CQI & Throughput

FORMULA DEL THROUGHPUT IN Huawei Tratto da http://www.slashdocs.com/xuqiw/hsdpa-troughput-formula.html VS_DataOutput_Mean

thput_mac_c_ack = ----------------------------------------------------------------VS_DataTtiRatio_Mean - VS_HSDPA_INACTIVEDataTtiRatio_Mean

where VS_DataTtiRatio_Mean is the average time ratio of the HSDPA UE queues which have data to be transmitted in the cell. It includes the time even if the data is presented in the buffer but not actually scheduled. while VS_HSDPA_INACTIVEDataTtiRatio_Mean counts the time when the data is buffered but not scheduled, in other words is the average time radio of the HSDPA UE queues witch have data to be tramitted but no PHY transmission in the cell, which means data is buffered but no scheduling. So the difference VS_DataTtiRatio_Mean - VS_HSDPA_INACTIVEDataTtiRatio_Mean indicates the average time of the HSDPA UE queues which has data scheduled and ready to be transmitted in the cell.

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). Regarding the influce of CQI on total throughput, refer to CQI vs Throughput in "Throughput" page.

6A.2.1 CQI definition when the UE is not configured in MIMO mode and not configured in MIMO mode with four transmit antennas This definition of CQI applies only when the UE is not configured in MIMO mode and not configured in MIMO mode with four transmit antennas. Based on an unrestricted observation interval, the UE shall report the highest tabulated CQI value for which a single HS-DSCH sub-frame formatted with the transport block size, number of HS-PDSCH codes and modulation corresponding to the reported or lower CQI value could be received with a transport block error probability not exceeding 0.1 in a 3-slot reference period ending 1 slot before the start of the first slot in which the reported CQI value is transmitted. Depending on the UE category as derived by higher layers in [5], either Table 7A, 7B, 7C, 7D, 7E, 7F or 7G should be used. For the purpose of CQI reporting, the UE shall assume a total received HS-PDSCH power of

PHSPDSCH  PCPICH     in dB, where the total received power is evenly distributed among the HS-PDSCH codes of the reported CQI value, the measurement power offset  is signalled by higher layers and the reference power adjustment  is given by Table 7A, 7B, 7C, 7D, 7E, 7F or 7G depending on the UE category.

Ideal MAX throughput for UMTS UE

CQI vs Throughput for UMTS In live network for HSDPA, Network sends data with different transport block size depending on CQI value reported by UE. For this mechanism to work properly, there should be a certain level of agreement between UE and the network about "which CQI value means which transport block size". These agreement is defined in the following tables of TS 25.214.      

Table Table Table Table Table Table

7A: CQI mapping table A. 7B: CQI mapping table B. 7C: CQI mapping table C. 7D: CQI mapping table D. 7E: CQI mapping table E. 7F: CQI mapping table F.



Table 7G: CQI mapping table G

Then next question is which table do I have to use for which case ? The answer is in the following table from 24.214. As you see, we use different table depending on UE Category, Modulation Scheme, MIMO. For example, if a UE is Category 14 device and uses 64 QAM and does not use MIMO, it use Table G for CQI-Transport Block Size Mapping as shown below.

I put Table 7G as an example. As you see in the table, the range of CQI value is 0~30. 30 means the best channel quality and lower number indicates poorer channel quality. And Network has to send the data with the proper transport block size according to the CQI values. For example, i) If UE report CQI value 15, it is expected for Network to send data with transport block size of 3328 bits/TTI which is equivalent to around 1.6 Mbps. ii) If UE report CQI value 30, it is expected for Network to send data with transport block size of 38576 bits/TTI which is equivalent to around 19 Mbps.

One thing you would notice that the transport block size for the highest CQI value is not amount to the ideal MAX throughput defined in 25.306 Table 5.1a. It implies that you wouldn't get the ideal Max throughput in any case with live network condition which may operate according to the CQI table defined in 3GPP. (It would not be any problem in real communication environment since your device would not report CQI 30 in most case). However, many UE manufacturer/developer wants to see if their device can really reach the ideal max throughput. In that case, we normally use a special network simulator which allows us to set the largest transport block size for each UE category. It would be even better if the network simulator allows us to define CQI-transport block mapping table arbitrarily. Fortunately I have access to this kind of the equipment and I did an experiment as shown below using the network simulator and a HSDPA Category 10 UE. First I defined a CQI-transport block size table very similar to Table 7D, but I changed the transport block size for high end CQI (30, 29, 28, 27) to allocate larger transport block than the ones specified in Table 7D to push the ideal MAX throughput. I programmed Network Simulator so that I decrease the downlink power by a certain steps. As downlink power (Cell Power) gets down, UE would report lower CQI and Network Simulator would transmit lower transport block size. The result is as follows.

