c-i based ho evaluation description

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C/I Based Handover Candidate

Evaluation

DN9814226Issue 3-0 en

# Nokia Siemens Networks 1 (22)

BSC3119

Nokia BSC/TCSM, Rel. S12, Product

Documentation, v.3



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2 (22) # Nokia Siemens Networks DN9814226Issue 3-0 en

C/I Based Handover Candidate Evaluation



Contents

Contents 3

List of tables 4

List of figures 5

Summary of changes 7

1 Overview of C/I Based Handover Candidate Evaluation 9

2 Functionality of C/I Based Handover Candidate Evaluation 11

3 Handover strategy of C/I Based Handover Candidate Evaluation 133.1 Processing of radio link measurements 153.2 Threshold comparison 153.3 HO decision algorithm 163.3.1 C/I evaluation 163.3.2 C/I-based handover candidate evaluation 17

4 User interface of C/I Based Handover Candidate Evaluation 21



Contents



List of tables





List of figures



List of figures



Summary of changes

Changes between document issues are cumulative. Therefore, the latest

document issue contains all changes made to previous issues.

Changes made between issues 3 and 2

Chapter Implementation of C/I Based Handover Candidate Evaluation has

been removed. The contents of this chapter have been added to other

chapters. In addition, editorial changes have been made. No changes

have been made in the document due to changes in the software.

Changes made between issues 2 and 1b-0

The document has been revised throughout to comply with the latest

documentation standards.

Updated information on the identification of the reference cell in the

chapter Adjacent cell parameters.

Changes made between issues 1b-0 and 1a-0

The document has been revised throughout to comply with the latest

documentation standards. No changes have been made in the document

due to new features.



Summary of changes



2 Functionality of C/I Based Handover

Candidate Evaluation

When C/I Based Handover Candidate Evaluation is employed by the BSC,the BSC is able to estimate the level of the potential co- and adjacent

channel interference on each candidate cell at the current location of the

mobile station. The interference each mobile is exposed to is used by the

BSC when it determines the most appropriate neighbouring cell where the

call can be handed over.

The carrier/interference (C/I) ratio on the handover candidates affects both

the target cell evaluation done according to radio criteria and the order of

preference of the target cells. The BSC removes those handover

candidates from the list of target cells whose potential C/I ratio is below an

acceptable level. The potential C/I ratio on the cells affects the order of preference of the target cells in the following way:

. good C/I ratio of a cell keeps the priority of the cell unchanged or

increases the priority of the cell;

. bad C/I ratio of a cell reduces the priority of the cell.

C/I Based Handover Candidate Evaluation is activated on a cell-by-cell

basis by means of the O&M (parameter C/I estimation method).

Downlink C/I ratio of the handover candidate

The BSC estimates the potential downlink C/I ratio of the handover

candidate by means of various adjustable parameters and the

measurement results reported by the MS via the BTS.

By comparing the downlink signal level of the candidate cell (carrier) and

the downlink interference level caused by the interfering (reference) cells,

the BSC can calculate the C/I ratio on the candidate cell at the current

location of the mobile station.



Functionality of C/I Based Handover Candidate Evaluation



It is possible to define 5 reference (interfering) cells at most for each of the

adjacent cells by means of the O&M. The reference cells must be adjacentto the serving cell (see Radio Network Configuration Management ),

otherwise the mobile station is not able to measure and report the signal

levels of the reference cells.

. Directly-measured interference level

The most common situation is that the potential source of

interference is a cell which has one or more radio frequencies

identical to the candidate cell and, furthermore, the location of the

interfering cell is close enough to cause interference. In this situation

the downlink signal level of the interfering (reference) cell

corresponds directly to the interference level on the candidate cell

caused by the interfering cell.

. Estimated interference level

The signal level of the interfering cell may not always be available.

The reason usually is that the MS is able to report the measurement

results of the six strongest neighbouring cells it receives best, and

the interfering cells are often weaker than the six strongest

neighbouring cells, coupled with the possibility of BSIC decoding

failure. Also, if the potential source of interference is a cell which has

BCCH frequency identical to the handover candidate, the

corresponding interference level cannot be measured and reportedby the MSs because of identical BCCH frequencies. In these cases

the BSC can only estimate the interference level caused by the

interfering cell by means of the signal levels which the MSs can

measure and report.

