Handovers in GSM

Submitted By: Anum Almas

If a phone entered in a new cell... If the radio channel is not providing the good quality to the mobile

equipment... When there is no capacity for the new calls and the new call or a on going

call is from overlapped area... If the call faces the interferance... If the call doesn’t find a appropriate cell for connection...

Then will call faces congestion or drop?????

What If.....

Solution of all these is..

Handover

Handover is the process of switching a radio connection from one BS to another in order to maintain seamless radio connection during a

mobile station movement.

Different Forms of Handover...

There are three different forms of Handovers

Hard Handover Soft Hnadover Inter-Technology Handover

  A Hard Handover is one in which the channel in the source cell is released

and only then the channel in the target cell is engaged. Thus the connection to the source is broken before or 'as' the connection to the target is made—for this reason such handovers are also known as break-before-make. Hard handovers are intended to be instantaneous in order to minimize the disruption to the call.

Application:

A Hard handoff can be practically employed with more efficiency in FDMA and TDMA (GSM) network access systems

Hard Handover

A Soft Handover is one in which the channel in the source cell is retained and used for a while in parallel with the channel in the target cell. In this case the connection to the target is established before the connection to the source is broken, hence this handover is called make-before-break. The interval, during which the two connections are used in parallel, may be brief or substantial.

Application

Mostly CDMA (Code Division Multiple Access) based technologies employ Soft handoffs.

Soft Handover

If a mobile phone leaves a cell and no new cell can be found in the same system, the Base Station can handover the appropriately equppied mobile phone to another system. These intersystem hanndovers are highly complexed because two technically disparate systems must be combined with each other.

Application

These are empoyled to tranfer a from GSM to CDMA or vice-versa.

Inter-Technology Handover

Handover Dtection Strategies

Mobile-Controlled Handoff(MCHO) Network-Controlled Handoff(NCHO) Mobile-Assisted Hnadoff(MAHO)

MCHO: In this strategy, the MS continuously monitors the radio signal strength and

quality of the surrounding BSs. When predefined criteria are met, then the MS checks for the best candidate BS for an available traffic channel and requests the handoff to occur. MCHO is used in DECT and PACS.

NCHO: In this strategy, the surrounding BSs, the MSC or both monitor the radio

signal. When the signal’s strength and quality deteriorate below a predefined threshold, the network arranges for a handoff to another channel. NCHO is used in CT-2 Plus and AMPS.

MAHO In this strategy, the network directs the MS to measure the signal from the

surrounding BSs and to report those measurements back to the network. The network then uses these measurements to determine where a handoff is required and to which channel. MAHO is used in GSM and IS-95 CDMA.

GSM Handovers

There are four different types of handovers in GSM, which involve tranferring a connection channels.

Intra-Cell Handover. Intra-BSC Handover. Inter-BSC Handover. Inter-Msc Handover.

This form of Handover occurs if it is required to change the frequency or slot being used by the mobile because of interferance, or other reasons. In this type of handover, the mobile remains attached to the base station tranciever, but changes the channel or slot.

Also known as Intra BTS Handover.

Intra-Cell Handover

Signalling Sequence of Intra-Cell Handover

The intra-BSC handover is performed when the MSC changes the BTS but not the BSC. The intra- BSC handover is entirely carried out by the BSC, but the MSC is notified when the handover has taken place.

Intra-BSC Handover

Signalling Sequence of Intra-BSC Handover

In the inter BSC handover, when the BSC decides that handover is required but the targeted cell is controlled by different BSC then it needs assistance form the connected MSC. The MSC contacts the targeted BSC for allocation of the required resources and inform the BSC when they are ready. After the successful resources allocation the MSC instructed to access the new channel and the call is transferred to the new BSC.

Inter-BSC Handover

Signalling Sequence of Inter-BSC Handover

The inter-MSC handover is performed when the two cells belongs to different MSC in the same system. The serving MSC/VLR must get help from the new MSC/VLR to carry out the handover.

Inter-MSC Handover

Signalling Sequence of Inter-MSC Handover

Flow of Handover Algorithm

Handover decisions is given following order priority:

RX_QUAL RX_LEV DISTANCE PBGT

Criteria for Handover

If downlink quality is worse then HO Thresholds Qual DL parameter, then an immediate quality handover takes place. This parameter is generally set to 3.2%–6.4%.

If uplink quality is worse then HO Thresholds Qual UL parameter, then an immediate quality handover takes place. This parameter is generally set to 3.2%–6.4%.

