Emerging optical code-division multiple access communicationsystems

All-optical systems, which perform signal processing functions optically so that the signal conversion from optical to electrical is done only when desired, are considered. Three such schemes are discussed, namely, fiber-optic code-division multiple-access (CDMA) systems, neuromorphic CDMA systems, and ultrashort-light-pulse CDMA systems. Fiber-optic CDMA systems in which incoherent optical signal processing techniques are used to establish optical CDMA systems are first examined. In particular, a novel class of sequences for incoherent fiber-optic CDMA systems, called optical orthogonal codes, are discussed. The codes are then applied to neuromorphic optical systems, and various applications of the networks are examined. Recent experiments that demonstrate the ability to encode and decode extremely fast, femtosecond optical pulses and that suggest the possibility of ultrahigh-speed CDMA systems based on ultrashort light pulses are described     

Quasi-synchronous code-division multiple access with high-ordermodulation

Code-division multiple access (CDMA) is a multiplexing technique where a number of users simultaneously access a transmission channel by modulating and spreading their signals with preassigned codewords. This paper studies the performance of CDMA signals with orthogonal (Walsh-Hadamard) codewords and synchronization errors smaller than the chip time. Two high-order modulation techniques, M-level quadrature amplitude modulation (M-QAM) and M-level phase-shift keying (M-PSK) are compared with respect to bit-error rate (BER). The results are especially important for the return channel of cable TV networks and summarized as follows: 1) Synchronization errors between transmitters lead to interference noise, whereas synchronization errors between the transmitter and the receiver lead to a decreased amplitude of the received user signal. Both effects have significant impact on the system performance. 2) Closed expressions are obtained for the BER of a CDMA signal with M-PSK and M-QAM with a given maximum synchronization error. 3) The higher the modulation order, the more sensitive the system gets for synchronization errors. 4) The BER is highly dependent on the assigned codewords out of the Walsh-Hadamard code set. 5) The BER performance of M-QAM outperforms that of M-PSK     

双极性光码分复用系统的研究

对光码分多址（OCDMA）系统中的地址码，包括单极性码和双极性码的容量和性能进行了分析比较，给出了4种双极性编码方案。单极性系统无论是在确定误码率条件和同时传输用户数方面，还是在系统总用户数方面，都远不及双极性系统，双极性系统是OCDMA系统实现大容量的有效途径之一。     

Optical code-division multiple access using broad-band parametrically generated light

A novel approach for an optical direct-sequence spread spectrum is presented. It is based on the complementary processes of broad-band parametric down-conversion and up-conversion. With parametric down-conversion, a narrow-band continuous-wave (CW) optical field is transformed into two CW broad-band white-noise fields that are complex conjugates of each other. These noise fields are exploited as the key and conjugate key in optical direct-sequence spread spectrum. The inverse process of parametric up-conversion is then used for multiplying the key by the conjugate key at the receiver in order to extract the transmitted data. A complete scheme for optical code-division multiple access (OCDMA) based on this approach is presented. The salient feature of the approach presented in this paper is that an ideal white-noise key is automatically generated, leading to high-capacity versatile code-division multiple-access configurations.     

Extended carrier-hopping prime codes for wavelength-time optical code-division multiple access

A two-dimensional wavelength-hopping time-spreading coding scheme has been recently studied for optical code-division multiple access, owing to maturity in fiber-Bragg grating (FBG) and arrayed-waveguide grating (AWG) technologies. Most of the optical codes designed for the scheme were based on an assumption, which may not be true in high data-rate systems, that the number of chips is higher than the number of available wavelengths. To provide a flexible code design without the assumption, this paper studies and analyzes a new family of carrier-hopping prime codes with expanded cardinality and ideal correlation properties. Flexible implementation of programmable AWG- and FBG-based coding hardware by using the wavelength-shift property of the codes is discussed.     

