IJRIT International Journal of Research in Information Technology, Volume 2, Issue 1, January 2014, Pg: 84-88

International Journal of Research in Information Technology (IJRIT) www.ijrit.com

ISSN 2001-5569

Role of advanced modulation formats Somnatha, Kamaljit Singh Bhatiab, Kulwinder Singhc a,cDepartment of Electronics and communication, Bhai Maha Singh College of Engg., Sri Muktsar Sahib, 152026, India bDepartment of Electronics Engineering, Sri Guru Granth Sahib World University, Fatehgarh Sahib, India

Abstract: In this paper role of different modulation formats is elaborated. After comparing the performance of RZ and NRZ modulation formats. The ability to modulate optical signals in formats different of the common RZ and NRZ like duobinary, modified duobinary, carrier suppressed RZ, DPSK and DQPSK is explained

1. Introduction In the past, the optical modulation format employed in lightwave communication systems was non-return-to-zero (NRZ). At that time, the most deleterious effect of an optical fiber was the dispersion, which increases with the bit rate. However, at high bit rate, such as 40 Gb/s, the effect of nonlinearity becomes more important, and the RZ signal format is proving to be superior to NRZ at such high bit rates. The advantages of the RZ signal format over the NRZ are discussed below. Due to the fact that the RZ signal format has larger bandwidth than the NRZ, the RZ pulses are broadened more rapidly by dispersion. However, this turns out to be beneficial because as a pulse is broadened, its peak decreases. The severe pulse broadening in the case of RZ signals makes it more robust to the effect of nonlinearity [1]-[5] due to the fact that the nonlinear effect is proportional to the signal intensity. This is very important in high-bit-rate systems in which high launched power is required to provide adequate SNR at the receiver. The effect of accumulated dispersion can be removed by employing dispersion compensation, which in effect recovers the waveform back to its original form. Since the pulse width of the RZ signal is narrower than that of the NRZ signal, the RZ pulse has higher peak power than the NRZ for a given average power. Thus, the eye opening of the RZ signal format is wider than that of the NRZ, resulting in better receiver sensitivity than the NRZ for a given average power [6-8]. This implies that for a required receiver sensitivity, the transmitted power can be lowered by employing the RZ signal format rather than the NRZ [7]. The better receiver sensitivity in the case of the RZ signal also suggests that the transmission distance can be increased compared with the NRZ signal for the same transmitted power [8]. When the SPM is considered, its interaction with dispersion depends strongly on the pulse Somnath, IJRIT

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 1, January 2014, Pg: 84-88

width. In the case of the NRZ signal format, the transmitted signal consists of pulses having different pulse widths depending on the data pattern. Thus, the effect of SPM depends on the data pattern. On the other hand, the transmitted RZ signal consists of a sequence of identical pulses corresponding to the data pattern; hence, the patterndependence of SPM-induced waveform distortion can be avoided [9-12]. In addition, the interaction between the SPM and dispersion in the anomalous dispersion regime can be exploited effectively in the case of the RZ signal, resulting in uniform pulse compression.

2. System description: To generate the optical signals we have used a CW laser source, Mach-Zehnder modulators, NRZ pulse pattern generator and a sinusoidal electrical signal generator. Furthermore, we have used a duobinary precoder for the duobinary and MDRZ signals in order to avoid recursive decoding in the receiver. Figure 1 shows a system transmitting a Duobinary signal at 40 Gb/s. The Duobinary was generated by first creating an NRZ doubinary signal using a precoder and a duobinary pulse generator. The generator drives the first MZM, and then concatenates this modulator with an second modulator that is driven by a sinusoidal electrical signal with the frequency of 40 GHz.The duobinary precoder used here was composed of an exclusive-or gate with a delayed feedback path.

Figure 1: System Set-up

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 1, January 2014, Pg: 84-88

3. Results and discussions The optical signal at the output of the second MZ modulator is shown in Figure 2. The CSRZ signal is generated in a similar way to the RZ format. However, the frequency of the sinusoidal electrical signal applied in the second MZM has half of the bit rate, 20 GHz. The second MZM was biased in a way to provide alternating optical phases between 0 to for the neighboring time slots.

