a novel sir to channel-quality indicator (cqi)

8/9/2019 A Novel SIR to Channel-Quality Indicator (CQI)

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Abstract — To support very high data rate services that requirehigher system capacity, the high speed downlink packet access

(HSDPA) was proposed in the UMTS standard. One of keytechniques supporting the HSDPA services is the adaptivemodulation and coding (AMC) in which the modulation scheme

and the coding rate are adaptively changed to match the currentchannel condition reported by the user equipment (UE).Therefore, the mapping between the channel quality indicator(CQI) and signal to interference ratio (SIR) is closely related tothe accuracy of AMC and the performance of HSDPA. This paperproposes a novel SIR to CQI mapping method that satisfies the3GPP requirements. In order to verify the performance of the

proposed mapping method, we implemented the link-levelsimulator which is composed of all the physical layer blocks

depicted in the 3GPP standard. With the proposed mappingmethod, we show that UE can report the exact channel condition

and the system can yield performance exceeding the requirementsin the 3GPP technical specification.

Index Terms — AMC, CQI, HARQ, HSDPA, SIR, UMTS

I. I NTRODUCTION

Mobile cellular devices, what first started out as a tool for

sending voice through wireless environments, now can receive

text messages and multimedia data, and provide the interactive

environment for playing games. To satisfy the demand for more

data at higher data rates, many of the cellular systems created

high speed data access schemes such as enhanced data rate for

GSM evolution (EDGE) for GSM systems, EV-DO and

EV-DV for CDMA 2000 systems, and high speed downlink

packet access (HSDPA) for WCDMA systems.

HSDPA is a new scheme in the standard air interface created by 3GPP. The main idea of HSDPA is to use user diversity on a

shared link channel. HSDPA utilizes powerful channel coding

method called the turbo coding with adaptive modulation and

coding (AMC) mechanism and a hybrid ARQ (HARQ) scheme

to maximize throughput.

This entire HSDPA mechanism is based on the fact that the

user equipment (UE) can provide the Node-B with the channel

quality indicator (CQI). The 3GPP specification does not state

how CQI should be generated. It is entirely up to the UE

designer. The only requirement it must satisfy is that the block

error rate (BLER) with CQI fed from UE must be under 10%.

In this paper we propose a novel method to create CQI values

using signal to interference ratio (SIR) and the mapping

procedure. We make the SIR to CQI mapping graph for three

different SIR measurement methods and confirm that the CQI

generation using this method performs well within the 3GPP

specifications through simulation.

The remaining part of this paper is organized as follows:

Section II introduces the key features of the HSDPA system,

and Section III discusses the SIR measuring techniques. In

Section IV, we propose the SIR to CQI mapping method.

Section V describes the simulation environments and evaluates

the performance of the proposed scheme. Finally, conclusions

are made in Section VI.

II. HSDPA SYSTEM

The HSDPA system is a new system that has been included

in the Release 5 of the 3GPP specifications. The main idea of

HSDPA is to use multi-user diversity. Since HSDPA has a

single downlink channel shared by multiple users, each user

may experience different channel conditions. In HSDPA, each

UE reports back to the Node-B of its channel quality with a CQI

value. Then the Node-B can decide how to allocate time slots in

the shared downlink channel to UEs. Usually it allocates time

slots in the shared downlink channel to the user with the most

excellent channel conditions to increase the overall throughput

of the system.

A. AMC

As HSDPA exploits multi-user diversity, it requires users to

report back each user’s channel condition by means of CQI.

With the use of CQI values at the Node-B, it can send data to

the user at the optimum channel coding rate for that particular

channel quality. These channel coding rates can be achieved

using turbo encoding with bit puncturing or bit repetition. AMC

utilizes this system and formats the data block to be transported

to the user with a specific channel coding rate and modulation

scheme according to the CQI value received from the user.

