Why CQI

Why CQI? Some Questions answered

Prepared by Faraz Husain

Why CQI Several times we’ve asked ourselves these questions: Q: Why do we get poor Ec/No when conducting HSDPA Drive Test? Q: What is the calculations behind Ec/No going low when HSDPA session is initiated? Q: Why do we consider CQI as a Quality Metric when conducting HSDPA Drive Test instead of Ec/No? • Take a look at the following slides

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Why CQI Note the CQI value is 27 (good) while EcNo is about to explode (-12db)

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Let’s go back to developing Basics Lets start by establishing some basics: Ec/No for most of you is a Quality & Capacity measuring Metric. It generally gives us the idea how good or bad the link Quality is. However, by definition, its quite confusing – CPICH Ec/No = Pilot channel quality energy per chip over total received power spectral density OR

WHAT?!?!?

RSCP = RSSI + Ec/No EcNo = RSCP - RSSI Prepared by Faraz Husain

Ok…..

Misconceptions about Ec/No • It is believed that Ec/No is an expression of signal-to-noise ratio and is therefore, an indicator of signal quality.

• IT IS NOT ! • The “N0” consists of three things: interfering (non-orthogonal) power, thermal noise, and, non-interfering (orthogonal) power. • In situations where N0 consists mainly of "in-cell" power (e.g., during HS-DSCH bursts), Ec/N0 can be "low", whereas the objective signal quality is high. This will be shown in the following slides

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Misconceptions about Ec/No •

Further, Ec is an expression of power in the CPICH, not in the downlink DPxCH.

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Though the dynamic range of downlink DPxCH power is often expressed relative to the CPICH, downlink power control will do its best to ensure that downlink SNR is whatever it needs to be to meet the transport channel quality target.

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The point being: The traffic channel SNR has almost nothing to do with pilot Ec/N0.

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Basics……. Don’t be shy. I know we’re still Confused ….. 

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For the sake of simplicity, just imagine that you’re in a test-bed, being served by a single cell

• Then, Ec/No for a UE is – The measure of PCPICH (code power) over Total Wideband Power on that particular carrier • Measure of PCPICH = RSCP dBm (right?) • Measure of Total Wideband power = RSSI dBm

– So our Ec/No will become Ec/No = RSCP / RSSI

(cannot divide

dBm values so we have to apply logarithmic rules)

– Ec/No = RSCP – RSSI (db) (dBm – dBm = value in dB)

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Are you there yet…? Hope you’re able to make a picture of how Ec/No works……. If yes, proceed further to find what happens when an HSDPA session is initiated If not, I suggest you discuss this concept first before proceeding. Prepared by Faraz Husain

Cell Power Distribution •

Let’s take a look at how the POWER of a CELL is distributed among different channels

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For simplicity, check the diagram below

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Notice that HSDPA uses the REMAINING power of the cell after power is allocated to Common and Dedicated channels (check more detailed figure in the next slide)

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Total available cell power

HSDPA

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Dedicated channels (power controlled)

Common channels (not power controlled)

Cell Power Distribution The figures highlight the following points: 1) Common Channels have the highest priority for Power. Their power is fixed 2) DCH come second in priority – Power control is applicable 3) All that remains after DCH utilization can be used up for HSDPA power 4) So if a cell is less loaded, a single UE HS session can utilize the whole HSDPA power space which is lying unused.

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Ec/No Computation – No Load • • • •

Now imagine yourself conducting a test in Test Bed on a cell having ZERO DCH or HS Traffic. Assuming only 10% cell power is dedicated to CPICH (33dBm = 2watt) Cell MAXTXPOWER is 20W or 43dBm Ideally, if you have NO traffic on this cell, your UE will calculate the CPICH Ec/No in the following manner – Ec/No = 10 log (CPICH power / Total Transmit power) – Ec/No = 10 log (2w/2w) = 10 log (1) = 0 dB (VERY high, but that’s IDEAL!!!) OR – Ec/No = RCSP – RSSI – Ec/No = 33 (dBm) – 33 (dBm) = 0

