Chip-level detection in optical code division multiple access

A new detector for optical code-division multiple-access (CDMA) communication systems is proposed. This detector is called the chip-level receiver. Both ON-OFF keying (OOK) and pulse-position modulation (PPM) schemes, that utilize this receiver, are investigated in this paper. For OOK, an exact bit error rate is evaluated taking into account the effect of both multiple-user interference and receiver shot noise. An upper bound on the bit error probability for pulse-position modulation (PPM)-CDMA system is derived under the above considerations. The effect of both dark current and thermal noises is neglected in our analysis. Performance comparisons between chip-level, correlation, and optimum receivers are also presented. Both correlation receivers with and without an optical hardlimiter are considered. Our results demonstrate that significant improvement in the performance is gained when using the chip-level receiver in place of the correlation one. Moreover the performance of the chip-level receiver is asymptotically close to the optimum one. Nevertheless, the complexity of this receiver is independent of the number of users, and therefore, much more practical than the optimum receiver     

一种新有用于光码分多址的光相关器

本文提出了一种新的用于光码分多址系统的光相关器,并对光同步和异步系统给出了误比特率公式。数值仿真结果表明,无论是异步还是同步系统,这种光相关器的误比特率性能都优于传统光相关器的性能。     

光码分多址相位编/解码器

介绍了几种基于相干光的光码分多址(OCDMA)相位编/解码器.分别给出了采用空间光调制器(SLM)、平面光波导(PLC)、超结构光纤布喇格光栅(SSFBG)技术的OCDMA编/解码器的工作原理及最新研究进展,并分析了它们的特点.基于SSFBG的编/解码器在产生超长光学码方面是最好的;平面光波导编/解码器由于可实现单片集成,也是一种很有前途的器件.     

Performance evaluation of optical code division multiple access systems

In this paper, a new mechanism of faster routing known as message priority and fast routing (MPFR) mechanism in optical code division multiple access (OCDMA) is proposed for optimization of routing performance and to provide effective data transfer. In this proposed work, we have reduced the size of the header bits which are used in MPFR mechanism. In proposed algorithm, with accordance to packet header, the message size is being increased and priority bit is being added which leads to faster data transfer rate and effective data transfer in OCDMA. The transmitter sent the 2D codes to the MPFR which is then added priority bit. If the cover bit in any codes is enabling then the code is considered to be a priority packet and its superior bit then the code is considered to be routing. A modern novel approach which reduces the overhead for fast data transfer is proposed on name of MPFR. Thus the routing can be faster and the codes to be sent are being priorities and send. Thus the OCDMA can now become much faster to be used in critical areas. MPFR protocol, which prioritizes the code with high priority and also it reduces the header processing time of the codes with increase in message size. The prioritization of the code helped to process the particular code first and then rest of the codes and with increase in message size, help to reduce the transmission and processing time of the packet header.     

Performance evaluation of coherent optical code division multiple access

A coherent optical CDMA network employing Gold sequences for the spreading and despreading operations is analysed in terms of the signal to noise ratio penalty as a function of the number of simultaneous users incurred to maintain a given value of the bit error rate. The binary PSK and OOK modulation schemes are considered. Exact results in the presence of noise and interfering signals (interferers) are obtained.<>     

A semiclassical analysis of optical code division multiple access

A model noncoherent, optical, asynchronous, CDMA system is described. The error rate for a single-user matched-filter receiver that is valid for arbitrary photomultipliers and signature sequence sets, adheres to the semiclassical model of light, and does not depend on approximations for large user groups, strong received optical fields, or chip synchronism is analyzed. The exact minimum probability of error and optimal threshold are compared to those obtained with user-synchronism and multiple-access interference (MAI) distribution approximations. For the special case of unity-gain photodetectors and prime sequences, it is shown that the approximation of chip synchronism yields a weak upper bound on the exact error rate. It is demonstrated that the approximations of perfect optical-to-electrical conversion and Gaussian-distributed MAI yield a poor approximation to the minimum error rate and an underestimate of the optimal threshold. Arbitrarily tight bounds are developed on the error rate for unequal energies per bit. In the case when the signal energies coincide, these bounding expressions are considerably easier to compute than the exact error rate     

