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ICOP-2009: INTERNATIONAL CONFERENCE ON OPTICS AND PHOTONICS CHANDIGARH, INDIA, 30TH OCT – 1ST NOV.2009 ALL-OPTICAL FO-CDMA NETWORK: PERFORMANCE ANALYSIS E. S. Shivaleela S. Ganesh and T. Srinivas Indian Institute of Science, Bangalore, India-560012. [email protected], [email protected] , [email protected] Abstract: Optical CDMA technology is being explored as a multiplexing technology for high speed access networks, with optical fibre as transmission medium and also processing ie. the spreading/dispreading of the CDMA sequences. For fibre-optic code division multiple access (FO-CDMA) scheme to be a practically implement able one as a high speed access network, two main challenges are, design of (1) codes with high spectral efficiency and cardinality and (2) high speed encoders/decoders which are simple and efficient to implement. As a solution to the first requirement, we have formulated wavelength/time singlepulse-per-row (W/T SPR) codes which are energy efficient and of weight/row WP = 1, for a given weight of the code W, in addition have good 1) cardinality 2) spectral efficiency and minimal 3) correlation values, for given wavelength and time dimensions. In this paper, we have simulated W/T SPR network and also Optical orthogonal codes (OOC) network using a commercially available optical simulation package. Keywords: Fibre optics communication, optical code division multiple access, incoherent optical communications, wavelength/time codes I. INTRODUCTION Spread sequences1 used in FO-CDMA use ultra short pulses which are prone for nonlinear effects in optical fibres. To overcome this problem of one dimensional codes in FOCDMA, several types of encoding of the CDMA sequences such as frequency-hopping (FH), time-space (T/S) and W/T have been proposed. FH codes2 are not suitable for FOCDMA3, as systems using Frequency-Encoding CDMA technique suffer from optical beat noise that appears between the frequency slices at the photo detector 4, 5. Simulation results show that T/S systems are limited by the skew in associated ribbon fibers6, 7. W/T coding of the CDMA sequence is suitable and several W/T codes have been proposed, either by crossing one type of sequence with another such as prime-hop8, and extended quadratic congruence/prime9 (eqc/prime) or converting 1-D sequences to 2-D sequences. Mendez et al. 10 have described the design of pseudoorthogonal codes based on folding of optimum Golomb rulers and these codes have arbitrary cardinality. Yang and Kwong 11 have constructed 2-D Optical Orthogonal signature pattern codes (OOSPC) from 1-D OOCs. The cardinality and correlation properties of OOSPCs are same as that of 1-D OOCs. Raymond12 et al. have described a depth-first search algorithm for the generation of W/T 2-D codes, which have unit out-of-phase autocorrelation and peak cross correlation. We reported earlier the design of Addition Modulo L_T (AML) codes13, where L is the temporal length. AML codes are single-pulse-per-row (SPR) codes with and unit peak cross correlation values. As T/S systems are limited by the skew in associated ribbon fibres, AML codes can be encoded in W/T dimensions instead of T/S, where wavelength encoding is done in place of space. AML codes encoded in W/T are called W/T SPR codes. We have also reported the basic principles of W/T MPR. We report in this paper, the simulation of OOC and also W/T SPR networks for 4 users. In Figure 1 the schematic diagram of the all-optical CDMA network with optical encoding of W/T SPR codes is shown. Wavelengths derived from a laser source are given to an external modulator. A data source (PRBS) modulates a NRZ pulse generator, which in turn is used as the control signal in external modulator, Mach-Zhender interferometer and thus the amplitude modulated signal is obtained. Optical encoding of the