When the RF signal profile of an adjacent cell is similar to the

interference profile within the coverage area of the handover

candidate, it is possible to define the adjacent cell as the interfering

(reference) cell instead of the true source of interference. The RF

signal profile is considered identical to the interference profile, when

the ratio between the RF signal level and the interference level (for

example 6 dB, represented by means of a parameter leveladjustment (L1-L5) set for each reference cell) remains

approximately unchanged within the service area of the handover

candidate.

To increase the reliability of the estimation, several reference cells

may be used for calculating the estimated downlink interference

level.

For an overview, see Overview of C/I Based Handover Candidate

Evaluation.





3 Handover strategy of C/I Based

Handover Candidate Evaluation

The main functions of the C/I Based Handover Candidate Evaluation arecontrolled by the RF Power Control and Handover Algorithm, which is

responsible for processing radio link measurements and for the threshold

comparison and decision of Power Control (PC) and Handover (HO) in the

BSC. For more information, see RF Power Control and Handover

Algorithm.

C/I Based Handover Candidate Evaluation has special requirements for

the HO decision algorithm, that is, the target cell evaluation procedure.

Special requirements are not, however, directed towards the measurement

processing and the threshold comparison.

The handover decisions made by the BSC are based on the measurement

results reported by the MS/BTS and various adjustable parameters.

C/I based handover candidate evaluation

The C/I based handover candidate evaluation is activated on a cell-by-cell

basis by means of the parameter C/I estimation method.

The C/I ratio on the handover candidates at the current location of the

mobile station affects both the target cell evaluation done according to

radio criteria and the order of preference of the target cells.

When the C/I based handover candidate evaluation procedure is

employed in the serving cell, the HO decision algorithm verifies, in addition

to the conventional signal strength conditions, that the C/I ratio of

handover candidates meet the requirements of a good radio connection

quality. The BSC removes those handover candidates from the list of

target cells whose C/I ratio is below an acceptable level.

The BSC ranks the handover candidates (target cells) which meet the

requirements for the radio link properties on the basis of the following

properties:



Handover strategy of C/I Based Handover Candidate Evaluation



1. the priority levels of the handover candidates

2. the load of the handover candidates which belong to the local BSS

3. the C/I ratio on the handover candidates at the current location of the

mobile station

Both the load of the cells and the C/I ratio on the cells affect the order of

preference of the target cells by means of the priority levels. The HO

algorithm considers the C/I ratio of a handover candidate at the current

location of the mobile station in the following way:

. good C/I ratio of a cell keeps the priority of the cell unchanged or

increases the priority of the cell. bad C/I ratio of a cell decreases the priority of the cell

The crucial principle is that high-priority cells are considered better than

low priority cells, that is, a cell is ranked higher than another cell if it has a

higher priority level even though its signal strength condition is worse.

Applicable handovers

The C/I-based handover candidate evaluation procedure concerns inter-

cell handovers caused by the following reasons:

. conventional radio criteria such as power budget, low signal level,

and bad signal quality

. traffic control between macrocells and microcells during a call, that

is, umbrella handover

. traffic control between regular and super-reuse frequencies during a

call (underlay-overlay handover) when the handover to be performed

is an inter-cell handover from a parent cell to a child cell (for more

information, see Intelligent Underlay-Overlay.

The crucial principle is that the BSC applies the C/I-based handover candidate evaluation procedure when the handover candidates for a

handover are ranked mainly according to the priority levels. If the handover

candidates for a handover are ranked only according to radio link

properties and priority levels are not used (see RF Power Control and

Handover Algorithm, imperative handover, traffic reason handover), the C/

I-based handover candidate evaluation procedure is not applied.





3.1 Processing of radio link measurements

C/I-based handover candidate evaluation does not have any special

requirements for the following basic averaging procedures (for more

information, see RF Power Control and Handover Algorithm):

. weighted averaging of quality and level

. averaging of distance

. bookkeeping and averaging of the RXLEV of the adjacent cell.

The BSC uses exactly the same averaging procedure for the bookkeeping

and averaging of the signal level (RXLEV) of the reference cell as for theadjacent cell.

There is the following correspondence between the measurement resultsof the reference cells of the adjacent cell (n) and the averaged results:

M EA SU RE ME NT R ES UL T A VE RA GE D R ES UL T

RXLEV_REF1 (n) AV_ RXLEV _RE F1 (n)





Note

The reference cells must be adjacent to the serving cell, otherwise the

mobile station is not able to measure and report the signal levels of the

reference cells (see Radio Network Configuration Management ).

3.2 Threshold comparisonC/I Based Handover Candidate Evaluation does not have any special

requirements for the threshold comparison.






