27-12-2012, 03:02 PM
Performance Analysis of Optical OFDM Systems
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Abstract
Orthogonal Frequency Division Multiplexing (OFDM)
has been proposed as a promising technology for high speed
optical communication [1]. In this paper, we provide a
performance analysis of direct detection optical OFDM (DDOOFDM),
coherent optical OFDM (CO-OFDM) and self coherent
optical (SCO-OFDM). We first review the theoretical
fundamentals for Optical OFDM and the differences between the
three systems. Then, phase noise (PN) effects for various laser
linewidths and the effect of the received optical power (ROP) on
the system performance for these optical OFDM systems are
compared. In this article, we provide the first comparative
analysis between the three existing optical OFDM systems, This
analysis is simulated using VPItransmissionMaker™ V8.5.
Finally, the best optical OFDM system with high performance is
shown as a future trend.
INTRODUCTION
OFDM is a modulation technology which is used in many
broadband wired and wireless communication systems to
combat the multipath fading. The application of OFDM in
optical communication system has been investigated recently.
The optical OFDM systems can be classified into three
approaches according to the detection scheme. These
approaches are: (I) DDO-OFDM was first investigated in 2006
by Lowery [2]. (II) CO-OFDM, which can mitigate the
chromatic dispersion (CD), was also investigated in 2006 by
Shieh [3]. In 2007, Shieh proved that CO-OFDM can mitigate
polarization mode dispersion (PMD) [4]. (III) SCO-OFDM was
investigated in 2008 by Xu [5]. This scheme modifies COOFDM
in so far as it extracts the optical carrier from the
optical OFDM signal at the receiver for coherent interference.
Recently, an increasing number of papers on the simulation and
experimental analysis for high data rate optical OFDM have
been published. For DDO-OFDM, simulation results of 10
Gbps over 4000 km transmission standard single mode fiber
(SSMF) were presented in [2], while experimental results of 24
Gbps over 800 km transmission SSMF were reported in [6].
For CO-OFDM, the transmission of 10 Gbps over 3000 km
SSMF was simulated in [3], experimental results of 8 Gbps
transmission over 1000 km SSMF are reported in [7], the
transmission of 25.8 Gbps over 4160 km SSMF was
demonstrated in [8], and simulation results of 100 Gbps backto-
back transmission are reported in [9].
ARCHITECTURE DESIGN
This section represents the design concepts used in the three
systems with the main differences between them. Fig. 1 shows
the block diagram of three existing optical OFDM back-toback
transmission, including four main blocks [2], [5], [11]:
RF-OFDM-transmitter (Tx), RF-to-optical (RTO) upconverter,
optical-to-RF (OTR) down-converter and RFOFDM-
receiver (Rx). The RF-OFDM-Tx and RF-OFDM-Rx
are common parts between three existing systems while the
RTO up-converter and OTR down-converter blocks differ. A
very important assumption for OFDM is the linearity in
modulation and demodulation [12]. A linear RTO up-converter
and linear OTR down-converter can be obtained in principle by
biasing Mach Zehnder modulator (MZM) at quadrature or null
point in order to obtain a linear transformation between RF
signal and optical signal [3], [11], [13]. Fig. 1 shows the details
of the considered system setups. In the next section we will
expand the setups.
RF-OFDM-Tx
The input high-bit-rate serial data are converted to
low-bit-rate parallel blocks of bits where is the
number of parallel data paths. These blocks of bits contain
information symbols o “subcarriers”. The information
symbols are mapped by quadrature amplitude modulation
(QAM) onto orthogonal carriers with equally spaced
frequencies. To avoid aliasing due to the sampling process of
the digital-to-analog converter (DAC), zero padding (ZP) is
needed. This shifts the aliases away from the OFDM signal. By
inverse fast Fourier transformation (IFFT), one obtains the
time-domain OFDM signal. The IFFT size determines the
numbers of subcarriers and the numbers of ZP. The ZP may be
inserted in the middle of the IFFT sequence or at its edges.
Usually half of the input IFFT sequence is used for ZP while
the other half is used for subcarriers because of the required
Hermitian symmetry. The IFFT size usually lies between 128
[7] and 1024 [2]. Increasing the IFFT size makes the signal less
susceptible to intersymbol interference (ISI) between OFDM
symbols.
CONCLUSION
In this paper, we have analyzed the performance of three
exiting optical OFDM systems: DDO-OFDM, CO-OFDM and
SCO-OFDM. In particular the effect of a narrow OBPF in
SCO-OFDM transmission was investigated. Compared to COOFDM
at 20 Gbps, about 3.6 dB OSNR improvement can be
achieved, and 10.9 dB compared with DDO-OFDM. The
simulation results show that SCO-OFDM has a good tolerance
to laser linewidth.