Performance evaluation of a DPSK/SCM combined modulation scheme for optical label switching

Transcription

1 Performance evaluation of a DPSK/SCM combined modulation cheme for optical label witching C. Díaz Giménez, I. Tafur Monroy, J.J. Vega Olmo, S. Sale, A.M.J. Koonen COBRA Reearch Intitute, Faculty of Electrical Engineering, Eindhoven Univerity of Technology, Den Dolech, PO Box 513, 5600 MB, Eindhoven, the Netherland. c.diaz.gimenez@gmail.com A combined modulation cheme for optical label witching uing DPSK/In-band- SCM ha been tudied. We preent imulation reult for the performance of a ytem with payload data in DPSK modulation at 10 Gb/ and the label modulated on a ubcarrier frequency SCM at bitrate of 155 Mb/. Higher bit rate for the label ignal have alo been conidered and it impact on the payload ignal detection ha been aeed. Moreover, by mean of imulation and mathematical development we how that the frequency for the inband ubcarrier that minimize the crotalk between the label and the payload i a frequency equal to half the bit rate of the DPSK ignal. Furthermore, an optimum value for the modulation index of the ubcarrier data label taking into account the induced power penalty over the payload data ignal i given. The imulation reult how that the propoed cheme i uitable for an OLS network becaue of it robutne againt non-linear impairment in the tranmiion link and the implicity of the independent detection of label and payload. 1. Introduction Several combined modulation cheme have been already tudied a technique to label optical ignal in Optical Label Switching (OLS) network [1]. However, another technique ha been recently propoed which combine angle modulation and ubcarrier multiplexing. Thi technique, DPSK/SCM, conit in encoding the payload data in DPSK modulation format (tranmitting the high bit rate payload data) and tranporting the label information by uing SCM technique. The feaibility of conveying the payload in DPSK and the label modulated with SCM technique in the ame band of frequency i due to the fact that a phae modulated ignal keep contant amplitude envelope and the information i carried in the phae change. On the other hand, an amplitude modulated ignal keep contant phae and ha change of amplitude where the information i conveyed. For thee reaon, both phae and amplitude of a ingle optical carrier can be ued to convey information. Uing a combined SCM/DPSK modulation cheme offer a few advantageou feature [, 3]: 1 The ubcarrier frequency may be choen to be in the ame frequency

2 band than the payload data, which i appropriate for pectral efficiency while keeping the complexity of the RF ignal generation/detection low. Uing the SCM method, the label and the payload travel imultaneouly through the network, and there i no need to maintain tight ynchronization at any node, achieving implified network control []. 3 SCM labelling method i relatively efficient in term of wavelength channel utilization. The SCM label do not occupy any additional time lot or wavelength channel [], and hence, it i only neceary that the label fit within the boundarie of the packet [5]. The ubcarrier ignal may be detected by imple photodetection and it i not affected by the DPSK information a the photodiode doe not react to the optical ignal phae []. 5 Robutne againt chromatic diperion, polarization mode diperion (PMD), and cro-phae modulation (XPM) effect offered by the ue of DPSK modulation format [6, 7]. 6 High capacity and link reach optical tranmiion by uing DPSK modulation format. 7 DPSK offer better receiver enibility than other modulation format [6]. 8 It i a labelling technique for a tranparent protocol, compatible with legacy and emerging network technologie by adopting the GMPLS frame work for a unified control plane. However, there are alo ome iue requiring pecial attention. The main drawback of the SCM technique i the RF fading effect coming from the interaction between the RF ubcarrier and the chromatic diperion in optical fiber [9], but the optical-frequency-domain filtering method eliminate the fading effect [8]. Moreover non-linearitie may caue intermodulation ditortion, cauing interference into other channel [1]. In thi paper we preent the imulation reult of a DPSK/In-band-SCM combined modulation. The paper i organized a follow. In ection the etup cheme i preented. In ection number 3 we how ome reult of the performance analyi relate to the SC modulation index, the SC label frequency and it effect in the payload, and the bit rate label data. Finally, in ection we how the concluion of the tudy.. Setup The performance analyi of the combined modulation cheme ha been carried out through imulation in a commercially available photonic numerical imulator (VPI Tranmiion Maker). Baically, the way of combining the two ignal (payload in DPSK and label in SCM) i hown in the next cheme:

3 Figure 1. Scheme of tranmitter and receiver of DPSK/SCM combined ignal The payload data i differentially encoded by mean of an XOR operation and modulated by a phae modulator. It modulate the phae of the light from the laer depending on the incoming electrical ignal, with a phae deviation of 180 degree. The label data i modulated on a ubcarrier and it i inerted into the payload ignal by mean of an electroabortion modulator (EAM). The EAM modulate the amplitude of the optical ignal with the electrical modulation ignal: the label. The receiver for the label ignal conit of a ingle photodetector and a mixer to downconverter the label to baeband. In the cae of the DPSK payload ignal, a Mach-Zehnder Interferometer with a delay of one bit in one of it arm i needed and two photodetector to get balanced detection. The bit rate payload data ued ha been 10 Gb/ and the label ha been convoyed with a bit rate of 155 Mb/. Anyway, higher bit rate for the label have been alo tudied and their effect on the payload detection. 3. Performance Analyi The preence of the SCM ignal affect to the DPSK one. The effect that will be tudied in the next ection are the dependence of the DPSK performance on the modulation index of the EAM in the payload detection, the dependence on the value of the ubcarrier frequency and the effect of different data rate of the label ignal DPSK/SCM performance dependence on the SCM modulation index The modulation index define the amplitude difference between a tranmitted one and a zero power level. It decribe how much lower in power a tranmitted zero mut be repect to the maximum value of the incoming light. A DPSK ignal ha contant amplitude but if thi ignal i modulated again in amplitude, thi contant value i lot in the output of the EA-modulator. The bit error rate curve meaured in both receiver (payload and label) are hown in figure. The reult indicate that the BER of the DPSK ignal i wore when the modulation index i higher, wherea the BER of the label i better.

4 (a) (b) Figure. (a)ber curve for the DPSK receiver after balanced detection for different index modulation value. The SC frequency value ued for the label i 5 GHz. (b) BER curve for the label receiver for different value of the modulation index. The SC frequency value i 5 GHz. Figure 3 how the performance for the payload and label detection depending on the modulation index. The receiver enitivity penalty ha been repreented for payload and label repect to the DPSK back-to-back ytem without any ubcarrier (without label) and the SCM tranmiion without payload and with a modulation index of 0.5 repectively. Figure 3.Receiver enitivity penalty curve for payload and label detection depending on the modualtion index. SC frequency: 5 GHz Due to the fact that the payload and label behaviour when the modulation index increae i oppoite, a compromie mut be found to chooe the mot uitable index in the EA-modulator. Figure 3 how a value of 0.38 yield the ame power penalty for both ignal. However, an index modulation of 0.3 ha been ued in the main imulation carried out for the analyi. With thi value the penalty in the detection of the label i higher than in the payload but on the other hand, the detection of the label doe not depend on the ubcarrier frequency. Thi mean that there will be no extra penalty if other value of frequency are choen for the ubcarrier. Later in thi ection, it will be hown that the performance of payload receiver, however, depend on the value of the ubcarrier frequency.

5 3.. DPSK performance dependence on the ubcarrier frequency One of the aim of thi tudy ha been to find the mot uitable ubcarrier frequency value, upporting in-band labelling, with a DPSK ignal. Operating the payload data to 10 Gb/, ubcarrier frequencie of 1 to 10 GHz were conidered. The power penalty repect to the DPSK Back-to-Back ytem can be appreciated in the figure. Thi penalty i the power difference between the received power in every cae to get a BER of 1e-9 and the received power to get the ame BER when only the DPSK ignal i tranmitted without fiber, attenuation or degrading effect. Power Penalty (db) 5,5 3,5 3,5 1,5 1 0, Subcarrier frequency (GHz) Figure. DPSK Balanced receiver enitivity power penalty v. SC frequency. Mod. Index=0.3 Two ubcarrier frequency value how low power penaltie for the DPSK ignal: 5 GHz and 10 GHz. In general, when the frequency of the ubcarrier i higher the interaction between label and payload i maller [10]. Thi trend i due to the DPSK payload pectral ditribution of power. For thi reaon, a a ubcarrier frequency of 10 GHz overlap with a notch of the DPSK pectrum, thi i a good frequency. Higher frequencie are already out of band. Regarding the other frequency value, although at 5 GHz there i a high pectral component of the DPSK payload, it reult in al low crotalk for the DPSK ignal. We preent below a mathematical derivation demontrating the reult above Subcarrier frequency equal to half bit rate of the DPSK payload. Mathematical demontration. The expreion for the electric field at each output port of the MZI in the DPSK demodulator i given by: 1 Eout1 ( t) = ( Ein( t τ) Ein( t) ) Detructive Port j Eout ( t) = ( Ein( t τ) + Ein( t) ) Contructive Port where τ i the delay applied in the MZI and it correpond to one bit of the label. The incoming ignal i the output of the EA-modulator where both ignal (DPSK payload and SCM label) are combined. The DPSK ignal ha contant amplitude and change in the phae, wherea the SCM ignal ha change in the amplitude but contant phae. For thi reaon, the incoming ignal into the DPSK demodulator can be written a:

6 where: E in( t) j φk ( 1+ m label( t) in( ω t ) e = ) m: index modulation label(t): binary ignal with the label information ω : π (ubcarrier frequency) j k e φ : phae of the DPSK ignal The bit rate ued for the label ha been: BitRateLabel(SCM) = BitRatePayload( DPSK) 6 So label (t) = contant by a period of 6 bit of payload. In thi time, the incoming ignal E in (t) can be repreented a: E in( t) j φk ( 1+ m in( ω t ) e = ) Analyzing the reultant photo-current, reulting from detection at each output port of the MZI, we have the following expreion: Detructive Port: I ( t) = R P ( t) =R E ( ) I ( t) E ( ) out1 out1 out1t out1 out1t E out1 ( t) = = 1 1 j φk 1 j φk [ ( 1+ m in( ω ( t τ) )) e ( 1+ m in( ω t) ) e ] j φ k 1 j φk [ P ( 1+ m in( ω ( t τ) )) e P ( 1+ m in( ω t) ) e ] = 0 0 = P 0 { + m in( ω ( t τ) ) + m in( ω t) 1+ m in ( ω ( t τ) ) 1+ m in( ω t) co( φ φ )} k 1 k By mean of Taylor erie approximation, thi current can be written a: Eout1( t) { + m ( 1 co( φk 1 φk) ) in( ω( t τ) ) + m 1 co( φk 1 φk) in ω t m in( ω( t τ) ) in( ω t) co( φk 1 φk) co( φk 1 φk) ( ) ( ) It i known that: in ( ω ( t τ) ) in( ω t) in( ω t)

7 For thi reaon, uing a low-pa filter after photodetection, thi term diappear and the reult i: Irx_ out1( t) ( 1 co( φk 1 φk) ) { + m in( ω( t τ) ) + m in( ω t) } Contructive Port Following a imilar proce than in the detructive port, the output current can be expreed like: Irx_ out( t) ( 1+ co( φk 1 φk) ) { + m in( ω( t τ) ) + m in( ω t) } Balanced Detection After balanced detection the photocurrent i given by; Ibalanc( t) co( φk 1 φk) { + m in( ω( t τ) ) + m in( ω t) } The objective i to find the frequency that made the interaction between payload and label a mall a poible. To minimize thi crotalk, it i needed that: ( ω t) = in( ω ( t τ) ) in So: 1 BitRateDPSK ω τ = π f = = τ Tranmitting at 10 Gb/, the ubcarrier frequency that minimize the interaction between label and payload i 5 GHz DPSK performance dependence on the label bit rate BER BER payload (Subcarrier = 3 GHz) 1,00E-0 1,00E-05 1,00E-06 Label: 156 Mb/ 1,00E-07 Label: 655 Mb/ 1,00E-08 Label: 1.5 Gb/ 1,00E-09 1,00E-10 1,00E-11 1,00E-1 1,00E-13 1,00E-1 1,00E Received Power (dbm) 1,00E-05 1,00E-06 1,00E-07 1,00E-08 1,00E-09 1,00E-10 1,00E-11 1,00E-1 1,00E-13 1,00E-1 BER payload (Subcarrier = 5 GHz) 1,00E Received Power (dbm) (a) (b) Figure 5. BER curve at the DPSK receiver for different bit rate label data, when the ubcarrier frequency where the label i convoyed i 3 GHz(a) and 5 GHz (b) BER Label: 156 Mb/ Label: 655 Mb/ Label: 1.5 Gb/ The curve of the bit error rate of the DPSK payload ignal are hown in figure 5. The value of the ubcarrier frequencie of the label are 3 GHz and 5 GHz, repectively. According to the imulation reult hown in thee graphic, for a BER of 1e-9, the difference in the receiver enitivity power penalty for the DPSK receiver for the lower and the higher label bit rate i about 1 db, when uing both value of the ubcarrier frequencie. We oberve that the value of

