PAPR Reduction of OFDM Signal Using Error Correcting Code and Selective Mapping by ijcsiseditor


									                                                                              International Journal of Computer Information Systems,

                     Anshu                                                              Er. Himanshu Sharma
                 ECE Deptartment                                                     Lecturer, ECE Department
        Mahrishi Markendeshwar University                                        Maharishi Markendeshwar University
                  Mullana, India                                                            Mullana, India

                                                                        of selective mapping (SLM) [1], in which a set of independent
                       I.   ABSTRACT                                    sequences are generated by some means from the original
   Orthogonal frequency division multiplexing (OFDM)                    signal, and then the sequence with the lowest PAPR is
technique is a promising technique to offer high data rate and          transmitted. To generate these sequences we use code en
reliable communications over fading channels. The main                  encoder. Using error correcting coding will offer two
implementation disadvantage of OFDM is the possibility of               advantages,significant PAPR reduction and astonishing bit
high peak to average power ratio (PAPR). This paper presents            error rate (BER) performance.
a novel technique to reduce the PAPR using errorcorrecting
coding and selective mapping (SLM).We show that the                        The rest of the paper is organized as follows: The problem
probability of the PAPR of OFDM signal with 100 subcarriers.            of high PAPR of OFDM signal is briefly defined in section 2.
                                                                        Section 3 introduces the proposed technique. Some simulation
Keywords-OFDM,SLM,CCDF, PAPR, PAR.                                      results are shown in section 4. Finally, the conclusions are
                                                                        drawn in section 5.
                 II.    INTRODUCTION
                                                                                    III.   PROBLEM DEFINITION
   OFDM, orthogonal frequency division multiplexing, is a
multicarrier communication technique, where a single data                  We suppose an OFDM transmission scheme, where a block
stream is transmitted over a number of lower rate subcarriers.          of N complex symbols is first over-sampled using over
OFDM has become tangible reality, it has been employed for              sampling factor J and then transformed into time domain using
wire-line communications and also has been employed in                  the inverse fast Fourier transform (IFFT). This results in the
wireless local area network (WLAN) e.g. IEEE 802.11. Other              following signal:
applications of OFDM are digital audio broadcasting (DAB)
and digital video broadcasting (DVB).
   Unfortunately, OFDM has the drawback of a potentially
high peak to average power ratio (PAPR). Since a multicarrier           Where sk is the data to be transmitted and 1 < t < NJ.
signal consists of a number of independent modulated
subcarriers that can cause a large PAPR when the subcarriers                    The PAPR is defined as the ratio between the
are added up coherently.                                                maximum powers occurring in OFDM symbol to the average
                                                                        power of the same OFDM symbol:
   To reduce the PAPR different techniques were proposed.
These techniques can be categorized into the following,
clipping and filtering [1], coding [2], phasing [3], scrambling                                                                     (2)
[4], interleaving [5], and companding [6].

   In this paper we propose and examine a technique for                 WhereE [.] denotes expectation.
reducing the probability of a high PAPR, based on part on a             When the OFDM signal with high PAPR passes through a
method proposed in [1] and [8]. This technique is a variation           non-linear device, (power amplifier working in the saturation

                                                                                                   ISSN 1947-5500
                                                                                International Journal of Computer Information Systems,
region), the signal will suffer significant non-linear distortion         same information. In order to achieve a PAPR reduction, the
[9]. This non-linear distortion will result in in-band distortion         symbol with the lowest PAPR is transmitted. We define
and out-of-band radiation. The in-band distortion causes
system performance degradation and the out-of-band radiation                                                                            (6)
causes adjacent channel interference (ACI) that affects
systems working in the neighbour bands. To lessen the signal
distortion, it requires a linear power amplifier with large
dynamicrange. However, this linear power amplifier has poor               In SLM, we use log2 D bits side information to indicate the
efficiency and is so expensive.                                           phase weighting. As this side information is of highest
                                                                          importance to recover the data, it should be carefully protected
                                                                          by channel coding.

