CQI – Channel Quality Indicator

2 Feb

CQI

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

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

ii) 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 transmit the data with larger transport block size and vice versa.

What if network sends a large transport block even though UE reports low CQI, it is highly probable that UE failed to decode it (cause CRC error on UE side) and UE send NACK to network and the network have to retransmit it which in turn cause waste of radio resources.

What if UE report high CQI even when the real channel quality is poor ? In this case, network would send a large transport block size according to the CQI value and it would become highly probable that UE failed to decode it (cause CRC error on UE side) and UE send NACK to network and the network have to retransmit it which in turn cause waste of radio resources.

How UE can measure CQI ? This is the most unclear topic to me. As far as I know, there is no explicit description in any standard on the mechanism by which the CQI is calculated, but it is pretty obvious that the following factors play important roles to CQI measurement.

  • signal-to-noise ratio (SNR)
  • signal-to-interference plus noise ratio (SINR)
  • signal-to-noise plus distortion ratio (SNDR)

It is unclear how these factors are used and whether there is any other factors being involved. I was told the detailed CQI measurement algorithm is up UE implementation (chipset implementation).

In LTE, there are 15 different CQI values randing from 1 to 15 and mapping between CQI and modulcation scheme, transport block size is defined as follows (36.213)

If you are an engineer in Network (eNodeB) programming, you need to know the number of resource blocks and MCS for each CQI value to properly allocate the resources for each of UEs. With the modulation scheme in the table, you would get a certain range of MCS you can use for each CQI index. But you cannot pinpoint a specific MCS and Number of RBs. You need another condition to get the proper MCS and N RBs and it is ‘Code Rate‘ shown in the table. But still there is not a single formula that would give you a single/determined value for MCS and NRB. You have to come up with a set of MCS and N RB that meet the modulation scheme and Code Rate requirement in the table. One example case can be as follows.

CQI

Modulation

Bits/Symbol

REs/PRB

N_RB

MCS

TBS

Code Rate

1

QPSK

2

138

20

0

536

0.101449

2

QPSK

2

138

20

0

536

0.101449

3

QPSK

2

138

20

2

872

0.162319

4

QPSK

2

138

20

5

1736

0.318841

5

QPSK

2

138

20

7

2417

0.442210

6

QPSK

2

138

20

9

3112

0.568116

7

16QAM

4

138

20

12

4008

0.365217

8

16QAM

4

138

20

14

5160

0.469565

9

16QAM

4

138

20

16

6200

0.563768

10

64QAM

6

138

20

20

7992

0.484058

11

64QAM

6

138

20

23

9912

0.600000

12

64QAM

6

138

20

25

11448

0.692754

13

64QAM

6

138

20

27

12576

0.760870

14

64QAM

6

138

20

28

14688

0.888406

15

64QAM

6

138

20

28

14688

0.888406

Note 1 : Refer to Throughtput Calculation Example for determining N_RB, MCS, TBS determination.

Note 2 : REs/PRB varies depending on CFI value as follows.

CFI

REs/PRB

1

150

2

138

3

126

Note 3 : I used the following formula explained in Code Rate section.

v_CodingRate := (int2float(p_TBSize + 24)) / (int2float(p_N_PRB * tsc_REs_Per_PRB * v_BitsPerSymbol));

CQI is carried by PUCCH or PUSCH depending on the situation as follows.

  • Carried by PUCCH : Periodic CQI
  • Carried by PUSCH : Aperiodic CQI.

 

Regarding CQI report period and configuration, refer to CQI, PMI, RI Reporting Configuration part.

 

Source: http://www.sharetechnote.com/html/Handbook_LTE_CQI.html

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