Long-reach SSB-OFDM-PON Employing
Fractional Sampling and Super-Nyquist
Image Induced Aliasing
Changjian Guo, Jiawei Liang, and Rong Li
Abstract—We investigate frequency diversity introduced
by deliberate aliasing of in-band signals using super-
Nyquist images in an optical single-sideband (SSB) modu-
lated and directly detected orthogonal frequency-division
multiplexing passive optical network (OFDM-PON) system.
Fractional sampling and a low-complexity per-subcarrier
maximum-ratio-combining (MRC) algorithm are proposed
to combine the diversity. We show that the computation
complexity of the MRC can be well controlled by adding a
2× resampling after digital filtering without compromising
performance. We experimentally demonstrate that for the
downlink transmission of an 8 GHz, 16-QAM mapped, and
SSB modulated OFDM-PON system, the residual super-
Nyquist images can be used for boosting the signal-to-noise
ratio of the higher frequency subcarriers at the receiver and
that the receiver sensitivity can be significantly improved
after 24.4 km and 80 km transmission. We also demonstrate
a significant increase in system capacity for an adaptively
modulated SSB OFDM-PON when using the proposed
scheme without any modification in the transmitter.
Index Terms—Adaptive bit loading; Maximum ratio
combining; Optical OFDM; Optical single-sideband;
Super-Nyquist image.
I. INTRODUCTION
L
ong-reach passive optical networks (LR-PONs) are
widely acknowledged as one of the most promising
candidates for next generation access networks because
they combine the capacity of metro and access networks
by extending the coverage span from the traditional
20 km range to up to 100 km. Thus, LR-PONs can signifi-
cantly reduce cost and simplify architecture [1,2]. In
LR-PONs, optical intensity modulation and the direct-
detection (IM/DD) scheme are still preferred over coherent
ones due to their simple implementation and low cost.
Optical orthogonal frequency-division multiplexing (O-
OFDM) [3] is also a promising modulation and multiplexing
scheme due to its high spectral efficiency, its robustness
against inter-symbol interference (ISI), and its modulation
flexibility when combined with adaptive modulation
schemes [4,5]. In IM/DD OFDM-PONs using digital-to-
analog converter (DAC) based transmitters, super-Nyquist
spectrum components (images) are generated along with
the in-band signal [6]. The super-Nyquist images are usu-
ally removed at either the transmitter or receiver side.
Transmitter-side analog anti-aliasing filtering may lower
the transmitter capacity because an excessive oversampling
ratio (or a number of zero-padded subcarriers) may be re-
quired [7–9] due to the non-ideal response of the filter.
Recently, we proposed taking advantage of the super-
Nyquist image instead of removing it at the receiver to com-
pensate for the chromatic dispersion (CD) induced power
fading in double-sideband (DSB) modulated LR-PONs
[10]. In [10], CD compensation was accomplished by select-
ing a sampling point with the best bit error rate (BER) per-
formance at the receiver; the other sampling points were
discarded. In this work, we propose and investigate in detail
the use of fractional sampling and a low-complexity diver-
sity combining technique to further enhance the signal-
to-noise ratio (SNR) of the higher frequency subcarriers
in single-sideband (SSB) modulated OFDM-PONs. We show
that a penalty will not be introduced by adding a 2× resam-
pling after digital filtering. Therefore, the complexity of the
proposed algorithm can be well controlled. We experimen-
tally demonstrate this fractional sampling and deliberate
aliasing induced frequency diversity via a 16-QAM mapped
and SSB modulated OFDM-PON in 24.4 km and 80 km
transmission cases. Without any modification at the trans-
mitter, significant improvement in receiver sensitivity can
be obtained by using the proposed scheme, even in long-
reach (80 km) scenarios. We also show that the achievable
data rate can be increased when adaptive modulation is
employed.
II. PRINCIPLE OF OPERATION
The output signals in sampled systems, such as conven-
tional DAC based optical transmitters, have the following
properties: 1) the signals are real-valued, which indicates
that they have both positive and negative frequencies with
XωX
−ω; 2) both positive and negative frequencies
are replicated along the frequency axis as a result of the
sampled nature; and 3) zero-order hold operation used in
DACs has a window-like time-domain impulse response
that results in a sinx∕x envelope in the frequency domain.
These properties are illustrated in Figs. 1(a) and 1(b),
which show a typical sine signal generated using a DAC
with a fundamental output frequency f
OUT
of 6.4 GHz
http://dx.doi.org/10.1364/JOCN.7.001120
Manuscript received August 3, 2015; revised October 14, 2015; accepted
October 16, 2015; published November 16, 2015 (Doc. ID 247208).
The authors are with South China Normal University, Guangzhou
510006, China (e-mail: changjian.guo@coer‑scnu.org).
1120 J. OPT. COMMUN. NETW./VOL. 7, NO. 12/DECEMBER 2015 Guo et al.
1943-0620/15/121120-06$15.00/0 © 2015 Optical Society of America
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