ENCODING TECHNIQUES IN OPTICAL CDMA Jaswinder Singh

Transcription

1 Review Article ENCODING TECHNIQUES IN OPTICAL CDMA Jaswinder Singh Address for Correspondence Dept. of Electronics & Communication Engineering Beant College of Engineering & Technology, Gurdaspur, India E Mail: j_singh73@rediffmail.com ABSTRACT The demand for higher data rates along with variable QOS and variable bandwidths has led to the remarkable advancements in the communication technology. To meet the above-listed objectives, whereas the TDMA and WDMA require complex protocols and extensive hardware, the CDMA systems are much simpler and very easily support the above-mentioned services. Since optical CDMA systems assigns unique codes to different users, encoding/decoding becomes a very important stage which mainly determines the performance of optical CDMA systems. This paper reviews the various encoding techniques reported in the literature for optical CDMA. 1. INTRODUCTION Optical CDMA is a very promising candidate from multiple access systems. The users are assigned unique codes which act as addresses. The codes are unique in the sense that the matched receiver with a matched code word will receiver the transmissions correctly whereas the receiver with unmatched code will not be able to listen to the transmission. Hence optical CDMA becomes a suitable multiple access technology where only the desired receiver will be able to listen to the transmission. The codes used as addresses are obtained from some code family designed to satisfy some basic properties which are Off-peak autocorrelation, cross-correlation, code weight and code length so as to yield the desired system performance at the given data rates. 2. TECHNIQUES FOR ENCODING Ever since its introduction by Davies and Shaar [1] in 1983, many approaches for introduction of fiberoptic CDMA (FO-CDMA) have been considered. According to Yim [2], FO-CDMA has been classified into the following categories where direct or differential detection can be used. Incoherent amplitude encoding Spectral encoding Coherent phase encoding Spatial encoding Matrix encoding The other classifications are coherent/non-coherent and synchronous/asynchronous systems, which will be discussed in a later section. 3. INCOHERENT AMPLITUDE ENCODING The incoherent amplitude encoding is very economical as only the power of the signal is detected and any phase information of the signal is not considered. The performance of such a system depends on the crosscorrelation properties of the code used i.e. the interference among the codes. Obviously, sparsely spaced and longer-length codes (for low cross-correlation) have been used for encoding such as prime sequences constructed by Shaar and Davies [3]. Optical codes have also been proposed by many other authors. Chung et al. in 1989 developed optical orthogonal codes and presented their analysis and applications [4]. Salehi and Brakett (1989) [5] evaluated the code performance of the codes proposed by Salehi [6] in 1989 using various parameters like data rate, code length, code weight, number of users and the receiver threshold. They also showed that ideal optical hard-limiter improves the system performance by reducing the multiple access interference. Kwong et al. (1991) [7] used optical delay lines to encode the optical signals as shown in Figure 1. Chung and Kumar (1990) have proposed a family of optical codes with cross-correlation of 2 [8].

2 Optical delay time Coupler Figure 1 Optical Tapped Delay Line Encoder. Gamiero (2000) studies the sequences based on Gold codes for use in Quasi-synchronous CDMA (QS-CDMA) [9]. The codes are shown to be more tolerant to the timing jitter in the user s clock. 4. Spectral Encoding Spectral encoding can use modulation of amplitude or phase of the spectral components in the signal to embed the information. Zaccarin and Kavehrad in 1993 [10] and Kavehrad and Zaccarin in 1995 [11] reported the spectral encoding scheme using a low-cost broadband optical source which is shown in Figure 2. The two types of spectral encoding are discussed below. 4.1 Spectral Amplitude Encoding Nguyen et al. (1997) have demonstrated experimentally the non-uniform spectral amplitude encoding to compensate for the uneven spectral shape of the laser source. Nonuniform spectral encoding improves orthogonality between the coded waveforms of the multiple users [12]. For spectral amplitude encoding, the mask used in the encoder is an amplitude mask as shown in figure 2. The complementary mask is used in the decoder but such integrated structures lead to significant insertion losses as shown in Fathallah in 1999 [13]. A grating separates the various spectral components which are focused by a lens at the amplitude mask imprinted with the desired code. A new code family for spectral amplitude encoding using fiber brag grating arrays has been proposed by Wei et al. in 2001 and also proposed a transmitter/receiver structure [14]. Djordjevic and Vasic (2004) have proposed the construction of optical orthogonal codes for spectral amplitude coded optical CDMA systems using combinatorial methods [15]. Djordjevic et al. (2004) have also designed multi-weight codes to be used for multimedia applications for spectral amplitude coded optical CDMA systems [16]. Park et al. in 2004 have used differential detection with spectral coding to reduce multiple access interference [17]. Pham et al. in 2005 have used a heterodyne detection receiver in their work on demonstration of spectralamplitude-encoding optical CDMA system [18]. Wen et al. (2006) have employed perfect difference codes (PDC) in asynchronous spectral optical CDMA by employing the interference cancellation scheme. Non-uniform spectral phase encoding has been proposed by Du et al. (2006) to compensate for the non-ideal uneven spectra of optical sources. The scheme improves the orthogonality of the encoded waveforms between the multiple users. The authors have considered the synchronous systems only and mode-locked lasers. 4.2 Spectral Phase Encoding In spectral phase encoding, a phase mask with a pseudo-random spatial phase pattern is inserted between two lenses in figure 2. Salehi et al. (1990) have proposed a spectral phase coding optical CDMA system in which the user addresses are generated by spectral phase coding of the source [21]. The first lens separates the different spectral components spatially. The phase mask is placed at mid-point between the lenses where the spatial distance between the different spectral components is maximum. It introduces the pseudo-random phase shifts into the spectral components.

