HSDPA – CQI & Ec/No

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 is initiated?

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. OR

RSCP=RSSI + Ec/NoEc/No =RSCP-RSSI Quite confusing…….

OK

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 includesall components received, including the signals from the current and neighboring cellson 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 distinguishedfrom the overall noise. The EcNo is usually expressed in dB as it’s a relative value. Thevalue 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 istherefore, an indicator of signal quality.

Misconception about Ec/No

IT IS NOT (JUST THAT)

The “No” consists of three things: interfering (non orthogonal) power, thermalnoise and non-interfering (orthogonal power).

In situations where No consists mainly of “in-cell” power (e.g., during HS-DSCH bursts), Ec/No can be “low” , 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 DPxCH.

Though the dynamic range of downlink DPxCH power is often expressedrelative 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 channelquality target.

The point being: The traffic channel SNR has almost nothing to do with pilot Ec/No.

It is also to be noted that UE is only capable of receiving Pilot RSCP andRSSI on the carrier. Ec/No is calculated by the UE by the formula(RSCP-RSSI) and sent in the UL.

Still confusion……

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

Then,

Ec/No for a UE is

Hope you’re able to make a picture of how Ec/No works then proceed forwhat 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 different channels.

For simplicity, check the diagram

Notice that HSDPA usesthe remaining power of the cell after power is allocatedto Common & Dedicatedchannels (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.

Why CQI…….

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/nmany 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.

Conclusion……

• So, what is the ALTERNATIVE to Ec/No in HS session ?

• Its called Channel Quality Indication.

CQI

Why CQI…….

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). 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 throughputthat 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.

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 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%.

STEPS.

• 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)

HSDPA-UE categories 1-12

Thanks for Reading