A Novel SIR to Channel-Quality Indicator (CQI)Mapping Method for HSDPA System

Kyungsu Ko, Daewon Lee, Moohong Lee and Hwang Soo Lee

Department of EECS, Division of Electrical Engineering, KAIST373-1, Guseong-dong, Yuseong-gu, Daejeon, 305-701, Republic of Korea

Tel: +82-42-869-5428, Fax: +82-42-869-8670

e-mail: [email protected]

1-4244-0063-5/06/$20.00 ©2006 IEEE



B. HARQ

The term ‘Hybrid’ comes from the fact HARQ is essentially

a hybrid of ARQ system and the soft combining technique. The

main idea of HARQ is not to waste packets even if there are

some errors in them. All data packets that are sent to the users

are appended with a cyclic redundancy check for error

detection. So when a corrupted data packet is received by theuser, the user sends back a NACK signal through the uplink and

request for a retransmission of that particular packet. Then the

Node-B retransmits the packet to the user, the user does soft

combining the newly received packet with the old corrupted

packet using chase combing technique or incremental

redundancy technique.

Chase combining is used when the retransmitted packet is

exactly the same as the corrupted packet that was received

before. It combines the two packets using maximal ratio

combing. Incremental redundancy is another HARQ technique

wherein instead of sending simple repeats of the entire coded

packet, additional redundant information is incrementally

transmitted if the decoding fails on the first attempt.

III. SIRMEASUREMENT

Although there might be many ways to measure the wireless

channel conditions, we believe SIR is the best measure for

quantifying the quality of the channel. The main reason SIR is

good measurement reference for CQI is because the transport

block size selection in HSDPA targets a 1dB step size in SIR in

AWGN channel conditions for a BLER of 10% [8]. SIR can be

measured differently according to different methods of

measuring interference and noise. For the CQI generationmethods we use three different SIR measuring techniques.

A. Conventional SIR Measurement

The conventional SIR measurement technique is to measure

squared mean of the input signal and divided it by the variance

of the input signal.

[ ]

[ ]

2

( ) . E x

SIR xVar x

= (1)

We have used the CPICH pilot symbols for the input signal.

The reason why we used pilot symbols instead of data symbolsis because in HSDPA there are too many data symbols, when

using pilot symbols the entire SIR measuring process is much

easier. Also since the data symbols are modulated into QPSK or

16 QAM symbols, the variance of the input signal might be

affected by different input bit sequences. The CPICH pilot

symbols in HSDPA were all mapped into a single QPSK

symbol. This means the pilot symbols are very constant signals.

If there are variations within the signal, they are due to the

wireless channel effects and noise. These facts make the

CPICH pilot symbols an ideal input source for the SIR

measurement.

Equation (1) can be developed as follows

= =

=

−

= N

k

N

k

k k

N

k

k

x N

x N

x N

xSIR

1

2

1

2

1

11

1

)( (2)

where N is the number of CPICH symbols in a TTI, and xk is

CPICH pilot symbols.

B. Modified SIR Measurement

The conventional SIR measurement defines signal power as

the mean of the signal squared. Although this represents pretty

accurate picture, there are problems when the noise power is

high. One of the problems is that SIR means signal to

interference and noise ratio, but in the conventional SIR

measurement the signal power actually contains noise and

interference power. This effect acts as an offset in the SIR and

when the noise and interference power increases it deviates

from the actual SIR. The modified SIR measurement method

compensates for this problem by estimating the interference power and subtracting it from the signal mean power as follows

−

=

+−

−=

1

1

2

1)1(2

1 N

k

k k s x x N

I (3)

=

=

N

k

k s x N

E 1

1 (4)

N

I E E s

s s −=′ (5)

s

s s

s

s

I N

I E N

I

E x RSI

⋅

−⋅=

′=′ )( (6)

where xk is the CPICH pilot symbol and N is the number of

CPICH pilot symbols in one TTI [1].

C. Vector Based Interference Projection SIR Measurement

The vector based interference projection (VBIP) SIR is an

SIR measurement method for CDMA systems [7]. It utilizes the

orthogonal codes used in CDMA systems. VBIP uses the

unused code in the downlink channel to project the interference

and noise on to that code. Since all codes must be applied on to

chips not symbols, and processing chip information into

symbols and equalizing them takes time, this method has the

advantage of calculating SIR quickly. This method is also

different from other method since it does not utilize the CPICH

pilot symbols at all. It only needs information on which codes

are being used in the CDMA system and which codes are not

being used.

