1.1.1 Other Tuning Considerations
With WCDMA RAN P6 release, a number of new optional features are introduced to enhance the performance of a R99 network. This sub-section aims to introduce these features in brief and discuss their effect in Initial Tuning.
1.1.1.1 Extended Range
This feature introduces the possibility of extending the cell range to 200km. The requirements to implement this feature are as follows:
1. Good Propagation conditions i.e. open and flat terrain, open seas etc.
2. High antenna height
3. Support of extra hardware as described in [x].
4. Valid license for different range.
To activate this feature, the following parameter is to be set.
cellRange
To activate the Extended Range feature, this parameter will need to set more than the default value of 35000. This determines the intended cell range that the cell will transmit, provided all the requirements set above are met. To turn off the Extended cell range feature, any value less than or equal to 35000 is to be set.
Beside the parameter setting, it is important to take note of the CPICH power setting of the cells which have the feature turned on. A higher pilot CPICH power setting will be required.
With Extended Range, extra propagation delay is introduced at call setup. It is therefore required to set the AICH and RACH parameters to compensate for the extra delay.
powerOffsetPO
The power ramp step for the preamble when no acquisition indicator is received. Default is 3.
powerOffsetPpm
The power offset between the last transmitted preamble and the control part of the random access message. Default is -4.
preambleRetransMax
The maximum number of preambles sent in one RACH preamble ramping cycle.
maxPreambleCycle
The maximum number of preamble ramping cycle.
The Extended Cell Range allows the operator to deploy R99 services in areas such as Rural areas, open seas where the main objective is to provide the large area coverage. In such case, capacity is not an issue.
If the feature is deployed by the operator, the initial tuning process flow will be similar to the process described in Section 2.
1.2 GSM and R99
In an environment where both GSM and WCDMA network co-exist, it is necessary to implement IRAT definition to realize a seamless handover between both networks. Seamless handover between both networks enable subscribers to reach R99 services on WCDMA while using GSM as a fallback access technology outside WCDMA coverage. This lessen drop call occurrence where WCDMA coverage is limited (WCDMA cell border and coverage holes within WCDMA system) and thus increase subscribers’ experience in WCDMA services.
When implementing IRAT in the network, one needs to be careful to avoid unnecessary disturbance on both current GSM network and new WCDMA network. Improper planning may cause negative impacts on both networks e.g. ping pong effect, unnecessary signalling load and handover, etc. Therefore, it is important to understand the consequences from activation of the functionality. The values of the relevant IRAT parameters mentioned in this chapter are in Appendix 8.2.
1.2.1 IRAT Strategies
The IRAT strategies discussed under this chapter will include those in idle mode, connected mode for CS and PS services.
1.2.1.1 Idle Mode
The choice of system for idle mode camping is important since a mobile terminal will be paged in the system where it is currently camping on. A recommended strategy would be camping on WCDMA whenever the coverage is available (WCDMA biased). This minimizes the chances of UE camping in GSM and unable to access WCDMA services. For example, a UE in GSM is unable to make or receive video call.
This strategy is done by setting sRatSearch and qQualMin to a recommended value of 4dB and -18dB respectively, which give a quality threshold of -14dB. UE in idle mode will remains in WCDMA as long as the quality level is above sRatSearch + qQualMin. Only when the quality level falls below sRatSearch + qQualMin, then the UE will starts to measure GSM cells and rank WCDMA and GSM cells. A cell reselection will be initiated when a GSM cell is ranked the strongest. In the other direction, when the UE is in GSM idle mode, the UE will always make measurements on WCDMA system by setting the parameter QSI to 7. When the measured quality in WCDMA is above FDDQMIN, the UE will cell reselect from GSM back to WCDMA.
Newer UE that support 3GPP CR130 on TS 25.304 Rel 5 and the latest release of 3GPP 44.018 is able to use RSCP thresholds in idle mode (using sHcsRat and FDDRSCPMIN).
