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International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012
DOI : 10.5121/ijcseit.2012.2103 23
Effect of Interleaved FEC Code on Wavelet Based
MC-CDMA System with Alamouti STBC in
Different Modulation Schemes
Rifat Ara Shams1
, M. Hasnat Kabir1
, Sheikh Enayet Ullah2
1
Department of Information and Communication Engineering, University of Rajshahi
Rajshahi-6205, Bangladesh.
swarna601@gmail.com
2
Department of Applied Physics and Electronic Engineering, University of Rajshahi
Rajshahi-6205, Bangladesh.
ABSTRACT
In this paper, the impact of Forward Error Correction (FEC) code namely Trellis code with interleaver on
the performance of wavelet based MC-CDMA wireless communication system with the implementation of
Alamouti antenna diversity scheme has been investigated in terms of Bit Error Rate (BER) as a function of
Signal-to-Noise Ratio (SNR) per bit. Simulation of the system under proposed study has been done in M-ary
modulation schemes (MPSK, MQAM and DPSK) over AWGN and Rayleigh fading channel incorporating
Walsh Hadamard code as orthogonal spreading code to discriminate the message signal for individual
user. It is observed via computer simulation that the performance of the interleaved coded based proposed
system outperforms than that of the uncoded system in all modulation schemes over Rayleigh fading
channel.
KEYWORDS
DWT, MC-CDMA, Trellis Code, Alamouti, AWGN, Rayleigh Fading
1. INTRODUCTION
The most important objectives in the 4th
generation wireless mobile communication systems are
to enable the integration of existing technologies in a unified platform, to take care of the severe
Inter-Symbol Interference (ISI), to provide high speed data transmission rate in a spectrally
efficient manner with utilizing the available limited bandwidth and to achieve low cost and
reduced complexity receivers and their signal processing solutions for high quality
communication [1-2]. Multicarrier techniques on multi-path fading environment can be one of the
best solutions to achieve the above performance. Due to the limitations of Orthogonal Frequency
Division Multiplexing (OFDM) and Code Division Multiple Access (CDMA), Multi-Carrier
Code Division Multiple Access (MC-CDMA) which is a multiple access scheme generated by the
combination of OFDM and CDMA has received much attention [2]. Each user symbol in the
frequency domain is spreaded by the MC-CDMA. In other word, each user symbol is phase
shifted according to a code value and carried over multiple parallel sub-carriers. However, the
code values differ per sub-carrier and per user. All sub-carrier signals are merged at the receiver
and undo the code shift. Then the receiver separates signals of different users. So, the network can
accommodate many users within a given frequency band [1,3]. It has many other advantages like
robustness to channel dispersion, high frequency spectrum efficiency and high data transmission
International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012
24
[2]. In Discrete Fourier Transform (DFT) based conventional MC-CDMA, the sub-carrier
orthogonality and Synchronization are very sensitive to signal phase offset and frequency errors
which will cause severe performance degradation due to the inter-channel interference (ICI),
inter-symbol interference (ISI) and multiple access interference (MAI) [4,5]. For better
performance, a number of improved MC-CDMA systems have been proposed. Among them,
wavelet based MC-CDMA attracts much interests due to its powerful ability to combat multi-path
interference (MPI) and ISI than conventional MC-CDMA [5]. Wavelet based MC-CDMA avoids
the influence of delayed waves and eliminates the ISI by ensuring smaller sub channel bandwidth
than the channel coherence bandwidth and by using a guard interval in multi-path environment
[6]. In our previous work, the performance of the Wavelet based MC-CDMA system has been
investigated using antenna diversity such as Alamouti STBC scheme [6] according to the
suggestion of Salih M. [7]. Therefore, it is required to judge the performance of the system after
implementing the Forward Error Correction (FEC) Code
In this paper, we propose a wavelet based MC-CDMA system using Forward Error Correction
(FEC) with interleaving in different modulation schemes on fading environment. We consider
error control coding technique to compensate the performance loss due to narrower bandwidth
caused by multi-carrier technique. The reason for choosing FEC is to reduce the receiver
complexity, eliminate the MAI effectively and give the receiver an ability to correct errors
without needing a reverse channel to request retransmission of data [8]. Nevertheless, in this
system, interleave can increase the performance by arranging data in a non-contiguous way.
Interleave spreads the source bits out in time. Therefore, the important bits from a block of source
data are not corrupted due to noise burst [9].
2. FORWARD ERROR CORRECTION USING TRELLIS CODE
In telecommunication, information theory and coding theory, forward error correction (FEC) is a
technique used for controlling errors in data transmission caused by corruption from the
unreliable or noisy communication channels, whereby the sender adds systematically generated
redundant data to its messages, also known as an error-correcting code (ECC). The American
mathematician Richard Hamming pioneered this field in the 1940s and invented the first error-
correcting code, the Hamming code in 1950 [10].
Automatic repeat request (ARQ) and FEC mechanisms are well-known and widely used error
detection and correction mechanisms for data transmission. In ARQ mechanism, if the sender
fails to receive an acknowledgement before the timeout, it usually retransmits the lost packets to
the receiver until the sender receives the explicit retransmission request from the receiver within
certain expiration time or exceeds a predefined number of retransmissions. This mechanism is bad
for multicast and is impracticable because of the retransmission delay [11]. In contrast to the
ARQ mechanism, the FEC mechanism is considered more suitable for error correction. In a
communication system that employs FEC, the information source sends a data sequence to an
encoder and the encoder inserts redundant bits using a predetermined algorithm. This process
generates a longer sequence of code bits, called a codeword, and such codeword is then
transmitted to a receiver, which uses a suitable decoder to retrieve the original data sequence
either during the process of transmission or on storage [12].
