基于混沌序列的相干OCDMA系统性能分析

文章把二进制的双极性混沌序列应用到两个相干光码分多址(OCDMA)系统中,推导了信噪比和误码率的表达式,仿真分析了误码率随用户数的变化曲线,并和使用Gold序列的同一方案进行了比较,研究结果表明基于混沌序列的系统有较低的误码率和较高的安全性.     

异步BPSK调制DS／CDMA通信系统的性能分析

DS/CDMA是扩频技术应用的一种重要形式，在无线个人通信和蜂窝移动通信领域受到越来越多的关注。本文介绍了异步BPSK调制DS/CDMA系统模型，应用随机过程理论，分析得到了系统BER性能的解析表达式，并进行了计算机模拟验证。     

异步多径条件下空时CDMA多用户检测

本文利用最小均方误差准则,推导了异步多径条件下空时CDMA多用户检测算法,分析了多用户检测与单用户检测关系,提出空时CDMA多用户检测接收机结构。基于SAGE迭代算法进行空时多用户检测,并利用CDMA信号的有限字符特性和上行信道的导频信息对SAGE算法提出改进。仿真结果说明了本文提出的空时CDMA多用户检测算法性能。     

Fading Margin Analysis for Asynchronous CDMA Systems

In this paper, a detailed theoretical analysis of fading margin in an asynchronous code division multiple access (A-CDMA) system is discussed. Rayleigh and Rician frequency-selective slowly fading channels are considered. Probability distribution and density functions of the probability of error are derived for Rayleigh and Rician fading channels. The fluctuations in the channel capacity are proved to be directly proportional to the signal-to-noise ratio (SNR) variations. Fading margin is calculated for both Rayleigh and Rician fading channels as a function of the probability of error specification and the probability of unsatisfactory operation.     

Phase-Tracking Performance of Direct-Detection Optical Receivers

The performance in estimating a Wiener process that modulates the phase of a subcarrier in a direct-detection optical communication system is studied. An approximation is derived for the steady-state probability density of the estimation error. The meansquare estimation error predicted using this density is compared to theoretical lower bounds on the error and to the sample meansquare error obtained by simulation.     

Performance of SCHT Sequences in Asynchronous CDMA System

In this letter, the performance of a new set of spreading sequences for a direct sequence (DS) code division multiple access (CDMA) system is examined. These new sequences are derived from sequency-ordered complex Hadamard transform (SCHT). Various correlation properties are presented and the performance of the new sequence is analyzed by comparing it with the other well-known existing spreading sequences, such as UCHT, WHT, Gold, small set of Kasami and m-sequences. According to the simulation results, it is shown that the SCHT sequences provide smaller mean square cross correlation values and offer better average bit error rate for an asynchronous CDMA system over the multipath fading channel.     

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.      

Performance of Optical Receivers with Avalanche Photodetection

This paper reports the results of a computer study to numerically evaluate the digital performance of an optical receiver with avalanche photodetectors. Exact and approximate APD statistics were implemented and error probabilities in the rangel0^{-1}tol0^{-4}were computed. Both shot-noise and receiver-noise limited conditions were examined, and on-off keying, binary pulse comparison, multilevel intensity, and PPM signaling formats are considered.     

双硬限幅器异步光码分多址系统误码性能分析

针对基于单硬限幅器（OHL）异步光码分多址（OCDMA）系统的解码器不能完全消除信道串扰产生的误码,提出了具有双OHL的解码器结构来优化系统传输性能。通过数值分析给出了地址码（光正交码,OOC）的码长、码重、用户数与系统误码率的关系,结果表明：双OHL异步OCDMA系统误码率相对于单OHL系统误码率下降明显,系统传输性能得到改善。     

Performance of RAKE-LMMSE Receivers in Wideband Communication Systems

High speed multimedia services with flexible data rate are main cause of popularity of 3 G WCDMA. WCDMA air interface of UMTS has the bandwidth of 5 MHz and chip rate of 3.84 Mcps. The flexibility in data rate is achieved by varying the length of its spreading code. WCDMA supports seamless connections of dynamic data rate ranging from 15 Kbps to 1 Mbps and respective spreading factors are 256 and 4 respectively for indoor and outdoor applications. The conventional RAKE receiver is near far and interference limited. Its performance at high data is limited due to short spreading factor of a code. A simple MIMO receiver structure of LMMSE in downlink asynchronous direct-sequence code division multiple access is proposed in this paper. This paper investigates RAKE receiver characteristics and does the analysis of zero forcing and LMMSE equalizer for MIMO channels.     

