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Some Questions answered
Prepared by Faraz Husain
Several times we've asked ourselves these questions:
Q: Why do we get poor Ec/ No when conducting HSDPA
0: 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
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 = Pllot channel quality energy per chip over total received power
spectral density o
RSCP = RSS1 + Ec/ No
EcNo . -. RSCP - RSSI
Let's go back to developing Basics
R809: 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.
nssi: 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
lrequency. in an isolated cell having only CPICH power to transmit and with none oi the other
channels utilized. the effective FtSSl as seen by the UE would be (traction of) PCPICH power.
However, the RSSI changes when the 5Mhz carrier will carry power for other dedicated or control
channels. In that case, the eilective power perceived by the UE would be the transmitted power of
the serving cell + the power transmitted by neighbor cells over the same carrier lrequency.
1: That's the received energy per chip (Ec) ot the pilot channel divided by the total noise
power density (No). In other words the EcNo is the RSCP divided by the FiSSl. And again in other
words: The better this value the better can a signal oi 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. D
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 (just that)!
- The “NO” consists of three things: interfering (non-orthogonal)
power, thermal noise, and, non-interfering (orthogonal) power.
- In situations where NO 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
- Further. Ec is an expression of power in the CPlCH, not in the downlink
- Thouh 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 trafﬂc 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
RSSl on the carrier. EcNo is calculated by the UE by the formula (RSCP-
RSSI) and sent in the UL. D
For the sake of simplicity, just imagine that you're
in a test—bed. being served by a single cell
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)
~: .a. V
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. D
Cell Power Distribution
- Let's take a look at how the POWER of a CELL is distributed among
- 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)
Dedicated channels (power controlled)
Total available cell power
Bnlnrnnn nlsannnln Innﬂ manna’ nnnO'nIIn: I . ... _I
Cell Power Distribution
The figures highlight the following points:
Common Channels have the highest priority for Power. Their power is
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.
3 . ... ... ...
E Dedi ctiennt Detailed channels
" into time
witnout HSDPA mt, "sop. Reference 
. — . . . atlas
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, it 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! !!)
— Ec/ No = RCSP — FiSSl
- Ec/ No = 33 (dBm) — 33 (dBm) = o
Ec/ No Computation — One HSDPA
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
— Eci‘No = 10 log (CPICH power i’ Total Transmit Power)
— EcfNo = 10 log (2W / 20W)
— Ec/ No = 10 log (0.1) = -10dB (Very low - agai very IDEAL! )
on (in dBm)
— Ec/ No = RCSP — RSSI
— Ec/ No = 33 (dBm) — 43 (dBm) = -10 dB
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 ~10d§. It cannot report ANY better than
. . 0
Hence, that means, in a live network where resources of
cell are shared between many u rs, and Interference
from other cells also plays its pa , 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?
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
béllvnozx l’ ~ ACK NAUK
1"‘/ (AK ll ln, |'l'r1l
HSDPA utilizes link adaptation techniques to substitute pov-verfcontrol 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-DPCCg~| )
The CQI tells the NodeB scheduler. the data rate the UE expects to be able
to receive at a given point in time. 0
NOTE: The HSDPA system defines a different CQI mapping table for difte t
categories of UEs. The category is determined according to the capability o E
In addition to
CQI. UE also
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HSDPA - CQI involvement
Below figure shows the steps how CQI hebs 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 SS0 HSDPA terminals. .. . slxng -1;
(‘QI = Sl. ''R[oll! ’; 4» 13:. -1!‘ < SINK -: 20.’:
10 slxlt 3 was
- CQI value for a given SNR is done via a linear as shown in figure
- Contd. ... .
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- The CQI values are used by the link adaptation algorithm at T , ‘T _T ’ T (“T . ,.M
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 : ‘ _ . - um
- Example of a UE category 10 CQI values from O to 30 are - W - W - ‘ - UN
shown in the figure. Other UE categories are shown in the ~ ' ‘ ~
ll . 33"‘: ‘ I ultxx
- 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
- 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): res (bits)/ TTI (sec) "(1-BLER)