                                                            x             With , the transmitted signal is          . In the receiver, X can
                                                                          be recovered with
      v                                              x
  Label                                                  Select
                  encod      Mapp         IFFT
                                                                          To recover X it is necessary for the receiver to have a table of
                                                                          all     .

                     Figure: 1 System model                               The phase sequence combination that results in the lowest
                                                                          PAR of the time-domain signal is used for transmission. Here
                                                                          we encode the information fist with forward error correcting
 IV.       SELECTIVE MAPPING USING ERROR                                  code and then do SLM. The technology combines SLM, which
              CORRECTING CODING                                           aims at PAPR reduction, and coding(basically we use
                                                                          hamming code, rsc code, and convolution code), which are
Selected mapping (SLM) is a specific scheme for                           excellent in error control and are play excellent role in further
PAPRreduction that was introduced in [13].SLM takes                       reduction in PAPR. In this paper we show comparison of all
advantage of the fact that the PAR of an OFDM signal is very              the three coding with SLM.
sensitiveto phase shifts in the frequency-domain data. PAR
reduction is achieved by multiplying independent phase                    After coding the process is done just like what is applied on
sequences to the original data and determining the PAR of                 the carriers in the usual SLM algorithm. The process is given
each phasesequence /data combination. The combination with                in figure1. Finally all of the different sequences, after serial to
the lowest PAR is transmitted. In other words, the data                   parallel conversion, pass through the IFFT block to produce D
sequence X is element-wise phased by D N-length phase                     block of time domain signal. The block with the lowest PAPR
sequences,                                                                is to be sent to the receiver through the channel.

                                                         (3)                           V.    SIMULATION RESULTS
                                                                          In this section the results obtain through the simulations using
Where d is an integer such that                            After          MATLAB are examined. Quarter phase shift keying (QPSK)
phasing, the D possible frequency-domain                  OFDM            is used. The results are given here in terms of PAPR-CCDF.
                                                                          First we show comparison of PAPR with SLM and the original
symbols                        where      is the element-wise             OFDM signal. Secondly the comparison with SLM and coded
multiplication.                                                           SLM.

We assume that                                                            In the following the performance of the system for both
                                                                          conventional SLM and coded are examined.
                                                         (4)                A. CCDF-PAPR
                                                                          Figure 1 shows the difference between the results of SLM and
                                                                          Original OFDM signal in terms of CCDF-PAPR. In this we
So that                                                  (5)              consider the OFDM signal with number of subcarriers N=64.
                                                                          The hamming encoder has constraint length K=16 is used.
  Define the D candidate time-domain OFDM symbols x(d) =                    B. CCDF-PAPR
IFFT{X(d)}. Note that all of the candidate symbols carry the

                                                                                                      ISSN 1947-5500
                                                                             International Journal of Computer Information Systems,
Figure 2 in this section, the performance of the system for            reduces the hardware complexity of the system. An excellent
each coded SLM i.e. SLM using hamming code,SLM using                   style manual for science writers is [7].
rsc code and SLM using convolution code. In this the OFDM
signal with number of subcarriers N=100 and hamming                                VII.   ACKNOWLEDGEMENT
encoder has constraint lengthK=16 is used.                             I would like to express my gratitude toward those who have
                                                                       supported me through the work. First, I would like give my
                                                                       sincerest thanks to my guide, Er. Himanshu Sharma, for his
                                                                       unreserved guidance, support and encouragement. He has
                                                                       looked out for me at every time and has given me every
                                                                       opportunity to succeed; for that, I am truly grateful. I consider
                                                                       myself lucky to be one of her students. I want to thank my
                                                                       thesis committee, Dr.AmitGarg and Dr. H.P Sinha, for taking
                                                                       the time to review and critique my work. Additionally, I would
                                                                       like to thank my fellow research group members for their
                                                                       friendship and insightful comments. Finally, I want to thank
                                                                       my family, who have supported me through every step of my

                                                                                          VIII.   REFERENCES
                                                                       [1] X. Li and L. J. Cimini Jr., ”Effects of clipping and filtering
                                                                       on the performance of OFDM,” IEEE Communication Letters,
                                                                       pp. 131-133, May 1998.

