基于OCDMA over WDMA光纤网络

基于OCDMA(optic code division multiple access)在网络扩张方面的优势,考虑采用相干OCDMA方法提出了OCDMA over WDMA(wavelength division multiple access)光网的结构,并对其网络特性、连接协议、优点及其在光网络中应用时系统出现的问题加以讨论.最后通过计算机仿真,研究了OCDMA overWDMA网络的阻塞特性与时间延迟.     

多波长OCDMA与WDMA系统的性能分析和比较

分析了色散效应对多波长光码分多址(MWOCDMA)系统的归一化吞吐量性能的影响.光脉冲信号的切普宽度越小,色散效应就越大,MWOCDMA系统的归一化吞吐量性能也就下降得越严重.在考虑色散效应和多用户干扰的情况下,比较了MWOCDMA系统与波分多址(WDMA)系统的归一化吞吐量性能.结果表明:当负载较小时,MWOCDMA系统的性能不如WDMA系统;当负载较大时,MWOCDMA系统的性能优于WDMA系统.     

基于OOC的WDM+OCDMA系统性能研究

基于光正交码,研究了WDM+OCDMA系统的性能。采用4个波长信道和3个码字的光正交码OOC(37,4,1),对12个用户的WDMA+OCDMA进行了系统仿真,用户数据速率为1Gbit/s,单模光纤长为50km,色散补偿光纤长为5km。如需增加WDMA+OCDMA的用户数,可以通过增加波长信道数或码字容量来实现。分析了色散效应对WDMA+OCDMA系统性能的影响,研究了用户之间相对延时对多址干扰和系统性能的影响。     

System performance comparison of optical CDMA and WDMA in a broadcast local area network

The performance of optical code-division multiple-access (CDMA) systems with wavelength-hopping/time-spreading codes is compared to that of a wavelength-division multiple-access (WDMA) system. The multiple-access techniques are applied in a time-slotted broadcast local area network. The utilization, defined as the throughput per unit of time-domain bandwidth expansion, and packet delay are used as metrics of performance. When more than seven wavelengths are available, optical CDMA systems using asymmetric prime-hop codes and all-optical signal processing are shown to have higher peak utilization and lower corresponding delay than a WDMA system with the same number of wavelengths. When the encoders/decoders operate at the chip rate, the utilization of optical CDMA exceeds that of WDMA at high offered loads; however, the peak utilization of the WDMA system is still superior.     

A new family of space/wavelength/time spread three-dimensionaloptical code for OCDMA networks

A new family of space/wavelength/time spread three-dimensional (3-D) optical codes for optical code-division multiple-access (OCDMA) networks has been proposed. Two types of 3-D codes have been constructed: 3-D codes with single pulse per plane and 3-D codes with multiple pulses per plane. Both codes are based on the prime sequence algorithm and have shown improved performance compared to the previously proposed two-dimensional (2-D) prime code. Effective implementation of the 3-D code has also been proposed. In order to eliminate the requirement of fiber ribbons and multiple star couplers in space/wavelength/time spread 3-D code based optical networks, a wavelength²/time scheme has been suggested, in which the periodic property of an arrayed waveguide grating (AWG) is used. It has been shown that the system performance can be maximized for given resources with a proper choice of the wavelength²/time scheme. Due to the improved performance of the 3-D code and the effective architecture of the wavelength²/time scheme, the feasibility of the OCDMA network is much enhanced     

Design of FBG En/Decoders in Coherent 2-D Time-Wavelength OCDMA Systems

A novel fiber Bragg grating (FBG)-based en/decoder for the two-dimensional (2D) time-spreading wavelength-hopping optical coding is proposed. Compared with other 2D en/decoders, the proposed en/decoding for an optical code-division multiple-access (OCDMA) system uses a single phase-encoded FBG and coherent en/decoding. Furthermore, combined with reconstruction-equivalent-chirp technology, such en/decoders can be realized with a conventional simple fabrication setup. Experimental results of such en/decoders and the corresponding system test at a data rate of 5 Gb/s demonstrate that this kind of 2-D FBG-based en/decoders could improve the performances of OCDMA systems.     

Performance of Asynchronous Time-Spreading and Spectrally Coded OCDMA Systems

The performance of asynchronous coherent time-spreading optical code-division multiple-access (OCDMA) systems is evaluated semi-analytically and the results are compared with those of spectral coding OCDMA systems using ultrashort pulses. The fundamental multi-access interference limited performances are predicted to be identical.      