Quality Handover

If downlink level is worse then HO Thresholds Lev DL parameter, then an immediate level handover takes place. This parameter is set to –95dBm as default.

If uplink level is worse then HO Thresholds Lev UL parameter, then an immediate level handover takes place. This parameter is set to –105dBm as default. .

Level Handover

MS-BS distance Handover(timing advance) is triggered when the distance between the MS and BTS has been increased above the threshold distance ,which is measured from timing advance.

MS Distance Handover

When signal strength difference between serving cell and neighbor cell

exceeds Power Budget Handover margin which is set in Handover parameters, the call is handed over to the neighboring cell. This margin is usually set to 3 to 6 dB. Power Budget HO feature should be enabled for this type of Handover.

In case of ping–pong handovers between the same two cells, power budget handover margin between the two could be increased to reduce number of handovers. Margin should be decreased if faster handover decision is wanted.

Power Budget Handover

Handover ProcessThe Handover Process in GSM consists of following four steps:

Measurement Hnadover Request Handover Decision Handover execution

Following are the parameters for which threshold value is to be set for handover:

Uplink receive level (HoULLevThs/ HoULLevN/ HoULLevP). Downlink receive level(HoDLLevThs/HoDLLevN/HoDLLevP). Uplink receive quality(HoULQualThs/HoULQualN/HoULQualP). Downlink receive quality(HoDLQualThs/HoDLQualN/HoDLQualP). Uplink receive level of Intra-HO(IntraHoULLevN/IntraHoULLevP). Downlink receive level of Intra-HO(IntraHoDLLevN/IntraHoDLLevP). Good C/I (GoodCIThs/GoodCIN/GoodCIP). Bad C/I (BadCIThs/BadCIN/BadCIP). Rapid handover (RapidHoThs/RapidHoN). Distance handove (DistanceN/DistanceP). PBGT handover (PBGTHoN/PBGTHoP).

Handover Threshold Parameters

Timers

GSM resource allocation and termination depends primarily on timers. The

functionality and protocols may vary, but the presence of timer based

signaling and resource allocation is ubiquitous. Timers effect call setup

procedures, handovers, call clearing, data usage (GPRS), VAS facilities; in short, everything that a customer might do to access and utilize the GSM network. Therefore, for a high quality network, it is imperative to have impeccable timing. Precise synchronization and timing improves call signal hand over performance between base stations; and, as an added benefit, reduces customer complaints. GSM networks are dependent on strict synchronization of the base stations to facilitate call handovers.

The Timer timing is in seconds for efficent working of the network.

BSS End Handover TimersT3103 The timer starts when the initiate Handover command is sent to the BTS by the

BSC.

Timer is stopped when handover complete message is received by the BSS entity.

In case the timer expires, the handover procedure is aborted and the Handover

attempt is termed as unsuccessful.

It is recommanded to be set 5 sec.

T8 The timer starts when the Handover command is sent to the MS via the BSS entity.

The Timer is stopped when handover complete message is received by the BSS

entity.

In case the timer expires, the handover procedure is aborted and the Handover

attempt is termed as unsuccessful.

T7 The timer starts when a Handover Request message is sent to the MSC for

an external BSC handover. The Timer is stopped when handover response message is received by the

BSS entity. In case the timer expires, the handover procedure is aborted and the

Handover attempt is termed as unsuccessful.

TQHQ This timer defines the maximum Queuing time for a Handover request. The timer starts when the BSS places a Handover request in the resource

queue for allocation. The timer is stopped when the Handover request is allocated the required

resource by the BSS entity. In case of an expiry event of this timer, the handover request is removed

from the queue and the handover procedure is aborted.

T3121

This timer IS related with inter BSC and 2G/3G handovers. The timer is started when a request is sent for an inter‐BSC or 2G/3G

handover. The timer is stopped when a Handover complete message is received by

the BSS entity. In case of an expiry event of this timer, Clear request is sent and the

handover procedure is aborted.

Handover Success Rate(HOSR)

FormulaHO Success/HO Attempt*100%

HO Success:C900060098+C900060102+C900060120+C900060094+C900060096

HO AttemptC900060097+C900060213+C900060214+C900060215+C900060099+C900060100+C900060101+C900060216+C900060119+C900060093+C900060095

There are number of reasons for unsuccessful handovers. Some of them are:

Missing Neighbors. Co-BCCH BSIC clashes. Unavailable time slots because of high traffic. Low signal strength of neighbors. Hardware issues at target cell.

Handover Failure

Thank You