A LONG HAUL CO-OFDM SYSTEM INCORPORATING SDC MODULE FOR ISI COMPENSATION

Research Paper

Engineering

E-ISSN No : 2454-9916 | Volume : 3 | Issue : 5 | May 2017

A LONG HAUL CO-OFDM SYSTEM INCORPORATING SDC MODULE FOR ISI COMPENSATION 1

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Anu Chauhan | Dr. Atul Kumar | Parath Singh Chauhan

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1

Assistant Professor, Physics, Lingayas University, Faridabad, India - 122002.

Dean Academics, Physics, Ansal University, Gurgaon, India -122001.

3 Retd. Block Education Officer, Education, Ministry of Education, Gurgaon, India - 122001.

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ABSTRACT In this work, we accentuated on the design of long haul coherent optical orthogonal frequency division multiplexing system (CO-OFDM) at high data rate. At 50 Gbps, CO-OFDM signals suffers from pulse width broadening (PWB) or inter-symbol interference (ISI) and in order to prolong the system reach, an optimal ISI compensation technique is required. Symmetrical dispersion compensation module (SDCM) is a prominent and well competent technique to combat the ISI effects and operational in our proposed architecture. Moreover, to make system bandwidth efficient, 4-QAM with grey coding is employed in the system and receiver phase manual adjustments are also considered to decrease the EVM. OFDM data stream is successfully communicated over 3000 km long stretch to cater the demands of long haul services. Performance of system is investigated for different launched powers, link lengths and laser linewidths and evaluated in terms of Q factor, signal-tonoise ratio and error vector magnitude (EVM).

KEYWORDS: Coherent optical orthogonal frequency division multiplexing (CO-OFDM), Inter-symbol interference (ISI), Pulse width broadening (PWB), Symmetrical dispersion compensation module (SDCM), Error vector magnitude (EVM) Introduction Orthogonal frequency division multiplexing is emerged as a prominent and well competent technology to overcome RF microwave multipath fading and also a potential advanced modulation for wired and wireless services [1]. In recent times, an alternate of radio frequency called coherent optical OFDM has been demonstrated and evolved as a promising technology for pulse broadening resilient transmission and combat fiber dispersion [2]. OFDM is widely implemented because of the less susceptible to frequency fading and interferences in broadband communication. FFT in OFDM exhibits the advantage of wide spectral efficiency and also performs equalization. For long haul optical communication system, OFDM is very attractive technology and has numerous advantages such as bandwidth efficiency and enables the equalization of dispersion [3]. In an OFDM system, to mitigate the effects of inter symbol interference (ISI) the cyclic prefix (CP) is inserted between symbols. In long haul communication systems, the major degrading factor is ISI and it arises due to chromatic dispersion. OFDM has two flavors and referred as direct detection OFDM and coherent detection OFDM at the reception end. Numerous research works has been reported in the context of the comparison of two types of OFDM and concluded that CO-OFDM is better than direct detection [4] [5]. CO-OFDM is less prone to ISI and ICI effects due to phase matching at receiver and extensively employed in the long reach communication systems.

data followed by QAM in order to convert serial bit stream into parallel bit stream. OFDM modulator with 512 subcarriers is incorporated for conversion of signal into orthogonal frequency in order to utilize bandwidth. Two arms MZM modulators are placed to modulate OFDM signal into optical domain and combined by a power combiner. Output of OFDM signal then fed to the loop consisting of SMF, DCF and EDFA. Symmetric dispersion compensation is employed to mitigate the effects of dispersion. Local Oscillator(LO) is used for phase matching centered at freq. 193.1THz at power 10dBm.Signal fed to OFDM demodulator and then m-array threshold detector is to detect multilevel signal followed by QAM decoder. A constellation visualizer is incorporated to check error vector magnitude EVM i.e. deviation of pulse from ideal point to another point.BER analyzer is to analyze the OFDM signal in terms of Q-factor and BER. SDCM module Symmetrical dispersion compensation is consisting of single mode fiber (SMF) each of length 25 Km followed by calculated Gain optical amplifier EDFA to overcome effects of attenuation caused by optical fiber. Dispersion compensation fiber (DCF) of 10 Km is placed right after SMF along with repeating EDFA of gain 5dB. In order to realize SDCM (symmetrical dispersion compensation module) identical SMF of length 25Km again linked to the DCF. The length of fiber can be increased by increasing the number of loops as depicted in fig 2.

Many studies have been reported so far in the OFDM transmission domain and diverse techniques have been suggested to extend the reach and speed of the system. In [6], author proposed a long reach system and recovered a coherent signal reliant on OFDM after 900 picoseconds and 1000 km via single mode fiber without pulse width reduction or ISI. Inter symbol interference compensation with FBG is reported in [7] for 250 km. These studies encouraged researchers to suggest the ways to mitigate the effect of ISI as OFDM is inherently resilient to dispersion, however, long distances gets dispersion accumulated. Design of long haul system is only possible by compensating the effects of ISI with dispersion compensation fiber. In [8] author investigated pre-, post- and symmetrical compensation in 10 Gbps NRZ links using DCF. Investigation of QPSK is also studied in [8] along with pre, post and symmetrical dispersion compensation at 10 Gbps and post compensation found out to be best for 145 km transmission. In spite of many constructive outcomes, OFDM systems are either limited in terms of speed and distance accomplishment or used complex techniques to quell ISI. In this research article, a long haul CO-OFDM system is proposed with intersymbol interference compensation to achieve prolonged distance. Symmetrical pulse width reduction is incorporated in the transmission line to re-time the optical OFDM signal pulses. A 50 Gbps OFDM stream line is communicated over 3000 km long stretch to cater the demands of long haul services.

