Impact of Mobility on CDMA Call Admission Control

Interference is the key parameter in a code-division multiple access (CDMA) network. As users move around, the interference will flow with the movement. The objective of this paper is to study the impact of the moving interference (mobility) on systems' performance and design parameters. In this paper, we develop a detailed CDMA interference model with soft handoff, power-control errors, and shadowing effect. We combine this interference model with a Markov model to study the performance of CDMA systems under various mobility and traffic conditions. We also present a call admission control (CAC) scheme that sets aside some interference cushion for handling mobility and compare this scheme with other CAC schemes in terms of the effectiveness of mitigating the negative impact of mobility on system capacity     

通用多载波CDMA无线传输体制及其研究进展

为实现高速率无线多媒体信息传输,近年来提出了一种新的无线传输体制通用多载波码分多址(GMC CDMA)。在GMC CDMA中,通过把信息符号流转换成符号块后,在符号块级冗余扩频,消除了多用户间干扰和符号间干扰,从而具有对信道衰落不敏感、频谱效率高等优点,提高了系统误比特率性能和容量,是新一代无线通信系统中极具发展前景的技术体制。该文综述了GMC CDMA的原理、设计、性能特点以及研究进展等,并讨论了几个热点研究问题。     

Scheme for enhancing performance of smart antennas in variableprocessing gain packet CDMA systems

Owing to the finite beamwidths of smart antenna systems, interference exists between signals belonging to different beams in CDMA systems. A new scheme is proposed for reducing the interference between the signals of smart antennas for variable processing gain packet CDMA systems. Numerical results show that the proposed scheme considerably improves the system performance     

Spread spectrum multiple access for mobile meteor burstcommunications

The paper considers the application of a direct sequence code division multiple access (CDMA) scheme to a mobile meteor burst communication network. It is shown that a CDMA scheme solves the significant multiple-access problem caused by low channel diversity when remote station nodes are closely spaced. The CDMA scheme is shown to provide low waiting times and high throughput for networks of many thousands of nodes. A comparison of the CDMA scheme to a random frequency division multiple access scheme is provided where the offered traffic load, network size, system bandwidth, and interference levels are varied     

Synchronous CDMA based on the cyclical translations of a CAZAC sequence

A new scheme of synchronous CDMA is introduced in the paper. The new scheme is based on a code made by all the cyclical translations of a basic sequence having constant amplitude and white discrete spectrum. Such a code is proposed here for the first time as a code for CDMA. According to the proposed scheme, a cyclic prefix is appended to the multiplexed signal. The proposed scheme has a property that none of the known CDMA schemes has: in a multipath environment, it allows multiuser interference to become cyclic intersymbol interference. Noticeably, the memory of the finite state machine that describes the ISI model is equal to memory of the multipath channel. The main advantage of our proposed scheme is that optimal and suboptimal detectors can be obtained from detectors proposed in the past for the ISI channel, which are much easier to implement than conventional multiuser detectors of classical CDMA schemes. Another advantage of our scheme is that it leads naturally to a signal processing architecture similar to that of OFDM systems, hence based on the efficient FFT/IFFT algorithm.     

Soft Linear Precoding for the Downlink of DS/CDMA Communication Systems

In this paper, we propose a transmitter-based linear-precoding scheme that outperforms conventional precoding by making use of a portion of the interference between the users in a code-division multiple-access (CDMA) system downlink. The utilization of part of the interference is achieved by selectively orthogonalizing the desired symbols to destructive interference by means of precoding while allowing interference that constructively contributes to the useful signal's energy. The existence and exploitation of constructive interference effectively spreads the signal constellation and enhances the signal-to-interference-plus-noise ratio (SINR) at the receiver. SINR improvement is attained with no need for additional power-per-user investment at the transmitter since energy that is inherent in the CDMA system is utilized. The scheme introduced in this paper applies to the downlink of cellular phase-shift keying (PSK)-based CDMA systems. Theoretical analysis and comparative simulations show that significant performance improvement can be attained with the proposed technique.      

海面的红外被动成像

本文在研究海面的红外成像特性中,建立了基于刚体的双尺度成像模型,从被动成像方面讨论了海面辐射特性的变化规律.该模型仿真结果很好地说明了风向对海面红外被动成像特性的影响.     

一种新的光码分多址技术

通常ＣＤＭＡ系统接近波分复用的性能,但是也面临一些有待解决的问题,例如多用户时的谱干扰、散弹噪声等。文中提出了一种实现光ＣＤＭＡ的新方案,它应用平衡发射和接收技术,同时使用Ｍａｃｈ－Ｚｅｈｎｄｅｒ干涉链来进行地址编码。按新方案实施可以在非相干调制光系统中得到地址码的真正正交,从而改善系统的性能     

低扩频比CDMA系统中符号间干扰抵消的Viterbi均衡算法研究

本文提出了一种低扩频比CDMA系统中Rake合并与软输出Viterbi均衡的联合算法以及相应的接收机结构。首先通过Rake接收得到符号序列,然后进行软输出Viterbi均衡,在计算复杂度增加不多的情况下,有效地克服蜂窝CDMA系统中传输高速数据业务时所存在的多径干扰和符号间干扰的影响,从而显著地提高CDMA接收机的性能。     

