A look at working of Soft Handover (SHO)

Zahid Ghadialy

Last Updated: 29/03/2005

Introduction:

Soft Handover is one of the most interesting feature of the WCDMA operation. In this case the UE always has radio link connection. In case of GSM only hard handover was possible where the radio connection is lost before new connection is established. In UMTS hard handover is also possible when we are changing frequencies but generally soft handover will be used when in the same frequency.

Hard handover is possible in the same frequency (Intra-frequency) scenario when RAB is being added and there are not enough codes available on the current cell then the network might ask the UE to move to new cell (scrambling code).

In this tutorial we will not metion Softer handover seperately as it is a special case of Soft Handover. For more details please see Handover Tutorial. Softer handover will be referred to as Soft Handover in this document.

Please note that Soft handover procedure could include Radio link that belongs to the same Node B, different Node B but the same RNC or a Node B that belongs to dirrent RNC (Drift RNC). For more details please see Handover Tutorial.

Also note that the Soft Handover procedures does not involve the Core Network. It just involves the RNC, Node B and the UE. The Soft Handover procedure is only applicable to UEs in FDD mode of operation.

Basic Definitions:

Before we begin there are some terms that need to be discussed. The UE measures the cells on the Intra Frequency in order for the network to decide which cells to add in the Active Set. An Active Set contains all the Cells that the UE is listening to and Transmitting to. The Cells that UE measures belong to one of the three categories:

  1. Cells, which belong to the active set. User information is sent from all these cells. In FDD, the cells in the active set are involved in soft handover. The UE shall only consider active set cells included in the variable CELL_INFO_LIST for measurement; i.e. active set cells not included in the CELL_INFO_LIST shall not be considered in any event evaluation and measurement reporting.
  2. Cells, which are not included in the active set, but are included in the CELL_INFO_LIST belong to the monitored set.
  3. Cells detected by the UE, which are neither in the CELL_INFO_LIST nor in the active set belong to the detected set. Reporting of measurements of the detected set is only applicable to intra-frequency measurements made by UEs in CELL_DCH state.

Entering CELL_DCH:

When RRC Connection is setup, UE generally moves to CELL_DCH. This is based on the network configuration and it is the network that will decide if the UE has to enter CELL_DCH or CELL_FACH. For simplicity we assume that the network puts the UE in CELL_DCH. In CELL_DCH state UE generally starts with one Radio Link that belongs to its Active Set.

In CELL_DCH state UE will generally start Intra Frequency Measurements. These measurements are either setup via System Information broadcasted on BCH or through a Measurement Control Message (MCM).

The System Information Block Sib11/12 or MCM contains the Cell Info List that the UE has to store to perform measurements. All the cells in the Cell Info List are either in Active Set and if not in Active Set then they are in Monitored Set. The network might ask the UE to report Detected Cells. These are thye cells that UE can see but were not sent in the Cell Info List. If asked then the UE reports them whenever Measurement Reports are sent out.

Intra-Frequency Measurements

These Measurements can be either Periodic or Event Based. For SHO, Event Based Measurements are generally used.

The Intra Frequency Measurement Events are from 1A to 1I. For SHO Event 1A, Event 1B and Event 1C are important. Event 1A and Event 1C could be periodic Event Based Measurements. In this case the network might ask that if an Event is trigerred than the UE should keep sending the report N number of times; where N is specified in the Measurement Control message or Sib11/12.

The figure above demonstrates a simple (cannot happen in practice though) scenario when Cell 1 has same power but Cell 2 has its power increasing till a certain point after which it decreases.

Now the Intra Frequency Event 1A notifies to UTRAN that a particular Cell has reached power level above the threshold. Similarly Event 1B notifies to UTRAN that a particular cell has power level below the threshold. For Event 1A (and 1C) UTRAN can request more than one report for a single event that has occurred. This is to make sure that only suitable cells are added to active list.

