Performance Evaluation of Space-Time Turbo Code Concatenated With Block Code MC-CDMA Systems by ijcsis


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									                                                          (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                           Vol. 9, No. 1, January 2011

Performance Evaluation of Space-Time Turbo Code
Concatenated With Block Code MC-CDMA Systems

              Lokesh Kumar Bansal                                                           Aditya Trivedi
     Department of Electronics & Comm. Engg.,                             Department of Information and Comm. Technology
             N.I.E.M., Mathura, India                                               ABV-IIITM, Gwalior, India
      e-mail:                                          e-mail:

Abstract—In this paper, performance of a space-time turbo              emphasizes the need for improved spectral efficiency and
code (STTuC) in concatenation with space-time block code               higher Quality of Service (QOS) over current systems [5]-
(STBC) in multi-carrier code-division multiple-access (MC-             [7]. The above requirements can be fulfilled by multicarrier
CDMA) system with multi-path fading channel is considered.             modulation techniques. Single carrier systems give good data
The performance in terms of bit error rate (BER) is evaluated          rate but are limited in performance in multi path fading
through simulations. The corresponding BER of the                      channels. Improved performance in multipath fading channel
concatenated STTuC-STBC-MC-CDMA system is compared                     conditions, high data rates and efficient bandwidth usage are
with STTuC-MC-CDMA system and STBC-MC-CDMA                             the main advantages of multicarrier modulation. Space-time
system. The simulation results show that the STTuC-MC-                 coding (STC) techniques incorporate the methods of
CDMA system performance is better the STBC-MC-CDMA                     transmitter diversity, channel coding, and provide significant
system and it can be further improved by employing                     capacity gains over the traditional communication systems in
concatenation technique i.e. STTuC-STBC-MC-CDMA
                                                                       the fading wireless channels. Here, STC has been developed
                                                                       along two major directions: space-time block coding
                                                                       (STBC) and space-time turbo coding (STTuC).
     Keywords-MMSE; Multi-path channel; MAP Decoder;
MIMO; Space-time code; Space-time turbo-code; Space-time                   In this paper, the STBC, STTuC, and STTuC-STBC code
trellis-code; Viterbi Decoder                                          techniques are studied and applied in MC-CDMA systems.
                                                                       These techniques are employed with multiple input multiple
                                                                       output (MIMO) antenna diversity in multi-path fading
                       I. INTRODUCTION                                 channel. At the receiver side minimum mean-square error
    The wireless communication market is increased                     detection (MMSE) technique is used by employing
exponentially in recent years. A lot of interest has been              maximum a posteriori (MAP) algorithm for turbo code
developed in modulation techniques like Orthogonal                     decoding purpose on full load. The performance in terms of
Frequency Division Multiplexing (OFDM), Code Division                  bit error rate probability (BER) is obtained in presence of
Multiple Access (CDMA) and Multicarrier Code Division                  perfect channel state information (CSI).
Multiple Access (MC-CDMA). MC-CDMA is seen as a
possible candidate for Fourth Generation (4G) wireless                     The rest of the paper is organized as follows: in Section
communication systems that demand higher data rate for                 II, MC-CDMA system is presented. In Section III, space-
voice and data transmissions. CDMA technique is widely                 time coding technique is described. The space-time block
used in current Third Generation (3G) wireless                         code scheme is given in section IV. In Section V, the
communication systems. The principle of spread spectrum                mathematical representation for the space-time turbo code is
technology behind CDMA was popularly used in military                  explained. Space-Time turbo code in concatenation with
communications for improving secrecy and low probability               space-time block code MC-CDMA system model is given in
of interception during transmission. Now, CDMA                         section VI. Simulation for error rate performance of MC-
technology is also increased in civilian markets due to high           CDMA systems in presence of perfect CSI are carried out in
capacity and better performance. The rapid growth of video,            Section VII. The conclusions are presented in Section VIII.
voice and data transmission through the internet and the
increased use of mobile telephony in today’s life have the                                II. MC-CDMA SYSTEMS
necessity for higher data rate transmissions over the wireless             CDMA communication system allows multiple users to
channels [1]-[4].                                                      transmit at the same time in the same frequency band.
    In 3G systems we have higher data rate i.e. 64kbps –               Traditional multiple access techniques like time division
2Mbps as compared to 9.6kbps – 14.4kbps used in 2G                     multiple access (TDMA) and frequency division multiple
systems. The 4G systems that include broadband wireless                access (FDMA) are based on the philosophy of letting no
services require data rate up to 20Mbps. This also                     more than one transmitter occupy a given time-frequency