International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012
25
Figure 1: Block diagram of the operation of Forward Error Correction (FEC)
As shown in figure 1, the FEC encodes k packets with (n-k) redundant packets to form a FEC
block with the n packets at the sender. Then the FEC can tolerate the loss of (n-k) packets in
networks and recover the k packets with the help of the decoder from the FEC block at the
receiver. It is advantageous that codes which introduce a large number of redundancy convey
relatively little information per individual code bit, because it reduces the probability that all of
the original data will be wiped out during transmission [10]. FEC has other advantages like it
avoids multicast problems, sometimes no feedback channel is necessary, has long delay path and
is one-way transmission. But a major disadvantage of it is the increase in channel bandwidth.
Trellis coded modulation (TCM) is a combined binary convolution codes of rates (r, r+1) and
modulation technique which introduces forward error correction coding without increasing the
bandwidth of the channel signal. TCM, invented by Gottfried Ungerboeck in 1970s [13], is highly
efficient technique because it provides high coding gain and little descending data rate by coding
and error correction.
3. ALAMOUTI STBC SCHEME
Space time block coding (STBC) is a technique used in wireless communications to transmit
multiple copies of a data stream simultaneously across different transmit antennas and to exploit
the various received versions of the data to improve the transmission reliability, to achieve a full
diversity order and to enjoy the simple maximum likelihood decoding [14-15]. For scattering
environment the STBC is a practical, effective and widely applied technique for mitigating the
effect of multipath fading because the transmitted signal must traverse a difficult environment
with scattering, reflection, refraction and so on, and may then be further corrupted by thermal
noise in the receiver means that some of the received copies of the data will be better than others,
then STBC combines all the copies of the received signal in an optimal way to extract as much
information from each of them as possible [14].
The Alamouti code, suggested by Mr. Siavash M Alamouti in his landmark October 1998 paper,
is the only STBC that can achieve its full diversity gain using two transmit antennas and one
receive antenna without needing to sacrifice its data rate and can provide the same diversity order
as maximum-ratio receiver combining (MRRC) with two antennas at the receiver [16].
International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012
26
Figure 2: Conceptual block diagram of Alamouti STBC scheme
Figure 2 shows a wireless communication system with two transmit antennas and a single receive
antenna. The transmitted symbols from the two transmitter antennas arrive at the receiver via two
different channels and the discrete baseband received signal can be written as
y = h1x1 + h2x2 + n
where y is the received signal, h1 and h2 are the respective channel coefficients from the
transmitter antenna 1 and 2 to the receive antenna, x1 and x2 are two corresponding transmitted
symbols from these two antennas and n is a circularly symmetric complex Gaussian noise with
variance [17].
Let us consider, each set of two transmitted symbols spans over two consecutive time slots. In the
first time slot, the received signal is,
In the second time slot, the received signal is,
Where y1, y2 is the received symbol on the first and second time slot respectively and n1, n2 is the
noise on first and second time slots respectively.
For convenience, the above equation can be represented in matrix notation as follows:
Let us define
To solve for , , we know that we need to find the inverse of H.
For a general m x n matrix, the pseudo inverse is defined as,
International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012
27
The term,
Since this is a diagonal matrix, the inverse is just the inverse of the diagonal elements, i.e.
The estimate of the transmitted symbol is,
The bit error rate is,
Where,
Since (HH
H) is a diagonal matrix, hence there is no cross talk between x1, x2 after the equalizer
[16].
4. SYSTEM MODEL
We consider, the wavelet based MC-CDMA transmitter includes M-branches and each of
them consists of an up-sampler followed by a synthesis filter. The impulse response of this
filter is derived from the wavelet orthogonal condition which generates a specific wavelet pulse.
Here we propose conventional simulated wavelet based MC-CDMA system with interleaved
forward error correction and implementation of Alamouti diversity scheme over AWGN and
Rayleigh fading channel using different modulation techniques which is depicted in Figure 3.
International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012
28
Figure 3: Block diagram of proposed wavelet based MC-CDMA system with implementation of
FEC and Alamouti STBC scheme.
The proposed system with Nc subcarriers and K users, all with the same spreading factor G, is
considered. Each user is assumed to have similar encoding, modulation and spreading. User data
is frequency-division multiplexed over different orthogonal subcarriers, whereas a separate
signature code sequence based on pseudo noise codes is assigned to each user to enable user
separation. Here the synthetically generated binary bit stream for four users are fed into encoder
to produce the required sequence of binary output vectors and then interleaver to obtain time
diversity in a digital communications system without adding any overhead. Its output bits are
mapped to a stream of coded symbols and the data are then converted into complex symbols
using digital modulation and these symbols are converted into serial to parallel form and fed into
copier section and multiplied with assigned individual Walsh Hadamard code. The coded symbols
are fed into inverse discrete wavelet transformation whose output is a series of wavelet
coefficients. Then the output is fed into the Alamouti space time block encoder whose output is
then passed through the channel. Next the inverse operations are performed to retrieve the
original data source signal at the receiver. Here the bit error rate (BER) performance as a function
of signal-to-noise ratio (SNR) is examined.
