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ISSN: 2278 7798 International Journal of Science, Engineering and Technology Research (IJSETR) Volume 2, Issue 5, May 2013 1119 All Rights Reserved © 2013 IJSETR AbstractIn wireless communication systems, the design and analysis of an orthogonal frequency division multiplexing (OFDM) system signal is highly affected by the nonlinear effects of high power amplifier (HPA). The nonlinear effects of HPA on the OFDM system are the most critical because it is not only reducing the system performance but also creates interference on adjacent channels. This paper explores the analysis of nonlinear distortion effects in an OFDM system when the signal is passed through a nonlinear high power amplifier (HPA). Throughout this paper we also present the AM/AM analysis of HPA on OFDM. Finally we present the simulation results to show the nonlinear effects of HPA on OFDM. Index TermsAM/AM, HPA, OFDM and Nonlinear effect. I. INTRODUCTION In recent years, there has been a tremendous demand for reliable, high speed digital wireless communications. Besides the cellular phone, there are wireless modems, high definition television (HDTV) and digital radios. Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation technique that’s considered as one of the high spectral efficient modulation techniques. Although OFDM plays an important role in wide band transmission schemes, but its performance is degraded by various factor that is very common in any wireless communication system [1]. However, one of the major problems associated with OFDM is the nonlinear distortion in the transmitted OFDM signal when it is passed through a nonlinear HPA. This nonlinear distortion causes serious in-band distortion as well as adjacent channel interference due to spectrum re-growth in the transmitted signal. The performance of wireless communication is highly related with the power amplifier. Orthogonal frequency division multiplexing (OFDM) signalling exhibits high peak-to-average power ratio (PAR) which results in large sensitivity to nonlinear distortion created by the use of a high power amplifier (HPA) at the end of a wireless transmitter [2]-[5]. In [6] the authors addressed the effects of HPA phase distortion on system performance, in which they discussed mainly on the influence of AM/PM distortion of HPA by evaluating BER performance. Modern power amplifier Manuscript received April, 2013. Tushar Kanti Roy, Department of Electronics and Telecommunication Engineering, RUET, Rajshahi, Bangladesh. Monir Morshed, Department of Information and Communication Technology, Mawlana Bhashani Science and Technology University, Tangail, Bangladesh. designer face much tougher environment [7]. Nonlinearity in power amplifier response leads to nonlinear amplification of OFDM signal. The nonlinearity at the output of a PA was modeled as AM/AM (amplitude distortion which depends on the amplitude of the input). It presumed that the nonlinear noise at the receiver can be modeled as a complex Gaussian process uncorrelated with the input process and hence evaluated the OFDM system performance with nonlinear distortions in Additive White Gaussian Noise (AWGN) channels. To achieve maximum efficiency the power amplifier should be driven near the saturation region, but since the OFDM signal has high PAPR these power amplifiers will cross over to the nonlinear region causing serious in-band distortions as well as adjacent channel interference with spectrum re-growth in the transmitted signal. To minimize the nonlinear effects it is desirable to operate HPA in the linear region i.e. with huge back off, but in this case, efficiency has to be compromised. So this degrades the performance of an OFDM system. A measure of the degradation can be very helpful in evaluating the performance of a given system and in designing a signaling set that avoids degradation. Throughout this paper, we analyze the effects of nonlinearity of HPA in the OFDM system which degrades the system performance. The rest of this paper is organized as follows. Section II, briefly describes the basic concepts of OFDM transceiver. Section III, introduces the power amplifier model. Section IV, shows the non linearity analysis and simulations of OFDM system for non linear power amplifier. Finally, the paper is concluded in Section V. II. SYSTEM DESCRIPTION In this section, we describe the OFDM transceiver system. Orthogonal Frequency Division Multiplexing (OFDM) is a broadband multicarrier modulation method that has superior performance and benefits over older, more traditional single-carrier modulation methods (Laakso et. al., 1971). Before transmitting information bit in AWGN or Rayleigh fading channel through OFDM transmitter, the incoming modulated serial bits are converted into parallel streams by using a serial to parallel converter. Its then transforms this spectral representation of the data into the time domain using an Inverse Fast Fourier Transform for baseband OFDM modulation. To prevent overlapping of the data at the receiver cyclic prefix is inserted whose duration is one fourth of the total OFDM symbol duration. At the receiver side, firstly the data is received through linear receivers followed by a linear combiner. This linear combiner is designed in such a way that the output SNR is maximized at each instant of time. Then this data is converted again to the digital domain by passing it through an analog to digital converter. After removing the High Power Amplifier Effects Analysis for OFDM System Tushar Kanti Roy and Monir Morshed
Transcript

