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Ryu et al.:
PAPR Reduction Using Soft Clipping and ACI Rejection in OFDM System
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PAPR REDUCTION USING
SOFT
CLIPPING AND ACI REJECTION IN OFDM SYSTEM
Heun g-Gyo on Ryu, Byoung-l l
J in
and In -Bae
Kim
Depar t men t
of
Elect ronic Engineer ing, Chungbu k National
University
San 48 Kaes i n -dong , Cheon g j u Chungbuk , 361-763, Republ ic of Korea
E-mail:
bravo1 @hanmai l .net , h i ber@hanbox .com
Abstract
OFDM(orthogona1 frequency division multiple-
xing) is usefully applied for the DAB (digital
audio broadcasting) and DVB(digita1 video
broadcasting) system due tu the high spectral
efficiency and the robustness to the
IS1
(intersymbol interference) and multipath fading.
However, the OFDM signal has a large PAPR
(peak to average power ratio), which results in the
significant nonlinear distortion when it passes
through a HPA(high power amplifier) such as
TWTA(trave1ing wave tube amplifier) and
SSPA(so1id state power amplifier). We propose a
new PAPR reduction method using soft clipping
and filtering. Unlike the conventional bard
clipping,
soft
clipping has non-zero slope in the
clip region. Next, filtering which uses the
additional FFT and IFFT transform processes
is
applied in order to reject the out-of-band clip
noise. Although the filtering removes the out-of-
band noise, it oppositely increases PAPR in small
quantity. Since HPA (high power amplifier)
increases the pow er consumption according to the
linearity range,
soft
clipping for SSPA lowers the
power consumption and
is
more efficient than the
Contributed Paper
Manuscript received October 29, 2001
0098 3063/00 10
conventional hard clipping method. By the results
o f proposed method, the wanted PAPR can b e
obtained a nd there is no out-of-band radiation
caused by clipping process.
1.
Introduction
Due to a high spec tral efficiency, the robustnes s
to the intersymbol interference
ISI)
and
multipath fading, OFDM(orthogona1 frequency
division multiplexing) is a very useful system for
the DAB (digital audio broadcasting) and
DVB(digita1 video broadcasting) applications. In
spite of many advantages, a major drawback of
OFDM is a high PAPR problem. When the
OFDM signal with high PAPR passes through
nonlinear device, the signal may suffer significant
nonlinear distortion[l]. To lessen the signal
distortion, it requires a linear amplifier with a
large dynamic range .. However, this linear
amplifier has poor power efficiency and is so
expensive.
To reduce the PAPR, several techniques have
been proposed, such as clipping[2][3], block
coding[4], ph ase shift[5][6]. C lippin g is the
simplest technique and an effective PAPR
.OO 2002
IEEE
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IEEE Transactions on Consumer Electronics,Vol. 48 No.
1,
FEBRUARY 2002
18
reduction method. However, it causes a serious that PAPR
is
reduced
to
about 6.9dB at
in-band
and
out-of-band clipping noise.
This
CCDF(comp1ementary cumulative distribution
makes BER performance degradation and function)
=
10”
,
the required SNR at
adjacent channel interference(AC1). Block coding BER= 10.’
is about 12.2dB in AWGN. The
seem s attractive because it does not create any proposed method can be applied to the other cases.
out-of-band radiation, but there is no proper
coding solutions which can maintain
a
reasonable
2. SSPA and soft clipping
coding rate for arbitrary large number
of
subcarriers. Phase shift method is another
representative and flexible PAPR reduction
method without signal distortion. However, it
requires so many IFFT stages equivalent to
.subblocks and comnlex structure in OFDM
The SSPA(solid state power amplifier) has the
transfer characteristic that is constant over the
linear region, which is different form the
TWTA(trave1ing wave tube amplifier). The SSPA
output is written as[3]
(1)
k A
transmitter. In addition, the spectral efficiency
d A I =
gets worse since the side informations for the
phase shift should be transmitted. For PAPR
reduction, a companding method that decreases
system complexity and has good spectral
efficiency has been proposed[7], but PAPR
reduction
in
this method is not great.
In this work, we propose
a
new method that is
composed of the
soft
clipping and filtering. The
filtering method uses additional FFT and IFFT
transform stages[S]. Unlike the conventional
hard clipping, soft clipping limits in a skew line,
,where
A
is
the input signal, v k is the small
signal am plification,
p
is the model parameter
and
A,
is the output amplitude at the saturation
point(
A , .
t
is
depicted
in
Fig.
1
for
k I
and different values of p : p I 2 ,4 , and IO .