Where σx
2
is the mean square of the baseband signal and σe
2
is the mean square difference
between the original and reconstructed signals.
5. SIMULATION RESULTS AND DISCUSSION
In our study, a simulation for MC-CDMA based on DWT has been made by using MATLAB 7.5.
The developed program provides graphical display of various waveforms generated by the model.
It calculates the bit error rate (BER) and also capable for simultaneous representation of both
transmitted and received message signals. The simulation parameters are shown in Table 1. In
different modulation schemes the bit error probability performance of DWT based MC-CDMA
system is compared over AWGN channel. Moreover, the bit error probability performance of
coded DWT based MC-CDMA system is determined and compared with the respective uncoded
Data
source
Mapp-
ing
Modula
tion
S/P Symbol
repe-
tition
Encod-
ing
Inter-
leaving
Walsh code
spreading
IDWT
Alamouti
STBC
Channel
DWT
Walsh
code de-
spreading
Symbol
repe-
tition
P/SDemod
uation
De-
mapping
Deinterl
eaving
DecodingRetrieved
data
source
International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012
29
system under the fading environment namely Rayleigh fading channel in different modulation
schemes.
Table 1: The parameters of the simulation model
Parameter Parameter Values
User 4
No. of bits used for synthetic data 10,000
SNR 0-10 db
Mother wavelet Daubechies
Modulation and Demodulation BPSK, QPSK,4QAM, DBPSK
Spreading code Walsh-Hadamard Code
FEC code Trellis code
Wireless channel AWGN channel, Rayleigh fading channel
Processing gain 8
Figure 4 shows the comparison of the performance of different modulation schemes (BPSK,
QPSK, 4QAM and DBPSK) for wavelet based MC-CDMA system in the presence of Additive
White Gaussian Noise with implementation of Alamouti antenna diversity scheme for wide range
of SNR from 0dB to 10dB. From the figure it is seen that the bit error rate (BER) is inversely
proportional to the signal-to-noise ratio (SNR) as expected. At low SNR, BER reduction rate is
small but with the increase of SNR the rate of BER reduction increases dramatically. In
comparison with other modulation scheme, 4QAM shows much better BER performance than
that of others in this system. At 2 dB SNR, the BER value of 4 QAM is approximately 5, 8 and 20
times lower than BPSK, DBPSK and QPSK, respectively.
0 1 2 3 4 5 6 7 8 9 10
10
-6
10
-5
10
-4
10
-3
10
-2
Signal to Noise ratio, EbNo(dB)
BER
MISO MCCDMA using BPSK over AWGN
MISO MCCDMA using QPSK over AWGN
MISO MCCDMA using 4QAM over AWGN
MISO MCCDMA using DBPSK over AWGN
Figure 4: Comparison of different modulation schemes for Wavelet based MC-CDMA over
AWGN channel.
International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012
30
To improve the performance of the proposed wireless communication system, to achieve the
desired bit error probability and to minimize the burst error rate, interleaved FEC code can be
used. The performance of interleaved trellis coded and uncoded wavelet based MC-CDMA
system over Rayleigh fading channel using BPSK, QPSK, 4QAM and DBPSK modulation
schemes is presented by figure 5, 6, 7 and 8 respectively.
0 1 2 3 4 5 6 7 8 9 10
10
-3
10
-2
10
-1
Signal to Noise ratio, EbNo(dB)
BER
MISO DWT based MCCDMA using BPSK over Rayleigh(uncoded)
MISO DWT based MCCDMA using BPSK over Rayleigh(coded)
Figure 5: Comparison of coded and uncoded Wavelet based MC-CDMA using BPSK modulation
scheme over Rayleigh fading channel.
From figure 5, it is seen that interleaved convolutional coding in discrete wavelet transform
(DWT) based MC-CDMA with deployment of Alamouti diversity scheme using BPSK
modulation can give better performance under Rayleigh fading channel. Contrast to uncoded
DWT based system, coded one shows approximately 1 order of magnitude better BER
performance at a typical SNR value of 10dB. Here, it is remarkable that the BER value of coded
system decreases linearly with the increase of SNR.
0 1 2 3 4 5 6 7 8 9 10
10
-3
10
-2
10
-1
Signal to Noise ratio, EbNo(dB)
BER
MISO DWT based MCCDMA using QPSK over Rayleigh(uncoded)
MISO DWT based MCCDMA using QPSK over Rayleigh(coded)
Figure 6: Comparison of coded and uncoded Wavelet based MC-CDMA using QPSK modulation
scheme over Rayleigh fading channel.
International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012
31
The simulation result in figure 6 shows that interleaved coded DWT based MC-CDMA using
QPSK modulation over Rayleigh fading channel has better BER performance than uncoded one
as expected. For proposed coded communication system, when SNR is low, the rate of BER
reduction is small but with the increase of SNR the BER reduction rate becomes high. At 3 dB
SNR, the gain is roughly calculated which is almost 7 dB in the present system. On the other
hand, it is not so significant al lower SNR.