Macrodiversity in DS/CDMA Asynchronous Systems with Decorrelating Detectors

This paper analyzes the application of macrodiversity in Code DivisionMultiple Access (CDMA) asynchronous systems with decorrelatingdetection in the reverse link. It is shown that decorrelatingdetectors allow predetection combining and, utilizing this property,several combining strategies are studied. We obtain the set ofcombining factors which minimize the bit error probability, andbased on these results, we propose the Simplified Minimum ErrorCriterion (SMEC), which has little impact on the overallcomplexity of the system and outperforms theMaximal Signal to Interference plus Noise Ratio Combiner(MSINRC). Using this technique, it is possible to perform a distributeddetection in the area influenced by the mobile, allowing thecoexistence of heterogeneous cells and the effective useof microcells.     

Simulation Studies on Nonlinear Bit Synchronizers in APD-Based Optical Receivers

This paper examines the problem of self bit synchronization employing nonlinear timing extraction schemes in optical receivers with avalanche photodetectors (APD) through digital computer simulation. A direct algorithm is developed for the simulation of sample functions of the APD output process with due consideration to intersymbol interference (ISI). Simulation of all the functional blocks used for synchronization is carried out in the discrete time domain. Performances of the nonlinear timing recovery schemes are evaluated in terms of the rms timing jitter for nonreturn-to-zero (NRZ) as well as return-to-zero (RZ) signaling formats.     

Performance Analyses of OFDM–CDMA Receivers in Multipath Fading Channels

Orthogonal frequency-division multiplexing (OFDM) is a promising scheme for multicarrier (MC) transmission. The combination of OFDM and code-division multiple access (CDMA) which is referred to as OFDM–CDMA, has recently attracted much research interest in wireless communications. In this paper, we evaluate the downlink performance for two forms of adaptive OFDM–CDMA receivers in multipath fading channels: 1) MC–CDMA and 2) MC–direct sequence (DS)–CDMA. We propose theoretical minimum mean square errors (MMSEs) for MC–CDMA and MC–DS–CDMA to compare the performances in different situations. Moreover, one drawback of OFDM schemes is the rate reduction due to the cyclic prefix (CP) overhead, which significantly affects the channel utilization in MC–DS–CDMA systems. We propose adaptive receiver structures with both subcarrier- and code-interval equalizations to improve the performance of MC–DS–CDMA systems with shorter or no CP. Simulation results demonstrate the effectiveness of the proposed receivers and the correctness of the theoretical derivations.      

Design of Turbo-MUD Receivers for Overloaded CDMA Systems by Density Evolution Technique

This paper deals with a turbo multiuser detector suitable for applications in overloaded coded DS-CDMA systems. The turbo-MUD receiver is based on the use of a linear MMSE detector in the first iteration and a parallel interference cancellation scheme in the successive ones. The inputs of the interference cancellator are both the detector outputs and the soft information from a bank of turbo decoders. The performance of the proposed receiver has been derived by means of computer simulations and applications of the density evolution theory: in particular, this technique permits to properly evaluate the number of MMSE iterations, simplifying the overall receiver design.     