                                                                                                  ISSN 1947-5500
                                                          (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                           Vol. 9, No. 1, January 2011

slot. In a CDMA based system, users are assigned different               demonstrated to significantly increase system performance
signature wave forms or codes. Each transmitter sends its                as well as capacity. The merit of using multiple antennas or
data stream by modulating its own signature waveform as in               space diversity is that no bandwidth expansion or increase in
a single-user digital communication system. The receiver                 transmitted power is required for capacity and performance
does not need to concern itself with the fact that the signature         improvements [14].
waveforms overlap both in frequency and time, because their
orthogonality ensures that they will be transparent to the                                  III. S-T CODING TECHNIQUE
output of the other user's correlator [8], [9]. CDMA still has               In most wireless communication systems, the number of
a few drawbacks, the main one being that capacity is limited             diversity methods are used to get the required performance.
by the multiple access interference (MAI). The W-CDMA                    According to the domain, the diversity techniques are
supports high data rate transmission, typically 384 kbps for             classified into time, frequency, and space diversity [15]. S-T
wide area coverage and 2 mbps for local coverage for                     coding technique is designed for use with multiple transmit
multimedia services. Thus, W-CDMA is capable of offering                 antennas. There are various techniques in coding structures,
the transmission of voice, text, data, picture (still image) and         which include Alamouti STC, STBC, STTC, STTuC, and
video over a single platform. However, in addition to the                layered space-time (LST) codes. S-T coding with multiple
drawbacks arising from the mobile environment and multiple               transmit and receive antennas minimizes the effect of multi-
access interference, high bit rate transmission causes inter-            path fading and improves the performance and capacity of
symbol interference (ISI) to occur. The ISI, therefore, has to           digital transmission over wireless radio channels [16].
be taken into account during transmission.
                                                                             STBC can achieve a maximum possible diversity
    Multi-carrier modulation is being proposed for 4G                    advantage with a simple decoding algorithm. It is very
wireless communication systems for high data rate                        attractive because of its simplicity. However, no coding gain
application to reduce the effect of ISI and adapt to channel             can be provided by STBC. STTuC is able to combat the
conditions. A number of MC-CDMA systems have been                        effects of fading. However, STTuC have a potential
proposed lately. These systems solve the ISI problem by                  drawback due to the fact that its decoder complexity (MAP
transmitting the same data symbol over a large number of                 decoder) grows with the number of iterations.
narrow band orthogonal carriers. The number of carriers
equals or exceeds the pseudo-noise (PN) code length [10]-                   A base band S-T coded system with nT transmit
[12]. In MC-CDMA system, each data symbol is transmitted
over N narrowband sub-carriers, where each sub-carrier is                antennas and n R receive antennas is shown in Figure 1. The
encoded with a 0 or π phase offset. An MC-CDMA signal is                 transmitted data are encoded by a S-T encoder. At each time
composed of N narrowband sub-carrier signals each with                   instant, a block of m binary information symbols, denoted by
symbol duration, Tb, much larger than the delay spread, Td,
hence MC-CDMA signal does not experience significant ISI.
                                                                         a ( n ) = ( a1 (n),a 2 (n),a 3 (n),.....,a m ( n) ) is fed into the
Multiple access is achieved with different users transmitting            S-T encoder. The S-T encoder maps the block of m binary
at the same set of sub-carriers but with spreading codes that            input data into nT modulation symbols from a signal set of
are different to the codes of other users. Initially, the data           M =2m points. The coded data are applied to a serial to
stream is serial to parallel converted to a number of lower
rate streams. Each stream feeds a number of parallel streams             parallel (S/P) converter producing a sequence of nT parallel
with the same rate. On each of the parallel streams, bits are            symbols, arranged into a nT × 1 column vector
interleaved and spread by a PN code with a suitable chip
rate. Then, these streams modulate different or orthogonal
carriers with a successively overlapping bandwidth.                                              (
                                                                                      X(n) = x1 (n), x 2 (n),........., x nT (n)         )T

