The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'06)

1-4244-0330-8/06/$20.002006 IEEE

Radio Link Protocol Based SCH Power Control in cdma2000

Jian Gu Xiangguang Che Giridhar D. Mandyam Beijing University of Posts and Telecom Nokia Research Center Nokia Mobile Phone

No 10 Xitucheng Road No 11 He Ping Li Dong Jie 6000 Connection Drive Beijing 100876 P.R.China Beijing 100013 P. R. China Irving, Texas 75039 USA e-mail: jian_gu@163.com e-mail: xiangguang.che@nokia.com e-mail: giridhar.mandyam@nokia.com

ABSTRACT Fast closed-loop power control is widely used in code division multiple access (CDMA) systems to ensure a minimum received signal quality for circuit-switched services, foremost among which is voice. However, third-generation wireless systems such as cdma2000 often accommodate multiple concurrent services with different quality-of-service constraints. Nevertheless, typical closed loop power control methods still are biased towards only one service. In the cdma2000 reverse link, concurrent services may be accommodated over code-multiplexed channels (namely the fundamental channel FCH and supplemental channel SCH). This paper provides a method for optimizing power control for the SCH in the presence of an FCH when a radio link protocol (RLP) is used. This method achieves less transmission power in most of cases, as well as less RLP delay.

I. INTRODUCTION

cdma2000, a 3G technology, offers circuit switched voice and packet data services in wireless environment. Its Fundamental CHannel (FCH) carries voice, signaling and low-speed data while its Supplemental CHannel (SCH) supports medium and high-speed packet data services. Fast closed loop power control is an important feature of CDMA systems to overcome the negative effects caused by slow fading and partial negative effects caused by fast fading. Fast closed loop power control comprises inner and outer loop power control. Outer loop power control sets the SIR (signal-to-interference ratio) target, while the inner loop power control determines the command of increasing or decreasing the transmit power according to the values of the SIR target and the measured SIR. In data transmission with active FCH and SCH in reverse link, outer loop power control is normally driven by the reception of FCH alone. That is to say, the decoding of Reverse Fundamental CHannel determines the SIR target that, together with the measured SIR, determines power control commands. The power control commands directly control the power of Reverse FCH and Reverse Pilot Channel, as well as indirectly the power of Reverse SCH through fixed traffic-to-pilot-power ratios (T/P) for the reverse link [1].

Radio Link protocol (RLP) is a selective repeat request, negative

acknowledgment (NAK) based retransmission mechanism in media access control (MAC) layer of cdma2000 system. It retransmits the lost RLP frames for a limited number of times to alleviate high frame error rate in wireless links. In RLP Type 3, the number of NAK rounds and the number of retransmissions in each round are variable [2].

Most relevant research work mainly focuses on power control with the consideration of physical layer performance. For instance, an RLP based power control to reduce the number of RLP transmissions through increasing SIR target at RLP retransmissions is proposed in [3]. But the method assumes that SCH drives outer loop power control. Furthermore, a systematic way for setting an additional SIR target is not presented in [3].

The paper provides a power control method for the SCH by adaptively adjusting T/P of SCH based on RLP NAK feedback and RLP NAK round. This method achieves the optimal T/P of the SCH in an adaptive manner. This adaptive method better meets the QoS requirements of data services carried by SCH.

This paper is organized as follows. Problems of the fixed T/P of SCH are analyzed and a new adaptive method to adjust the T/P of SCH is proposed in section II. At the same the parameters of the proposed method is optimized on basis of theoretical analysis. Numerical results are presented in section IV to show the new method’s advantages. Finally section V concludes the paper.

II. PROBLEMS AND SOLUTION

A. Problems The current solution, adjusting SCH power through fixed T/P of SCH, is not the optimal solution in three aspects, i.e., different error correcting capabilities in FCH and SCH, different physical layer frame error rate (FER) requirements in FCH and SCH, and different QoS requirements of data services carried by SCH.

The FCH and SCH are received independently and use different error-correcting channel codes. A turbo code used by SCH has a more powerful error-correcting capability than the convolutional code used by the FCH. With the current system, the power for the FCH is well controlled, but the corresponding power for the SCH may be too large or too small in different scenarios.

