Optical coherent burst-mode receivers with
delayed channel equalisation feedback from
parallel and pipelined design
ISSN 1751-8628
Received on 28th July 2014
Accepted on 17th December 2014
doi: 10.1049/iet-com.2014.0735
www.ietdl.org
Bo Xu
✉
, Xinyu Liu, Chenyu Wu, Kun Qiu
Key Laboratory of Optical Fiber Sensing and Communications (Education Ministry of China), School of Communication and Information
Engineering, University of Electronic Science and Technology of China, Chengdu 611731, People’s Republic of China
✉ E-mail: xubo@uestc.edu.cn
Abstract: A digital optical coherent burst-mode receiver (BMR) with 112-Gb/s polarisation-division multiplexing-quaternary
phase-shift keying (QPSK ) modulation is p roposed with parallel digital signal processin g and pipelined design. The
convergence rate property of the digital channel equaliser with non-negligible computational delay is studied extensively
using simulations. Owing to the interaction between the channel equalisation and frequency offset estimation (FOE), a
novel equaliser adaptation method using the latest available equaliser output and the latest available FOE result is
proposed for a faster convergence than the conventional method. Simulation results reveal that the computational delay
have a great impact on the convergence rate. A smaller step size for tap coefficient updating with slower convergence rate
should be used to avoid divergence. Compared with the FOE module, the computational delay from the channel equaliser
has the dominant effect on the convergence. The design of the equaliser with short computational delay is the key for a
fast convergence rate. The parallel and pipelined digital optical coherent BMR after successful initialisation has negligible
error rate performance loss when compared with the ideal serial signal processing within a wide frequency offset range.
1 Introduction
Optical burst transmission is an efficient way to build elastic optical
network with sub-wavelength granularity [1–3]. In such a network,
the data is transmitted in the form of optical bursts from different
source nodes to different destination nodes and different optical
bursts share one wavelength channel with time multiplexing.
With the fast development of coherent detection and polarisation-
division multiplexing (PDM) to improve the channel efficiency,
optical burst-mode receiver (BMR) with coherent detection for
PDM-QPSK signals is expected to be one of the key technologies
for optical burst transmission [4, 5].
In an optical coherent BMR with digital signal processing (DSP),
adaptive equalisation is used to compensate fibre dispersion,
polarisation-mode dispersion (PMD) etc. Different from
conventional optical coherent receivers working in a continuous
mode, an optical coherent BMR should be able to make fast
re-initialisation for receiving the next optical burst which may
come from a different source node in the network and encounter
different channel condition. The fast initialisation of the adaptive
equaliser in an optical BMR is crucial for improving the
transmission efficiency. A training sequence-based initialisation
method with least-mean square (LMS) algorithm for coefficients’
adaptation is thus used in this work [6, 7].
Real-time implementation of DSP functions is the bottleneck for
an optical coherent BMR at a transmission rate of 100-Gb/s or
above [8]. Parallel processing with pipelined structure must be
used [9, 10]. Some early works have proposed ways for block
implementation of adaptive digital filters which can process a
block of signals simultaneously [11–13]. The effect of delays on
the block adaptive LMS digital filters were then found to slow
down significantly the convergence speed of the equaliser through
simulations [6, 7, 14]. However, these early works did not
consider the interaction of the adaptive LMS filters with the
frequency offset which is another limiting factor for optical
coherent receivers. A detailed simulation study on the effect of
parallelism and pipelined structure is included in this paper.
This paper is organised as follows. The pipelined structure of the
proposed optical BMR with parallel signal processing is given in
Section 2. Section 3 explains the key signal processing functions
used in the BMR. Section 4 studies the convergence speed of the
equaliser taking into account the parallelism and computational
delay in the pipelined structure. Simulation results on the error rate
performance of the optical coherent BMR are also given in
Section 4. Section 5 concludes the paper.
2 Structure of the optical coherent BMRs
Fig. 1 gives the schematic of the optical coherent BMR with parallel
DSP. Under the assumption that a 28 G baud rate PDM-QPSK signal
is transmitted over the channel and sampled at a rate of two samples
per symbol at the receiver side, the DSP modules should be able to
process the discrete-time signals at a rate of 56 Gsamples/s. Current
electronic processing digital circuits cannot deal with such a high
speed in a serial way and parallel signal processing is imperative.
For instance, the optical coherent BMR in Fig. 1 has 128 parallel
branches and is designed to process 128 symbols simultaneously.
The clock rate for the DSP modules can then be effectively
reduced from the 28 G baud rate for the original signals to a value
of 218.75 MHz. This paper focuses on two DSP modules, the
channel equaliser (EQ) and the frequency offset compensation and
estimation (FOC and FOE) module as shown in Fig. 1.EachEQ
is build with a butterfly structure for simultaneous channel
equalisation and polarisation division demultiplexing with a
schematic shown in Fig. 1b. The four sub-equalisers, H
xx
,H
xy
,
H
yx
and H
yy
, are assumed to have equal number of taps in this
study. The structure of the sub-equaliser H
xx
is given in Fig. 1c
with 11 taps as an example. A data-aided LMS-based adaptation
algorithm is used on the EQ’s tap coefficients in this study for a
faster convergence speed.
Carrier phase estimation (CPE) is also a commonly-used module
in optical coherent receivers because of non-negligible laser
linewidths. For small laser linewidth <1 MHz, the laser linewidth
induced phase variation from one block of 128 symbols to the
next block of 128 symbols has a variance <0.03 for the studied 28
G baud rate PDM-QPSK signals. Such a small phase variation can
be tracked by the data-aided LMS-based coefficient updating of
IET Communications
Research Article
IET Commun., 2015, Vol. 9, Iss. 7, pp. 975–981
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