COOPERATIVE PARTIAL TRANSMIT SEQUENCE FOR PAPR REDUCTION IN SPACE FREQUENCY BLOCK CODE MIMO-OFDM SIGNAL

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COOPERATIVE PARTIAL TRANSMIT SEQUENCE FOR PAPR REDUCTION IN SPACE FREQUENCY BLOCK CODE MIMO-OFDM SIGNAL Powered By Docstoc
					            INTERNATIONAL Communication Engineering & Technology AND
International Journal of Electronics andJOURNAL OF ELECTRONICS(IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME
   COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)

ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 3, Issue 2, July- September (2012), pp. 321-327
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        COOPERATIVE PARTIAL TRANSMIT SEQUENCE FOR
       PAPR REDUCTION IN SPACE FREQUENCY BLOCK CODE
                    MIMO-OFDM SIGNAL

                              1
                                Bharti Rani, 2 Mrs Garima Saini
                            1
                              Scholar, M.E, NITTTR, Chandigarh
                         2
                           Assistant Prof. ECE, NITTTR, Chandigarh
                                Email id erbharti2@gmail.com


ABSTRACT

Orthogonal frequency division multiplexing (OFDM) is a popular method for high data rate
wireless transmission. OFDM is combined with antenna to increase the diversity gain to
enhance the system capacity on time-variant and frequency-selective channels, resulting in a
multiple-input multiple-output (MIMO) configuration. To better handle fading channel,
Space frequency block code (SFBC) MIMO-OFDM is preferred. In this paper space
frequency block code (SFBC) is used for reducing PAPR using Co-operative partial transmit
sequence (Co-PTS).

Index Terms: Space frequency block code (SFBC), Multiple-input multiple-output (MIMO),
Orthogonal frequency division multiplexing (OFDM), peak-to-average power ratio (PAPR)

INTRODUCTION

With the constant demand of high spectral efficiency and high transmission speed for audio,
video and internet applications, MIMO-OFDM has become the most promising technology
combination for present and future wireless communications [1]. MIMO offers spatial
diversity and therefore increase the capacity while OFDM allow systems to work in time
varying or frequency selective environment.      Recently, Multiple input multiple output
(MIMO) Orthogonal frequency division multiplexing (OFDM) with space frequency block
code (SFBC) has attracted increasing attention because it is robust to time selective fading
channels [2]. However, SFBC MIMO-OFDM signal also inherit disadvantages from OFDM
techniques e.g sensitivity to synchronization errors and high peak-to-average power ratio
(PAPR).Therefore many PAPR reduction methods have been introduced.




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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME

Especially, the signal scrambling methods such as partial transmit sequence(PTS), selective
mapping, ploy phase interleaving and inversion(PII) ,cross-antenna translation and partial
shift sequence method [3,4,5,6,7] .All the PAPR reductions methods have some drawbacks
such as increase in transmit power, high computational complexity, high bit error rate(BER),
, reduction in bit transmission rate of the system and high peak-to-average power ratio. In this
paper, a co-operative partial transmit sequence (Co-PTS) is proposed for SFBC MIMO-
OFDM signal [8]. In Co-PTS, alternate optimization and spatial sub block circular
permutation are combined. The use of alternate optimization results improvement in
performance for PAPR reduction.

OFDM and PAPR

In OFDM modulation, a block of N symbols Xn (n=0,1,2,…N-1) is transmitted in parallel,
and each of them modulates a group of N subcarriers fn(n=0,1,2…,N-1). The subcarriers are
orthogonal to each other, and fn = n∆f, where ∆f= 1/T and T is the symbol period. The
resulting baseband OFDM signal x(t) can be written as:

                            ∆
   x(t)   =                           0≤ <           (1)


In real implementation, the digital transmission signal may be generated by the Inverse Fast
Fourier Transform (IFFT) in the transmitter and restored by Fast Fourier Transform (FFT) in
the receiver. The PAPR of the OFDM signal can be defined as:

                            {     }
 PAPR(X) =10           {    }
                                                     (2)

Where E{.} denotes the expectation operation. Because independent phases of subcarriers
may align with each other, OFDM signals often exhibit a high PAPR, and the PAPR
increases proportionally with the number of subcarriers. The complementary cumulative
distribution function (CCDF) is often used to characterize the probability that PAPR of an
OFDM signal exceeds a given threshold (PAPR0), which can be expressed as

CCDF =        Pr(PAPR>PAPR0)                   (3)



SFBC MIMO-OFDM System

The SFBC MIMO-OFDM system that employs Alamouti scheme, although is an extension to
more than two transmits antennas [7, 9]. A block diagram of such scheme is shown in Fig 1.
A data symbol vector X = [X0, X1, . . . , XN−1] is encoded with space-frequency encoder into
two vectors X1 and X2,




