A subcarrier multiplexed CDM optical local area network, theory andexperiment

A hybrid scheme is proposed for the purpose of suppressing the effects of external modulation and/or laser nonlinearities in subcarrier multiplexing (SCM) fiber optic communications systems. Hybrid CDMA/FDMA combines two schemes in such a way that the resulting hybrid network is robust against interference and is much more spectrally efficient than a CDMA system. Several possible architectures are introduced for the hybrid CDMA/FDMA subcarrier fiber optic local area network (LAN). The networks utilize CDMA and SCM, an asynchronous multiple access scheme. Direct sequence spread spectrum multiple access (DS/SSMA), the most common form of CDMA is employed. It is shown that by using the code sequence sets for which the shift-and-add property holds, intermodulation products (IMPs) and harmonics have a similar interference-like effect as nonmatching sequences do. Owing to the fact that the shift-and-add property holds for conventional spreading sequences, suppression of nonlinear distortions is examined. An average error probability performance evaluation of the selected configuration is presented for a transceiver pair. In analysis of the system, the authors assume the interference term arising from other users is Gaussian distributed. The results are compared to that obtained by exact evaluation of interference distribution using Gauss quadrature rule integration method. The authors compare the performance of this scheme for two different code sequence lengths. They also present some preliminary experimental results on the proposed LAN implementation as well as its measured transmission performance     

Optical spectral amplitude CDMA communication

We report the first demonstration of bipolar coding techniques in the optical spectral domain for incoherent optical code division multiple access (CDMA) communication. Based on the modulation and detection principles that we have developed, the power spectrum of an erbium-doped superfluorescent fiber source was encoded using bipolar codes and decoded using an optical bipolar correlator. A CDMA testbed consisting of two encoders and one decoder was implemented with bulk optics and free-space transmission. Our measurements verify the correlations between the bipolar codewords and demonstrate the rejection of multiple access interference     

采用光码分多址技术的高速计算机局域网

本文提出一种可行的适于高速计算机网络的光码分多址技术，该网络采用光正交码和光信号处理，以保证实时数据通信。提出的全光结构的快速可调光正交码编码器和解码器，可在光域中用电光开关和光延迟线实现，能支持超高速吞吐量且重构时间很短     

Performance comparison of asynchronous and synchronouscode-division multiple-access techniques for fiber-optic local areanetworks

Synchronous code-division multiple access (S/CDMA) is investigated for fiber-optic local area networks. It is shown that the large bandwidth expansion required by spread-spectrum techniques, such as CDMA, can be accommodated by using a fiber-optic channel for transmission and incoherent optical signal processing for code generation and correlation. Prime sequence codes, previously developed for a fiber-optic network using (asynchronous) CDMA, are modified to fit a synchronous transmission format. A performance comparison of CDMA and S/CDMA systems reveals that S/CDMA can accommodate a larger number of subscribers and more simultaneous users than CDMA. An environment for which S/CDMA would be suited is discussed     

Ultrashort Optical Pulse Detection for High-Speed Asynchronous Optical CDMA Networks

Like its wireless counterpart, optical code-division multiple access (optical CDMA) offers greater scalability than other optical multiplexing schemes and provides flexible quality of service, physical layer privacy and asynchronous access . However, unlike wireless CDMA, high bit-rate optical CDMA networks use much higher bandwidth, which cannot be effectively processed with modern electronics rendering many earlier developed detection schemes inapplicable. In this paper we show both theoretically and experimentally that conventional electronics-based detection is inefficient in optical CDMA networks and limits the total network throughput by the bandwidth of the photodetector used. As a solution, we show that network performance can be greatly improved using ultrafast all-optical signal processing for signal detection. Recently developed all-optical thresholding devices performing cubic transformation allow for more than seven times increase in throughput for typical network parameters. A comprehensive comparison of different detection methods for optical CDMA including optimized electronics-based and all-optical signal processing-based is given for the first time.      

飞秒脉冲时空变换整形技术

飞秒激光脉冲通过时空变换技术将时域信息变换到空域,通过空域的处理再返回到时域,是实现飞秒激光脉冲整形、测量和控制的一项重要技术。在应用方面,可产生各种所需要的波形．已广泛应用在飞秒化学、信号处理、安全通信、生物学和医学成像等方面。介绍了飞秒时空变换脉冲整形的几种方法。     

Performance evaluation of optical CDMA systems using non-coherentdetection and bipolar codes

