Performance Enhancement of WDM Optical System with Coherent Detection by Rohit Bhimjibhai Patel

Guided by: Dr. D. K. Kothari With Synopsis and CD 11EXTPHDE61

ABSTRACT:
Wavelength division multiplexed (WDM) optical communication is rapidly growing
with deployment of multilevel modulation formats. Efficient utilization of optical fiber
bandwidth with high spectral efficiency is the key advantage of this technology for long
transmission reach. However, fiber linear and nonlinear impairments are still the
challenges which prevent the growth of WDM optical communication at high data rates.
Coherent detection technique with digital signal processing (DSP) at the receiver end
has shown the capability to overcome these challenges. Further, multi-carrier
generation at transmitter and the use of polarization division multiplexed (PDM) multilevel
modulation format are the solutions to enhance the spectral efficiency of the
system with narrow spacing between channels.
In this thesis, multi-carrier generation concept is applied at transmitter side employing
coherent detection technique with DSP at receiver end to enhance the performance of
WDM optical system. Polarization division multiplexed quadrature phase shift keying
(PDM QPSK) and polarization division multiplexed 16- quadrature amplitude
modulation (PDM 16-QAM) are used to realize the WDM optical system for long reach
at high transmission rates. Hybrid transmission approach has also been considered in
designing the system to represent the system performance in the dual modulation
environment. Q-factor, Log of Estimated symbol error (Log (ESE)) and error vector
magnitude (EVM) are considered as a performance metrics. Simulation results are
obtained using optisystem software.
The performance of 12 x 160 Gb/s (1.92 Tb/s) wavelength division multiplexed WDM
optical system employing dual carrier and coherent detection with DSP is investigated
for three different cases: gray coding, with differential coding and without any coding.
Results are compared for 100 km transmission using polarization division multiplexed
quadrature phase shift keying (PDM-QPSK) format. The results exhibit that system
with differential coding performs better in comparison with gray coding and without
any coding technique. Improvement of 2 to 5 dB in Q factor is found for system using
differential coding. Moreover, performance evaluation for long transmission distance
up to 8000 km is carried out for WDM optical system with differential coding technique
with maintaining the Q factor beyond the FEC limit (BER value 3.8 x 10-3). An effort is made to investigate the performance of 4 x 200 Gbps (800 Gb/s) coherent
WDM Optical systems employing dual carrier concept for polarization division
multiplexed 16-quadrature amplitude modulation (PDM 16-QAM) modulation format.
Results are compared for the system with nonlinearity compensation and without
nonlinearity compensation at the receiver end. Improvement in Q factor is observed
almost 2.43 dB to 1.95 dB for system with nonlinearity compensation than system
without nonlinearity compensation at 6 dBm power per channel for 200 km to 1000 km
transmission reach. Spectral efficiency of 4 b/s/Hz is achieved by keeping 50 GHz
channel spacing between generated sub-carriers. Another approach is made to evaluate
10 x 100 Gbps coherent WDM optical system using hybrid modulation. WDM system
is designed using PDM QPSK and PDM 16-QAM modulation formats in combination
with dual carrier concept. Results are reported with ULAF fiber and SSMF fiber for
WDM optical system. System with ULAF fiber shows better results due to less
influence of fiber nonlinearity at 2500 km transmission reach.
A Comparative analysis is carried out of of 23 x 100 Gbps (2.3 Tb/s) WDM Optical
systems employing multicarrier generation for three different fibers: ultra large area
fiber (ULAF), standard single mode fiber (SSMF) and large effective area fiber
(LEAF). Enhancement in Q-factor is observed almost 3.21 dB and 4.63 dB for system
with ULAF fiber than system with SSMF and LEAF fibers respectively for 5000 km
transmission reach. Performance comparison is also carried out using ultra large area
fiber (ULAF) and large effective area fiber (LEAF) for 39 x 100 Gbps (3.9 Tb/s) hybrid
transmission WDM optical system employing multicarrier generation. Among 39
carriers, 20 sub-carriers utilize PDM-QPSK modulation format and 19 sub-carriers use
PDM 16-QAM format. It is found that deployment of ULAF fiber outperforms than
LEAF fiber in the system at long transmission reach.