How does UE calculate CQI value?

CQI stands for Channel Quality Indicator. As the name implies, it is an indicator carrying the information on how good/bad the communication channel quality is. This CQI is for HSDPA. (LTE also has CQI for its own purpose).

CQI is the information that UE sends to the network and practically it implies the following two:

1) Current Communication Channel Quality is this-and-that..

2) I (UE) wants to get the data with this-and-that transport block size, which in turn can be directly converted into throughput

In HSDPA, the CQI value ranges from 0 ~ 30. 30 indicates the best channel quality and 0,1 indicates the poorest channel quality. Depending which value UE reports, network transmit data with different transport block size. If network gets high CQI value from UE, it transmits the data with larger transport block size and vice versa.

CQI index

modulation

code rate x 1024

efficiency

0

out of range

1 QPSK

78

0,1523

2 QPSK

120

0,2344

3 QPSK

193

0,377

4 QPSK

308

0,6016

5 QPSK

449

0,877

6 QPSK

602

1,1758

7 16QAM

378

1,4766

8 16QAM

490

1,9141

9 16QAM

616

2,4063

10 64QAM

466

2,7305

11 64QAM

567

3,3223

12 64QAM

666

3,9023

13 64QAM

772

4,5234

14 64QAM

873

5,1152

15 64QAM

948

5,5547

CQI Periodicity

Following is an example when CQI only is being transmitted. Npd and N_OFFSET,CQI configured by RRC Connection Setup and RRC Connection Reconfiguration and the value itself came from Table.

10*nf +[ns/2] – Noffset,CQI) mod (Npd)=0

n= SFN

n= Slot number within a radio frame

Noffset,CQI = Reporting Period Offset in subframes

Npd = Reporting Period in subframes

The equation itself will be quite simple, but just to double check your understanding, let me give you a short quiz. Don’t panic -:) Here you go.

Let’s try with some example for a FDD case.

If cqi-pmi-Configindex in RRC message is 13. What is Npd and N_OFFSET,CQI ?

This maps to the row of the table as follows. You can read Npd directly. It is 10. Then you need a little bit of math to get N_OFFSET,CQI.  The value for this example is specified to be (I_CQI/PMI-7). It is “13 -7” which is 6. Now the last step is to apply these two values to the equation above to calculate the exact sub-frame number for CQI/PMI transmission.

ICQI/PMI

Value of Npd

Value of Noffset,CQI

0 ≤ICQI/PMI≤1

2

ICQI/PMI

2 ≤ICQI/PMI≤6

5

ICQI/PMI-2

7 ≤ICQI/PMI≤16

10

ICQI/PMI-7

17 ≤ICQI/PMI≤36

20

ICQI/PMI-17

37 ≤ICQI/PMI≤76

40

ICQI/PMI-37

77 ≤ICQI/PMI≤156

80

ICQI/PMI-77

157 ≤ICQI/PMI≤316

160

ICQI/PMI-157

ICQI/PMI=317

Reserved

318 ≤ICQI/PMI≤349

32

ICQI/PMI-318

350 ≤ICQI/PMI≤413

64

ICQI/PMI-350

414 ≤ICQI/PMI≤541

128

ICQI/PMI-414

542 ≤ICQI/PMI≤1023

Reserved

Additional info

You might be interested in some other interesting topics concerning cellular networks engineering, such as:

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