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(in dBm) *

Ec/No Computation – One HSDPA user • Now assume you start an HS session.Since there is no DCH traffic, all the REMNANT power is given to HSDPA •

Cell Transmit power is now 2W for CPICH and REMNANT power (18W) for HSDPA – so ideally, Cell is utilizing almost 100% power

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So UE will measure the Ec/No in the following manner – – –

Ec/No = 10 log (CPICH power / Total Transmit Power) Ec/No = 10 log (2W / 20W) Ec/No = 10 log (0.1) = -10dB (Very low – again very IDEAL!)

OR (in dBm) – Ec/No = RCSP – RSSI – Ec/No = 33 (dBm) – 43 (dBm) = -10 dB

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Why CQI • From above computations, we establish the following: – In Idle mode and with no resources allocated, a UE will measure as low as 0 dB Ec/No – In HS Mode and with no resources allocated on DCH, a UE will measure as low as -10dB. It cannot report ANY better than -10dB

• Hence, that means, in a live network where resources of cell are shared between many users, and Interference from other cells also plays its part, the Ec/No will always give a FALSE value for an HSDPA user. And it will show a very poor value What you should be asking yourself: 1) What happens when more users join in to share the REMNANT power? 2) Will Ec/No improve or degrade further with addition of DCH or HS user?

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Why CQI So, we establish that, – The CPICH Ec/No measurements collected during HSDPA Transmission could mask the true radio conditions – To avoid this, we have to monitor the Ec/No in IDLE mode only, which is a true reflection of Interference and coverage problems – HSDPA session adds to the overall load of the cell which is taken into consideration during computing Ec/No. As calculations above proved that this does not, necessarily, depict the TRUE radio Conditions.

• So, what is the ALTERNATIVE to Ec/No in HS session? • Its called Channel Quality Indication

CQI Prepared by Faraz Husain

What is CQI • • • •

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HSDPA utilizes link adaptation techniques to substitute power-control and variable spreading factor The HS-DSCH link-adaptation algorithm at the Node-B is very dynamic, and adjusts the transmit bit rate on the HS-DSCH every 2-ms TTI. The UE periodically sends a CQI to the serving HS-DSCH cell on the uplink high-speed dedicated physical control channel (HS-DPCCH) The CQI tells the NodeB scheduler, the data rate the UE expects to be able to receive at a given point in time. NOTE: The HSDPA system defines a different CQI mapping table for different categories of UEs. The category is determined according to the capability of UE

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In addition to CQI, UE also sends HARQ on HS-DPCCH

HSDPA – CQI involvement Below figure shows the steps how CQI helps in determining the initial throughput that a UE terminal demands from the system STEPS • SINR is used to evaluate the channel quality as observed by the receiver, where a standard single antenna Rake is used because it is the most common in the SISO HSDPA terminals. •

CQI value for a given SINR is done via a linear mapping, as shown in figure

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Contd…..

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HSDPA – CQI involvement STEPS

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The CQI values are used by the link adaptation algorithm at the Node-B. Every CQI value reported corresponds to the Transport Block Size (TBS) that can be granted on a particular Modulation type and Number of codes

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Example of a UE category 10 CQI values from 0 to 30 are shown in the figure. Other UE categories are shown in the following slide

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The CQI is further defined as the TBS that can be supported with a BLER no greater than 10% -

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HSDPA – CQI involvement STEPS •

While CQI is sending UE’s requirement to the system in UL, there is BLER calculation going on and UL HARQ (Hybrid Auto Repeat Request) mechanism is helping in maintaining the BLER to below 10%.

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So the system (NodeB) is essentially calculating the Data Rate to be scheduled to the user based on CQI reports and BLER which it receives from the UE

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So DATA RATE (bps) = TBS (bits)/ TTI (sec) * (1-BLER)

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HSDPA – UE Categories 1-12

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Thank you for reading

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