2 *

1 1

1 1 1( ) ( ) ( ) .

N SF H

S I N

k n

y E y y y SF k n y SF k nM N SF

σ + +

= =

= = ⋅ + ⋅ ⋅ +

(7)

2*2

21 1

1 1( ) ( ) ( ) .

H T N SF

I N

k n

y c c y y E c n y SF k n

N SF cσ

+

= =

⋅ ⋅= = ⋅ ⋅ +

(8)



2

2( ) 1 .S I N

I N

SIR x σ

σ

+ +

+

′′ = − (9)

Equation (9) represents VBIP SIR measurement, where c(n)

is the unused CDMA code, SF is the spreading factor of c(n),

y(n) is the input chip signal, and N is the number of chips in one

TTI divided by SF .

IV. PROPOSED SIR TO CQI MAPPING METHOD

The method to generate CQI values and make a mapping

table from the generated CQI values is based on the fact that the

transport block size (TBS) selection in HSDPA targets a 1 dB

step size in SIR in AWGN channel conditions for a BLER of

10%. It is desired that transmitter sends data as many as

possible within the BLER 10% criterion. A high CQI value

means a high SIR and a good channel condition, and the higher

CQI value UE reports, the larger transport block is transmitted.

If Node B transmits a TBS larger than the one suited to current

channel condition, the block will be corrupted. So the goal is tofind the optimal CQI value from the estimated SIR at UE. This

is same as finding the optimal transport block size, number of

multicodes and modulation scheme in the system.

The flowchart of the SIR to CQI value mapping algorithm is

depicted Fig. 1. The procedure starts from setting the channel

condition, such as PA3, PB3, VA30 and VA120 that show ITU

pedestrian A with 3km/h, pedestrian B with 3km/h, vehicular A

with 30km/h and vehicular A with 120km/h respectively. Then

set the HS-PDSCH_Ec/Ior (HS-PDSCH power to transmitted

power ratio) -3dB or -6dB, Ior/Ioc (transmitted power to

interference power ratio) -50~10dB and CQI value according to

condition we want to simulate. After setting the transport blocksize, modulation scheme, number of multicodes based on

chosen CQI value, we run simulation (in case of PA3, 15000

cycles or frames, and in other cases 1500 cycles or frames) and

record SIR estimations for each transport block. Because

BLERs of the target systems is near 10%, we calculated all the

BLERs for all the simulations. If BLER is beyond the range

between 9.8 ~10.2%, then adjust Ior/Ioc value properly and run

the simulation repeatedly until BLER in the range of

9.8~10.2% is found. Here, if BLER is larger than 10.2%,then

we should increase Ior/Ioc since excessive interference causes

block errors deviating from the standard, and if the BLER is

smaller than 9.8%, then we should decrease the Ior/Ioc since

there is a margin in the block error rate. Once BLER near 10%(9.8 ~ 10.2%) is found, draw a histogram of SIR distribution for

the simulation and find average SIR. Finally match this average

SIR to the CQI value used for the simulation. Repeat these steps

for all CQI values 1 through 30.

V. SIMULATION

A. HSDPA Link Level Simulator

Currently the HSDPA system is designed for WCDMA

which is a specification created by 3GPP. The simulator

consists of a Node-B transmitter, a wireless channel and a UE

receiver. The Node-B transmitter consists of a bit-rate

processing (BRP) block and a chip-rate processing (CRP)

block. The UE receiver consists of blocks complement to the Node-B transmitter. This simulator is designed with an

equalizer instead of a conventional rake receiver usually used in

a CDMA system. The reason why equalizer is used is because

as the wireless channel effects increase the performance of the

rake receiver decreases significantly. A good Equalizer can

cancel out the wireless channel effects and have better

performance than a conventional rake receiver. The SIR

measurement equations are slightly modified to be used with an

equalizer. The equalizer type used is the conjugate gradient

algorithm.

