Locating algorithm

Agenda

Introduction

Flow chart of locating steps

Steps description and related parameters

Example

Main controling parameters & ranges

Conclusion

Introduction

Mobility is the main important issue to maitain in a GSM netwok through handover from a cell to another, but what could make handover runs succesffully?

The locating algorithm serves the purpose of providing a list of possible cell candidates, in descending ranking order, for handover.

The locating algorithm is feature available for Ericsson GSM Systems. It serves as the basis for a number of other radio network features(for example HCS , CLS and IHO). These features are mentioned as auxiliary radio network functions.

Flow chart

The algorithm consists of eight stages, corresponding to the eight boxes in the main flow chart.

Penalty list

Measurement result from GSM

Organizing the list

Urgency Condition

Auxiliary radionetwork functions

evaluations

Allocation reply

Basic Ranking

Filtering

Initiations

Sending the list

Initiation

At a change of channel a new locating individual is created and the old one is terminated.

If the new locating individual is a result of handover, a list of penalties is transferred from the old locating individual.

Penalty list

Measurement result from GSM

Organizing the list

Urgency Condition

Auxiliary radionetwork functions

evaluations

Allocation reply

Basic Ranking

Filtering

Initiations

Sending the list

filtering Locating is based on a number of measurements reports contain The signal strength, signal quality and timing advance that are reported to the BSC every SACCH period (0.48 s).

Penalty list

Measurement result from GSM

Organizing the list

Urgency Condition

Auxiliary radionetwork functions

evaluations

Allocation reply

Basic Ranking

Filtering

Initiations

Sending the list

filtering

Sourcesignal strength

downlink own cell full set MS

signal strength

downlink own cell subset MS

signal strength

downlink MS

quality downlink own cell full set MSquality downlink own cell subset MSquality uplink own cell full set BTSquality uplink own cell subset BTS

BTSBSC (MS)

BTS

MS

DTX used by base station during the measurement periodDTX used by mobile during the measurement period

Data description

six strongest neighbours

timing advanceMS power capability (according to classmark)

these measured values are filtered by performing an averaging of a number of consecutive measurements.

filtering

1- Signal Strength:

1 general FIR 22 general FIR 63 general FIR 104 general FIR 145 general FIR 18

7 recursive exponential SSLENSI, SSLENSD

8recursive 1st. order Butterworth SSLENSI, SSLENSD

9 median SSLENSI, SSLENSD

SSLENSI, SSLENSDrecursive straight average6

Filter selection parameter value

SSEVALSI, SSEVALSD Filter type

Filter length, SACCH periods

Default and recommended

2- Signal Quality: it is the same as signal strength (QEVALSD, QEVALSI, QELENSI and QELENSD). 3- Timing Advanced : straight average filter with a length specified by TAAVELEN is the only type of the filter used for timing advanced.

Urgency conditions

Two types of urgency condition are evaluated: bad signal quality and excessive timing advance.

Penalty list

Measurement result from GSM

Organizing the list

Urgency Condition

Auxiliary radionetwork functions

evaluations

Allocation reply

Basic Ranking

Filtering

Initiations

Sending the list

Bad quality urgency condition:

or

Excessive timing advance urgency condition:

rxqual(uplink) > QLIMUL

rxqual(downlink) > QLIMDL

TA > TALIM

Basic Ranking• The basic ranking consists of

four stages:• Correction of base station

output power for downlink measurements,

• Evaluation of the minimum signal strength condition,

• Subtraction of signal strength penalties,

• Ranking according to signal strength.

Penalty list

Measurement result from GSM

Organizing the list

Urgency Condition

Auxiliary radionetwork functions

evaluations

Allocation reply

Basic Ranking

Filtering

Initiations

Sending the list

Basic RankingCorrection of base station output power for downlink measurements:Neighbouring Cells:

Serving Cell:BCCH Frequency: Frequency Hopping:

where N is the number of frequencies in the hopping sequence.

