Even though LTE is considered to be the next global standard. It still does not cover the entire globe yet. Forget entire globe, it’s not even covered everywhere in US.
In fact, US deployment of 4G LTE started initially with deployment in big cities such as Dallas, New York, Orlando etc.
Well it definitely makes sense to have such a deployment strategy. It does meet the financial needs of the operators, takes away the risk, keeping supply demand where customers are. Most of the revenue comes from the same big cities.
To keep business growing and revenue coming in, it’s a reasonable business model to start LTE deployment in islands.
A pain point for the Operators and Performance Engineers:
We as customers want to consume, Netflix, Facebook , email and Apps all the time through our smart phones. In order to fuel this consumption — access to Packet data services is needed. Customers get wild access to high speed data on 4G.
Operators’ preference is to give customers access to 4G most of the time. Data access is the biggest treat for customers and the money maker for the operator, while voice revenues are declining.
The only problem is, not all areas are covered by 4G. This creates a challenge for performance engineers to keep the UE on 4G LTE as much as possible to have a better user experience.
Therefore, in this context, it is important to understand what methodologies are used to keep the UE on LTE 4G, even in the presence of 3G networks.
Idle and Connected Mode
It is important to understand, and configure parameters in such a way that UE should prefer 4G over 3G, both in case of Idle and Connected Mode.
In this discussion, we will focus only on IDLE MODE, with the mission that UE should prefer LTE in presence of 3G.
IDLE MODE TECHNIQUES
In idle mode, preference on LTE over other technologies should starts from the point UE is powered on. That is to say in idle mode, LTE ought to be preferred during
When UE is powered on. It can see different operators’ carriers, technologies broadcasting PLMN. Ideally UE wants to select Home Public Land Mobile Network (HPLMN).
How does UE know HPLMN ?
IMSI (SIM card of the UE) has this information and defines the Home PLMN (HPLMN)
How to prioritize a certain PLMN over the other ?
The following ways an operator or a subscriber can prioritize one PLMN over other PLMNs.
PLMN can be selected either automatically or manually. Most of the cases it is automatic selection.
Default Automatic PLMN Selection Algorithm:
In case of Automatic PLMN selection. Any specific Radio Access Technology is selected by the UE in the following order.
*PLMN are reported to NAS as high quality if their RSRP >= -110 dBm
Cell Selection in Idle Mode takes place using S-Criteria. I am not going to explain S-criteria over here. Topic for another discussion.
In idle mode, our objective is to make UE prefer LTE over any other technology. In Idle mode UE performs measurements whenever it is awake in Idle mode.
There are some thresholds defined in the standards using which we can force the UE to only do measurements on LTE, not on other technologies such as WCDMA in idle mode.
This way we can make UE delusional that it does do any measurements on other RAT but on LTE only.
These thresholds/parameters are available to disable UE to perform any measurements on WCDMA compared to LTE.
Let’s talk about those parameters.
UE can measure Reference Signal Received Quality (RSRQ) or Reference Signal Received Power (RSRP) in idle mode.
For this purpose, we use two four variants of the parameters. Two for LTE related measurements and other two for IRAT related measurements.
For measurements on inter-frequency and Inter-RAT such as WCDMA , the following parameters are introduced.
Both SintraSearchP, SnonIntraSearchP , SintraSearchQ, and SnonIntraSearchQ are signaled using values within the range 0 to 31.
Only For SintraSearchP and SnonIntraSearchP :
The values (0 – 31) equals = signaled value x 2
Which means their actual range is 0 to 62.
Enable /Disable Measurement Settings:
For UE to perform idle mode measurement on LTE alone even in presence of WCDA for example.
The recommended value to setup for is :
SintraSearchP = higher value (e.g., 62)
SintraSearchQ = higher value (e.g., 31)
Why I recommend the above values ?
Because Condition to Enable LTE Intra-Freq Measurements on LTE is:
(Srxlev and Squal are the outcome of S-critera in case of RSRP and RSRQ measurments respectivley. They need to be greater > 0 for S-criteria to be fulfilled )
So for example, if you set SintrasearchP and SintraSearchQ equal to 62 and 31 respectively, then the condition 1 and 2 will remain true and UE will not need to perform measurements on any LTE intra frequency cell and remain camping on the cell selected.
If the condition is not true, then UE will have to perform measurements again for LTE cell ReSelection.
The exhaustive list of options. For UE to perform measurements on Intra LTE in idle mode or not is shown in the picture below.
