Full optical encoders/decoders for photonic IP routers

Photonic Internet protocol (IP) routers can support higher throughput and have lower end-to-end transfer delay, with respect to electrical IP routers. The IP addresses are directly mapped onto the optical layer and processed by performing optical correlations in the time domain between the incoming code and the entries of the address bank; a different tapped delay-line encoder/decoder (E/D) is used for each photonic label. In the present paper, an innovative compact low-loss full E/D is presented that generates/processes a set of orthogonal codes (OCs) simultaneously. A tree of Mach-Zehnder interferometers (MZIs) furnishes the whole set of photonic labels at the same time; it is also possible to add/drop labels without modifying the remaining code sequences. The correlation performances of the OCs can be further enhanced by inserting additional phase shifters in the MZI's arms.     

Design of optical full encoders/decoders for code-based photonic routers

This paper demonstrates that standard multiplexers as generalized Mach-Zehnder interferometers or waveguide grating routers can be designed to generate/process a set of orthogonal optical codes (OCs) with very high-correlation performances. The same device can be used at the ingress node of a generalized multiprotocol label switching network to generate the photonic labels and at each routing node to perform all the correlations simultaneously. To enhance the code cardinality, without increasing the code length, this paper shows that it is possible to use the proposed encoder/decoder architectures to generate/process multidimensional OCs.     

Memristor-based combinational circuits: A design methodology for encoders/decoders

The crossbar architecture is viewed as the most likely path towards novel nanotechnologies which are expected to continue the technological revolution. Memristor-based crossbars for integrating memory units have received considerable attention, though little work has been done concerning the implementation of logic. In this work we focus on memristor-based complex combinational circuits. Particularly, we present a design methodology for encoder and decoder circuits. Digital encoders are found in a variety of electronics multi-input combinational circuits (e.g. keyboards) nowadays, converting the logic level ‘1’ data at their inputs into an equivalent binary code at the output. Their counterparts, digital decoders, constitute critical components for nanoelectronics, mainly in peripheral/interface circuitry of nanoelectronic circuits and memory structures. The proposed methodology follows a CMOS-like design scheme which can be used for the efficient design and mapping of any 2ⁿ×n (n×2ⁿ) encoder (decoder) onto the memristor-based crossbar geometry. For their implementation, a hybrid nano/CMOS crossbar type with memristive cross-point structures and available transistors is elaborated, which is a promising solution to the interference between neighboring cross-point devices during access operation. Circuit functionality of the presented encoder/decoder circuits is exhibited with simulations conducted using a simulator environment which incorporates a versatile memristor device model. The proposed design and implementation paradigm constitutes a step towards novel computational architectures exploiting memristor-based logic circuits, and facilitating the design and integration of memristor-based encoder/decoder circuits with nanoelectronics applications of the near future.     

Art of constructing low-complexity encoders/decoders forconstrained block codes

A rate p : q block encoder is a dataword-to-codeword assignment from 2^p p-bit datawords to 2^p q-bit codewords, and the corresponding block decoder is the inverse of the encoder. When designing block encoders/decoders for constrained systems, often, more than 2^p codewords are available. In this paper, as our main contribution, we propose efficient heuristic computer algorithms to eliminate the excess codewords and to construct low hardware complexity block encoders/decoders. For (0,4/4) and (0,3/6) PRML constraints, block encoders/decoders generated using the proposed algorithms are comparable in complexity to human-generated encoders/decoders, but are significantly simpler than lexicographical encoders/decoders     

二维光纤码分多址通信系统

介绍了三种二维光纤码分多址（OCDMA）系统。分析表明,它们的性能都优于一维OCDMA。同时也比较了WDM＋OCDMA和多波长OCDMA系统的性能。     

Coherent optical CDMA networks

Recent work in optical code-division multiple access (CDMA) is reviewed, progressing from incoherent to coherent techniques. It is shown that under appropriate conditions, coherent CDMA can in principle rival wavelength-division multiplexing (WDM) in terms of aggregate network throughput. Furthermore, it is shown that at high data rates, some of the components for WDM are coherent CDMA networks are nearly identical, indicating a similarity between the two approaches. CDMA retains a coding aspect which may prove attractive in security applications     

Quantizers with uniform decoders and channel-optimized encoders

Scalar quantizers with uniform decoders and channel-optimized encoders are studied for a uniform source on [0,1] and binary symmetric channels. Two families of affine index assignments are considered: the complemented natural code (CNC), introduced here, and the natural binary code (NBC). It is shown that the NBC never induces empty cells in the quantizer encoder, whereas the CNC can. Nevertheless, we show that the asymptotic distributions of quantizer encoder cells for the NBC and the CNC are equal and are uniform over a proper subset of the source's support region. Empty cells act as a form of implicit channel coding. An effective channel code rate associated with a quantizer designed for a noisy channel is defined and computed for the codes studied. By explicitly showing that the mean-squared error (MSE) of the CNC can be strictly smaller than that of the NBC, we also demonstrate that the NBC is suboptimal for a large range of transmission rates and bit error probabilities. This contrasts with the known optimality of the NBC when either both the encoder and decoder are not channel optimized, or when only the decoder is channel optimized.     

