LTE: Carrier Aggregation - SCell Measurements


In order to configure/add the secondary cell (SCell), the eNodeB can make use of measurement event A3 (Neighbour becomes offset better than PCell).
Once an SCell is added to the set of serving cells (PCell + one or more SCells), the UE can perform measurements on the SCell (and on all the serving cells) without a need for measurement gaps.

The Secondary component carrier may be activated and deactivated by MAC-CE commands as explained in detail here. The applicable performance requirements depend on whether the SCell is activated or deactivated.
When SCell is active, the requirements are similar to that of PCell. On the other hand, the RRC parameter measCycleSCell controls the measurement requirement when the SCell is in deactivated state.


Measurement Event A6
The eNodeB always tries to configure the UE with best cell (radio conditions) as SCell, in order to increase the spectrum efficiency. This is only possible, if the UE can detect and report best cell(s) on the concerned SCell’s carrier frequency.

None of the reporting events until Release-9 are applicable for the UE to detect and report best cell on the SCell’s carrier frequency.
A new measurement event A6 is introduced in Relese-10 for Carrier Aggregation needs.

Measurement event A6 is defined as ‘Neighbour becomes offset better than SCell’ which is intended for intra-frequency measurement events on SCell's carrier frequency i.e., event A6 compares the neighbor cell(s) and the SCell that are on the same carrier frequency (illustrated below).


The eNodeB, after receiving measurement report with event A6, will likely to release the old SCell and add the best reported cell in the measurement report as the new SCell.
Event A6 is similar to event A3, but event A3 can be used for PCell (intra and inter-frequency) whereas event A6 is used only for SCell (intra-frequency only).

As a result of the addition of the new event A6, ReportConfigEUTRA in Release-10 contains the following structure for event A6.

Supporting of event A6 is optional for the UEs. The UE shall indicate the support for Measurement reporting trigger Event A6 in the field featureGroupIndRel10 (index: 111) in the IE UE-EUTRA-Capabilityv1020-IEs.
An example measurement configuration for event A6 is given here. An example of measurement report event A6 from the UE is shown below.


The eNodeB configures the UE with event A6 only if the concerned SCell is configured. 

The UE shall remove the measId corresponding to event A6 from it's measurement configuration, if the concerned SCell is not configured or released. This procedure is called as ‘Measurement identity autonomous removal’.
Reference: 3GPP TS 36.133 and 36.331
Posted by KP
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19 comments:

  1. AnonymousNovember 6, 2014 at 10:42 AM

    Hey Kumar,

    I have one question about - "In order to configure/add the secondary cell (SCell), the eNodeB can make use of measurement event A3 (Neighbour becomes offset better than PCell)"?

    Are you sure that it is good solution to configure mentioned event to add sCell? I think it does not. UE will send MR for sCell only when it will be offset better than pCell. (let assume a3-offset 4, pCell RSRP -80dBm, sCell - 78dBm - good signal area, but even that no sCell will be added via RRC, because criteria are not fullfiled)

    I think event A4 (Neighbour becomes better than treshold RSRP>-105dBm) is proper to informs eNB about sCell (to trigger sCell addition). To release event A2 can be used.


    Best Regards,
    Lucas

    ReplyDelete
    Replies
    1. KPNovember 6, 2014 at 12:11 PM

      Hello Lucas,

      Good point.

      I think that A3 is still a good choice as compared to A4. The most important goal of adding an sCell is to increase the bandwidth so that throughput can be maximized. If the decision is just based on results on SCell's frequency, the UE may end up having good sCell and but very bad PCell. It is very important to maintain a good link quality on the PCell.

      Similar to Handover, the decisions for adding SCell may be based on relative RSRP of the PCell...

      To address the problem in your example, can a negative 'a3-offset' be used?

      I have also checked in conformance test cases (36.523) where 3GPP was using A3.

      What do you think?

      Delete
  2. AnonymousNovember 7, 2014 at 8:51 AM

    Hi again,

    I do not say, that your statement is wrong. Above explanation is based on live air implementation (many European operators).

    "If the decision is just based on results on SCell's frequency, the UE may end up having good sCell and but very bad PCell. It is very important to maintain a good link quality on the PCell. "

    Regarding above statement - I agree with you, but to avoid weak radio condition on pCell A3 is configured. So when A3 (pCell mobility) and A4(sCell addition) are set up (with proper parameters) this is unlikely to have weak pCell and good sCell.
    I mean if pCell becomes weak (new cell - better then offset is found), UE sends A3 and NW triggers HO (with release of sCell). Then if sCell is better then treshold, UE reports it and adds them.

    Please take a look - http://www.google.com/patents/WO2013169834A1?cl=en paragraph [0078] [0079].

