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C H A P T E R 1
Introduction and Review
1.1 Introduction
The development of the economy and society needs improved performances of communication networks, in particular optical communication networks. Optical backbone networks have been able to afford the requirement of bandwidth, whereas access networks are still the bottleneck and will be of most importance in network design and construction. Optical code division multiple access (OCDMA) is one of the emerging technologies for future multiple access networks along with wave- length division multiple access (WDMA) [1] and optical time division multiple access (OTDMA) [2]. The trend of increased research on OCDMA is accelerating due to fiber penetration in the first mile and the establishment of passive optical network (PON) technology as a pragmatic solution for residential access. The con- cept of OCDMA is based on that of the widely used code division multiple access (CDMA) technology in microwave wireless communications where users are as- signed spreading codes or signature sequences. The concept of CDMA [3] involves
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sharing of a common communication channel among multiple users. Similarly, in OCDMA a common optical communication channel is shared among multiple users. Networks employing OCDMA may be asynchronous in which users can transmit their assigned codes at any instant of time, or synchronous in which users transmit their assigned codes at previously allotted time periods.
According to [4], the use of multiple high-capacity fibers for communications networking with optical correlation by fiber tapped delay lines provides speedy and easy-to-implement decoders. Thus the individual user obtains a transparent low-speed channel by code multiplexing. The reliability of this low-speed channel can be enhanced by redundantly coding the patterns sent by the user, for which the encoding and decoding processes can be performed electronically. The elec- tronic encoding of data followed by optical encoding is simple to implement, highly reliable at reasonable throughput, and provides asynchronous access with simple protocol. Even though both synchronous OCDMA [5,6,7] as well as asynchronous OCDMA [6, 8, 9, 10] have been reported, this thesis is limited to the asynchronous case.
Spread spectrum CDMA allows asynchronous multiple access to a local area network (LAN) with no waiting. The additional bandwidth required by spread spectrum can be accommodated by using a fiber-optic channel and incoherent optical signal processing. New CDMA sequences are designed specifically for op- tical processing. Gold codes are not suitable for OCDMA systems because they exhibit a large crosscorrelation variance [11]. In OCDMA, an optical code (OC) represents a user address and signs each transmitted data bit. Optical coding is the process by which a code is inscribed into, and extracted from, an optical sig- nal. Although a prerequisite for OCDMA, optical coding boasts a wide range of novel and promising applications, such as OC label switching. The advantages of OCDMA are often specific to particular technologies, techniques, or components,
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1.2 One-Dimensional OCDMA Systems
thus implying drawbacks and tradeoffs. For instance, the cheapness of incoherent light sources implies a limit on the network reach (due to dispersion), bit-error rate (BER), and user bandwidth [12].
Optical communication channels for OCDMA may be either wireless [13] or fiber-optic. We have confined our research work to fiber-optic CDMA. The digital technology available in the electrical domain enables the use of bipolar codes for CDMA, which have good correlation properties with large number of users.
In the optical domain, bipolar codes are restricted to optical phase shift keyed communications like phase encoded OCDMA [8, 14, 15]. But fiber-optic phase communication is prone to error as optical fibers are unable to maintain the phase of an optical signal. Intensity modulation and direct detection (IM/DD) [16]
is an established fiber-optic communication technology. The IM/DD technology requires the use of unipolar codes to give low error OCDMA systems.
One of the ways in which an OCDMA system can be integrated into a fiber optic network is shown in Fig. 1.1. Different optical LAN and/or metropolitan area network (MAN) networks can be interconnected by using all-optical CDMA technology. The optical LAN/MAN networks may be any of electrical - opti- cal - electrical CDMA (EOE CDMA), WDMA or OTDMA or a hybrid EOE CDMA+WDMA network.
