Iteration SSII cancellation in DD-OFDM PON upstream scheme
Cheng Ju
n
, Na Liu, Xue Chen
State Key Lab of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), P.O. Box 128,
#10 XiTuCheng Road, HaiDian District, Beijing 100876, China
article info
Article history:
Received 13 September 2015
Received in revised form
5 November 2015
Accepted 6 November 2015
Available online 19 November 2015
Keywords:
Intensity modulation
Direct detection
Iteration interference cancellation
Subcarrier to subcarrier intermixing
interference
abstract
Iteration interference cancellation algorithm is proposed in direct detection OFDM PON upstream
scheme to mitigate subcarrier to subcarrier intermixing interference (SSII) caused by dispersion and
square-law photo-detection. The receiver sensitivity is improved by 1 dB in 20-Gbps, 16-QAM OFDM PON
upstream experiment after 100-km standard single mode fiber (SSMF) transmission.
& 2015 Elsevier B.V. All rights reserved.
1. Introduction
Optical orthogonal frequency division multiplexing (OFDM) can
support a data rate of 4 40 Gbps with a reach of 4 100 km by
using direct detection (DD). Therefore, DD-OFDM long-reach pas-
sive network (PON) has become one of the most promising can-
didates to economically build future-proof high-capacity and
wide-coverage access networks [1,2]. However, intensity mod-
ulation and direct detection OFDM system will meet two bottle-
necks: subcarrier to subcarrier intermixing interference (SSII) and
power fading [3]. By modeling SSII [4], it could be mitigated by
using iteration cancellation technique in the receiver [5], and our
previous works have shown that SSII and power fading could be
both mitigated by using vestigial-sideband (VSB) filter and itera-
tive technique in the receiver [6] or symbol predistortion in the
transmitter [7]. SSII cancellation algorithms of the previous works
mainly focus on OFDM-PON downstream scenario, which essen-
tially is a point-to-point (P2P) transmission scheme. However, SSII
and power fading still considerably limits the transmission band-
width and reduces the capacity in OFDM-PON upstream scheme.
OFDM upstream scenario essentially is a multipoint-to-point
(M2P) transmission scheme, where the SSII is caused by the
photoelectric conversion from multiple optical network unit
(ONU). Nevertheless, to the author’s best knowledge, no inter-
ference cancellation algorithm has been given to mitigate the SSII
in OFDM-PON upstream.
In this work, iteration interference cancellation method is used
to mitigate SSII caused by square-law photo-detection in OFDM-
PON upstream scheme for the first time. The SSII essentially is the
linear superposition of beating interference from multiple ONU.
The SSII reconstruction process should consider the difference of
the received power and symbol synchronization from different
ONU. Iteration interference cancellation algorithm is experimen-
tally demonstrated to achieve 1-dB sensitivity improvement at a
BER of
×
−
.8 10
3
(the Forward Error Correction limit) in a 20-Gbps
OFDM-PON upstream system with two ONU over 100-km un-
compensated SSMF. The impact of symbol synchronization on the
algorithm performance is evaluated in the following experiment.
2. Principle of SSII cancellation in OFDM-PON upstream
scheme
The principle and explicit analysis of SSII cancellation with
iteration technique in OFDM-PON downstream system have been
given in Refs. [5,6]. In this paper, we first introduce the cause of
SSII in OFDM-PON upstream scheme, and then explain the op-
eration principle of iteration SSII cancellation. The SSII in optical
line terminal (OLT) part is the linear superposition of beating in-
terference from multiple ONU. In the upstream scheme, different
wavelengths for different ONUs are necessary for preventing from
optical beating interference. The frequency of the beating inter-
ference between ONUs exceeds the range of photo-detection re-
sponse. Therefore, the wavelength spacing of ONUs is considerably
larger than the bandwidth of OFDM signal. Subcarriers of different
frequencies in one OFDM symbol period are assigned to the
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journal homepage: www.elsevier.com/locate/optcom
Optics Communications
http://dx.doi.org/10.1016/j.optcom.2015.11.014
0030-4018/& 2015 Elsevier B.V. All rights reserved.
n
Corresponding author.
E-mail address: jucheng719@bupt.edu.cn (C. Ju).
Optics Communications 364 (2016) 9– 12