    I am going to check conformance test cases, because I am not familir with them.
    I will reply soon.

    Have a nice. Thanks.
    BR


    ReplyDelete
  3. Telecom Protocol IssuesDecember 21, 2015 at 6:50 AM

    Event A6 is similar to event A3, but event A3 can be used for PCell (intra and inter-frequency) whereas event A6 is used only for SCell (intra-frequency only).

    A6 is Not for inter frequency ..?,can any one explain why it will not support why A6 is only for intra Freq ANy reason

    ReplyDelete
    Replies
    1. KPDecember 23, 2015 at 1:52 PM

      A6 is introduced for sCell Intra-frequency measurement purposes only. A3 could be used both for PCell and SCell inter-frequency measurements.

      Moreover, SCell inter-frequency measurement results are of not that much interest as PCell is the one which maintains the connection.

      Delete
  4. AnonymousJanuary 27, 2016 at 6:49 AM

    Hi Experts,

    Have you observed cases where UE gets A6 event detail in RRC connection Reconfiguration but even after condition fulfill doesn't rigger A6 event?

    I confirmed from FGI-111 bit UE can support A6 event.

    Thanks
    Sanjeev

    ReplyDelete
  5. HaroonFebruary 29, 2016 at 2:23 PM

    whats the downside if we don't configure A6 reporting on 2 Carrier CA site. Or don't configure at all

    ReplyDelete
    Replies
    1. KPMarch 1, 2016 at 11:04 AM

      If A6 is not configured, measurement gaps on primary frequency may be needed in order to measure sCell-intra frequencies.

      Delete
  6. Sadanand KhedkarOctober 7, 2016 at 10:51 AM

    Hi Kumar,

    I have simple question, to add SCell, UE need to measure SCell configured frequeny and when A3/A4 event satify it will send MR. Do UE need measurement gap for scanning SCell frequency( Assuming Scell frequency is different than Pcell)?

    ReplyDelete
    Replies
    1. KPOctober 10, 2016 at 9:41 AM

      UE needs measurement gap for measuring any other frequency than pCell. So, before sCell is configured, the UE obviously needs measurement GAP. However, once the sCell is configured, the UE doesn't need measurement GAP to measure sCell frequency.

      Delete
    2. AnonymousJanuary 10, 2018 at 12:25 PM

      Dear,
      Assume that UE is on Pcell Only (No SCell configured). Wich param will trigger the GAP Measurement to measure possible SCell ?
      Is it the same A2 event for Interfreq HO ? Is it Periodic Measurement ? If the same Interfreq HO A2 and the PSCell in good condition (no need for interfreq HO) how to trigger CA GAP Measurement ?

      Delete
    3. KPJanuary 17, 2018 at 11:43 AM

      Most of the time (live network) I have experienced the network configuring the measurement objects/configurations for SCell carrier frequency along with PCell Carrier frequency. This means that the UE doesn't need to send any Measurement report in order for it to receive Measurement Configuration for SCell carrier frequency measurements.

      Delete
  7. AnonymousJuly 11, 2017 at 9:08 PM

    Hi Experts,

    I am not seeing any A6 measurement report during SCell deactivation with Neighbor cell offset better.

    I have one basic doubt, by any chance NW controls A6 measurement reporting, means is there any message sent by NW to UE saying please report your A6 measurements or its only decided by UE to send sends A6 measurements.

    ReplyDelete
    Replies
    1. KPSeptember 6, 2017 at 10:16 PM

      Sorry for the delayed response.
      As usual, the network configures A6 measurements...based on the measurement configuration the UE shall peform and report A6 MR.

      Delete
  8. UnknownAugust 25, 2019 at 10:17 AM

    Hi Kumar can you please explain that how ue will add secondary cell when both primary and secondary cell having different cqi based on different RSRp

    ReplyDelete
  9. AnonymousNovember 19, 2019 at 6:48 PM

    Hello Kumar,

    I would like to know in general how the eNodeB vendors do the 2CC and 3CC measurements intern adding the Scell on UE. The reason being asking this question is that if we take the incremental approach of adding 2cc and then going for 3CC may result in lot of RRC signalling over head. Please tell your views on it.

    ReplyDelete
  10. Prabhat VarshneyMarch 24, 2021 at 6:38 PM

    Very Informative

    ReplyDelete
  11. AnonymousJanuary 31, 2023 at 6:37 PM

    what are the parameters related to the UE measurements on the PCell in relation to the A1/A2 and A3 reporting.

    ReplyDelete
  12. AnonymousMay 4, 2023 at 8:04 AM

    So can UE use CA with different Pcell and Scell ? I mean they are located in different cells

    ReplyDelete

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