The SCH has a looser physical layer FER requirement than

The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'06)

FCH. This is due to the type of service that usually runs over the SCH (data traffic) and over the FCH (circuit-switched voice). Typically, the SCH requires a FER of 5%, while FCH requires 1%. The outer loop power control driven by FCH converges too slowly, especially for the variable-rate SCH.

SCH carries data services with different QoS requirements, including different delay and error rate requirements. Different values of RLP parameters are applied to different services. Delay sensitive services usually use a small number of NAK rounds and achieve medium residual FER, while delay tolerant services usually use a large number of NAK rounds and achieve low residual FER. Fixed T/P value of SCH, ignoring RLP, is different to accurately achieve the required QoS.

B. Solution The paper proposes a mechanism in a transmitter that operates like outer loop power control in a receiver. The proposed method adjusts T/P with correcting factor (CF) according to NAK feedback and NAK round counter.

Whenever a new RLP frame is sent out, the CF of T/P ratio, 1+iCF , is updated at the time of i +1 according to

),11

max( min1

1 CFFER

CFCF ii −−=+

δ (1)

where iCF is the CF of T/P ratio at the time of i +1, δ is the step parameter, the parameter minCF is the minimum allowable CF of T/P ratios and the parameter 1FER is the required target FER of the initial RLP frame.

Whenever the NAK of the initial RLP frame is received at the time of i , it is updated at the time of i +1 according to

),min( max1 CFCFCF ii δ+=+ (2)

where maxCF is the maximum allowable CF of T/P ratio.

If the packet to be transmitted does not contain the RLP retransmission frames at the time of i +1, the T/P ratio (in dB),

PT / , is given by

( ){ }min_)1(max_)1(1)1( /,/,/minmax/ ++++ += iririir PTPTCFPTPT

(3) where )1( +ir is the data rate selected by packet scheduler at the time of i +1, and

min_)1(/ +irPT and max_)1(/ +irPT , respectively,

are the minimum and maximum allowable T/P ratio (in dB) for the data rate of )1( +ir .

If the packet to be transmitted contains the RLP packet(s) of the nr1

th, nr2th, …, nrm

th NAK round at the time of i +1, the T/P ratio (in dB), PT / , is given by

( )( )

+∆++

=+

+++

min_)1(

max_)1(1)1(

/

,/,)1(/minmax/

ir

irnriir

PT

PTirCFPTPT

(4) where

i

m

inrnr

1max

== , ( ))1( +∆ irnr

is the additional power for the

data rate of )1( +ir in the nrth NAK round relative to initial RLP transmission.

Like outer loop power control, the proposed solution can help RLP frame error probability in the first transmission converge to

1FER , because an CF up jump is, on the average, followed by average 1/1 1 −FER CF down jumps.

III. OPTIMAL PARAMETERS

A. Configurations When initial correcting factor, ∆ and ( ))1( +∆ irnr

are zeros, the proposed method is the same as the current solution. This section provides optimal parameters for the proposed solution. The analysis is based on the following assumptions. The physical layer FER, )0(EbNfer , is assumed to be [4]

<

=−

otherwise00

00if1)0(

λ

init

init

EbNEbN

EbNEbNEbNfer (5)

where 0EbN and initEbN0 are, respectively, the received linear

Eb/N0 and the linear Eb/N0 for the inflection point of the FER curve, and λ is the slope of the FER curve. The FER curve is illustrated in fig. 1. The typical value of λ is 15~45 for all data rates in cdma2000 system in different environments[4]. The second assumption is that all RLP transmissions of an RLP protocol data unit (PDU) are transmitted with the same physical layer data rate, i.e., the same λ and

initEbN0 .

10*l og10( EbN0 i ni t )

10*l

og10

(fer

)

10*l og10( Eb/ N0)

i ni t i alt r ansmi ssi on

Fig. 1 the physical layer FER curve

The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'06)

According to the upper bound of turbo code over exponentially correlated Rayleigh channel in [6] and the simulation results over Rayleigh channel in [5], the assumption of broken-line FER is reasonable for turbo codes in the FER range of (10-3,1).

