Friday, 12 July 2013

What is Rank Indicaton in LTE


Rank Indication is one of the important input to eNB , in selection of the transmission layer in downlink data transmission. Even though the system is configured in transmission mode 3 (or open loop spatial multiplexing) for a particular UE and if the same UE report the Rank Indication value 1 to eNB, eNB will start sending the data in Tx diversity mode to UE . If UE report Rank Indication 2 , eNB will start sending the downlink data in MIMO mode (Transmission Mode 3).

Why we need this RI in LTE concept? When UE experience bad SNR and it would be difficult (error prone) to decode transmitted downlink data  it gives early warning to eNB by stating Rank Indication value as 1. When UE experience good SNR it pass this information to eNB by indicating rank value as 2.

Because of this reason, you might have observed that some time data transmitted by eNB is in Tx diversity mode, though MIMO was configured and hence you may have observed less downlink throughput than expected one. 

However, it is not necessary that eNB will always change the transmission mode based on RI value, it could be implementation specific decision. 
Questions are welcome.

13 comments:

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  2. why some vendors recommend tm3 for TDD?

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  3. Dear Ahmed,

    Good question. Here is my logic behind it . As we know TM does depend on the channel conditions. In TDD UL and DL are on same frequency , hence channel characteristics are same . Because of that a feedback is not needed from the UE when a suitable Uplink signal is present that the base station can use to estimate the channel (using uplink SRS).

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  4. Hi,could u tell the reasons for poor MIMO Utilization?

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  6. Rank indicator 2 definition

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  7. I got issue for throughput. UE still get RI 1 even RF conditions are good (RSRP and SINR) and the network MIMO are configured.

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    Replies
    1. Hi...are you able to resolve your issue ? Can you shed some inputs on how did you debug ? Thanks in advance.

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    2. You can check this on the device side whether proper UE Category is set on the device. Once I faced this issue and it came out that the UE Category was wrongly set as '1' on the device hence didn't report Rank2.

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  8. which scenario Rank 3 & Rank 4 Coming in picture?

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  9. hi,

    i have an issue regarding RI Vs TM.
    my 99% sample of Transmission mode is TM3.
    but RI shows RI1 55% and RI2 45%.
    so how TM is dependent RI.

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    Replies
    1. WELL how TM is dependent RI.??

      Poor SINR Poor RI, vendor set the SINR thershold for switching RI + hyst means "N" times consecutive request for TM change then convert
      suppose 15dbm set the criteria for TM3 condition tx diversity as UE get 15dbm or greater the range signal run the hyst count "N" , as fullfill N times request convert TM2 to TM3

      also have option to set SINR + FER thershold for TM switching

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  10. Can any one explain me about transmission modes

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