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PERFORMANCE ANALYSIS OF NEW BINARY ORTHOGONAL CODES FOR DS-CDMA COMMUNICAT

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PERFORMANCE ANALYSIS OF NEW BINARY ORTHOGONAL CODES FOR DS-CDMA COMMUNICAT Powered By Docstoc
					         INTERNATIONAL and Communication Engineering & Technology (IJECET),
International Journal of Electronics JOURNAL OF ELECTRONICS AND
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME
 COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)

ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
                                                                            IJECET
Volume 4, Issue 4, July-August, 2013, pp. 248-254
© IAEME: www.iaeme.com/ijecet.asp                                          ©IAEME
Journal Impact Factor (2013): 5.8896 (Calculated by GISI)
www.jifactor.com




   PERFORMANCE ANALYSIS OF NEW BINARY ORTHOGONAL CODES
       FOR DS-CDMA COMMUNICATION OVER AWGN CHANNEL

                          K P Ravali1, K Usha2, K Varshini3, K Gayatri Devi4
                      1
                        (Dept. of ECE, MVSR Engg. College, Hyderabad, India)
                      2
                        (Dept. of ECE, MVSR Engg. College, Hyderabad, India)
                      3
                        (Dept of ECE, MVSR Engg. College, Hyderabad, India)
                      4
                        (Dept. of ECE, MVSR Engg. College, Hyderabad, India)



ABSTRACT

        Direct Sequence - Code Division Multiple Access (DS-CDMA) is a communication system
in which a particular random signal is multiplied with message signal to produce noise like spectrum.
These random signals are called user codes. There are two types of user codes, orthogonal and non-
orthogonal codes. Orthogonal codes are very important in direct sequence CDMA communication,
since they ensure minimum correlation error. There are many types of user codes available with
different advantages. In this paper we have put immense efforts to highlight their performance in
comparison with new orthogonal codes. MATLAB simulation software is used in order to find
autocorrelation and cross-correlation properties of each user code. Bit Error Rate (BER) is also
considered as a very important parameter for comparing the performance of all user codes. We have
limited our studies to single and two users over AWGN channel in order to work with all popular
codes such as Gold, Kasami, Hadamard–Walsh and new orthogonal sequence with an intention of
highlighting adaptive nature and increased capacity of new orthogonal sequence for DS-CDMA.

Keywords: AWGN channel, Binary orthogonal codes, DS CDMA.

1. INTRODUCTION

        In the communication sector, spectral resources are often very limited. So it becomes very
crucial to make sure that they are used to maximum productivity. In order to use the spectrum
efficiently, multiple access techniques are used. Basically there are three major multiple access
techniques. They are Frequency Division Multiple Access (FDMA), Time Division Multiple Access
(TDMA) and Code Division Multiple Access (CDMA). CDMA has upper hand over FDMA and
TDMA since it can accommodate infinite number of users (theoretically). Eventually performance


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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME

decreases as number of users increase. In order to ensure good performance user codes are supposed
to have good cross correlation property in order to avoid cross-correlation error.
       In CDMA, it is a well known fact that users are distinguished by their codes. CDMA system
performs correlation operation between message and user code to introduce encryption in transmitter
and again correlation is done between received signal and synchronizing code. To obtain perfect
recovery cross-correlation error is supposed to be zero. So each code should be orthogonal with all
other user codes. For specific number of bits, there is limited number of codes with perfect
orthogonal property. In order to increase the capacity, we have an algorithm that produces maximum
possible number of orthogonal codes.
       Some of the popular user codes are Gold codes, Hadamard Walsh code and Kasami codes.
Gold codes are generated by modulo addition of two preferred polynomials of maximal length
sequences. Gold sequences are nearly orthogonal with non-linear phase. As they are not perfectly
orthogonal, cross-correlation error is not zero. Since limited number of preferred polynomials is
available, only limited number of user codes can be obtained and so the capacity is limited.
Hadamard Walsh codes are generated in powers of 2 by using Hadamard matrices recursively. Walsh
codes are perfectly orthogonal, linear phase codes with unique number of zero crossings within the
set and are a subset of orthogonal codes present in n-length sequence. Length of Walsh sequence is
always even. Use of Walsh is limited to synchronous communication only. Kasami code is binary
sequence which has good cross-correlation. Kasami sequence is obtained by decimation of two
sequences. There are two types of Kasami sequences, small set and large set. Small Kasami sets are
used for reverse channels in IS 95 mobile communication standard. Cross-correlation of Kasami
approaches nearly the Welch lower bound. Hardware implementation of Kasami sequence is very
complex and requires very high frequency of clock.