Wideband code division multiple access

To satisfy ever-increasing demands for higher data rates, as well as to allow more users to simultaneously access the network, interest has peaked in what has come to be known as wideband code division multiple access (WCDMA). We discuss those basic characteristics of WCDMA waveforms that make them attractive for high data rate transmission over wireless and mobile channels. We emphasize how the choice of spread bandwidth affects the bit error rate of the system, as well as how it affects the reliability of various subsystems, such as those that perform coarse acquisition and adaptive power control. We then discuss how some of the waveforms being considered for cellular WCDMA systems differ from those in use in the narrowband CDMA cellular system, and emphasize, as an example, multicarrier CDMA. Finally, we discuss other potential enhancements to WCDMA systems, such as the use of interference suppression at the receiver, or multiple antennas at the transmitter     

基于跳频扩时光正交码的光码分多址系统性能分析

提出了一种基于光正交码（OOC）的跳频方案，与OOC系统相比，采用该跳频码字的光码分多址（OCDMA）系统的可容纳用户数目得到了极大提高，并且多用户干扰（MAI）大大降低，从而系统整体性能得到提高。同时提出了使用该跳频码的多比特传输光码分多址系统方案，该方案利用了跳频码字数目的充裕性，使得系统传输速率成倍提高。     

光码分多址多比特传输系统性能分析

:多比特传输是光码分多址通信的一种方案。本文对光码分多址多比特传输系统的误码性能进行了理论分析和计算机数字仿真。在系统中存在光电检测器散粒噪声、暗电流噪声、背景光、热噪声的情况下 ,分别对无光硬件限幅器和引入一个光硬件限幅器的多比特传输系统进行讨论 ,得出相应的误比特率表达式 ,该表达式明确、具体。分析结果表明 ,当接收功率较大时 ,多比特传输系统的误码性能优于相应的单比特传输系统。     

Chip-interleaved block-spread code division multiple access

A novel multiuser-interference (MUI)-free code division multiple access (CDMA) transceiver for frequency-selective multipath channels is developed. Relying on chip-interleaving and zero padded transmissions, orthogonality among different users' spreading codes is maintained at the receiver even after frequency-selective propagation. As a result, deterministic multiuser separation with low-complexity code-matched filtering becomes possible without loss of maximum likelihood optimality. In addition to MUI-free reception, the proposed system guarantees channel-irrespective symbol detection and achieves high bandwidth efficiency by increasing the symbol block size. Filling the zero-gaps with known symbols allows for perfectly constant modulus transmissions. Important variants of the proposed transceivers are derived to include cyclic prefixed transmissions and various redundant or nonredundant precoding alternatives. (Semi-) blind channel estimation algorithms are also discussed. Simulation results demonstrate improved performance of the proposed system relative to competing alternatives     

Direct-sequence code division multiple access: From radio communications to optical networks

A direct sequence optical code division multiple access technique is investigated. This paper introduces, first, the principles definitions of Code Division Multiple Access in radio communications. After that, the performances of Optical Code Division Multiple Access are evaluated in association with a realistic amplified optical channel model. To determine the performances obtained by this combination, different detection methods are investigated in an OpticalCDMA system. Threshold and multi-user detections are developed in association with Optical Orthogonal Codes:ooc and Prime Sequences:ps, specially designed to be adapted to fiber optic channel.     

Novel algorithm for time division multiple access in broadband ISDN passive optical networks

For shared access to a broadband communication network using the asynchronous transfer mode (ATM) a passive optical network (PON) has previously been proposed. A novel algorithm to evaluate the time division multiple access (TDMA) protocol which controls the traffic from the various subscribers towards the shared access equipment (upstream direction) is proposed. It uses counters to estimate the moment of a data packet arrival at the subscriber side. By this ‘look-ahead’ technique no extra upstream channels are needed to initiate the polling by the subscriber. The algorithm guarantees fair access for all subscribers and minimizes the necessary overhead. Both the maximum access delay and overhead may be readily calculated, and the obtained results were supported by numerical simulation. Applicability in the standard broadband network environment has been proved by hardware design simulation.     

光码分多址系统中二维QCC码的误码性能分析

对基于一维二次同余码构成的QCC/QCC跳频扩时码和QCC/OOC跳频扩时码进行了详细的码字性能分析.通过计算不同情况下码字的碰撞次数,得出了互相关均值,并对这两种二维QCC码进行了误码率的仿真比较和理论分析,数值模拟表明,当码字长度确定时,QCC/OOC码的码字性能较QCC/QCC码更加优良,码字容量更大,但对跳频码片的要求较高.当码字容量确定时,较大p值的QCC/QCC码的码字性能更加优良,但码字长度相对较长.     