1.2 One-Dimensional OCDMA Systems
The working of a basic OCDMA system is explained in [11]. In OCDMA, a mode- locked laser produces a low duty cycle, high intensity pulse stream at the data rate. This sequence of pulses is modulated by an optical gate, such as a directional coupler switch, which is driven by the information waveform. Using single-mode optical fiber delay lines, each short laser pulse generates the appropriate code
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1
2 3
4 N
1 2 3
4 N
Fiber−optic Interexchange/
Long−haul Network (All−optical CDMA) 2
3 Gateway
Central Office
(EOE−CDMA/WDMA/OTDMA) Fiber−optic LAN/MAN
N
1
Figure 1.1: Application of a OCDMA systems in a fiber optic network sequence. The optical fiber delay lines are configured so that K taps can be selected from any ofT positions, according to the address of the desired receiver.
At the receiver, correlation is performed by optical fiber delay lines. In order to reduce the bandwidth requirements of the detector, the narrow autocorrelation peak is used to trigger a bistable or monostable optical switch, with decay time equal to the bit width. The slowly decaying signal is detected and processed at the rate of the original data. Increasing the number of chips per bit, by using optical processing, allows an increase in capacity of an OCDMA LAN.
Among the many different types of code sequences reported, prime sequences were first discussed for OCDMA having maximum crosscorrelation values greater than 1 [17]. Kwong and Prucnal [18] have given the construction of prime codes
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1.2 One-Dimensional OCDMA Systems
and the overall system for one-dimensional (1D) OCDMA including encoders and decoders of the prime codes. Tunable prime code encoders and decoders for OCDMA are reported in [19]. A special class of 2n code, derived from prime sequences for OCDMA is constructed in [20] along with a hybrid coding architec- ture comprising a modified tunable prime encoder and 2n decoder. A technique for constructing codes is presented that provides a family of optimal optical or- thogonal codes (OOCs) having correlation λa = λc = 2 [21]. The parameters (T, K, λa, λc) are respectively (p2m−1, pm+ 1,2,2), wherep is any prime and the cardinality ispm−2.
Algebraically designed OOCs, named as extended quadratic congruence codes having better correlation properties than prime sequences are reported in [22].
The algebraic construction of quadratic congruence codes for use in CDMA fiber- optic LANs is reported [23]. For every odd prime p, (p−1) codes exist. The sequences are of length T = p2, and correlation properties are λa ≤ 2, λc ≤ 4.
Various techniques for algebraically constructing 2n codes [20] of weight 4 are presented in [24]. The upper bounds on the cardinalities of these codes are also provided. Although, the cardinalities of the codes are not as good as that of the non-symmetric codes, it is important to note that the coding architectures used by those non-symmetric codes create so severe power loss that OCDMA systems are not feasible no matter how good the cardinalities of those codes are. A general theorem on the cardinality of the 2n prime-sequence codes is provided in [25].
These codes possess the algebraic properties of both prime-sequence and 2ncodes.
Optical encoding and decoding structures to optimize the system parameters of these OCDMA networks are described.
Code families for OCDMA having low autocorrelation and crosscorrelation values are termed as OOCs [22, 26, 27, 28, 29]. The fundamental principles and systems performance analysis of OCDMA are given in [30, 31]. An OOC intended
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for OCDMA is a collection of (0,1) sequences with good correlation properties, i.e., the autocorrelation of each sequence exhibits the thumbtack shape and the crosscorrelation between any two sequences remains low throughout. The use of OOCs enables a large number of asynchronous users to transmit information efficiently and reliably. The thumbtack-shaped autocorrelation facilitates the de- tection of the desired signal, and low-profiled crosscorrelation reduces interference from unwanted signals. Methodologies in the design and analysis of OOCs with tools from projective geometry, the greedy algorithm, iterative constructions, al- gebraic coding theory, block design, and various other combinatorial disciplines are discussed in [26]. An algorithm based on the extended set concept enables the design of OCDMA codes with best achievable correlation properties [27]. Two systematic OOC design techniques based on extended sets are presented in [32].
The first technique is a deterministic design approach where the OOC sequences are generated in a single run resulting in sequences of relatively short length. The second technique is semi-random and may require multiple iterations until all OOC sequences are generated converging to the optimal OOC.
