Coherent Optical OFDM For Long-haul Communication System

With the development of global internet,in the up-gradation of optical networks,significant amount of effort has been made in order to keep up with the huge increase of traffic demand across the globe.Orthogonal frequency division multiplexing(OFDM)acknowledged countless consideration of the researchers next it was proposed as a modulation technique by the researchers in the region of long-haul communication networks in coherent detection and direct detection both.Nevertheless,Coherent optical OFDM is the next trend in optical communication technology since it has the ability of integrating with wavelength division multiplexing(WDM)system.By doing so,the bandwidth of communication can be enormously increased(Tbps),having significant data rates,and high spectral efficiency while maintaining the system overall cost.WDM system transmits multiple wavelengths over a single channel which results into increase in the bandwidth.The aim of this research is to design a coherent optical OFDM transmission system for high data rate.The study starts with the designing a 10Gbps single channel transmission across 100km SSMF which later extends to implanting the current CO-OFDM system into WDM system.The proposed schemes can transmit different type of data rates starting from 10Gbps to 50Gbps across 100 to 200km SSMF.For encoding purposes,4-QAM encoder is utilized at the OFDM transmitter side,whereas the signal is received via a CO-OFDM receiver and decoded with a 4-QAM decoder.At transmitter optical I/Q modulation is adopted,and at the receiver side coherent heterodyne detection scheme is operated.The system performance is analyzed in terms of its constellation diagrams and it is observed in the simulation results that the signal is almost fully recovered having minimum distortion.1)In the first scheme,the explanation of 10Gbps based single channel direct detection(DD)-OFDM transmission system for 100km SSMF and 200km SSMF optical fiber transmission channel is explained and implemented.It is found that the results are clear and constellation diagrams has minimum distortion.2)In the second proposed scheme,a heterodyne detection scheme is proposed for lOGbps single CO-OFDM transmission system.The length of fiber for the given scheme is tested at 100km SSMF and 200km SSMF.A 20dB erbium doped fiber amplifier(EDFA)is used in the transmission channel to achieve the transmission across the said fiber length.The constellation diagram shows clear in-phase diagrams with minimum distortion.3)In the third proposed scheme,the integration of CO-OFDM system with WDM system is proposed and explained.In this scheme,4 channels are used to transmit a multiplexed signal via the transmission channel.This scheme is proposed to achieve higher data rates.4 channels are used in the simulations where each channel has lOGbps data which results into transmission of 40Gbps transmission in long-haul optical communication networks.Moreover,a novel optical frequency comb(OFC)source at the transmitter side of the OFDM system is proposed and integrated in WDM system.The OFC can produce 5-flat comb lines.Overall,50Gbps data can be transmitted with each carrier transmitting lOGbps data across 60km SSMF and 120km SSMF as well.CO-OFDM with heterodyne receiver scheme is designed for this scheme and the results shows clear constellation diagrams with minimum distortion.