The smoothness of the transition into the
saturation region can be adjusted by the param eter
p
as indicated in Fig.
1.
its slope is made suitable to SSPA characteristic.
Next, filtering is applied in orde r to reject the out-
of-band clip noise. Although the filtering removes
the out-of-band noise, it oppositely increases
PAPR in small quantity. When the amplitude of
the input OFDM signal excesses the linear region,
the output signal is decreased in TWTA(trave1ing
wave tube amplifier), but is constant in SSPA.
-10 5 0 5
,“phi P m r dB1
Thus, soft-clipping
is
more applicable to SSPA.
When the subcarrier number
is
16 and QPSK
Fig. I Characteristics of the typical S SPA.
modulation
is used,
the
proposed
method shows
Fig. 2 shows
the
transfer characteristic of soft
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Rvu el
al.:
PAPR ReductionUsing Soh
Clipping and ACI
Rejection in OFDM
System
clipping and hard clipping. Hard clipping is the
conventional clipping scheme. The clip ratio is
defmed as the ratio of the clip level to the average
signal amplitude.
Fig.
2.
The
t r a n s f e r c h a r a c t e r i s t i c of soft
c l i p pi n g a n d h a r d c l i p pi n g .
Soft clipping has non-zero slope which
i s
different from the conventional hard clipping, as
shown in Fig. 2. In other words, it may have the
middle properly between the two hard clipping
cases. Relationship of the input and output is
expressed as
s t ) = s , t )
,
O l s ; t ) < C ,
s; t)-Q
+c,c s
t ) L .
-.
u c
L-c
2)
,where s , f ) is the input signal, L is the
peak amplitude of IFFT output signal, nd
c
are the start and end points of soft clipping region.
Fig.
3
shows block diagram of the proposed
OFD M system including PAPR reduction. OFDM
transmit signal is the sum of N independent
QPSK suhsignals with identical bandwidth.
QPSK
symbol
is converted in serial-to-parallel
block and is modulated into OFDM symbol in
IFFT block. Let X , ( o k N - I ) he a
complex QPSK symbol
Fig. 3. B l o c k diagram o f t h e p r o p o s e d
OFDM system
The n-th O FDM signal can be written as
1
N-I
i =O
j 2 m k i N
x
= - E X , e
, O < n < N - l .
3)
,where
N
denotes the number of suhcarriers.
Hereafter, the OFD M signal is soft- clipped. Then,
the PAPR is expressed as
,where
x,,
is the soft-clipped signal and
E{*)
i s
expectation.
The soft-clipped signal passes through FFT and
IFFT block which functions as filtering [7].
Filtering has the bandpass property to get rid of
the out-of-band soft clip noise, which is made by
nulling the input of subcarriers of the second
IFFT equivalent to the out-of-band. Out-of-band
soft clip noise is a spectral regrowth component
which becomes ACl(adjacent channel interfer-
ence).
PAPR after filtering is given by
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IEEETransactionson Consumer Electronics, Vol. 48, No. 1,
FEBRUARY
2002
Fig. 4. The spectrum comparison before and
after filtering.
As shown in Fig. 4(a), the spectrum regrowth
of soft clipping is medium between the
two
hard
clipping cases of clip ratio = 0.8 and
1.0.
Fig. 4(b)
PAPR/i,mi, = 5 )
where x, is the filtered signal
In this paper, the OFDM system is assumed
that N=16 and QPSK modulation format is used.
An OFDM signal is over-sampled by a factor of 8.
The proposed soft clipping system is compared
with the conventional hard clipping system whose
clip ratio=O.S and 1 O as shown in Fig
2 .
Fig. 4
shows the spectrum of the softhard-clipped signal
before and after filtering.
a) Before filtering
shows that the spectrum of the filtered signal
becomes identical to the original OFDM spectrum.
The filtering removes the out-of-band clip noise,
so
that the ACl(adjacent channel interference)
may not be occurred. However, it can't remove
the in-band noise from the clip process, which
causes a little BER degradation similar to the
conventional hard clipping. In Fig. 5 , we can see
CCDF (complementary cumulative distribution
function) of PAPR. Sof tdi pp ing reduces PAPR to
the medium level between the two hard clipping
of clip ratio
=
0.8 and
1.0.
Soft clipping has the
medium BER performance. Table 1 shows the
performance comparison between the original
OFDM and the proposed OFDM system. there is
a little BER degradation that is similar to the
conventional hard clipping method due to a clip
noise.
11 . . . . . . . . ., T T T . .
i . . . .. : 7 . . . . . .
