基于剩余数系统算法的映射序列扩频通信方式及特性

本文建议了一种基于剩余数系统的映射序列扩频通信（ＲＮＳＭＳ－ＳＳＭＡ）方式。在这种通信方式下，系统中任一用户根据剩余数系统算法选择３个被调制的扩频序列处理成映射序列进行扩频通信，能同时传送多比特信息。分析了信道中存在加性高斯白噪声和多用户干扰时系统的差错特性和多址能力     

正交码混合DS─—SFH扩频通信系统及性能

本文提出了一种正交码混合ＤＳ－ＳＦＨ扩频通信系统。分析了当信道中存在加性高斯白噪声和多址干扰时，系统采用随机签名直扩序列，无记忆跳频图案情况下，系统的差错特性。分析结果证明，在低信噪比时，混扩系统的差错概率小于纯直扩系统的差错概率；当信噪比较大时，和纯跳频系统相比，混合ＤＳ—ＳＦＨ扩频通信系统的特性好得多，因而可以大大弥补纯跳频系统多址能力差的缺点。     

正交码混合DS—SFH扩频通信系统及性能

本文提出了一种正交码混合ＤＳ－ＳＦＨ扩频通信系统，分析了当信道中存在加性高斯白噪声和多址干扰时，系统采用随机签名直扩序列，无记忆跳频图案情况下，系统的差错特性。分析结果证明，在低信噪比时，混扩系统的差错概率小于纯直扩系统的差错概率；当信噪比较大是地，和纯跳频系统相比，混合ＤＳ－ＳＦＨ扩频通信系统的特性好得多，因而可以大大弥补纯跳频系统多址能力差的缺点。     

直序扩频通信系统的研究与System View仿真

扩频通信系统与常规的通信系统相比,具有很强的抗干扰能力,并具有信息隐蔽、多址保密通信等特点,正从军事应用向民用通信发展。理论分析了直接序列扩频通信系统的原理,并用功能强大的仿真软件System View对直接序列扩频通信系统进行建模,仿真分析了系统的抗干扰特性等特点,对扩频通信系统的实现有着重要意义。     

新的混沌扩频序列抗多址性能的Simulink仿真

针对混沌扩频通信中提升混沌码性能的问题,通过重构Bernoulli映射相空间,得到一种改进的Bernoulli混沌映射,计算证明该映射具有弱结构特性,并在Simulink平台搭建了多用户混沌直接扩频通信系统的仿真模型,并与原Bernoulli混沌序列的抗多址性能做了比较。Simulink动态仿真结果表明,改进后的混沌扩频序列具有较好的抗多址干扰能力。     

虚拟式直接序列扩频通信电台的研究

文中提出的虚拟式直接序列扩频通信电台将直接序列扩频通信、软件无线电和虚拟技术相结合。使用虚拟技术，系统具有易于实现、开发快捷、性能改进灵活、容易与其它应用结合、有利于新标准的推广等优点。文中论述了虚拟式直接序列扩频通信电台原理，包括发射方、接收方的结构和多址通信的原理，并详细讨论了电台实现硬件和软件情况，最后进行了计算机模拟仿真，结果令人满意。     

AR自编码扩频系统中扩频序列的特性研究

在扩展频谱通信中,扩频序列的性能与系统的抗多址干扰、抗多径衰落的能力,和信号的低截获率概率有密切关系,也关系到捕获与同步系统的实现。扩频序列的特性对扩频系统的性能有重要影响。在自编码扩频通信系统中,自编码扩频序列的特性对于自编码扩频系统的性能同样非常重要。该文详细分析了采用AR滤波方式产生的扩频序列的扩频特性。计算机仿真说明,用AR滤波提取自编码扩频序列完全满足自编码扩频通信的要求。     

扩频通信技术浅谈

文中阐述了扩频通信系统的特点,分析了扩频通信可行性的理论基础,即信息论中关于信息容量的香农公式和柯捷尔尼可夫关于信息差错概率公式,指出了扩频通信的主要性能指标,给出了扩频通信的数学模型和物理模型,最后分析了几种典型的扩频方式(包括直接序列扩频、跳频扩频、跳时扩频和直扩/跳频混合扩频)及其应用领域。     