ISSN: 2278 – 7798

International Journal of Science, Engineering and Technology Research (IJSETR)

Volume 2, Issue 5, May 2013

1119 All Rights Reserved © 2013 IJSETR

Abstract— In wireless communication systems, the design and

analysis of an orthogonal frequency division multiplexing

(OFDM) system signal is highly affected by the nonlinear effects

of high power amplifier (HPA). The nonlinear effects of HPA on

the OFDM system are the most critical because it is not only

reducing the system performance but also creates interference

on adjacent channels. This paper explores the analysis of

nonlinear distortion effects in an OFDM system when the signal

is passed through a nonlinear high power amplifier (HPA).

Throughout this paper we also present the AM/AM analysis of

HPA on OFDM. Finally we present the simulation results to

show the nonlinear effects of HPA on OFDM.

Index Terms—AM/AM, HPA, OFDM and Nonlinear effect.

I. INTRODUCTION

In recent years, there has been a tremendous demand for

reliable, high speed digital wireless communications. Besides

the cellular phone, there are wireless modems, high definition

television (HDTV) and digital radios. Orthogonal Frequency

Division Multiplexing (OFDM) is a multicarrier modulation

technique that’s considered as one of the high spectral

efficient modulation techniques. Although OFDM plays an

important role in wide band transmission schemes, but its

performance is degraded by various factor that is very

common in any wireless communication system [1].

However, one of the major problems associated with OFDM

is the nonlinear distortion in the transmitted OFDM signal

when it is passed through a nonlinear HPA. This nonlinear

distortion causes serious in-band distortion as well as adjacent

channel interference due to spectrum re-growth in the

transmitted signal. The performance of wireless

communication is highly related with the power amplifier.

Orthogonal frequency division multiplexing (OFDM)

signalling exhibits high peak-to-average power ratio (PAR)

which results in large sensitivity to nonlinear distortion

created by the use of a high power amplifier (HPA) at the end

of a wireless transmitter [2]-[5].

In [6] the authors addressed the effects of HPA phase

distortion on system performance, in which they discussed

mainly on the influence of AM/PM distortion of HPA by

evaluating BER performance. Modern power amplifier

Manuscript received April, 2013.

Tushar Kanti Roy, Department of Electronics and Telecommunication

Engineering, RUET, Rajshahi, Bangladesh.

Monir Morshed, Department of Information and Communication

Technology, Mawlana Bhashani Science and Technology University,

Tangail, Bangladesh.

designer face much tougher environment [7]. Nonlinearity in

power amplifier response leads to nonlinear amplification of

OFDM signal. The nonlinearity at the output of a PA was

modeled as AM/AM (amplitude distortion which depends on

the amplitude of the input). It presumed that the nonlinear

noise at the receiver can be modeled as a complex Gaussian

process uncorrelated with the input process and hence

evaluated the OFDM system performance with nonlinear

distortions in Additive White Gaussian Noise (AWGN)

channels. To achieve maximum efficiency the power

amplifier should be driven near the saturation region, but

since the OFDM signal has high PAPR these power amplifiers

will cross over to the nonlinear region causing serious in-band

distortions as well as adjacent channel interference with

spectrum re-growth in the transmitted signal. To minimize the

nonlinear effects it is desirable to operate HPA in the linear

region i.e. with huge back off, but in this case, efficiency has

to be compromised. So this degrades the performance of an

OFDM system. A measure of the degradation can be very

helpful in evaluating the performance of a given system and in

designing a signaling set that avoids degradation. Throughout

this paper, we analyze the effects of nonlinearity of HPA in

the OFDM system which degrades the system performance.

The rest of this paper is organized as follows. Section II,

briefly describes the basic concepts of OFDM transceiver.

Section III, introduces the power amplifier model. Section IV,

shows the non linearity analysis and simulations of OFDM

system for non linear power amplifier. Finally, the paper is

concluded in Section V.

II. SYSTEM DESCRIPTION

In this section, we describe the OFDM transceiver system.

Orthogonal Frequency Division Multiplexing (OFDM) is a

broadband multicarrier modulation method that has superior

performance and benefits over older, more traditional

single-carrier modulation methods (Laakso et. al., 1971).

