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First than anything, let me assure you I am fully aware that no one REALLY knows HOW UE manufacturers specifically implement the CQI reports sent on the UL, but there should certainly be a better engineering answer to my question.

This is particularly important on today's reality where wireless operators are doing great efforts to improve the RF environment (important for high capacity venues such as Stadiums and Arenas) to provide the best customer experience (generally measure by the throughput granted to users) and throughput is directly dependent upon on LTE UE CQI reports back to the eNodeB.

Some background context.

PART 1: Everywhere where we look we see articles like the one below. Basically, everyone states mobiles measure RS-SINR to define the CQI value to be reported back to the eNodeB. This is where the “magic” statements comes in.

http://testfirstforkevin.blogspot.com/2013/11/lte-cqi.html?m=1

PART 2: The RS-SINR is defined by the specification(3GPP TS 36.213 V14.4.0 /2017-09) as:

Reference signal-signal to noise and interference ratio (RS-SINR), is defined as the linear average over the power contribution (in [W]) of the resource elements carrying cell-specific reference signals divided by the linear average of the noise and interference power contribution (in [W]) over the resource elements carrying cell-specific reference signals within the same frequency bandwidth. For RS-SINR determination, the cell-specific reference signals R0 according TS 36.211 [3] shall be used. The reference point for the RS-SINR shall be the antenna connector of the UE. If receiver diversity is in use by the UE, the reported value shall not be lower than the corresponding RS-SINR of any of the individual diversity branches. If higher-layer signalling indicates certain subframes for performing RS-SINR measurements, then RS-SINR is measured in the indicated subframes.

And the specification (3GPP TS 36.214 V15.0.0 /2017-09) also defines the CQI as per the paragraphs below. Pay special attention to the BOLDED sentence.

7.2.3 Channel Quality Indicator (CQI) definition

The CQI indices and their interpretations are given in Table 7.2.3-1 for reporting CQI based on QPSK, 16QAM and 64QAM. The CQI indices and their interpretations are given in Table 7.2.3-2 for reporting CQI based on QPSK, 16QAM, 64QAM and 256QAM. The CQI indices and their interpretations are given in Table 7.2.3-3 for reporting CQI based on QPSK and 16QAM. For a non-BL/CE UE, based on an unrestricted observation interval in time unless specified otherwise in this Subclause, and an unrestricted observation interval in frequency, the UE shall derive for each CQI value reported in uplink subframe n the highest CQI index between 1 and 15 in Table 7.2.3-1 or Table 7.2.3-2 which satisfies the following condition, or CQI index 0 if CQI index 1 does not satisfy the condition:

  • A single PDSCH transport block with a combination of modulation scheme and transport block size corresponding to the CQI index, and occupying a group of downlink physical resource blocks termed the CSI reference resource, could be received with a transport block error probability not exceeding 0.1.

The CQI definition above seems to imply that there are not RS-SINR measurements but, rather, the UE defines the CQI by looking at the transport blocks and selecting the highest CQI by a sort of reverse mapping from TBS (those that perform with a BLER < 10%) to MCS to CQI Mapping (because by knowing the TBS and MCS, it’d be relatively straightforward to make assumptions to CQI mapping).

QUESTION: What is the actual approach given by the specification? CQI based on RS-SINR or CQI based on reverse mapping from TBS to CQI?

I hope one of you can help me on my quest!!

Thanks a lot! Lamberto

PD: I am a newbie in the community so I could not add the tags: lte,cqi,ue to the question but these would be key tags to identify this question. If others can add it, it'd be of great help to other members.

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I support your assumption that CQI is NOT actually based on RS-SINR. In fact I saw strong evidence of this in practice. We applied negative CRS gain in our LTE (RS -3dB lower than PDSCH), that immediately led to poorer RSRQ and increase in L1 radio link failure declarations (because L1 RLF is based exactly on RS-SINR). But unlike L1 RLF, CQI has actually increased (improved)! And that's an evidence that CQI is obviously estimated NOT based on RS-SINR, for otherwise it would have degraded as well as L1 RLF.

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