SS_DOWN n = rxlev n + BSTXPWR n - BSPWR n

SS_DOWN n = rxlev s + BSTXPWR S - BSPWR s

SS_DOWN s = rxlev s + (BSTXPWR s - BSPWR s ) /N

Basic Ranking• Power control:• BCCH frequency:

• Frequency hopping:

• • where PWR_USED is the output power used during the corresponding SACCH

period.• Evaluation of the minimum signal strength condition

• or

• where n refers to a neighbouring cell and SS_DOWN is the downlink signal strength after correction

SS_DOWN s = rxlev s + (BSTXPWR - PWR_USED)

SS_DOWN s = rxlev s + (BSTXPWR - PWR_USED) * (N - 1 )/N

SS_DOWN n >= MSRXMIN n

SS_UPn >= BSRXMIN n

Basic Ranking

Subtraction of signal strength penalties:

where p refers to the punished cell.LOC_PENALTY : is given by one of the following three reasons:Handover failure :PSSHF (size), PTIMHF (duration);Bad quality urgency handover :PSSBQ (size), PTIMBQ (duration);Excessive timing advance urgency handover : PSSTA (size), PTIMTA (duration).HCS_PENALTY : is a punishment which is associated with the feature Hierarchical Cell Structures (PSSTEMP:size , PTIMTEMP:duration).

p_SS_DOWN p = SS_DOWN p - LOC_PENALTY p - HCS_PENALTY p

Basic Ranking

The penalties:

The locating penalty list can contain a maximum of three cells at any time for a connection.

Each punished neighbouring cell in this list can only be punished

with one of the three types of penalty at a time.

A cell can be included in the two lists at the same time. In this case the two punishments are added.

Basic Ranking

Ranking according to signal strengthIn the ranking algorithm, an offset value and a hysteresis value is used when ranking neighbouring cells. The ranking value is given by:For serving cell:

For neighbouring cell:

where s denotes serving cell and n neighbouring cells.

Basic ranking and penalty evaluation the cells are ordered in a basic ranking list.

RANK s = SS_DOWN s

RANK n = p_SS_DOWN n - OFFSET s,n - HYST s,n

Auxiliary radio network functions evaluations

Six auxiliary radio network functions are incorporated in the locating software:

Assignment to Other Cell, Hierarchical Cell Structures, Overlaid/Underlaid Subcells, Intra-cell Handover, Extended Range, Cell Load Sharing.

Penalty list

Measurement result from GSM

Organizing the list

Urgency Condition

Auxiliary radionetwork functions

evaluations

Allocation reply

Basic Ranking

Filtering

Initiations

Sending the list

Organizing the list

Categorization

Categorization 1:Better cell: Cell with a higher ranking value than serving cell in the candidate list. Worse cell: Cell with a lower ranking value than serving cell in the candidate list. Serving cell: The cell serving the connection.

Categorization 2:Above S: Neighbouring cell above serving cell in the candidate list. Below S: Neighbouring cell below serving cell in the candidate list S: Serving cell.

Penalty list

Measurement result from GSM

Organizing the list

Urgency Condition

Auxiliary radionetwork functions

evaluations

Allocation reply

Basic Ranking

Filtering

Initiations

Sending the list

Sending the list

The resulting candidate list forms the basis for cell selection. empty list :there is no better than remaining on the current channel.Candidates list :The first cell is ideally the cell to which the MS should be connected. If there are no channels available in that cell, an attempt to allocate a channel in the next cell in the candidate list is made, etc.

Penalty list

Measurement result from GSM

Organizing the list

Urgency Condition

Auxiliary radionetwork functions

evaluations

Allocation reply

Basic Ranking

Filtering

Initiations

Sending the list

Allocation reply

The result channel allocation can be either a success or a failure. Success : means that an available radio resource was found in one of the cell candidates, and that the transfer of the connection to the new channel was successful.Failure :can be either due to congestion or to a signalling failure. The result determines a number of actions, such as setting of penalties and enabling of certain timers.

Penalty list

Measurement result from GSM

Organizing the list

Urgency Condition

Auxiliary radionetwork functions

evaluations

Allocation reply

Basic Ranking

Filtering

Initiations

Sending the list

THE END