0 1 2 3 4 5 6 7 8 9 10
10
-3
10
-2
10
-1
Signal to Noise ratio, EbNo(dB)
BER
MISO DWT based MCCDMA using 4QAM over Rayleigh(uncoded)
MISO DWT based MCCDMA using 4QAM over Rayleigh(coded)
Figure 7: Comparison of coded and uncoded Wavelet based MC-CDMA using 4QAM
modulation scheme over Rayleigh fading channel.
Figure 7 shows that, the coded proposed communication system using 4QAM modulation scheme
gives better BER performance in contrast to uncoded system as expected. The rate of BER
reduction for coded system is higher than that of uncoded one. The bit error probability 0.001 is
obtained for coded proposed system at 10 dB SNR where as it is 0.01 for uncoded system at the
same SNR. By comparing figure 4 and 7, it is found that the BER value of 4QAM modulation
scheme is much lower in AWGN channel in contrast to Rayleigh fading channel.
0 1 2 3 4 5 6 7 8 9 10
10
-3
10
-2
10
-1
Signal to Noise ratio, EbNo(dB)
BER
MISO DWT based MCCDMA using DBPSK over Rayleigh(uncoded)
MISO DWT based MCCDMA using DBPSK over Rayleigh(coded)
Figure 8: Comparison of coded and uncoded Wavelet based MC-CDMA using DBPSK
modulation scheme over Rayleigh fading channel.
International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012
32
It is also seen from figure 8 that the BER performance of interleaved coded system using DBPSK
modulation scheme is much better than that of uncoded one. A desirable gain which is slightly
less than 9dB can be achieved from this proposed model using DBPSK.
6. CONCLUSION
In this paper the performance of wavelet based MC-CDMA system with Alamouti STBC using
different modulation schemes over AWGN and Rayleigh fading channel has been studied. An
approach has been presented that successfully applies FEC codes in the system to combat against
the noise in channels and to improve the performance in contrast to uncoded system. It is seen
that interleaved FEC coded wavelet based MC-CDMA system has better performance than that of
uncoded system in fading channel irrespective of modulation schemes. From simulation results it
is observed that in AWGN channel 4QAM modulation scheme has the best performance among
others. On the other hand, in Rayleigh fading channel BPSK modulation technique provides the
best performance among others though 4QAM has almost the same performance as BPSK in
fading environment.
REFERENCES
[1] Yee N., Linnartz J-P and Fettwis G., (1993) "Multicarrier CDMA in indoor wireless radio networks",
Proc. of IEEE PIMRC, Yokohama, Japan, pp. 109-113.
[2] Hara, S. and Prasad, R., (1997) "An overview of multicarrier CDMA", IEEE Commun. Mag., 35, pp.
126-133.
[3] Fazel K. and Papke L., (1993) "On the performance of convolutionally-coded CDMA/OFDM for
mobile communication system", Proc. of IEEE PIMRC, Yokohama, Japan, pp. 468–472.
[4] Muayyadi A. and Abu-Rgheff M.A., (2003) "Wavelet based multicarrier CDMA system and its
corresponding multi-user detection", IEEE Proc.-Commun., Vol. 150, No. 6.
[5] Xiangbin Yu, Xiaodong Zhang and Guangguo Bi, (2004) "Performance of complex wavelet packet
based multi-carrier CDMA system with precoding technique", 10th Asis-Pacific Conference on
Communications and 5th International Symposium on Multi-Dimensional Mobile Communications,
IEEE.
[6] Matiqul Islam Md., Hasnat Kabir M. and Enayet Ullah Sk., (2011) "Performance analysis of wavelet
based MC-CDMA system with implementation of various antenna diversity schemes", International
Journal of Research and Reviews in Computer Science (IJRRCS), Vol. 2, No. 2.
[7] Salih M. Salih (2009) “Two-Dimensional Wavelet Transform Model for MC-CDMA”, IEEE 9781-
4244-3941-6/09.
[8] Clark, George C., Jr.; Cain, J. Bibb (1981). Error-Correction Coding for Digital Communications.
New York: Plenum Press. ISBN 0-306-40615-2.
[9] Golam Rashed Md., Hasnat Kabir M., Selim Reza Md., Matiqul Islam Md., Rifat Ara Shams, Saleh
Masum, Sheikh Enayet Ullah, (2011) “Transmission of Voice Signal: BER Performance Analysis of
Different FEC Schemes Based OFDM System over Various Channels”, International Journal of
Advanced Science and Technology Vol. 34.
[10] http://en.wikipedia.org/wiki/Forward_error_correction
[11] Ming-Fong Tsai, Ce-Kuen Shieh, Senior Member, IEEE, Tzu-Chi Huang, and Der-Jiunn Deng,
Member, IEEE, (2011) "Forward-Looking Forward Error Correction Mechanism for Video Streaming
over Wireless Networks", IEEE SYSTEMS JOURNAL, DOI: 10.1109/JSYST.2011.2165595
International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012
33
[12] Charles Wang, Dean Sklar, Diana Johnson (Winter 2001/2002), "Forward Error-Correction Coding",
The Aerospace Corporation magazine of advances in aerospace technology 3 (1).
[13] Ungerboeck G., (1982) “Channel coding with multilevel phase signals” IEEE Trans. Inform. Theory,
vol. IT-28, pp. 55-67.