3.3 HO decision algorithm

3.3.1 C/I evaluation

The BSC calculates the downlink C/I ratio of those handover candidates

only which have specified reference cells. The BSC calculates the

downlink C/I ratio of the handover candidate by means of the processed

measurement results (averages) and the relevant parameters.

Processed measurement results are the following:

.

downlink RXLEV of the adjacent (candidate) cell. downlink RXLEV of the reference cells

Parameters are the following:

. C/I estimation method

. level adjustment

. C/I estimation weight

By comparing the downlink RXLEV of the candidate cell and the downlinkRXLEV of the reference cells, the BSC can calculate the C/I ratio of the

candidate cell. If the average downlink RXLEV of a reference cell is not

available, that is, the reference cell is not among the six best neighbouring

cells measured by the MS, the C/I evaluation procedure ignores this

reference cell of the handover candidate. If there is no averaged RXLEV

available of any reference cell, the BSC is not capable of calculating the

downlink C/I ratio of the handover candidate.

The HO algorithm uses two alternative methods to calculate the potential

downlink C/I ratio on the candidate cell:

. average taking method;

. maximum taking method.

The C/I estimation method is selected by means of the parameter C/Iestimation method and administered on a cell-by-cell basis by means

of the O&M.

Average taking method

The downlink C/I ratio CI_EST(n) of a handover candidate (n) is calculated

by means of the average taking method in the following way:





C I _ E S T (n ) =

(W 1 ( n ) * ( A V _ R X L E V _ NC E L L ( n) - A V _ R XL E V _ R E F1 ( n ) - L E V _ A DJ _ R E F 1 (n ) ) +

W 2 ( n ) * ( A V _ R X L E V _N C E L L (n ) - A V _ R XL E V _ R E F2 ( n ) - L E V _ A DJ _ R E F 2 (n ) ) +



W 5 ( n ) * ( A V _ R X L E V _N C E L L (n ) - A V _ R XL E V _ R E F5 ( n ) - L E V _ A DJ _ R E F 4 (n ) ) ) /

( W 1 (n ) + W 2 (n ) + W 3 (n ) + W 4 (n ) + W 5 (n ) )

AV_RXLEV_NCELL(n) is the average downlink RXLEV of the handover

candidate (n) and AV_RXLEV_Rx(n) is the average downlink RXLEV of its

reference cell x. LEV_ADJ_REFx(n) is the adjustment parameter (leveladjustment) of the reference cell and Wx(k) is the weighting coefficient

of the reference cell (parameter C/I estimation weight, set for each

reference cell).

Maximum taking method

The downlink C/I ratio CI_EST(n) of a handover candidate (n) is calculated

by means of the maximum taking method in the following way:

C I _ E S T (n ) =

M I N ( ( A V _ R X LE V _ N C EL L ( n ) - A V _ R X LE V _ R E F1 ( n ) - L E V _ A DJ _ R E F 1 (n ) )

, ( A V _ R X LE V _ N C E LL ( n ) - A V _ R X LE V _ R E F 2 (n ) - L E V _ A DJ _ R E F 2 (n ) )



, ( A V _ R X LE V _ N C E LL ( n ) - A V _ R X LE V _ R E F 5 (n ) - L E V _ A DJ _ R E F 5 (n ) ) )

The principle of this method is exactly the same as in the average taking

method except that the maximum taking method takes into account the

worst C/I ratio and the maximum interference instead of the weightedaverage.

3.3.2 C/I-based handover candidate evaluation

The C/I-based handover candidate evaluation is activated on a cell-by-cell

basis by means of the parameter C/I estimation method. When the C/

I-based handover candidate evaluation procedure is employed in theserving cell, the BSC verifies, in addition to the conventional signal

strength conditions, that the C/I ratio of specified handover candidates

meets the requirements for a good radio connection quality. The BSC

removes those handover candidates from the list of target cells whose C/I

ratio is below an acceptable level.






The candidate cells which meet all the required radio link properties are

ranked according to priority levels. Both the load of the cells and the C/Iratio on the cells affect the order of preference of the target cells by means

of the priority levels. The crucial principle is that high-priority cells are

considered better than low-priority cells, that is, a cell is ranked higher than

another cell if it has a higher priority level.

The C/I-based handover candidate evaluation concerns only those

handover candidates which have specified reference cells. The C/I-based

handover candidate evaluation is not applied to those handover

candidates whose C/I ratio estimate CI_EST(n) is not available for some

reason or which have not specified reference cells. Instead the BSC

evaluates and ranks those handover candidates according to the

conventional criteria.