The wireless channel is implemented using the improved

Jake’s fader [6] with a tapped delay line model. The power

delay profiles are used from the 3GPP specification channeltesting conditions [1].

B. Simulation Results

Figs. 2, 3, and 4 show the results from many simulations. The

dots represent values of average SIR where the system has

BLER of 10% for a specific CQI.

The simulation results show that the modified SIR values are

the most consistent throughout the different channel conditions.

The SIR dots not shown in the result figures such as SIR values

Start

Set the channel modeout of PA3, PB3, VA30,

VA120

Set Ior/Ioc

-50dB ~ 10dB

Choose CQI value = 1

Set TB size, modulation

scheme, No.# ofmulticodes based on

chosen CQI

Run simulation 1500

cycles (PA3:15000cycles)

Measure and store each

SIR estimation for eachcycle

Calculate accumulated

BLER over 1500 cycles

(PA3:15000)

BLER > 0.102IncreaseIor/Ioc by 2dB

BLER < 0.098 Decrease

Ior/Ioc by 2dB

Draw a histogram ofmeasured SIR distribution

and find average SIR

Match this average SIR tothe CQI value used for

simulation

Increase CQI value by 1

1

2

2

1

2

Set HS-PDSCH Ec/Ior =-3dB (or -6db)

CQI value <=30

End

Yes

No

Yes

No

Yes

No

Fig. 1. Flowchart of SIR to CQI value mapping algorithm



at CQI 20 in channel condition VA120 do not exist, because at

VA120 channel condition no matter how the noise and

interference level decrease the HSDPA system will not achieve

BLER of 10% or lower. So points that can not be represented

are not shown. VBIP SIR values show non-linear properties in

fitting data points between SIR and CQI.

The average SIR values of different channel conditions for

each CQI value are shown in Figs. 5, 6, and 7. It can be seen

that the simulation result curves tend to saturate when the CQI

values gets higher than 20. Therefore we considered this fact to

make a second linear fitting curve around CQI values over 20

using results from the AWGN channel. The two linear fitting

curves can be seen in Figs. 5 and 6, and the non-linear fitting

curve and linear fitting curve for higher CQI values can be seen

in Fig. 7. Through these fitting curves the mapping table

between SIR and CQI is determined.

The 3GPP specifications state that the shared downlink

HS-PDSCH channels can either take -3dB or -6dB of the entire

Node-B transmission. The simulation results of Figs. 1 to 3

were done when the HS-PDSCH channels take up -3dB of the

entire Node-B transmission power. When the HS-PDSCH

channels take up -6dB of the entire Node-B transmission

power, the SIR to CQI linear fitting curve is expected move by

3dB since the signal power is being decrease by 3dB.

Simulation results have shown that almost every point between

SIR and CQI values when system had BLER of 10%, moved by

close to 3dB.

0 2 4 6 8 1 0 12 1 4 1 6 1 8 20 2 2 2 4 26 2 8 3 0

5

10

15

20

25

30

35

40

U p p e r P a r t

Y = 1 2 . 6 2 3 6 7 22 0 9 + 0 . 6 9 7 1 5 8 3 5 4 X

L o w e r P a r t

Y = 3 . 6 9 8 8 4 4 42 5 + 1 . 1 3 3 9 9 8 6 5 8 X

S I R

( d B )

C Q I

Con ven t i ona l S IR , Ec / I o r - 3dB

Con ven t i ona l S IR , Ec / I o r - 3dB L i nea r F i t o f Lower Pa r t

L i nea r F i t o f Uppe r Pa r t

Conven t i ona l S IR Es t ima t i on vs . CQI

Fig. 5. Conventional SIR to CQI mapping by linear fitting

0 2 4 6 8 1 0 12 1 4 1 6 18 20 2 2 24 2 6 2 8 30

5

10

15

20

25

30

35

40

Upper Pa r t

Y =14 .004686135+0 .640014549 X

Lower Pa r t

Y =5 .249450552+1 .07142637 X

Louay ' s Mod i f i ed S IR Es t ima t i on vs . CQI

S I R ( d

B )

