A Novel Timing Synchronization Method for U-
OFDM-Based Visible Light Communication
Zhiyuan Du, Xian Zhou, Keping Long
Beijing Engineering and Technology Research Center for Convergence Networks and Ubiquitous Services
University of Science and Technology Beijing (USTB)
Beijing, China
zhouxian219@gmail.com
Abstract—Unipolar orthogonal frequency division
multiplexing (U-OFDM) is compatible with optical intensity
modulation and attract attention recently. Existing
synchronization methods cannot be used directly or do not
perform well in U-OFDM systems because U-OFDM signals are
unipolar. Therefore, a novel synchronization algorithm, i.e.,
negative mirror image (NMI) algorithm, is proposed, where the
training sequence is re-designed based on the characteristics of
U-OFDM. Analysis and simulation results demonstrate that the
proposed NMI method has excellent synchronization
performance for the U-OFDM systems.
Keywords—visible light communication; U-OFDM; timing
synchronization
I. INTRODUCTION
Visible light communication (VLC) systems employ white
light-emitting diodes (LEDs) as a light source, therefore the
performing high-speed communication and providing
illumination can be achieved at the same time. Since it is
conventional intensity modulation/direct detection (IM/DD)
optical communication system and the intensity of light is
modulated in VLC systems, the transmitted signal must be real
and positive [1].
Orthogonal frequency division multiplexing (OFDM), a
multi-carrier modulation, is an effective solution to multipath
dispersion and it has high spectral efficiency. However,
conventional OFDM signals are bipolar and they should be
modified to be transmitted in VLC channels, due to the
intensity of light cannot be negative. In the IM/DD systems, to
ensure that the time domain OFDM vector at the output of the
IFFT is real, the input date must have Hermitian symmetry [2].
There are two popular ways to ensure the real signals to be
unipolar: At the time of electro-optical conversion, adds a DC
bias to convert the bipolar real values to strictly positive
signals (DCO-OFDM); the other way is clipping the real
signals at zero to give unipolar samples (ACO-OFDM).
However, with the purposes of DCO-OFDM, DC bias
increases the average optical power of the transmission optical
signal while reducing the modulation depth, resulting in poor
power efficiency of the system. In comparison, ACO-OFDM
is more power efficient than DCO-OFDM, however, in which
only the odd subcarriers are loaded with date and it has half of
the spectral efficiency of DCO-OFDM and a 3 dB
performance disadvantage for bipolar signals when compared
to OFDM [3]. Therefore, a novel modulation scheme——U-
OFDM receives much concern, it uses an innovative
rearrangement of the OFDM frame to become unipolar
required by the VLC systems. U-OFDM is better than the
DCO-OFDM in terms of optical power and closes the 3 dB
gap between OFDM and ACO-OFDM for bipolar signals [4].
It is well known that OFDM systems are very sensitive to
synchronization error. There are a lot of classic
synchronization algorithms for conventional RF-based OFDM
systems [5], [6]. For the synchronization issues of DCO-
OFDM systems and ACO-OFDM systems, many scholars
have done much targeted research and published a lot of
excellent papers and arithmetic achievements [7], [8], [9].
Nevertheless, U-OFDM is a brand-new method to generate
unipolar signals used in IM/DD optical systems. Existing
synchronization methods cannot be used directly or don not
perform well in U-OFDM system because of its special
modulation method and structure. There are not many research
results for its synchronization problems.
In this paper, a novel training sequence structure is
constructed for U-OFDM synchronization. The rest of the
paper is organized as follows. In Section II, the U-OFDM
system is briefly described. Section Ⅲ proposes a novel
training sequence for U-OFDM. The comparison between
Simulation results and conventional synchronization
algorithms performance is presented in Section IV. Finally,
the concluding remarks are made in the Section V.
II. U-OFDM SYSTEM
Fig. 1. Block diagram of an U-OFDM system
Date input
Serial to
parallel and
mapping
N/4
Hermitian
symmetry
IFFT(N/2) P/S
U-OFDM
conversion
Add
CP
N/2
Intensity
Modulator
(optical)
Optical
channel
Direct
Detection
(optical)
Novel timing
synchronization
Become a bipolar
signal using
subtraction
N
Remove CP
and serial to
parallel
N-point
FFT
Demapping
and parallel to
serial
Date out
2015 24th Wireless and Optical Communication Conference (WOCC)
978-1-4799-8854-9/15/$31.00 ©2015 IEEE 63