The efficiency of an asynchronous optical CDMA (code-division multiple access) system is addressed. Two types of data modulations are considered: ASK and large-deviation CPFSK. Each user's narrow-band information is spread by the use of an electrooptic phase modulator. The sequences considered are bipolar Kasami and Gold codes. The receiver consists of an optical filter of bandwidth ≈1/T, to filter the interfering signals, followed by simple, noncoherent detection. We thereby avoid all the difficulties usually met with coherent detection. Moreover, this type of demodulation is particularly suitable for a hybrid WDMA/CDMA (wavelength-division multiple access/code division multiple access) system where the spread-spectrum technique allows multiple users to share the same wavelength, thereby increasing the capacity of the network. Results are given in terms of the number of interfering signals that can be tolerated for different code periods N, and for an acceptable P_e. Comparison is made relative to the Gaussian approximation on the distribution of the interfering signals     

Asynchronous fiber optic local area network using CDMA and optical correlation

An experimental fiber optic local area network utilizing code division multiple access (CDMA) and fiber optic delay-line signal processing is reported. The throughput-delay characteristics are compared to ALOHA and CSMA/CD. Since CDMA allows many users to access the network simultaneously, it can achieve higher throughput with no delay.     

无线局域网中的预约CDMA接入技术

在传统的无线ＭＡＣ接入协议（如ＡＬＯＨＡ,ＣＳＭＡ／ＣＡ,ＴＤＭＡ和ＣＤＭＡ）的基础上,重点探讨了基于码分多址的预约ＣＤＭＡ无线多址接入协议。该协议的在无线局域网接入时,无论在时间延迟还是吞吐量特性方面均比传统的无线接入协议有很大的改善。     

Channel reuse strategies for indoor infrared wirelesscommunications

We examine systems of fixed-channel reuse for base stations in an indoor infrared wireless communication system. The following techniques are compared: time-division multiple access (TDMA) using on-off keying (OOK) or pulse-position modulation (PPM); frequency-division multiple access (FDMA) using binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK); code-division multiple access (CDMA) using OOK with direct-sequence spreading by m-sequences or optical orthogonal codes (OOCs). We define a parameter γ, which equals the signal-to-noise ratio (SNR) for unit optical path gain and is proportional to the square of the transmitted average optical power. Using measured pathloss data, it is found that in a system using hexagonal cells and a reuse factor of three, for cell radii above 3 m, TDMA with OOK or 2-PPM, and CDMA using OOCs all require approximately the same γ to achieve a worst-case bit-error rate (BER) of 10^-9 within a cell. Using TDMA with 4-PPM results in a 6-dB decrease in the required value of γ. CDMA using m-sequences requires an increase in γ of 5 dB over TDMA using OOK, and FDMA with BPSK requires an increase of 12 dB. For a given reuse factor N in the noise-limited regime, the required value of γ decreases in inverse proportion to N² for TDMA schemes and inversely with N for FDMA and CDMA schemes. For cell radii below 3 m, cochannel interference dominates the systems using TDMA, FDMA, and CDMA with an OOC, resulting in an irreducible BER above 10^-9 at cell radii below 1.5 m. Only CDMA with m-sequences does not develop an irreducible BER, making it the only choice for cell radii below 1.5 m     

Multicarrier CDMA for future generation mobile communication

Future generations of wireless communication systems will be designed with the aim of making the best possible use of the limited radio spectrum in order to further increase throughput as well as user-capacity. In this paper, the application of multicarrier CDMA (code division multiple access) within mobile communication systems is discussed. An overview of the main types of multicarrier spread-spectrum systems is given. The multicarrier CDMA system is described in detail, including its relationship with OFDM (orthogonal frequency division multiplexing), its general performance in comparison with direct sequence CDMA and some specific implementation problems. The paper also presents some results comparing the performance of a selection of algorithms that can be used to isolate the individual signal of each user. Potential capacity increases achievable with adaptive loading in time and frequency slots is briefly demonstrated. By allocating users multiple codes, very high data rates can be achieved, and the paper gives a comparison of multicode, multicarrier CDMA with OFDM-based wireless LAN systems     

Ultrafast All-Optical Synchronous Multiple Access Fiber Networks

Two synchronous multiple access schemes, TDMA and CDMA, are proposed for fiber optic networks using optical signal processing. Network synchronization is achieved by using a central modelocked laser which also serves as the source for each station. The data are converted into a high-bandwidth optical signal using electrooptic modulators. The accessing schemes use optical fiber delay lines. The feasibility of these schemes is discussed.     

