Spread-time code-division multiple access

An alternative code-division multiple-access (CDMA) scheme to spread spectrum (SS), called spread time (ST) is proposed for bandlimited multiple-access channels. ST-CDMA can be considered the time-frequency dual of SS-CDMA. In ST-CDMA pseudorandom (PN) sequences are assigned to each user, and the Fourier transform of the transmitted pulse for a given user is determined by modulating the phase of the desired transmitted spectrum by the user's PN-sequence. The transmitted data for a particular user can be recovered by sampling the output of a filter matched to the user's pulse. Implementations are described in which surface acoustic wave devices are used to perform the matched filtering or Fourier transformation. Averaged signal-to-interference plus noise ratio (SIR) and spectral efficiency are computed for both asynchronous ST and direct-sequence SS-CDMA systems, assuming an arbitrary channel transfer function H(f), which is the same between all pairs of users. The results are the same for SS and ST provided that the magnitude of the Fourier transform of the chip shape in the SS system is the same as the magnitude of the Fourier transform of the ST pulse shape. The main advantage of the ST technique Is the flexibility with which the transmitted spectrum can be selected. We derive the transmitted spectrum that maximizes the SIR subject to an average power constraint     

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     

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     

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.     

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。     

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     

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.     

2n prime-sequence codes and coding architecture foroptical code-division multiple-access

Recent study shows that optical code-division multiple-access (CDMA) networks cannot be evaluated or designed by only considering the performance (i.e., correlation properties) of the optical pseudo-orthogonal codes selected. The structures of optical encoders and decoders are another important factors to consider and are needed to coordinate with the selected optical codes as much as possible. A special family of 2ⁿ codes, so-called 2ⁿ prime-sequence codes, is constructed. A general theorem on the cardinality of the new codes is provided. The properties and performance of the codes are also studied. Since these codes pose the algebraic properties of both prime-sequence and 2ⁿ codes, new optical encoding and decoding structures are designed to optimize the system parameters (e.g., power budget and cost) of these optical CDMA networks. This new configuration is particularly attractive for ultrafast optical processing and waveguide implementation for tile future high-capacity, low-loss, all-optical CDMA networks     

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。     

User-separating demodulation for code-division multiple-accesssystems

A user-separating (US) demodulator for a multiple-access system with digital transmission is defined to be a demodulator that, without knowledge of the channel codes of the various users, provides the decoder for each user with a scalar-valued output for each symbol period that permits maximum-likelihood decoding of that user's data. It is shown that such a US demodulator exists in general only for approximations to the true statistics of the modulator-input sequences of the interfering users. It is argued that approximating the interfering modulator-input sequences as independent, white Gaussian processes is the practical compromise between accuracy and simplicity in code-division multiple-access (CDMA) systems. The US demodulator for this approximation is shown to consist of a kind of matrix whitening fitter followed by a kind of matched-filter for the user in question. It is further shown that this US demodulator can often be well approximated as the symbol-by-symbol demodulator that makes the linear minimum mean-squared error estimate of each modulator-input symbol for the user in question. Simulation results are presented to confirm the theory of US demodulation and to illustrate its practical utility     

Universal coding for multiple access channels

The problems of universal coding and decoding for multiple-access channels are examined. The hypothetical mutual information functions are used to prove that rate vectorsR=(R_{1},R_{2})in a subregionhat{cal C}of the capacity regioncal Care universally achievable in the sense that there exist codes of rateRthat are asymptotically optimum for all multiple-access channels with finite fixed input alphabetsX_{1},X_{2}, and output alphabetY.     

Quasi-orthogonal sequences for code-division multiple-accesssystems

The notion of quasi-orthogonal sequence (QOS) as a means of increasing the number of channels in synchronous code-division multiple-access (CDMA) systems that employ Walsh sequences for spreading information signals and separating channels is introduced. It is shown that a QOS sequence may be regarded as a class of Bent (almost Bent) functions possessing, in addition, a certain window property. Such sequences while increasing the system capacity, minimize interference to the existing set of Walsh sequences. The window property gives the system the ability to handle variable data rates. A general procedure of constructing QOSs from well-known families of binary sequences with good correlation, including the Kasami and Gold (1967) sequence families, as well as from the binary Kerdock code is provided. Examples of QOSs are presented for small lengths. Some examples of quaternary QOSs drawn from Family A are also included     

Toward management systems for emerging hybrid fiber-coax accessnetworks

Advances in networking and hardware are motivating the development of high bandwidth local access networks that connect subscriber communities to the Information Superhighway. Hybrid fiber-coaxial (HFC) technology is emerging as one of the few economical viable technologies for designing new local access networks. These HFC local access networks are expected to support a variety of services in different spheres in an integrated manner, by drawing upon diverse technologies from the cable, computer, and telecommunications industries: the tree-and-branch network topology is typical of the cable network; intelligence in the end devices is a characteristic of the computer industry; and addressability, global interconnectivity, and quality of service (QoS) are characteristic of the telecommunications industry. It is this combination of diverse services and technologies that poses fundamental challenges in architecting a management system for HFC networks. The design of a management framework for HFC access networks is discussed     

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。     

Wideband coding for uncoordinated multiple access communication

The authors review many of the coding techniques which have been proposed for use in spread-spectrum communication links. In these systems, narrow-band data accesses a common wideband channel, with each subscriber assigned to a unique access code. This code division multiple access (CDMA) system permits uncoordinated access to the channel with the interference level set by the total number of active subscribers. The authors investigate pseudo-noise (PN) and related sequence sets, specifically the auto- and crosscorrelation responses are examined both with and without data transmissions. These are quantified in terms of peak sidelobe errors, characteristic probability density function and bit error ratio performance in the CDMA multi-user traffic environment. This allows the CDMA capacity to be predicted given the bit rate requirements, total available bandwidth and required bit error ratio     

Low-complexity multiple access protocols for wavelength-divisionmultiplexed photonic networks

Media access control protocols for an optically interconnected star-coupled system with preallocated wavelength-division multiple-access channels are discussed. The photonic network is based on a passive star-coupled configuration in which high topological connectivity is achieved with low complexity and excellent fault tolerance. The channels are preallocated to the nodes with the proposed approach, and each node has a home channel it uses either for data packet transmission or data packet reception. The performance of a generalized random access protocol is compared to an approach based on interleaved time multiplexing. Semi-Markov analytic models are developed to investigate the performance of the two protocols. The analytic models are validated through extensive simulation. The performance is evaluated in terms of network throughput and packet delay with variations in the number of nodes, data channels, and packet generation rate     

Customer and network operations for broadband and narrowband accessnetworks

Effective operations of today's access networks based on intelligent, computer-controlled network elements is a key issue. Due to the increased complexity of operation and the cost pressure in a competitive telco environment, a possible answer for these challenges is the introduction of sophisticated network operations (network management) technologies. On the other side, the costs of customer operations in connection with access networks are also very substantial. Any improvement in this field can increase the capabilities of the access operator to maximize the revenue stream. Another issue is that in the near future all operators will be forced to introduce next-generation customer operation systems simply to be able to survive. This article discusses the general issues of both access network management and customer operations in a tutorial manner     

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.