PERFORMANCE EVALUATION OF VFFT-OFDM SYSTEM IN THE PRESENCE OF CARRIER FREQUENCY OFFSET

Ni Made Ary Esta Dewi Wirastuti, I Made ArsaSuyadnya, Duman Care Khrisne

Study Program of Electrical Engineering

Faculty of Engineering

2018

Outline:

• Motivation

• Introduction

• Objectives

• DFT, FFT dan VFFT

• CFO

• Simulation

• Results and Analysis

• Conclusion

2

Motivation (1)

Next generation mobile:

• System complexity and energy efficieny

• Chips next generation mobile: cheaper, faster dan smaller

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Motivation (2)

4

Introduction (1)

o Need of fast data access in Multimedia communication

o Orthogonal Frequency Division Multiplexing(OFDM)

o one of the main technologies that implemented in wireless communication

o main candidate for future mobile generationo Application: DAB, DVB, Wireless LANs, broadband

wireless internet (802.16), LTE.

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Introduction (2)

o One of components OFDM that needs intensive calculation: inverse Fast Fourier Transform (IFFT) dan Fast Fourier Transform (FFT)

o VFFT offer lower complexity implementation that implemented in various to replace FFT

o Reducing FFT complexity ➔ improve power efficiency ➔ VFFT-OFDM (G-OFDM)

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Pendahuluan (4)

o OFDM communication systems are sensitive to the frequency synchronization in form of carrier frequency offset (CFO). CFO severely degrades the performance of the OFDM systems

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Objectives

o To model OFDM and VFFT-OFDM system using LTE parameters over AWGN and Rayleigh fading channels

o To analyze the effect of carrier frequency offset to the OFDM and VFFT-OFDM performance

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OFDM

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Conventional

multi-carrier

modulation

• Parallel data,

• Non-overlapping

sub-channels,

• FDM

Orthogonal multi-

carrier modulation

• Parallel data,

• Overlapping sub-

channels,

• OFDM

DFT Implementation

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Typical TX OFDM

Typical RX OFDM

Carrier Frequency Offset (1)

• One of the main disadvantages of the OFDM system is the sensitivity to offset frequencies

– Caused by jitter on the carrier signal

– Doppler effect caused by movement of both the TX and Rx.

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Carrier Frekuensi Offset (2)

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• Synchronization is an important part of the receiver

– because the time difference in the clock and frequency of the local oscillator

– can cause a decrease in performance on the system.

Carrier Frequency Offset (3)

– Time synchronization is used to select the boundary of the symbol and frequency synchronization to equalize the oscillator between the receiver and the transmitter.

– The orthogonal subcarrier properties will disappear and ICI appears if both synchronizations are not performed.

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Carrier Frekuensi Offset (4)

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• Frequency synchronization error or the difference in frequency between the transmitter and receiver oscillator causes frequency offset.

• The result : the demodulation process by DFT sampling the wrong position.

Carrier Frekuensi Offset (5)

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• The carrier signal power on the sampled channel decreases ("+") and the ICI appears in the adjacent subcarrier ("O") signal.

• This results in a decrease in the SNR value

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Simulation (1)

• OFDM parameter in LTE, modulation Quadrature Phase Shift Keying (QPSK).

• Observe the presence of offset frequency, shifted subcarrier termed as normalized frequency offset (ε) : 0.2 dan 0.05

• Comparing the performance of OFDM and VFFT-OFDM systems without ICI reduction methods on AWGN and Rayleigh fading channels using the BER vs. parameter. Eb / No.

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Simulation (2)

o Simulation model

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Serial to

parallel

converter

Coding & Mapping

QPSK IVFF

T

Guard

interval

insertion

VFFT

Parallel to

serial

converter

Serial to

parallel

converter

Guard

interval

removal

De-coding & Demapping

QPSK

Parallel to

serial

converter

Received

data

BER evaluation

BER

evalua

tion RECEIVER

TRANSMITTER

MEASUREMENT TOOL

RADIO CHANNEL

Rayleigh fading channel

AWGN channel

Transmitted Data

Simulation (3)

o Parameter:

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Parameter Nilai yang digunakan

N frame 5000 Clipping Ratio (CR) Classical Clipping = 1,4

Deep Clipping = 0,6

FFT Size 64

N of data subcarriers 64

Guard periode type Cyclic prefix

N bits per OFDM symbol

64

N symbol 1 N Cyclic prefix ¼ (N of data

subcarriers)

Total symbol N of data subcarriers + N cyclic prefix

Mbit 4

Results and Analysis (1)

• Frequency Offset model on OFDM over AWGN channel

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x(n)

ej2n/N

n(n)

y(n)

IDFT

DFT X(m) Y(k)

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( ) ( ) ( )2πnε

y n = x n +n nj

Ne ,

( ) ( )( )

( )2 m l+ k 2 mk1

l 0

11

m 0 m 0

1Y k X l + n m

− − −

=

−−

= =

= j j

N N

N NNe e

N.

( )( )2 n l+ k1

n 0

1l k

−−

=

= jN

N

Ne- ,

( ) ( ) ( ) ( ) ( ) ( )1

l=0,l k

1Y k X k 0 + X l l k + N k

−

= N

N- ,

Results and Analysis (2)

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( )1

1

N

N

a rS

r

−=

−,

( )( )

( )( ) ( )

( ) ( )

l+ k l+ k l+ kl+ k

l+ k l+ kl k

− − − −− −

− −−

−

−

=j j j

jN

j j

N N

e ee

e e

- .

x x2 sin x −= −j jj e e ,

Results and Analysis (3)

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( )( ) ( )( )

( )

1l+ k sin l + k

l + ksin

l k−

−

−

−

=N

jN

NN

e- .

+=y x n

+= -1Y F F X N

Results and Analysis (4)

• Offset Frequency model in OFDM system over Rayleigh fading channel

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x(n)

exp(j2n/N)

n(n)

y(n) X(l) Y(k)

h(n)

Results and Analysis (5)

• AWGN

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Results and Analysis (6)

• Fading

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Conclusion

• The decrease in Bit Error Rate (BER) due to the carrier frequency offset has been successfully simulated in this study.

• For the decrease of BER 0.5x10-4 which can be ignored, the maximum tolerable frequency offset is less than 1% of the sub-carrier spacing on the AWGN channel.

• Whereas in the fading channel, the decrease in BER is 0.3x10-2 which can be ignored, maximum tolerable frequency offset is smaller than 4% of the sub-carrier spacing.

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