                                                   Table 1
                                                                             7-length Kasami Code
           7-length Gold Code set         8-length Walsh code set
                                                                                       set
       -     -   -   -                                                     -        - - -         -
                         1   1   -1   1    1   1    1     1   1   1   1         1             1
       1     1   1   1                                                     1       1 1 1         1
             -   -           -             -        -         -            - -             - - -
       1             1   1       1    1        1          1       1   -1           1 1
             1   1           1             1        1         1            1 1             1 1 1
       -         -       -   -                 -    -             -                 -         - -
             1       1           -1   1    1              1   1       -1   1 1          1 1
       1         1       1   1                 1    1             1                1          1 1
             -           -                 -   -              -   -        - - - -            -
       1         1   1       1   -1   1             1     1           1                    1     1
             1           1                 1   1              1   1        1 1 1 1            1
                 -   -   -                                -   -   -             -       -         -
       1     1               1   1    1    1   1    1                 -1   1       1       1 1
                 1   1   1                                1   1   1             1       1        1
       -                                   -        -     -       -                     - - -
             1   1   1   1   1   1    1        1              1       1    1 1 1                 1
       1                                   1        1     1       1                     1 1 1
       -     -       -   -   -                 -    -     -   -            -
                 1               1    1    1                      1   1         1 1 1 1 1 1
       1     1       1   1   1                 1    1     1   1            1
                                           -   -          -
                                      1             1         1   1   -1
                                           1   1          1

       New orthogonal codes are flexibly available for both even and odd symmetry. This flexibility
has expanded the search area for orthogonal code in binary sample space. Due to availability of good
computational resources, we have adopted a complex algorithm to explore new orthogonal codes in
the given sample space. Proposed code has good cross-correlation and BER performance which is
explained in the later part of this paper.

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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME

       Rest of the paper is organized as follows: Section-2 describes the procedure to generate
binary orthogonal codes. In Section-3 time, frequency, auto-correlation and cross-correlation
properties of all user codes are discussed. Section-4 presents the BER performance comparison of
the proposed codes with the existing popular spreading codes. The paper is concluded finally with
the applications and merits of the proposed code in Section-5.

2. DESIGN METHODOLOGY FOR BINARY ORTHOGONAL CODES

        The design methodology discussed here is similar to the construction technique discussed in
[1]. This design methodology can be applied to construct orthogonal code set of any length. In this
paper, we are restricting ourselves to fixed power orthogonal codes of length 8 bits. Though for 8
bits a total of 256 combinations are possible, only the first half of the numbers excluding the DC
component are considered as the second half set of binary numbers are complement to the first half.
Thus orthogonal search is performed in the region 1 to 127.
Step-1: Initially all the numbers from 1 to 127 are represented in radix-2 format or binary format.
Step-2: As the binary spreading codes have two chip levels [1, -1], all the binary representations are
converted to Bipolar Non Return to Zero (NRZ) form (i.e. 0 as -1 and 1 as 1) and are stored in two
matrices of size 127X8 each.
Step-3: For generation of the desired mutually orthogonal set, an exhaustive computer search is
performed. For a code length of 8, 8 mutually orthogonal codes are obtained.
Step-4: Orthogonal code sets with good correlation properties are limited for 8 bit code words
because of the small sample space.
Step-5: Taking into consideration zero mean and linear phase conditions, a total of 22 codes are
available in the entire sample space for a code length of 8.
        One such orthogonal code set is displayed in Table 2. Orthogonal code set of any length can
be constructed using this technique.