[14] Gerard J. Foschini and Michael. J. Gans (1998), "On limits of wireless communications in a fading
environment when using multiple antennas", Wireless Personal Communications Vol. 6, No. 3, pp.
311–335
[15] Tarokh, V., Jafarkhani, H., And Calderbank, A.R., (1999) “Space-time block codes from orthogonal
designs”, IEEE Trans. Inf. Theory , Vol. 45, pp. 1456–1467
[16] Siavash M. Alamouti, (1998) "A simple transmit diversity technique for wireless communications",
IEEE journal on select areas in communications, Vol. 16, No. 8, pp. 1451-1458.
[17] Lin Zhou, Jian-Kang Zhang, Member, IEEE, and Kon Max Wong, Fellow, IEEE, (2007) "A Novel
Signaling Scheme for Blind Unique Identification of Alamouti Space-Time Block-Coded Channel",
IEEE Transactions On Signal Processing, Vol. 55, No. 6.

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Effect of Interleaved FEC Code on Wavelet Based MC-CDMA System with Alamouti STBC in Different Modulation Schemes

  • 1. International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012 DOI : 10.5121/ijcseit.2012.2103 23 Effect of Interleaved FEC Code on Wavelet Based MC-CDMA System with Alamouti STBC in Different Modulation Schemes Rifat Ara Shams1 , M. Hasnat Kabir1 , Sheikh Enayet Ullah2 1 Department of Information and Communication Engineering, University of Rajshahi Rajshahi-6205, Bangladesh. swarna601@gmail.com 2 Department of Applied Physics and Electronic Engineering, University of Rajshahi Rajshahi-6205, Bangladesh. ABSTRACT In this paper, the impact of Forward Error Correction (FEC) code namely Trellis code with interleaver on the performance of wavelet based MC-CDMA wireless communication system with the implementation of Alamouti antenna diversity scheme has been investigated in terms of Bit Error Rate (BER) as a function of Signal-to-Noise Ratio (SNR) per bit. Simulation of the system under proposed study has been done in M-ary modulation schemes (MPSK, MQAM and DPSK) over AWGN and Rayleigh fading channel incorporating Walsh Hadamard code as orthogonal spreading code to discriminate the message signal for individual user. It is observed via computer simulation that the performance of the interleaved coded based proposed system outperforms than that of the uncoded system in all modulation schemes over Rayleigh fading channel. KEYWORDS DWT, MC-CDMA, Trellis Code, Alamouti, AWGN, Rayleigh Fading 1. INTRODUCTION The most important objectives in the 4th generation wireless mobile communication systems are to enable the integration of existing technologies in a unified platform, to take care of the severe Inter-Symbol Interference (ISI), to provide high speed data transmission rate in a spectrally efficient manner with utilizing the available limited bandwidth and to achieve low cost and reduced complexity receivers and their signal processing solutions for high quality communication [1-2]. Multicarrier techniques on multi-path fading environment can be one of the best solutions to achieve the above performance. Due to the limitations of Orthogonal Frequency Division Multiplexing (OFDM) and Code Division Multiple Access (CDMA), Multi-Carrier Code Division Multiple Access (MC-CDMA) which is a multiple access scheme generated by the combination of OFDM and CDMA has received much attention [2]. Each user symbol in the frequency domain is spreaded by the MC-CDMA. In other word, each user symbol is phase shifted according to a code value and carried over multiple parallel sub-carriers. However, the code values differ per sub-carrier and per user. All sub-carrier signals are merged at the receiver and undo the code shift. Then the receiver separates signals of different users. So, the network can accommodate many users within a given frequency band [1,3]. It has many other advantages like robustness to channel dispersion, high frequency spectrum efficiency and high data transmission
  • 2. International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012 24 [2]. In Discrete Fourier Transform (DFT) based conventional MC-CDMA, the sub-carrier orthogonality and Synchronization are very sensitive to signal phase offset and frequency errors which will cause severe performance degradation due to the inter-channel interference (ICI), inter-symbol interference (ISI) and multiple access interference (MAI) [4,5]. For better performance, a number of improved MC-CDMA systems have been proposed. Among them, wavelet based MC-CDMA attracts much interests due to its powerful ability to combat multi-path interference (MPI) and ISI than conventional MC-CDMA [5]. Wavelet based MC-CDMA avoids the influence of delayed waves and eliminates the ISI by ensuring smaller sub channel bandwidth than the channel coherence bandwidth and by using a guard interval in multi-path environment [6]. In our previous work, the performance of the Wavelet based MC-CDMA system has been investigated using antenna diversity such as Alamouti STBC scheme [6] according to the suggestion of Salih M. [7]. Therefore, it is required to judge the performance of the system after implementing the Forward Error Correction (FEC) Code In this paper, we propose a wavelet based MC-CDMA system using Forward Error Correction (FEC) with interleaving in different modulation schemes on fading environment. We consider error control coding technique to compensate the performance loss due to narrower bandwidth caused by multi-carrier technique. The reason for choosing FEC is to reduce the receiver complexity, eliminate the MAI effectively and give the receiver an ability to correct errors without needing a reverse channel to request retransmission of data [8]. Nevertheless, in this system, interleave can increase the performance by arranging data in a non-contiguous way. Interleave spreads the source bits out in time. Therefore, the important bits from a block of source data are not corrupted due to noise burst [9]. 2. FORWARD ERROR CORRECTION USING TRELLIS CODE In telecommunication, information theory and coding theory, forward error correction (FEC) is a technique used for controlling errors in data transmission caused by corruption from the unreliable or noisy communication channels, whereby the sender adds systematically generated redundant data to its messages, also known as an error-correcting code (ECC). The American mathematician Richard Hamming pioneered this field in the 1940s and invented the first error- correcting code, the Hamming code in 1950 [10]. Automatic repeat request (ARQ) and FEC mechanisms are well-known and widely used error detection and correction mechanisms for data transmission. In ARQ mechanism, if the sender fails to receive an acknowledgement before the timeout, it usually retransmits the lost packets to the receiver until the sender receives the explicit retransmission request from the receiver within certain expiration time or exceeds a predefined number of retransmissions. This mechanism is bad for multicast and is impracticable because of the retransmission delay [11]. In contrast to the ARQ mechanism, the FEC mechanism is considered more suitable for error correction. In a communication system that employs FEC, the information source sends a data sequence to an encoder and the encoder inserts redundant bits using a predetermined algorithm. This process generates a longer sequence of code bits, called a codeword, and such codeword is then transmitted to a receiver, which uses a suitable decoder to retrieve the original data sequence either during the process of transmission or on storage [12].
