KEYWORDS: Digital signal processing, Receivers, Polarization, Nonlinear optics, Kerr effect, Single mode fibers, Modulation, Complex systems, Optical filters, Signal detection
The hybrid mid-link spectral inversion (H-MLSI) and digital signal processing techniques to compensate for the optical Kerr effects in 224 Gbit/s DP-16QAM transmission over 640 km of single-mode fiber are numerically evaluated. Digital signal processing methods, i.e., electronic dispersion compensation (EDC) and digital backward propagation (DBP) techniques, are implemented. The system is evaluated for diverse signal input launch powers for both single-channel and multichannel transmission in which five channels are multiplexed with a channel spacing of 100 GHz with central wavelength at 1550 nm. The system performance is enumerated by monitoring the bit error ratio. From the results, it is clear that the nonlinear threshold point is improved by 2 and 3 dBm signal power by using H-MLSI and DBP, respectively, with 20 steps per fiber span as compared to EDC. Furthermore, we have also evaluated the DBP complexity as compared to H-MLSI and the resultant impact on maximum transmission distance. Moreover, the performance penalty coming from the span-offset of H-MLSI can be reduced by employing DBP to compensate for the residual Kerr effects.
We have numerically evaluated the transmission performances of digital backward propagation (DBP) algorithms based on the step-size selection methods: (a) constant step-size based modified DBP (M-DBP) and (b) logarithmic step-size based DBP (L-DBP). The nonlinear tolerance of homogeneous dispersion un compensated fiber links are compared in 224 Gb/s DP-16QAM transmission over 656 km of diverse fiber types, i.e., standard single mode fiber (SMF), non-zero dispersion shifted fiber (NZDSF), large effective area pure-silica-core-fiber (LA-PSCF), large effective area fiber, and true wave classic. In the case of homogeneous fiber links, LA-PSCF exhibits higher non linear threshold and gains 32% more transmission distance as compared with SMF by employing M-DBP, while we have observed an additional 13% increase in transmission distance by employing L-DBP.
KEYWORDS: Receivers, Digital signal processing, Nonlinear optics, Dispersion, Wavelength division multiplexing, Single mode fibers, Data communications, Telecommunications, Modulation, Transmitters
We have numerically investigated the impact of non-linear impairments on the performance of 400Gbit/s DP-RZ-
QPSK transmission system over 1200km standard single mode fiber (SMF-28) having an average span
loss of 16dB and with no in-line optical dispersion compensation in the transmission link. Digital backward
propagation (DBP) algorithm based on split-step Fourier method (SSFM) is employed along with the coherent
receiver to compensate the fiber transmission impairments i.e. chromatic dispersion (CD) and non-linear (NL)
impairments. The system performance is monitored in terms of Q-value (calculated form BER) for various
signal input launch powers. We further quantify the impact of inter-channel non-linear impairments such
as cross-phase-modulation (XPM) and four-wave-mixing (FWM) on the performance of DBP algorithm by
investigating the multiple-channel transmission, i.e. 8x400Gbit/s DP-RZ-QPSK system. The results depict
efficient performance of DBP algorithm as compared to the system where only linear dispersion compensation
is implemented. This shows the promising impact of digital backward propagation algorithm on the high data-rate
transmission systems such as 400Gbit/s per single channel which is expected to be a possible data rate for
long-haul optical communication systems after 100Gb Ethernet in near future.
KEYWORDS: Digital signal processing, Optical filters, Single mode fibers, Nonlinear filtering, Linear filtering, Digital filtering, Filtering (signal processing), Nonlinear optics, Computing systems, Computer simulations
We have investigated a new method to reduce the complexity of the digital backward propagation algorithm
(DBP). A logarithmic step-size based split-step Fourier method (SSFM) is investigated in this paper to compensate
fiber transmission impairments i.e. chromatic dispersion (CD) and non-linearities (NL) in dual-polarization
quadrature phase shift keying (DP-QPSK) system. The algorithm is numerically investigated for coherently-detected
multiple channel DP-QPSK system over 2000km (25 spans) standard single mode fiber (SMF-28)
with un-compensated transmission link. The algorithm is numerically evaluated for: (a) 20 channel 56Gbit/s
(14GBaud) with 25GHz channel spacing; (b) 10 channel 112Gbit/s (28GBaud) with 50GHz channel spacing
and (c) 5 channel 224Gbit/s (56GBaud) with 100GHz channel spacing. Each simulation configuration has
the bandwidth occupancy of 500GHz and a total transmission capacity of 1.12Tbit/s. The logarithmic DBP
algorithm (L-DBP) shows efficient results as compared to the conventional DBP method based on modified
SSFM (M-DBP). The results depict efficient mitigation of CD and NL, therefore improving the non-linear
threshold point (NLT) upto 4dB. Furthermore by implementing a low-pass-filter (LPF) in each SSFM step,
the required number of DBP stages to compensate fiber transmission impairments can be significantly reduced
(multi-span DBP) by 75% as compared to L-DBP and by 50% as compared to M-DBP. The results delineate
improved system performance of logarithmic step size based filtered DBP (FL-DBP) both in terms of efficiency
and complexity which will be helpful in future deployment of DBP algorithm with real-time signal processing
modules for non-linear compensation.
Recent numerical and experimental studies have shown that coherent transmission with advanced modulation
formats i.e. DP-QPSK and QAM are the promising candidates for next-generation systems with data rates
of 100Gbit/s and above. Coherent detection is considered efficient along with digital signal processing (DSP)
to compensate many linear effects in fiber propagation i.e. chromatic dispersion (CD) and polarization-mode
dispersion (PMD). Despite of fiber non-linearities (NL), which are the major limiting factors, next-generation
optical systems are employing higher order modulation formats in order to fulfil the ever increasing demand
of capacity requirements. However, the channel capacity is limited at higher signal input powers because the
system is operating in the non-linear regime. Due to this phenomenon the compensation of non-linearities is
a topic of great interest and research these days, especially for long-haul fiber transmission. Digital backward
propagation (DBP) algorithm has emerged as a promising and potentially capable candidate, which can jointly
compensate fiber dispersion and non-linearities along with the coherent receiver. In this paper we give a
detailed overview on the advancements in DBP algorithm based on different types of mathematical models i.e.
Wiener (Asymmetric Method) and Wiener Hammerstein models (Symmetric Method). We also discuss the
importance of optimized step-size selection, i.e. constant step-size and logarithmic step-size based split step
Fourier methods, for simplified and computationally efficient implementation of DBP algorithm. Moreover, by
means of numerical investigations we refer to recent system investigations to further improve the performance
of DBP algorithm.
Digital Backward Propagation (DBP) algorithm for mitigating fiber dispersion and non-linearities based on modified
non-iterative symmetric split-step Fourier method (M-SSFM) is implemented and numerically evaluated.
The algorithm is modified by shifting the calculation point of non-linear operator (r) together with the optimization
of dispersion (D) and non-linear coefficient (γ) to get the optimum system performance. DBP is evaluated
for 10x10Gbit/s wavelength division multiplexed (WDM) system (a total transmission capacity of 100Gbit/s)
with RZ-DQPSK encoded signals over a transmission length of 1600km standard single mode fiber (SMF) with
no in-line optical dispersion compensation. Furthermore, we quantify the impact of optical add-drop multiplexers
(OADMs) in the transmission link. Modification of DBP parameters and bandwidth of optical filters associated
with OADMs give significant improvement in the system performance.
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