International Journal of Engineering Trends and Technology (IJETT) – Volume 21 Number 3 – March 2015
Multi-Carrier CDMA Wireless
Communication System with Channel Deterioration
DMA
J.Sai Manikanta#1, B.Bindu madhavi*2, B.Nanda Kishore#2, K.Anil Chandra*4, R.Divya Kanthi#5
#*
#
Student, Department of ECE, Lendi Institute Of Engineering And Technology, Vizianagaram, India.
Assistant professor, ECE Department, Lendi Institute Of Engineering And Technology, Vizianagaram, India.
Abstract— In this paper, Analysis of the Bit Error Rate (BER)
will be carried out for a Multi Carrier CDMA wireless
communication link considering the effect of channels limitations
like fading, delay spread etc. When transmission over fading
channel multi-cell interference occurs and this degrades the
performance of the system. Multi Carrier Code Division Multiple
Access (MC-CDMA) is an attractive choice for high speed
wireless Communication. The expression of Bit Error Rate
(BER) is found with respect to number of users using bandwidth.
Performance degradations due to above system deterioration
will be evaluated and optimum system design parameters will be
determined.
Keywords—MC-CDMA;
BER;
fading;
deterioration; wireless communication;
delay
spread;
I. INTRODUCTION
Broadband wireless access for evolving mobile internet and
multimedia services are driving a surge of research on future
wireless communication systems, which have to be highly
spectral efficient in order to support multi-user access and
high data rates. Due to this growth of multimedia
communication, the users demanded high date rate
communication systems in wireless environment where the
spectral resource is scarce. Future technology must be able to
allow users to efficiently share common resources, whether it
involves the frequency spectrum, computing facilities,
databases, or storage facilities. With so many more users to
accommodate, more efficient use of bandwidth is a priority
among cellular phone system operators. Equally important is
the security and reliability of these calls. One solution that has
been offered is a Code Division Multiple Access system.
Multiple Access is a technique where many subscribers or
local stations can share the communication channel at the
same time or nearly so despite the fact originate from widely
different locations. The multi carrier modulation scheme is
easily adaptable to the multiuser environment to provide code
division multiplexing. In multicarrier CDMA signatures are
formed in the frequency domain, by controlling the amplitudes
and phases of subcarriers in a user-specific manner. A channel
can be defined as a portion of the limited radio resource,
which is temporarily allocated for a specific purpose or user,
such as someone’s phone call. Unique codes are used
distinguish different users using same frequency band is the
basic idea behind CDMA. Development of spreading codes
had drawn a great interest since inception of CDMA with an
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objective to increase in user capacity with low multiple access
interference (MAI).
A multiple access method is a definition of how the radio
spectrum is divided into channels and how the channels are
allocated to the many users of the system. One of the major
disadvantages of multicarrier systems is their high sensitivity
to frequency offsets. Frequency offsets occur due to frequency
mismatch between the carrier oscillators at the transmitter and
the receiver or due to mobility of the radio channel. In the
process, motivations for this new technique are discussed.
Next, implementation of this modulation scheme will be
discussed with a possible transmitter and receiver model. In
the receiver model, different frequency equalization
techniques are considered.
A. Basic Concept
In code division multiple access (CDMA) system, the
narrowband massage signal is multiplied by a very large
bandwidth signal called the spreading signal. Here multiple
number of users share the same channel bandwidth at the same
time through the use of (near) orthogonal spreading codes. So
it has potential to provide higher user capacity compared to its
other close competitors such as time division multiple access
(TDMA) and frequency division multiple access (FDMA). All
CDMA users use the same carrier frequency and may transmit
simultaneously. Each user has its own pseudorandom
codeword. The receiver performs a time correlation operation
to detect only the specific desired codeword. All other
codeword appear as noise. Each user operates independently
with no knowledge of the other users.
The near far problem occurs when many mobile users share
the same channel. In general, the strongest received mobile
signal will capture the demodulator at a base station. In
CDMA, stronger received signal levels raise the noise floor at
the base station demodulators for the weaker signals, thereby
decreasing the probability that weaker signals will be
received. To overcome this problem, power control is used.
