Handover Analysis

HUAWEIHUAWEI

Handover analysis

Prepared by:

ETISALAT MS section

HUAWEI Radio Network Planning

15-Aug-04

Handover analysis

Context

1 Introduction..............................................................................................................................52 Handover principle...................................................................................................................52.1 Intra-Frequency measurement..........................................................................................52.2 Hard handover measurement............................................................................................72.3 Handover algorithms review..............................................................................................8

2.3.1 Soft handover algorithms.............................................................................................82.3.2 Inter system handover algorithms review....................................................................8

2.4 Handover parameters analysis..........................................................................................92.4.1 Soft handover parameters...........................................................................................92.4.2 Hard handover and compressed mode parameters..................................................11

3 Handover optimization...........................................................................................................133.1 Soft handover optimization..............................................................................................13

3.1.1 Soft handover optimization overview.........................................................................133.1.2 SHO Optimization target............................................................................................143.1.3 Drives Routes.............................................................................................................143.1.4 Optimization solution..................................................................................................15

3.2 SHO Optimization Analysis Approach.............................................................................153.2.1 Small handover area..................................................................................................153.2.2 Ping-Pong handover..................................................................................................16

3.3 Inter-system handover optimization review.....................................................................163.3.1 GSM neighbor list configuration problem...................................................................16

Page 2 , Total18 Huawei Confidential

Handover analysis

Table Context

table 1 XXX..............................................................................................................................3


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Figure Context

figure 1 Example of Event 1A...................................................................................................6figure 2 Different Optimisation Phases...................................................................................14figure 3 Scan CPICH SC of best service................................................................................16figure 4 Measure report of GSM neighbor..............................................................................17figure 5 3G handover to 2G required......................................................................................17figure 6 the road of the wrong configuration GSM neighbor...................................................18


Handover analysis

1 Introduction

In WCDMA system, there are soft handover, Intra frequent hard handover, Inter frequent

hard handover, Inter system handover. Handover is the most important part of mobile manage. It

plays very important role in RNO of WCDMA system.

In this report, the principle of handover, and the optimization of the handover are presented.

2 Handover principle

A typical handover process has measurement control, measurement report, handover

evaluation, handover implement. The types of measurement involved in handover include intra-

frequency measurement, inter-frequency measurement and inter-system measurement, which

will be discussed in the following paragraphs. The handover algorithms and parameters also are

presented.

2.1 Intra-Frequency measurement

UTRAN uses the measurement control message to inform the UE what events need to

trigger measurement reporting. All intra-frequency measurement report events are identified with

1X.

Event 1A: A primary pilot channel enters the reporting range

When the measurement values satisfy the following formulas, the UE deems that a primary

pilot channel has entered the reporting range, the trig condition for Ec/Io:

Where,

MNew is the measurement result of the cell that has entered the reporting range

CIOnew is the cell individual offset which is configured for neighbor cells of servicing cell

Mi is the measurement result of the cells in the active set

NA is the number of cells in the current active set

MBest is the measurement result of the best cell in the current active set

W is the weight factor

R is the reporting range. With the signal strength as an example, R equals to the signal strength

of the best cell in the current active set minus a value

H1a is the hysteresis value of event 1A

In order to reduce the signaling traffic flow of the measurement report, the TIME-TO-

TRIGGER parameter is used so that the UE will not trigger measurement reporting before the

primary pilot enters the reporting range and is maintained for a certain period of time. This

parameter is also used in other events. An example of measurement reporting triggered by event


Handover analysis

1A is given below:

figure 1 Example of Event 1A

Generally, if event 1A is triggered, the UE will send a measurement report to UTRAN, and

UTRAN will deliver an ACTIVE SET UPDATE signaling message to update the active set.

However, UTRAN may give no response after the UE sends the measurement report (for

example, due to insufficient capacity). In this case, the UE will shift from event reporting to

periodic reporting mechanism, and the content of the measurement report includes the

information of the cells in the active set and the cells in the monitored set that has entered the

reporting range. The UE will not stop sending periodically the measurement report until this cell is

successfully added into the active set or leaves the reporting range.