The required total aggregate linear Eb/N0 is given by

∑=

∆+NR

nrnrnr FERNnr

11

10/1 10 γγ (6)

where 1γ is the received Eb/N0 of the initial RLP frame, 1FER is the FER of the initial RLP frame, iFER is the FER after i-1 RLP rounds, NR is the maximum number of NAK rounds transmitted by RLP, i.e., NUM_ROUNDS in [2], and Nnr is the number of NAKs to send in the nrth NAK round, i.e., NAK_COUNT[nr] in [2].

nr∆ is the additional Eb/N0 (in dB) required for the nrth RLP round relative to initial RLP transmission.

In order to minimize transmission power, the optimal parameters can be expressed as

+ ∑=

∆

∆∆

NR

nrnrnr

FERFERNnr

NR 11

10/1

,...,10minarg

11

γγ

subject to

λ

γ−

=

initEbNFER

01

1, (7)

1

110/

2

11

010 FEREbN

FERN

init

λγ

−∆

= , (8)

…

1

110/ 1

1

010

−

−∆ −−

= NR

N

initNR FER

EbNFER

NRNR

λγ , (9)

resFERFEREbN

FER NR

N

initNR

NRNR

=

=

−∆

+

λγ

010 1

10/

1, (10)

where the constant resFER is the required residual FER after all RLP retransmissions. The optimization of parameters is a Lagrange problem.

The value of T/P affects the performance of the coherent receiver, but the curves of T/P Ratio vs. the required Eb/N0 for specified FER are very flat around the optimal points. The loss is less than 0.2dB when T/P is ±2dB around the optimal point [7]. The effect of adaptive T/P to the performance of coherent receiver is very small, as long as

min_)1(/ +irPT and max_)1(/ +irPT

are well set.

B. Optimal configuration Optimal values of

1∆ , 2∆ ,

3∆ and 1FER are shown in fig 2 for the case of resFER =10-6, NR=3, N1=1, N2=2 and N3=3. In this case,

1∆ is nearly 0.1 dB. 2∆ is -0.25~-0.15 dB.

2∆ is -0.45~-0.35 dB. So

T/P in initial RLP transmission and the first NAK round need be larger than that in the rest NAK rounds. 1FER also decreases from 6% to 2%, when λ increases from 15 to 45. For most of the cases, 1FER is smaller than 5%. That is to say, the proposed method brings smaller RLP delay than the conventional method in most cases. At the same time, the proposed method requires less average transmission power than the conventional method because it minimizes average transmission power.

In all, optimal values of NR∆∆ ,,1 … are small, thus they do not

enlarge the dynamic range of T/P a lot. Optimal values of NR∆∆ ,,1 … are relatively fixed for different λ , so

NR∆∆ ,,1 … is, in practical, suitable for all data rates. Higher data rates requires smaller 1FER , since λ increases with data rate in the same environment [4][5].

IV. SIMULATION RESULTS

The simulations are conducted for cdma2000 1x [1]. The selected data rates are 19.2, 76.8 and 307.2 kbps. In cdma2000, power control frequency is 800Hz. Inner loop power control step is 0.5dB. Power control bit error rate of 4% is modeled. The up and down step of outer loop power control step are, respectively, 0.5dB and 0.5/99dB. The working frequency is 2GHz. The T/P values for concurrent services in [1] are 5.75, 10.875 and 17.25dB for 19.2, 76.8 and 307.2 kbps, respectively. The new method uses the fixed T/P values as the initial values. The dynamic ranges T/P are [3.75, 7.75], [8.875, 12.875] and [15.25, 19.25] dB for 19.2, 76.8 and 307.2 kbps, respectively. The step, δ , equals 0.3dB. The 5 types of fast fading channels, Mode A, B, C, D and E, are modeled according to [8]. Table 1 gives some details of fading channels. The simulations finish until 50 residual erroneous frames are collected or 5×106 new RLP frames are transmitted.

The residual FER after retransmissions in radio link error control protocol and the average transmission energy per PDU are two important performance indices.

In this case of NR=3, N1=1, N2=2 and N3=3. The required resFER is 10-6.

1∆ =0.1, 2∆ =-0.2,

3∆ =-0.4 and 1FER =0.05. The

residual erroneous PDUs resort to TCP retransmissions.