  • 3. International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012 25 Figure 1: Block diagram of the operation of Forward Error Correction (FEC) As shown in figure 1, the FEC encodes k packets with (n-k) redundant packets to form a FEC block with the n packets at the sender. Then the FEC can tolerate the loss of (n-k) packets in networks and recover the k packets with the help of the decoder from the FEC block at the receiver. It is advantageous that codes which introduce a large number of redundancy convey relatively little information per individual code bit, because it reduces the probability that all of the original data will be wiped out during transmission [10]. FEC has other advantages like it avoids multicast problems, sometimes no feedback channel is necessary, has long delay path and is one-way transmission. But a major disadvantage of it is the increase in channel bandwidth. Trellis coded modulation (TCM) is a combined binary convolution codes of rates (r, r+1) and modulation technique which introduces forward error correction coding without increasing the bandwidth of the channel signal. TCM, invented by Gottfried Ungerboeck in 1970s [13], is highly efficient technique because it provides high coding gain and little descending data rate by coding and error correction. 3. ALAMOUTI STBC SCHEME Space time block coding (STBC) is a technique used in wireless communications to transmit multiple copies of a data stream simultaneously across different transmit antennas and to exploit the various received versions of the data to improve the transmission reliability, to achieve a full diversity order and to enjoy the simple maximum likelihood decoding [14-15]. For scattering environment the STBC is a practical, effective and widely applied technique for mitigating the effect of multipath fading because the transmitted signal must traverse a difficult environment with scattering, reflection, refraction and so on, and may then be further corrupted by thermal noise in the receiver means that some of the received copies of the data will be better than others, then STBC combines all the copies of the received signal in an optimal way to extract as much information from each of them as possible [14]. The Alamouti code, suggested by Mr. Siavash M Alamouti in his landmark October 1998 paper, is the only STBC that can achieve its full diversity gain using two transmit antennas and one receive antenna without needing to sacrifice its data rate and can provide the same diversity order as maximum-ratio receiver combining (MRRC) with two antennas at the receiver [16].
  • 4. International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012 26 Figure 2: Conceptual block diagram of Alamouti STBC scheme Figure 2 shows a wireless communication system with two transmit antennas and a single receive antenna. The transmitted symbols from the two transmitter antennas arrive at the receiver via two different channels and the discrete baseband received signal can be written as y = h1x1 + h2x2 + n where y is the received signal, h1 and h2 are the respective channel coefficients from the transmitter antenna 1 and 2 to the receive antenna, x1 and x2 are two corresponding transmitted symbols from these two antennas and n is a circularly symmetric complex Gaussian noise with variance [17]. Let us consider, each set of two transmitted symbols spans over two consecutive time slots. In the first time slot, the received signal is, In the second time slot, the received signal is, Where y1, y2 is the received symbol on the first and second time slot respectively and n1, n2 is the noise on first and second time slots respectively. For convenience, the above equation can be represented in matrix notation as follows: Let us define To solve for , , we know that we need to find the inverse of H. For a general m x n matrix, the pseudo inverse is defined as,
  • 5. International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012 27 The term, Since this is a diagonal matrix, the inverse is just the inverse of the diagonal elements, i.e. The estimate of the transmitted symbol is, The bit error rate is, Where, Since (HH H) is a diagonal matrix, hence there is no cross talk between x1, x2 after the equalizer [16]. 4. SYSTEM MODEL We consider, the wavelet based MC-CDMA transmitter includes M-branches and each of them consists of an up-sampler followed by a synthesis filter. The impulse response of this filter is derived from the wavelet orthogonal condition which generates a specific wavelet pulse. Here we propose conventional simulated wavelet based MC-CDMA system with interleaved forward error correction and implementation of Alamouti diversity scheme over AWGN and Rayleigh fading channel using different modulation techniques which is depicted in Figure 3.