C Q I

Louay 's Modi f ied SIR, Ec/ Ior -3dB

Louay 's Modi f ied SIR, Ec/ Ior -3dB

L i nea r F i t o f Lower P a r t

L i nea r F i t o f Uppe r P a r t

Fig. 6. Modified (Louay) SIR to CQI mapping by linear fitting

0 5 10 1 5 2 0 25 30

24

26

28

30

32

34

36

38

40

42

44

46

48Upper Pa r t

Y =23 .946519945+0 .634683004 X

Lower Pa r t

Y =25 .351344552 -0 .213147367 X+0 .04327612 X2

S

I R ( d

B )

C Q I

VB IP S IR Es t ima t i on , Ec / Io r - 3dB

VB IP S IR Es t ima t i on , Ec / Io r - 3dB 2nd O rde r F i t o f Lowe r Pa r t

L Inea r F i t o f Uppe r Pa r t

Ch ip -based VB IP S IR Es t ima t i on vs . CQI

Fig. 7. VBIP SIR to CQI mapping by 2nd order non-linear fitting

0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 2 2

1

10

1 00

S I R

C Q I

P A 3 - E c / I or - 3 d B

P B 3 - E c / I or - 3 d B

V A 3 0 - E c / I o r - 3 d B V A 1 2 0 - E c / I o r -3 d B

C Q I v s . C o n v e n t i o n a l S I R

Fig. 2. Conventional SIR vs. CQI at system BLER of 10%

0 2 4 6 8 10 12 14 1 6 1 8 20 22

1

10

10 0

S I R

C Q I

PA3 - Ec / Io r -3 d B

PB3 - Ec / Io r -3 d B

VA3 0 - Ec / Io r -3 d B

VA1 2 0 - Ec / Io r -3 d B

CQI v s . L o u a y S IR Es t ima t ion

Fig. 3. Modified (Louay) SIR vs. CQI at system BLER of 10%

0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 2 2

10

10 0

1 0 0 0

S

I R

C Q I

P A 3 - E c / Io r - 3 d B

P B 3 - E c / Io r - 3 d B

V A 3 0 - E c / Io r - 3 d B

V A 1 2 0 - E c / I o r -3 d B

C Q I v s . V B I P S I R E s t i m a t i o n

Fig. 4. VBIP SIR vs. CQI at system BLER of 10%



C. Simulation Verification through 3GPP Specification Requirement Testing

There are two types of tests that the UE CQI reporting must

satisfy. These two tests are designed by the 3GPP to confirm

the operation of CQI reporting. The first test is done on the

AWGN channel and the second test is done on the fading

channel. All the procedures of the test are written in [4]. Table

I, II and III are the CQI reporting test simulation results for the

three different SIR measurement methods. The CQI reportingcreated using conventional SIR and modified (Louay) SIR

measurements passed all the criteria of the 3GPP specification

CQI reporting tests. The CQI reporting created using VBIP SIR

passed the CQI reporting test fading channel conditions but

failed the AWGN channel conditions.

VI. CONCLUSION

In this paper, we proposed a method to generate CQI values

using SIR and the mapping procedure between them. In the

proposed method, the simulator in which all the physical layer

blocks are implemented is used to estimate the SIR at UE,report the CQI to Node B, and verify our SIR to CQI mapping

method in the exact HSDPA service environment. The SIR is

estimated by three different techniques, so the SIR to CQI

mapping table is created for three different SIR measurement

methods. With the proposed method, UE can report the optimal

CQI values which represent the exact downlink channel

conditions and the system can yield throughput exceeding the

requirements of the 3GPP specifications.

ACKNOWLEDGMENT

This research was supported in part by MIC (Ministry of

Information and Communication) & IITA (Institute for

Information Technology Advancement), Korea, through

TI-KAIST international joint program conducted by MMPC

(Mobile Media Platform Center) of KAIST

R EFERENCES

[1] 3GPP TS 25.101, V5.11.0, “3rd Generation Partnership Project;

Technical Specification Group Radio Access Network; User Equipment(UE) Radio Transmission and Reception (FDD)”.