1.2.1.2 Connected mode for CS services
In connected mode, IRAT handover should only take place in order to prevent dropped call when the UE is moving out of WCDMA coverage and GSM coverage is still good. Unnecessary compressed mode usage in good WCDMA coverage should be avoided to improve the overall system performance. One way of doing this is to identify cells that IRAT handover is likely to happen and only enable IRAT handover functionality in these cells. This is done by setting the cell level parameter hoType (GSM_PREFERRED or NONE).
When the UE moves towards an area with poor WCDMA coverage, the UE will go into compress mode and start measuring GSM signals. If the condition to handover is fulfil, IRAT handover will take place. The IRAT handover evaluation criteria are [20]:
Start compressed mode and measurement of GSM if any of the following condition is fulfilled for the best AS cell:
CPICH Ec/No < usedFreqThresh2dEcno OR
CPICH RSCP < usedFreqThresh2dRscp OR
UeTxPower reaches maximum[1]
Stop compressed mode and measurement of GSM if all of the following conditions are fulfilled for the best AS cell:
CPICH Ec/No > usedFreqThresh2fEcno AND
CPICH RSCP > usedFreqThresh2fRscp AND
UeTxPower < ueTxPowerThresh6b
Triggering of handover attempt
If compress mode is triggered by CPICH Ec/No,
GSM RSSI with BSIC verified > gsmThresh3a AND
CPICH Ec/No < usedFreqThresh2dEcno+utranRelThresh3aEcno -hysteresis3a/2
If compress mode is triggered by CPICH RSCP,
GSM RSSI with BSIC verified > gsmThresh3a AND
CPICH RSCP < usedFreqThresh2dRscp+utranRelThresh3aRscp -hysteresis3a/2
If compress mode is triggered by UE Tx power event6d,
GSM RSSI with BSIC verified > gsmThresh3a AND
CPICH RSCP < usedFreqThresh2dRscp + utranRelThresh3aRscp + utranRelThreshRscp - hysteresis3a/2
G2U handover: QSC defines when WCDMA neighbours will be monitored for handover. The parameter is set per cell. It is recommended for those GSM cell with WCDMA neighbours to set the QSC to 7 (Always).
When measurements on WCDMA have begun two criteria must be fulfilled for a GSM to WCDMA inter-RAT handover to happen:
· Percentage of idle TCH (traffic channel) in the serving GSM cell ≤ ISHOLEV (ISHOLEV set per GSM cell) AND
· P-CPICH Ec/No> MRSL
The first criteria will be fulfilled when the percentage of idle TCH (traffic channels) in GSM is lower than or equal to the threshold ISHOLEV, which is set per GSM cell. When the first criterion is fulfilled the second criterion is tested separately for each neighbouring WCDMA cell. The WCDMA neighbours having a measured Ec/No higher than MRSL are added to the handover candidate list.
Handover to GSM from WCDMA can be prevented by setting the parameter IHOLEV to 99 (that makes the corresponding value to 99%). This would mean that WCDMA cells are always prioritized when measured. Handover to WCDMA will take place as soon as the WCDMA P-CPICH Ec/No exceeds MRSL. The value of MRSL should be set so that an IRAT handover to WCDMA does not result in a ping-pong handover back to GSM. It may also be preferred to not go into compressed mode immediately when arriving in WCDMA, the following relation should avoid this:
MRSL > usedFreqThresh2dEcno + an hysteresis
A suggested initial setting could be MRSL= usedFreqRelThresh2fEcno + used FregThresh2dEcno, which suggests MRSL = -9dB.
Depending on the strategy of the operator, it may prefer to terminate the call in GSM and cell reselection back to WCDMA in idle mode after the call is terminated. This strategy helps to ensure that ping pong handover will never happen in connected mode. This can be done by setting ISHOLEV to 0 and QSC to 15 (Never).
The connected mode strategy is to set the lowest possible parameter values without causing the connections to drop prior to an IRAT handover. Thus, the setting of the parameter values would differ from one network to the other. The recommended parameter values should therefore be used as a starting point and tune accordingly to meet the objective.