   In a MC-CDMA system, the numbers of carriers are                      where T denotes the transpose of a matrix. The nT parallel
typically chosen to be large enough so that the signal on
                                                                         outputs are simultaneously transmitted by nT different
each sub-carrier is propagated through a channel, which
behaves in a nonselective manner. The fading processes on                antennas, where by symbol x (n), 1 ≤ i ≤ nT , is
each sub-carrier for each user must be estimated, which can              transmitted by antenna i and all transmitted symbols have
be used in forming the MMSE filter. This approach is
                                                                         the same duration of T sec . The vector of coded modulation
shown to perform close to ideal for sufficiently high vehicle
                                                                         symbols from different antennas is called a S-T symbol. The
speeds, up to which the normalized Doppler rate is about
1percent [13].                                                                                                          rb
                                                                         spectral   efficiency       of   the    system    =mis   η=
   Providing high data rate transmission of the order of                                                                B
several megabits per second (mbps) is important for future               bits / sec/ Hz ,where rb is the data rate and B is the
wireless communications. In recent years, antenna systems
which employ multiple antennas at both the base station                  channel bandwidth. This spectral efficiency is equal to the
(BS) and mobile station (MS), have been proposed and                     spectral efficiency of a reference uncoded system with one
                                                                         transmit antenna.

                                                                                                          ISSN 1947-5500
                                                           (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                            Vol. 9, No. 1, January 2011

                                                                                                         x1 (n)          r 1 (n)
                                                                                                         x ( n)             2                  e
                                                              X(n)                                                       r ( n)                            a(n)
                                 a (n )                                                                                                        c
              Information                  Space-Time                                                                                 .        e
                Source                      Encoder                        S/P               .                                        .
                                                                                             .                                                 i
                                                                                                                                      .        v
                                                                                                         x nT (n) r nR (n )                    r

                                                  Figure 1- A base band system model

The multiple antennas at both the transmitter and the receiver
create a MIMO channel. For wireless mobile                                                           (
                                                                                     r (n) = r 1 (n), r 2 (n),........., r nR (n)             )T

communications, each link from a transmit antenna to a
receive antenna can be modeled by flat fading, if we assume               Thus, the received signal vector can be represented as,
that the channel is memoryless. The MIMO channel                                     r ( n ) = H ( n ) X (n ) + v ( n )                                   (2)
with nT transmit and n R receive antennas can be represented
                                                                          It is assumed that the decoder at the receiver uses a
by an (n R × nT   ) channel matrix H and it can be given as               maximum likelihood algorithm to estimate the transmitted
                                                                          information sequence with receiver has perfect channel state
              h1,1 (n ) h1, 2 (n) ..... h1,nT (n)                       information (CSI) on the MIMO channel. At the receiver, the
                                                                        decision metric is computed based on the squared Euclidean
              h2,1 (n) h2,2 (n ) ...... h2.nT (n)                       distance between the hypothesized received sequence and the
    H ( n) =  :              :        :          :                      actual received sequence as
                                                      
              :              :        :          :                                  nR                      nT                     2

             hnR ,1 (n) hnR ,2 (n ) ...... hnR ,nT (n)
                                                                                   ∑r
                                                                                      j =1
                                                                                                     (n) − ∑ h j ,i (n) x (n)
                                                                                                              i =1

where the ( ji ) element, denoted by h j ,i ( n) , is the fading
                                                                          The decoder select a codeword with the minimum decision
attenuation coefficient for the path from transmit antenna                metric as the decoded sequence [4], [16].
i to receive antenna j .
                                                                                       IV. SPACE-TIME BLOCK CODE (STBC)
  It is further assumed that the fading coefficients h j ,i ( n)
                                                                             STBC first introduced by Alamouti with two transmit
are independent complex Gaussian random variables. At the                 antennas. Figure 2 shows the block diagram of the Alamouti
receiver, the signal at each of the n R receive antennas is a             S-T encoder. It is assumed that a M − ary modulation
noisy superposition of the   nT transmitted signals degraded              scheme is used. In the Alamouti S-T encoder, each group of
by channel fading. The n
                                       received signal at
                                                                           m information bits is first coded, where m = log 2 M .
                                           j                              Here, the encoder takes a block of two modulated symbols
antenna j ( j = 1,2,...., nR ) denoted by r ( n) , is given by
                                                                           x1 and x 2 in each encoding operation and maps them to the
                     nT                                                   transmit antennas according to a code matrix given by
      r j (n) = ∑ h j ,i (n)x i (n) + v j (n)                 (1)
                  i =1                                                                         x                  − x2 
                                                                                             X= 1                   * 
where v (n) is the noise component of receive antenna j                                         x2                 x1 
at time n , which is an independent noise sample of the zero-             The encoder outputs are transmitted in two consecutive
mean complex Gaussian random variable with the one sided                  transmission periods from two transmit antennas. During the
power spectral density of N o . r ( n) is the received signal             first transmission period, two signals x1 and x 2 are
sequence from n R receive antennas of n R × 1 column                      transmitted simultaneously from antenna one and antenna
                                                                          two, respectively.

