Blanking of subframes in LTE

[thomas christiansen]

Hi all,

I'm working on a ABS framework for LTE HetNets and i'm trying to control the TxPower of the given subframes to facilitate Inter-Cell Interference Coordination. The problem is that the CQI calculated in the victim cell is unchanged even though the SINR fired from the trace gives a substantially higher value. The cqi should be higher since the channel quality is much improved but it isn't.

I was wondering if someone with any knowledge of the phy-layer of LTE could please explain?

A big thanks to anyone who tries to look into it!

/Thomas

[Vikram Chandrasekhar]

Hello Thomas,

Essentially, you have to ensure that the CQI computed is reflective of the true interference condition over protected subframes. If I were to guess what you are doing, you are not scheduling any users at the aggressor cell, so as to minimize interference to the victim cells during protected subframes.

Strictly speaking, you can "hack" the correct behavior by ensuring PDCCH is not transmitted during protected subframes of aggressor cell and using existing CQI computation mechanism in NS3, and maintain two sets of state information corresponding to cqi computed over protected and unprotected subframes.

A more "correct" implementation would be the following:

1. Ensure that PDCCH is not transmitted during protected subframes. This means you have to not schedule any users at aggressor cell (not just lower transmit power). This can be ensured via modifying void LteEnbPhy::SendControlChannels (std::list<Ptr<LteControlMessage> > ctrlMsgList)
2. Compute the downlink cqi relative to the SINR computed over PDSCH (not PDCCH as in the current implementation). You need to modify LteDataSinrChunkProcessor::End () to call the associated function void LteUePhy::GenerateDataCqiReport (const SpectrumValue& sinr).Note that by default, inside NS3, the downlink CQI is derived in lte-ue-phy.cc based on the UE's SINR as computed over the physical downlink control channel (PDCCH) via LteUePhy::GenerateCtrlCqiReport (const SpectrumValue& sinr). Note that this cqi report is called from within lte-sinr-chunk-processor.cc via LteCtrlSinrChunkProcessor::End ().
3. Modify void LteUePhy::GenerateDataCqiReport to compute te downlink CQI. A simple fix would be to call GenerateCtrlCqiReport from within GenerateDataCqiReport.
4. You also need to ensure you compute two sets of cqi, one over protected and one over unprotected subframes. This is to accurately model resource restricted CQI measurements, as introduced during Rel-10. Otherwise, the cqi are computed over unprotected subrames will result in pessimistic link adaptation.

Good luck,

- Vikram

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[thomas christiansen]

Thank you so much, I'll give it a shot.

[mailm...@gmail.com]

Hello Thomas,

I am also working on this. Are you able to implement what Vikram has said?. Because I have a doubt that simply calling GenerateCtrlCqiReport from within GenerateDataCqiReport will solve the problem.

Regards
Archana

[mailm...@gmail.com]

Hello Vikram,

I tried to implement what you have told to Thomas. But in the step 2. I amfacing some problem. I have changed the code LteDataSinrChunkProcessor::End () to call the associated function void LteUePhy::GenerateDataCqiReport (const SpectrumValue& sinr). But my problem is that LteDataSinrChunkProcessor::End is not called for LteUePhy. Could you please help me in implementing this.

Thanks
Archana

[Stefania Zinno]

Hi have you implemented something yet? I'm interesting in new about a ABS framework to simulate Hetnes. Cheers