A Hierarchical Power Control Scheme in CDMA Systems Zhisheng NIU, Xiang DUAN, Tao SHU, Junli ZHENG

State Key Lab on Microwave and Digital Communication Dept. of Electronic Engineering, Tsinghua University, Beijing, 100084, CHINA

niuzhs@ tsinghua.edu.cn, (xiang, shut, zhengil} @atm.mdc.tsinghua.edu.cn

Abstract Power control is one of the key technologies of resource management in CDMA systems to provide QoS guarantee to multimedia applications. In this paper we propose a hierarchical power control scheme for multimedia CDMA systems, where the power control process is divided into two steps, i.e., power adjustment, in a short span, and power scheduling, in a long span. In the power control scheme, we take the dependence between user parameters and user QoS constraints into consideration. Numerical results show that with the proposed power control scheme, user QoS can be guaranteed and better system resource utilization is achieved.

I. Introduction

Providing end-to-end quality-of-service (QoS) guarantees for multimedia applications is required in the emerging mobile multimedia systems. Due to the advantages in user capacity, code-division multiple access (CDMA) has been widely accepted as one of the major multiple access schemes for the 3‘d generation mobile communication systems. Since all users in a CDMA system communicate at the same time and same frequency, each user’s transmission power is regarded as a part of other users’ multiple access interference (MAI). Therefore, power control is considered to be one of the key technologies to guarantee user QoS, which aims at enhancing the soft capacity of CDMA systems by dynamically allocating the system resources. Due to the existence of MAI, power control in CDMA system should be a system-wide algorithm, i.e., react according to all users’ condition.

To date, power control in CDMA wireless systems has been extensively studied. In [I], authors proposed a power allocation scheme that satisfies user required data rate and received bit energy to interference ratio ( E , , / I ~ ). This scheme dynamically assigns the transmission power of each user at each power control point, according to the path loss of each user and the background noise received at the base station, together with user terminal’s transmission power limit. This scheme

can be applied in multi-rate wireless CDMA systems. Similar work is done in [2] and [3]. In [4], authors proposed a power control algorithm based on a convergence criterion for multi-rate traffic.

As we know, in wireless mobile channels, there are two kinds of fading: fast fading and slow Sadmg [5]. Fast fading, mainly caused by small Doppler shifts, makes the path loss fluctuate fast. Slow fading, a result from location dependent shadowing effects, has strong time dependence in fading depth. For combating fast fading in wireless channels, user’s transmission power is required to be adjusted at a fast speed, for example, 800 to 1600 times per second in WCDMA of IMT-2000 systems [6]. However, it is difficult for power control to optimally allocate resources to all users at such a fast speed, for resource allocation is a system-wide optimization problem that leads to high computational complexity and overhead. How to solve the conflict is an essential issue for power control in CDMA systems. Unfortunately, this problem is left untouched in the literatures mentioned above [ I -41.

In order to solve the probletn, we propose a hierarchical structured power control scheme. Power control process in CDMA systems is divided into two levels of operations, power adjustment and power scheduling, in order to combat the fast fading in wireless channels as well as to allocate system resources optimally. Thc remainder of this paper is organized as follows. Section I1 described the proposed power control scheme. The system model is studied in section 111. Section N presents the numerical result, and then we conclude the paper in section V.

11. Hierarchical Power Control Scheme for CDMA Systems

As mentioned above, due to the co-existence of fast fading and slow fading in wireless channcls, the conflict between power adjustment and resource allocation in wireless CDMA systems always exists. Power adjustment need to react fast to combat channel fast fading, while resource allocation scheme consumes more time and

overhead to calculate the system-wide optimal scheme. Therefore we propose a new power control scheme with a hierarchical control structure for CDMA systems. Here we consider the uplink, which is often considered as the limiting one [Sj. Our power control scheme i b divided into two steps: power adjustment and power scheduling. Power adjustment operates at each individual user, while power scheduling aims at the system-wide optimal rcsource utilization and operates on the base of all users. Fig.1 is illustrates the structure of the scheme. We will describe the scheme in detail as follows.

2.1 Power Adjustment

Power adjustment adjust.; user's transmission power in a fast speed in order to stabilize the received power to a predetennined value. By Fig. 1 we can see that, in power adjustment, each user terminal increases or decreases its transmission power according to its received power detected at the base station er . If the er is less than a predetermined value 4 , the transmission power is increased. Otherwise it is decreased. The target of power adjustment is stabilizing er to . Whether the transmission power should be increased or decreased can be indicated by one indicating bit carried by the downlink control channel (e.g., TPC bit in WCDMA DPDCH 161). Since each user terminal adjusts its transmission power only based on its own received power at the base station and regardless of other users, power

adjustment operation will not expense much time and overhead, so that it can react at a relatively fast speed to combat the fast fading.