When Event 1A is sent to the UTRAN, UTRAN will add the new cell in the UE's active cell list and will send an ACTIVE SET UPDATE message that will add the cell to the UE's active set. Similarly when Event 1B is sent then UTRAN will send ACTIVE SET UPDATE message to remove the cell from UE's active set. When Event 1C is sent, UTRAN will send ACTIVE SET UPDATE message that will remove one or more cells and will add one or more cells. The only restriction for 1C is that there should be atleast one radio link that is not affected by the procedure. This radio link will exist before and after ACTIVE SET UPDATE message.

To avoid problems, Event 1B can only occur is there are more than one active radio links. Similarly there is an onpion available in the Measurement Criteria IE that deactivates Event 1A after certain number of cells are added in the active set. This is to avoid sending 1A and 1C for the same scenario. Similarly Event 1C only becomes applicable if there are more than one active radio links. The network can decide how many radio links are needed in active set for Event 1C trigerring to start.

The figure above shows the scenario of triggering of Event 1C. In this case when the first Event 1C is triggered UTRAN sends ACTIVE SET UPDATE to add Cell 4 and delete Cell 2. Similarly when the second Event 1C is triggered Cell 1 is replaced by Cell 3.

Signalling Examples

Lets look at an example where Cell 2 has triggered Event 1A and the network decides to add it in the Active Set


  UE                               Node B                            RNC
------                           ---------                       ----------
   |                                  |                               |
   |    RRC: Measurement Report (Event 1A: Cell 2)                    |   
   |----------------------------------------------------------------->|
   |                                  |  NBAP: RL Setup Request       |
   |                                  |<------------------------------|
   |                                  |                               |
   |                             Start RX                             |   
   |                                  |     NBAP: RL Setup Response   |
   |                                  |------------------------------>|
   |                                  |   ALCAP: ERQ                  |
   |                                  |<------------------------------|
   |                                  |              ALCAP: ECF       |
   |                                  |------------------------------>|
   |                                  |         NBAP: RL Restore Ind  |
   |                                  |------------------------------>|
   |                                  |      DCH-FP: Downlink Sync    |
   |                                  |<------------------------------|
   |                                  |       DCH-FP: Uplink Sync     |
   |                                  |------------------------------>|
   |                               Start TX                           |
   |   RRC: Active Set Update         |                               |
   |<-----------------------------------------------------------------|
   |        (add Cell 2)              |                               |
   |                          RRC: Active Set Update Complete         |
   |----------------------------------------------------------------->|
   |                                  |                               |
   |                                  |                               |

Similarly suppose Cell 2 has now triggered Event 1B and UTRAN decides that Cell 2 has to be removed than the signalling flow will be as follows:


  UE                               Node B                            RNC
------                           ---------                       ----------
   |                                  |                               |
   |    RRC: Measurement Report (Event 1B: Cell 2)                    |   
   |----------------------------------------------------------------->|
   |   RRC: Active Set Update         |                               |
   |<-----------------------------------------------------------------|
   |        (remove Cell 2)           |                               |
   |                          RRC: Active Set Update Complete         |
   |----------------------------------------------------------------->|
   |                                  |                               |
   |                                  | NBAP: RL Deletion Req         |
   |                                  |<------------------------------|
   |                                  |     NBAP: RL Deletion Resp    |
   |                                  |------------------------------>|
   |                                  |  ALCAP: REL                   |
   |                                  |<------------------------------|
   |                                  |                  ALCAP: RLC   |
   |                                  |------------------------------>|
   |                                  |                               |
   |                                  |                               |
   

The signalling for Event 1C will be combination of the above.

References:

[1] 3GPP TS 25.331: RRC Protocol Specification

[2] 3GPP TS 22.129: Handover requirements between UTRAN and GERAN or other radio systems

[3] 3GPP TS 23.009: Handover Procedures

[4] 3GPP TS 25.401: UTRAN Overal Description

[5] 3GPP TR 25.832: Manifestations of Handover and SRNS Relocation

[6] 3GPP TR 25.936: Handovers for real time services from PS domain










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