A UE that supports both downlink 4x4 MIMO and 256 QAM may not meet the requirements for 256 QAM after entering 4x4 MIMO mode.
As a result, 256 QAM may offer lower or even no gains.
Is it really difficult to reach 4 layer 256 QAM?
256 QAM is subject of high SINR.
4x4 MIMO is subject of rich multipath, uncorrelated paths between enodeB and UE.
Have nothing to do one with another.
Right on SINR, in addition it is also based on AMC - those with best channel gain will get 256-QAM - normally nearer to the Tx antenna.
Is it possible to reach 4 layers in low SINR?
No, it is not possible.
I’ve heard in Huawei when sinr > 20, eNB can start schedule rank4.
I am talking about Rank Indicator from UE.
If you look at drivetests you will see rank indicator 4 even fro SINR 2-3 dB.
I see rank4 at sinr < 10, but very few times.
You will see samples of rank indicator 4 even when sinr is less than 10 dB.
Generally rank4 is rare.
This is why you don’t get many samples.
My opinion is that 256 QAM and rank indicator are independent from each other.
How to measure uncorrelated signals?
UE receivers does this on each of the 4 antenna based on csi-rs reference signals.
I can see RI = 2, csirs measurement by 4 antennas of ue.
But do not see explanation of how rank is calculated as 2.
Rank indicator is for future tti i.e. what gnodeB will schedule in next tti.
In 5G UE calculates rank indicator based on N1, N2, O1, O2 and i1,1 i1,2 and i2.
Also PMI is calculated also on those things: N1, N2, O1, O2 and i1,1 i1,2 and i2.
Whether eNB schedule 4 layers but UE just measured rank1/2/3?
Also rank indicator is based on air interface matrix.
Svd of it and rank of diagonal matrix.
If cqi/pmi/ri is reported periodically 40 slots, firstly rank3 reported, so enb/gnb will always schedule 3 layers during next 40 slots?