Event 1B: A primary pilot channel leaves the reporting range

When the following formulas are satisfied, the UE deems that a primary pilot channel has left the reporting range

Where,

MOld is the measurement result of the cell that has left the reporting range

CIOold is the cell individual offset which is configured for neighbor cells of servicing cell

Mi is the measurement result of the cell in the active set

NA is the number of cells in the current active set

MBest is the measurement result of the best cell in the current active set

W is the weighted factor

R is the reporting range

H1b is the hysteresis value of event 1B

If several cells satisfy the reporting condition simultaneously after the trigger delay, the UE

will sort the cells according to the measurement values and report all the measurement results.


Handover analysis

Event 1C: The primary pilot channel in a non active set is better than the primary pilot

channel in an active set

When the following formulas are satisfied, a non-active set cell replaces an old active set cell and becomes

an active set cell.

MNew is the measurement result Ec/Io of the cell not included in the active set.

CIONew is the individual cell offset for the cell becoming better than the cell in the active set if an

individual cell offset is stored for that cell. Otherwise it is equal to 0.

MInAS is the measurement result Ec/Io of the cell in the active set with the lowest measurement

result.

CIOInAS is the individual cell offset for the cell in the active set that is becoming worse than the

new cell

H1c is the hysteresis parameter for the event 1C, which is Hystfor1C.

Event 1D: The best cell changes

When the following formulas are satisfied, the best cell will be changed.

where:

MNotBest is the measurement result Ec/Io of a cell not stored in "best cell".

MBest is the measurement result Ec/Io of the cell stored in "best cell".

H1d is the hysteresis parameter for the event 1D, which is Hystfor1D.

2.2 Hard handover measurement

Inter-frequency measurement events are identified with 2X. The frequency quality estimation

involved in events 2A, 2B, 2C, 2D, 2E and 2F is defined as follows:

Where, Qcarrierj is the logarithmic form of the estimated quality value of frequency jMcarrier j is the estimated quality value of frequency jMi j is the measurement result of cell i with the frequency of j in the virtual active set NA j is the number of cells with the frequency of j in the virtual active set MBest j is the measurement result of the best cell with the frequency of j in the virtual active set Wj is the weight factorH is the hysteresis value

Event 2D: The estimated quality value of the used frequency is lower than a certain threshold


Handover analysis

Event 2D can be used to enable the compressed mode to perform inter-frequency measurement. This threshold is specified by IE “Threshold used frequency” in the measurement control message delivered by UTRAN.

Event 2F: The estimated quality value of the used frequency is higher than a certain thresholdEvent 2F can be used to disable the compressed mode to stop inter-frequency measurement. This threshold is specified by IE “Threshold used frequency” in the measurement control message delivered by UTRAN.

2.3 Handover algorithms review

The soft handover algorithms example and Inter-RAT handover algorithms example will be

introduced. Handover algorithms control measurement control and handover evaluation.

2.3.1 Soft handover algorithms

1) When event 1A report is received, if the active set is not full, then links are sequenced

and added in the order of good quality to poor quality (CPICH Ec/No) (in case that multiple cells

report event 1A), until the active set is full; if the active set is already full, no processing will be

made.

2) When event 1B is received, if there are more than one links in the active set, then the

braches are sequenced and removed in the order of poor quality to good quality (CPICH Ec/No)

(in case that multiple cells report event 1B), until only one link is left; if there is only one in the

active set, no processing will be made.

3) In case of event 1C, the UE will report the replacing and replaced cells in the event

trigger list. If the active set is not full, then the triggered cell link will be added; if the active set is

already full at this moment and the replaced cell is not the best cell in the active set, then this cell

link will be removed.

4) In case of event 1D, if the triggered cell is an active set cell, then it will be marked as the

best cell and measurement control is updated; if the triggered cell doe not belong to the active

set, then this cell link will be added (if the active set is full, one of the non-best cell will be

removed before this link is added) and marked as the best cell, with measurement control

updated.

2.3.2 Inter system handover algorithms review

In the early time of WCDMA system, Inter-system handover from WCDMA to GSM is very

important, the algorithms can be :

1) Inter-system handover is enabled only in cells located at the verge of WCDMA FDD


Handover analysis

system coverage.

2) For inter-system handover, CPICH RSCP is used as the physical measurement quantity

and events 2D and 2F are used to decide enabling or disabling the compressed mode.

