Flexible multidimensional modulation formats based on
PM-QPSK constellations for elastic optical networks
Zilong He (何子龙)
1,
*, Wentao Liu (刘文涛)
1
, Bailin Shen (沈百林)
2
, Xue Chen (陈雪)
1
,
Xiqing Gao (高夕晴)
1
, Sheping Shi (施社平)
2
, Qi Zhang (张 琦)
2
,
Dongdong Shang (尚冬冬)
2
, Yongning Ji (吉勇宁)
2
, and Yingfeng Liu (刘英峰)
2
1
State Key Lab of Information Photonics and Optical Communications,
Beijing University of Posts and Telecommunications, Beijing 100876, China
2
ZTE Corporation, Beijing 100191, China
*Corresponding author: hzl_bupt@163.com
Received September 5, 2015; accepted February 23, 2016; posted online March 18, 2016
We demonstrate flexible multidimensional modulation formats, polarization multiplexed k-symbol check quad-
rature phase shift keying (PM-kSC-QPSK), based on PM-QPSK constellations for elastic optical networks. The
experimental results show a significant optical signal noise ratio (OSNR) tolerance improvement for PM-2SC-
QPSK and PM-4SC-QPSK over PM-QPSK in both back-to-back and 500 km transmission scenarios at the
expense of spectral efficiency reduction. This flexible modulation method can be used in elastic optical
networks to provide a trade-off between the spectral efficiency and OSNR tolerance.
OCIS codes: 060.1660, 060.2360, 060.4080.
doi: 10.3788/COL201614.040602.
Currently, elastic optical networks
[1]
and software defined
networks
[2]
have been touted as solutions for enhanced
spectral efficiency (SE) and optimized network resource
utilization. These architectures require transceivers with
a tunable modulation format and symbol rate to support
trade-offs among optical reach, bit rate, and SE
[3]
. There
exist several approaches to realize flexibility in both
reach and bit rate. In principle, reach can simply be
traded against SE and bit rate by employing polarization
multiplexed m-ary quadrature amplitude modulation
(PM-mQAM) with varying size m of the modulation
alphabet
[4–7]
. However, because available modulation for-
mats, such as PM quadrature phase shift keying (QPSK)
and PM-16QAM, have a big difference in terms of SE and
achievable transmission distance, the resulting options in
terms of deployment are insufficient for flexible systems.
The granularity of this trade-off can be further refined by
time domain interleaving of symbols belonging to different
sizes m of the modulation alphabet. The resulting modu-
lation formats are referred to as time-domain hybrid
QAM
[8]
. Additionally, rate adaptive forward error correc-
tion (FEC) codes can be employed for an extra degree of
freedom
[9]
. In such schemes, the FEC code rate and the size
of the modulation alphabet are adjusted to optimally
support the desired net bit rate over a requested reach.
However, some studies suggest that rate adaptive FEC
requires a significant additional hardware effort, thus
increasing the transceiver cost
[8]
. Another option for
achieving a finer granularity is the application of alterna-
tive modulation formats in four-dimensional (4D) signal
space, which have recently received a lot of attention
due to their interesting features. For instance, polarization-
switched QPSK (PS-QPSK) was identified as the most
power-efficient modulation format
[10]
in four dimen sions
and can be derived from PM-QPSK by using the
Ungerboeck’s set-partitioning scheme
[11]
in four dimen-
sions, where the minimum Euclidean distance between
the constellation points is increased by
2
p
≈ 1.76 dB after
one partition
[12,13]
. Similarly, another popular 4D format
128-SP-QAM can be derived from PM-16QAM by set par-
titioning, i.e., a subset of M ¼ 128 points out of 256 are
chosen from the PM-16QAM constellation by increasing
the minimum Euclidean distance
[14]
.
In this Letter, we first present the flexible multidimen-
sional modulation method, PM-kSC-QPSK (SC: symbol
check), and then describe the experimental investigations
of the back-to-back (B2B) and 500 km transmission per-
formance of PM-2SC-QPSK and PM-4SC-QPSK, with
PM-QPSK as the reference at the same symbol rate of
32 Gbaud (i.e., at the bit rates of 96 Gb/s for PM-2SC-
QPSK, 112 Gb/s for PM-4SC-QPSK, and 128 Gb/s for
PM-QPSK). The results show that PM-2SC-QPSK and
PM-4SC-QPSK reduce the optical signal noise ratio
(OSNR) required to achieve a bit error rate (BER) of
10
−3
with 3 and 2 dB, respectively, compared to PM-
QPSK in 500 km transmission. This method can be used
in elastic optical networks to enable trade-off between SE
and OSNR tolerance.
Figure
1 shows the schematic diagram of the flexible
multidimensional modulation method based on the PM-
QPSK constellations (this method also can be applied
on other kind of constellations, such as PM-16QAM).
We use the single-parity check (SPC) to correlate k con-
secutive symbols at the transmitter, shown in Fig.
1(a),
and each group of k consecutive symbols are decoded to-
gether by using a joint-decision algorithm at the receiver.
The signal space is expanded to 2 k-dimensional, since
these k-related QPSK symbols can be regarded as a whole.
COL 14(4), 040602(2016) CHINESE OPTICS LETTERS April 10, 2016
1671-7694/2016/040602(4) 040602-1 © 2016 Chinese Optics Letters