Progress In Electromagnetics Research Symposium Proceedings, Cambridge, USA, July 5–8, 2010
101
Power Control and Diversity Performance Analysis in CDMA
Systems
Baso Maruddani1, 2 and Adit Kurniawan2
1
2
Electrical Engineering Department, State University of Jakarta, Indonesia
School of Electrical Engineering and Informatics, Bandung Institute of Technology, Indonesia
Abstract— In wireless communications systems, fading can degrade a system’s performance.
In this paper, the problem that will be observed is the increase of system’s performance when
the antenna diversity techniques combined with power control in CDMA communication system to overcome fading. This research simulates the performance of the system, which power
control algorithm is fixed step algorithm and combination of diversity is selective diversity combining. The simulation results show that combined power control and antenna diversity gives
more performance improvement than using power control only. In the cases where the user of
Doppler frequency is greater than 1/10 of the power control rate value, performance improvement also occurred, but the BER curves of Eb /I0 tend toward the direction of the channel fading
characteristics.
1. INTRODUCTION
CDMA as a wireless communication technique has some unique features such as high spectrum
efficiency, soft hand-off, low transmit power, and multipath mitigation. A system design description
for CDMA applied to digital cellular radio communications is described in [1], where a multiple
pseudonoise (PN) receiver is employed to demodulate the received signals.
The main problem in CDMA is multiple access interference because all users use the same
frequency at the same time and the PN code is not orthogonal. The distance between base station
and mobile station also affects the signal strength received by the base station. Users that were
located closer to the base station will be more dominating than the user who is relatively further.
This problem can be overcome by power control by the process so that the average received power
at base station is same for each user.
Power control to mitigate Rayleigh fading in CDMA system has been studied in [2]. And
evaluation about CDMA performance using diversity technique is shown in [3]. The purpose of
this research is to determine the performance improvement for system when use combined power
control and diversity technique on the receiver using 2-branch and 3-branch antenna.
In this research the problem that will be observed is the increase of system’s performance when
diversity techniques combined with power control to overcome the deep and shallow fading that
experienced by signals. The idea is the received signal that experience fading is overcome by
diversity technique then power control works to make fading shallower.
The simulation in this research using fixed step power control algorithm combined with selective
diversity algorithm. This paper will evaluate the performance about combined power control and
diversity technique with 2-branch antenna and with 3-branch antenna.
2. POWER CONTROL AND CHANNEL MODEL IN CDMA
Power control in CDMA can be classified into several categories. According to the algorithm used,
power control can be divided into open-loop power control, and closed-loop power control.
2.1. Open Loop Power Control
Near-far effect is a problem that occurs because the distance of each mobile station to base station
is not the same. Signals from each mobile station will experience different propagation attenuation,
depends on the distance between the mobile station to base station. Open-loop power control is
designed to ensure that the amount of power received from each user at the base station will be the
same (on average). In open-loop algorithm, the mobile station can calculate the required transmit
power by using the estimated downlink signal (not required feedback information). This is because
the large scale propagation attenuation is reciprocal on the uplink and downlink channels.
To overcome the problems of near-far effect is simply to say that the mobile station is located
far from the base station should transmit a signal with greater power than mobile stations that are
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closer to the base station. Signals sent by the mobile station must have the power of [1]:
Pt = −Pr + Poff + Pp
(1)
with:
Pt (dBm) = power to be emitted MS,
Pr (dBm) = received power at the MS,
Poff (dB) = parameter offset power,
Pp (dB) = power parameter adjustment
2.2. Closed Loop Power Control
Closed-loop power control aims to eliminate signal fluctuation due to small scale propagation attenuation. Unlike the large scale propagation attenuation, small-scale propagation attenuation on
the uplink and downlink do not have any correlation. Therefore, to control the uplink fading on the
uplink channel, the base station must estimate the information and then give the feedback to the
mobile stations, so the mobile stations can adjust the power emitted in accordance with feedback
information. To obtain the uplink channel information, the base station can estimate the SIR.
However, in CDMA system, power control based on SIR is preferable than power control based on
received signal level because in CDMA is interference limited [1].
Power Control based on the SIR is more preferable because SIR value shows BER value. Especially in uplink channel, power control based on SIR has ability to respond the change of interference
on base station for each user[4].
Figure 1. Closed-loop power control model [1].
2.3. Channel Model
Generally, the wireless channel can be modeled as a superposition of several paths with different
amplitudes and time delays which can be expressed as:
h(t) =
L−1
X
βl δ(t − τl )ejθl
(2)
l=0
where βl is the magnitude of the l−th path impulse response at time τl , δ(t − τl ) is an impulse at
t = τl , and θl is the phase shift of the l-th path. If the transmitted baseband signal of the user is
expressed as:
√
x(t) = 2P bs(t)
(3)
where 2P is the transmitted power, b is the transmitted symbol and s(t) is the binary waveform,
then the received signal can be expressed as:
r(t) = x(t) ∗ h(t) = h(t) =
L−1
X√
2P bβl s(t − τl )ejθl
l=0
(4)
Progress In Electromagnetics Research Symposium Proceedings, Cambridge, USA, July 5–8, 2010
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The received signal in (4) may experience fluctuations due to constructive and destructive interference from L paths and also will suffer from time spreading due to different arrival time instants
of L paths. When all signal paths have approximately the same time delay, the effect of delay
spread is not significant and the channel can be said as a frequency nonselective or flat fading,
known as a Rayleigh fading channel.
