Why CQI-Related Questions answered
Q: Why do we get poor Ec/No when conducting HSDPA Drive Test?
Q: What is the calculation behind Ec/No going low when HSDPA session
Q: Why do we consider CQI as a Quality Metric when conducting HSDPA
Drive Test instead of Ec/No?
Lets a look on the drive test in the next slide_____
Note the CQI value is 27 (good) while EcNo about to explode (-12dB)
Lets start with 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.
RSCP=RSSI + Ec/No
Ec/No =RSCP-RSSI Quite confusing««.
Let¶s go back to some developing Basics.
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 can be seen by the UE would be
(fraction of) PCPICH power. However 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.
It is believed that Ec/No is an expression of signal-to noise ratio and is
therefore, an indicator of signal quality.
Misconception about Ec/No
IT IS NOT (JUST THAT)
The ³No´ consists of three things: interfering (non orthogonal) power, thermal
noise and non-interfering (orthogonal power).
In situations where No consists mainly of ³in-cell´ power (e.g., during
HS-DSCH bursts), Ec/No can be ³low´ , whereas the objective signal
quality is high. This will be shown in the following slides.
Misconception about Ec/No
Further, Ec is an expression of power in the CPICH, not in the downlink
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
The point being: The traffic channel SNR has almost nothing to do with
It is also to be noted that UE is only capable of receiving Pilot RSCP and
RSSI on the carrier. Ec/No is calculated by the UE by the formula
(RSCP-RSSI) and sent in the UL.
For the sake of simplicity, just imagine that you are in a test-bed, being
served by a single cell.
Ec/No for a UE is
Hope you¶re able to make a picture of how Ec/No works then proceed for
what happens when a HSDPA session is initiated«.
Cell power distribution
Let¶s take a look at how the POWER of a CELL is distributed among
For simplicity, check the diagram
Notice that HSDPA uses
the remaining power of the
cell after power is allocated
to Common & Dedicated
channels (check more detail
In next slide).
Cell power distribution
Without HSDPA With HSDPA
From the figure
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 the 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.
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 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.
From above computations we establish the following conclusion..
1. In Idle mode and with no resources allocated, a UE will measure as
low as 0 dB Ec/No.
2. In HS Mode and with no resources allocated on DCH, a UE will
measure as low as -10dB. It cannot report ANY better than -10 dB.
What you may 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?
Hence, it means in a live network when resources of cell are shared b/n
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 poor value.
So, what is the ALTERNATIVE to Ec/No in HS session ?
Its called Channel Quality Indication.
What is CQI ?
HSDPA utilizes link adaptation techniques to substitute power control and variable
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). In addition to CQI, UE
also sends HARQ on 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 defined according to the capability of UE.
HSDPA ± CQI involvement
Below figure shows the steps how CQI helps in determining the initial throughput
that a UE terminal demands from the system.
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.
HSDPA ± CQI involvement
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
Example of a UE category 10 CQI values from 0 to 30 are shown in
figure. Other UE 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%.
When 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)