Bounds on the size of OOCs with unequal autocorrelation and crosscorre- lation values are developed and construction techniques for building them are demonstrated in [33]. From results, an increase in the code size is possible by letting the autocorrelation value exceed the crosscorrelation value. Among the constructions given, the (T, K,2,1) codes are near-optimal, their cardinality is Nmax = 2(T −1)/K2 and it is impossible to get more than 2(T −1)/(K2 −K) codewords. Upper bounds on the size of an OOC are discussed in [34]. Sev- eral constructions for optimal OOCs with weight 4 and correlation constraints λa = λc = 1 are described by means of optimal cyclic packings. An equivalence between optimal OOCs and optimal cyclic t-packings is established, whch allows construction of optimal OOCs by way of optimal cyclic t-packings.
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1.2 One-Dimensional OCDMA Systems
Some combinatorial constructions for optimal OOCs having λa = λc ≤ 1 are given in [28, 35, 36, 37]. The known techniques for constructing constant weight codes are surveyed, and a table of (unrestricted) binary codes of lengthT ≤28 is given in [38]. Three constructions for OCDMA code families having cyclic constant weight are presented in [39]. All code families are asymptotically optimum, which in turn means that, as the length of the sequences within the family approaches infinity, the ratio of family size to the maximum possible under the Johnson up- per bound approaches unity. A recursive construction for (T, K, λa, λc) OOCs is presented in [40]. For the case of λa =λc, the recursive construction enlarges the original family with λc unchanged, and produces a family of asymptotically opti- mal codes [39], if the original family is optimal. Some combinatorial constuctions of optimal OOCs havingλa=λc ≤2 are given in [41, 42].
Constructions of difference families applicaple in 1D OCDMA are given in [43,44]. Constructions of optimal (T,4,1,1) OOCs [45] are shown by using perfect difference families and cyclic pairwise balanced designs. A construction of OOCs which is a generalization of the well-known construction of distinct difference set by Bose and Chowla is provided in [29]. The construction is optimal with respect to the Johnson bound and has parametersT =qa−1, K =q, and λa=λc = 1.
The application of an optical hard limiter in an OCDMA receiver to reduce the effect of MAI is explained in [31, 46, 47]. Performance of asynchronous OCDMA systems with double optical hard-limiters using OOCs is analyzed under the as- sumption of Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists [48]. Performance analysis of OCMDA systems using OOCs and considering all major noise sources, i.e., quan- tum shot-noise, dark current noise, and Gaussian circuit noise is discussed in [49].
Optical hard-limiters and high-speed integrate and dump circuits are two impor- tant factors which make power efficient fiber-optic CDMA receivers realizable.
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Asynchronous OCDMA systems with double optical hard-limiters have good per- formance even when the number of simultaneous users is large. The exact bit error probability of OCDMA systems employing optical hard limiters [47] is found to be the generalization of the analysis in [46]. For λc = 1 codes, the result is not restricted to the case for threshold T h = K. The parallel interference can- cellation (PIC) technique [50] can be used to remove MAI. Four direct sequence OCDMA receivers based on the PIC technique with hard limiters placed before the nondesired users or before the desired user receiver, or both are studied. For the ideal synchronous case, the theoretical upper bound of the error probability for the four receivers is given. The demonstration of synchronous OCDMA is re- ported in [5]. A comparison of asynchronous and synchronous OCDMA based on cardinality using prime codes is reported in [6]. Synchronous OCDMA systems are also reported in [8, 9, 10, 51, 52, 53].
An optical AND logic gate receiver [54], which, in an ideal case, e.g., in the absence of any noise source, except the optical MAI, is optimum. Direct and exact solutions for OOCs with λc = 1,2,3, . . . K, with the optical AND logic gate as receiver are given. In most practical cases, OOCs with λc = 2,3 perform better than OOCs with λc = 1, while having a much bigger cardinality. An arrayed waveguide grating (AWG)-based multiport optical encoder/decoder (E/D) and forward error correction (FEC) technique are applied in an OCDMA system [55].