Before transmitting information bit in AWGN or Rayleigh

fading channel through OFDM transmitter, the incoming

modulated serial bits are converted into parallel streams by

using a serial to parallel converter. Its then transforms this

spectral representation of the data into the time domain using

an Inverse Fast Fourier Transform for baseband OFDM

modulation. To prevent overlapping of the data at the receiver

cyclic prefix is inserted whose duration is one fourth of the

total OFDM symbol duration. At the receiver side, firstly the

data is received through linear receivers followed by a linear

combiner. This linear combiner is designed in such a way that

the output SNR is maximized at each instant of time. Then this

data is converted again to the digital domain by passing it

through an analog to digital converter. After removing the

High Power Amplifier Effects Analysis for

OFDM System

Tushar Kanti Roy and Monir Morshed

ISSN: 2278 – 7798

International Journal of Science, Engineering and Technology Research (IJSETR)

Volume 2, Issue 5, May 2013

1120

All Rights Reserved © 2013 IJSETR

cyclic prefix, data is again converted into serial to parallel by

a serial-to-parallel converter. These parallel bit streams are

demodulated using Fast Fourier Transform (FFT) to get back

the original data by converting parallel bit streams into serial

bit streams.

In Orthogonal Frequency Division Multiplexing, multiple

sinusoidal with frequency separation T/1 is used where, T is

the active symbol period. The information kg to be send on

each subcarrier k is multiplied by the corresponding

carrier Tktj

k etg2

)( and the sum of such modulated

sinusoidal form the transmit signal. Therefore, the sinusoidal

used in OFDM can be defined as (Roy et. al., 2012):

)()(2

twetg Tktj

k

(1)

where, k =0, 1, ...N-1 correspond to the frequency of the

sinusoidal and w(t) =u(t)-u(t-T) is a regular window over [0,

T]. Since OFDM uses multiple sinusoidal having frequency

separation 1/T, therefore each sinusoidal is modulated by

independent information. Mathematically we can write the

transmit signal through the channel is,

)(1

)(

)()()()(

1

0

2

1

0

111100

tweN

tg

tgtgtgtS

N

k

Tktj

k

N

k

kk

NN

(2)

where, k is the kth

symbol in the message symbol sequence

for k in [0, N-1], N is the number of carriers. In an OFDM receiver, we will multiply the received signal

with a bank of correlator and integrate over the period T.

Therefore, the information sends on subcarrier

km

kmgetS

T

kT

m tj

T,0

,)(

12

(3)

where, m takes values from 0 to k-1.

III. POWER AMPLIFIER MODEL

In this section we describe the model of the high power

amplifier for OFDM systems. High power amplifier

nonlinearities significantly affect the performance of OFDM

systems. In this section, the effects of amplifier nonlinearities

are modelled in an OFDM system. Consider an input signal in

polar coordinates as (Hardeep et. Al., 2012)

jex (4)

where, 𝜌 is the amplitude and is the phase of input signal.

The output of the power amplifier can be written as

))(()()( pjeMxg

(5)

where, g(𝑥) represents the output of the power amplifier,

)(M represents the AM/AM conversion and 𝑝(𝜌)

represents the AM/PM conversion characteristics of the

power amplifier.

IV. SIMULATION RESULT

In this section, numerical simulation results are presented to

investigate the nonlinear effects of HPA on an OFDM system.

HPA nonlinearity may have bad influence on OFDM signals

mainly on two aspects: (a). Out-of-band distortion, which will

cause the OFDM power spectrum distortion, i.e. the spectral

spreading of the amplified signal and introduce adjacent

channel interference (ACI). Requirements on ACI for RF

systems are very strict especially for large number of

subscribers; therefore, it is of great importance to decrease the

out-of-band distortion. (b). In-band distortion, which may

disturb the OFDM constellations. PSD is utilized to evaluate

the effects of AM/AM distortions on OFDM signals. Fig.1 (a)

shows the time domain waveform of a typical OFDM signal

before passing through the HPA.

Fig.1(a): OFDM waveform before HPA

Fig.1 (b): OFDM waveform after HPA

0 10 20 30 40 50 60 70-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

Time

Am

plitu

de

OFDM Signal before HPA

0 10 20 30 40 50 60 70-2.5

-2

-1.5

-1

-0.5

0

0.5

1

Time

Am

plitu

de

OFDM Signal after HPA

ISSN: 2278 – 7798

International Journal of Science, Engineering and Technology Research (IJSETR)

Volume 2, Issue 5, May 2013

1121 All Rights Reserved © 2013 IJSETR

Fig. 2: Power spectrum of OFDM signal before and after amplifier

Fig.3: AM/AM response of OFDM signal

As is mentioned earlier, HPA nonlinearity may introduce the

spectral spreading of OFDM signals; for that we make a

simulation that shown in Fig.2 to illustrate the out-of-band

distortion caused by AM/AM distortions. From Fig.2, it is

clear that the AM/AM distortion has great influence on HPA

power spectral density performance.