Modeling of a Time-Spreading OCDMA System Including Nonperfect Time Gating, Optical Thresholding, and Fully Asynchronous Signal/Interference Overlapping

We present an extended theoretical model for time-spreading optical code-division multiple-access (OCDMA) coherent systems. We have updated well-known model to evaluate multiuser interference to include arbitrary encoded/decoded chip shape, receiver transfer function, and optical thresholding before detection. Full asynchronous regime is also assumed to exploit statistical benefits over the dominant primary beat noise. The model provides clear improvements in terms of number of users and required intercodes crosstalk, leading to more feasible OCDMA systems.     

Study of Phase Modulations With Dual-Balanced Detection in Coherent Homodyne Optical CDMA Network

In this paper, we examine the performance of two different phase modulation methods in coherent optical code-division multiple-access (OCDMA) systems. As optical spreading sequences, a recently introduced prime code family hereby referred to as double-padded modified prime code has been employed and analyzed. The OCDMA system operates under synchronous homodyne dual-balanced detection scheme. In this analysis, binary phase-shift keying format is deployed where the phase is modulated by either a Mach–Zehnder interferometer external phase-modulator or an injection-locking of distributed feedback laser diode's driving current. The performances of the two phase-modulation techniques in the presence of multiple-access interferences in a shot-noise limited regime are studied. Phase limitations and reasonable consequence associated with the limited phase excursion are also investigated.      

Spectral efficiency of OCDMA systems with coherent pulsed sources

We present a model to evaluate the upper limit of the spectral efficiency of optical code-division multiple-access (OCDMA) systems with coherent sources. Phase-encoded and direct-sequence OCDMA systems are evaluated using this model. The results show that a spectral efficiency of 2.24/spl times/10/sup -2/ b/s/spl middot/ Hz can be achieved with a maximum bit error rate of 10/sup -10/ in these systems of the number of users. This result demonstrates that the maximum spectral efficiency of OCDMA systems with coherent sources is at least a factor of 5 higher than OCDMA systems with incoherent sources.     

Novel Encoder/Decoder Using Subsampled Bragg Grating for a WDM-Compatible OCDMA System

Optical code-division multiple-access (OCDMA) technology is attractive for optical broadband access network. It is very interesting to use the super-structure fiber-Bragg-grating-based en/decoders to construct a wavelength-division-multiplexing (WDM)-compatible OCDMA system. Based on the equivalent phase-shift technique, we have proposed a subsampled-Bragg-grating (S-SBG)-based en/decoder. Based on the proposed method, only a single uniform phase mask can be used to accommodate different WDM channels with relevant central wavelengths and spectrum width. Using the S-SBG-based en/decoder, we can make different WDM channels have similar as well as better encoding efficiency and correlation performance. S-SBG-based en/decoders with 127 chips for two different wavelength channels are fabricated and measured. The experiments indicate the advantages revealed by the theoretical simulation.     

IP Routing and Traffic Analysis in Coherent Optical CDMA Networks

This paper proposes a novel IP routing schemes over coherent optical code-division multiple-access (OCDMA) network. As spreading code, flexible in length double-padded modified prime code (DPMPC) has been deployed. The performance has been analyzed in terms of the users' channel utilization factor in the network. The results are comparable with previous schemes and indicate that the architecture is power efficient and capable of accommodating greater number of active users with advantage of shorter code length (i.e., higher bit rate). It is shown that for a maximum bit rate the network performance can be improved by reducing the channel utilization. However, when each user in the network has a fixed bit rate, optimal channel utilization can be set for the overall network performance. Additionally, each IP packet is buffered only at the edge of this network, thus the buffer delay is significantly reduced compared with traditional routing schemes. Since the optical encoder is adjusted for number of packets rather than individually, the encoder adjusting time is also significantly reduced. Analytical results clearly indicate that coherent OCDMA technique can be a fine candidate for the future ultra fast optical IP networks.     

波分复用OCDMA系统的性能分析

分析了波分复用(WDM)光码分多址(OCDMA)系统的归一吞吐量性能。当同时用户数均匀分配到不同的波长时,得到了WDM OCDMA系统归一化吞吐量性能的上界;当同时用户数尽量分配到同一个波长时,得到了WDM OCDMA系统归一化吞吐量性能的下界。在所有用户均匀分配到不同波长信道的条件下,比较了WDM OCDMA与多波长(MW)OCDMA系统的归一化吞吐量性能。结果表明,当负载较小时,两者的性能基本相当;当负载较大时,WDM 0CDMA的性能优于MW OCDMA系统。同时,WDM OCDMA的归一化吞吐量峰值高于MW OCDMA系统的归一化吞吐量峰值。     

