Performance Comparison of Power Control Methods That Use Neural Network and Fuzzy Inference System in CDMA

International Journal of Innovation Engineering and Science Research
Open Access
41|P a g eVolume 2 Issue 3 May 2018
ABSTRACT
Performance Comparison of Power Control
Methods That Use Neural Network and Fuzzy
Inference System in CDMA
Yalcin Isik
Silifke-Tasucu Vocational High School, Selcuk University
Mersin, Turkey
In the cellular communication applications of Code Division Multiple Access (CDMA) system, each user signal
can be received in the different power levels in the input of the base station due to different distances of the users.
In that case, signal of the user that is closer to the base station increases the communication errors of the far
users. To solve this problem, open or closed loop power control system is used to make each user signal equals
in the input of the base station. Power prediction and power control have been performed with various methods in
the literature. In this study, two different methods will be investigated and will be compared with each other in
terms of power control performance. The power at the output of the matched filter is predicted using neural
network and fuzzy inference system, power control is realized according to the predicted values.
Keywords—CDMA; Power Control; Neural Network; Fuzzy Inference System
I. INTRODUCTION
In a code division multiple access (CDMA) system, a number of users simultaneously transmit
information over a common channel, each user`s signal is assigned a different signature waveform,
and the received signal is the superposition of the signals transmitted by each user. In the receiver,
user data is obtained by multiplying the received signal with its own spreading code. In the system,
when the signal powers of the users are different, errors of the user data that has weak signal power
increase; this situation is called as the near-far problem. The power controlsystem is used to overcome
this problem. The power control system assures that all signal levels are the same in the input stage of
the base station.
The power control is performed as open-loop or closed-loop. In the open-loop control, the power of the
mobile user`s transmitter is adjusted by itself evaluating signal power that is transmitted from the base
station. In this method, path losses are assumed as the same from the base station to mobile user and
from the mobile user to the base station. However, open-loop control is not sufficient method, because
these losses can not be the same in the real application. In the closed-loop control, base station
detects the power level of the mobile user and transmits power control signal to the mobile user to
adjust its power. In this way, all the mobile user`s signals are received in the base station with the
same power level due to control signals that is transmitted from the base station system. However,
determination of the signal power of each mobile user in the base station is not easy process, because
all the user`s signals are combined as one CDMA signal. There are various studies about this subject
in the literature, but generally they consist of complex structures. A FIR filter was used for prediction of
the signal power in [1]. A predictive low pass filtering was used to improve power estimation in [2,3,4].
A fuzzy system was used to make power level estimation in [5]. However, an optimized neural network
was used for power prediction in [6], and Elman neural network were used to control the power in [7]
with the complex structures. Neural Network was used to predict the power from output of the matched
filter in [9] and Fuzzy Inference System was used to predict the power from output of the matched filter
in [10].

Yalcin Isik “International Journal of Innovation Engineering and Science Research”
Volume 2 Issue 3 May 2018 42|P a g e
In this study, Neural Network (NN) and Fuzzy Inference System (FIS) will be compared in terms of
power control performance in CDMA system. The power level prediction is realized with NN and FIS by
evaluating outputs of the matched filters, and the closed-loop power control was used to adjust the
power of the mobile user by small steps.
II. SYSTEM MODEL
We consider synchronous CDMA system with binary phase shift keying (BPSK) modulation in an
additive white Gaussian noise (AWGN) channel. Data for each user as random series in form of -1,+1
is generated and multiplied with its spreading code to obtain a CDMA signal. The CDMA signals of all
users and AWGN are added in the channel. At the receiver, the received CDMA signal with the K users
is multiplied with kth user signature waveform and integrated in one bit period to make an estimation
for kth user bit. The output of the kth matched filter in one symbol interval yk is given by
kjk
K
kj
j
jjkkk nbAbAy  



