4 handover access bursts in the target cell using an uplink TBF provided by the PS Handover Command. If multiple uplink TBFs are provided by the PS Handover Command message the MS sends access bursts using just one of these TBFs (i.e. at minimum one uplink TBF must be provided in the PS Handover Command message). The handover reference number is included within each access burst. This message is always sent for the case of non-synchronised handover and may still be sent for the case of synchronised or pre-synchronised handovers (determined by the target BSS during the PS handover preparation phase) to allow the target BSS to verify the accessing MS.

– Packet Physical Information message – sent by the target BSS to the MS in the target cell in response to the PS Handover Access message for the case of unsynchronised cells. Whether or not this message is sent in case of synchronised cells is indicated by the PS Handover command message (see 3GPP TS 44.018 [25]). It is sent using the downlink PACCH associated with the uplink TBF used to send the access bursts and provides the MS with physical layer information (i.e. Timing Advance). The time alignment information received in this message applies to all uplink TBFs allocated to the MS in the PS Handover Command message. The target BSS only sends this information if it receives the expected handover reference number in a PS Handover Access message.

– Uplink RLC Data Blocks – sent on uplink TBFs allocated by the PS Handover Command message after the MS receives Packet Physical Information message as follows:

– For inter-SGSN PS handover the MS, prior to sending any uplink user plane payload for any PFC, shall send an XID Response message .

– Downlink RLC Data Blocks – sent on downlink TBFs allocated by the PS Handover Command message as follows:

– For inter-SGSN PS handover for a given PFC the new SGSN may begin downlink N-PDU transmissions for that PFC prior to receiving a PS Handover Complete message (i.e. blind transmission may be used) or it may wait for a PS Handover Complete message before beginning downlink N-PDU transmissions for that PFC.

– GA-PSR HANDOVER INFORMATION message (for PS handover from GAN mode) – sent from the MS to the GANC to provide a list of target GERAN A/Gb mode cells, identified by CGI, in order of preference for PS handover, and includes the received signal strength for each identified GERAN A/Gb mode cell.

– GA-PSR UPLINK QUALITY INDICATION message (for PS handover from GAN mode) – sent from the GANC to the MS to provide it with PS service related information as perceived by the GANC which may result in the MS triggering the PS handover procedure (i.e. sending a GA-PSR HANDOVER INFORMATION message).

6.1.2.2 UTRAN/GERAN Iu mode to GERAN A/Gb mode/GAN mode PS Handover

When performing a PS Handover from UTRAN/GERAN Iu mode to GERAN A/Gb mode or from UTRAN to GAN mode the following the following information is sent over the Uu, Up and Um interfaces:

– RRC Message – sent to the MS in the source cell and includes the same information described for the PS Handover Command in clause 6.1.2.1.

– For PS handover from UTRAN to GERAN A/Gb mode/GAN mode the Handover from UTRAN Command (3GPP TS 25.331 [17]) message is used in CELL_DCH and CELL_FACH state when only PS RABs are established.

– For PS handover from GERAN Iu mode to GERAN A/Gb mode the HANDOVER FROM GERAN IU COMMAND message is used in RRC-Cell_Dedicated (MAC Dedicated or MAC DTM state) or RRC-Cell_Shared state when only PS RABs are established.

– PS Handover Access message (for PS handover to GERAN A/Gb mode) – sent as described in clause 6.1.2.1.

– Packet Physical Information message (for PS handover to GERAN A/Gb mode) – sent as described in clause 6.1.2.1.

– Uplink RLC Data Blocks (for PS handover to GERAN A/Gb mode) – sent as described in clause 6.1.2.1.

– Downlink RLC Data Blocks (for PS handover to GERAN A/Gb mode) – sent as described in clause 6.1.2.1.

– The target BSS/GANC may begin downlink LLC PDU transmissions prior to confirming the successful completion of the PS handover procedure (i.e. blind transmission may be used) or it may wait for an uplink RLC data block (target BSS) or a GA-PSR HANDOVER COMPLETE message (GANC) before beginning downlink N-PDU transmissions.

– GA-PSR ACTIVATE UTC REQ message (for PS handover from UTRAN to GAN mode) – sent to the MS from the target GANC to activate the transport channel to be used in the GAN cell.

– GA-PSR ACTIVATE UTC ACK message (for PS handover from UTRAN to GAN mode) – sent from the MS to the target GANC to confirm allocation of the transport channel to be used in the GAN cell.