Compensating fiber gratings for source flatness to reduce multiple-access interferences in optical CDMA network coder/decoders

A fiber-Bragg-grating (FBG)-based optical code-division multiple-access (OCDMA) network coder/decoder (codec) is investigated for its interference suppression induced by nonflattened broad-band lightwave sources. Since each network user with different signature address code has different spectral distribution, the nonflattened light sources will cause multiple-access interference (MAI). Flatness compensation schemes are proposed to solve the MAI effects induced by nonflattened broad-band light sources. By arranging the same coding scheme but in different spectral coding band, spectral chips from FBG coder/compensator will incoherently power summed in the photodetectors to approach a more flattened power level. Signal-to-interference ratio (SIR) performances are evaluated with such compensation method for the discussed OCDMA network.     

Multiclass optical orthogonal codes for multiservice optical CDMA networks

Optical code division multiple access (CDMA)-based networks are an interesting alternative to support various traffic types of multimedia applications with highly variable performance targets. Generally, multilength codes are designed to support multirate services, while the multiweight codes are designed to support differentiated quality of service (QoS) for multimedia applications. However, existing optical orthogonal codes (OOCs) are limited to single class or multiclass with restricted weight and length properties. Therefore, there exists a lack of flexibility in the existing OOCs to support arbitrary rate and QoS. This paper presents a proposal of generation procedure and performance analysis of joint multiweight multilength strict OOCs. The approach used in this paper is to apply a methodology strongly relying on developed analytical theory that is supported by computer optimization, because it has turned out that it is mathematically intractable to construct unconstraint joint multilength multiweight OOCs using pure algebraic techniques. The generated code set fulfills the conditions of strictly OOCs, namely, the maximum nonzero shift autocorrelation and the maximum cross correlation constraints of one. The mark position difference (MPD) approach is used to generate in a flexible way the multiclass code set. The MPD results in the simple evaluation of multiclass code set cardinality. Furthermore, the multiple-access interference (MAI) in a multiclass OOC system is evaluated by modeling the interference per class as a Poisson distribution to simplify performance evaluation with acceptable accuracy.     

Quantizers with uniform encoders and channel optimized decoders

Scalar quantizers with uniform encoders and channel optimized decoders are studied for uniform sources and binary symmetric channels. It is shown that the natural binary code (NBC) and folded binary code (FBC) induce point density functions that are uniform on proper subintervals of the source support, whereas the Gray code (GC) does not induce a point density function. The mean-squared errors (MSE) for the NBC, FBC, GC, and for randomly chosen index assignments are calculated and the NBC is shown to be mean-squared optimal among all possible index assignments, for all bit-error rates and all quantizer transmission rates. In contrast, it is shown that almost all index assignments perform poorly and have degenerate codebooks.     

OCDMA编解码器性能与光源占空比的关系

对OCDMA系统性能与光源占空比的关系进行了理论分析,提出了分析等效相移超结构光纤光栅编解码器的结构参数与光源脉冲宽度关系的定量分析方法,利用MATLAB仿真软件计算分析了光源占空比对编解码器性能的影响.计算分析表明:光源占空比直接影响到编解码性能.考虑到窄脉宽光源高昂成本的因素,提出了在低成本高占空比的情形下的改进设想.     

Two-dimensional optical orthogonal codes for fiber-optic CDMA networks

Several constructions of two-dimensional (2-D) codes have been proposed to overcome the drawbacks of nonlinear effects in large spread sequences of one-dimensional (1-D) unipolar codes in fiber-optic code-division multiple-access (FO-CDMA) networks. Wavelength-time (W/T) encoding of the 2-D codes is practical in FO-CDMA networks. W/T codes reported so far can be classified mainly into two types: 1) hybrid codes, where one type of sequence is crossed with another to improve the cardinality and correlation properties and 2) conversion of 1-D sequences to 2-D codes to reduce the "timelike" property. This paper describes the basic principles of a new family of wavelength/time multiple-pulses-per-row (W/T MPR) codes, for incoherent FO-CDMA networks, which have good cardinality, spectral efficiency, and minimal cross correlation values. In addition, an expression for the upper bound on the cardinality of W/T MPR codes is derived. Another feature of the W/T MPR codes is that the aspect ratio can be varied by a tradeoff between wavelength and temporal lengths. The correlation properties of W/T MPR codes are verified by simulation using Matlab. For given wavelength /spl times/ time dimensions, various W/T codes, whose cardinalities are known, are compared, and it is shown that the W/T MPR family of codes have better cardinality and spectral efficiency than the other W/T codes. Performance analysis of the W/T MPR codes and their limiting cases is carried out for various parameter variations such as the dimensions of wavelength, time, and weight of the code.     

