9 Mobility procedures

25.3083GPPHigh Speed Downlink Packet Access (HSDPA)Overall descriptionRelease 17Stage 2TS

While in CELL_DCH state, the UE may be allocated one or more HS-PDSCH(s), allowing it to receive data on the HS-DSCH(s).

Mobile evaluated hard-handover and soft-handover mechanisms provide the RRC connection mobility in CELL_DCH state. The mobility procedures are affected by the fact that the HS-PDSCH allocation for a given UE belongs to only one of the radio links assigned to the UE, the serving HS-DSCH radio link. The cell associated with the serving HS-DSCH radio link is defined as the serving HS-DSCH cell.

A serving HS-DSCH cell change facilitates the transfer of the role of serving HS-DSCH radio link from one radio link belonging to the source HS-DSCH cell to a radio link belonging to the target HS-DSCH cell.

Figure 9-1: Serving HS-DSCH cell change

The serving HS-DSCH cell change may be further categorised in regards to whether the decision of the target HS-DSCH cell is made by the UE or by the network. In Release 5, only network controlled serving HS-DSCH cell changes shall be supported.

In case of a network-controlled serving HS-DSCH cell change the network makes the decision of the target HS-DSCH cell, and the decision could be based on UE measurement reports and other information available in the network. A network controlled HS-DSCH cell change is performed as an RRC layer signalling procedure and is based on the existing handover procedures in CELL_DCH state.

9.1 Serving HS-DSCH cell change

With regard to the way a serving HS-DSCH cell change is performed with respect to the dedicated physical channel configuration, the following categories exist:

1. Serving HS-DSCH cell change while keeping the dedicated physical channel configuration and the active set;

2. Serving HS-DSCH cell change in combination with an establishment, release and/or reconfiguration of dedicated physical channels (note: this may by definition imply an update of the active set);

3. Serving HS-DSCH cell change in combination with active set update in soft handover.

With respect to synchronisation between UE and UTRAN as to when transmission and reception is stopped and re-started, two possibilities for a serving HS-DSCH cell change exist:

1. Synchronised serving HS-DSCH cell change: Start and stop of HS-DSCH transmission and reception is performed at a certain time typically selected by the network;

2. Unsynchronised serving HS-DSCH cell change: Start and stop of HS-DSCH transmission and reception is performed "as soon as possible" (stated by UE performance requirements) at either side.

The serving HS-DSCH cell change may also be categorised with respect to the serving HS-DSCH Node B:

1. Intra-Node B serving HS-DSCH cell change: The source and target HS-DSCH cells are both controlled by the same Node B. The serving HS-DSCH Node B is not changed.

2. Inter-Node B serving HS-DSCH cell change: The Node B controlling the target HS-DSCH cell is different from the Node B controlling the source HS-DSCH cell.

The cell-Node B relations shall remain transparent for the UE and the UE should therefore shall not be aware of whether the serving HS-DSCH cell change procedure is of a intra-Node B or inter-Node B nature.

At an Inter-Node B serving HS-DSCH cell change, a serving HS-DSCH Node B relocation needs to be performed at the UTRAN. Serving HS-DSCH Node B relocation and serving HS-DSCH cell change are two separate procedures, even if serving HS-DSCH Node B relocation cannot be performed without a serving HS-DSCH cell change (but the other way is possible).

Figure 9.1-1: Inter-Node B serving HS-DSCH cell change combined with serving HS-DSCH Node B relocation

During a serving HS-DSCH Node B relocation, the HARQ entities located in the source HS-DSCH Node B belonging to the specific UE are deleted and new HARQ entities in the target HS-DSCH Node B are established. Different CRNCs may control the source and target HS-DSCH Node B.

9.2 Serving HS-DSCH cell change mechanisms

In the case of AM RLC mode, the polling function either pre- or post- HS-DSCH cell change can be utilised to obtain the status of the data transmission to the UE at the RLC level.

9.3 Intra-Node B synchronised serving HS-DSCH cell change

Figure 9.3-1 illustrates an intra-Node B serving HS-DSCH cell change while keeping the dedicated physical channel configuration and the active set, using the Physical channel reconfiguration procedure. The transition from source to target HS-DSCH cell is performed synchronised, i.e. at a given activation time.

In this example, the UE transmits a MEASUREMENT REPORT message containing intra-frequency measurement results, here assumed to be triggered by the event 1D "change of best cell". When the SRNC has performed the handover decision, the Node B is prepared for the serving HS-DSCH cell change at an activation time indicated with CPHY-RL-Commit-REQ primitive. The SRNC then sends a PHYSICAL CHANNEL RECONFIGURATION message, which indicates the target HS-DSCH cell and the activation time to the UE. Since the same Node B controls both the source and target HS-DSCH cells we assume there is no need to reset the MAC-hs or MAC-ehs entities. When the UE has completed the serving HS-DSCH cell change it transmits a PHYSICAL CHANNEL RECONFIGURATION COMPLETE message to the network.