The AWG-based OCDMA E/D with high power contrast ratio between autocorre- lation and crosscorrelation values can significantly suppress the interference noises in an asynchronous OCDMA system without using ultra-long optical codes and optical thresholder.
A limitation of 1D OOCs is that the length of the sequence increases rapidly when the number of users or the weight of the code is increased, which means large bandwidth expansion is required if a large number of codewords is needed.
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1.3 Two-Dimensional OCDMA Systems
This can be overcome by the use of larger bandwidth in terms of using additional dimensions to spread the codewords as discussed below.
1.3 Two-Dimensional OCDMA Systems
Two-dimensional OCDMA systems can be those which use any two domains among space, wavelength, polarization, time and phase for spreading. This thesis concentrates mainly on wavelength - time systems with IM/DD fiber optic com- munication technology. A 2D OOC family is a set of W ×T matrices with (0, 1) elements having low autocorrelation and crosscorrelation values. A brief review of phase based 2D systems concludes this section.
An architecture for code-empowered OCDMA lightwave networks, based on reconfigurable optically transparent paths among users of the network to provide high-bandwidth optical connections on demand over small areas such as LANs or access networks is presented in [56]. The network operates on the transmission of incoherent OCDMA codes, each network station being equipped with an OCDMA encoder and decoder. The routing at a network node is based on the OCDMA code itself. The destination address, as well as the next node on the path, is given by the code as in a code-empowered network. Commonly available delay lines enable the tunability of the encoder, decoder, and router for a reconfigurable and flexible network. A power analysis and focus on the performance issues of dynamic routing is presented. The effect of coding, topology, load condition, and traffic demand is analyzed using simulations. Routing rules, which are very unique to OCDMA networks, are presented.
Multiwavelength OOCs (MWOOCs) [57] consist of 2D codewords with every pulse of a codeword encoded in a distinct wavelength. Three classes of MWOOCs based on OOCs, prime codes and Reed-Solomon codes have been constructed.
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Using multiple wavelengths, the requirements of fiber ribbons and multiple stars in space - time OCDMA networks are eliminated. A 2D OCDMA code family having orthogonal properties in both the wavelength and time domains, which can be physically implemented by using an array of Bragg gratings is reported in [58].
Multiple pulse per row (MPR) codes with optimum threshold detection maximizes the cardinality and spectral efficiency [59]. A computationally efficient design algorithm for MPR codes with optimum threshold detection is developed and a simple receiver to enable real-time network optimization is shown. A construction of (W×T, λ+2, λa, λc) [60] MWOOCs with the number of available wavelengthsW, codeword lengthT, and constant Hamming weightλ+ 2 that have autocorrelation and crosscorrelation values not exceedingλa =λc is shown. There is no constraint on the relationship between the number of available wavelengths and the codeword length, and it is also possible to use an arbitrary λc. The code is optimal for λc
= 1. The basic principles and the upper bound on the cardinality of a family of wavelength-time MPR codes, for incoherent OCDMA networks, which have good cardinality, spectral efficiency, and minimal crosscorrelation values are analyzed in [61]. A family of one-coincidence frequency hop code (OCFHC)/OOC [62]
employs OCFHC and OOC as wavelength-hopping and time-spreading patterns respectively. An algorithm to construct wavelength-time MPR codes, starting with distinct 1D OOCs of a family as the row vectors of the code is given in [63].