– GA-PSR HANDOVER COMPLETE message (for PS handover from UTRAN to GAN mode) – sent from the MS to the target GANC to indicate handover completion.

6.1.2.3 GERAN A/Gb mode to GERAN Iu mode PS Handover

When performing a PS Handover from GERAN A/Gb mode to GERAN Iu mode the following information is sent over the Uu and Um interfaces:

– PS Handover Command message – sent to the MS in the source cell and includes the following:

– System broadcast information applicable to the target cell.

– Indication of the radio resources for uplink and downlink TBFs to be used in the target cell for each RAB receiving PS handover treatment (i.e. the reverse path container created by target BSS). Note that the MS will map the N-SAPI associated with each of its active PDP Contexts directly to a RABid (i.e. N-SAPI = RABid).

– Indication of a PS handover reference number to be used when the MS arrives in the target cell.

– PS Handover Access message – sent as described in clause 6.1.2.1.

– Packet Physical Information message – sent as described in clause 6.1.2.1.

– RRC message (e.g. Physical Channel Reconfiguration Complete) – content is GERAN Iu mode specific.

– Uplink RLC Data Blocks – sent on uplink TBFs allocated by the PS Handover Command message after the MS receives Physical Layer Information (content is GERAN Iu mode specific).

– Downlink RLC Data Blocks – sent on downlink TBFs allocated by the PS Handover Command message (content is GERAN Iu mode specific).

6.1.2.4 GERAN A/Gb mode/GAN mode to UTRAN mode PS Handover

When performing a PS Handover from GERAN A/Gb mode/GAN mode to UTRAN mode the following information is sent over the Uu, Up and Um interfaces:

– PS Handover Command message (for PS handover from GERAN A/Gb mode) or GA-PSR Handover Command message (for PS handover from GAN mode) – sent to the MS in the source cell. It includes the reverse path container created by the target RNC that consists of the RRC message (i.e. the Handover to UTRAN Command) required to perform PS handover to UTRAN.

– MS Detected by Target RNC – exact procedure and information transfer is UTRAN specific.

– RRC message (e.g. Physical Channel Reconfiguration Complete) – information content is UTRAN specific.

– GA-PSR HANDOVER INFORMATION message (for PS handover from GAN mode) – sent from the MS to the GANC to provide a list of target UTRAN cells, identified by a combination of LAI and 3G Cell Identity, in order of preference for PS handover, and includes the received signal strength for each identified UTRAN cell.

6.1.2.5 GERAN A/Gb mode to GAN mode PS Handover

When performing an inter-SGSN PS Handover from GERAN A/Gb mode to GAN mode the following information is sent over the Um and Up interfaces.

– PS Handover Command message – sent to the MS in the source cell and includes the following:

– optionally, the NAS container for PS HO.

– RN information which consists only of the mandatory fields within the PS Handover Radio Resources IE carried within the PS Handover Command message.

– The target GANC may begin downlink LLC PDU transmissions prior to confirming the successful completion of the PS handover procedure (i.e. blind transmission may be used) or it may wait for a GA-PSR HANDOVER COMPLETE message before beginning downlink N-PDU transmissions.

– GA-PSR ACTIVATE UTC REQ message – sent to the MS from the target GANC to activate the transport channel to be used in the GAN cell.

– GA-PSR ACTIVATE UTC ACK message – sent from the MS to the target GANC to confirm allocation of the transport channel to be used in the GAN cell.

– GA-PSR HANDOVER COMPLETE message – sent from the MS to the target GANC to indicate handover completion.

6.1.2.6 GERAN A/Gb mode to E-UTRAN PS Handover

When performing a PS Handover from GERAN A/Gb mode to E-UTRAN the following information is sent over the radio interface:

– PS Handover Command message (for PS handover from GERAN A/Gb mode) to the MS in the source cell (on the Um interface). It includes the reverse path container created by the target eNB that consists of the RRC message (i.e. the RRCConnectionReconfiguration message) required to perform PS handover to E-UTRAN.

– MS Detected by Target eNB- the exact procedure and information transfer on the Uu interface is E-UTRAN specific 3GPP TS36.300 [30], 3GPP TS36.331 [31].

– RRC message on the Uu interface – information content is E-UTRAN specific TS36.331 [31].