  • 6. International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012 28 Figure 3: Block diagram of proposed wavelet based MC-CDMA system with implementation of FEC and Alamouti STBC scheme. The proposed system with Nc subcarriers and K users, all with the same spreading factor G, is considered. Each user is assumed to have similar encoding, modulation and spreading. User data is frequency-division multiplexed over different orthogonal subcarriers, whereas a separate signature code sequence based on pseudo noise codes is assigned to each user to enable user separation. Here the synthetically generated binary bit stream for four users are fed into encoder to produce the required sequence of binary output vectors and then interleaver to obtain time diversity in a digital communications system without adding any overhead. Its output bits are mapped to a stream of coded symbols and the data are then converted into complex symbols using digital modulation and these symbols are converted into serial to parallel form and fed into copier section and multiplied with assigned individual Walsh Hadamard code. The coded symbols are fed into inverse discrete wavelet transformation whose output is a series of wavelet coefficients. Then the output is fed into the Alamouti space time block encoder whose output is then passed through the channel. Next the inverse operations are performed to retrieve the original data source signal at the receiver. Here the bit error rate (BER) performance as a function of signal-to-noise ratio (SNR) is examined. Where σx 2 is the mean square of the baseband signal and σe 2 is the mean square difference between the original and reconstructed signals. 5. SIMULATION RESULTS AND DISCUSSION In our study, a simulation for MC-CDMA based on DWT has been made by using MATLAB 7.5. The developed program provides graphical display of various waveforms generated by the model. It calculates the bit error rate (BER) and also capable for simultaneous representation of both transmitted and received message signals. The simulation parameters are shown in Table 1. In different modulation schemes the bit error probability performance of DWT based MC-CDMA system is compared over AWGN channel. Moreover, the bit error probability performance of coded DWT based MC-CDMA system is determined and compared with the respective uncoded Data source Mapp- ing Modula tion S/P Symbol repe- tition Encod- ing Inter- leaving Walsh code spreading IDWT Alamouti STBC Channel DWT Walsh code de- spreading Symbol repe- tition P/SDemod uation De- mapping Deinterl eaving DecodingRetrieved data source
  • 7. International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012 29 system under the fading environment namely Rayleigh fading channel in different modulation schemes. Table 1: The parameters of the simulation model Parameter Parameter Values User 4 No. of bits used for synthetic data 10,000 SNR 0-10 db Mother wavelet Daubechies Modulation and Demodulation BPSK, QPSK,4QAM, DBPSK Spreading code Walsh-Hadamard Code FEC code Trellis code Wireless channel AWGN channel, Rayleigh fading channel Processing gain 8 Figure 4 shows the comparison of the performance of different modulation schemes (BPSK, QPSK, 4QAM and DBPSK) for wavelet based MC-CDMA system in the presence of Additive White Gaussian Noise with implementation of Alamouti antenna diversity scheme for wide range of SNR from 0dB to 10dB. From the figure it is seen that the bit error rate (BER) is inversely proportional to the signal-to-noise ratio (SNR) as expected. At low SNR, BER reduction rate is small but with the increase of SNR the rate of BER reduction increases dramatically. In comparison with other modulation scheme, 4QAM shows much better BER performance than that of others in this system. At 2 dB SNR, the BER value of 4 QAM is approximately 5, 8 and 20 times lower than BPSK, DBPSK and QPSK, respectively. 0 1 2 3 4 5 6 7 8 9 10 10 -6 10 -5 10 -4 10 -3 10 -2 Signal to Noise ratio, EbNo(dB) BER MISO MCCDMA using BPSK over AWGN MISO MCCDMA using QPSK over AWGN MISO MCCDMA using 4QAM over AWGN MISO MCCDMA using DBPSK over AWGN Figure 4: Comparison of different modulation schemes for Wavelet based MC-CDMA over AWGN channel.
  • 8. International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012 30 To improve the performance of the proposed wireless communication system, to achieve the desired bit error probability and to minimize the burst error rate, interleaved FEC code can be used. The performance of interleaved trellis coded and uncoded wavelet based MC-CDMA system over Rayleigh fading channel using BPSK, QPSK, 4QAM and DBPSK modulation schemes is presented by figure 5, 6, 7 and 8 respectively. 0 1 2 3 4 5 6 7 8 9 10 10 -3 10 -2 10 -1 Signal to Noise ratio, EbNo(dB) BER MISO DWT based MCCDMA using BPSK over Rayleigh(uncoded) MISO DWT based MCCDMA using BPSK over Rayleigh(coded) Figure 5: Comparison of coded and uncoded Wavelet based MC-CDMA using BPSK modulation scheme over Rayleigh fading channel. From figure 5, it is seen that interleaved convolutional coding in discrete wavelet transform (DWT) based MC-CDMA with deployment of Alamouti diversity scheme using BPSK modulation can give better performance under Rayleigh fading channel. Contrast to uncoded DWT based system, coded one shows approximately 1 order of magnitude better BER performance at a typical SNR value of 10dB. Here, it is remarkable that the BER value of coded system decreases linearly with the increase of SNR. 0 1 2 3 4 5 6 7 8 9 10 10 -3 10 -2 10 -1 Signal to Noise ratio, EbNo(dB) BER MISO DWT based MCCDMA using QPSK over Rayleigh(uncoded) MISO DWT based MCCDMA using QPSK over Rayleigh(coded) Figure 6: Comparison of coded and uncoded Wavelet based MC-CDMA using QPSK modulation scheme over Rayleigh fading channel.