3) For inter-system handover, three compressed mode style sequences are used for

concurrent measurement of GSM RSSI, BASIC identification and BASIC reconfirm, and the

configuration of parameters is oriented to the cell type, namely, the parameters can be selected

and configured based on the cell characteristics and user mobility statistics characteristics.

4) Periodic measurement reports are used for inter-system handover, and the RNC decides

whether to implement hard handover according to the measurement reports.

5) Handover judgement, while the compressed mode is started and this following formulas is

satisfied, Inter-system handover will be implemented.

Mother_RAT + CIO Tother_RAT + H/2

Where:

Mother_RAT is the estimated quality value RSSI of inter-system frequency Tother_RAT is the GSM RAT measurement init_threshold, the minimum signal Rxlev requirement for GSM

cell as a handover target cell.CIO is Cell individual offset , the measurement signal Rxlev offset for GSM cellH is the hysteresis for GSM RSSI threshold.

2.4 Handover parameters analysis

2.4.1 Soft handover parameters

The main parameters of SHO are: intra-frequecy relative report range, filter coefficient,

hysteresis, time to trigger, cell individual offset.

1. Report range (SHO relative threshold)

These parameters define the difference between the quality of a cell and the overall quality

of the active set (if w=0, then it is the quality of the best cell). The relative threshold parameters

for soft handover include IntraRelThdFor1A (relative threshold for event 1A) and

IntraRelThdFor1B (relative threshold for event 1B).

The parameters determine the size of the soft handover area and the soft handover

subscriber proportion. In a CDMA system, it is required that the UE proportion in soft handover

should be 30% to 40%. If report range is too larger, monitor cell adds to active set cell very

easily, soft handover area is too larger and the forward capacity will be waste serious; if the 1A

and 1B report range is too small, monitor cell is difficult to join to active cell and SHO can’t

implement smoothly, and handover success rate may be low.


Handover analysis

2. Inter-frequency measurement filter coefficient

The filter is configured at Layer 3 intra-frequency measurement report. This parameter used

to smooth the influence of shadow fading and some fast fading burrs. The formula is used to filter

layer1 measurements:

nnn MaFaF 1)1(

Where,

Fn: the updated measurement result after filtering processing.

Fn-1: the old measurement result of the previous moment after filtering processing.

Mn: the latest measured value received from the physical layer.

= (1/2)(k/2), where, k comes from "Filter coefficient", namely the local FilterCoef. When

is set to 1, it means there is no Layer 3 filtering.

We can find, the bigger the filter coefficient is, the stronger the smoothing capability of the

burr will be, but the lower the signal tracing capacity will be. If if this parameter is too small,

unnecessary soft handover and ping pong handover will influence the system; if the filter is too

bigger, soft handover can’t implement in time and the call will always dropped in handover area.

3. hysteresis

To limit the amount of event-triggered reports, a hysteresis parameter may be connected

with each reporting event given above. The value of the hysteresis is given to the UE in the

Reporting criteria field of the Measurement Control message.

The bigger the hysteresis is, the stronger the signal fluctuation resistance capability will be,

and the better the ping pong effect will be suppressed; however, the response speed of the

handover algorithm on signal changes will be decreased. Therefore, the radio environment (slow

fading characteristics), the actual handover distance and the user moving speed should be fully

considered for the setting of this parameter.

4. time to trigger

The effect of the time-to-trigger is that the report is triggered only after the conditions for the

event have existed for the specified time-to-trigger.

Mobile stations moving at different speeds respond differently to the time-to-trigger value.

The call drop rate is more sensitive to the time-to-trigger value when the mobile station is in high-

speed movement, while it is less sensitive when the mobile station is in low-speed movement,

and ping-pong handover and mis-handover are suppressed to a certain extent. Therefore, for

cells where there are more high-speed moving mobile stations, this value can be relatively small,

while for cells where there are more low-speed moving mobile stations, this value can be


Handover analysis

relatively big.

5. cell individual offset.

This offset mechanism provides the network with an efficient tool to change the reporting of

an individual primary CPICH.