1
(1)
where bk is the input bit of the kth user (desired user), bk{-1, +1}, bj is the input bit of the jth user, Ak
is the received amplitude of the kth user, Aj is the received amplitude of the jth user, jk
is the cross-
correlation coefficient between desired user and the jth user, and nk is additive white Gaussian noise.
The second term in Eq.(1) is the multiple access interference (MAI) that is effect of the other active
users. In the base station, the power level of each user is determined with the output of the matched
filter as:
2
1
)( )(
1



M
j
jkk y
M
p (2)
Where pk is the power of the kth user, k is the user`s number and M is the number of the bits that are
considered in the calculation of the power. The power estimation and the control are done with the
outputs of the matched filters as in Fig. 1.
Fig. 1. The structure for the power estimation from the outputs of the matched filters and the control of the power level.
Channel
Output
r (t)
Matched Filter
(K.user)
Matched Filter
(2.user)
Matched Filter
(1.user) Neural
Network
or
Fuzzy
Inference
System
Power
Estimator
y1
y2
yK
P1
P2
PK
If Pk <Reference, increase Ak 1 step
If Pk >Reference, decrease Ak 1 step
Transmitter
Amplitude
Adjustment

III. ARTIFICIAL NEURAL NETWORKS
The neural networks are constructed with neurons that connected to each other. Each connection has
a weight factor and these weights are adjusted in a training process. There are many types of the
neural networks for various applications in the literature. A common used one of these is the
multilayered perceptrons (MLP) [8]. MLPs consist of input, hidden and output layers and they have
feedforward connections between neurons. Neurons in the input layer only act as buffers for
distributing the input signals to the neurons in the hidden layer. There are various activation functions
that are used in the neurons. The weights are changed with various learning algorithm for getting
proper output. The basic structure of the neural network with N input and one output is shown in Fig.
2. In this study, the Levenberg-Marquardt algorithm is used as the learning algorithm for the MLPs [8].
The basic structure of the neural network with N input, one output.
IV. FUZZY INFERENCE SYSTEM
Fuzzy Inference System (FIS) constitute the outputs by evaluating the inputsaccording to the rules
defined before.The structure of Fuzzy Inference System with r rules, n inputs and m outputs is shown
in Fig. 2. Fuzzy inference process is made of 5 parts. Adjustment of the inputs variables to the fuzzy
values, application of the fuzzy logic operands (AND or OR) to the adjusted values, obtaining the end
values from the initial values, evaluating of the obtained values according to the rules, and back
transformation from the fuzzy logic values.
In this study, Sugeno type FIS is used after the matched filter, the character of proper membership
function for FIS is determined by calculating the power of an user in equation 2. In this receiver;
Gaussian has been used for all of the input membership functions, triangle type has been used for
output membership functions, and VE operator has been used between the inputs. Rules are set as
depended on the power values which are calculated from equation 2 different power level matched
filter output, and are depended on the number of user. For 3 users, 16 rules are being applied and for
this reason 16 bit training data is sufficient. Output power for every user is determined according to
these 16 rules. These rules can be determined for 2 different power levels like this:
Rule(j): IF y1j and y2j and ...........yKj THENoutputk=Pi
where k=1,2,…,K (K number of users), j=1,2,…,J (J number of bit J=2x2K
), i=1 and 2, P consists of
output power and different power levels used in training for user.
Fig. 2. The structure of Fuzzy Inference System with n inputs, r rules and m outputs.
output
input1
input2
İnputN
Σ
Σ
Σ
Rule 1
Rule 2
Rule r
Output1
Output 2
Output m
Input 1
Input 2
Input n