光码分多址网络技术及其研究进展

介绍了光码分多多的基本概念、技术及其特点，重点综述了光码分多址网的光编码和光解码技术及其研究进展，最后指出光码分多址网络技术是实现全光网的重要技术，应用前景广阔。     

多维码OCDMA通信系统的研究

提出了OCDMA系统中所使用的多维码的概念并对其性能进行了分析．重点介绍了光素数码、二维光正交码、二维素数码及三维码．     

Frame time-hopping fiber-optic code-division multiple access using generalized optical orthogonal codes

A new spreading technique for intensity modulation direct detection fiber-optic code-division multiple-access (FO-CDMA) communication systems is proposed. This new spreading technique is based on generalized optical orthogonal codes (OOC) with large cardinality and minimal degradation in performance when compared with a more optimum system, namely, an optical CDMA system using OOC with autocorrelation and cross-correlation value bounded by one, i.e., OOC (/spl lambda/=1). To obtain the performance of such systems, we use a communication scheme, namely, frame time-hopping (FTH)-CDMA with random coding. Systems with generalized OOC patterns and random time-hopping coding are close in structure and performance. Furthermore, the performance of such systems is near the performance of optical CDMA with optimum but low cardinality OOC (/spl lambda/=1), which further renders the practicality of the proposed technique with very large cardinality. Two new receiver structures for FO-CDMA, namely, chip-level detector with optimum comparator threshold and correlation receiver with an electrical hard limiter, are also proposed. The performance analysis for a binary pulse position FTH-FO-CDMA network is considered for the correlation receiver, chip-level detector, correlation receiver with an optical hard limiter, optimum receiver, and the two newly proposed receiver structures. The results also show that a chip-level detector with optimum comparator threshold is superior to a chip-level detector for received low signal powers, and predict that the performance of the correlation receiver with an electrical hard limiter is superior to all other considered receiver structures, e.g., requiring one third of transmission power to achieve a desired bit error rate when compared with other receiver structures.     

光码分多址接入技术中单、多维编解码器的实现

在简要介绍光码分多址（OCDMA）接入技术中各种单、多维光扩频编码原理的基础上,对相应各种光编／解码器的结构和特点进行了分析和比较,并对光编／解码器的发展前景作了探讨。     

时域相位光码分多址隐藏通信性能分析

提出并建立了基于时域相位光码分多址的单波长宿主信道光隐藏通信系统.通过对系统仿真结果分析,论证了隐藏信道的隐藏性及可用性、隐藏信道与宿主信道功率比平衡点,以及隐藏信道的加入对宿主信道的功率代价.     

二维空频域光码分多址编解码方法

以哈达玛(Hadamard)矩阵为基础,给出一种二维空间相位码的编码方法,分析它的各种性能,并用这种二维空间相位码作为用户的地址码,构成一种新的空间-相位码分多址系统,使光码分多址编解码在空频域内进行,由于对空频域内的信息只作相位调制,大大提高了光传递效率。     

Lighting the local area: optical code-division multiple access and quality of service provisioning

Asynchronous, high-speed multiple-access is proposed as a natural solution to achieving asynchronous, high-speed connectivity in a local area network environment. Optical CDMA is shown to be competitive with other networking technologies such as WDMA and TDMA, but has the benefit of more flexibility, simpler protocols, and no need for centralized network control. The limitations of one-dimensional optical orthogonal codes for CDMA have motivated the idea of spectral spreading in both the temporal and wavelength domains. If the constraints on constant weight in these two-dimensional codes are relaxed, differentiated levels of service at the physical layer become possible. Areas for further research are suggested which may allow quality of service levels to be guaranteed at the physical layer.     

Novel optical fiber code-division multiple access networkssupporting real-time multichannel variable-bit-rate (VBR) videodistributions

we design novel code-division multiple access (CDMA) networks which use strict optical orthogonal codes (OOCs) and incoherent optical processing for multichannel variable-bit-rate (VBR) video broadcasting. The proposed techniques possess the characteristics of fast access tunability and bit-rate flexibility. This in turn can facilitate real-time VBR video distributions for entertainment HDTV applications. Compared with conventional OOCs, the use of strict OOCs can ensure both auto- and cross-correlation constraints to be minimum (i.e., “1”) for multichannel VBR video broadcasting and high-speed photonic switching applications. A parallel-serial hybrid architecture for all-optical tunable decoders (or encoders) is also presented to strict OOCs, which has a fast reconfiguration time to support real-time video applications in high-speed optical fiber CDMA networks     

Analysis of the performance of time-spread optical code-division multiple access systems with integrate-and-dump receivers