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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME



                                                               TX1

                                                   IFFT & CP
             X       S/P       Space freq                      TX2

                               Encoder             IFFT & CP



                                                                     Y
            ^X
                     P/S       Space freq       PR & FFT
                               Decoder


                   Fig 1. Block Diagram of SFBC MIMO-OFDM System

X1 = [X0, -X1*…... XN−2,-X*N-1]
X2 = [X1, X0*…... XN−1,-X*N-2]                  (4)

Which are fed to the IFFT Cyclic Prefix (CP) blocks and sent simultaneously from antennas
TX1 and TX2, respectively. The demodulated signal at the receiver is then given by

Y = Λ1X1 + Λ2X2 + Z,                            (5)

After decoding the Y signal we get the signal ^X

Proposed Co-PTS Schemes

In this paper, a cooperative partial transmit sequence (co-PTS) is proposed with SFBC
MIMO-OFDM signal [8, 10]. In cooperative partial transmit sequence (Co-PTS) , alternate
optimisation and spatial sub block circular permutation are combined, where the use of
alternate optimisation results in reducing computational complexity and the use of spatial sub
block circular permutation across all the transmitting antenna is able to increase the number
of candidate sequences, which equivalently improves the performance for PAPR reduction.
                                                TX1
 D/S      S/P       SFBC    SSCP      IFFT
                                                TX2


Fig 2. Proposed Co-PTS with SFBC MIMO-OFDM

The proposed design is shown in Fig 2. Firstly the input sequence is modulated by QPSK
modulation and converting into parallel form. The modulated signal X is coded into two
vectors X1(n) and X2(n) by space frequency encoder block and a factor spatial sub-block
circular permutation (SSCP) & alternate partial transmit sequence are introduced to achieve
the modulate PAPR reduction performance.

Algorithm:-
 Step1:- The modulated signal X, X(n) = [X1, X2, X3,………….. XN-1] the frequency domain
sequence of OFDM system with N subcarriers


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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME

Step2:- The data symbol vector Xv (n) is coded into two vectors X1(n) and X2(n) by space
frequency encoder block as

X1 (n) = [X0(n), −X1*(n),…….., XN−2(n),-X*N-1(n)]

X2 (n) = [X1(n), X0*(n),…….., XN−1 (n),-X*N-2(n)]

Step3:- To achieve the minimum PAPR, a factor for spatial sub-block circular permutation
(SSCP) and alternate optimisation are used.

Step4:- All SSCP data block are transformed into time domain to get transmitted symbol or
simply take IFFT of that sequence, Finally, the one signal with the minimum PAPR is
selected for transmission
SIMULATIONS AND RESULTS
The complementary cumulative distribution function (CCDF) of the PAPR of OFDM signals
with 128 subcarriers, 4 & 8 sub block and phase weighting factors (±j) & (±1) are used to
evaluate the PAPR reduction performance of Co-PTS compared with original OFDM and
SFBC MIMO-OFDM Signal. In the following results, 105 random QPSK modulated OFDM
signals are generated for simulation and the oversampling factor L is four.
                                  0
                                 10
                                                               co-pts sfbc mimo ofdm
                                                               sfbc mimo ofdm
                                                               orignal
                                  -1
                                 10
                          ccdf




                                  -2
                                 10




                                  -3
                                 10




                                  -4
                                 10
                                       2   4   6       8      10         12            14
                                                   papr(dB)


   Fig 3. PAPR performance of SFBC MIMO-OFDM signal in case 128 subcarriers, 4 sub
                          block and phase weighting factor ±1

                                  0
                                 10
                                                               co-pts sfbc mimo ofdm
                                                               sfbc mimo ofdm
                                                               orignal
                                  -1
                                 10
                          ccdf




                                  -2
                                 10




                                  -3
                                 10




                                  -4
                                 10
                                       2   4   6       8      10         12            14
                                                   papr(dB)




   Fig 4. PAPR performance of SFBC MIMO-OFDM signal in case 128 subcarriers, 8 sub
                          block and phase weighting factor ±1


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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME

                                  0
                                 10
                                                               co-pts sfbc mimo ofdm
                                                               sfbc mimo ofdm
                                                               orignal
                                  -1
                                 10




                          ccdf
                                  -2
                                 10




                                  -3
                                 10




                                  -4
                                 10
                                       2   4   6       8      10         12            14
                                                   papr(dB)


   Fig 5. PAPR performance of SFBC MIMO-OFDM signal in case 128 subcarriers, 4 sub
                          block and phase weighting factor ±j

                                  0
                                 10
                                                               co-pts sfbc mimo ofdm
                                                               sfbc mimo ofdm
                                                               orignal
                                  -1
                                 10
                          ccdf




