IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 58, NO. 4, MAY 2009 1673
OFDM and Its Wireless Applications: A Survey
Taewon Hwang, Chenyang Yang, Senior Member, IEEE,GangWu,Member, IEEE, Shaoqian Li, and
Geoffrey Ye Li, Fellow, IEEE
(Invited Paper)
Abstract—Orthogonal frequency-division multiplexing
(OFDM) effectively mitigates intersymbol interference (ISI)
caused by the delay spread of wireless channels. Therefore,
it has been used in many wireless systems and adopted by
various standards. In this paper, we present a comprehensive
survey on OFDM for wireless communications. We address
basic OFDM and related modulations, as well as techniques to
improve the performance of OFDM for wireless communications,
including channel estimation and signal detection, time- and
frequency-offset estimation and correction, peak-to-average
power ratio reduction, and multiple-input–multiple-output
(MIMO) techniques. We also describe the applications of OFDM
in current systems and standards.
Index Terms—Channel estimation, frequency-offset estima-
tion, intercarrier interference (ICI), multicarrier (MC), multiple-
input–multiple-output (MIMO) orthogonal frequency-division
multiplexing (OFDM), peak-to-average power reduction, t ime-
offset estimation, wireless standards.
I. INTRODUCTION
H
IGH-DATA-RATE transmission over mobile or wireless
channels is required by many applications. However, the
symbol duration reduces with the increase of the data rate, and
dispersive fading of the wireless channels will cause more se-
vere intersymbol interference (ISI) if single-carrier modulation,
such as in time-division multiple access (TDMA) or Global
System for Mobile Communications (GSM), is still used. From
[1], to reduce the effect of ISI, the symbol duration must be
much larger than the delay spread of wireless channels. In
orthogonal frequency-division multiplexing (OFDM) [2]–[4],
the entire channel is divided into many narrow-band subchan-
nels,
1
which are transmitted in parallel to maintain high-data-
Manuscript received February 14, 2008; revised June 16, 2008. First pub-
lished August 26, 2008; current version published April 22, 2009. The review
of this paper was coordinated by Dr. W. Zhuang.
T. Hwang is with the School of Electrical and Electronic Engineering, Yonsei
University, Seoul 120-749, Korea.
C. Yang is with the School of Electronics and Information Engineering,
Beihang University, Beijing 100083, China.
G. Wu and S. Li are with the National Key Laboratory of Communications,
University of Electronic Science and Technology of China, Chengdu 610054,
China.
G. Y. Li is with the School of Electrical and Computer Engineering,
Georgia Institute of Technology, Atlanta, GA 30332-0250 USA (e-mail:
liye@ece.gatech.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TVT.2008.2004555
1
It is also called the subcarrier or tone in the literature.
rate transmission and, at the same time, to increase the symbol
duration to combat ISI.
OFDM is a special form of multicarrier (MC) that dates back
to 1960s. The concept of MC transmission was first explicitly
proposed by Chang [5] in 1966. A detailed description of MC
can also be found in [6] and [7]. Before Chang, Doelz et al. [8]
had implemented a special MC system for a single-sideband
voice channel in 1957, and Holsinger [9] had implicitly intro-
duced the MC system in his dissertation at the Massachusetts
Institute of Technology in 1964. In 1971, Weinstein and Ebert
[3] proposed time-limited MC transmission, which is what
we call OFDM today. The implementation of MC systems
with equalization was investigated by Hirosaki et al. [10] and
[11] and Peled and Ruiz [12]. Zimmerman and Kirsch [13]
published one of the earliest papers in the application of MC in
HF radio in 1967. More materials on the HF application of MC
can be found in [14] and the references therein. The capacity of
OFDM was investigated in [15] and [16]. In 1985, Cimini first
applied OFDM in mobile wireless communications [2]. In [17],
Casas and Leung discussed the application of MC over mobile
radio FM channels. Bingham [18] studied the performance and
complexity of MC modulation and concluded that the time for
MC has come. The application of original OFDM, clustered
OFDM, and MC code-division multiple access (CDMA) in
mobile wireless systems can be found in [19]–[26].
The flexibility of OFDM provides opportunities to use ad-
vanced techniques, such as adaptive loading, transmit diver-
sity, and receiver diversity, to improve transmission efficiency.
Shannon’s classical paper in 1948 [27] suggested that the
highest data rate can be achieved for frequency-selective chan-
nels by using an MC system with an infinitely dense set of
subchannels and adapting transmission powers and data rates
according to the signal-to-noise ratio (SNR) at different sub-
channels. Based on his theory, a water-filling principle has been
derived [28]. Cioffi and his group have extensively investigated
OFDM with performance optimization for asymmetric digital
subscriber line, which they more often called discrete multiple
tone (DMT). Some of their earlier inventions on practical
loading algorithms for OFDM or DMT systems were in [29].
More results on this topic can be found in [30]–[32].
The capacity of a wireless system can significantly be
improved if multiple transmit and receive antennas are
used to form multiple-input–multiple-output (MIMO) channels
[33]–[37]. It is proved in [33] that, compared with a single-
input–single-output (SISO) system, a MIMO system can im-
prove the capacity by a factor of the minimum number of
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