In this example it is assumed that HS-DSCH transport channel and radio bearer parameters do not change. If transport channel or radio bearer parameters shall be changed, the serving HS-DSCH cell change would need to be executed by a Transport channel reconfiguration procedure or a Radio bearer reconfiguration procedure, respectively.

Figure 9.3-1: Intra-Node B synchronised serving HS-DSCH cell change

9.4 Inter-Node B synchronised serving HS-DSCH cell change during hard handover

Figure 9.4-1 illustrates a synchronised inter-Node B serving HS-DSCH cell change in combination with hard handover. The reconfiguration is performed in two steps within UTRAN. On the radio interface only a single RRC procedure is used.

Here we assume the UE transmits a MEASUREMENT REPORT message containing intra-frequency measurement results, triggered by the event 1D "change of best cell". The SRNC determines the need for hard handover based on received measurement reports and/or load control algorithms (measurements may be performed in compressed mode for FDD).

In the first step, the SRNC establishes a new radio link in the target Node B. In the second step this newly created radio link is prepared for a synchronised reconfiguration to be executed at a given activation time indicated in the CPHY-RL-Commit-REQ primitive. After the first step, the target Node B starts transmission and reception on dedicated channels. At the indicated activation time, transmission of HS-DSCH is started in the target HS-DSCH Node B and stopped in the source HS-DSCH Node B.

The SRNC then sends a TRANSPORT CHANNEL RECONFIGURATION message on the old configuration. This message indicates the configuration after handover, both for DCH and HS-DSCH. The TRANSPORT CHANNEL RECONFIGURATION message includes a flag indicating that the MAC-hs or the MAC-ehs entity in the UE shall be reset. The message also includes an update of transport channel related parameters for the HS-DSCH in the target HS-DSCH cell.

The UE terminates transmission and reception on the old radio link at the activation time indicated in the TRANSPORT CHANNEL RECONFIGURATION message, and configures its physical layer to begin reception on the new radio link. After L1 synchronisation has been established, the UE sends a TRANSPORT CHANNEL RECONFIGURATION COMPLETE message. The SRNC then terminates reception and transmission on the old radio link for dedicated channels and releases all resources allocated to the considered UE.

Note that in this inter-Node B handover example, RLC for transmission/reception on HS-DSCH is stopped at both the UTRAN and UE sides prior to reconfiguration and continued when the reconfiguration is completed. It is furthermore assumed in this example that the TRANSPORT CHANNEL RECONFIGURATION message indicates to the UE that the MAC-hs or MAC-ehs entity should be reset. A reset of the UE MAC-hs or MAC-ehs entity triggers the delivery of the content in the re-ordering buffer to higher layers.

Figure 9.4-1: Inter-Node B synchronised serving HS-DSCH cell change during hard handover

9.5 Inter-Node B synchronised serving HS-DSCH cell change after active set update (radio link addition)

Figure 9.5-1 illustrates an inter-Node B serving HS-DSCH cell change performed subsequent to an active set update. In this example it is assumed that a new radio link is added which belongs to a target Node B different from the source Node B. The cell which is added to the active set is assumed to become the serving HS-DSCH cell in the second step. This combined procedure is comprised of an ordinary Active Set Update procedure in the first step and a synchronised serving HS-DSCH cell change in the second step.

We assume the UE transmits a MEASUREMENT REPORT message containing intra-frequency measurement results. The SRNC determines the need for the combined radio link addition and serving HS-DSCH cell change based on received measurement reports and/or load control algorithms (measurements may be performed in compressed mode for FDD).

As the first step, the SRNC establishes the new radio link in the target Node B for the dedicated physical channels and transmits an ACTIVE SET UPDATE message to the UE. The ACTIVE SET UPDATE message includes the necessary information for establishment of the dedicated physical channels in the added radio link (but not the HS-PDSCH). When the UE has added the new radio link it returns an ACTIVE SET UPDATE COMPLETE message.

The SRNC will now carry on with the next step of the procedure, which is the serving HS-DSCH cell change. The target HS-DSCH cell is the newly added radio link, so far only including dedicated physical channels. For the synchronised serving HS-DSCH cell change, both the source and target Node Bs are first prepared for execution of the handover at the activation time indicated with CPHY-RL-Commit-REQ primitive.

The SRNC then sends a TRANSPORT CHANNEL RECONFIGURATION message, which indicates the target HS-DSCH cell and the activation time to the UE. The message may also include a configuration of transport channel related parameters for the target HS-DSCH cell, including an indication to reset the MAC-hs or MAC-ehs entity.

Since source and target HS-DSCH cell are controlled by different Node Bs, MAC-hs or MAC-ehs in source and target Node B need to be released and setup, respectively, which is assumed to be done with CMAC-HS-Release-REQ and CMAC-HS-Setup-REQ primitives. These MAC-hs and MAC-ehs control primitives are assumed to be carried on the same NBAP/RNSAP messages, which carry the CPHY-RL-Reconfig-REQ primitives. Execution of release and setup of MAC-hs or MAC-ehs entities shall also be performed at the indicated activation time.

When the UE has completed the serving HS-DSCH cell change it returns a TRANSPORT CHANNEL RECONFIGURATION COMPLETE message to the network.