System Description: To investigate the proposed CO-OFDM system, a simulation tool Optiwave's OptiSystem is used. This tool allows user to simulate, comprehend and provide a practical scenario for the evaluation of results. Figure1 depicts the system block diagram of proposed system architecture consisting of a CW laser source centered at frequency 193.1THz at 10dB power. A PRBS is used to generate random

Figure 1: Proposed system architecture of long haul CO-OFDM system employing SDCM

CopyrightŠ 2016, IERJ. This open-access article is published under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License which permits Share (copy and redistribute the material in any medium or format) and Adapt (remix, transform, and build upon the material) under the Attribution-NonCommercial terms.

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Research Paper

E-ISSN No : 2454-9916 | Volume : 3 | Issue : 5 | May 2017 Figure 3(b) depicts the illustration of signal to noise ratio at different launched powers for diverse link lengths. SNR is a measure of signal power to noise power accumulated in the system and SNR should be high. SNR at 10 dBm is maximum and for 8 dBm is minimum due to the reason that more power and less noise appears in former case and less power with more noise is emerged in later case.

Error vector magnitude is the deviation of the received signal from its ideal position and should be low as possible. EVM (rms) % is shown with respect to distances at different launched powers in fig 3(c). Minimum EVM is reported in the case of 10 dBm launched power level and moderate for 12 dBm because of symmetrical dispersion compensation and less power dependent effects of optical fiber such as nonlinear effects in aforementioned case.

Figure 2 Internal structure of SDCM module Table 1 System specifications of proposed architecture Parameters

Values

Data Rate

50 Gbps

Frequency

193.1THz

Launched Power

10dBm

Local Oscillator power

10dBm

QAM

4QAM

No. of Subcarrier

512

FFT Points

1024

Amp.

EDFA

Gain

5dB

NF

4dB

Results and Discussions Design of coherent orthogonal frequency division multiplex system for long haul communication is proposed and investigated for different link lengths and launched power levels. Results are evaluated in terms of error vector magnitude (EVM), Q-factor, BER and signal to noise ratio. Figure 3(a) depicts the performance of proposed system at different transmission distances with respect to the launched powers and also represents the Q factor. Results revealed that there is significant degradation in Q of the signal as the link length increases. Properties of attenuation, dispersion and nonlinear effects come into play and affect the signal. Launched powers are investigated in the system such as 8 dBm, 10 dBm and 12 dBm. It is seen that at 8 dBm Q factor is not as high as 10 dBm and 12 dBm due to less power transmission over SDCM. Results enhanced as power level increased from 8 dBm and maximum Q is obtained at 10 dBm due to optimal power lever that cannot influenced by nonlinearities on larger extent. However, nonlinearities degrade the system performance for 12 dBm launched power level. So, 10 dBm power is recommended for proposed system.

Figure 5 Graphical representation of (a) Distance versus Q factor (b) Distance versus SNR at different laser linewidths Figure 5(a) represents the linewidth effects of laser on proposed system setup and depicts the Q factor versus distance curve. At 0 MHz laser linewidth, performance of the system is moderate and trend of the curve reveals that Q decreases with the increase of the system. Here, performance of 0.1 MHz is excellent due to fact that increase in linewidth cause less emergence the nonlinear effects over the transmission fiber span, however at 10 MHz, results degraded because more linewidth cause more pulse broadening. Signal to noise ratio for 10 MHz is found out to be maximum (113.2 dB) and for 0 MHz is minimum (112.1 dB) as shown in fig 5(b).

Figure 6 (a) depicts the constellation diagram of 4-QAM at 3600 km for 10 dBm launched power. It is observed that maximum EVM % is reported at this particular instant. However, proposed system works within acceptable range of Q factor i.e. 3 for 3000 km and beyond 3000 km, Q do not exhibits Q of 3. Figure 6 (b) represents the constellation of QAM at 0.1 MHz laser linewidth for 3600 km link distance.

Figure 6 Constellation diagram of 4 QAM at receiver for (a) 10dBm at 3600 km (b) linewidth 0.1 MHz at 3600 km Conclusion CO-OFDM is a potential technology in wired and wireless services and very attractive candidate to combat ISI effects in optical fiber. In this research article, investigations of long haul ISI compensated CO-OFDM has been done by incorporating symmetrical dispersion compensation modules. Results revealed that proposed system covered 3000 km distance successfully at 50 Gbps with manual constellation adjustment and SDCMs. All the dispersion compensation fibers and EDFAs are used in calculated manner to re-shape the pulse and boost the signal respectively. It is observed that launched power of 10 dBm and laser linewidth of 0.1 MHz is optimal for system and minimum EVM% has been reported at these parameters. Q factor, SNR is also analyzed and found that 4QAM grey coded signal at 10 dBm exhibits best performance.

REFERENCES [1] A.J. Lowery, J. Armstrong, “Orthogonal-frequency division multiplexing for dispersion compensation of long-haul optical systems”, Opt. Express, vol. 6, no. 14, pp, 2079–2084, 2006.

Figure 3 Graphical representation of (a) Q-factor (b) SNR (c) EVM (rms) % versus Distance at different launched powers

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