A new technique for multi-cell joint channel estimation in time division code division multiple access based on reduced rank singular value decomposition

A new technique for multi-cell joint channel estimation (MCJCE) in time division code division multiple access based on singular value decomposition (SVD) reduced rank technique is proposed in this paper. MCJCE is one of the effective solutions to improve the mobile system performance throughout mitigate the inter-cell interference form the neighboring cells. The increasing complexity of multi-cell system model due to the additional processing of the interferer users will be solved by using SVD reduced rank technique, where a limited number of parameter that really need it to describe the channel matrix will be estimated. Two models of multi-cell approaches are discussed, the first one depended on reconstruct the convolutional midamble matrix of inactive users in serving cell by the strongest interferer users from the neighboring cells. The second one will be more inclusive to user traffic scenarios in mobile systems and will be expanding to contain all detected users. The simulation results prove the validity of the proposed reduced rank technique for precision channel estimation (6.4 and 5 dB) and (9 and 7 dB) for case 1 and 2 respectively; BER performance improvements over the conventional estimators.     

Design and analysis of a dynamic code division multiple access communication system based on tunable optical filter

A dynamic optical code division multiple access (DOCDMA) communication system is proposed for high-bandwidth communication systems. An implementation of the system is proposed based on a fast tunable optical filter (TOF) in each encoder and decoder. This technique actively modulates the central wavelength of a TOF according to a functional code at the transmitter during the bit period before the transmission of the data. The system is modeled and analyzed taking into account multiple access interference (MAI), thermal noise, and phase-induced intensity noise (PIIN). The performance of this system is compared to that of a spectral amplitude coding system that uses either a Hadamard code or a modified quadratic congruence (MQC) code. The results show that the proposed DOCDMA system reduces the PIIN effect on the performance of the system and improves the bit error rate (BER) performance at a large number of users. Furthermore, it is found that when the effective power is large enough, the MAI becomes the main factor that limits system performance, whereas when the effective power is relatively low, both thermal noise and PIIN become the main limiting factors with thermal noise having the main influence.     

码分多址系统中多用户检测算法的性能分析

多用户检测算法是码分多址系统中解决多址干扰的关键技术之一，近年来人们对此做了许多研究。现分析了几种典型的多用户检测算法的特性，给出了它们的性能特点。     

Synchronous fibre-optic code division multiple access networks with error control coding

For a fibre-optic synchronous code division multiple access (S/CDMA) network, the number of users able to simultaneously utilise the network is a major concern. A theoretical analysis of the S/CDMA network employing error control codes to enhance the reliability of the network and to increase the number of concurrent users is presented. The error probabilities for both the uncoded and coded systems are analysed. The example shows that by using a (63, 36) BCH code the influence of interference arising from other users can be greatly reduced and the number of active users can also be increased significantly.<>     

Spreading code allocation strategy for downlink multicarrier code division multiple access transmission in a correlated Rayleigh fading channel

In this paper, we address the problem of multiple access interference (MAI) in a downlink multicarrier code division multiple access system. Because in the realistic case of correlated faded subcarriers, MAI greatly depends on the codes assigned to users, one way of improving performance without increasing receiver complexity consists in an appropriate code selection. We propose a code allocation strategy with the following properties: possible use with any code sequence and equalizer, low complexity, and efficient management of load variations. The allocation problem is formulated as a minimization problem of a cost function related to MAI. First, we provide analytical expressions for the channel frequency correlation function after maximum ratio combining, equal gain combining, and minimum mean square error equalization and for the MAI power. Then, by approximating these expressions, we define a simple cost function and build an iterative algorithm on the basis of the minimum maximum criterion to select the spreading codes. Finally, a complete analysis of the allocation efficiency is provided versus key parameters, in particular, the degree of correlation between the faded subcarriers, the system load, and the equalization techniques. Copyright © 2012 John Wiley & Sons, Ltd.     

Slotted ALOHA and code division multiple access techniques forland-mobile satellite personal communications

The throughput and delay characteristics of a land-mobile satellite channel are analyzed for both slotted ALOHA. And slotted direct-sequence CDMA (code division multiple access), using binary phase shift keying (BPSK) modulation and forward error correction coding (FEC). In the case of CDMA, the application of path diversity techniques-maximal ratio combining and selection diversity-is also taken into account. Packet success probabilities are derived for both slow and fast fading, in order to evaluate the throughput and delay. Numerical results are presented for arbitrary code lengths and for specific values of the number of resolvable paths. It is shown that CDMA can offer a substantial improvement over slotted ALOHA, especially when the chip time is less than the delay spread