In an OCDMA system using 2D single pulse per row (SPR) codes, a single choice of the number of wavelength channels can accommodate different num- bers of users with maximal spectral efficiency [64]. A fixed-hardware network can readily be adapted in response to changes in the number of users and traffic load by a readily scalable network or a time-dependent network. A family of 2D wavelength-hopping time-spreading codes, which employs wavelength hopping al- gebraically under prime-sequence permutations on top of time-spreading OOCs, is
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1.3 Two-Dimensional OCDMA Systems
studied and analyzed [65]. These codes allow the number of wavelengths and code length to be chosen independently and the code cardinality is a quadratic function of the number of wavelengths without sacrificing the maximum crosscorrelation value. A novel wavelength-aware detector for wavelength-hopping time-spreading codes is discussed and shown to provide improved code performance. Wavelength - time code families constructed with short code lengths [66] are given for OCDMA networks. A family of 2D codes, having λa = 0, λc ≤ 1, constructed by combin- ing frequency-hop and time-spreading codes is presented in [67], which employ a M-ary signalling scheme to increase the data transmission rate. A wavelength- time coding scheme for high-speed OCDMA networks is reported in [68], and a large number of new codes with asymptotically optimal cardinalities are gener- ated by this coding scheme. This coding scheme has potential for applications of secure optical networks. An incoherent OCDMA transceiver design, employing a double-padded modified prime code family as spreading sequences, based on the 2D optical modulation scheme deploying frequency and polarization shift keying is given in [69].
A 2D OOC construction scheme using the difference family (DF) is given in [70]. The performance of the codes based on the received signal power is compared with other codes. The system performance with double optical hard limiters by using the Markov-chain method is also analyzed. The combinatorial properties of 2D OOCs are revealed and an equivalent combinatorial description of a 2D OOC is given in [71]. A special case of SCP called strictly T-cyclic balanced incomplete block design (SCBIBD) is used to obtain optimal 2D OOCs. By using (W×T, K�,1)-SCBIBDs, new infinite classes of optimal (W×T, K�,1)-OOCs can be obtained.
Flexibility of wavelength-time codes is investigated in [72], providing clarity on the tradeoff between key code factors, specifically the number of available wave-
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lengths and time chips. Since the number of available codes is always>> number of active users at any given time for any truly asynchronous OCDMA system that employs quasi-orthogonal codes, there is always a set of unused codes. These un- used codes can be exploited to increase the spectral efficiency of the system [73] by exclusively assigning to each user a set ofM codes which represent a log2(M)-tuple of bits so that each user effectively uses a multi-dimensional modulation (multiple information bits per code are conveyed).
Replacing the SUM detector with the AND detector, the spectral efficiency can be at least doubled with the same bandwidth, cardinality and probability of error as reported in [74]. With respect to MAI, the AND detector is the optimum single-user detector for any code with any dimension and weight. Practical clock and data recovery (CDR) [75] for wavelength-time OCDMA provides an acceptable BER penalty as compared to optimum sampling with a global clock. Results show that MAI is not detrimental to practical CDR. A receiver without global clock [76]
providing quantization (to eliminate MAI), CDR, return-to-zero to non-return-to- zero conversion (for OCDMA compatibility with digital logic), framing (for byte synchronization), and FEC using a (255, 239) Reed-Solomon decoder, more than doubles the number of supported users at a bit-error rate < 10−10. The receiver supports an information rate of 156.25 Mb/s.
The temporal-spatial addition modulo T SPR codes having zero out-of-phase autocorrelation and crosscorrelation value of one are constructed in [77]. A temporal- spatial SPR prototype network with optical encoding and decoding using tapped delay lines is built to test the autocorrelation and crosscorrelation properties of these codes. The hard-limiting performance of 2D optical codes is analyzed under the chip-asynchronous assumption in [78]. The experimental set-up of a three- node 2D wavelength-time incoherent OCDMA system and study of output pulse sequences of encoders and correlators for different number of active users and influ-
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1.3 Two-Dimensional OCDMA Systems
ence of erbium doped fiber amplifier at input of correlators is demonstrated in [79].
The theoretical and experimental elimination of MAI in an incoherent OCDMA system by using an incoherent dual code OCDMA receiver incorporating an ul- trafast all-optical time gate is investigated in [80]. Experimental measurements confirmed by simulations show error-free [BER < 10−12] operation for up to four users. An integrated-photonic decoder [81] for 2D wavelength-time OCDMA, com- posed of three AWGs, eight variable delay lines, and a 3 dB coupler is given. The decoder utilizes complementary code processing and balanced detection to reduce unwanted interference without using a threshold or time-gating device.