3. DIVERSITY TECHNIQUE
Diversity is one technique to exploit the same information from several independent and uncorrelated signal paths than can be resolved and combined by the diversity receiver. If the received paths
are correlated, no diversity gain can be attained. Generally, there are three diversity techniques;
time diversity, frequency diversity and space diversity. In time diversity, a signal path came with
the same information but arrived at a different time slot, then signals are combined. Arrival time
difference between signal paths with the other signals is uncorrelated, so the benefits from the use
of diversity can be obtained. In frequency diversity, frequency diversity gained obtained from a
signal path that came to bring the same information but using different frequencies carrier, and
then signal are combined. The separation of several different carrier frequencies must exceed the
bandwidth of the channel. In space or antenna diversity, several signal paths bearing the same
information that come from different antennas are combined. The separation between one antenna
and another must exceed the coherence distance of the channel.
In our simulation, we choose to use antenna diversity to evaluate the performance of CDMA
system when it is combined with power control. We will consider reception diversity at the base
station because we will use this technique in conjunction with power control on the uplink.
Figure 2. Simple model for antenna diversity.
Diversity combining in Fig. 2 can be selective diversity when the weight factor ws is selected to
the strongest signal xs (t), equal gain diversity when the weight factor w1 , w2 , . . . are set to unity,
or maximal ratio combining when the weight factor w is set proportional to the value of SIR.
To investigate the effect of diversity techniques on power control, we repeat our power control
simulation, but now we consider diversity channels at the base station. We simulate selective
diversity combining (SDC) algorithm with 2-elements antenna array and 3-elements antenna array.
With its simple algorithm, SDC is enough to alleviate the deep fade that experienced by signal.
SDC select only the strongest signal in its branch. We considered using 2-branch and 3-branch
antenna to see how significant this method to improve the performance compared with no diversity.
Our previous work just compares system with no diversity and system with 2-branch antenna
diversity [5].
4. SIMULATION AND RESULT
In this simulation, the assumption is the problems of near-far effect and shadowing already overcome
by open-loop power control so that the average received signal level by the mobile station is constant.
Open-loop power control algorithm is overcome the fluctuations caused by Rayleigh distributed flat
fading only. For closed loop power control, the process is based on SIR measurement on the base
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station. This research uses some parameter for simulation. The parameters that used can be seen
on Table 1 and the simulation result for fading channel can be seen on Fig. 3.
Figure 3(a) show the fading channel for Doppler frequency is 16.67 Hz and Fig. 3(b) shows the
fading channel for Doppler frequency 116.67. Doppler frequency depends on user’s velocity.
Simulation result for SIR can be seen on Fig. 4. For controlled SIR value that controlled by
power control only can be seen on Fig. 4(a). The variation between the highest SIR value and the
lowest SIR value is still wide. But when power control combined with diversity, the variation SIR
is getting narrowed. Controlled SIR value for 2-branch antenna and 3-branch antenna diversity can
be seen on Fig. 4(b) and Fig. 4(c) respectively.
Simulation result for diversity block can be seen on Fig. 5. This figure shows that selective
diversity combining algorithm works successfully to overcome fading, which this algorithm select
the strongest signal in each branch for each time slot. Fig. 5(a) shows the diversity block for
2-branch antenna and Fig. 5(b) shows the diversity block for 3-branch antenna.
Based on parameter simulation, we will investigate user which moves with velocity 10 km/h and
70 km/h. For user with velocity is 10 km/h, the simulation result can be seen in Fig. 6.
Figure 6 shows that using 3-antenna diversity can improve performance better than using 2antenna diversity or not using diversity. It’s because 3-antenna diversity is more efficient to eliminate deep fades. But the performance improves significantly from 1-antenna (not using diversity)
to 2-antenna diversity compared than from 2-antenna diversity to 3-antenna diversity. To achieve
BER of 10−3 , the required Eb /I0 for 3-antenna diversity system is 5 dB higher than for AWGN
channel. To achieve BER of 10−3 , the required Eb /I0 for 2-antenna diversity system is 5.5 dB higher
than for AWGN channel. And to achieve BER of 10−3 , the required Eb /I0 for no diversity system
Table 1. Simulation parameter.