3 Grating Lens Mask Source Mirror Figure 2 Spectral Encoder. The second lens will collect the phase shifted spectral components and gives out a composite spectral phase coded signal. This introduces phase shifts among different spectral components of the transmitted signal. A conjugate phase mask is used to extract the signal at the desired receiver which removes the phase shifts. An unmatched decoder further re-spreads the signal. 5. Coherent Phase Encoding Phase modulated narrow optical pulses are generated using electro-optic modulators. Pulses are then encoded by using tapped optical delay lines introducing different delays and phaseshifts in the different branches. Griffin et al. (1992) have studied the optical phase coding and employed differential detection to reduce multiple access interference. They use a master encoder to provide a reference for the system. Huang et al. (2000) describe the coherent optical CDMA systems and give their performance comparison with the OTDMA system. They also propose a hybrid OCDMA/WDMA system for better performance [23]. 6. Spatial Encoding Spatial encoding systems use multiple fibers as space channels for encoding in optical CDMA systems. Many implementations of such system have been proposed like two-dimensional optical codes by Kitayama in 1994 [24] and the spatial optical CDMA by Hui in 1985 [25]. Park et al. have in 1992 proposed Temporal/Spatial codes. The performance of the temporal/spatial codes is better than temporal codes as these allow for smaller bit times [26]. Lin et al. in 2005 have proposed non-coherent spatial/spectral codes, named 2-D perfect difference codes [27]. 7. Matrix Encoding A wavelength-time encoder using superstructured fiber-brag grating (SSFBG) is shown in figure 3. The broadband light pulse is used with a SSFBG so that the brag wavelengths of the gratings are reflected back so as to appear at the output of the encoder as the coded signal. The different spectral components are timeshifted due to the positions of the gratings in the fiber. A conjugate of the SSFBG is used at the decoder to recover the signal. Fathallah et al. have used strain to tune the grating to different wavelengths [13]. Kim (2000) have proposed a cyclic encoder/decoder using arrayed waveguide grating [28]. Arrayed waveguide gratings (AWG) alongwith metal reflection delay lines have been used to carry out wavelength/time encoding/decoding by Yu et al [29]. Kwong and Yang in 2003 have reported the use of arrayed waveguide grating for constructing programmable wavelength-time optical CDMA coders. The design is based on the phenomenon of wavelength periodicity of the AWG s. The fiber brag gratings alongwith delay lines have also been used for coder implementation in [31] whereas AWG s and delay lines have been used in [32] for designing the coders.

