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Distortion Compensation Scheme for PAPR Reduction of OFDM Signals

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Distortion Compensation Scheme for PAPR Reduction of OFDM Signals Powered By Docstoc
					A Novel Clipping Integrated Into ACE for PAPR
        Reduction in OFDM Systems


 Yang Zhou and Tao Jiang Wuhan National Laboratory
  for Optoelectronics Huazhong University of Science
                and Technology Wuhan,

    Wireless Communications & Signal Processing,
                2009. WCSP 2009

        Reporter: HSIU CHI,KOU (97325515)
Outline

• Introduction

• OFDM System with Conventional ACE for PAPR
  Reduction

  A. Definition of Peak-to-Average Power
     Ratio
  B. Conventional Active Constellation
     Extension
  C. ACE with Repeated Clipping

• Simulations and Results

• Conclusion
INTRODUCTION
• Orthogonal frequency division multiplexing (OFDM) has been
  standardized in many wireless applications with highs peed
  data transmission.

• However, one of the major challenges in the design of
  practical OFDM systems is its high peak-to-average power
  ratio (PAPR) of the time domain transmitted OFDM signals
  when the number of sub-carriers is large.

• Among all existing techniques of PAPR reduction, active
  constellation extension (ACE) is a very attractive scheme due
  to its good performance of PAPR reduction for constellation
  with small size and no restriction of the number of the sub-
  carriers.
• Commonly, conventional ACE is combined with the repeated clipping
  method (ACE-RC) to obtain active constellation points.

• However, ACE-RC suffers from a huge challenge, i.e., its
  convergence rate is too slow due to the peak regrowing, resulting
  in large increasing of its computation complexity.

• In this paper, we propose a novel method, called as repeated-
  enlipping (RE), to integrate into conventional ACE to effectively
  reduce the PAPR of OFDM signals.
OFDM SYSTEM WITH CONVENTIONAL ACE FOR
PAPR REDUCTION
A. Definition of Peak-to-Average Power Ratio
•   An OFDM signal is produced by a block of N data symbols
    X = {Xk, k = 0, 1, · · · ,N −1} that are typically chosen from
    an M-ary phase-shift keying (PSK) or quadrature amplitude
    modulation (QAM) constellations.

•    The resulting time domain OFDM signal is expressed as




     where T is OFDM signal duration, and △f = 1/T is two
     adjacent subcarriers separation.
• The oversampling by a factor of L is used to approximate the real
  PAPR of x(t), where L is an integer larger than . The L-time over-
  sampled signal is given by

                                  2kn
                     1 N 1     j
            x[n]         X k e LN , 0  n  LN
                     LN k  0

• Therefore, the PAPR of the transmitted OFDM signals is defined
  as




   where E[|x(n)|2] is the average power.
B. Conventional Active Constellation Extension

• With conventional ACE, some data symbols in an OFDM data block
  are changed to reduce the PAPR, and the peak-reduced signals
  could be expressed as




   where x (t ) is the peak power reduced signals, and   X is the new
   data block that are changed by ACE method.

• Let set R be the location of symbols that are not changed. Let set
  Rc be the complement of R in N = {0, 1, · · · ,N − 1}, X could be
  obtained as



  where Xk represents the changed data symbol.
• For example, as shown in Fig. 1, there are four constellation points, i.e.,
  1+j,−1+j,−1−j, 1−j, and their corresponding shaded regions are region1,
  region2, region3 and region4 when the modulation is 4-QAM.
C. ACE with Repeated Clipping


• Commonly, conventional ACE is combined with the repeated clipping
  (RC) method, called as ACE-RC method, to reduce the PAPR of the
  time domain OFDM signals.

• The time domain signal x(n) is first clipped to a predefined threshold
  A. If the amplitude of x(n) is larger than A, we clip it to A without
  changing its phase. If the amplitude of x(n) is smaller than A, the x(n)
  keeps the original amplitude. After the clipping, we get the new time
  domain signal x(n).
         X
• Then, we deploy an inverse fast Fourier transform (IFFT) operation to
  the new generated time domain signal x(n), and thus we have the
  new data block .
• Compare X and X , we find the changed data symbols in new
  data block X that may maintain the minimum distance and
  replace the original symbols with the changed data symbols.

• Obviously, the ACE-RC scheme could largely reduces the
  PAPR of OFDM signals.

• However, one issue of the ACERC scheme is its slow
  convergence rate, which leads to a tradeoff between the
  complexity and the performance of PAPR reduction in OFDM
  systems.
PROPOSED ACE WITH REPEATED ENLIPPING METHOD

  • In this section, a novel clipping method, called as repeated
    enlipping (RE), to integrate into conventional ACE to
    effectively reduce the PAPR of OFDM signals.

  • The difference between the original clipping and the proposed
    novel RE method is that the novel RE method not only clips
    the large signals but also enlarges the small signals.




    where A is the predefined threshold and θ(n) is the phase of
    x(n).
• According to above discussions, We describe the steps of the
  proposed ACE-RE scheme as follows.
  Step 1: Deploy an IFFT operation to the data block X to get the time
          domain signals x(n);
  Step 2: According to proposed novel RE method x(n) is clipped and
          the new time domain
          signal x(n) could be obtained;
  Step 3: Deploy a FFT operation to x(n) to obtain the new data block
           ~
           Xk ;
  Step 4: In this step, we take a processing to data block X to reduce
           the PAPR of the transmitted signals. When a data symbol ˜Xk
           is in the extension region corresponding to Xk, we replace Xk
                  ~
           with X k . Otherwise, we keep Xk unchanged. After all data
           symbols are processed by ACE, we have the new data block
           X .
•   Step 5: Deploy an IFFT operation to X and go to step 2, repeat the steps
    until the number of iterations equals L.

•   Fig. 2 shows the constellation of a data block with ACE scheme to reduce its
    PAPR. It is obvious for us to find that the constellation has been changed
    after using ACE compared with the original signals.
SIMULATIONS AND RESULTS

• To evaluate the performances of the proposed ACE-RE
  scheme in terms of PAPR reduction and convergence rate in
  OFDM systems, 104 OFDM signals are randomly generated
  and are modulated by 4QAM with the number of sub-carriers
  N = 256 and the clipping threshold A = 7.99-dB throughout
  simulations.

• Fig. 3 shows different performances of the PAPR reduction by
  the ACE-RC scheme, where the maximum number of
  iterations is 16.

• Fig. 4 shows that the performance of PAPR reduction is
  improved by using the proposed ACE-RE scheme in OFDM
  systems.
• Fig. 5 depicts that the performance comparisons in terms of the
  convergence rate and PAPR reduction between the ACERC
  and the proposed ACE-RE scheme.
CONCLUSION

• In this paper, a novel clipping scheme, called as RE, is
  proposed to integrate into the conventional ACE scheme for
  PAPR reduction in OFDM systems.

• To evaluate the PAPR reduction performance and
  convergence rate, we conduct some simulations and the
  results show that the proposed scheme, i.e., ACE-RE method,
  can provide a better PAPR reduction with a faster
  convergence rate in OFDM systems, compared with the ACE-
  RC scheme.
REFERENCES

• B. S. Krongold, and D. L. Jones. PAR reduction in OFDM via active
  constellation extension, IEEE Transactions on Broadcasting, vol. 49,
  no. 3, pp. 258-268, Sept. 2003.

• J. Armstrong. Peak-to-average power reduction for OFDM by
  repeated clipping and frequency domain filtering, IEE Electronics
  Letters, vol. 38, pp. 246-247, May. 2002