                                                                       [2] A. E. Jones, T. A. Wilkinson and S. K. Barton, ”Block
                                                                       coding scheme for reduction of peak to mean envelope power
Figure2: Comparison between the original OFDM &SLM                     ratio of multicarrier transmission schemes,” Electronic Letters,
                                                                       pp. 2098-2099, Dec. 1994.

                                                                       [3] V. Tarokh and H. Jafakhani, ”On the computation and
                                                                       reduction of peak to average power ratio in multicarrier
                                                                       communications,” Proc. IEEE 53rd Vehicular Technology
                                                                       Conference, vol.2, pp. 37-44, Jan 2000.

                                                                       [4] P. Van Eetvelt, G. Wade and M. Tomlinson, ”Peak to
                                                                       average power reduction for OFDM schemes by selective
                                                                       scrambling,” Electronic letters, vol. 32, pp. 1963-1994, Jan

                                                                       [5] A. D. S. Jayalath and C. Tellambura ” The use of
                                                                       interleaving to reduce the peak to average power ratio of an
                                                                       OFDM signal,” Global Telecommunication

                                                                       Conference, 2000, GLOBECOM ’00. IEEE, vol. 1, pp. 82-86,
                                                                       Nov.-1 Dec. 2000.

Figure3: Comparison among the various coding with SLM                  [6] Xiao Huang, Jianhua Lu, Justin Chuang, and JunliZheng,
                                                                       ”Companding transform for the reduction of peak to average
                 VI.   CONCLUSIONS                                     power ratio of OFDM signal,” IEEE Trans. On Common. vol.
                                                                       48, pp. 835-839, May 2001.
We have shown that, coding and SLM can be combined to
reduce the PAPR of OFDM signal with quite moderate
                                                                       [7] R. B¨auml, R. Fischer and J. Huber, ”Reducing the peak to
additional complexity. The advantage of the proposed scheme
                                                                       average power ratio of multicarrier modulation by selected
is that, the coding is used for two purposes, error correction
                                                                       mapping,” Elect. Letts., vol. 32, pp. 2056-2057, Oct. 1996.
and PAPR reduction. Here we see that combination of RS
code and SLM gives the better results instead of hamming
with SLM and convolution with SLM as shown in fig 2.This

                                                                                                   ISSN 1947-5500
                                                                      International Journal of Computer Information Systems,
[8] N. Carson, T. A. Gulliver, ”PAPR reduction of OFDM
using selected mapping, modified RA codes and clipping”, in
Proc. IEEE VTC, vol.2, pp. 1070-1073, Sep. 2002.

[9] S. Merchan, A. Garcia Armada and J. L. Garcia,” OFDM
performance in ampli- er nonlinearity,” IEEE Trans. On
Broadcasting, vol. 44 Issue:1, pp.106-114, Mar. 1998.

[10] M. Breiling, S. M¨uller-Weinfurtner and J. Huber, ”Peak-
power reduction in OFDM without explicit side information,”
Proc. 5th Int. OFDM Workshop, Germany, pp- 28.1-

[11] C. Berrou, A. Glavieux and P. Thitimajshima, ”Near
Shannon limit error correcting coding and decoding: Turbo
Codes.” in Proc. of ICC’93, (Geneva, Switzerland), pp. 1064-
1070, May 1993.

[12] S. Benedetto, G. Montorsi, D. Divsalar, and F. Pollara,
”A soft-input soft-output maximum a posterior (MAP) module
to decode parallel and serial concatenated codes”, TDA
progress report 42-127, Jet Propulsion Lab., Pasadena, CA,
Nov. 15 1996.

[13] K.Kasiri&M.j.Dehghani, “A Blind SLM Scheme for
Reduction of PAPR in OFDM System.”

Author Profile:

M.Sc., Kurukshetra University
Pursuing M.Tech at Mahrishi Markendeshwar University,

Er. Himanshu Sharma
Currently he is a Lecturer, ECE Department
Mahrishi Markendeshwar University, MULLANA
Experience in Wireless Communication.

                                                                                         ISSN 1947-5500

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