In the upper plot, you see three traces - Green, Red, Blue. Green trace means the average CQI value within 500ms that UE reported. Red trace indicates the the amount of data in Kbps that the network emulator transmitted to UE within a second. Blue trace indicates the amount of data in Kbps that UE successfully decoded. If the Red trace and Blue traces overlaps, it implies that UE successfully decoded all the data transmitted by the network. If the Blue trace is lower than the Red Trace, UE failed to decode some of the data transmitted by the network. The black line shown in section A, B, C is the data rate defined in Table 7D, but I intentionally allocated the higher data rate for section A,B,C to push the data rate closer to the ideal Max throughput. In the lower plot, you see three traces - Green, Red, Blue. Green trace means the average CQI value within 500ms that UE reported. Red trace indicates the amount of ACKs within 500 ms and Blue trace indicates the amount of NACKs within 500 ms. There are a couple of things you may notice (The notes here may be different from what you observed from your device and test setting) i)

Section A is the only region in which UE shows 100% data decoding without any failure. It means that you have to make it sure that your test equipment configuration, cable connection between the test equipment and UE is configured properly so that the channel quality belongs to this area. (I would say "CQI should be much higher than 30". I know 30 is the max CQI value. What I mean is that the channel quality should be much better than the quality in which UE barely reports CQI 30).

ii) In Section B, you see huge drops in terms of throughput and huge increase in terms of number of NACKs. Main reason would be that I allocated too large transport block size for CQI 29, 28. There would also be some UE issues with this range. Section C,D,E shows a kind of normal trends, but ideally we should expect exact overlapping of rad trace and blue trace, but reality never goes like ideal -:)

User Equipment (UE) categories HSDPA comprises various versions with different data speeds. The following table is derived from table 5.1a of the release 11 of 3GPP TS 25.306[6] and shows maximum data rates of different device classes and by what combination of features they are achieved. The per-cell per-stream data rate is limited by the Maximum number of bits of an HS-DSCH transport block received within an HS-DSCH TTI and the Minimum interTTI interval. The TTI is 2 ms. So for example Cat 10 can decode 27952 bits/2 ms = 13.976 MBit/s (and not 14.4 MBit/s as often claimed incorrectly). Categories 1-4 and 11 have inter-TTI intervals of 2 or 3, which reduces the maximum data rate by that factor. Dual-Cell and MIMO 2x2 each multiply the maximum data rate by 2, because multiple independent transport blocks are transmitted over different carriers or spatial streams, respectively. The data rates given in the table are rounded to one decimal point.

Max. 3GPP number of Modulation[note MIMO, Category 1] Release HS-DSCH Multi-Cell codes Release 5 1 5 16-QAM Release 5 2 5 16-QAM Release 5 3 5 16-QAM Release 5 4 5 16-QAM Release 5 5 5 16-QAM Release 5 6 5 16-QAM Release 5 7 10 16-QAM Release 5 8 10 16-QAM Release 5 9 15 16-QAM Release 5 10 15 16-QAM Release 5 11 5 QPSK Release 5 12 5 QPSK Release 7 13 15 64-QAM Release 7 14 15 64-QAM Release 7 15 15 16-QAM MIMO 2x2 Release 7 16 15 16-QAM MIMO 2x2

Code rate Max. data at rate max. data [Mbit/s][note 3] rate[note 2] .76 1.2 .76 1.2 .76 1.8 .76 1.8 .76 3.6 .76 3.6 .75 7.2 .76 7.2 .70 10.1 .97 14.0 .76 0.9 .76 1.8 .82 17.6 .98 21.1 .81 23.4 .97 28.0

15 15 15 15

64-QAM 16-QAM 64-QAM 16-QAM

MIMO 2x2

.82 .81 .98 .97

17.6 23.4 21.1 28.0

19

15

64-QAM

MIMO 2x2

.82

35.3

20

15

64-QAM

MIMO 2x2

.98

42.2

21 22 23 24

15 15 15 15

16-QAM 16-QAM 64-QAM 64-QAM

.81 .97 .82 .98

23.4 28.0 35.3 42.2

Release 9

25

15

16-QAM

.81

46.7

Release 9

26

15

16-QAM

.97

55.9

Release 9

27

15

64-QAM

.82

70.6

Release 9

28

15

64-QAM

.98

84.4

Release 10

29

15

64-QAM

.98

63.3

Release 10

30

15

64-QAM

.98

126.6

Release 10

31

15

64-QAM

.98

84.4

Release 10

32

15

64-QAM

.98

168.8

Release 11

33

15

64-QAM

.98

126.6

Release 11

34

15

64-QAM

.98

253.2

Release 11

35

15

64-QAM

.98

168.8

Release 11

36

15

64-QAM

.98

337.5

Release 11

37

15

64-QAM

.98

168.8

Release 11

38

15

64-QAM

Dual-Cell Dual-Cell Dual-Cell Dual-Cell Dual-Cell + MIMO 2x2 Dual-Cell + MIMO 2x2 Dual-Cell + MIMO 2x2 Dual-Cell + MIMO 2x2 Triple-Cell Triple-Cell + MIMO 2x2 Quad-Cell Quad-Cell + MIMO 2x2 Hexa-Cell Hexa-Cell + MIMO 2x2 Octa-Cell Octa-Cell + MIMO 2x2 Dual-Cell + MIMO 4x4 Quad-Cell + MIMO 4x4

.98

337.5

Release 7

17

Release 7

18

Release 8 [note 4] Release 8 [note 5] Release 8 Release 8 Release 8 Release 8

MIMO 2x2

Notes: 1. 16-QAM implies QPSK support, 64-QAM implies 16-QAM and QPSK support. 2. The maximal code rate is not limited. A value close to 1 in this column indicates that the maximum data rate can be achieved only in ideal conditions. The device is therefore connected directly to the transmitter to demonstrate these data rates. 3. The maximum data rates given in the table are physical layer data rates. Application layer data rate is approximately 85% of that, due to the inclusion of IP headers (overhead information) etc.

4. Category 19 was specified in Release 7 as "For further use". Not until Release 8 simultaneous use of 64QAM and MIMO were allowed to obtain the specified max. data rate. 5. Category 20 was specified in Release 7 as "For further use". Not until Release 8 simultaneous use of 64QAM and MIMO were allowed to obtain the specified max. data rate.