Fig.3 demonstrates the comparison of idealized and practical

AM/AM response of power amplifier at the RF transmitter

side of OFDM system. From the input =1, non linearity starts

for the high power amplifier. AM/AM distortions have

distinct effects on OFDM signal constellations that shown in

Fig.4. It is clear to see from simulation Fig.4 that the AM/AM

distortion causes not only the dispersion but also the rotation

of signal constellations which greatly degrade the

performance of the OFDM system.

V. CONCLUSION

In this paper, we have highlighted the effects of

nonlinearities in the power amplifier over OFDM system. So

it is obvious from the simulation results that the performance

of the OFDM system will be greatly degraded if we will not

minimize the nonlinear effects of HPA. It is also clear from

the simulation results that the distortion in OFDM system due

to high power amplifier, either limiting or nonlinearity effects

are highly related to the distribution parameter of signal which

controls the dynamic range itself rather than the clipping level

or the saturation level of the amplifier. It is noticed that the

effects of nonlinearity of the high power amplifier depends

upon the type of modulation used in OFDM system which is

our next research task.

REFERENCES

[1] A. Behravan and T. Eriksson (2002), ―PAPR and Other Measures for

OFDM Systems with Nonlinearity‖, In the Proceedings of Wireless

Personal Multimedia Communications (WPMC), Vol. 1, pp. 149-153.

[2] E. Costa, M. Midrio, and S. Pupolin (1999), ―Impact of amplifier

nonlinearities on OFDM transmission system performance,‖ IEEE

Commun. Let., vol. 3, pp 37–39.

[3] A. A. Saleh (1981), ―Frequency independent and frequency dependent

nonlinear models of TWT amplifiers,” IEEE Trans. Commun., vol.

COM29, pp. 1715–1720.

[4] D. Dardari, V. Tralli, and A. Vaccari(2000), ―A theoretical

characterization of nonlinear distortion effects in OFDM systems,‖

IEEE Trans. Commun., vol. 48, pp 1755–1764.

[5] P. Banelli (2003), ―Theoretical analysis and performance of OFDM

signals in nonlinear fading channels,‖ IEEE Trans. Wireless Commun.,

vol. 2, pp. 284–293.

[6] AI Bo, ZHANG Tao-tao, PAN Chang-yong, YANG Zhi-xing, WANG

Yong, ZHAO Huai-xun (2007), ―Effects of HPA Phase Distortion on

System Performance,‖ Journal of System Simulation, Vol. 19, No. 2.

[7] Kenington (2000), ―High-Linearity RF amplifier Design,‖ Artech

House,2000.

[8] F. H. Gregorio and T. I. Laakso (2005), ―The Performance of

OFDM-SDMA Systems with Power Amplifier Non-Linearities‖,

Proceedings of the 2005 Finnish Signal Symposium FINSIG'05,

Kuopio, Finland.

[9] Roy T. K (2012), ―Comparative BER Performance Analysis of OFDM

System using BPSK Modulation Technique over AWGN and Rayleigh

Fading Channel,‖ IJAR-CSIT 1(3): p. 9 – 16.

[10] Amanjot Singh and Hardeep Kaur (2012) ―Non Linearity Analysis of

High Power Amplifier in OFDM system,‖ International Journal of

Computer Applications (0975 – 8887), Vol. 37, No. 2.

-10 -8 -6 -4 -2 0 2 4 6 8 10-5

0

5

10

15

20

25

30

Frequency

sp

ectru

m sig

na

ls

Spectrum Effects

Befor Amplifier

After Amplifier

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

1

2

3

4

5

Input Amplitude

Ou

tpu

t A

mp

litu

de

AM/AM response of power amplifier

Linear Response

Amplifier Response

Mean of OFDM Amplitude

-1.5 -1 -0.5 0 0.5 1 1.5-1.5

-1

-0.5

0

0.5

1

1.5

I channel

Q c

ha

nn

el

signal Constelleations

After Amplifier

Before Amplifier


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