September 10, 2010 / Vol. 8, No. 9 / CHINESE OPTICS LETTERS 875
Phase pre-emphasis for PAPR reduction in optical OFDM
systems based on OIDFT or time lens
Jing Shao (
ªªª
), Wei Li (
êêê
)
∗
, and Xiaojun Liang (
)
Wuhan National Lab for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China
∗
E-mail: weilee@hust.edu.cn
Received May 10, 2010
Phase pre-emphasis is theoretically studied and introduced to reduce peak-to-average p ower ratio (PAPR)
in optical orthogonal frequency division multiplexing (OFDM) systems. In intensity modulated (IM)
systems, simulations show noticeable PAPR reductions: 4.14 dB (N = 16) and 15.48 dB (N = 512) in
time lens-based OFDM, N is the number of subcarriers. An equation is developed to calculate phase
values and is proved to be effective. Optical implementing methods are proposed and analyzed. In a time
lens-based OFDM system, phase pre-emphasis reduces fiber nonlinearity and results in a 5.2-dB increase of
launch power at the bit error rate (BER) of 10
−6
. Simulations also show similar PAPR reduction and fiber
nonlinearity mitigation in optical inverse discrete Fourier transformer (OIDFT) based OFDM systems.
OCIS codes: 060.4230, 060.4510, 190.4370.
doi: 10.3788/COL20100809.0875.
Optical orthogonal frequency division multiplexing
(OFDM) has been a research interest in recent years.
It is well treated as a promising candidate for long-haul
and high-speed transmission, due to its high spectral
efficiency, relatively low bit rate, and advanced robust-
ness against chromatic dispersion and polarization mode
dispersion
[1−5]
. Optical OFDM has also been applied to
wavelength-division multiplexing passive optical network
(WDM-PON)
[6]
and radio-over-fiber (ROF)
[7]
systems.
High peak-to-average power ratio (PAPR) is a seri-
ous intrinsic defect of optical OFDM systems, exacer-
bating nonlinear impairments in optical fibers and thus
deteriorating system performance. Therefore, reduction
of PAPR and mitigation of nonlinear impairments have
drawn great attention
[8−13]
. In wireless OFDM, many
PAPR reduction schemes have been proposed and widely
studied, including clipping and filtering
[14]
, partial trans-
mit sequence
[15]
,coding
[16]
, selected mapping (SLM)
[17]
,
and so on. Most schemes depend on electrical process-
ing, which needs additional digital signal processing or
electrical circuits, and thus increase system complexity
and cost. This letter focuses on OFDM systems that im-
plement inverse Fourier transform in optical domain and
studies PAPR reduction by optical methods. It should
be noticed that SLM is also related with adding different
phase rotations to subcarriers. However, the phase pre-
emphasis stated in this letter differs from SLM mainly
in two aspects: 1) only one combination of phase shifts
and one inverse fast Fourier transform (IFFT) operation
are needed; 2) optimized phase values are obtained by
calculating an equation proposed in this letter.
Phase pre-emphasis means that optimized phase values
are pre-chirped to optical pulses on different subcarriers
before IFFT in OFDM systems. In this letter, PAPR
reduction by phase pre-emphasis scheme is analyzed in
two kinds of optical OFDM systems. We propose to use
pre-emphasis for PAPR reduction in optical inverse dis-
crete Fourier transformer (OIDFT) based optical OFDM
systems, where only one system with 16 subcarriers was
studied
[8]
. Here, a more extensive study is given, with
two kinds of optical OFDM systems and different num-
bers of subcarriers. It is found that phase pre-emphasis is
highly effective in PAPR reduction when intensity mod-
ulation (IM) is employed, both in time lens-based optical
OFDM
[18]
and in OIDFT based optical OFDM
[19−21]
.
An equation is developed to calculate optimized phase
pre-emphasis values. Through simulation, the phase val-
ues obtained by calculating the equation are proved to
be almost as effective as those values optimized by re-
peated testing. Moreover, optical implementing methods
of phase pre-emphasis are proposed and analyzed. Phase
pre-emphasis reduces fiber nonlinearity significantly. In
a time lens-based OFDM system, it results in a 5.2-dB
increase of launch power at the bit error rate (BER) of
10
−6
.
Figure 1 depicts a block diagram of an optical OFDM
system based on OIDFT
[19−21]
. A continuous-wave
(CW) laser is fed into a pulse carver and changes into
an optical pulse train. A power splitter divides the op-
tical pulse train into N identical trains, which are mod-
ulated with N parallel electrical signals coming from a
serial-to-parallel processor (not shown in Fig. 1), N is
the number of subcarriers. After IM, an OIDFT con-
verts the modulated optical pulses into OFDM symbols.
After fiber-link transmission, an optical discrete Fourier
transformer (ODFT) rebuilds the transmitted signals on
every subcarrier.
In the system shown in Fig. 1. OFDM symbols (x
n
)
canbeexpressedby
x
n
=IDFT{X
k
} =
1
√
N
N−1
k=0
X
k
exp
j
2π
N
nk
,
n =0, 1, ··· ,N − 1, (1)
Fig. 1. Block diagram of an OIDFT based optical OFDM
system. Rx: receiver.
1671-7694/2010/090875-06
c
2010 Chinese Optics Letters