Power control is provided by each base station in a cellular
system and assures that each mobile within the base station
coverage area provided the same signal level to the base
station receiver. In general, CDMA belongs to two basic
categories: synchronous (orthogonal codes) and asynchronous
(pseudorandom codes Code Division Multiplexing
(Synchronous CDMA)
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International Journal of Engineering Trends and Technology (IJETT) – Volume 21 Number 3 – March 2015
There are four ways to spread the bandwidth of the signal:
1. Frequency hopping
2. Time hopping
3. Direct sequence
4. Hybrid hoping
B. Multi Carrier Code Division Multiple Access
(MC-CDMA)
Multi Carrier Code Division Multiple Access (MC-CDMA)
is a relatively new concept. Its development aimed at
improved performance over multipath links. MC-CDMA is a
modulation method that uses multi carrier transmission of DSCDMA type signals. An MC-CDMA transmitter spreads the
original data stream in the frequency domain over different
sub carriers using a given spreading code. In this system the
sub carriers convey the same information at one time. The
MC-CDMA offers better frequency diversity to combat
frequency selective fading.
Fig. 2. MC-CDMA receiver
In fig.2 the MC-CDMA receiver is designed by the capacity
of i number of user. MC-CDMA receiver also receives the
transmitted signal as a summation of i number of users. At
first demodulates the received signal by the same career
frequency of each signal and then the signals multiply with the
specific codes given by the receiver code generator. Then we
get the signal of
user which is same for transmitter and
receiver. After that low pass filter remove the high frequencies
portion of the signal. Finally, the P/S converter presents the
actual digital data signal
.
II. OBJECTIVE OF THE THESIS
Work to be carried out for analysis of MC-CDMA system
with fading and interference. Different schemes of MCCDMA will be considered and performance result will be
evaluated. To evaluate the performance results in terms of Bit
Error Rate. To determine the optimum system parameters at a
given system BER.
Fig. 1. MC-CDMA transmitters
In fig.1 we see the transmitter MC-CDMA system for i
number of user. The MC-CDMA transmitter spreads the
original data stream using a given spreading code in the
frequency domain. The code generator creates different unique
codes for each different user and then combines together.
Then the frequency generator combines different carrier
frequency to the data signal and then combines the entire
signal together by a combiner. After combining all the signals
the CDMA antenna transmits the signals over the wireless
media.
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III. ANALYSIS OF CDMA
Here we discuss about the Bit Error Rate on the MC-CDMA
system. After combining the transmitted signal signals the
CDMA antenna transmits the signals over the wireless media.
In receiver side we get the all combing signal with some
unexpected signal which are MUI, ICI and Noise signal. So in
the receiver side after combining all sub-carrier signals we get
the received signal is
……………………………..[1]
Where,
= wanted signal;
= multi-user interference (due to imperfect restoration
of the sub-carrier amplitudes);
= inter-carrier interference (due to crosstalk ,
between and );
= noise;
We can write the wanted signal as,
…[2]
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International Journal of Engineering Trends and Technology (IJETT) – Volume 21 Number 3 – March 2015
Where,
N = number of subscriber,
n = subscriber number,
= crosstalk between the user.
= Sampling time.
= weight factors which is constant.
= orthogonal spreading codes
The variance of
became zero for large number of N, i.e.,
the system working like non fading channel.
The multi-user interference signal is
…………....[3]
We can write the
as,
……[4]
Where,
is the sets of orthogonal
code of the sub career index n
is the sets of orthogonal
code of the sub career index n
And
is the value of
So the variance of MUI,
….[5]
If we may assume that fading of the sub-carriers is
independent, we can write
…………....[6]
and,
………..[7]
Where
= variation of the signal power between of any two
subscriber.
The variance of the noise collected over all sub-carriers
weighted by
becomes,
………………….…………………..[13]
Now
………....[14]
Since we consider many different channels and
zero-mean complex Gaussian. So BER is,
B=
are
……………………………………………..[15]
IV. RESULTS AND DISCUSSION
Here, we take the number of subscriber N is 16; number of
chips per bit (code length) L is taken as 16. Bit rate
is
10000 bit per sample.
Noise power
has been taken 10 micro watts. We
take different Signal power in dB which is 10dB, 7dB, 5dB,
2dB, 0dB, -2dB, -5dB and -10dB.
The equation we use here to find,
…………………………………….…………..[16]
………………………..[17]
SNR=
………………………………………………....[18]
Here we take = 10% of ……………………………..[19]
Plots of Variance of multi-user interference versus number of
user is shown below.
= power of the signal.
= power of the noise signal.
