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
CPICH HSPDSCHP
P
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.
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 7A: CQI mapping table A.
Table 7B: CQI mapping table B.
Table 7C: CQI mapping table C.
Table 7D: CQI mapping table D.
Table 7E: CQI mapping table E.
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.
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 -:)