LTE IDLE MODE MEASUREMENTS ON INTER-FREQ AND IRAT
Measurements on Non-LTE i.e, WCDMA:
The objective is to make UE not to perform any measurements in Idle mode on Inter LTE or Inter Radio Access Technology (IRAT) such
The condition for this to happen are similar to earlier conditions as follows.
As an example, in this case we do not want UE to do measurements on WCDMA,therefore we can set the values in such a way
that condition 3 and 4 remain true.
SnonintraSearchP = lower value (e.g., 0, 5 etc)
SnonintraSearchQ = lower value (e.g., 0 etc.)
Actual values, depend on operator’s policies and what boundaries do they have for different Frequency bands and different IRAT in the neighborhood areas.
The table listing exhaustive list of relationships for setting different values for SnonIntraSEarch is shown in the table below.
PRIORITY BASED CELL-RESELECTION
Remember Cell-Reselection is different than Cell Selection. Once you get this point clear, we can proceed further to discuss. How priorities can be used
for cell-reselection in Idle mode.
Absolute priorities can be allocated to RF carriers belonging to LTE and other Radio Access Technologies (RAT). What does this mean is that even if two different LTE frequency cells are available in Idle mode. An operator can still give priority to one frequency over the other so on and so forth.
Same thing goes for Inter Radio Access Technologies selection.
These priorities can be broadcast in system information.
Absolute priorities can be allocated values between 0 and 7.
Allocation of equal priorities to different RAT is not supported.
Different vendors name this priority parameter differently. Few vendors call this priority as Cell Reselection Priority while others can use a different name for the same.
In order to make LTE is preferred over WCDMA. The solution is to have LTE be given a higher priority versus other technologies such as WCDMA in this case as depicted in the figure above .
Quick Points to Remember:
Absolute priorities can be broadcast within the System information
SIB3- absolute priority for the current LTE RF carrier
SIB5- absolute priorities for other LTE RF carriers
SIB6- absolute priorities for UMTS RF carriers
UE only perform cell reselection evaluation for LTE RF carriers and inter-RAT RF carriers which are defined within the system information and for which the UE has a priority.
Inter-System and Different Priority inter-Frequency Cell Re-selection
There are 2 rules for cell re-selection towards an inter-system cell , or towards an inter-frequency cell with a different priority to the current serving cell.
Before the two set of rules are presented. New parameters are introduced for it
*The values for Thresh,high, Thresh,low and Threshserving,low are signaled using values between 0 and 31 , while the actual value of these parmaeters is Signaled value *2 , which means the actual range for these threshold is 0 to 62 i.e., ( 0 to 62)
In new releases, the names for these parameters are varied a little bit.
Cell Reselection to a higher priority inter-frequency or inter-RAT :
The conditions are as follows
For LTE and UMTS FDD
if ThreshServingLowQ is broadcast by SIB3
neighbor cell Squal > Threshx,highQ during time Treselection
neighbor cell Srxlev > Threshx, highP during Treselection
Cell Reselection to a lower priority inter-frequency or inter-RAT :
For LTE and UMTS FDD
if ThreshServingLowQ is broadcast by SIB3
serving cell Squal < ThreshServingLowQ and neighbor cell Squal > Threshx low, Q during time Treselection
serving cell Squal < ThreshServingLowQ and neighbor cell Srxlev > Threshx, low,P during Treselection
In all cases, cell reselection is only permitted if more than 1 second has passed since the UE camped on the current serving cell.
If more than one candidate cell meets the criteria for cell reselection, the target cell is selected according to:
How to bring UE back to LTE from WCDMA in Idle Mode ?
In case of scenarios where UE goes onto WCDMA in idle mode. For example during CS Fallback etc. What can be done to bring UE back onto LTE for WCDMA in such a case.
This is where SIB 19 comes for the rescue.
It is system information block in WCDMA not in LTE. It contains Inter-RAT frequency and priority information to be used in the cell for the absolute RAT priority cell reselection algorithm.
SIB 19 is used to bring UE to go from WCDMA back to LTE in Idle Mode. Setting proper value of SIB 19, can also help UE to come back on to LTE with higher priority over other technologies.
SIB 19 E-UTRA Info List:
Provides eight different E-UTRA Frequency priority information entries , indexing from 1 to 8
Each entry allows you to configure the following fields:
In all the above discussion, we have focussed only on standard parameters used in Idle mode.
For connected mode, operators can have their own specific features defined to keep the UE remain on LTE most of the time , in addition to performing proper
connected mode mobility planning.
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