The BSC compares the C/I ratio estimation CI_EST(n) of the handover

candidate with 7 C/I bands whose limits are determined on a cell-by-cell

basis by means of the O&M (parameter lower C/I limit for band 1-

6). The comparison indicates either how much the priority level of the

handover candidate is decreased/increased when the C/I ratio of the

handover candidate is within a certain C/I band or that a handover to the

candidate cell is not allowed. Both decrease and increase in the priority

level and a handover prohibition are determined on a band-by-band basis

for each of the 7 C/I bands (parameter priority adjustment step for

band 1-7).

Decrease or increase in the priority level CI_PRIORITY_FACTOR(n) of the

handover candidate (n) depends on CI_EST(n) in the following way:

The final priority level PRIORITY(n) of the handover candidate (n) is

calculated in the following way:

CI_PRIORITY_FACTOR(n) CI_EST(n)

PriorityAdjStep for band 1 >= LowerCILimit 1

PriorityAdjStep for band 2 < LowerCILimit 1>= LowerCILimit 2

. .

. .

PriorityAdjStep for band 6 < LowerCILimit 5>= LowerCILimit 6

PriorityAdjStep for band 7 < LowerCILimit 6





P R I O R IT Y ( n ) = H O _ P R IO R I T Y _L E V E L (n ) - H O _ L O AD _ F A C TO R ( n )

+ C I _ P R IO R I T Y _F A C T O R (n )

HO_PRIORITY_LEVEL(n)-HO_LOAD_FACTOR(n) is the priority level of

the handover candidate when the load of the cell has been considered.

If the result of the calculation is greater than the maximum priority level,

PRIORITY(n) will take the value of the maximum priority. Respectively if

the result of the calculation is lower than the minimum priority level,

PRIORITY(n) will take the value of the minimum priority.

The BSC performs intra-BSC handovers autonomously according to the

list of preferred HO candidates. If there is an inter-BSC handover to be

performed, the BSC sends the list to the MSC and the MSC performs thehandover according to the list.


Evaluation.






4 User interface of C/I Based Handover

Candidate Evaluation

Parameters control the C/I-based handover candidate evaluationprocedure in the BSC. By changing the values of the parameters it is

possible to affect both the target cell evaluation done according to radio

criteria and the order of preference of the target cells. The parameters are

stored in the BSS Radio Network Configuration Database (BSDATA). All

parameters are administered on a cell-by-cell basis by means of the O&M

by using either the local MMI in the BSC site or Nokia NetAct.

Handover control parameters

The handover control parameters are administered on a cell-by-cell basis

. C/I estimation method defines the estimation method which will

be used in the C/I evaluation procedure to calculate the downlink C/I

ratio of a handover candidate. Alternative methods are the average

taking method and the maximum taking method. The C/I-based

handover candidate evaluation is not employed in the BTS when the

value is 'not in use'.

. Lower C/I limit for band 1-6 is used for calculating the final

priority level of the handover candidate for the C/I-based handover

candidate evaluation procedure. The parameter defines 6 limits

which determine 7 C/I bands. The comparison of the C/I ratio of the

handover candidate with the limits indicates the C/I band whichcorresponds to the C/I ratio of the handover candidate.

. Priority adjustment step for band 1-7 is used for

calculating the final priority level of the handover candidate for the C/

I-based handover candidate evaluation procedure. The parameter

defines a priority adjustment step on a band-by-band basis for each

of the 7 C/I bands. A priority adjustment step indicates how much the

priority of the handover candidate is decreased/increased when the

C/I ratio of the handover candidate is within a certain C/I band.



User interface of C/I Based Handover Candidate Evaluation



For more information, see Handover Control Parameter Handling MML (EH

command group).

Adjacent cell parameters

It is possible to define 5 reference cells for every adjacent cell and the

following parameters for each of the 5 reference cells.

. ReferenceCellId contains the identification of the reference cell. The

identification of the reference cell consists of cell global identification

(CGI) data comprising of mobile country code (MCC), mobile

network code (MNC), location area code (LAC) and cell identity (CI).

.

Level adjustment is the adjustment level of the reference cell.The parameter is used for calculating an interference level estimate

from the signal level of the reference cell.

. C/I estimation weight is the weighting coefficient of the

reference cell.

For more information, see Adjacent Cell Handling MML (EA command

group).


Evaluation.

22 (22) N ki Si N t k DN9814226


c-i based ho evaluation description

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