                                              Table 2
                                 Decimal
                     S.No                              NRZ Representation
                                Equivalent
                       2.           7          -1 -1 -1 -1 -1 1 1 1
                       3.          49          -1 -1 1 1 -1 -1 -1 1
                       4.          62          -1 -1 1 1 1 1 1 -1
                       5.          82          -1 1 -1 1 -1 -1 1 -1
                       6.          93          -1 1 -1 1 1 1 -1 1
                       7.          100         -1 1 1 -1 -1 1 -1 -1
                       8.          107         -1 1 1 -1 1 -1 1 1


3. TIME, FREQUENCY, AUTO AND CROSS-CORRELATION PROPERTIES

        This paper discusses the performance of the new set of proposed codes in comparison with
Walsh (8 bit length), Gold and Kasami (7 bit length) codes by using the parameters like auto
correlation, cross correlation, magnitude and phase plot. Fig.1 displays the time domain
representations of typical code words of all spreading codes. Fig.2 and Fig.3 represent magnitude
and phase functions of codes respectively. The proposed orthogonal codes are more evenly spread
when compared to other user codes like Gold and Kasami codes whereas phase is non-linear for both
proposed orthogonal codes and Gold codes.


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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME




                       Kasami 7 Proposed codes 8
                                                        1
                                                        0
                                                      -1
                                                             1       2       3        4              5        6         7      8

                                                        1
                                                        0
                                                      -1
                                                             1           2       3           4           5          6          7

                                                        1

                       Walsh 8
                                                        0
                                                      -1
                                                             1       2       3        4              5        6         7      8

                                                        1
                       Gold 7




                                                        0
                                                      -1
                                                             1           2       3           4           5          6          7
                                                                                          Time--->


  Fig.1 Time domain representation of typical 7-length Gold code, 8-length Walsh code, 7-length
                     Kasami code and 8-length proposed orthogonal code

                                                                                                                   Proposed codes 8
                                                            6
                                                                                                                   Kasami 7
                                                                                                                   Walsh 8
                                                                                                                   Gold 7
                                                            5



                                                            4
                                                   |x(k)|




                                                            3



                                                            2



                                                            1



                                                            0
                                                                 0   0.5     1        1.5       2            2.5    3       3.5
                                                                                     frequency (rad)

           Fig.2 Magnitude plot of Walsh, Gold, Kasami and proposed orthogonal codes

                                                            4                                                      Proposed codes 8
                                                                                                                   Kasami 7
                                                                                                                   Walsh 8
                                                            3
                                                                                                                   Gold 7

                                                            2


                                                            1
                                      arg(x(k))




                                                            0


                                                            -1


                                                            -2


                                                            -3


                                                            -4
                                                                 0   0.5     1        1.5       2            2.5    3       3.5
                                                                                     frequency (rad)

             Fig3. Phase plot of Walsh, Gold, Kasami and proposed orthogonal codes

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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME

        The ability of a DS-CDMA receiver to detect the desired signal relies to a great extent on the
auto-correlation properties of the spreading codes and on the other hand multi-user interference
rejection depends on cross correlation properties of the spreading sequences. In synchronous DS-
CDMA system, the code sequence in the receiver is exactly same as that in the transmitter.
Orthogonal codes are most suitable for synchronous communication. Two sequences are said to be
orthogonal when the cross-correlation (inter-code correlation) between them is zero. Codes with high
auto-correlation and low cross-correlation are preferred in synchronous communication. Auto-
correlation sequence for all user codes is shown in Fig.4.
                                                   Kasami 7 Proposed codes 8

                                                                                   1
                                                                                   0
                                                                               -1
                                                                                 1         2           3               4            5       6       7   8

                                                                                   1
                                                                                   0
                                                                               -1
                                                                                 1             2               3              4         5       6       7

                                                                                   1
                                                   Walsh 8




                                                                                   0
                                                                               -1
                                                                                 1         2           3               4            5       6       7   8

                                                                                   1
                                                   Gold 7




                                                                                   0
                                                                               -1
                                                                                 1             2               3              4         5       6       7
                                                                                                                            Delay