Power adjustment does not take the issuc of optimally resource utilization into consideration. In our hierarchical power control scheme, power scheduling, as described below, deals with the issue of resources allocation.

2.2 Power Scheduling

Power scheduling is a centralized radio resource allocation (RRA) algorithm that optimally allocates the limited resources to users in CDMA systems. As illustrated in Fig. 1, it takes all users' conditions, including average path loss, inaximum transmission power, etc., into consideration, so that a system-wide optimization of resource allocation can be achieved. Power scheduling calculates t., the received power that each user terminal should stabilize to, as well as determines the each user's data rate and other parameters. These parameters are broadcasted to users, and are used by power adjustment operation as a baseline (predetermined value) until the next power scheduling point.

Due to the strong time dependence property of slow fading, if the effect of fast fading is partly counteracted by the operation of fast power adjustment, the frequency of power scheduling operation can be deterniined by the fading speed of slow fading. Therefore, power scheduling can

1

1 I

I

1 --

Pwr Schedul i n y minCP,/L, ST. QoSConstrainB

Powerconstrains

Base St a t i on

P w Adjuszioent N

If so, TPC=T P; < P,?

. __ - - . - . - . - ~ -: BS : Base Station UT : User Terminal R, : Date rate e : Predetermined power Data Dita

Packet Packet allocated by Pwr Scheduling

8' : Received power at BS TPC: Terminal Power Con-

Data Rate: R, ... . - , i Instructions ( f a s t )

& 1nstruut ;or ls [slow; ~ ~ 4 : PwrScheduliny Info

Fig 1. Structure of Hierarchical Power Control Scheme

579

(3) ( i = 1 , 2 , - . . , N ) perform much slower than power adjustment.

@b/'O 1, =-- Hence, sufficient reaction time enables power R, ZP, + I ' scheduling to collect information of all users, J#I

predict the average path loss, and calculate the with the power constraint of: optimal resource allocation scheme. It also keeps a 054 .C P ~ ~ ~ ~ L , , ( i = 1 , 2 , . . . , ~ ) (4) IOW overhead for the centralized power scheduling algorithm to broadcast user parameters information to all users.

where P;',, is user terminal's transmission power limits.

The hierarchical power control scheme in CDMA systems releases optimization process of RRA from fast power adjustment operation. Power adjustment can react very fast to combat the fast fading. Meanwhile, there is enough time for the power scheduling to calculate the system-wide optimal resource allocation scheme. By dividing power control into power adjustment and power scheduling, resource allocation optimization can be achieved while guaranteeing the users' respective QoS requirements.

111. System Model of the Hierarchical Power Control Scheme

As analyzed above, power adjustment is used to combat the channel fast fading, and power scheduling is to deal with the resource allocation issue. In the system model, we analyze the system-wide optimal resource allocation problem of power scheduling and take the joint effect of fast fading and fast power adjustment into consideration.

Consider a single cell with N mobile users, where 4 ( i = 1,2, ..., N ) is each user's predetermined value of received power at the base qtation. R, (i=1,2 ,..., N ) represents the data rate, and L, represents the path loss. Also let w be the occupied bandwidth of the system, and I be the background interference received at the base station, including thermal noise and adjacent cell interferences. Let p,,, represents the bit-error-rate (BER). pb, can be mapped from E ~ / I ~ function of j, (.), which is a technical related function and is dependent on antenna, modulation, coding, etc. The system that provides QoS to users is formulated as

where

And

fQ"S ( 4 , I R, 1 2 0 (1)

4, = f,(Eb/IO), (2)

Eq. (1) describes the constraint dependence between phi and R~ . The function f nos ( . ) is called the QoS dependence function, which defines the dependence between user QoS constraints and user transmission parameters. If Eq. (1) is satisfied, it indicates that user QoS constraint can be satisfied.

As an example, if the traffic is bursty and real-time, which is assumed to be represented by an ON-OFF source of parameters (p- ' ,L-' ,G) , and the interested user QoS constraint is packet loss probability due to packet delay constraint, the QoS dependence function of data rate and received E b / l o is [71

TbR , f,,, (Pb, R ) = (1- (1 - e-")(l- P,)" p- - Pd I O (3

where IZ is the packet length, q, is the delay bound of packets, and 6 is the packet loss probability constraint, and

d IZl* ____ a = G-R(l-P,)" R(l-Pb)"

By considering the dependence between user QoS constraint and user parameters (such as received power and Eb /I, ) together, power scheduling algorithm may have larger feasible resource allocation space. As a result, power scheduling algorithm can select user parameters more flexible and this, obviously, will benefit the system resource utilization.