~ :: :
:::;Tj;\
:
....: :: ; ; I : I : : : : I : :I
: ; : :
j
......... ......
...........................
.............................
....................
......
1
.......
:
...........
..... ........,........E... l
. ~ ~ ~ ~ ~ > ~ ~ . ~ ~ . ~
\ \
0 2
4
6 8 10
12
PbF'RldBl
Fi g .
5.
PAPR reduction.
(b)
After filtering
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Ryu et
al. : PAPR Reduction Using Soft Clipping and ACI Rejection in OFDM System
21
4. Conclusion
In this paper, a new PAPR reduction method
using soft clinoine and filtering has been
reduced to 6.9dB at
m3
CCDF, when the
subcarrier number is 16 and QPSK modulation is
used. Also, the spectrum after filtering is identical
The original Hard clipping
OFDM
i
R
1
.O
~~~ ~~~
r v
to the original OFDM spectrum,
so
that there is
no spectral regrowth causing the ACI. However,
proposed. Clip slope of the soft-clipping is not
zero. The filtering method simDlv uses additional
PAPR
CCDF= 1
o ~ 10.1
dB 6.6 dB
7.2
dB
-
. .
there is a little BER degradation that is similar to
FFT and IFFT transform stages and removes the
6.9 dB
the conventional hard clipping.
out-of-band clip noise. By the
soft
clip method
between the clip ratio 0.8
and 1.0,
PAPR is
Table
1.
The PAPR comparison
PAPR reduction
3 5
dB 2.9 dB 3.2 dB
References
[ l ] E.
Bogenfeld,
R.
Valentine,
K.
Metzer, W.
Sauer-Greff, Influence of nonli near HPA
on trellis-Coded OFDM for terrestrial
broadcasting of digital HDTV, Global
Telecomm unications Conference, including a
Communications Theory Mini-Conference
GLOBECOM 93.. I E E E ,
vo1.3, pp.1433 -1438,
1993.
[ ] X. Li and L. J. Cimini Jr. Effects o f clipping
on the performance of OFDM with transmitter
diversity, in
Proc. IEEE Vehicular Technology
Con/ , vol.
3.
pp.1634-1638. May 1997.
[3] Xiaodong Li and Cimini,
L.J.
Jr., Effects of
clipping and filtering on the performance of
OFDM, IEEE Communications Letters,
pp.131- 133, May 1998.
[4] A.
E.
Jones , T.
A .
Wilkinson and
S . K.
Barton,
Block coding scheme for reduction of peak to
mean envelope power ratio of multicarrier
transmission schemes,
Electronics Letters,
vo1.30, pp.2098-20 99, Dec . 1994.
[ 5 ] A. E. Jones and T. A. Wilkinson, Comb ined
coding
for
error control and increased
robustness
to
system nonlinearities
in
OFDM,
Proc. IEEE 46th Vehicular Technology
Con/erence, pp.904-908, 28 April-I May 1996.
[6] V. Tarokh and H. Jafakhani, On the
computation and Reduction of the Peak-to-
Average Power Ratio in Multicarrier
Communications,''
IEEE Trans. on Commun.
vo1.48, no.1, pp.37-44, Jan.
2000.
[7]
Xiao Huang, Jianhua
Lu,
Junli Zheng,
I.
Chua ng, Jun Gu, Reduction of peak-to average
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IEEE Transactions on Consumer Electronics, Vol. 48 No. 1
FEBRUARY
2002
power ratio of OFDM signals with companding
transform, IEEE Electronics Letters, vo1.37,
110.8,pp.506 507, April 2001.
In-Bae Kim was born in
cheonan, Korea in 1978.
He received the B.S. degree
in the department of
electrical and electronics
engineering, Chungbuk
National University February 2000. He is
currently working towards M.S. degree on digital
communication system. His research interests are
I O G R A P H I E S
Henng-Gyoon Ryu was OFDM communication system, satellite
born in Seoul, Korea in
1959. He received the
B.S. and
M.S.
and Ph.D.
degrees in electronic
engineering, Seoul
communications and signal processing.
National University, 1982, 1984 and 1989. Since
1988, he has been with Chungbuk National
University, Korea. He is currently professor of
electrical and electronic department. His main
research interests are in the field of digital
communication systems, circuit design, spread
spectrum system and communication signal
processing.
Byoung-Il
Jin
was born
in cheonan, Korea in 1976.
He received the B.S.
degree in the department
of electrical and
electronics engineering,
Chungbuk National University February 2001. He
is currently working towards
M.S.
degree on
digital communication system. His research
interests are 4G mobile communication systems
and consumer electronic system