The average transmission energy per PDU is shown in fig. 3. The proposed method brings the gains of 0.95 and 1.6dB for 307.2kbps on channels D and E, respectively. The propose method brings -0.09~0.57dB gain in other cases. Small negative gains appear in some cases of 76.8 and 307.2kbps. The reason behind the negative gains is that 0.05 is not an optimal

1FER in some cases, especially high λ cases (i.e., the cases of high data rates). For a mixed environment with 30% mobile stations on Channel A, 30% on B, 20% on C, 10% on D and 10% on E [8],

The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'06)

the proposed method brings 0.35, 0.19 and 0.16 dB gain in transmission energy per PDU for 19.2, 76.8 and 307.2 kbps, respectively. The residual FERs of both methods on channel A, B and C are not shown in table 2, because they are smaller than 10-6 and almost of them are 0. From the residual FER shown in table 2, the conventional method cannot achieve the required residual FER on channel D and E. The residual FERs are quite high for 76.8 and 307.2kbps. But the proposed method can achieve much smaller residual FERs. Although the residual FERs of 76.8 and 307.2kbps on channel E are above 10-6, they are still acceptable.

V. CONCLUSIONS

This paper provides a power control for data channel (SCH in cdma2000) in presence of voice channel (FCH in cdma2000) based on radio link protocol (RLP). This method adaptively adjusts traffic-to-pilot power ratio of SCH according to RLP information. The optimal configuration is also achieved for this method. This method achieves less transmission power in most of cases, as well as less RLP delay.

REFERENCES [1] Physical Layer Standard for cdma2000 Spread Spectrum Systems, 3GPP2

standard, Release 0, Version 3.0, June 15, 2001. [2] Data Service Options for Spread Spectrum Systems: Radio Link Protocol

Type 3, 3GPP2 standard, Version 1.0, C.S0017-010-A, June 11, 2004. [3] G. D. Mandyam, "Power control based on radio link protocol in

cdma2000", in Proc. IEEE Wireless Com. and Networking Conf., pp.1368 - 1372 vol.3, 21-24 Sept. 1999.

[4] J. Gu, S. Nie, D. Wang, and X. Che, "Optimal HARQ procedures for Reverse Link", in proc. 15th IEEE Int. Symposium on Personal, Indoor and Mobile Radio Com., pp. 875 - 879 vol.2, Sept. 2004.

[5] 3GPP2, "Final R-SCH Short Term FER Curves", C30, November 20, 2002, available at ftp.3gpp2.org.

[6] E. K. Hall and S. G. Wilson, "Design and analysis of turbo codes on Rayleigh fading channels", IEEE J. Selected Area in Com., Vol.16, Feb 1998.

[7] S. Sarkar, T. Chen, G. Leung, L. Blessent, and E. Tiedemann, "cdma2000 reverse link: design and system performance", in Proc. 52nd IEEE Vehicular Tech. Conf., pp.2713 - 2719 vol.6 Sept. 2000.

[8] cdma2000 Evaluation Methodology, 3GPP2 standard C.R1002-0, version 1.0, Jan. 2005.

Table 1 parameters of fading channel models Channel Model

Multi-path model

Number of paths

Speed (km/h)

Fading

Model A Pedestrian A 1 3 Jakes Model B Pedestrian B 3 10 Jakes Model C Vehicular A 2 30 Jakes Model D Pedestrian A 1 120 Jakes Model E Single path 1 0.81 Rician

Factor=10

Table 2 the residual FERs on channels D and E Methods 19.2kbps 76.8kbps 307.2kbps

Conv. 3.00e-6 0.00359 0.201 Channel D New 0 0 0

Conv. 8.69e-5 0.0325 0.108 Channel E New 8.00e-6 2.76e-5 7.41e-5

λ

λ

∆∆∆

Fig 2. Optimal

1∆ , 2∆ ,

3∆ and 1FER for NR=3

1 1.5 2 2 .5 3 3.5 4 4 .5 515

20

25

30

cha nnel mod es

aver

age

tran

smis

sion

ene

rgy

per

PD

U

c onventional,19.2k bpsc onventional,76.8k bpsc onventional,307.2kbpsnew,19.2kbpsnew,76.8kbpsnew,307.2 kbps

AA CA DB E

Fig. 3 the average transmission energy per PDU