The bigger this parameter is, the more easily soft handover will occur, and the more UEs will

be in the soft handover state, but the more forward resources will be occupied; the smaller this

parameter is, the more difficultly soft handover will occur, which will be likely to affect the

receiving quality

2.4.2 Hard handover and compressed mode parameters

Presently, the main purpose of inter-RAT handover is to extend the signal coverage range,

so that the UEs at the verge of WCDMA coverage can implement handover to the GSM system

without communication interruption.

There have inter-frequency measurement filter coefficient, hysteresis, time to trigger,

compressed mode enable /disable threshold.

1. 2D event threshold

UE will start compressed mode and measure GSM signal as a handover target cell when the

value of RSCP is below the setting.

2. 2F event threshold

UE will stop compressed mode l when the value of WCDMA cell RSCP is higher than the

setting.

When a cell is at the verger of carrier frequency coverage, it will use RSCP measurement

value as the decision criterion for 2D and 2F. If the compressed mode is expected to start as

early as possible, set the 2D event threshold to a big value; otherwise set it to a small value. To

reduce ping pong start/stop of the compressed mode, increase appropriately the difference

between the 2D and 2F thresholds.

3. GSM RSSI threshold

The minimum signal Rxlev requirement for GSM cell as a handover target cell. The inter-RAT

handover decision threshold is Tother_RAT in the formula described before..


Handover analysis

4. Inter-RAT handover trigger time

If the inter-RAT quality always satisfies the inter-RAT handover decision condition within the

time specified by this parameter, the network will start the inter-RAT handover process.

This parameter and the hysteresis are jointly used to prevent mis-decision caused by burst

jitters of signals in inter-RAT handover decision.

In view that the UEs are already at the verge of the system, this parameter should not be too

big.In high-speed cells, as mobile stations usually move fast, they pass through the handover

area very quickly. In addition, as the shadow fading variance in a high-speed cell is small and

signal jitter after filtered by mobile stations is relatively small, this parameter can be set to

2000ms or lower for high-speed cells.

With reference to the simulation result in simulation reports, the time-to-trigger settings

2000ms and 4000ms satisfy the call drop rate requirement for medium-speed and low-speed

cells. However, considering the inter-RAT handover delay, it is more appropriate to set the time-

to-trigger parameter to 2000ms for medium-speed and low-speed cells.

The time-to-trigger setting should not be lower than the interval of periodic inter-RAT

measurement reports; otherwise it will make no sense. This value can be decreased

appropriately for high-speed cells, for example, to 1200ms, and increased appropriately for

medium-speed and low-speed cells. However, the setting should not exceed 4000ms; otherwise

the inter-RAT handover delay will be too long.

5. Cell individual offset

The measurement signal Rxlev offset for GSM cell. This parameter is used for the inter-RAT

handover decision process, and it is to be set according to the terrain characteristics of the GSM

cells. The UE use the sum of the original cell measured value and this offset as the

measurement result for the UE handover decision. It plays the role of moving the cell boarder in

the handover algorithm. The bigger this parameter is, the higher the handover priority of the GSM

cells will be

6. hysteresis

The hysteresis for GSM RSSI threshold and the setting of which can reduce the probability of

mis-decision caused by signal jitter. This parameter is used to prevent mis-decision caused by

burst jitters of signals in inter-RAT handover decision. This parameter and the inter-RAT quality

thresholds jointly determine whether to trigger an inter-RAT handover decision


Handover analysis

7. Filter Coefficient

The physical meaning and measurement model of this parameter are the same as those for

inter-frequency measurement. The report cycle is 480ms. For the specific analysis, refer to Intra-

frequency Measurement Filter Coefficient

3 Handover optimization

3.1 Soft handover optimization

3.1.1 Soft handover optimization overview

Parameter optimization should be conducted after RF optimization to resolve the drop calls

where the RF conditions should be good exclude the coverage problem. Soft handover

optimization is the most important stage. The network structure isn’t always idea, so some

network problems that RF optimization can’t be done, some drop calls, such as Ping-Pong

handover, small soft handover area, the soft handover optimization can be effective.

SHO optimization is a parameters tuning procedure.The report range can control the

handover area, the filter coefficient can confirm the measurement result, hsyteresis can limit

event report, the time to trigger cofrim the measurement result, CIO changes the report of the

individual cell.