After simplify all the equation we get the variance,
=
………………………..………..[8]
The ICI comes from the crosstalk between sub carriers. Intercarrier interference signal is,
….…………....[9]
Here,
Δ = distance of signal between two subscriber,
Now after putting
in the equation
……..[10]
So the variance of ICI,
……….[11]
After simplify the equation we get the variance
…………………………...………..[12]
Where
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Fig. 3 Plots of
versus number of user in MC-CDMA system
Fig.3 shows the plots of
versus number of user in MCCDMA system. This figure comes from equation [8]. We see
that if we increase the number of user then the interference
between different user increases. The variance of the multi
user interference depends on signal power. If we increase the
power then the interference increases gradually. Plots of
Variance of inter carrier interference versus number of user is
shown below.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 21 Number 3 – March 2015
Fig. 4. Plots of
versus number of user in MC-CDMA system
is found that BER decreases with respect to
for a
particular signal power. I signal power is very high than BER
decrease rapidly with respect to
.In below figure 11, if
we draw a axis with parallel to
from a particular BER
we see that the axis intersect the Po curve in 10dB, 7dB, 5dB
and 2dB.
From above graph we find the combinations are:
Db
dB
10
- 29.9 = -30 (approx)
7
-23.8 = - 24 (approx)
5
-19.8 = - 20 (approx)
2
-14.3 = -14 (approx)
0
-10.6= -11 (approx)
Plots of Bit Error Rate versus number of user shown below.
Fig.4 shows the plots of
versus number of user in MCCDMA system. This figure comes from equation [12]. We see
that if we increase the number of user then the variance of ICI
increase. The variance of the ICI also depends on the signal
power.Plots of
versus number of user is shown below.
Fig. 7 Plots of bit error rate versus number of user in MC-CDMA system
Fig. 5. Plots of
versus number of user in MC-CDMA system
Fig.5 shows the plots of
versus number of user in
MC-CDMA system. This figure comes from equation [14].
have an inverse relationship with no of subscriber. If
we increase number of user then
decreases. Plots of Bit
Error Rate versus
is shown below.
Fig.7 shows the plots Bit Error Rate versus number of user
in MC-CDMA system. Here we three different code length
which is 8bit , 16bit and 32 bit. It is show that the number of
user increases and at in wireless communication system and it
is essential to reduce higher the same time BER also increases.
From above graph we find the combinations are:
Code length (L)
Number of User (N)
8bit
0.8 = 1 (approx.)
16bit
1.2 =1 (approx)
32bit
12.3 = 12(approx.)
V. FUTURE SCOPE
1. Further research can be carried out on MC-CDMA
system considering the effect of fading and frequency offset
between the sub-carriers in the receiver.
2. Work can be carried out to find the improvement in
BER performance while using Rake Receiver to combat the
effect of fading and delay spread.
3. Works can be initiated to find optimum user code length
for a given number of users in a MC-CDMA system in
presence of channel effects.
Fig. 6. Plots of Bit Error Rate versus
in MC-CDMA system
Fig.6 shows the Plots of Bit Error Rate versus
in
MC-CDMA system. This figure comes from equation [15]. It
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VI. CONCLUSION
In this thesis paper we have used some basic equation to
find out our expected results. No new equation was deriving
on this thesis paper. We saw the performance result for Multiuser interference and Inter carrier interference for certain
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International Journal of Engineering Trends and Technology (IJETT) – Volume 21 Number 3 – March 2015
number of user. If we increase the user then the MUI and ICI
occurs more and more. By BER versus ratio of Energy of bit
and Noise density analysis we saw that if we increase the
value of
then the BER became low. In communication
system we cannot prefer high BER. If BER is low then the
channel can transfer the signal more perfectly We also saw
that how can the Code length effect the user capacity in the
system. For a particular accepted BER we can easily serve
more number of users if the code length is high.
. The major problem of MC-CDMA is Multi carrier
interference and inter carrier interference occur. Near far
problem and Multi-path fading also another disadvantage of
this system. We saw that in CDMA system due to code
difference between the users they can easily share the same
frequency. That’s why the capacity of serving user easily can
increase
ACKNOWLEDGMENT
We would like to thank and warmly acknowledge to our
supervisor for continuous encouragement, invaluable
supervision, timely suggestions and inspired guidance offered
by our guide R.Divya kanthi, Department of Electronics and
communication Engineering, Lendi institute of engineering
and technology, in the bringing this report to a successful
completion.
We also thankful to Mr R.V.Ch.Sekhar Rao, for their
inspiration and kind cooperation and guidance.
We are grateful to Dr. M.rajanbabu , Head of the
Department of Electronics and Communication Engineering
for permitting us to make use of the facilities available in the
department to carry out the project successfully
Finally we extend our gratefulness to one and all who are
directly or indirectly involved in the successful completion of
this project work.
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