Retainability KPI in LTE- RF Optimization Part I

//Retainability KPI in LTE- RF Optimization Part I

Retainability KPI in LTE

Retainability KPI in LTE- RF Optimization Part I

Retainability KPI in LTE

LTE Retainability is the measure of drops per second where the definition of drop is either a Mobility Management Entity (MME)- initiated E-RAB abnormal release or an eNB initiated E-RAB release.

In both cases, data must be present in either the DL or UL buffer for the release to be considered abnormal. In the case of an MME-initiated abnormal release, it is typically triggered by the eNB reception of an S1 E-RAB Release Command Message from the MME.

If the abnormal release is initiated by the eNB, it will be the transmission of an S1 E-RAB Release Indication message that will identify the drop. Session time is defined as the amount of time that the E-RAB is active (transmitting data) per QCI.

Two equations are provided by the 3GPP specification to calculate retainability.

Equation R1 is used when retainability for a single QCI is being calculated.

If the retainability for multiple QCIs is to be calculated, equation R2 should be used.

The difference between these two equations is in the manner in which the session time is calculated.

The calculation of retainability is accurate based on session time if only one QCI is considered.

If UEs have multiple E-RABs established, these multiple-RABs may be active at the same time, thus skewing the retainability KPI when aggregating the session times for each QCI.

The illustration in the figure illustrates this problem. In this example, UE1 has a single E-RAB established with QCI=X. However UE2, has two E-RABs simultaneously established with QCI=X and QCI=Y.

If the retainability for QCI=X was being calculated, the session time would be equal to seven activity periods and equation R1 could be used accurately. Likewise, if the retainability for QCI=Y was being calculated, the session time would be equal to five activity periods and equation R1 could be used accurately.

However, if the aggregate retainability for all QCIs was being calculated, equation R1 would yield inaccurate results with a total session time of 12 activity periods. This is because there are two cases when UE2 has simultaneous activity periods for QCI=X and QCI=Y.

To alleviate the problem, equation R2 can be used where session time is measured at the UE level. In this case, the UE session time would equal 10 activity periods.

You may also like to go through What is RF Optimization?, 4G-LTE KPIs and 3GPP Accessibility KPI.

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2018-12-05T10:39:38+00:00
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