                                                                                                                   ISSN 1947-5500
                                                                       (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                                        Vol. 9, No. 1, January 2011

                                                                                                                                                   X = x1 − x 2            ]
                                                             [x1            x2 ]                                   Encoder
           Data                     Modulator
                                                                                                                   x      − x2 
                                                                                                                              *                           TX2
         Sequence                                                                           [x1             x2 ] →  1       * 
                                                                                                                                                       X = x2     [     *
                                                                                                                                                                       x1   ]
                                                                                                                    x2     x1 

                                           Figure 2- Block diagram of the Alamouti space-time encoder

In the second transmission period, signal − x 2 is transmitted                             STBC. The rows of the transmission matrix X n T are
                                                 *                                         orthogonal to each other. This means that in each block, the
from transmit antenna one and signal x from transmit
                                                                                           signal sequences from any two transmit antennas are
antenna two [7], [16]. The main principle of the Alamouti
                                                                                           orthogonal. For example, if we assume that
scheme is that the transmit sequences from the two transmit
antennas are orthogonal, since the inner product of the                                                 (                      )
                                                                                           X i = xi ,1 , xi , 2 ,...., xi , p is the transmitted sequence from
sequences x1 and x2 is zero, i.e.                                                                  th
                                                                                           the i        antenna, i = 1,2,.........., nT , we have
                1   2         *    *
              X ⋅ X = x1 ⋅ x − x ⋅ x1 = 0
                              2    2                                                                                          p

This scheme may be generalized to an arbitrary number of                                                         Xi ⋅ X j = ∑ xi ,t ⋅x*j ,t = 0,
transmit antennas by applying the theory of orthogonal                                                                       t =1
designs. The generalized schemes are referred to as STBC.
The STBC can achieve the full transmit diversity specified                                                      i ≠ j, i, j ∈ { ,2,...., nT }
by the number of the transmit antennas nT , while allowing a                               where X i ⋅ X j denotes the inner product of the sequences
very simple maximum-likelihood decoding algorithm, based
only on linear processing of the received signals [8], [16].                               X i and X j . The orthogonality enables to achieve the full
                                                                                           transmit diversity for a given number of transmit antennas. In
  In general, a STBC is defined by a nT × p transmission                                   addition, it allows the receiver to decouple the signals
matrix X. Here nT represents the number of transmit                                        transmitted from different antennas and consequently, a
                                                                                           simple maximum likelihood decoding, based only on linear
antennas and p represents the number of time periods for
                                                                                           processing of the received signals [8], [16].
transmission of one block coded symbols. It is assumed that
the signal constellation consists of 2m points. At each
encoding operation, a block of km information bits are                                                   V. SPACE-TIME T URBO CODE (STTUC)
mapped into the signal constellation to select k modulated                                    Space-Time Turbo code can achieve outstanding
signals x1 , x 2 ,........, x k , where each group of m bits selects                       performance gain because it uses specific coding and
                                                                                           decoding structure. Here, in turbo code parallel
a constellation signal. The k modulated signals are encoded                                concatenation convolutional code is used, which is a recent
by a S-T block encoder to generate nT parallel signal                                      scheme of channel code developed, whose bit error
sequences of length p according to the transmission matrix                                 performance is close to the limit predicted by Shannon. In
                                                                                           coding side it uses a particular concatenation of two
X. These sequences are transmitted through nT transmit
                                                                                           Recursive Convolutional Codes (RCC), associated together
antennas simultaneously in p time periods.                                                 by the inter-leaver, while in decoding it delivered soft
                         (    2        2
             X ⋅ X H = α x1 + x 2 + .... + x k
                                                             )I   nT
                                                                                           information between two decoder components to make soft-
                                                                                           input and soft-output iterative decoding. The first decoder
                                                                                           must deliver a weighted soft decision to the second decoder.
where   α is a constant, XH is the Hermitian of X and I n                   T
                                                                                is         The      Logarithm       of    Likelihood      Ratio    (LLR)
an nT × nT identity matrix. The element of X in the i row
                                                                       th                   L1 (an ) associated with each decoded bit an by first decoder
        th                                                                                 is a relevant piece of information for the second decoder. For
and j column, xi , j , i = 1,2,....., nT , j = 1, 2,........, p,
                                                                                           information bit an , It’s LLR is defined as the natural log of
represents the signal transmitted from the antenna i at                                    its base probabilities.
time j . The orthogonal designs are applied to construct