Synchronous CDMA for optical subscriber systems using 'block interleave'

Synchronous code-division multiple access (S/CDMA) for optical subscriber systems is reported. 'Block interleave' for signals modulated by the spread spectrum method is proposed. Block interleave supports phase synchronisation of codes. S/CDMA is an exciting possibility for passive double stars because it minimises interference among channels. A theoretical analysis and one experiment show that the most important factor in selecting code sequences is the small size of the maximum absolute value of crosscorrelation.<>     

Code division multiple-access techniques for use in optical-fibrelocal-area networks

Optical fibre offers a virtually limitless communications bandwidth when used as the physical medium for a local area network (LAN). However, conventional multiple-access schemes are limited in their ability to exploit this potential. Direct-sequence spread-spectrum systems are a class of code division multiple-access (CDMA) systems that encode data using high-rate sequences to spread data over a much wider bandwidth than normally required for transmission. The authors first consider the problems to be overcome in the design of future high-speed optical networks and then suggest the application of CDMA techniques as a possible solution. There follows an introduction to basic CDMA principles which is then extended to cover the particular coding, modulation and processing considerations to be taken into account when applying CDMA to an optical medium. These considerations are then used to develop a classification scheme which forms the basis for a review of current approaches in the application of CDMA techniques to optical-fibre local-area networks     

Uplink capacity improvement through orthogonal code hopping in uplink-synchronized CDMA systems

We propose an orthogonal code hopping multiple access (OCHMA) scheme in order to improve the capacity of an uplink-synchronized code division multiple access (CDMA) systems. When orthogonal codes (OCs) are used for channelization in uplink-synchronized CDMA systems, a finite set of OCs tends to severely limit the capacity gain of the uplink-synchronized CDMA systems. The OCHMA system allows each user to use a different OC for each symbol according to an allocated hopping pattern (HP). It also allows multiple users to use the same OC at a specific symbol time, which is called an HP collision. Thus, the proposed OCHMA scheme can accommodate more users than the number of available OCs. We analyze the capacity of the OCHMA scheme and compare the performance of the OCHMA with that of conventional schemes including the system using multi-scrambling codes (MSC) which have also been proposed to overcome a code-limited situation.     

交织多址与迭代多用户检测

介绍了一种新的多址方式IDMA，它采用不同的交织序列作为用户的标识特征。作为一种特殊的CDMA，IDMA继承了CDMA的很多优点，而且它以非常低的解码代价支持多用户，具有复杂度低、运算量小的特点。IDMA与迭代多用户检测技术相结合，在计算复杂度增加较少的情况下，有效提高了频谱利用率。     

A wireless multimedia CDMA system based on transmission powercontrol

This paper proposes a wireless multimedia code division multiple access (CDMA) system based on transmission power control. When we transmit multimedia information (i.e., speech, image, and data), differences in information rate, traffic performance, and required quality should be taken into account. The wireless CDMA system can achieve a flexible balance if transmission power distribution is controlled by using channel measurement information, i.e., traffic. In this paper, the optimal control of transmission power is derived by linear programming and nonlinear programming. Numerical results using a LAN demonstrate that the proposed method can satisfy the required quality     

Cyclic optical encoders/decoders for compact optical CDMA networks

For compact optical code-division multiple access (CDMA) networks, a novel cyclic optical encoder/decoder based on an arrayed waveguide grating (AWG) router is proposed. The proposed encoder/decoder treats multiple codewords of the two-dimensional Reed-Solomon code simultaneously using the cyclic property of the AWG router and the code, and multiple subscribers can share the encoder/decoder in networks. With the proposed encoder/decoder, optical CDMA networks become compact and cheap, since the number of encoders/decoders required in the network is reduced. The feasibility of the proposed encoder/decoder is demonstrated experimentally     

Microcellular direct-sequence spread-spectrum radio system usingN-path RAKE receiver

A microcellular local area network (LAN) for indoor communications is proposed using code-division multiple access (CDMA) and differential phase-shift keying (DPSK) for data modulation. The pseudonoise (PN) codes in the transmitters of the base station are mutually synchronized. For this purpose, sets of Gold code sequences having low cross correlation have been found by an exhaustive computer search. Together with wideband measurements of the indoor radio channel at 900 MHz, a five-path RAKE receiver was designed to combat fading effects and to process the time diversity by using multipath signal reception. Each receiver path is demodulated independently. Several methods of diversity combining of these paths have been investigated. Acquisition and tracking of the spreading code in the receiver are controlled by a digital signal processor (DSP). Experimental results of the CDMA system are presented, showing the behavior in a multipath environment     

Energy-add multistage detector: a novel multiuser detection methodfor synchronous CDMA systems

In synchronous code division multiple access (CDMA) systems, both multiple access interference (MAI) and intersymbol interference (ISI) arise. MAI is present due to the CDMA format while ISI is present due to channel multipath. One approach to overcome these problems is to detect the entire package of data for all users at once. This implies working with matrices with dimensions proportional to the product of the data package length and the number of active users. In this paper, we present an alternative approach. This alternative consists of a type of multistage detector that overcomes the MAI and ISI problems; in addition, it has complexity proportional to the number of CDMA users, is computationally efficient, and is suitable for pipeline implementation which allows fast data processing. Simulations are compared to a derived single user bound for the average bit error probability and the results prove the validity of the proposed detection method