Frequency Synchronization in OFDM System 139

racy. The estimation range can be made as large as de-

sired without the need of the second training symbol [4].

Fredrik Tufvesson et al. compared and analyzed the

preambles for OFDM systems based on repeated OFDM

data symbols or repeated short pseudo noise sequences.

Synchronization based on PN-sequence preambles of-

fered greater power reductions in stand-by model [5].

Minn et al. [6] compared the performance of timing off-

set estimation methods with modification in the training

structure and found a smaller estimator variance in his

scheme. Ren et al. [7] proposed the modified preamble in

WLANs with a typical structure weighted by the pseudo-

noise sequence which enlarged range of frequency offset

estimation to ±4. Hlaing Minn et al. [8] presented a fre-

quency offset estimation approach using a maximum-

likelihood principle with a sliding observation vector

(SOV-ML). Chin-Liang Wang et al. [9] proposed a

method to make a modulatable orthogonal sequence par-

tially geometric for large CFO estimation. Wei Zhong

[10] proposed a novel integral frequency offset (IFO)

estimation method which examined the phase changes of

synchronization signals in frequency domain. This

method provided excellent IFO estimation performance

with very low computational complexity. Sung-Ju Lee et

al. [11] proposed the carrier frequency offset mitigation

scheme in wireless digital cooperative broadcasting sys-

tem using multi-symbol encapsulated orthogonal fre-

quency division multiplexing (MSE-OFDM), which uses

one cyclic prefix (CP) for multiple OFDM symbols.

Adegbenga B. Awoseyila et al. [12] proposed a novel

technique for 3GPP LTE specifications using only one

training symbol with a simple structure of two identical

parts to achieve robust, low-complexity and full-range

time-frequency synchronization in OFDM systems. E. C.

Kim et al. [13] enhanced the performance frequency off-

set compensation by adding a ternary sequence to OFDM

signals in the time domain which finds application in

design of synchronization block of OFDM scheme for

wireless multimedia communication services. The power

level of the ternary sequence to be added needs to be low

enough in order not to affect the normal operation of the

OFDM system. Ilgyu Kim et al. [14] proposed an effi-

cient synchronization signal structure for OFDM-based

Cellular Systems The sequence used for the Primary

Synchronization signal is generated from a frequency-

domain ZC sequence for high rate and multimedia data

service systems such as LTE in the 3GPP. Ji-Woong

Choi et al. [15] described the joint ML estimation using

correlation of any pair of repetition patterns, providing

optimized performance.

In Moose method, the limit of the estimation for the

CFO is ±1/2 the subcarrier spacing. In Schmidl and Cox

method, the limit of the estimation for the CFO is ±1 the

subcarrier spacing. In Minn method, the limit of the es-

timation for the CFO is ±2 the subcarrier spacing. In Ren

method, the limit of the estimation for the CFO is ±4 the

subcarrier spacing. The method using ZC sequence as

preamble, the limit of the estimation for the CFO is ±30

the subcarrier spacing. Hence the Carrier Frequency

Offset estimation range is large when compared to the

previous methods.

This paper is based on preamble-aided methods that

can be applied to both burst-mode and continuous

OFDM applications. The organization of the paper is as

follows. In Section I, the OFDM system model and

importance of frequency offset estimation is described.

In Section II, the frequency offset estimation of previous

methods is explained. The algorithm for frequency offset

estimation using ZC sequence is given in Section III.

Simulation results and discussions are presented in

Section IV. Finally, in Section V, conclusions are drawn.

3. The OFDM System Model

The incoming input binary streams are first mapped into

constellation points according to any of the digital

modulation schemes such as QPSK/QAM. In QPSK

(Quadrature Phase Shift Keying) modulation, the incom-

ing binary bits are combined in the form of two bits and

are mapped into constellation point. After mapping into

constellation points, the incoming serial bits are con-

verted into parallel bits transmitting N OFDM samples at

a time. The OFDM signal is generated using N subcarri-

ers. The total bandwidth is divided into 64 sub channels.

The N constellation points are modulated using N sub-

carriers whose carrier frequencies are orthogonal in na-

ture. The modulation is similar to taking inverse dis-

crete/fast fourier transform (IDFT/IFFT) operation. The

output of N point (IFFT) block is the OFDM signal. Now

the N OFDM signal samples are combined and then

transmitted i.e., the parallel samples are now converted

into serial sequence and then it is transmitted. The

OFDM baseband signal at the transmitter is expressed as

in (1)

12

0

1

()(). 01

Njnk

N

k

xnX knN

e

N

(1)

where

n -time domain sample index

X (k) -modulated QPSK data symbol on the kth

subcarrier

N -total number of subcarriers and

x (n) -OFDM signal.

In order to maintain a signal to noise ratio (SNR) of 20

decibels or greater for the OFDM carriers, offset is limited

to 4% or less than the inter carrier spacing which is

simulated in Figure 1. The lower bound for the SNR at

the output of the DFT for the OFDM carriers in a channel

with AWGN and frequency offset is derived as in [1] and

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