                                                                                                                                    ISSN 1947-5500
                                                               (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                                Vol. 9, No. 1, January 2011

                               P(an = +1 )                                             P(s',s, y) = P(yf s)P(s',s, yp , yn ) .
                L(an )= ln
                               P(an = -1 )                                   Now apply Baye’s rule to the last term
                                                                                           P(s',s, yp , yn ) = P(s, yn s ', yp )P(s ', yp )
   If an has two values +1 and -1, with equally likely, then
this ratio is equal to zero. If the binary variable is not equally                        P(s',s,y)= P(y f s)P(s,yn s' ,yp )P(s',yp )
likely then P(an = -1)= 1- P(an = +1 ) .                                     Let           αn-1(s') = P(s ', yp ) βn (s) = P( y f s)
                                                                                                                           ,                     ,
                            P(a n = +1)                                      and          γ n (s',s) = P(s, yn s ', yp )
             L(an ) = ln
                           1-P(a n = +1)                                     Then         P(s ', s, y) = αn−1 (s')βn (s)γ n (s',s)
The LLR of N bit sequence is formulated as below. The                        The LLR equation for MAP algorithm can be written as
lower indicator of y means it starts at time 1 and the upper
indicator means the ending point at N.
                                                                                     ∑α  an =+1
                                                                                                  n−1   (s')βn (s)γ n (s',s)
                                                                             L(a ) =                                              , where   α n−1 ( s ') is   called
          L1(an )= ln
                           P(an = +1 y1N )
                                                                                     ∑α  an =−1
                                                                                                  n−1   (s')βn (s)γ n (s',s)
                            P(an = -1 y1N )
Here, first the S-T Turbo encoder encodes the source data.
                                                                             the Forward metric,             β n ( s) is        called the Backward metric,
Next, the encoded data is applied to interleaver, and then                   and γ n ( s ', s ) is called Transition metric. At the receiver,
mapped according to the desired signal constellation.                        the received data sequence is combined according to the
Finally at each time interval, the signals are modulated and                 combining techniques described for STC-MC-CDMA
transmitted simultaneously over different transmit antennas                  system. The soft output of the combiner is applied directly
[16]-[18]. Using Baye’s rule, the above equation can be                      to the deinterleaver, and then finally, it is applied to S-T
reformulated as                                                              Turbo decoder, such as the MAP algorithm, to decode the
                P(y1N ,an =+1)/P(y1N )     P(y N ,a =+1)                     data.
   L(an )=ln                            =ln 1N n
                P(y1N ,an = -1)/P(y1N )    P(y1 ,an = -1)                    L1(an )= L(a - priori) + L(channel) + L(extrinsic)
This LLR includes joint probabilities between the received                   The L(a - priori) is a-priori information about an ,
bits and the information bit, the numerator of which can be                   L(channel) is the channel value calculated from the
written as
                                                                             knowledge of the SNR and received signal. The third term is
           ∑P(a    n   = +1, y1 ) = ∑P(s ', s, y1N )
                                                                             called the a-posteriori term, also called the extrinsic L value.
                                                                                                                  r   r
            N                         N
where s’ is starting state and s is ending state of trellis.                                                 (s').β n (s).γn (s',s)

                                   ∑ P(s',s,y1N )
                                                                                   L(extrinsic)= ln a r           r
                                                                                                                  u                                              (6)
       L(an )= ln
                  P(y1N ,an =+1) an=+1
                                 =                                                                 ∑                       e
                                                                                                       α n-1 (s').β n (s).γn (s',s)
                  P(y1N ,an = -1) ∑ P(s',s,y1N )                                                                 a-

