Handout11

Copyright © 1999 by Ericsson Radio Systems AB2882-EN/LZT 123 4038 Rev C

Overlaid and Underlaid Subcells


Overlaid and Underlaid Subcells

UL subcell

OL subcell covering only a part of the original cell.

Normal cell

f1, f2, f3, f4

f1

f2, f3, f4


Why OL/UL Subcells?

• The frequencies allocated to the OL subcell cause andsuffer less from interference. An OL subcell make itpossible to use forbidden frequencies.

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Increased Overall Capacity

• A tighter frequency re-use can be applied in theOL subcells giving more frequencies per cell.

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Subcell Change UL to OL (SubCell LoadDistribution, SCLD, Inactive)

• A subcell change will be requested when:downlink path loss is equal or below LOL - LOLHYSTANDtiming advance is lower than TAOL - TAOLHYST.


Subcell Change OL to UL (SCLDInactive)

• A subcell change will be requested when:Downlink path loss exceeds LOL + LOLHYST ORtiming advance is equal to or increases above TAOL +TAOLHYST.


OL/UL Subcell Change (SCLD Active)

• UL subcell as first choice so if capacity exist,use UL

• If OL cell has lower C/I than UL due to tighterfrequency re-use, this is good

• Every SCLD interval, specified by parameterSCLDTIMEINT, the traffic load in the UL subcellthat are active for SCLD are examined for SCLD.


Subcell Load Distribution

Percentage ofactive TCHs

SCLDUL

SCLDLL

Subcell change OL to UL(no other condition)

No subcell change

Subcell change UL to OL(L < LOL, ta < TAOL)

When the percentage of idle full rate capable TCHs in the UL subcell equals orfalls below SCLDLL subcell changes from the UL to the OL subcell are initiated.When the same percentage exceeds SCLDUL then subcell changes from the OLto the UL are initiated. In between no subcell changes are initiated.

Percentageof idle TCHs



Apart from OL to UL subcell change due to Subcell LoadDistribution the mobile station can also be requested tomove to the UL subcell because of excessive timingadvance and/or path loss. The following condition is used:

L > LOL + LOLHYST or

ta ≥ TAOL + TAOLHYST

This condition is checked by Locating every SACCH period.



• No SDCCHs should be configured in OL subcells which makeuse of Subcell Load Distribution. This is because Subcell LoadDistribution is limited to traffic channels, i.e. the SDCCHchannel in the OL subcell can not be accessed.

• Full rate connections as well as half rate connections can bemoved by Subcell Load Distribution. High Speed CircuitSwitched Data (HSCSD) connections can not be moved, but willremain in the UL subcell.


Handovers

Subcell Load Distribution inactive

• Possible to make handover from either the OL or the UL subcell in the servingcell to either the OL or the UL subcell in the target cell.

• Handover to an OL subcell is only allowed if the the OL subcell is co-sited(indicated by the parameter CS).

Subcell Load Distribution active• Handovers to another cell are only possible to the UL subcell.


Dimensioning Considerations

• The output power and the size of the OL subcell mustbe so small that the OL subcell not causes or suffersfrom interference.

• The OL subcell must not be too small. Then it will notserve any traffic and capacity will be wasted.

• It is important to dimension the OL subcell so it will becongested before the UL cell since traffic can “spillover” from the OL subcell to the UL subcell.


Subcell Change and HandoverPossibilities at Co-Siting

CS = YES


Subcell Change and HandoverPossibilities if not Co-Sites

CS = NO


Note

• The immediate assignment is always carried out in theUL subcell.

• Assignment to a another cell is not allowed to an OLsubcell unless the cells are co-sited (if SCLD activeassignment to another cell is only possible to the ULsubcell).

• At congestion in the UL subcell it is possible to assigna traffic channel in the OL subcell as a last resort.

• Intra-cell handover between subcells is allowed atcongestion. An intra-cell handover from the OL to theUL subcell is also allowed if MAXIHO IHOs have beenreached.


Drawbacks with OL/UL Subcells

• In order to improve the interference situation forthe frequencies in the OL subcell substantially,the OL subcell often must be made very small.

• Dependence of a stationary traffic distributionbetween the OL and UL subcell.

• The gain of frequency hopping is reduced whenthe available frequencies are divided into twochannel groups.


Parameter Summary

Main controlling parameters• SCTYPE identifies the subcell type, overlaid or

underlaid, within a cell.• LOL is the path loss threshold for the serving area of

the overlaid subcell within a cell. This parameter is setper overlaid subcell.

• TAOL is the timing advance threshold for the servingarea of the overlaid subcell within a cell. Thisparameter is set per overlaid subcell.


Additional Controlling Parameters

• LOLHYST is the path loss hysteresis for the serving areaof the overlaid subcell within a cell. This parameter is setper overlaid subcell.

• TAOLHYST is the timing advance hysteresis for theserving area of the overlaid subcell within a cell. Thisparameter is set per overlaid subcell.

• BSTXPWR is the base station power at the reference pointfor the locating algorithm on the non-BCCH frequencieswithin a cell. This parameter is set per subcell.

• BSPWRT is the base station output power after the poweron the non BCCH frequency within a cell. This parameteris set per subcell.


Parameter Controlling Dynamic OL/ULSubcells

OL/UL subcells

Parameter Default Value range UnitSCLD OFF ON, OFFSCLDTIMETNT 100 100 to 1000 msSCLDLL 20 0 to 99 %SCLDUL 30 0 to 100 %


• TSC is the Training Sequence Code for the specifiedsubcell within a cell. This parameter is set per subcell, butit is not available for cells without an overlaid/underlaidsubcell structure. Note that it is recommended not tochange TSC in the underlaid subcells.

• CS indicates if a cell shares the same site as itsNeighbour. This parameter is set per neighboring cellrelation and it can take the values:

• YES, the cell is co-sited with its Neighbour.• NO, the cell is not co-sited with its Neighbour.


Value Ranges and Default Values

Parameter name

Default value

Recomm-endedvalue

Valuerange

Unit

SCTYPE UL, OLLOL 0 to 150 dBTAOL 0 to 63 bit periodsLOLHYST 3 2 0 to 63 dBTAOLHYST 0 0 0 to 63 bit periodsBSTXPWR 0 to 80 dBmBSPWRT dBmTSC BCC1 BCC 0 to 7CS NO NO YES, NO

Hardware dep.

1) The TSC is defined by the BSC in the BSIC forthe cell (initially)

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