The optimization flow can be found from figure 2 .


Handover analysis

Cluster of sites ready?

New Sites Integrated

Re-optimisationNeeded?

No

NoYes

RF Optimisation

Single Site Verification

Services Testing &Parameter Optimisation

Regular Reference RouteTesting & Stats Analysis

Yes

figure 2 Different Optimisation Phases

3.1.2 SHO Optimization target

In the early phase of the network, from KPI target, soft handover success rate for event

1A&1B&1C should be larger than 95%, and the call drop rate should be smaller than 5%.

3.1.3 Drives Routes

In the early phase of the network, because KPI result don’t change in evidence prior and post optimization, GPS drive test is very important .It is essential to use identical drive routes prior and post any optimization changes in order to accurately quantify the impact of such changes.

When possible, two-way drive tests should be conducted. The follow figure is the routes of 5 sites SHO optimization at sharjah.


Handover analysis

figure 3 the routes of 5 sites SHO optimization at sharjah

The 5 sites are Layyah, Majaz, Shj ExpoCenter, Ansar Mall, Kentucky and the routes are all two-

ways.

3.1.4 Optimization solution

Many handover problems can be resolved through adjusting the soft handover parameters, such

as:

1A & 1B report range

time to trigger

intra-frequceny measurement filter coefficient

CIO

hysteresis

3.2 SHO Optimization Analysis Approach

Some RF problems such as high interference at handover area, small handover area that

can’t be solved by RF technique and handover parameters problem at some scenario or cell can

be resolved through SHO optimization.

Small handover area issues, Ping-Pong handover problem, services code error rate high at

handover area, soft handover parameters setting problem are always found.


Handover analysis

3.2.1 Small handover area

This issue may be RNP problem but sometime this can’t be resolved at RF optimization

stage and also could be due to soft handover configuration problem. This will be causing call

dropped .The parameters can be tuned:

Enlarge 1A &1B report range to make the UE monitor cell easily add to active set;

Short 1A time to trig to make the measurement report in time, also can optimize the intra-

frequency measurement coefficient;

This problem can be found at cell handover success rate of cell KPI.

In the early phase of network, the sites are not enough and some RF or RNP problem can’t

be solved by RF means. So the small handover area issues are always RF problem. RF

optimization ended at the following picture sites, we can see there exits some small handover

problems.

figure 4 Scan CPICH SC of best service

3.2.2 Ping-Pong handover

This issue may be RF problem, and also cell soft handover configuration problem.

Cells with many Ping-Pong handover may be causing call dropped and poorer cell capacity.

This can enlarge intra-frequency measurement coefficient and time to trigger.


Handover analysis

3.3 Inter-system handover optimization review

Presently, the main purpose of inter-RAT handover is to extend the signal coverage range,

so that the UEs at the verge of WCDMA coverage can implement handover to the GSM system

without communication interruption.

The configuration GSM neighbor list of WCDMA cells is very important and always make

wrong. At the same time, the parameters optimization is very important.

The optimization solution is parameters configuration such as: 2D & 2F Event threshold,

GSM RSSI threshold, Inter-RAT handover trigger time, intra-frequceny measurement filter

coefficient, CIO and Hysteresis.

3.3.1 GSM neighbor list configuration problem

UE measures the UE GSM neighbor RSSI and BSIC when UE wants to handover from

WCDMA cell to GSM cell, we can always find that one GSM cell frequency RSSI is very strong,

but the GSM cell is always non-Verified. This is always the GSM neighbor configuration problem.

There is a GSM neighbor configuration problem example from the fellow figures:

figure 5 Measure report of GSM neighbor

In this figure, we can see that the GSM neighbor cell RSSI of frequency 117 is very strong, but

the BSIC is non-verified.


Handover analysis

figure 6 3G handover to 2G required

From the target ID, we can find that the the BSIC and cell frequency of GSM cell: CID =

"0x140D",NCC = 6,BCC = 4,BCCHARFCN = 117, From the measure control we can judge that

the GSM neighbor cell BSIC is wrong configuration.

Also from the RNC message, we can found the the road and the WCDMA cell. The follow

figure is the route of GSM neighbor configuration problem.

figure 7 the route of the wrong configuration GSM neighbor


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Handover Analysis