Decision-free radius-directed Kalman filter for universal
polarization demultiplexing of square M-QAM and
hybrid QAM signals
Yanfu Yang (杨彦甫)
1
, Qun Zhang (张 群)
1
, Yong Yao (姚 勇)
1,
*, Guoliang Cao (曹国亮)
1
,
Kangping Zhong
2
, Xian Zhou
2
, Alan Pak Tao Lau
2
, and Chao Lu
3
1
Department of Electronic and Information Engineering, Shenzhen Gradua te School,
Harbin Institute of Technology, Shenzhen 518055, China
2
Photonics Research Center, Department of Electrical Engineering, The Hong Kong Polytechnic University,
Hong Kong, China
3
Photonics Research Center, Department of Electronic and Information Engineering,
The Hong Kong Polytechnic University, Hong Kong, China
*Corresponding author: yaoyong@hit.edu.cn
Received May 18, 2016; accepted September 9, 2016; posted online October 14, 2016
We propose and experimentally demonstrate a universal and blind polarization-state tracking scheme for
M-quadrature amplitude modulation (QAM) signals based on a decision-free radius-directed linear Kalman filter
(RD-LKF). The polarization tracking performance is investigated through simulation and experiments for both
quadrature phase-shift keying (QPSK) and 16-QAM signals. The influence of the filter parameters on the static
polarization demultiplexing performance and dynamic tracking capability are discussed via simulation. The
optimization strategy of the filter parameters in modulation-format-independent scenarios is proposed and sim-
ulations are carried out to evaluate the polarization demultiplexing penalty for QPSK, 16QAM, and hybrid
QPSK/16QAM signals. Finally, the proposed decision-free RD-LKF is experimentally compared to the respec-
tive algorithms of constant modulus algorithm and multimodulus algorithm for QPSK and 16-QAM and the
advantage of ultrafast polarization tracking ability is confirmed.
OCIS codes: 060.1660, 120.3930.
doi: 10.3788/COL201614.110601.
Rate adaptive transceivers have recently attracted much
attention due to their potential for improving overall
optical network efficiency under dynamic internet traffic
patterns in emerging data center or various cloud applica-
tions. The spectral efficiency of the transmitter may be
adjusted discretely by configuring quadrature amplitude
modulation (QAM) formats or may be tuned continuously
with time domain hybrid QAM (TDHQ) schemes
[1–3]
.
Therefore, universal digital signal processing (DSP)
schemes supporting dynamic modulation formats is gain-
ing more and more attention
[4,5]
. In a polarization-division-
multiplexing (PDM) coherent receiver, fast polarization
tracking and demultiplexing is an indispensable DSP mod-
ule to deal with rapid stokes vector movements caused by
external vibrations
[6]
. Compared with training sequence
(TS) methods, blind polarization demultiplexing may
be desired due to a high spectral efficiency and no frame
synchronization. Conventional blind polarization demulti-
plexing algorithms, including the constant modulus algo-
rithm (CMA)
[7]
, the multimodulus algorithm (MMA), and
their variants
[8]
, have been applied to PDM-quadrature
phase-shift keying (QPSK) and PDM-16QAM, respec-
tively. However, these schemes are modulation format
dependent and a universal scheme applicable for dynamic
optical signals is highly desired. Gao et al. proposed a
two-stage universal demultiplexing scheme employing
CMA-based preconvergence and decision-directed least
mean square (DD-LMS) together
[5]
. Recently, due to its
excellent estimation accuracy and tracking ability, the
Kalman filter has attracted much attention for coherent
optical systems. The reported application examples in-
clude clock tone enhancement
[9]
, carrier recovery
[10–12]
, and
polarization tracking
[13–15]
. We proposed a novel radius-
directed linear Kalman filter (RD-LKF) scheme for fast
polarization tracking that is immune to frequency offset
compared to the previous extended Kalman filter (EKF)
scheme
[13]
. Here, we further modify our proposed RD-LKF
scheme into a universal polarization-state tracking frame-
work for dynamic modulation formats.
In this Letter, a universal polarization-state tracking
based on a decision-free RD-LKF filter is proposed and
experimentally investigated for M-QAM formats. The
universal RD-LKF-based scheme has the constant radius
as a reference for the residual estimation regardless of the
modulation order of M-QAM signals. Consequently, the
scheme involves no radius decision operation and can work
in a universal manner. As an application extension of RD-
LKF, this universal scheme has inherently the advantages
of immunity to the carrier phase, frequency offset, and
singularity problem. The rest of the Letter is organized as
follows. First, the principle of the decision-free RD-LKF
scheme is introduced. The polarization demultiplexing
COL 14(11), 110601(2016) CHINESE OPTICS LETTERS November 10, 2016
1671-7694/2016/110601(5) 110601-1 © 2016 Chinese Optics Letters