一种卫星光CDMA/WDM混合网络

在光波分复用(OWDM)成熟技术的基础上,结合光码分多址(OCDMA)的优点,提出了一种适合卫星通信的CDMA/WDM混合网络.针对提出的网络方案,分析其关键技术,并指出存在的困难及挑战.     

Design of multilength optical orthogonal codes for optical CDMA multimedia networks

To simultaneously support multimedia services with different signaling rates and quality-of-service requirements in optical code division multiple access (CDMA) networks, a new class of multilength, constant-weight optical orthogonal codes (OOCs) with good correlation properties is constructed algebraically in this paper. The performance of these new OOCs in an optical CDMA system with double-media services is analyzed. In contrast to conventional CDMA, our study shows that the performance of these multilength OOCs worsens as the code length increases, allowing prioritization in optical CDMA. Finally, an application of these multilength OOCs to integrate different types of multimedia services is briefly discussed.     

'Synchronous' CDMA demonstration for fibre-optic networks with optical processing

'Synchronous' code-division multiple-access (S/CDMA) for fibre-optic networks with optical processing is investigated. A two-user experiment demonstrating the feasibility of such a network is performed at 10 Mbit/s (500 Mchip/s) using modified prime sequence codes and optical processing. Fibre-optic delay-lines are used for code sequence generation and correlation without using fast electronic components. An environment in which S/CDMA would be particularly suitable is discussed.<>     

Multistage interconnection networks for parallel Viterbi decoders

We propose new multistage interconnection networks (MIN) for scalable parallel Viterbi decoder architectures. The architecture consists of the desired number of processing elements (PE) connected by the suggested MINs, thus allowing a tradeoff between complexity and speed. The structure of the MIN is derived first by transforming the de Bruijn interconnection-based Viterbi algorithm trellis into the equivalent trellis with a perfect shuffle interconnection, and then applying a new decomposition of the perfect shuffle operator. This results in an efficient modular system and data flow is formed by the shuffling in a local PE memory and data exchange through a fixed interconnection between PEs. We suggest several solutions for 1/n and k/n rate codes, where k denotes the number of input bits shifting into k shift registers of the encoder and, at each cycle, the encoder produces n output bits as linear combinations of certain bits in the shift registers.     

The application of spectral-amplitude-coding optical CDMA in passive optical networks

One code family suitable for passive optical network based on optical code division multiple access is present. Though spatial coding is used for capacity enlargement, the corresponding encoding/decoding devices require less fiber ribbons than that of the spatial/spectral systems proposed previously and the code cardinality is maintained. In addition, the multiple access interference is eliminated in the decoders of the proposed network and the phase-induced intensity noise arising during the photo-detecting process is suppressed further. Therefore, larger capacity can be accommodated in the network with simplified coding hardware.     

Variable-weight optical orthogonal codes for CDMA networks withmultiple performance requirements

An optical orthogonal code (OOC) is a family of (0,1) sequences with good auto- and cross-correlation properties; they are used as a means to obtain code division multiple access (CDMA) on fiber optic networks. Previous work on OOC has assumed that the weight of each codeword is the same. We consider codes when this assumption is removed. We develop bounds on the size of such “variable-weight” OOC's and demonstrate techniques to build them. Furthermore, we provide an analysis of the performance of variable-weight OOC in a CDMA network. Varying the weight of a user's signature sequence affects that user's performance; therefore this approach is useful for CDMA fiber optic networks with multiple performance requirements     

Multiuser detections for optical CDMA networks based on expectation-maximization algorithm

In this paper, we introduce new unblind and blind multiuser detectors for an optical code-division multiple-access system. The detectors have two soft and hard stages. In the soft stage, a soft estimation of the interference is obtained by solving an unconstrained maximum-likelihood problem via the iterative expectation-maximization (EM) algorithm. Then, the hard stage detects the user information bit by solving a one-dimensional Boolean constrained problem conditioned on knowing the interference. Our results reveal that the proposed detectors have very low complexity, and are robust against changes in parameters. Moreover, the numerical results illustrate that despite of their simplicities, our detectors substantially outperform other well-known suboptimum detectors, such as multistage and decorrelating detectors.     

The role of optical CDMA in access networks

We investigate the possible role of optical CDMA (O-CDMA) in future access networks. We begin with a short review of the O-CDMA technique for those unfamiliar with the technology. Next, we investigate in detail those characteristics of O-CDMA that make it an attractive technology for application in metro access networks: fairness, flexibility, simplified network control and management, service differentiation, and increased security. Although O-CDMA has many favorable attributes, it also has several actual or perceived drawbacks. We discuss the technical, economic, and perception barriers that may have limited the widescale deployment of O-CDMA access networks. We try to determine which of these drawbacks may be surmountable in the near future and which may be true "showstoppers"