8 the ubcarrier frequency determine mainly the power penalty, while the increae in label bit-rate affect the DPSK ignal, relatively, in the ame manner.. Concluion In thi paper we report on a combined modulation cheme for optical label witching by uing DPSK/In-band-SCM. The propoed ytem ha been tudied by mean of imulation tool. Thi cheme offer a imple way of independent detection of label and payload. From our tudy we have found the mot uitable et of value for the SC modulation index and frequency yielding the le impairment on the payload detection. In particular, we how that an inband SC frequency value correponding to half bit rate of the payload minimize the SC crotalk on the DPSK ignal. Acknowledgement We would like to acknowledge the European Commiion by partially upporting thi work in the framework of the IST-STOLAS project and the E- photon/one network of excellence. Reference [1] I. Tafur Monroy, A. M. J Koonen, J. Zhang, Nan Chi, P. V. Holm-Nielen, C. Peucheret, J. J. Vega Olmo, G-D Khoe, Technique for Labeling of Optical Signal in Burt Switched Network in Firt International Workhop on Optical Burt Switching, WOBS'03, Dalla, Texa, U.S.A., 003. [] I. Tafur Monroy, J. J. Vega Olmo, A. M. J. Koonen, F. M. Huijken, H. de Waardt, and G-D. Khoe, Optical label witching by uing DPSK and in-band ub-carrier multiplexing modulation format, Opt. Eng. Vol. 3, nr. 7, 00. [3] J. J. Vega Olmo, I. Tafur Monroy, A. M. J. Koonen, In-band Time-to-Live ignalling ytem for combined DPSK/SCM cheme in OLS, IEEE Photonic Technol. Lett., vol. 16, no. 10, pp October 00. [] Zuqing Zhu, V. J. Hernandez, Min Yong Jeon, Jing Cao, Zhong Pan, and S. J. Ben Yoo, RF Photonic Signal Proceing in Subcarrier Multiplexed Optical-Label Switching Communication Sytem, J. Lightwave Technol., vol. 1, no. 1, pp , December 003 [5] Ezter Udvary, and Tibor Berceli, Optical Subcarrier Label Swapping by Semiconductor Optical Amplifier, J. Lightwave Technol., vol. 1, no. 1, pp , December 003 [6] Hoon Kim, and Alan H. Gnauck, Experimental invetigation of the Performance Limitation of DPSK ytem due to nonlinear Phae Noie, IEEE Photon. Technol. Lett., vol. 15, no., Feb.003 [7] M. Rohde, et al., Robutne of DPSK direct detection tranmiion format in tandard fibre WDM ytem, Electronic Letter, Vol. 36 Iue: 17, , 000 [8] H. J. Lee, V. Hernandez, V. K. Tui, and J. B. Yoo, Simple, polarization-independent, and diperion-inenitive SCM ignal extraction technique for optical witching ytem application, Electron. Lett., vol. 37, no. 0, pp , Sept [9] F. Devaux, Y. Sorel, and J. F. Kerdile, Simple meaurement of fiber diperion and of chirp parameter of intenity modulated light emitter, J. Lightwave Technol., vol. 11, pp , Dec [10] T. Flarup, C. Peucheret, J. J. Vega Olmo, Y. Geng, J. Zhang, I. Tafur Monroy, P. Jeppeen, Labeling of 0 Gbit/ DPSK Payload uing In-band Subcarrier Multiplexing, OFC 005, Anaheim, Lo Angele, OWB7 [11] Photonic and ignal proceing module manual. VPI Tranmiion-maker oftware v.6.0. VPI Photonic