                                           an =-1
                                                                             During each iteration the decoder produces the extrinsic
                                                                             value, this extrinsic value becomes the input to the next
          N                N
         y1 = y1k-1, yn , yn+1 = yp , yk , y f                               decoder. The decision is made about the bit by looking at
We take the N bit data sequence and separate this into three                 the sign of the L value. an = sign {L1 (an ) , The process
pieces, from 1 to n-1, then the    nth point, and then from n+1              can continue until the extrinsic values change becomes
                 N                                                           insignificant or the algorithm can allow for a fixed number
to N. P(s ', s, y1 ) = P(s ', s, y p , yk , y f ) where yp is past
                                          ,                                  of iterations.
sequence, which is the part that came before the current data
                                                                                  VI. STTUC CONCATENATED WITH STBC MC-CDMA
point. yn current data point and y f is the part that come
                                                                                                          SYSTEM MODEL
after the current point. Using Baye’s rule
                                                                               In order to further improvement in the performance of
            P(s ', s, y) = P(s ', s, yp , yn , y f )                         STTuC-MC-CDMA system, we can use S-T turbo code in
                                                                             concatenation with S-T block code MC-CDMA system. The
           = P(y f s' ,s, y p , yn )P(s',s, y p , yn )                       STTuC-STBC-MC-CDMA system provides both diversity
The term y f is the future sequence and we consider that it                  and coding gain with a reasonable increase in complexity.
                                                                             Figure 3 show the general block diagram of concatenated
is independent of the past and only depend on the present                    system. First, the STTuC encoder encodes the source data.
state s. We can remove these dependencies to simplify the                    Next, the encoded data is applied to S-T block encoder &
above equation.

                                                                                                                      ISSN 1947-5500
                                                                                    (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                                                     Vol. 9, No. 1, January 2011

                                                                                                     (k )

        ak ( n )                          xk ( n )
                        STTuC                           STBC                          :                                                          IFFT            P/S
                                                                           Copier                                       Interleaver
                        Encoder                        Encoder
                                                                                      :                (k )



                                                                                                                                                                                              v (n)
                                                            xk ( n )
  ak ( n )                   Decoder
                                                                       (MMSE)             Deinterleaver                       FFT               S/P

                                          Figure 3- Block diagram of STTuC Concatenated with STBC code MC-CDMA System

interleaver, and then mapped according to the desired signal
constellation. Finally at each time interval, the symbols are                                                           s ( k ) (1) = x ( k ) (1) , s ( k ) ( 2 ) = − x ( k )* ( 2 )
modulated and transmitted simultaneously over different                                                                     (k )                        (k )
transmit antennas. At the receiver, the received data is                                                                s          (1) = x (k ) ( 2 ) , s ( 2 ) = x( k )* (1)
combined according to the combining techniques described for
STBC. The soft output of the combiner is sent directly to the                                     and ( .) denotes the complex conjugate. The two columns
deinterleaver, and then finally, it is applied to a STTuC
decoder, such as the MAP algorithm, to decode the data.                                           of s             ( n ) will be transmitted in two consecutive time slots,
   Here, a base band system configuration of the STBC-MC-                                         with the first element of each column transmitted from Tx1
CMDA system employing the Alamouti’s S-T coding scheme                                            and the second element from Tx2, respectively. Throughout
at the transmitter is depicted in Figure 3, which involves two
                                                                                                  this paper,   is designated to quantities associated with Tx2.
transmit antennas, Tx1 and Tx2, and one receive antenna, Rx.
                                                                                                  In the current system, each user is assigned two distinct
At the transmitter, we assume K number of users transmit
simultaneously with STTuC in concatenation of STBC over                                           spreading codes to spread symbols transmitted from the two
MC-CDMA system from the two transmit antennas. The                                                                                                c ( ) =  c0 ) ,...., c(p −)1 
                                                                                                                                                     k                     k
                                                                                                  antennas.                         Let                                                          and
frequency selective channel between transmit and receive                                                                                                                       
antennas is divided into P subchannels such that each
                                                                                                             =  c 0 ,...., c p −1  are two spreading code sequences for
                                                                                                      (k )         (k )      (k )
subchannel is approximately flat. Let X ( k ) ( n ) be the S-T    {         }                                  
                                                                                                                                  
Turbo encoded output.                                                                             kth user with processing gain P which spread the symbols
                                                                                                  transmitted from antenna Tx1 & Tx2 respectively. Define
   For kth user, the output of S-T Turbo encoder is given to the
                                                                                                  u ( ) ( n) = s ( k ) ( n)c( ) is the signal associated with Tx1. The
                                                                                                        k                                  k
S-T block encoder that is represented by the following code
matrix                                                                                                                                                                                  (k)
                                                                                                  OFDM modulation can be implemented via IFFT on u                                            (n)
                              s                                                                                                    (k )
                                   (k )
                                      (1)        s     ( 2 )
                                                     (k )                                                                                        −1 ( k )
                                                                                                                                   y ( n ) = F u (n )                                            (8)
                        (n) =   (k )             (k )
                                                                                     (7)                                                                                                (k )
                               s (1)            s ( 2 )                                         Similarly we obtain the signal associated with Tx2 as y                                       (n) .
                                                           