(a)

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 1, January 2014, Pg: 84-88

Figure 2 Duobinary signal (a) Time domain, and (b) spectra Most installed fibers are standard single mode fibers (SMF) with high group velocity dispersion values () at 1.55 µm. To achieve a good level of bit-error-rate (BER) as well as to enable larger repeater spacing and larger signal-to-noise ratio (SNR) in this type of fiber, it is very important to consider the influence of group velocity dispersion, nonlinear effects, PMD, and their interplay on the transmitted signals. Increasing the capacity of optical systems may require either an increase in the bit rate, usage of WDM or ultimately both. At high bit rates, the modulation format, type of dispersion compensation scheme, and channel power become important issues for optimum system design. In particular, it has been demonstrated numerically and experimentally that the conventional nonreturn-to-zero (NRZ) modulation format is superior compared to the return-to-zero (RZ) modulation when dealing with large WDM systems, as RZ modulation causes a significant Eye Closure Penalty near end channels. The results obtained in this tutorial will be used to compare the Eye Closure Penalties for both NRZ and RZ cases, as well as the effects of nonlinearities.

4. Conclusions It is clearly seen that the RZ signal format provides many advantages over the NRZ signal in terms of tolerance to interchannel and intrachannel impairments. However, the system performance can be further improved by imposing some special characteristics onto the RZ signal. Each RZ optical modulation format has its own mechanism to combat the impairments caused by dispersion and nonlinearity in an optical fiber. Those mechanisms are spurious pulse suppression and pulse compression, which are obtained by manipulating the phase of the signal. However, the RZ pulse shape itself can also lead to performance improvement

5. References [1] Govind P. Agrawal, “Fiber-optic communication systems”, Third edition, 2002 [2] Govind P. Agrawal, “Nonlinear fiber optics”, Second edition, 1995 [3] Chidambaram Pavanasam, “Vestigial Side Band Demultiplexing for High Spectral Efficiency WDM systems”, Master thesis submitted to the Department of Electrical Engineering at the University of Kansas, 2004 [4] Takeshi Hoshida, Olga Vassilieva, Kaori Yamada, etc. “Optimal 40 Gb/s modulation formats for spectrally efficient long-haul DWDM systems”, Journal of lightwave technology, VOL. 20, NO. 12, December 2002 [5] Yukata Miyamoto, Akira Hirano, Kazushige Yonenaga, etc. “320 Gbit/s (8 x 40Gbit/s) WDM transmission over 367-km zero-dispersion-flattened line with 120-km repeater spacing using carrier-suppressed return-to-zero pulse format”, OSA TOPS Vol. 30 Optical Amplifiers and their applications, Jeff C. Livas, Gerlas Van den Hoven and Susumu Kinoshita (eds.) ©1999 Optical Society of America [6] Ron Hui, Sen Zhang, etc. “Advanced Optical Modulation Formats and Their Comparison in Fiber-Optic Systems”, A Technical Report to Sprint, by Lightwave Communication Systems Laboratory, The University of Kansas, 2004

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[7] Sen Zhang, Ron Hui, “Impact of optical modulation formats on SPM-induced limitation in dispersion-managed optical systems – A simplified modeling”, 2004 Workshop on advanced modulation formats, IEEE/LEOS, Paper FA3, San Francisco, 2004 [8] O.Vassilieva, T.Hoshida, S. Choudhary, etc. “Numerical comparison of NRZ, CS-RZ and IM-DPSK formats in 43Gbit/s WDM transmission”, Proc. LEOS 14th Annual Meeting, paper ThC2, pp. 673-674, 2001. [9] Keang-Po Ho, “Impact of nonlinear phase noise to DPSK signals: A comparison of different models”, IEEE photonics technology letters, VOL. 16, NO. 5, pp. 1403-1405, May 2004 [10] Ron Hui, Sen Zhang, Ashvini Ganesh, Chris Allen and Ken Demarest, “40Gb/s Optical Transmission System Testbed”, Technical report, ITTC, the University of Kansas, January 2004 [11] R. Billington, “A Report of Four-Wave Mixing in Optical Fibre and its Metrological Applications”, NPL Report COEM 24, ISSN COEM 1369-6807, National physical laboratory, Queens Road, Teddington, Middlesex, TW11 0LW, January 1999 [12] J. P. Gordon, L. F. Mollenauer, “Phase noise in photonic communications systems using linear amplifiers”, Optics letters, Vol. 15, No. 23, pp. 1351-1353, December, 1990

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