V. SIMILATION RESULTS
In the simulation of the CDMA system, 31 bits length spreading codes that have normalized cross-
correlation 0.2258 between each other were used. Cross-correlation value is selected bigger to create
a more severe near-far environment. The simulations were done in the three users synchronous
AWGN channel.
In the neural network that is used in the system, the number of the input and hidden layer nodes were
chosen as the number of the users, and the number of output node was chosen as 1. The network is
a feed forward network and it was trained by Levenberg-Marquardt algorithm. In the hidden layer
tangent sigmoid activation function was used and in the output layer pure linear activation function
was used. The power estimation and the control were done only for the first user that is selected as
the desired user. The power control was performed to make the powers of the all users as 2 watt in
the input of the base station. The powers of the second and the third users were assumed as 2 watt
during the training and in the simulations. The training data that is produced for the three different
power level as 1,2 and 3 watt were used during the training. The 8 bits training data that is all the
possible combinations of three users were used without noise for each power level.
In the simulation, the performance of the power estimation and control of the neural network and fuzzy
inference system was examined in the synchronous AWGN channel with 10 dB and 25 dB signal to
noise ratios (SNR) of the first user. The SNR of the first user is defined as:
2
2
1
1
2
A
powernoise
powersignal
SNR  (3)
where 2 is the variance of the Gaussian noise with the zero mean value, A1 is the amplitude of the
first user`s signal and Ak is the amplitude of user k’s signal. All the simulations were done for the three
methods as mean of the squares of the output of the matched filter of the first user, the neural network
and fuzzy inference system estimator. The powers of the second and the third user were assumed as 2
watt whereas the power of the first user was assumed as 1 watt at the beginning. The power control
was performed to make the power of the first user 2 watt in the input of the base station. The amplitude
of the first user was changed by the 0.1 steps depending on estimated power level. The results for the
case that is considered 1 bit and 50 bits for each estimation are shown in Fig. 4 and Fig. 5,
respectively, for 100 different estimation in the channel with 10 dB SNR value. As it is shown, the
estimation performance of the neural network is better than mean squares method, and also fuzzy
inference system is much better than neural network. Power control can be done between 0 watt-5 watt
with mean squares, 1.5 watt-3 watt with neural network and 1.8 watt-2.8 watt with fuzzy inference
system by considering 1 data bit for each estimation in the channel with 10 SNR value.Furthermore, in
the case that is considered 50 bits for each estimation, power control performance gets better for all
methods. Fuzzy inference system has still the best performance. A better estimation can be done by
considering more values for the estimation, but in that case estimation time increases.
Furthermore, the case that is considered 50 bits for each estimationin the channel with 25 dB SNR
value is shown in Fig. 6.As it is shown, the power control depends on neural network and fuzzy
inference system estimator can be performed very good especially for the bigger SNR values.Fuzzy
inference system has the best performance again.

Fig. 3. The power control performance considering 1 bit data foreach estimation in the channel with 10 dB SNR value.

VI. CONCLUSION
The power level can be defined by calculating mean-squares of the values that are taken from the
output of the matched filter. However, the number of the considered values for the calculation of the
power level must be high to get more correct power level. But considering more values causes slow
power control. The power control must be fast enough for the effective communication. Simulations
results show that the fast power control can be performed with NN and FIS approach even in 1 bit
period. However, the estimation performance of the Fuzzy Inference System is better than Neural
Network. Performances of the NN and FIS estimators get better for the bigger SNR values. A better
power estimation can be done by considering more values for the estimation, but in that case
estimation time increases.
REFERENCES
[1] J.M.A. Tanskanen, A. Huang, T.I. Laakso,S.J.Ovaska, "Prediction of received signal power in CDMA cellular systems,"
Proc. of 45th IEEE Vehicular Technology , Chicago, Illinois,pp. 922–926, 1995.
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48th IEEE Vehicular Technology Conference, Ottawa, Ontario, Kanada, pp. 1091–1095, 1998.
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delay," IEEE Journal on Selected Areas in Communications, Vol.14 (9), pp. 1818-1829, 1996.
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optimal Neural-Network Structure," IEEE Transactıons on Neural Networks, Vol.8 (6), pp. 1446-1455, 1997.
[7] X.Z. Gao, S.J. Ovaska, A.V. Vasilakos, "A modified Elman neural network based power controller in mobile
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[10] Y. Işık,N. Taşpınar,"Power control by using Fuzzy Inference System in the CDMA systems,"URSI-Turkey, pp. 470-472,
8-10 September 2004.

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