Atmospheric Optical CDMA Communication Systems via Optical Orthogonal Codes

We propose and consider using a class of multiple-access sequences, namely, optical orthogonal codes (OOCs) in atmospheric optical code-division multiple-access systems. We obtain analytical solutions to the error probability for various channel models using positive-intrinsic-negative diode and avalanche photodiode photodetectors. In our analysis, the effects of atmospheric turbulence, ambient light, thermal noise, and multiuser interference are considered, in the context of a semiclassical photon-counting approach. The performance of the systems taking advantage of space diversity and error-correcting codes are also evaluated. Two common and widely used optical modulations, on–off keying and pulse-position modulation, are considered. Receiver structures based on correlator and chip level are used for OOC detection. Unlike the traditional chip-level receiver, here a generalized form of chip-level structure with two threshold levels is considered. Upper and lower bounds on the error probability for the above-chip-level receiver structure is obtained. From our analytical results, we can deduce that the chip-level receiver outperforms a simple correlator in the absence or weak atmospheric fading; however, in a strong fading environment, the simple correlator outperforms the chip-level receiver.     

Strategies for Dense Optical CDMA Communication Systems

1　Introduction　Recently, considerable interest has been focused on Opti cal Code Division Multiple Access (OCDMA)[1~15], be cause of the advantages of flexible bandwidth management,asynchronous access, and the potential network scalabilityetc. In an OCDMA system, many users can simultaneouslyshare the same channel through the allocation of specific ad dress codes. All users signals are transmitted by the sameoptical fiber channel. At the receiver end, the matched de coder can re…     

Blind Schemes for Asynchronous CDMA Systems on Dispersive MIMO Channels

The problem of blind detection in a dispersive multiple-input multiple-output (MIMO) code division multiple access (CDMA) channel is considered in this paper. Unlike previous studies, each user is assigned one spreading code to be employed on all of the transmit antennas, which poses a problem of data re-association at the receiver-end in the absence of prior information as to the channel state. Focusing on the differential Alamouti scheme, we propose a two-stage receive structure. The first stage performs a linear interference-blocking transformation, which allows user separation and inter-symbol interference (ISI) suppression. The second stage is, instead, a novel differential space-time block (STB) decoder suitable for frequency-selective channels. Interestingly, the proposed detector allows decoupling of the decisions on the transmitted symbols, while its blind implementation only requires a cubic (in the processing gain) complexity. A thorough performance assessment is undertaken to investigate, on one hand, the capability of acquiring the missing information, such as the system and the encoder timings, on the other hand, the interplay between the diversity gain provided by the MIMO structure of the communication system, and the additional co-channel interference that multiple transmit antennas produce in multi-path, multiple-access channels.     

All-Optical Asynchronous Detection for a Compact Integrable Incoherent Optical CDMA System

In this paper, we investigate all-optical truly asynchronous detection without global clocking in an incoherent optical code-division multiple-access (CDMA) system. The implemented system is designed with an integrable optical source consisting of an electro-absorption modulator for pulse carving, compact coders consisting of fiber Bragg grating arrays for encoding and decoding, and receiver consisting of an all-optical thresholder for data and clock recovery. We compare three detection schemes: (1) synchronous detection with data from a photodetector and clock from an external source; (2) asynchronous detection with data and clock from the all-optically thresholded signal received by a clock and data recovery (CDR) unit; and (3) asynchronous detection with data from a photodetector and clock extracted from the all-optically thresholded signal using CDR. Error-free transmission is obtained for detection schemes (1) and (3). A combination of all-optical thresholding and CDR technology is demonstrated in an optical CDMA system for the first time.     

Linear Interference Cancellation of Asynchronous CDMA Systems by Preconditioned CG Method

It is well known that the iterative method is used for solving a set of linear equations corresponding to linear interference cancellation structures. An ideal computation of the decorrelating or the linear minimum mean-squared-error (LMMSE) detector requires order K ³ flops, where K is the number of users. To alleviate the computational complexity, iterative decorrelating and the LMMSE detectors are proposed. In this paper, the symmetric successive over relaxation (SSOR) preconditioning scheme is applied to the conjugate gradient (CG) method. The performance of the detectors is investigated and it is found that the SSOR preconditioned CG (PCG) method can provide significantly faster convergence than CG method.