OOCs and W/T SPR codes is done using optical delay lines. The AM signals from all the users in the broadcast network get combined in the passive star.. On the receiver side after demultiplexing the wavelengths, corresponding inverted (optical) delay lines (which functions as a matched filter) are used to recover the desired signal at a receiver. II. SIMULATION We verify the autocorrelation and cross correlation properties of OOCs and W/T SPR codes by simulation using optisys commercial software. The four OOC codes are shown below Table 1 which is a representation of the actual codes. The simulation has been run for number of users ranging from 1 to 4. The sequence length has been taken as 16 and 32 for all the OOC codes. The weight has been chosen as 2, temporal length = 16(case 1) or 32 (case 2) for all the users. Table 1: The number in bracket indicates the position of optical pulse in corresponding chips in length of 16 and 32. (1,2) (1,5) (1,8) (1,11) Similarly, the code description for 2 D codes is explained below. The two W/T SPR codes, in the Table 2 are ICOP-2009: INTERNATIONAL CONFERENCE ON OPTICS AND PHOTONICS CHANDIGARH, INDIA, 30TH OCT – 1ST NOV.2009 chosen. Each row of a code is encoded by distinct 2; the number of coincidences between the pulses is greatly wavelengths, which is equal to R and the columns represent reduced. So by increasing the temporal length, the Bit Error the time, L. The 1s in code (1} and code (2}, in the Table 2 Rate (BER), reduces. indicate the presence of optical pulses. Compare Figure 2.B and D, for number of users equal to four and code weight equal to two, temporal length is Table 2: W/T SPR codes 1 and 2 with R = 2, L = 8. increased from 16 to 32, and similar observations are made. Compare Figure 2.C and D, for temporal length is λ1 1 0 0 0 0 0 0 0 λ1 1 0 0 0 0 0 0 0 equal to 32 equal and code weight equal to two, when number λ2 1 0 0 0 0 0 0 0 λ2 0 1 0 0 0 0 0 0 of users are increased from 2 to 4, the received signal has In Figure 1 schematic diagram of All Optical (AO) more noise due to the increased number of interfering users. CDMA network, for W/T SPR codes is shown. Two So for the given number of users, by increasing the temporal transmitters, of different wavelengths of 1 mw power, length, the Multiple Access Interference (MAI) can be operating at 1 Giga bits per second, for different rows are used, reduced, for better performance. In Figure 3.A and B the auto whereas for OOC, it is one transmitter of wavelength 1550 nm and cross correlation plots are obtained respectively, for W/T SPR codes. Further a detailed study is under progress on and 1550.1 nm. The optical pulses from two transmitters are passed similar lines as that of OOCs codes and to compare the through different fibre delay lines, depending on the pulse performances of 1D and 2D codes. sequence to be produced. Then the sequences from two rows are multiplexed and sent over the channel. The signals after passing through the broad cast channel are demultiplexed and then fed to matched filters (inverted delay lines of the encoder), to obtain the required signal (despread signal). The unmatched signal will appear as noise (spread signal). The received signal from matched filter is photo detected to get the data in electrical signal. The electrical signal is analysed in Eye Diagram Analyzer. ( A ) L = 16 , N = 2 Figure 1: Schematic diagram of the all-optical CDMA network with W/T SPR code. In the plots shown in Figure 2, we have compared OOCs for different parameter variations. In Figure 2. (A) shows the received signal for two users, code length of 16 and weight equal to 2. In Figure 2.B, shows the received signal for four users, code length of 16 and weight equal to 2. In Figure 2.C, shows the received signal for two users, code length of 32 and weight equal to 2. In Figure 2.D, shows the received signal for