                                  -2
                                 10




                                  -3
                                 10




                                  -4
                                 10
                                       2   4   6       8      10         12            14
                                                   papr(dB)


   Fig 6. PAPR performance of SFBC MIMO-OFDM signal in case 128 subcarriers, 8 sub
                          block and phase weighting factor ±j

  Table1. Proposed PAPR values of 128 subcarrier for SFBC MIMO-OFDM with Co-PTS
                                      Technique

                     S.no Subcarrier Subblock Modulation PAPR
                      1      128        4      QPSK      4.7dB
                      2      128        8      QPSK      4.3dB
                      3      128        4      QPSK      5.8dB
                      4      128        8      QPSK      6.0dB




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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME

                     Table 2. Comparison between the various techniques
                   Techniques           Parameter     Modulation       PAPR
                                                          Type
                     PTS for          Subcarrier-128      QAM          6.4dB
                 MIMO-OFDM             Sub block-8
             PII for SFBC MIMO-       Subcarrier-128      QAM          8.2dB
                     OFDM              Sub block-16
            CAT for SFBC MIMO- Subcarrier-256             QAM          7.5dB
                     OFDM              Sub block-4
             Partial Shift Sequence   Subcarrier-128     QPSK          8.5dB
               for SFBC MIMO-          Sub block-4
                     OFDM
              Co-PTS for MIMO-        Subcarrier-128     QPSK          7.4dB
                     OFDM              Sub block-8
            SLM for MIMO-OFDM Subcarrier-128             QPSK          8.6dB
                                       Sub block-2



It is observed form table 2 that the PAPR ratio in reference literature [3,4,5,6,7] for MIMO-
OFDM & SFBC MIMO-OFDM was found large as compared to the proposed table 1. With
SFBC MIMO-OFDM by using Co-PTS technique. Hence, this proposed technique gives
better result of PAPR approximately 4.7dB with 8 subblock compared to previous techniques.

CONCLUSION

The proposed Co-PTS technique with SFBC MIMO-OFDM signal makes use of alternate
optimisation to reduce peak to average power ratio. At the same time, the number of
candidate sequences is increased by employing spatial sub block circular permutation, which
improves PAPR reduction performance equivalently.

REFERENCES


[1] Helmut Bolcskei, Eth Zurich, “MIMO-OFDM Wireless Systems: Basic, Perspectives, and
Challenges”, IEEE transaction on wireless communication, Vol 13, pp31-37, Aug, 2006

[2] Sen-Hung Wang; Chih-Peng Li, “A Low-Complexity PAPR Reduction Scheme for SFBC
MIMO-OFDM Systems” IEEE transaction on signal processing letters, Vol.16, No.11,
pp941-944, 2009.

[3] Phetsomphou, D.; Yoshizawa, S.; Miyanaga, Y, “A partial transmit sequence technique
for papr reduction in MIMO -OFDM Systems” IEEE Conferences on communications and
information technologies, pp672-676, 2010

            ,                ,
[4] Gao Jing Wang Jinkuan Xie Zhibin, “A New concurrent SLM algorithm for STBC
MIMO-OFDM System” IEEE Conferences on Intelligent Signal Processing and
Communication Systems, pp132-135, Nov.28-Dec.1, 2007



                                             326
International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME

 [5] Zoran Latinovi´c, Student Member, IEEE, and Yeheskel Bar-Ness, Fellow, IEEE,
“SFBC MIMO-OFDM Peak-to-Average Power Ratio Reduction by Polyphase Interleaving
and Inversion” IEEE, Communications Letters, Vol.10, No.4, pp266-268, April, 2006

[6] Yuan Ouyang, “Peak-to-Average Power Ratio Reduction by Cross-Antenna Translation
for SFBC MIMO-OFDM Systems”IEEE Conference on vehicular technology, Vol.9, No.7,
pp592-594, July 2009.

[7] Jae Wook Kwon, Sang Kyu Park, “Peak-to-average power ratio reduction by the partial
shift sequence methods for space frequency block code ofdm systems” IEEE International
Conferences on Network Infrastructure and Digital Content,pp890-893, 2009.

[8] L. Wang and J. Liu, “Cooperative PTS for PAPR reduction in MIMO-OFDM” IEEE
Electronics Letters, Vol. 47, No. 5, pp472-474, March 2011

[9] King F. Lee and Douglas B. Williams “A Space-Frequency Transmitter Diversity
Technique for OFDM Systems” IEEE Conferences on Global Telecommunications, Vol.3, pp
1473-1477, 2000

 [10] A. D. S. Jayalath, C. Tellambura and H. Wu “Reduced complexity PTS and new phase
sequence for SLM to reduce PAPR of an OFDM signal” IEEE Conferences on Vehicular
Technology,Vol.3,pp 1914 – 1917,2000




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