6.1.2.7 E-UTRAN to GERAN A/Gb mode PS Handover

When performing a PS Handover from E-UTRAN to GERAN A/Gb mode the following information is sent over the radio interface:

– RRC Message – sent to the MS in the source cell (on the Uu interface). It includes the same information described for the PS Handover Command in sub-clause 6.1.2.1.

– The MobilityFromEUTRACommand (3GPP TS 36.331 [31]) message is sent to the UE (on the Uu interface).

– PS Handover Access message (for PS handover to GERAN A/Gb mode) – sent on the Um interface as described in sub-clause 6.1.2.1.

– Packet Physical Information message (for PS handover to GERAN A/Gb mode) – sent on the Um interface as described in sub-clause 6.1.2.1.

– Uplink RLC Data Blocks (for PS handover to GERAN A/Gb mode) – sent on the Um interface as described in sub-clause 6.1.2.1.

– Downlink RLC Data Blocks (for PS handover to GERAN A/Gb mode) – sent on the Um interface as described in sub-clause 6.1.2.1.

– The target BSS may begin downlink LLC PDU transmissions prior to confirming the successful completion of the PS handover procedure (i.e. blind transmission may be used) or it may wait for an uplink RLC data block (target BSS).

6.1.3 RLC/MAC segmentation

RLC/MAC segmentation is a feature that provides an additional mechanism for sending control plane messages from the BSS to the MS. This feature can only be used after contention resolution is completed in the BSS and the BSS has to send an RLC/MAC control message that is from 3 to 9 radio blocks long.

6.1.4 Inter RAT/mode PS Handover to GERAN A/Gb

Void.

6.1.5 Inter RAT/mode PS Handover from GERAN A/Gb

Void.

6.2 Mechanisms for Initial Access in the Target Cell

6.2.1 General

This clause describes two possible approaches to the MS making access in the target cell. Both basic approaches assume that a handover procedure similar to that for the CS domain (Handover Access followed by Physical Information messages) takes place.

The effect of synchronised networks is examined for both approaches where the exchange of Handover Access and Physical Information is not required as the TA can be derived in advance.

The main assumptions applicable to the call flows are:

– USFs are required to schedule uplink data blocks.

– All identifiers and resources are known by the MS and target BSS before the MS makes the initial access in the target cell.

6.2.2 Synchronisation of handovers

The following synchronisation mechanisms are used for PS handover:

– Non-synchronised.

– Synchronised.

– Pre-Synchronised.

The non-synchronised cases are shown in figures 22 and 24 and are characterised by the requirement for the MS to obtain a valid uplink timing advance before it can transmit normal bursts. The MS shall notify its presence in the target cell through the transmission of access bursts to the BSS, and the BSS shall respond with a valid timing advance which in turn enables the MS to send normal bursts in uplink.

The synchronised and pre-synchronised cases are shown in figures 23 and 25 and have different mechanisms for the provision of the timing advance that are described in 3GPP TS 45.010 [26].

NOTE: The pseudo-synchronised case is not supported by the PS handover feature.

6.2.3 Option 1 – Downlink Data sent after performing access in the target cell

In this approach, downlink data is not transmitted until the BSS has been made aware of the presence of the MS via the reception of a PS Handover Access message.

6.2.3.1 Unsynchronised Networks Call Flow

The message flow for this option is shown in figure 22. The MS starts by sending PS Handover Access messages as four access bursts to the network (through USF allocation). As there is no contention, the network should receive at least one of the access bursts. A Handover Reference parameter is allocated by the target BSS in the PS Handover Command message and included in the PS Handover Access message to verify that the correct MS is accessing the resources. This is similar to the Handover Reference in the CS handover case.

The BSS receives the PS Handover Access message and detects that the correct MS has now made access in the target cell. It sends a Packet Physical Information message to indicate that the MS has been detected. The main purpose of this message is to give the Timing Advance information to the MS.

Once the MS has received the Packet Physical Information message it sends uplink LLC PDUs (user or signalling data, depending on the scenario). When correctly receiving the first RLC data block from the MS the target BSS verifies the mobile station, generates a PS Handover Complete message and sends it to the new SGSN.

The MS can start sending uplink RLC/MAC data blocks on the pre-allocated resources when scheduled with its USF. When the PS Handover Complete message has been received by the new SGSN, it can start sending downlink data blocks.