                                                                                                                                               ISSN 1947-5500
                                                                                               (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                                                                Vol. 9, No. 1, January 2011
At the receiver, the received signal after demodulating via FFT
and passing through deinterleaver is given as                                                                 In Figure 4 we present the BER of STTuC-MC-CDMA
                           K                                                                               system with different number of iterations, and it is observed
         Y (n) = ∑  s ( k ) (n)bc( k ) + s ( k ) (n)bc ( k )  + V (n)                                    that the performance is improved by 2 dB from 1 to 2
                 k =1
                                                                                                         iterations and again it is enhanced by around 2 dB from 2 to 5
                                                                                           T               iterations at 10-3 BER. From this we can state that the rate of
where b = Fh , and h =  h ( 0 ) ,...., h ( M − 1) , 0T − M 
                                                                                              is
                                                      P                                                    improvement in the performance decreases with increasing
channel                    vector              between               Tx1       &               Rx,         number of iterations. The performance of STBC-MC-CDMA,
                                                             T                                             STTuC-MC-CDMA, & STTuC-STBC-MC-CDMA systems
and v ( n ) = v0 ( n ) ,...., v p −1 ( n )  contains samples of the
                                                                                                         versus the SNR in a Rayleigh fading environment for K = 10
                                                                           2                               users and 20000 bit sequences are shown in Figure 5. It is
channel noise with zero mean and variance σ v .
                                                                                                           noted that at 10-3 BER the performance of STTuC-MC-CDMA
                  VII. SIMULATION RESULTS                                                                  system using MMSE detector is better than STBC-MC-
                                                                                                           CDMA system by around 1 dB. The performance of STTuC-
  The simulations are done for STTuC-MC-CDMA, STBC-                                                        MC-CDMA system can also be further improved by 1.5dB at
MC-CDMA, and STTuC-STBC-MC-CDMA systems with K =                                                           10-3 BER by using STTuC-STBC-MC-CDMA systems in
10 users. The user symbols are drawn from a unit-energy                                                    presence of perfect channel state information (CSI).
BPSK (binary phase shift keying) constellation. Walsh-
Hadamard codes with processing gain P = 32 are used for                                                                               VIII.      CONCLUSION
spreading. We assume noise samples as i.i.d. complex
                                                        2                                                     In this paper, the performance of concatenated STTuC-
Gaussian random variables with zero mean and variance σ n .                                                STBC-MC-CDMA, STTuC-MC-CDMA and STBC-MC-
                                                                                                           CDMA systems are obtained using MMSE detection
                                                                                                           technique employing MAP algorithm for turbo code decoding.
                                                                                                           Simulation results are presented in presence of perfect CSI. It
                                                                 STTuC-MCCDMA-Iter-5                       is noted that the performance of STTuC-MC-CDMA system
              -1                                                                                           increases with more number of iterations. However, the rate of
                                                                                                           improvement in the performance decreases with increasing
                                                                                                           number of iterations. It is observed that the performance of
                                                                                                           STTuC-MC-CDMA system with 2 numbers of iterations is

                                                                                                           better than STBC-MC-CDMA system by around 1 dB. The
                                                                                                           performance gain can be further improved by around 1.5 dB
                                                                                                           using STTuC-STBC-MC-CDMA system at 10-3 BER.

                   0   1           2       3       4     5       6         7   8       9                   [1]   V. Tarokh, N. Seshadri, and A. R. Calderbank, “Space-time codes for
                                                   SNR(dB)                                                       high data rate wireless communication: performance criterion and code
                                                                                                                 construction,” IEEE Transactions on Information Theory, vol.44, no. 2,
Figure 4- BER performance of STTuC-MC-CDMA System for 1, 2, & 5                                                  pp. 744-765, March 1998.
number of iterations with perfect CSI.                                                                     [2]   Hemanth Sampath, and S. Talwar, J. Tellado, “A Fourth-Generation
                                                                                                                 MIMO-OFDM            Broadband Wireless System: Design, Performance,
         10                                                                                                      and Field Trial Results,” IEEE Comm. Magazine, pp.143-149,
                                                                                                                 September 2002.
                                                                                                           [3]   M. Juntti, M. Vehkapera, J. Leinonen, “MIMO MC-CDMA
         10                                                                                                      Communications for Future Cellular Systems,” IEEE Comm. Magazine,
                                                                                                                 pp. 118-124, Feb.2005.
                                                                                                           [4]   S. Hijazi, B. Natarajan, and Z. Wu., “Flexible Spectrum use and Better
          -2                                                                                                     Coexistence at the Physical Layer of Future Wireless Systems via a
                                                                                                                 Multicarrier Platform”, IEEE Wireless Comm., April 2004, pp. 64-71.