几种光传送网技术的比较

随着业务需求特别是宽带业务的增长,人们对高容量系统,特别是光传送网技术进行了大量的研究。目前研究的内容主要涉及到波分复用光网络,光时分复用技术和光码分复用技术。波分复用技术已经在网络中广泛采用,是目前唯一成熟且付诸实用的超大容量光传输系统;光时分复用的研究近几年取得了较大的进展,是未来极具潜力的技术,但远不如波分复用成熟;光码分复用可大大提高光纤的利用率,降低网络成本,简化网络管理,具有较高的网络     

BER analysis of coherent optical CDMA communication systems with transfer function matrix

We analyze the bit error rate (BER) of coherent optical code-division multiple-access (CDMA) communication systems with a transfer function matrix (TFM). The coherent optical CDMA is a promising system for an access network due to its advantages of asynchronous transmission, information security, multiple-access capability. TFM is defined as the Fourier transform of the impulse response of an optical fiber, which interconnects each CDMA system. TFM has parameters such as the state of polarization (SOP), phase difference, power distribution between two orthogonal modes over an optical frequency range. We first analyze the statistic of a received signal via an optical fiber with TFM, and then apply the statistic to the BER analysis. The analytical results show that BER increases with the bandwidth of a signal, and that selecting a proper center frequency leads to the BER reduction.     

DS-OCDMA encoder/decoder performance analysis using optical low-coherence reflectometry

Direct-sequence optical code-division multiple-access (DS-OCDMA) encoder/decoder based on sampled fiber Bragg gratings (S-FBGs) is characterized using phase-sensitive optical low-coherence reflectometry (OLCR). The OLCR technique allows localized measurements of FBG wavelength and physical length inside one S-FBG. This paper shows how the discrepancies between specifications and measurements of the different FBGs have some impact on spectral and temporal pulse responses of the OCDMA encoder/decoder. The FBG physical lengths lower than the specified ones are shown to affect the mean optical power reflected by the OCDMA encoder/decoder. The FBG wavelengths that are detuned from each other induce some modulations of S-FBG reflectivity resulting in encoder/decoder sensitivity to laser wavelength drift of the OCDMA system. Finally, highlighted by this OLCR study, some solutions to overcome limitations in performance with the S-FBG technology are suggested.     

Performance analysis of WS-EWC coded optical CDMA networks with/without LDPC codes

One extended Welch-Costas (EWC) code family for the wavelength-division-multiplexing/spectral-amplitude coding (WDM/SAC; WS) optical code-division multiple-access (OCDMA) networks is proposed. This system has a superior performance as compared to the previous modified quadratic congruence (MQC) coded OCDMA networks. However, since the performance of such a network is unsatisfactory when the data bit rate is higher, one class of quasi-cyclic low-density parity-check (QC-LDPC) code is adopted to improve that. Simulation results show that the performance of the high-speed WS-EWC coded OCDMA network can be greatly improved by using the LDPC codes.     

Performance comparison of multiwavelength CDMA and WDMA+CDMA forfiber-optic networks

A “multiwavelength” scheme has been proposed to support large numbers of subscribers and simultaneous users in optical code-division multiple-access (CDMA) networks without using very large bandwidth expansion or the need of complicated and not-yet-feasible optical processing. In this paper, multiwavelength optical orthogonal codes (MWOOCs), which consist of two-dimensional codewords (or matrices) with every pulse of a codeword encoded in a distinct wavelength, are constructed for this scheme. MWOOCs have larger cardinality than the one-dimensional codes used in the hybrid wavelength-division multiple-access (WDMA) and CDMA scheme. With the same hardware configuration, our analysis shows that the multiwavelength scheme, in general, performs better than the hybrid scheme, particularly when the traffic load is heavy. However, if a central controller (i.e., under the best scenario) is used to uniformly distribute all available wavelengths to simultaneous users in the hybrid scheme, both schemes have comparable performance for a medium traffic load and the hybrid scheme can theoretically achieve error-free transmission when the load is light. In addition, using multiple wavelengths, the requirements of fiber ribbons and multiple stars in temporal/spatial optical CDMA networks are eliminated     

OCDMA技术的研究现状及发展

OCDMA技术是新一代的全光技术,可以广泛用于光信道多路复用、光网络多址接入、光码标记交换核心网、光信号安全传输、光纤传感以及光栽无线系统等.概述了光码分多址(OCDMA)技术的研究现状、主要关键技术,并展望其发展趋势和应用前景.     

Distributed-Phase OCDMA Encoder–Decoders Based on Fiber Bragg Gratings

We propose and demonstrate new optical code-division multiple-access (OCDMA) encoder-decoders having a continuous phase-distribution. With the same spatial refractive index distribution as the reconfigurable optical phase encoder-decoders, they are inherently suitable for the application in reconfigurable OCDMA systems. Furthermore, compared with conventional discrete-phase devices, they also have additional advantages of being more tolerant to input pulsewidth and, therefore, have the potential of bandwidth saving