Long Haul QAM-16 Fiber Optic Coherent Communication Systems

OCSim Modules

Modern Fiber Optic Communication Systems Simulations with Advanced Level Matlab Modules


Module 15


Long Haul QAM-16 Fiber Optic Coherent Communication Systems

Company Researchers & Developers

Integrate the Modules with your in-house and Commercial Software & Hardware Products


(1) Use the Existing Modules / Components for Your Research & Development.
(2) Modify the Modules / Components to the Next Level for Your Research & Development.
(3) Integrate Different Modules / Components in the OCSim Package to Realize Your Own Fiber Optic Communication Systems.
(4) Modify the Modules for Co-Simulations with the Third Party Commercial Optical Communication Systems Softwares.


Main Module


Currently commercial long haul fiber optic communication systems are based on QPSK, which are moving towards QAM-16. The dominant impairments in such systems are (i) chromatic dispersion and (ii) nonlinear effects. In this module, QAM-16 data is transmitted over a fiber optic system consisting of N spans of single-mode fibers and N in-line amplifiers. The transmission fiber is characterized by the parameters (i) loss coefficient \alpha (ii) nonlinear coefficient \gamma (iii) second order dispersion coefficient  \beta{{_2}}  and (iv) third order dispersion coefficient  \beta{{_3}}.

This Module calls the following Sub Modules and Components:

(1) power_meter.m
Calculates the average power in dBm.

(2) constellation_diagram.m
Plots the constellation.

(3) QAM_modulation.m
This code generates the QAM data. Arbitrary QAM-M can be generated. For example, when XX=4 and YY=4, we get QAM-16. When XX=8 and YY=8, we get QAM-64. This code can also generate QPSK data by setting XX=2 and YY=2.

(4) fiber_prop.m
Solves the Nonlinear Schrödinger Equation (NLSE) using a split-step Fourier scheme.

(5) amp.m
Inline amplifier is realized using this function.

(6) gauss.m
A Gaussian bandpass filter is introduced. The half-bandwidth (‘bw’) should be specified. This could also be used as a low pass filter and in this case, ‘bw’ is the 3-dB bandwidth.

(7) down_sample_CD.m
The number of samples per symbol for modeling the fiber optic system may be different from that used in DSP. This code provides down-sampling of the received signal by a factor that is suitable for DSP processing.

(8) ber_calc_qam.m
Calculates the BER by comparing the received bit pattern with the transmitted bit pattern.


Explore Further this Module:

15.1. Pick XX=4 and YY=4 so that the QAM-16 data is generated. Turn off the nonlinearity (set ‘gam’=0) and plot the BER as a function of transmission distance. Choose the launch power (av_power_dBm = -6 dBm). Change the transmission distance from 5X80 km to 15X80 km. Change the number of bits, if needed. Observe that BER increases with distance.

15.2. Repeat experiment 15.1 at a higher launch power (say 0 dBm). Observe that BER in experiment 15.2 is lower than that in experiment 15.1 since higher launch power implies better performance in a linear system.

15.3. Turn on the nonlinearity (set ‘gam’=1.1 W-1 km-1). Fix the transmission distance as 10X80 km. Change the launch power from -8 dBm to 3 dBm (with an increment of 2 dBm) and plot the BER vs launch power. Observe that the BER decreases initially (linear regime) and then starts to increase (nonlinear regime).

15.4. Pick XX=8 and YY=8 so that the QAM-64 data is generated. Repeat 15.1 to 15.3. Adjust the total transmission distance so that the BER is in the range of 1e-3 to 5e-2.



Selected Simulated Results Using this Module 


Fig.1_Long Haul QAM-16 FO Systems Constellation at Tx

Constellation Diagram at the Tx

(Long Haul QAM-16 Fiber Optic Communication Systems)



Fig.2_Long Haul QAM-16 FO Systems Output at FO Link


Constellation Diagram at the Output of FO Link

(Long Haul QAM-16 Fiber Optic Communication Systems)



Fig.3_Long Haul QAM-16 FO Systems Constellation after CD Compensation


Constellation Diagram after the CD Compensation

(Long Haul QAM-16 Fiber Optic Communication Systems)



Fig.4_Long Haul QAM-16 FO Systems k-factor vs BER


k-factor vs BER

(Total nonlinear phase accumulated over the fiber optic link due to SPM is k times
nonlinear phase. k depends on the system parameters such as CD and loss and
hence, it needs to be optimized)

(Long Haul QAM-16 Fiber Optic Communication Systems)



Fig.5_Long Haul QAM-16 FO Systems Constellation after CD & SPM Compensations


Constellation Diagram after the CD & SPM Compensations

(Long Haul QAM-16 Fiber Optic Communication Systems)




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