cqi

1. · Why CQI?Some Questions answered
· 2 . 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
· 3 . Why CQI
Note the CQI value is 27 (good) while EcNo is about to explode (-12db)
· 4 . 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
RSCP = RSSI + Ec/No
EcNo = RSCP - RSSI
Let’s go back to developing Basics
WHAT?!?!?
Ok…..
· 5 . RSCP: The Received Signal Code Power: That's the power level the pilot channel of a
cell is received with and usually expressed in dBm (mW on a logarithmic scale). With this
parameter, different cells using the same carrier can be compared and handover or cell
reselection decisions can be taken.
RSSI: In UMTS that's the signal power over the complete 5 MHz carrier which includes all
components received, including the signals from the current and neighboring cells on the same
frequency. In an isolated cell having only CPICH power to transmit and with none of the other
channels utilized, the effective RSSI as seen by the UE would be (fraction of) PCPICH power.
However, the RSSI changes when the 5Mhz carrier will carry power for other dedicated or control
channels. In that case, the effective power perceived by the UE would be the transmitted power of
the serving cell + the power transmitted by neighbor cells over the same carrier frequency.
EcNo: That's the received energy per chip (Ec) of the pilot channel divided by the total noise
power density (No). In other words the EcNo is the RSCP divided by the RSSI. And again in other
words: The better this value the better can a signal of a cell be distinguished from the overall
noise. The EcNo is usually expressed in dB as it's a relative value. The value is negative as the
RSCP is smaller than the total received power. As the RSCP this value can be used to compare
different cells on the same carrier and handover or cell reselection decisions can be taken.
Let’s go back to developing Basics
· 6 . 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 (just that)!
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
Misconceptions about Ec/No
· 7 . Further, Ec is an expression of power in the CPICH, not in the downlink DPxCH.
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.
The point being: The traffic channel SNR has almost nothing to do with pilot Ec/N0.
It is also to be noted that UE is only capable of receiving Pilot RSCP and RSSI on the carrier.

2. EcNo is calculated by the UE by the formula (RSCP-RSSI) and sent in the UL.
Misconceptions about Ec/No
· 8 . JDon’t be shy. I know we’re still Confused …..
Basics…….
· 9 . 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)
For the sake of simplicity, just imagine that you’re in a test-bed, being served by a single cell
· 10 . 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.
Are you there yet…?
· 11 . Let’s take a look at how the POWER of a CELL is distributed among different channels
For simplicity, check the diagram below.
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)
HSDPA
Cell Power Distribution
Dedicated channels (power controlled)
Total available cell power
Common channels (not power controlled)
· 12 . Cell Power Distribution
The figures highlight the following points:
Common Channels have the highest priority for Power. Their power is fixed
DCH come second in priority – Power control is applicable
All that remains after DCH utilization can be used up for HSDPA power
So if a cell is less loaded, a single UE HS session can utilize the whole HSDPA power space
which is lying unused.
· 13 . 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 (in dBm)
Ec/No = RCSP – RSSI
Ec/No = 33 (dBm) – 33 (dBm) = 0 *
Ec/No Computation – No Load
· 14 . 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
So UE will measure the Ec/No in the following manner
Ec/No = 10 log (CPICH power / Total Transmit Power)

3. 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
· 15 . 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:
What happens when more users join in to share the REMNANT power?
Will Ec/No improve or degrade further with addition of DCH or HS user?
· 16 . 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
Why CQI
· 17 . What is CQI
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
In addition to CQI, UE also sends HARQ on HS-DPCCH
· 18 . Below figure shows the steps how CQI helps in determining the initial throughput that
a UE terminal demands from the system
STEPS
SINRis 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
Contd…..
HSDPA – CQI involvement
· 19 . HSDPA – CQI involvement
STEPS
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
Example of a UE category 10 CQI values from 0 to 30 are shown in the figure. Other UE

4. categories are shown in the following slide
The CQI is further defined as the TBS that can be supported with a BLER no greater than 10% -
· 20 . 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%.
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
So DATA RATE (bps) = TBS (bits)/ TTI (sec) * (1-BLER)
HSDPA – CQI involvement
· 21 . HSDPA – UE Categories 1-12
· 22 . Thank you for reading