Figure 9.5-1: Inter-Node B synchronised serving HS-DSCH cell change after active set update

9.6 Serving HS-DSCH cell change with target cell pre-configuration

Figure 9.6-1 illustrates the inter-Node B serving HS-DSCH cell change procedure with target cell pre-configuration. Target cell pre-configuration adds robustness to the serving HS-DSCH cell change procedure by allowing the network to send the serving HS-DSCH cell change command either over the source cell and/or over the target cell using the HS-SCCH. The use of target cell pre-configuration is configured by the network during the active set update procedure.

The UE transmits a MEASUREMENT REPORT 1A or 1C message containing intra-frequency measurement results requesting the addition of a new cell into the active set. The SRNC establishes the new radio link in the target Node B for the dedicated physical channels and transmits an ACTIVE SET UPDATE message to the UE. The ACTIVE SET UPDATE message includes the necessary information for establishment of the dedicated physical channels in the added radio link. If SRNC decides to preconfigure the target cell, the ACTIVE SET UPDATE message will also include the HS serving cell related configuration (e.g. H-RNTI, HS-SCCH configuration, etc.) of the new cell. When the UE has added the new radio link it returns an ACTIVE SET UPDATE COMPLETE message.

In a second step, the UE transmits a MEASUREMENT REPORT 1D to request the change of the HS-DSCH serving cell to a target cell.

If enabled by the network, the serving HS-DSCH cell change with target cell pre-configuration procedure can also be applied to a MEASUREMENT REPORT 1C requesting the change of current serving HS-DSCH cell; this would apply only to an Event 1C indicating the current serving HS-DSCH cell as the only active CPICH triggering this event 1C report. In such case, the UE transmits a MEASUREMENT REPORT 1C triggering the change of the HS-DSCH serving cell to a target cell, which is the best cell in the current active set according to the measured results in the Event 1C measurement report. If more than one cell in the active set share the same quality, the target cell is determined by the first one listed in the Event 1C measured results.

The further steps and procedures described below apply to a HS-DSCH serving cell change triggered by MEASUREMENT REPORT 1D or 1C, unless mentioned otherwise.

The MEASUREMENT REPORT message may include a calculated Activation time of the requested cell change, that the UE has calculated using an offset signalled in the ACTIVE SET UPDATE message before, if configured by the NW in the ASU message. The UE then starts monitoring one HS-SCCH channel in the target cell in addition to the HS-SCCH channels it is already monitoring in the serving HS-DSCH cell and secondary serving HS-DSCH cells if configured. The SRNC then may send a RADIO BEARER SETUP, RADIO BEARER RECONFIGURATION, TRANSPORT CHANNEL RECONFIGURATION or PHYSICAL CHANNEL RECONFIGURATION message, which indicates the target HS-DSCH cell and possibly an activation time to the UE depending on the configuration. The message may also include a configuration of transport channel related parameters for the target HS-DSCH cell, including an indication to reset the MAC-hs or MAC-ehs entity.

In parallel, the target Node B may transmit the HS-SCCH in the target cell to initiate the serving HS-DSCH cell change. If the UE has not received a RADIO BEARER SETUP, RADIO BEARER RECONFIGURATION, TRANSPORT CHANNEL RECONFIGURATION or PHYSICAL CHANNEL RECONFIGURATION message, it will upon receiving the HS-SCCH in the target cell execute serving HS-DSCH cell change.

Since source and target HS-DSCH cells may be controlled by different Node Bs, MAC-hs or MAC-ehs in source and target Node B need to be released and setup, respectively, which is assumed to be done with CMAC-HS-Release-REQ and CMAC-HS-Setup-REQ primitives. These MAC-hs and MAC-ehs control primitives are assumed to be carried on the same NBAP/RNSAP messages, which carry the CPHY-RL-Reconfig-REQ primitives. Execution of release and setup of MAC-hs or MAC-ehs entities may also be performed at the indicated activation time.

When the UE has completed the serving HS-DSCH cell change it returns a RADIO BEARER SETUP COMPLETE, RADIO BEARER RECONFIGURATION COMPLETE, TRANSPORT CHANNEL RECONFIGURATION COMPLETE or PHYSICAL CHANNEL RECONFIGURATION COMPLETE message to the network, regardless if the cell change was triggered by reception of RADIO BEARER SETUP, RADIO BEARER RECONFIGURATION, TRANSPORT CHANNEL RECONFIGURATION or PHYSICAL CHANNEL RECONFIGURATION message in the source cell or HS-SCCH in the target cell. The response message type, including the RRC transaction Id that shall be used, are provided in the configuration given in the ACTIVE SET UPDATE message.

After the procedure of enhanced serving cell change for Event 1C is completed, an ACTIVE SET UPDATE message can be initiated to remove the old serving HS-DSCH cell and add the new cell as per MEASUREMENT REPORT 1C.

Figure 9.6-1: Inter-Node B serving HS-DSCH cell change with target cell pre-configuration

NOTE: The procedure also applies to the case of periodical reporting of event 1C.