Parameter
Carrier frequency
Number of user
Vehicle’s speed
Doppler frequencies
Symbol rate for QPSK
Chip rate
Power control period
Processing gain
Number of symbol/time slot
Symbol
fc
K
v
fd
Rs
Rc
Tp
M
B
Value
1.8 GHz
10 user
10, 40, and 70 km/h
17, 67, and 117 Hz
60 ksps
3,84 Mcps
0,667 ms
64
40
Simulasi Kanal Fading Kec 10 km/jam
Simulasi Kanal Fading Kec 70 km/jam
10
20
5
10
Level sinyal yang diterima (dB)
Level sinyal yang diterima (dB)
0
-5
-10
-15
-20
0
-10
-20
-30
-25
-40
-30
-50
-35
0
200
400
600
800
1000
1200
0
200
400
600
Waktu (ms)
Waktu (ms)
(a)
(b)
Figure 3. Fading channel simulation.
800
1000
1200
21
21
19
19
17
17
15
15
13
13
11
11
9
9
7
7
5
5
SIR (dB)
SIR (dB)
Progress In Electromagnetics Research Symposium Proceedings, Cambridge, USA, July 5–8, 2010
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1
105
3
1
-1
-1
-3
-3
-5
-5
-7
-7
-9
-9
-11
-11
Uncontrolled SIR
Controlled SIR (target 15 dB)
-13
0
100
200
300
400
500
600
700
800
900
Uncontrolled SIR
Controlled SIR (target 15 dB)
-13
-15
1000
-15
0
100
200
300
400
500
Waktu (ms)
Waktu (ms)
(a)
(b)
600
700
800
900
1000
21
19
17
15
13
11
9
SIR (dB)
7
5
3
1
-1
-3
-5
-7
-9
-11
Uncontrolled SIR
Controlled SIR (target 15 dB)
-13
-15
0
100
200
300
400
500
600
700
800
900
1000
Waktu (ms)
(c)
Figure 4. Controlled SIR simulation.
Sinyal Keluaran SDC 3 Antena
4
5
2
0
0
-5
-2
Level daya (dB)
Level daya (dB)
Sinyal Keluaran SDC 2 Antena
10
-10
-15
-20
-4
-6
-8
-25
-10
Sinyal Path 1
Sinyal Path 1
-30
Sinyal Path 2
-12
Sinyal Path 2
Sinyal Path 3
Level sinyal keluaran SDC 3 antena
Level sinyal keluaran SDC 2 antena
-35
0
50
100
150
200
250
300
-14
0
50
100
150
Waktu (ms)
Waktu (ms)
(a)
(b)
Figure 5. Diversity block output.
200
250
300
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Perfrmance on fDTp = 0,011
0
-1
-1
10
10
-2
-2
10
BER, Bit Error Rate
10
BER, Bit Error Rate
Performance on fDTp = 0,089
0
10
10
-3
10
-4
10
-3
10
-4
10
-5
-5
10
10
AWGN Channel
AWGN Channel
Fading Channel
2 Antena, SDC, Fixed-step
Fixed-step PC
3 Antena, SDC, Fixed-step
-6
10
-7
Fading Channel
-6
2 Antena, SDC, Fixed-step
10
3 Antena, SDC, Fixed-step
Fixed-step PC
-7
10
10
0
2
4
6
8
10
12
14
16
18
20
Eb/Io (dB)
Figure 6. BER Performance with fDTp = 0.011.
0
2
4
6
8
10
12
14
16
18
20
Eb/Io (dB)
Figure 7. BER Performance with fDTp = 0.078.
is 10.2 dB higher than for AWGN channel. User with velocity 10 km/h, the experienced fading is
slow fading. It makes the power control and antenna diversity is enough to handle the fading and
improve the performance.
For user with velocity is 70 km/h, the simulation result can be seen in Fig. 7. User with velocity
70 km/h experienced fast fading. In fast fading, diversity technique can eliminates deep fades but
power control cannot works perfectly because power control rate is too slow relative to the fading
rate. From figure, power control seems unable to improve the fading channel performance. However,
improvement can be seen when it combined with diversity technique.
5. CONCLUSION
Under ideal conditions, power control can turn the fading channel into an AWGN channel performance. Power control can improve the BER performance of slow fading channel. In fast fading
channel, power control failed to combat multipath fading. Diversity technique, on the other hand,
also can improve the performance.
From our simulation, in slow fading condition, using diversity can improves the performance
significantly and using 3-antenna diversity at receiver gives performance better than 2-antenna
diversity.
REFERENCES
1. Kurniawan, A., Predictive Power Control in CDMA Systems, Institute for Telecommunications
Research, University of South Australia, 2003.
2. Salmasi, A. and K. Gilhousen, “On the system design aspects of code division multiple access
(CDMA) applied to digital cellular and personal communications networks,” Proc. 41th IEEE
VTC Conf., 57–62, 1991.
3. Kurniawan, A., “Power control to combat rayleigh fading in wireless mobile communications
systems,” Proceedings of APT Workshop on MCMT 2002, Jakarta, October 2002.
4. Naghian, S., M. Rintamaki, and R. Baghaie, “Dynamic step size power control in UMTS,” Proc.
of the 13th. IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, Vol. 4, 1606–1610, 2002.
5. Kurniawan, A. and B. Maruddani, “Performance evaluation of the combined power control
and antenna diversity in CDMA systems,” Proceeding ICEEI 2007, Bandung, Indonesia, June
2007.