A wavelength-time OCDMA modulation scheme that does not use spread- ing sequences for information transmission is presented in [82]. The proposed transmitter sends coded data directly through the optical channel and exploits a probabilistic method to reduce the amount of MAI. Simulations demonstrate that the scheme can support large numbers of active users and is robust to the effects of channel noise.
A space - wavelength OCDMA system [83] based on quadratic congruence code matrices (QCCM) is given. According to the in-phase crosscorrelation of the QCCM, MAI can be fully eliminated by using balanced photodetectors. Simula- tions show efficient suppression of thermal noise, shot noise, and phase-induced intensity noise (PIIN) in the receiver. A 2D space - wavelength code for spectral- amplitude coding an OCDMA system is proposed in [84]. The corresponding E/D pairs are based on the tunable fiber Bragg gratings cooperating with optical split- ters/combiners. For the performance analysis, the effects of PIIN, shot noise, and thermal noise are considered simultaneously. The BER performance compared with that of the system using M−matrix codes allows larger number of active users under a given BER.
Rapidly reconfigurable optical phase encoders and decoders based on fiber
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Bragg gratings are shown in [85]. Spectral phase-encoded time-spreading (SPECTS) OCDMA systems are described in [86, 87, 88, 89]. Spreading sequences for asyn- chronous spectrally phase encoded OCDMA are given in [90]. Phase-wavelength OCDMA systems are described in [8, 53, 91, 92, 93]. Encoders and decoders for phase-wavelength OCDMA systems are shown in [14, 94]. The performance of phase-wavelength OCDMA systems in dispersive fiber medium is analyzed in [95].
Coherent phase-wavelength coding techniques are given in [96]. Wavelength divi- son multiplexing (WDM)-compatible spectrally phase-encoded OCDMA systems are discussed in [97]. A full-duplex bidirectional spectrally interleaved OCDMA/dense WDM system is described in [98].
Design, simulation, and experimental investigations of OCDMA networking using SPECTS is discussed in [86]. Analysis has shown that nonuniform phase coding can increase the orthogonality of the code set, thereby reducing the impact of MAI. An experiment is conducted in a SPECTS OCDMA testbed incorporating a highly nonlinear thresholder demonstrated error-free operation for four users at 1.25-Gb/s/user and for two users at 10-Gb/s/user. Walsh codes demonstrate superior performance than m-sequences in the synchronous case, and the codes achieve synchronous error-free operation at 1.25 Gb/s.
Asynchronous OCDMA over WDM systems have been experimentally demon- strated using superstructured fiber Bragg gratings (SSFBG) and multi-port OCDMA E/D in [99]. The total throughput is above 380 Gbit/s with a spectral efficiency of about 0.32. Combined WDM/OCDMA systems using SSFBG are described in [15, 100, 101].
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1.4 Three-Dimensional OCDMA Systems
1.4 Three-Dimensional OCDMA Systems
Three-dimensional OCDMA systems can be those which use any three domains among space, wavelength, polarization, time and phase for spreading. This thesis concentrates mainly on space - wavelength - time systems with IM/DD fiber optic communication technology. The 3D space - wavelength - time OCDMA code families can be represented as (S×W ×T, K��, λa, λc).
A family of space - wavelength - time spread 3D optical codes for OCDMA networks is reported in [102]. Codes with single pulse per plane (SPP) and multiple pulses per plane (MPP), based on a prime sequence algorithm are shown. In order to eliminate the requirement of fiber ribbons and multiple star couplers, a wavelength2 - time scheme has been suggested, in which the periodic property of an AWG is used. For a small number of simultaneous users, the 3D MPP code shows better performance due to dominant effect of increased threshold. The 3D SPP code shows lower error probability for a large number of simultaneous users since the effect of reduced crosscorrelation probability becomes dominant.
