PAPR Reduction Using Soft Clipping and ACI Rejection in Ofdm System

8/15/2019 PAPR Reduction Using Soft Clipping and ACI Rejection in Ofdm System

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Ryu et al.:

PAPR Reduction Using Soft Clipping and ACI Rejection in OFDM System

17

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:

[email protected],

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

mailto:hanmail.netmailto:[email protected]:[email protected]:hanmail.net


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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

PAPR Reduction Using Soft Clipping and ACI Rejection in Ofdm System

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