four users, code length of 32 and weight equal to 2. Compare Figure 2.A and C, we see that when temporal length is increased from 16 to 32 for the same number of users equal to ( C ) L = 32 , N = 2 Figure 2. (A) (B) (C) (D): ( B ) L = 16 , N = 4 ( D ) L = 32 , N = 4 W=2 for OOCs codes. ( A ) Auto Correlation ( B ) Cross correlation Figure 3: L = 8 , W =2 for W/ T SPR codes. ICOP-2009: INTERNATIONAL CONFERENCE ON OPTICS AND PHOTONICS CHANDIGARH, INDIA, 30TH OCT – 1ST NOV.2009 8. L. Tancevski and I. Andonovic, "Wavelength Hopping/Time Spreading code division multiple access III. SUMMARY systems," One of the challenges of FO-CDMA to be practically Electron. Lett., vol.30, No.17, pp.1388-1390, Aug 1994. successful in high-speed access networks is the availability of codes with good cardinality and spectral efficiency. W/T SPR 9. L. Tancevski and I. Andonovic, "Hybrid Wavelength is one such family of codes suitable for FO-CDMA networks. Hopping/Time Spreading Schemes for Use in Massive Optical Networks with Increased Security", IEEE/OSA J. Lightwave We have analysed the Optical CDMA network for Technol., vol.14, No.12, pp.2636-2647, Dec 1996. OOCs codes in detail and verified the autocorrelation and cross correlation properties of W/T SPR codes, by simulation using optisys, and shown to be minimal for unipolar codes, 10. A. J. Mendez, R M. Gagliardi, V. J. Hernandez, C. V. which is desired to keep multiple access interference low in Bennett, and W. J. Lennon, "Design and Performance broadcast networks. As a result of which, better performance Analysis of Wavelength/Time (W/T) Matrix Codes for is obtained in broadcast networks, wherein MAI is the main Optical CDMA", IEEE/OSA J. Lightwave Technol., Special Issue on Optical Networks, vol.21, No.11, pp.2524-2533, Nov cause for errors. 2003. REFERENCES 1. J. A. Salehi, "Code division multiple access techniques in optical fiber networks Part I:Fundamental Principles," IEEE Trans. Commun., vol.37, No.8, pp.824-833, Aug 1989. 2. L. Bin, "One-coincidence sequences with specified distance between adjacent symbols of frequency-hopping multiple access", IEEE Trans. Commun., vo1.45, No.4, pp.408-410, April 1997. 3. H. Fathallah. L. A. Rusch, and S. LaRochelle, "Passive Optical Fast Frequency-hop CDMA Communications Systems", IEEE/OSA J. Lightwave Technol., vol.17, No.3, pp.397-405, Mar 1999. 4. E.D.J. Smith, P.T. Gough and D.P. Taylor, "Noise limits of optical spectral-encoding CDMA systems," Electron. Lett., vol.31, No.17, pp.1469-1470, Aug 17, 1995. 5. E.D.J. Smith, R.J. Blaikie and D.P. Taylor, "Performance enhancement of spectral-amplitude-encoding optical CDMA using pulse-position modulation," IEEE Trans. Commun., vol.46, No.9, pp.1176-1185, Sept 1998. 6. P. K. Pepeljugoski, B. K. Whitlock, D. M. Kuchta, J. D. Crow, and Sung-Mo KIng, "Modeling and simulation of the OETC optical bus", Conf Proc. IEEE/LEOS '95 Meeting, vol.1, pp.185-186 (1995). 7. B. K. Whitlock, P. K. Pepeljugoski, D. M. Kuchta, J. D. Crow, and S.-M. KIng, "Computer modeling and simulation of the Optoelectronic Technology Consortium (OETC) optical bus", IEEE J. Select. Areas in Commun., vol.15, No.4, pp.717-730, May 1997. 11. G. C. Yang and W. C. K wong, "Two-dimensional spatial signature patterns", IEEE Trans. Commun., vol.44, No.2, pp.184-191, Feb 1996. 12. R. M. H. Yim, L. R. Chen, and J. Bajcsy, "Design and Performance of 2D Codes for Wavelength-Time Optical CDMA", IEEE Photon. Tech. Lett., vol.14, No.5, pp.714-716, May 2002. 13. E.S. Shivaleela, Kumar N Sivarajan, and A. Selvarajan, "Design of a new family of two-dimensional codes for fiberoptic CDMA networks, " J. Lightwave Technol., vol.16, No.4, pp.501-508, April 1998. 14. E.S. Shivaleela, A. Selvarajan and T. Srinivas, "Twodimensional Optical Orthogonal codes for Fiber-Optic CDMA networks, " J. Lightwave Technol., vol.23, No.2, pp.647-654, Feb 2005.