4 Broadband input pulse SSFBG Coded signal 8. CONCLUSIONS Optical CDMA is a simple and versatile multiaccess technique requiring very simple protocols compared to TDMA and WDMA. It supports variable QOS and variable data rate applications. The encoding/decoding is the prime stage in optical CDMA since it determines the performance of optical CDMA systems. The different techniques of encoding/decoding inoptical CDMA systems have been reviewed in this paper. REFERENCES 1. Davies, P. A. and Shaar A. A. (1983). Asynchronous multiplexing for an optical fiber local area network. Electronic Lett. 19: Yim, R. (2002). New approaches to optical code-division multiple access. M.E. thesis, McGill University, Montreal, Canada. 3. Shaar, A. A. and Davies, P. A. (1983). Prime sequences: quasi optimal sequences for or channel code division multiplexing. Electronic Lett. 19: Chung, F. R. K., Salehi J. A. and Wei V. K. (1989). Optical orthogonal codes: Design, analysis and applications. IEEE Trans. Inform. Theory. 35: Salehi, J. A. and Brakett C. A. (1989). Code division multiple access techniques in optical fiber networks Part II: System performance analysis. IEEE Trans. Commun. 37: Salehi, J. A. (1989). Emerging optical codedivision-multiple-access communication systems. IEEE network. pp Kwong, W. C., Perrier, P. A. and Pruncal, P. R. (1991). Performance comparison of asynchronous and synchronous code-division Figure 3 Wavelength-Time Encoder. multiple-access techniques for Fiber-optic local area networks. IEEE Trans. Commun 39: Chung, H. and Kumar P. V. (1990). Optical orthogonal codes new bounds and an optimal construction. IEEE Trans. Inform. Theory. 36: Gamiero, A. (2000). Synchronous and quasisynchronous optical CDMA with balanced complementary receivers. IEE Proc. Optoelectronics. 147: Zaccarin, D. and Kavehrad, M.(1993). An optical CDMA system based on spectral encoding of LED. IEEE Photon. Technol. Lett. 4: Kavehrad, M. and Zaccarin D. (1995). Optical code division multiplexed systems based on spectral encoding of non coherent sources. J. Lightwave Technol. 13: Nguyen, L., Dennis, T., Aazhang, B. And Young, J. F. (1997). Experimental demonstration of bipolar codes for optical spectral amplitude CDMA communication. J. Lightwave Technol. 15: Fathallah, H., Rusch, L. and LaRochelle, A., S. (1999). Passive optical fast-frequency hop cdma communications systems. J. Lightwave Technol. 17: Wei, Z., Shalaby H. M. H and Shiraz, H. G. (2001). Modified quadratic congruence codes for FBG based spectral amplitude coding Optical cdma systems. J. Lightwave Technol. 19: Djordjevic, I. B., Vasic, B. (2004). Combinatorial constructions of optical orthogonal codes for ocdma systems. IEEE Commun. Lett. 8: Djordjevic, I. B., Vasic, B. and Rorison, J. (2004). Multi-weight unipolar codes for multimedia spectral-amplitude-coding optical

5 cdma systems. IEEE Commun. Lett. 8: Park, S., Kim, B. K and Kim, B. W. (2004). An ocdma scheme to reduce multiple-access interference and enhance performance for optical subscriber access networks. ETRI J., 26: Pham, A. T., Miki, N., Yashima, H. (2005). Spectral-amplitude-encoding optical-codedivision-multiplexing system with a heterodyne detection receiver for broadband optical multiple-access networks. J. Optical Networking. 4: Wen, J. H., Jhou, J. S., Lin and J. Y. (2006). Optical spectral amplitude coding CDMA systems using perfect difference codes and interference estimation. Proceedings Optoelectron., Vol. 153, No. 4, p.p Du, Y., Yoo, S. J. B. and Ding, Zhi. (2006). Nonuniform spectral phase encoding in optical cdma networks. IEEE Photon. Technol. Lett. 18: Salehi, J. A., Weiner, A. M. and Heritage, J. P. (1990). Coherent ultrashort light pulse codedivision multiple access communication system. IEEE J. Lightwave Technol. 8: Griffin, R. A., Sampson, D. D. and Jackson, D. A. (1992). Optical phase coding for codedivision-multiple-access networks. IEEE Photon. Technol. Lett. 4: Huang, W., Andonovic, I. and Tur, M. (2000). Coherent optical CDMA (OCDMA) systems used for high-capacity optical fiber networks system description, OTDMA comparison and OCDMA/WDMA networking. IEEE J. Lightwave Technol. 18: Kitayama, K. (1994). Novel spatial spread spectrum based Fiber-optic CDMA networks for image transmission. IEEE J. Select. Areas Commun.12: Hui, J. (1985). Pattern code modulation and optical decoding a novel code divisionmultiplexing technique for multifiber networks. IEEE J. Select. Areas Commun. 3: Park, E., Mendez, A. J. and Garmire E. M. (1992). Temporal/Spatial optical CDMA networks Design, demonstration and comparison with temporal networks. IEEE Photon. Technol. Lett. 4: Lin, C. L. and Wu, J. (2000). Channel interference reduction using random Manchester codes for both synchronous and asynchronous Fiber-optic CDMA systems. J. Lightwave Technol. 18: Kim, S. (2000). Cyclic optical encoders/decoders for compact optical CDMA networks, IEEE Photon. Technol. Lett. 12: Yu, K., Shin, J. and Park, N. (2000). Wavelength time-spreading OCDMA system using wavelength multiplexers and mirrored fiber delay lines. IEEE Photon. Technol. Lett. 12: Singh, J. and Singh, M. L. (2009). A New Family of Three-Dimensional Codes for Optical CDMA Systems with Differential Detection. J. Optical Fiber Techn. 15: Singh, J. and Singh, M. L. (2010). Design of 3- D Wavelength/Time/Space Codes for Asynchronous Fiber-Optic CDMA Systems. IEEE Photon. Technol. Lett. 22: