Performance Improvement of All Optical WDM Systems on Binary Asymmetric Channel

Transcription

1 Performance Improvement of All Optical WDM Systems on Binary Asymmetric Channel H.S. Mruthyunjaya Department of Electronics & Communication Engineering, Manipal Institute of Technology, Manipal, India. Tel.: ; Fax.: ; Abstract- In a highly asymmetric channel it may be necessary to correct only the errors result from incorrect transmission. A Long haul incoherent optical multichannel communication systems with EDFAs employing N N Wavelength Division Multiplexing (WDM) in presence of Stimulated Raman Scattering (SRS) and other receiver noises, including channel beat noise, is analyzed. Error control coding techniques such as Block, RS, Golay and rate 1/ convolution codes are employed to counter system degradation due to these limiting factors. Without coding BER of 1-9 can be achieved at the maximum of 7 channels while, with coding, it can be achieved beyond 1 channels. In other words the power penalty for 1 channels reduces from.6db to.778db with coding in presence of both SRS and channel beat noise. Index Terms- Binary Asymmetric Channel (BAC), Wavelength Division Multiplexing (WDM), Stimulated Raman Scattering (SRS), Bit Error Rate (BER), Error control coding, channel crosstalk. I. INTRODUCTION Single mode optical fiber with its enormous bandwidth provides attractive option for transferring digital data at high bit rate to longer distances and to exploit this bandwidth fully, WDM schemes are accommodated. Among all the nonlinearities, the SRS effect causes the spectral gain to be wavelength dependent with longer wavelengths having higher gain [1]-[]. When signal power is more than mw per channel, the nonlinear fiber interaction will cause significant SRS and results in severe signal-to-noise ratio (SNR) differential among many WDM channels after propagating long distances [4] [6]. The crosstalk variance of SRS in WDM system is found to be Gaussian [7]. Crosstalk in optical network systems can be classified as either heterodyne or homodyne. Homodyne crosstalk can be further subdivided into coherent crosstalk, between phase correlated signals, and incoherent crosstalk, between signals which are not phase correlated. Several studies have been reported in the literature, addressing different crosstalk contributions in optical networks [8]-[1]. Studies in [8] show that incoherent crosstalk may cause fluctuation of signal power because it can be a coherent combination of crosstalk contributions. The results in [11] revealed that when the crosstalk power exceeds a critical value, the error rate due to this can dominate all other affects inducing errors, and lead to poorer BER. The crosstalk must be less than -4dB for system to keep the power penalty below 1dB at BER of 1-9 [1]. Along with the nonlinear affect SRS, we have considered system with receiver shot and thermal noises. In this work we propose to introduce redundancy in the transmitted data bits by employing error control coding []-[] techniques and show considerable improvement in the system performance. II. SYSTEM CONSIDERATION In most of the electrical communication systems, the probabilities of the crossover

2 1 from bit 1 (ON state) to (OFF state) and vice-versa are the same. This is because the variance of noise effecting bit and bit 1 are the same. In other words the noise is stationary. Such systems are modeled as Binary Symmetric Channel (BSC) and error correcting codes for BSCs have been studied extensively []. In optically amplified lightwave communication systems the shot, channel beat and Amplified Spontaneous Emission (ASE) noises affects bit 1 more severely compared to bit. This results in variance of noise affecting bit 1 to be more compared to variance of noise affecting bit. Consequently, even after appropriately selecting the detection threshold, the probability of error in bit 1 (P 1 ) and probability of error in bit (P ) does not turns out to be the same generally. Under such circumstances the channel modeled more suitably as that of a Binary Asymmetric Channel (BAC) as shown in Fig.1. It is worth mentioning here that this is closely Z channel wherein either P or P 1 is zero. In such cases, conventional error correcting coding technologies may result in under utilization of the codes capacity.

3 Consider an intensity-modulation and direct detection (IMDD) N channel equally spaced WDM system operating in 1.5µm window. Let bits s and 1 s to be equally likely. Let all channels fall within the triangular Raman gain profile of coefficient g = 7x1-14 m/w with its slope of dg/df = 4.67x1-5 m/w/hz. We have taken thermal noise current of 1nA [] and shot noise current of 1nA [] in our calculations. The single mode fiber is assumed to have loss coefficient of.db/km with effective link length of 1.7km [], core effective area of 5µm []. The input power per channel P is taken as 1mW. As shown in Fig., the minimum power required by a PIN receiver is.6µw for BER to be 1-9 for 16 х 16 WDM systems in the absence of noise. In presence of SRS, the receiver needs around.75µw and obviously the SRS dominates the contribution to degradation of system performance. As shown in Fig., in order to have power penalty due to SRS less than 1dB with spacing df = GHz, the maximum value of N is around 1. Interchannel crosstalk arises when an interfering signal comes from a neighboring channel that operates at a different wavelength and can be possibly removed by the demultiplexer at the receiver end. The intrachannel crosstalk is more severe as it falls completely within the receiver bandwidth. Such severe degradation is due to beat noise caused by the interference of crosstalk light. The studies on beat noise have already been reported in [14], [16]. As reported in [17], the impact of same wavelength crosstalk as small as - db imposes a db power penalty. As shown in Fig.4, for power penalty due to beat noise to be less than 1dB, the multiplexer crosstalk must be less than -, -9 and -4dB for number of channels of 16, 64 and 18 respectively. III. ERROR CORRECTING CODES The performance of single-mode fiber optic systems has been mainly determined by BER impairments. Usually practical modulation schemes puts limit on the value of SNR and don t provide acceptable data quality. The acceptable option for maintaining data quality for particular SNR is error correcting coding. In this paper we show that the dispersion limited systems employed with a single- and multiple- error correcting codes can be more effective. Studies have been conducted in [18]-[1] and demonstrated in [] that the error correction can produce BER reductions equivalent to an 18 db increase in optical source power. The error detection and correction codes uses redundant or parity bits which are added to the data bits. In block codes the source data are segmented into blocks of k data bits and each block represents one of k distinct messages [], []. The encoder transforms each k bit data into larger block of n bits, referred as (n, k) code word with (n-k) bits as redundant or parity bits. Here (k/n) is the code rate. The error detecting and correcting capabilities of a code word is determined using minimum distance between the code words []. The addition of redundant bits influences on faster rate of transmission, which means more bandwidth. For the transition probabilities shown in Fig. 1 for the channel modeled as BAC, for two bit message signal, the error probability can be expressed as PA + PB Pe = (1) 4 where P A = ( 1 P ) + (1 ) + P + P B = P (1 P ) + P (1 P ) + P (1 P ) + P (1 P ) + (1 P )( 1 P + P P [ ] ) The probability of error for a three bit sequence is derived. In a similar fashion the error probability for a Hamming code is derived and plotted. 1

4 The BER with ½ rate convolution coding is derived with the help of Viterbi decoding algorithm [] as ' ' Pa + Pa + 1Pb + Pb + 5Pc + 5Pc Pe = () where P a = 1P + P P 4 ) + 5P ) ) + 6P )(1 ) + ) + P ) P P b = (1 + P (1 + ' P 5 (1 P ) P c = 6 P (1 ) + 4 (1 4 + ) + 4 (1 ) P + P )(1 P P P ) P a = P a with P replaced by P 1 P b = P b with P and P 1 are interchanged. In a similar fashion the error probability for other codes are derived and plotted. IV. RESULTS AND DISCUSSIONS Figures 5 to 8 shows improvement in system performance after employing above said error correcting coding techniques. We have used a bit rate of 1G bits/sec, thermal noise current of 1nA, [] shot noise current of 1nA, [] channel spacing of GHz for uncoded system Fig.5 shows SNR performance and comparison is made between all the above said coding techniques. As shown in Fig.6, the power penalty due to channel beat noise reduces for a particular value of multiplexer cross talk value. At -5 db multiplexer cross talk, the BCH codes [] reduces the power penalty from 5.5dB (without coding) to.4db (with BCH) at BER of 1-9 and N = 64 as can be seen from Fig. 6. As shown in Fig. 7, at BER of 1-9 with fiber link length of 1km, Hamming coding cause increase in channel handling capacity from 4 to 117 while it is from 4 to 14 with BCH coding. As shown in Fig. 8, with say 1 channels employed, the power available at km fiber length increases effectively by 7.5dB with BCH code at BER of 1-9. These performance enhancements are obtained at the cost of increase in system complexity and bandwidth expansion. V. CONCLUSIONS In this paper we have shown the enhancement of system performance by

5 4 parameters. Without coding BER of 1-9 can be achieved at the maximum of 7 channels while with coding it can be achieved beyond 1 channels at 1 km fiber length keeping the same channel spacing. In other words the power penalty for 1 channels reduces from.6db to.778db with coding in presence of both SRS and channel beat noise. REFERENCES applying various types of error correcting codes to a WDM system. As shown in Figures 5 to 8, the BCH code is expected to give the best result when compared to the other codes. Fig.7 shows performance enhancement after employing error correcting codes in presence of both SRS and channel beat noise with above said [1]. A.R.Chraplyvy, A.R. Optical power limits in multichannel wavelength division multiplexed system due to stimulated Raman scattering, IEEE Electron. Lett., 5, pp , []. Chraplyvy, A.R. and R.W.Tkach, What is the actual capacity of single mode fibers in amplified lightwave systems, IEEE Photon. Technol. Lett., 5, pp , 199. []. A.R.Chraplyvy, A.R. and P.S.Henry, Performance degradation due to stimulated Raman scattering in wavelength division multiplexed optical fiber systems, Electron. Lett., 19, pp , 198. [4]. Zou, X.Y. et al., Compensation of Raman scattering and EDFA s nonlinear gain in ultra long distance WDM links, IEEE Photon. Technol. Lett., 8, pp , [5]. Hwang, S.M. and A.E.Willner, Guidelines for optimizing system performance for WDM channels propagating through a cascade of EDFA s, IEEE Photon. Technol. Lett., 5, pp , 199. [6]. Willner, A.E. and S.M.Hwang, Transmission of many WDM channels through a cascade of EDFA s in long distance links and ring networks, J. Lightwave Technol., Special issue on optical Amplifiers, 1, pp. 8-81, [7]. Ho, K.P. Statistical properties of stimulated Raman crosstalk in WDM systems, J. Lightwave Technol., 18, pp ,. [8]. Yenfeng Shen, Kejie Lu, and Wanyi Gu, Coherent and Incoherent crosstalk in WDM optical networks, J. Lightwave Technol., 17, pp , [9]. Fabrizio Forghieri, R.W. Tkach and A.R.Chraplyvy, Effect of modulation statistics on Raman crosstalk in WDM systems, J. Lightwave Technol., 7, pp.11-19, [1]. Li, C.S. and F.Tong, Crosstalk and interference penalty in all optical networks using static wavelength routers, J. Lightwave Technol., 14, pp , [11]. Blumenthal, D.J., P. Granestrand, and L. Thylen, BER floors due to heterodyne coherent crosstalk in space photonic switches for WDM networks. IEEE Photon. Technol. Lett., 8, pp , [1]. Dods, D., J. P. R. Lacey, and R. S. Tucker, Homodyne crosstalk in WDM ring and bus

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