混沌扩频通信系统在多径Rayleigh衰落信道中的性能分析

本文研究了混沌直接扩频信号在多径Rayleigh衰落信道中传输时的抗多径干扰和多址干扰的特性。对于所给出的混沌扩频通信系统,当加入多径干扰和信道噪声时,给出了理论误码率与实际误码率的比较,数值结果表明在多径衰落信道中,混沌扩频通信系统抗多径干扰的性能很好;同时对混沌直接扩频信号的奇、偶互相关函数进行了计算,结果表明混沌扩频通信系统具有良好的抗多址干扰能力。     

CIRC编码在DS-SFH系统中的应用及性能分析

在Nakagami衰落信道的基础上,对采用DPSK调制方式的混合DS-SFH扩频通信系统进行了差错概率计算,分析了衰落参数对系统性能的影响。为了提高系统在低信噪比下的性能,又采用CIRC码对系统进行编码处理。理论和数值分析显示,CIRC编码作为一种能够有效对付突发差错的编码方式,可以有效地提高系统的抗多径干扰和多址干扰能力。     

Performance Analyses on Hybrid MQC/M-Sequence Coding Over Frequency/Spatial Optical CDMA System

A new two-dimensional frequency/spatial code for spectral-amplitude coding a optical code-division multiple-access system is proposed. The corresponding coder/decoder pairs are based on the tunable fiber-Bragg gratings cooperating with optical splitters/combiners, and multiple-access interference can be eliminated through the proposed decoding mechanism. For the performance analysis, the effects of phase-induced intensity noise, shot noise, and thermal noise are considered simultaneously. Bit-error rate (BER) performance is compared with that of the former system using M-matrix codes. It is shown that the system using these new code matrices not only maintains most advantages of the former one, but also allows larger number of active users under a given BER     

Optical OOK-CDMA and PPM-CDMA systems with turbo product codes

The application of turbo product codes (TPC) in intensity-modulated direct-detection optical code-division multiple-access (CDMA) systems with ON-OFF keying or binary pulse-position modulation is proposed in this study. Optical multiple-access interference caused by other users is the main source of noise that degrades system performance and limits the total number of active users in an optical CDMA system. In this work, the original turbo product decoding algorithm is modified according to the binary memoryless channel model. It is shown that the implementation of TPCs in an optical CDMA system provides significant improvement of the bit-error rate, hence, permitting a higher number of active users with optical orthogonal codes (OOC) of less weight. The proposed TPC has about 20% overhead, but the reduction in the weight of the OOC reduces the system bandwidth to less than half to that of the comparable uncoded system.     

Modified quadratic congruence codes for fiber Bragg-grating-basedspectral-amplitude-coding optical CDMA systems

We have constructed a series of new code families for the spectral-amplitude-coding optical code-division multiple-access (CDMA) system, and proposed new transmitter and receiver structures based on tunable chirped fiber Bragg gratings (FBGs). The proposed system has been analyzed by taking into account the effects of phase-induced intensity noise, shot noise, and thermal noise. We have also compared the performance of this system with that of a former system where a Hadamard code is used. It has been shown that the new code families can suppress the intensity noise effectively and improve the system performance significantly. When the effective power is large (i.e., >-10 dBm), the intensity noise is the main factor that limits the system performance. When the effective power is not sufficiently large, thermal and shot noise sources become the main limiting factors and the effect of thermal noise is much larger than that of shot noise     

An adaptive CMMSE receiver for space-time block-coded CDMA systems in frequency-selective fading channels

A technique that can suppress multiple-access interference (MAI) in space-time block-coded (STBC) multiple-input-multiple-output (MIMO) code-division multiple-access (CDMA) systems is developed. The proposed scheme, called a constrained minimum mean square error (CMMSE) receiver, is an extension of the CMMSE receiver for a single-input-single-output system to MIMO systems. It is shown that the complexity of the proposed CMMSE receiver is almost independent of the number of transmitter antennas. The advantage of the proposed receiver over the existing receivers for STBC CDMA systems is demonstrated by comparing the closed-form expressions of the signal-to-interference plus noise ratio and simulated bit error rates. The results indicate that the proposed CMMSE receiver can provide a significant performance improvement over the conventional receivers and that the gain achieved by suppressing the MAI can be larger than that from increasing the transmitter or receiver diversity.     