[2] 3GPP TS 25.212, V5.9.0, “3rd Generation Partnership Project; Technical

Specification Group Radio Access Network; Multiplexing and ChannelCoding”.

[3] 3GPP TS 25.214, V5.9.0, “3rd Generation Partnership Project; Technical

Specification Group Radio Access Network; Physical Layer Procedures(FDD)”.

[4] 3GPP TS 34.121, V5.4.0, “3rd Generation Partnership Project; TechnicalSpecification Group Radio Access Network; Terminal Conformance

Specification; Radio Transmission and Reception”.[5] Jason Woodard and Rudolf Tanner, “WCDMA, Requirements and

Practical Design”, John Wiley & Soncs, 2004.

[6] Y. Li and Y.L. Guan, “Modified Jakes’ Model for simulating MultipleUncorrelated Fading Waveforms”, IEEE transactions onCommunications, 2000.

[7] L.C. Wang and C.W. Wang, “A Near Real-time Signal to InterferenceRatio Measurement Technique in A Frequency-Selective Multipath

Fading Channel for the WCDMA System”, VTC IEEE VTS 54th Vol.2, pp752-756, 2001.[8] Brouwer, F., de Bruin, I., Silva, J.C., Souto, N., Cercas, F. and Correia, A

, “Usage of link-level performance indicators for HSDPA network-level

simulations in E-UMTS”, IEEE Eighth International Symposium, pp844-848, 2004.

[9] S.K. Yong, J.S. Thompson and S. McLaughlin, “Implementation ofCOST 259 Channel Models Using Tapped Delay Line Model for Multiple

Antenna Receivers”, 3G Mobile Communication Technologies, May,2002.

[10] Jalloul, L.M.A., Kohimann, M., Medlock, J, “SIR estimation and

closed-loop power control for 3G”, VTC IEEE 58th Vol. 2, pp831-835,2003.

TABLE I3GPP CQI TESTING R EQUIREMENT R ESULT FOR CONVENTIONAL SIR

AWGN Fading

Test 1 Test 2 Test 3 Test 1 Test 2

Median 1841/2000 1827/2000 1842/2000 16 18

Median BLER 4.1% 3.6% 3.2% ~ ~

Median+2BLER

28% 27.7% 26.5% ~ ~

Median-1BLER

~ ~ ~ ~ ~

R1 event(Median-CQIBLER)

~ ~ ~ 0.7% 10%

R2 event

(Median-CQI+3 BLER)

~ ~ ~ 0% 0%

UE Pass/Fail PASSED PASSED PASSED PASSED PASSED

TABLE II3GPP CQITESTING R EQUIREMENT R ESULT FOR MODIFIED (LOUAY) SIR

AWGN Fading


Median 1894/2000 1903/2000 1896/2000 15 17

Median BLER 0.67% 0.5% 0.4% ~ ~

Median+2

BLER83% 81% 77% ~ ~

Median-1 BLER ~ ~ ~ ~ ~

R1 event(Median-CQI

BLER)

9% 6%

R2 event

(Median-CQI+3BLER)

~ ~ ~ 9% 0%

UE Pass/Fail PASSED PASSED PASSED PASSED PASSED

TABLEIII3GPP CQITESTING R EQUIREMENT R ESULT FOR VBIP SIR

AWGN Fading


Median 1710/2000 1691/2000 1745/2000 16 16

Median BLER 0.67% 0.5% 0.4% ~ ~

Median+2 BLER 83% 81% 77% ~ ~

Median-1 BLER ~ ~ ~ ~ ~

R1 event(Median-CQI

BLER)

0.06% 0.5%

R2 event

(Median-CQI+3BLER)

~ ~ ~ 0% 0%

UE Pass/Fail FAILED FAILED FAILED PASSED PASSED

a novel sir to channel-quality indicator (cqi)

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