In this model, if there exists any vector p = ( 4 , p *,... p,) and R = (e,p,,. ..pH) satisfying constraints (1) to (4), user QoS constraint can be satisfied. With the optimization subjective setting to minimizing users transmission power, the system model is

N

min q ~ , i=l

(7)

580

0 200 400 600 800 1000 1200 1400 1600 Power Adjustment Point (1/16OOs)

Description

Fig 2. Typical path loss niodel with the joint effect of fast fading and fast power adjustment

subject to constraints in Eqs. (1) to (4).

Traffic Type 1 1 11 I 111 Symbol

Now consider the path loss model. For channel loss L, caused by slow fading, we assume it log-nonnally distributed:

Due to the strong time dependence property of slow fading in wireless channels, L, can be viewed as unchanged during each power scheduling period (interva1 between two adjacent power scheduling points). Considering the joint effect of fast fading and fast power adjustment, we assume that the power adjustment has some deviations of controlling the received power. Hence, the received power eo at the base station does not perfectly equal the calculated value e and can be viewed as a log-uniform distribution:

log4 - N ( P , P Z ) (8)

loge:" - U ( l 0 g f : -A,,logI: + A , ) (9)

Data rate in ON states Pktlength

Fig. 2 provides a typical simulation model of the path loss with the consideration of the joint effect of power adjustment. Slow fading determines the average path loss of a relatively long period of time, which can be utilized by power scheduling. Power adjustment reacts fast to combat channel fast lading. Therefore path loss does not fluctuate drastically.

G (Kb/s) 13 56 144

II (bit) 32 32 128

IV. Numerical Results

w ( K w System occupied BW

In our simulation systems, there are 4 kinds of traffic, all of which are modeled as ON-OFF source. The traffic parameters of the traffic and other system parameters are listed in Table 1. The mapping of bit error rate and the received $/lo

are reflected by

3840

Parameter in Eq. (10)

Average time in ON states Average time in OFF states

1 p-' (s) I 0.4 1 0.2 ~ 8:; I xi (SI 0.6 0.8

time Pkt loss rate

Background 1.6 x lo-'

users

I 5 Std deviation

Parameter eo^ (9) < I

Parameter in

Power Adjustmcnt Frequency

Power Scheduling Frequency (time&) !

(10) which illustrates the fact that RER becomes smaller with the received E ~ / I ~ getting larger.

U" ( E d ' " ) e,, = f e ( 4 / 1 0 ) , = A . e -

Fig. 3 shows the control effect of the proposed hierarchical power control scheme for CDMA systems. Power adjustment alleviates the fast path loss fluctuations led by fast fading, while power scheduling determines user parameters in order to guarantee all users' QoS constraints as well as optimize resource utilization. From this figure we clearly see that, for each kinds of trafiic, user's QoS on packet loss rate is approximately guaranteed. Fig. 4 compares system performance on total transmission power of the system modcl with and without considering the dependence between user parameters and user constraints. It illustrates that if we consider the dependence in power scheduling scheme, the total transmission power is always less than that of not-considering the dependence. Less transmission power will obviously benefit system capacity and reduce the inter-cell interference.

581

0010 -. - _ _ _ - - - _- - __ -_ 0 10 20 30 40 50

r41 Power Scheduling Point (1150s)

Fig 3 Control effect of the hierarchical power control scheme

d Consideration of Dependence

[51

161

[71

0 5 1 , . , . , I , . , I , 0 10 20 30 40 50

[Sl Power Scheduling Point (1150s)

Fig 4. Compare of user transmission power with/without conaideration of dependence between user QoS constraints and

user parameters

V. Conclusion

In this paper, we propose a hierarchical power control scheme used for multimedia CDMA systems, by dividing the power control process into tow levels of operations, i.e., power adjubtment and power scheduling. Power adjustment reacts fast to combat the fast fading, while power scheduling operates relatively slow to calculate system-wide optimal resource allocation scheme. Furthermore, in the power control scheme, we consider the dependence between user parameters and user QoS constraints. Numerical results show that with the proposed power control scheme, user QoS can be guaranteed and better system resource utilization performance is achieved.

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