Handover

The process of changing cells during a call is called handover in GSM terminology. To choose the best target cell, the MS and the RBS perform measurements. Because the MS contributes to the handover decision, this type of handover is often called Mobile Assisted HandOver (MAHO).

Locating

An MS continuously measures signal strength and quality on it’s own cell and signal strength on the BCCH carriers of the neighboring cells. The measurements are carried out on the downlink while MS is in active mode. The measurement results are sent to the RBS on SACCH at regular intervals. The serving RBS measures signal strength and quality on the uplink.

Measurements sent to BSC

The measurements from the RBS and MS are sent to the BSC in the form of measurement reports. Based on these reports, the BSC decides if a handover is necessary and to which cell. This is called locating. As soon as a neighboring cell is considered to be better than the serving cell, a handover is attempted. Another reason for attempting a handover, apart from signal strength and quality, is when the Timing Advance (TA) used by MS exceeds a threshold value set by the operator. This usually happens when the MS is moving over the cell border to another cell. When the MS has changed cells, the new RBS informs the MS about the new neighboring BCCH carriers so measurements can be taken again. If the MS has also switched to a new LA, a location updating type normal takes place after the call has finished. Handover can be used for load balancing between cells. During a call setup in a congested cell, the MS can be transferred to a cell with less traffic if an acceptable connection quality is likely to be obtained. Another area where forced handover is a useful tool is maintenance. Channels can be released from traffic if necessary, e.g. for RBS maintenance reasons. There are several types of handover, including:

• Intra-cell handover

• Handover between cells controlled by the same BSC

• Handover between cells controlled by different BSC’s, but the same MSC/VLR

• Handover between cells controlled by different MSC/VLRs

Each of these traffic cases is described in greater detail below. In each case, the decision to perform a handover has already been made and a target cell has been identified.

Intra-Cell Handover

A special type of handover is the intra-cell handover. It is performed when the BSC considers the quality of the connection too low, but receives no indication from the measurements that another cell would be better. In that case the BSC identifies another channel1 in the same cell which may offer a better quality, and the MS is ordered to retune to it.

Handover between Cells Controlled by the Same BSC

When performing a handover between two cells controlled by the same BSC, the MSC/VLR is not involved. However, the MSC/VLR will be informed when a handover has taken place. If the handover involves different LA’s, location updating is performed once the call is finished.

Handover: cells controlled by the same BSC

1. The BSC orders the new RBS to activate a TCH.

2. The BSC sends a message to the MS, via the old RBS, containing information about the frequency and time slot to change to and also the output power to use. This information is sent to the MS using FACCH.

3. The MS tunes to the new frequency, and transmits handover access bursts in the correct time slot. Since the MS has no information yet on TA, the handover bursts are very short (only 8 bits of information).

4. When the new RBS detects the handover bursts, it sends information about TA. This is also sent via FACCH.

5. The MS sends a Handover Complete message to the BSC via the new RBS.

6. The BSC tells the old RBS to release the old TCH.

Handover between Cells Controlled by Different BSC’s but the Same MSC/VLR

When another BSC is involved in a handover, the MSC/VLR must also be involved to establish the connection between the two BSC’s.

Handover: different BSC’s but the same MSC/VLR

1. The serving (old) BSC sends a Handover Required message to the MSC containing the identity of the target cell.

2. The MSC knows which BSC controls this cell and sends a Handover Request to this BSC.

3. The new BSC orders the target RBS to activate a TCH.

4. The new BSC sends a message to the MS via the MSC and the old RBS.

5. MS tunes to the new frequency and transmits handover access bursts in the correct time slot.

6. When the new RBS sends information about TA.

7. MS sends a Handover Complete message to MSC via the new BSC.

8. MSC sends the old BSC an order to release the old TCH.

9. The old BSC tells the old RBS to release the TCH.

Handover between Cells Controlled by Different MSC/VLRs

Handover between cells controlled by different MSC/VLRs can only be performed within one PLMN and not between two PLMN’s. Cells controlled by different MSC/VLRs also means that they are controlled by different BSC’s.

Handover: cells controlled by different MSC’s

1. The serving (old) BSC sends a Handover Required message to the serving MSC (MSC–A), with the identity of the target cell.

2. MSC–A identifies that this cell belongs to another MSC, (MSC–B), and requests help.

3. MSC–B allocates a handover number to reroute the call. A Handover Request is then sent to the new BSC.

4. The new BSC orders the target RBS to activate a TCH.

5. MSC–B receives the information, and passes it on to MSC– A together with the handover number.

6. A link is set up to MSC–B, possibly via PSTN.

7. MSC–A sends a handover command to the MS, via the old BSC.

8. The MS tunes to the new frequency and transmits handover access bursts in the correct time slot.

9. When the new RBS detects the handover bursts it sends information about TA.

10. The MS sends Handover Complete message to the old MSC via the new BSC and the new MSC/VLR.

11. A new path in the group switch in MSC–A is established, and the call is switched through.

12. The old TCH is deactivated by the old BSC (not shown in the picture).

The old MSC, MSC–A, retains main control of the call until the call is cleared. This is because it contains the information about the subscriber and call details such as charging. After call release, the MS must perform location updating because a LA never belongs to more than one MSC/VLR service area. The HLR is updated by the VLR–B, and will in turn tell VLR–A to delete all information about the mobile subscriber.