The BER performance of asynchronous time-spread optical code-division multiple access (TS-OCDMA) systems employing integrate-and-dump receivers with partially coherent optical sources was analyzed. Monte-Carlo simulations of the system were performed to verify the validity of the model. Using the developed analytic model, the TS-OCDMA systems with integrate-and-dump receivers were compared with those employing optical thresholders. It was found that: (1) as the integration time (or gating time) increases, the systems with optical thresholders outperform those using integrate-and-dump receivers, (2) while the performance of both systems is improved as the optical sources become more incoherent, those with integrate-and-dump receivers benefit more, (3) when the systems are spectral-efficiency limited, the performance does not improve by using partially coherent sources.     

Applications of ultrashort pulse propagation in Bragg gratings forwavelength-division multiplexing and code-division multiple access

We discuss theoretical and experimental studies on the propagation of ultrashort pulses through fiber Bragg gratings. We also consider several applications in optical communications to be found by combining ultrashort pulses and fiber Bragg gratings: a multiwavelength source for wavelength-division-multiplexed systems and a means for implementing optical code-division multiple access     

二维光正交码OCDMA系统编解码器的设计

文章提出了二维光正交码的概念并对其性有进行了分析，重点介绍了二维光正交码光纤延迟线编解码器的设计，并且给出了二维光正交码光纤光栅编码原理。     

Coherence coding for photonic code-division multiple accessnetworks

We analyze a code-division multiple access technique where information is optically encoded by manipulating the coherence between a pair of transmitted signals. Key features are intrinsic security, operation of the receiver at only the bit rate of a single channel, and reconfiguration without switching optical delays, which are considerable advantages compared to previously proposed CDMA schemes. Experimental results demonstrating the basic operation of the scheme are presented. The performance limitations are calculated and novel implementations are proposed     

Coherent ultrashort light pulse code-division multiple accesscommunication systems

A new technique for encoding and decoding of coherent ultrashort light pulses is analyzed. In particular, the temporal and statistical behavior of pseudonoise bursts generated by spectral phase coding of ultrashort optical pulses is discussed. the analysis is motivated by recent experiments that demonstrate high-resolution spectral phase coding of picosecond and femtosecond pulses and suggest the possibility of ultrahigh speed code-division multiple-access (CDMA) communications using this technique. The evolution of coherent ultrashort pulses into low intensity pseudonoise bursts as a function of the degree of phase coding is traced. The results are utilized to analyze the performance of a proposed CDMA optical communications system based upon encoding and decoding of ultrashort light pulses. The bit error rate (BER) is derived as a function of data rate, number of users, and receiver threshold, and the performance characteristics are discussed for a variety of system parameters. It is found that performance improves greatly with increasing code length     

On the feasibility of a 2-Mb/s bearer service in a future cellularradio system using code-division multiple access

The universal mobile telecommunications system (UMTS) and other third-generation mobile radio systems will be required to support data rates up to 2 Mb/s. This paper examines whether a direct-sequence spread-spectrum air interface can support such a high data rate in an outdoor environment. A mathematical analysis of the operation of a RAKE receiver is presented. Monte-Carlo methods are used to establish the probability that a 2-Mb/s link is possible and the proportion of system capacity such a link would consume. It is concluded that with sufficiently complex RAKE receivers and a high enough chip rate, it is certainly practical for a direct-sequence code-division multiple-access (DS-CDMA) mobile radio system to offer a 2-Mb/s service and that the overall spectrum efficiency will be high provided that the majority of users of the system use much lower data rates     

Packet labeling technique using electronic code-division multiple-access for WDM packet-based access networks

We propose and experimentally demonstrate the feasibility of a packet labeling technique using electronic code-division multiple-access for a wavelength-division-multiplexing (WDM) packet-based access network, whereby each wavelength is assigned a unique electronic code-based label on a radio-frequency subcarrier. Such a technique allows individual wavelength channels to be electronically identified without requiring the use of a WDM demultiplexer. We experimentally demonstrate this technique with two WDM channels each with 1.25-Gb/s payload data and 10-Mb/s header coded onto an electronic code at 160 Mb/s. The experimental results and theoretical analysis show that this technique has the potential to support large numbers of WDM channels.