
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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