COL 10(11), 110601(2012) CHINESE OPTICS LETTERS November 10, 2012
Improving nonlinearity tolerance of 112-Gb/s PDM OFDM
systems with coherent detection using BLAST algorithm
Yanfei Xu (
MMMýýý
), Yaojun Qiao (
zzz
), and Yuefeng Ji (
VVV
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)
∗
State Key Laboratory of Information Photonics and Optical Communications,
Beijing University of Posts and Telecommunications, Beijing 100876, China
∗
Corresponding author: jyf@bupt.edu.cn
Received March 10, 2012; accepted May 17, 2012; posted online August 3, 2012
The use of Bell Laboratories layered space-time (BLAST) architecture as a digital signal processing algo-
rithm is proposed in this letter. It is aimed at improving the nonlinearity tolerance of a polarization division
multiplexing (PDM) coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. The
application of this channel estimation algorithm simulates system performance under different dispersion
compensation (DC) maps. Simulation results show that, compared with intra-symbol frequency-domain
averaging (ISFA) algorithm, at least 5-dB Q-factor improvement is achieved for the PDM CO-OFDM
system at 112-Gb/s data rate over an 800-km standard single-mode fiber (SSMF) without DC.
OCIS codes: 060.1660, 060.4370, 070.6020.
doi: 10.3788/COL201210.110601.
Orthogonal frequency division multiplexing (OFDM) is
generally susceptible to fiber nonlinearity and phase
noise due to its high peak-to-average power ratio
(PAPR)
[1]
. Therefore, it is critical to investigate and
improve the coherent optical OFDM (CO-OFDM) sy s-
tem tr ansmission performance, including fiber nonlinear-
ity, which affects all types of long-haul optical systems.
Various comp e ns ation algorithms that aim to solve fiber
nonlinearity have been proposed in recent years. Among
these algorithms is the traditional but complex digital
back propagation
[2]
algorithm. Fiber nonlinearity com-
pens ation implemented by periodic dispersion maps
[3]
has already been pro posed. An optimized optical phase
conjugation (OPC) configuration for nonlinear cancella-
tion has also been presented
[4,5]
. The new metho d incor-
porates biased clipping OFDM
[6]
. However, all of the al-
gorithms mentioned have a high degree of computational
complexity. In this letter, we propose a channel estima-
tion algorithm to enhance nonlinearity tolerance based
on the Bell Laboratories layered space-time (BLAST) al-
gorithm.
The layered space-time architecture pr oposed by
Foschini
[7]
, now known as BLAST, is one of the most
impo rtant approaches in improving wireless spectrum
efficiency in multiple-input multiple-output (MIMO)
systems. This architecture was originally designed for the
transmitter that lacked knowledge of the channel charac-
teristic within a Rayleigh fading environment. This ar-
chitecture yields a system with a capacity that increases
linearly with n for both fixed bandwidth and fixed total
radiated power
[7]
. This method is then used by Djord-
jevic et al.
[8]
as a polarizatio n-mode dispersion (PMD)
compensation scheme suitable for use in multilevel (M >
2) block-coded modulation schemes with coherent detec-
tion. This algorithm compensates for the PMD influence
effectively, although it has not yet been used in estimat-
ing fiber nonlinearity.
The intra-symbol frequency-domain averaging (ISFA)
algorithm, first proposed by Liu et al.
[9]
, is an efficient
channel estimation method for CO-OFDM. It is robust
against various transmission impairments, such as opti-
cal noise, chromatic disp e rsion (CD), P MD, polarization-
dependent loss (PDL), and fiber nonlinear ity.
The BLAST algorithm is proposed in this letter to im-
prove the no nlinearity tolerance in po larization division
multiplexing (PDM) CO-OFDM systems. The algorithm
is then compared with the ISFA. The application of this
channel estimation algorithm enables the simulation of
system performance under different dispersion compen-
sation (DC) maps. The simulation results show that,
compared with the ISFA a lgorithm
[10]
, at least 5-dB Q-
factor improvement is a chieved for the PDM CO-OFDM
system at a 112-Gb/s data rate over an 800-km standard
single-mode fiber (SSMF) without DC.
Polarization-multiplexed quadrature phase-shift keying
(QPSK) modulation transmission with a gross data rate
of 112 Gb/s for CO-OFDM systems were simulated in
VPI transmission Maker 7.6. This process was conducted
to evaluate the system performance. Figure 1 illustrates
the schematic of the optical OFDM transmitter and re-
ceiver setup.
Assuming a perfectly synchronized system, the received
signal R(k) of the kth subcarr ier is expressed as
[3]
R(k) = H(k)X(k) + n(k), H(k)
h
xx
h
xy
h
yx
h
yy
, (1)
where X(k) = [x
x,k
, x
y,k
]
T
denotes the transmitted sy m-
bol vector, n(k) = [n
x,k
, n
y,k
]
T
represents the noise vec-
tor dominantly determined by the amplified spontaneous
emission (ASE), a nd H(k) is the 2 × 2 channel matrix
determined by training-aided
[11]
channel estimation.
Using a 512-sized fast Fourier transform (FFT), 112-
Gb/s data bits at the trans mitter were encoded into the
baseband OFDM signals. The cyclic prefix (CP) ratio
was 1/8, which meant that 64 subcarriers were copied
from the back of an OFDM symbol to the front. Mean-
while, the zero-padding ratio was maintained at 1/8, thus
1671-7694/2012/110601(4) 110601-1
c
2012 Chinese Optics Letters