NOTE: The downlink RLC data blocks can be sent for any PFC receiving PS handover treatment after the target BSS has confirmed that the correct MS is present and after the PS Handover Complete message has been received by the new SGSN.

Figure 22: Option 1 – Downlink Data after MS contacts network

6.2.3.2 Synchronised Networks Call Flow

In the case of synchronous networks it is possible for the MS to calculate the TA of the target cell before it moves from the source cell.

Figure 23 shows the call flow in the case of synchronous networks. In this case it is possible for the MS to start transmitting and receiving messages as soon as it switches to the target cell.

As described in 3GPP TS 44.018 [25], handover access bursts may optionally be sent if indicated in the handover command message. If no access bursts are sent the MS begins sending uplink LLC PDUs . These PDUs are only sent to verify the MS’s presence in the new cell but do not trigger the sending of Packet Physical Information message. As blind transmission in the downlink is not being used in this scenario, the BSS must wait until a first RLC/MAC block is received (through USF allocation) before transmitting data in the downlink.

Figure 23: Option 1 – Downlink Data after MS contacts network, Synchronous Networks

6.2.4 Option 2 – Downlink Data sent before performing access in the target cell (Blind Transmission)

Blind transmission aims at minimising the interruption time on the downlink following handover. The target BSS starts transmitting downlink data on the newly reserved resources in the target cell before the MS has accessed this cell. The message flow for this scheme is shown in figure 24.

6.2.4.1 Unsynchronised Networks Call Flow

As the BSS must send the Packet Physical Information as an immediate response to the Packet Handover Access message, it is unable to send any downlink data at the same time. Therefore the BSS interrupts delivery of downlink data in order to send the Packet Physical Information to the MS.

NOTE 1: DL data is shown as being sent before the Packet Handover Access, but it may be sent by the target BSS at any time in the sequence. The PS Handover Access message is sent when being scheduled with the USF.

NOTE 2: Blind transmission assumes that the downlink data flow for a PFC receiving PS handover treatment may begin before the new SGSN receives the associated PS Handover Complete message from the target BSS and before the RAU procedure has started.

Figure 24: Option 2 – Blind Transmission in target cell

6.2.4.2 Synchronised Network Call Flow

In the case of blind transmission with synchronous networks, downlink data can be sent at the earliest opportunity. This is similar to the case for blind transmission without synchronous networks. However, uplink data transfer is also sped‑up as shown in the call flow of figure 25.

Figure 25: Option 2 – Blind Transmission in target cell, Synchronous Networks

The PS Handover Access message is the first uplink message sent if indicated in the handover command message as described in 3GPP TS44.018. Otherwise one or more RLC/MAC blocks that contain the XID Response message will be sent. This case is therefore the best in terms of reduced service interruption time.

NOTE: Blind transmission assumes that the flow of downlink data for a PFC receiving PS handover treatment may begin before the new SGSN receives the PS Handover Complete message from the target BSS and before the RAU procedure has started.

6.3 Methods for triggering PS Handover

A PS Handover is triggered by the BSS based on the received measurement reports or initiated by the reception of the Packet Cell Change Notification message from the mobile station. The BSS controls which of the two methods to use for initiating the PS Handover.

A PS Handover is triggered by the GANC based on the measurement reports received in the GA-PSR HANDOVER INFORMATION message or by other trigger conditions (see 3GPP TS 44.318 [28]).

When PS Handover is triggered by the BSS based on the measurement reports, the mobile station is in NC2 mode. The mobile station sends measurement reports to the network (BSS). When the network has found a new cell meeting the cell reselection criteria, and prepared for the PS Handover in that target cell, it sends the PS Handover Command message to the mobile station.

If not in NC2 mode, the mobile station may, by transmitting a Packet Cell Change Notification message according to the Cell Change Notification procedure, make the BSS aware it has found a new cell meeting the cell reselection criteria. The network then prepares for the PS Handover in the indicated target cell and sends the PS Handover Command message to the mobile station. In order to avoid that the mobile station performs autonomous cell reselection due to timeout of T3208, the network can order the mobile station to enter NC2 mode by transmitting the Packet Measurement Order message in response to the PCCN message. This will keep the mobile station in the cell until the PS Handover Command message is sent by the network, thus making it possible for the network to perform all necessary PS handover signalling and set up the radio resources in the target cell.

Annex A (normative):
Agreed handover principles