  • 9. International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012 31 The simulation result in figure 6 shows that interleaved coded DWT based MC-CDMA using QPSK modulation over Rayleigh fading channel has better BER performance than uncoded one as expected. For proposed coded communication system, when SNR is low, the rate of BER reduction is small but with the increase of SNR the BER reduction rate becomes high. At 3 dB SNR, the gain is roughly calculated which is almost 7 dB in the present system. On the other hand, it is not so significant al lower SNR. 0 1 2 3 4 5 6 7 8 9 10 10 -3 10 -2 10 -1 Signal to Noise ratio, EbNo(dB) BER MISO DWT based MCCDMA using 4QAM over Rayleigh(uncoded) MISO DWT based MCCDMA using 4QAM over Rayleigh(coded) Figure 7: Comparison of coded and uncoded Wavelet based MC-CDMA using 4QAM modulation scheme over Rayleigh fading channel. Figure 7 shows that, the coded proposed communication system using 4QAM modulation scheme gives better BER performance in contrast to uncoded system as expected. The rate of BER reduction for coded system is higher than that of uncoded one. The bit error probability 0.001 is obtained for coded proposed system at 10 dB SNR where as it is 0.01 for uncoded system at the same SNR. By comparing figure 4 and 7, it is found that the BER value of 4QAM modulation scheme is much lower in AWGN channel in contrast to Rayleigh fading channel. 0 1 2 3 4 5 6 7 8 9 10 10 -3 10 -2 10 -1 Signal to Noise ratio, EbNo(dB) BER MISO DWT based MCCDMA using DBPSK over Rayleigh(uncoded) MISO DWT based MCCDMA using DBPSK over Rayleigh(coded) Figure 8: Comparison of coded and uncoded Wavelet based MC-CDMA using DBPSK modulation scheme over Rayleigh fading channel.
  • 10. International Journal of Computer Science, Engineering and Information Technology (IJCSEIT), Vol.2, No.1, February 2012 32 It is also seen from figure 8 that the BER performance of interleaved coded system using DBPSK modulation scheme is much better than that of uncoded one. A desirable gain which is slightly less than 9dB can be achieved from this proposed model using DBPSK. 6. CONCLUSION In this paper the performance of wavelet based MC-CDMA system with Alamouti STBC using different modulation schemes over AWGN and Rayleigh fading channel has been studied. An approach has been presented that successfully applies FEC codes in the system to combat against the noise in channels and to improve the performance in contrast to uncoded system. It is seen that interleaved FEC coded wavelet based MC-CDMA system has better performance than that of uncoded system in fading channel irrespective of modulation schemes. From simulation results it is observed that in AWGN channel 4QAM modulation scheme has the best performance among others. On the other hand, in Rayleigh fading channel BPSK modulation technique provides the best performance among others though 4QAM has almost the same performance as BPSK in fading environment. REFERENCES [1] Yee N., Linnartz J-P and Fettwis G., (1993) "Multicarrier CDMA in indoor wireless radio networks", Proc. of IEEE PIMRC, Yokohama, Japan, pp. 109-113. [2] Hara, S. and Prasad, R., (1997) "An overview of multicarrier CDMA", IEEE Commun. Mag., 35, pp. 126-133. [3] Fazel K. and Papke L., (1993) "On the performance of convolutionally-coded CDMA/OFDM for mobile communication system", Proc. of IEEE PIMRC, Yokohama, Japan, pp. 468–472. [4] Muayyadi A. and Abu-Rgheff M.A., (2003) "Wavelet based multicarrier CDMA system and its corresponding multi-user detection", IEEE Proc.-Commun., Vol. 150, No. 6. [5] Xiangbin Yu, Xiaodong Zhang and Guangguo Bi, (2004) "Performance of complex wavelet packet based multi-carrier CDMA system with precoding technique", 10th Asis-Pacific Conference on Communications and 5th International Symposium on Multi-Dimensional Mobile Communications, IEEE. [6] Matiqul Islam Md., Hasnat Kabir M. and Enayet Ullah Sk., (2011) "Performance analysis of wavelet based MC-CDMA system with implementation of various antenna diversity schemes", International Journal of Research and Reviews in Computer Science (IJRRCS), Vol. 2, No. 2. [7] Salih M. Salih (2009) “Two-Dimensional Wavelet Transform Model for MC-CDMA”, IEEE 9781- 4244-3941-6/09. [8] Clark, George C., Jr.; Cain, J. Bibb (1981). Error-Correction Coding for Digital Communications. New York: Plenum Press. ISBN 0-306-40615-2. [9] Golam Rashed Md., Hasnat Kabir M., Selim Reza Md., Matiqul Islam Md., Rifat Ara Shams, Saleh Masum, Sheikh Enayet Ullah, (2011) “Transmission of Voice Signal: BER Performance Analysis of Different FEC Schemes Based OFDM System over Various Channels”, International Journal of Advanced Science and Technology Vol. 34. [10] http://en.wikipedia.org/wiki/Forward_error_correction [11] Ming-Fong Tsai, Ce-Kuen Shieh, Senior Member, IEEE, Tzu-Chi Huang, and Der-Jiunn Deng, Member, IEEE, (2011) "Forward-Looking Forward Error Correction Mechanism for Video Streaming over Wireless Networks", IEEE SYSTEMS JOURNAL, DOI: 10.1109/JSYST.2011.2165595
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