An Internally Coded TH/OCDMA Scheme for Fiber Optic Communication Systems and Its Performance Analysis—Part I: Using Optical Orthogonal Code

In this paper, a new internally coded time-hopping optical code-division multiple-access (TH/OCDMA) scheme for fiber-optic communication systems is proposed, and its multiple-access performance is evaluated using optical orthogonal codes (OOCs). In the proposed method, the duration of each bit is divided into N_s frames, each one containing L chips. Two signature codes, namely, one pseudonoise (PN) sequence and one OOC, are assigned to each user. During each bit interval, based on the output of an channel encoder and the user's dedicated PN sequence, one of the N_s frames is selected, in which W pulses are transmitted in W chips, marked by the user's OOC. We consider three detectors at the receiver front end, namely, correlation, soft chip-level, and hard chip-level detectors. We evaluate the multiple-access performance of the system for each detector considering the effects of shot noise, dark current, and thermal noise. We compare the results with those obtained for the conventional OCDMA systems. Our numerical results indicate that, for the same bit rate and bandwidth, our proposed method substantially outperforms the conventional OCDMA system. Our results also show that the soft chip-level detector outperforms the other detection techniques in all cases considered     

Optical CDMA random access protocols with and without pretransmission coordination

The link layer of an optical direct-detection code-division multiple-access (CDMA) packet network is considered. Two different protocols that need pretransmission coordination are proposed. A variation of the second protocol that does not need pretransmission coordination is discussed. Both system throughput and average packet delay are derived and investigated for two different receiver models: the correlation and chip-level receivers. Both multiple-access interference and the photodetector's shot noise are taken into account in the analysis. The case where the number of users exceeds the available number of CDMA codes is numerically investigated. Our results reveal that the proposed protocols yield competitive system throughputs when used with the correlation receivers. Further, significant improvement in the throughput is achieved when using chip-level receivers along with the second protocol.     

Robust Decorrelating Detector in Multi-Path Fading Under Impulsive Noise

In direct-sequence code-division multiple-access systems, impulsive noise leads to increased bit error rates and hence limits the performance. The main novel contribution of this paper is the improvement achieved on the decorrelating detector by the addition of a nonlinear clipper in order to eliminate impulsive components. The system is studied under multi-path fading with impulsive noise. It is assumed that the user signals have unequal powers (near/far effect) which make it challenging to determine the threshold in the clipper structure. These cases are studied and it is shown that the proposed robust receiver reduces the impact of impulsive noise by eliminating extreme amplitudes.     

Analysis of beat noise in coherent and incoherent time-spreading OCDMA

The effect of beat noise and other types of additive noise in time-spreading optical-code-division multiple-access (TS-OCDMA) networks is analyzed in this paper. By defining the coherent ratio kt, the ratio of the chip duration to the coherence time of the light source, TS-OCDMA systems are classified into incoherent, partially coherent, and coherent systems. The noise distributions and the bit-error rates are derived, and system performance is discussed for different cases. The performance of coherent systems is limited by the beat noise. With increasing kt, the effect of beat noise decreases in incoherent systems, and they eventually become free of beat noise. Possible solutions to the beat noise problem in coherent and partially coherent systems are also proposed and discussed.     

Coherent OCDMA system using DPSK data format with balanced detection

In this letter, the application of differential phase-shift keying data format in coherent optical code-division multiple-access (DPSK-OCDMA) has been proposed and investigated theoretically and experimentally to combat noise in the OCDMA system. The DPSK-OCDMA can also ease the receiver's threshold level setting and enhance the system confidentiality.     

Multiuser detection in impulsive channels

The present paper deals with the problem of data detection in direct-sequence code-division multiple-access (CDMA) systems with non-Gaussian ambient noise. This issue arises in pratical situations because many physical channels in which multiple-access communications is applied are known to be decidedly non-Gaussian, due largely to impulsive phenomena. (Such channels include urban and indoor radio channels, and underwater acoustic-modem channels). The optimum multiuser detector for the additive noise channel model is derived, and several suboptimal multiuser detectors are proposed. Moreover, for performance comparison purposes the robust multiuser detector, recently proposed in the literature, is also considered.