Communications and Network, 2013, 5, 43-47

doi:10.4236/cn.2013.52B008 Published Online May 2013 (http://www.scirp.org/journal/cn)

An Iterative Maximum Likelihood Synchronization

Method for OFDM System

Chi-Min Li, Wei-Tse Sun

Department of Communications, Naviga tion and Control Engineer ing, National Taiwan Ocean University, Keelung, Chinese Taipei

Email: cmli@ntou.edu.tw

Received 2013

ABSTRACT

OFDM system divides a wideband transmission bandwidth into several overlapped narrowband subcarriers to avoid the

serious frequency selectiv e fading problem. However, if Ti ming Offset (TO) and Carrier Frequency Offset (CFO) exist

between the Transmitter (Tx) and Receiver (Rx), OFDM system will suffer the Inter-Symbol Interference (ISI) and In-

ter-Carrier Interference (ICI) that degrade the system performance dramatically. In this paper, we propose an iterative

maximum likelihood method for synchronization. We also adopt the overlap concept to reduce the plateau problem.

Simulation results sh ow that the proposed meth od can predict the time delay and frequ ency offset correctly even under

the multipath fading scenarios.

Keywords: Synchronization; Maximum Likelihood; OFDM

1. Introduction

Orthogonal Frequency Division Multiplexing (OFDM)

has been widely adopted in the current wireless commu-

nication systems, such as IEEE 802.11 a/g, and Long

Term Evolution (LTE). To correctly accomplish symbol

detection, an OFDM system has to establish the time and

frequency synchronization [1]-[7] firstly to avoid the Bit

Error Rate (BER) degradatio n. Poor synchronization will

destroy the orthogonal property of the received signal

and system will suffer the Inter-Symbol Interference (ISI)

and Inter-Carrier Interference (ICI). Therefore, synchro-

nization is a very important issue for the transmission,

especially for the OFDM system.

In [3], authors proposed a Maximum Likelihood (ML)

method via adopting the correlation properties of the

Cyclic Prefix (CP) with the copied portion of an OFDM

symbol to achieve the time and frequency synchroniza-

tion. Its performance depends notably on the length of

the inserted CP. The longer of the length of CP, the better

of the synchronization can be obtained. Based on this

behavior, if we use a preamble with two repetitions to

conduct the synchronization, the ML method should

perform well. However, a serious plateau problem arises

that causes the Receiver (Rx) can not predict the arrival

time precisely. This problem comes from the inherent

correlation properties of the preamble with two or multi-

ple repetitions. Many methods have been proposed to

reduce the plateau problem [2,4].

In this paper, we propose an iterative ML synchroniza-

tion method that adopts the overlap concept to reduce the

plateau problem. This paper is organized as follows. In

Section II, the conventional ML synchronization method

and the proposed iterative method will be described in

detail. Simulation results of these methods are compared

in Section III. Finally, some conclusions for this paper

are given in Section IV.

2. Method Descriptions

In OFDM system, Transmitter (Tx) can insert a CP at the

Guard Interval (GI) of the OFDM signal to avoid the ISI.

Assume the received signal can be expressed as

2/

() ()()

jnN

rn xnewn

(1)

where

is the time delay,

is the normalized fre-

quency offset and is the Additive White Gau ssian

Noise (AWGN). ()wn

Consider an observation interval with 2N+L samples

(Figure 1), the interval contains a complete OFDM sym-

bol of N+L samples, where N is the number of subcarrier

and L is the length of the CP.

,1,...,-1 ',1,...,-1ILINNN

L

Figure 1. ML synchronization method.

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