A 3D OCDMA transmission system that encodes data on time, wavelength and polarization is experimentally demonstrated in [103]. This type of coding can increase the cardinality by a factor of approximately 2λc over a conventional 2D code. The performance of wavelength - polarization - time 3D OCDMA codes in terms of bit error rate and Q factor with 1 Gbps, 1.5 Gbps, 2 Gbps, 2.5 Gbps, 3 Gbps,3.5 Gbps & 4 Gbps is studied in [104]. Such systems may be ideal for use in short-distance optical LANs, where polarization states remain fairly stable.
Three dimensional perfect difference codes are constructed, and a correspond- ing system structure for space - wavelength - time OCDMA is described in [105].
The codes, generated from the perfect difference set, can suppress the PIIN and possess the MAI cancellation property. A family of 3D SPP codes for differ- ential detection (SPDD) for OCDMA systems (based on the 1D golomb ruler
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sequences), which achieve good cardinality and performance is presented in [106].
The improved BER performance is obtained by using two codes to encode ‘1’ and
‘0’ bits in the encoder and differential detection in the receiver. A family of 3D space - wavelength - time codes for asynchronous OCDMA systems with off-peak autocorrelation, λa = 0, and peak crosscorrelation, λc = 1, is reported in [107].
With W wavelengths and T time-slots, (W2T +W) codes are generated.
Though various types of 1D, 2D and 3D OCDMA code families and network architectures are reported, a practically realisable system for fiber to the home networks has not been reported so far. The thesis aims to bridge the gap between the present and practically realisable OCDMA systems. The objective of the thesis can be stated as:
• Design, development and testing of spreading codes for all-optical code di- vision multiple access communication systems that possess the property of low correlation, high cardinality and miniaturized generation.
1.5 Contributions made in the Thesis
The overall contributions of the thesis can be summarized below.
I. Construction, performance analysis and comparison of 2D MPR and 3D MPP OCDMA code families based on a novel RWOP algorithm used for wavelength and/or spatial allocation.
II. Construction, performance analysis and comparison of improved 2D MPR and 3D MPP OCDMA code families based on a novel CRWOP algorithm used for wavelength and/or spatial allocation.
III. Feasibility of lithium niobate based design of 1D, 2D and 3D miniature en- coders for OCDMA code generation.
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1.6 Organization of the Thesis
A brief description of the work carried out in the proposed thesis work is presented below.
1.6 Organization of the Thesis
The functioning of different types of OCDMA networks is discussed in Chap- ter 2. Some architectures of fiber based 1D, 2D and 3D OCDMA networks are explained. The code families of such OCDMA systems given in literature are dis- cussed. In Chapter 3, the construction and analysis of some of the existing 2D and 3D OCDMA code families are discussed followed by the proposition of a row- wise orthogonal pairs (RWOP) algorithm for wavelength and/or spatial allocation.
The RWOP algorithm is applied to construct 2D MPR and 3D MPP code families.
The performance of the constructed code families is analyzed and compared with existing 2D and 3D code families. Chapter 4 deals with the construction and per- formance analysis of 2D and 3D OCDMA code families based on a novel complete row-wise orthogonal pairs (CRWOP) algorithm. The CRWOP algorithm is also used for wavelength and/or spatial allocation. The construction of 2D MPR and 3D MPP code families is illustrated with the help of examples. The performance of these CRWOP-based code families is compared with the RWOP-based code families and other reported literature.
Based on some of the 1D, 2D and 3D OCDMA code families in Chapters 2, 3 and 4, miniaturization of 1D, 2D and 3D integrated-optic code generation is considered in Chapter 5. The discussed integrated-optic devices are based on Ti- tanium indiffused Lithium Niobate (Ti:LiNbO3) technology. Zero-gap directional couplers (ZDCs) are designed as TE-TM mode splitters to be able to use the bire- fringence property of LiNbO3. The simulation and design of a ZDC as a TE-TM mode splitter is proposed. The application of the TE-TM mode splitter in de-
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signing miniature 1D, 2D and 3D OCDMA code generators is worked out. The designed miniature OCDMA code generators are compared with similar devices in which the ZDC is replaced with a 3dB power splitting Y-junction. Finally, in Chapter 6, the conclusions derived from this thesis are discussed and some aspects of future directions of research are described.