                                                                                                           [5]   Essam A. Sourour, and Masao Nakagawa, “Performance of Orthogonal
                                                                                                                 Multicarrier CDMA in a Multipath Fading Channel,” IEEE Transactions
         10                                                                                                      on Communications, vol. 44, no.3, pp. 356-366, March 1996.
                                                                                                           [6]   S. Hara, and R. Prasad, “An Overview of Multi-Carrier CDMA,” IEEE
                                                                                                                 Comm. Magazine, vol. 35, no. 12, pp.126-133, Dec. 1997.
         10                                                                                                [7]   Scott L. Miller, and B. J. Rainbolt, “MMSE Detection of Multicarrier
                           STBC-MC-CDMA                                                                          CDMA,” IEEE J. on Selected Areas in Communications, vol. 18, no.11,
                           STTuC-MC-CDMA-Iter-2                                                                  pp. 2356-2362, Nov. 2000.
          -5               STTuC-STBC-MC-CDMA-Iter-2                                                       [8]   Kai-Kit Wong, Ross D. Murch, and Khaled Ben Letaief, “Performance
                  0    1       2       3       4      5      6       7     8   9   10                            Enhancement of Multiuser MIMO Wireless Communication Systems,”
                                                   SNR(dB)                                                       IEEE Transactions on Communications, vol. 50, no.12, pp. 1960-1968,
Figure 5- Performance comparision of STTuC-STBC, STTuC, and STBC-                                                Dec. 2002.
MC-CDMA Systems with perfect CSI.                                                                          [9]   Prokis, J. G. (2008). Digital Communication (5th ed.). New York: Mc-
                                                                                                                 Graw Hill.

                                                                                                                                           ISSN 1947-5500
                                                                          (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                                           Vol. 9, No. 1, January 2011
[10] A. F. Naguib, V. Tarokh, N. Seshadri, and A. R. Calderbank, “A space-
     time coding modem for high-data-rate wireless communications,” IEEE
     J. on Selected Areas in Comm., vol. 16, no. 8, pp. 1459 – 1478, Oct.
[11] S. M. Alamouti, “A simple transmit diversity technique for wireless
     communications,” IEEE J. on Selected Areas in Communications, vol.
     16, no. 8, pp. 1451 – 1458, Oct. 1998.
[12] Trivedi Aditya and Bansal Lokesh Kumar, “Comparative study of
     different space-time coding schemes for MC-CDMA systems”,
     International Journal of Communication, Network and System Sciences
     published by Scientific Research, vol. 3, no. 3, pp. 418-424, April 2010.
[13] Wei Sun, Hongbin Li, and Moeness Amin, “A Subspace-Based Channel
     Identification Algorithm for Forward Link in Space-Time Coded MC-
     CDMA Systems”, IEEE Proceedings, 2002 pp. 445-448.
[14] L. A. P. Hernandez and M. G. Otero, “A new STB-TCM coded MC-
     CDMA Systems with MMSE-SVA based Decoding and Soft-
     Interference Cancellation”, IEEE Proceedings 2005, pp. 113-116.
[15] Clude Berrou, Alain Glavieux, and Punya Thitimajshima, “Near
     Shannon Limit Error-Correcting Coding and Decoding: Turbo-Codes”,
     IEEE Proceedings 1993, pp. 1064-1070.
[16] R. Garello., P. Pierleoni, and S. Benedetto, “Computing the free distance
     of Turbo Codes and Serially Concatenated Codes with Interleavers:
     Algorithms and Applications”, IEEE J. on Selected Areas in Comm.,
     vol. 19, no. 5, pp. 800 – 812, May 2001.
[17] M. Modarresi and S. Sadough, “Turbo-Trellis Coded Modulation Under
     Channel Estimation Inaccuracies”, IEEE Proceedings of ICEE 2010,
     May 11-13, 2010.
[18] V. Tarokh, A. Naguib, N. Seshadri, and A. R. Calderbank, “Space-time
     codes for high data rate wireless communication: performance criterion
     in the presence of channel estimation errors, mobility, and multiple
     paths,” IEEE Transactions on Communications, vol.47, no. 2, pp. 199-
     207, February 1999.

                                                                                                               ISSN 1947-5500

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