                    .
           Fig.4 Auto-Correlation sequences of Walsh, Gold, Kasami and proposed codes

        Fig.5 displays the cross-correlation property of all user codes. It can be inferred from the
figure that auto-correlation and cross-correlation sequences of Gold and proposed code are almost
similar. Deviation value of Gold code is comparable to proposed code.
                      Kasami 7 Proposed codes 8




                                                  0.5
                                                                0
                                                  -0.5
                                                                               1       2           3               4                5       6       7   8

                                                  0.5
                                                                0
                                                  -0.5
                                                                               1           2               3                 4          5       6       7

                                                  0.5
                      Walsh 8




                                                                0
                                                  -0.5
                                                                               1       2           3               4                5       6       7   8

                                                  0.5
                      Gold 7




                                                                0
                                                  -0.5
                                                                               1           2               3                 4          5       6       7
                                                                                                                           Delay


           Fig.5 Cross-correlation sequences of Walsh, Gold, Kasami and proposed codes



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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME

4. BER PERFORMANCE OVER AWGN CHANNEL

        Bit Error Rate (BER) specifies the ratio of the number of bits which are received with error to
the number of bits transmitted. Very low value of BER is desired for good performance of the
system. In this paper, the code with decimal equivalent 49 and 107 are used for BER performance for
two users over AWGN channel. Fig.6 and Fig.7 show the BER performance of all user codes with
single and two users in the system respectively. BER curve of Walsh and proposed codes is very
closely spaced.




 Fig.6 BER performance of Walsh, Gold, Kasami and proposed codes for synchronous DS CDMA
                 communication over AWGN channel in a single user scenario




 Fig.7 BER performance of Walsh, Gold, Kasami and proposed codes for synchronous DS CDMA
                  communication over AWGN channel in a two users scenario



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International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME

5. CONCLUSION

       Flexible size orthogonal codes can be used in emerging applications of multiple carrier
CDMA communication system. The proposed orthogonal codes have better performance in
comparison to other user codes. It is clear that the capability of synchronous CDMA system can be
improved by employing the proposed orthogonal user codes. The extension of the proposed
orthogonal codes to channels with multiple users is to be investigated.

REFERENCES

 [1] Ali N. Akansu., Radha Poluri, Walsh-Like Nonlinear Phase Orthogonal Codes for Direct
     Sequence CDMA Communications, in IEEE Trans. on signal processing, Vol 55. No. 7, July
     2007.
 [2] Mosa Ali Abu – Rgheff, Introduction to CDMA Wireless Communications (Elsevier, 2005).
 [3] Sarwate D.V., Pursley M.B., Correlation properties of pseudo random and related sequences,
     Proc. Of IEEE, 68(5), 1980, pp. 593-619.
 [4] Walsh J.L., A Closed set of Normal Orthogonal Functions, American Journal of Mathematics,
     45, 1923.
 [5] Faîçal Baklouti and Rabah Attia, “Numerical Suppression of Linear Effects in an Optical
     CDMA Transmission”, International Journal of Electronics and Communication Engineering &
     Technology (IJECET), Volume 3, Issue 3, 2012, pp. 112 - 121, ISSN Print: 0976- 6464,
     ISSN Online: 0976 –6472.
 [6] Prof. B.M. Mohan and Sanjeeb Kumar Kar, “Optimal Control of Multi-Delay Systems Via
     Orthogonal Functions”, International Journal of Advanced Research in Engineering &
     Technology (IJARET), Volume 1, Issue 1, 2010, pp. 1 - 24, ISSN Print: 0976-6480,
     ISSN Online: 0976-6499.
 [7] J.Ravindrababu and Dr.E.V.Krishna Rao, “Performance Analysis and Comparison of Linear
     Multi-User Detectors in DS-CDMA System”, International Journal of Electronics and
     Communication Engineering & Technology (IJECET), Volume 3, Issue 1, 2012, pp. 229 - 243,
     ISSN Print: 0976- 6464, ISSN Online: 0976 –6472.




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