一种基于光纤布拉格光栅的新型多址干扰消除方案

在强度调制、直接探测的光码分多址(CDMA)通信系统中,为了有效克服并发用户数之间的多址干扰,提高系统的通信性能,根据组信息,针对由光正交码(OOC)和修正素数码(MPC)构成的扩时/跳频地址码MPC/OOC,提出一种基于光纤布拉格光栅(FBG)的多址干扰消除方案,并详细分析了采用这种多址干扰消除方案的光CDMA通信系统的性能。结果表明,这种基于光纤布拉格光栅的多址干扰消除方案,可以很好地消除光CDMA通信系统中的多址干扰,提高系统的用户数和误码率性能。     

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     

A CDMA scheme based on perfect Gaussian integer sequences

Both m-sequences and Gold sequences have been applied as the pseudonoise (PN) code in the code division multiple access (CDMA) scheme to enable distinct users to simultaneously share the available capacity of a common channel, which is called direct sequence CDMA (DS-CDMA). However, this scheme does not provide the required ideal periodic cross-correlation function property among various members of sequences within the same set. This paper proposes a new perfect Gaussian integer sequence (PGIS)-based CDMA scheme called PGIS-CDMA; in this scheme, the PN code of DS-CDMA is replaced by a set of PGISs. The intrinsic orthogonal property of PGIS can be applied to a CDMA system to achieve perfect cochannel interference separability under the ideal synchronism; it can also be adapted at the receiver to obtain diversity gain and improve the carry-to-interference ratio. These two concepts are further developed to implement two PGIS-CDMA configurations, and they can outperform DS-CDMA on the basis of Gold sequences that possess a favorable cross-correlation property for the CDMA scheme.     

Open-loop power control error in cellular CDMA overlay systems

Adaptive power control has widely been used in DS/CDMA systems to overcome the so-called “near-far” problem. This paper studies the adaptive open-loop power control of a cellular CDMA system, which is overlaid in the downlink by a narrowband signal. The effects of downlink power allocation schemes to power control error in the presence of narrowband interference are analyzed. In order to get a minimum power control error in the CDMA overlay situations, an optimum downlink power allocation scheme is used, which works well for a wide range of signal to narrowband interference ratio     

小扩频比的CDMA2000下行链路中Viterbi均衡算法的研究

针对CDMA2000—1X系统中存在的双小扩频比码道提出了Viterbi均衡和联合干扰抵消与Viterbi均衡两种算法，并比较了它们之间的性能差异。仿真结果表明，两种算法都能在计算复杂度增加不多的情况下，有效地克服蜂窝CDMA2000—1X系统中传输高速数据业务时所存在的多径干扰和多址干扰的影响，显著地提高了接收机的性能。     

基于CDMA技术的光接入网

提出了采用ＣＤＭＡ实现光纤搪入网的一般方案，讨论了光ＣＤＭＡ中的光扩频编码技术，信道间干扰的消除，功率控制等关键技术，并对光纤接入网的几种接入方法作了详细的分析比较。     

Analysis of a pipelined successive interference cancellation schemefor a DS/CDMA system

The bit error rate (BER) expression for a proposed pipelined successive interference cancellation (PSIC) scheme in a direct sequence/code division multiple access (DS/CDMA) system is derived analytically. The proposed PSIC scheme is a pipelined modification of a successive interference cancellation (SIC) scheme. The numerical results show that the proposed PSIC scheme outperforms the SIC scheme in viewpoints of the BER and the decoding delay performances     

同步CDMA系统中的MMSE检测器研究

分析了基于实域估计的最小均方误差(MMSE)检测器及其在同步CDMA系统的应用。首先对同步CDMA系统进行了描述,并在此基础上对这种新的MMSE检测器在同步CDMA系统中的设计和近似实现方法进行了探讨。     

A Novel Joint Detection of Frequency Offset and MAI for MC-CDMA Systems

While multicarrier CDMA techniques are used to reduce interference and improve the performance of the system in fading channel, carrier frequency offset and multiple access interference remain major obstacles for a multicarrier CDMA system's performance. In this article we propose a novel joint detection method called modified maximum likelihood multistage parallel interference canceller (M-ML-MPIC) to eliminate the frequency offset and MAI simultaneously. The main idea of our approach is to combine PIC multiuser detection and frequency offset estimation based on a maximum likelihood function with guard interval. We discuss our scheme and report on its performance using a set of simulation experiments.     

不完全信道状态信息条件下多用户Turbo-BLAST迭代检测算法

该文将空时多用户检测技术和Turbo-BLAST方案相结合,构造了基于CDMA技术的多用户Turbo- BLAST系统模型,提出了不完全信道状态信息条件下的解相关算法和迭代检测算法。在发送端将V-BLAST结构与CDMA技术相结合实现多路复用,在接收端利用空时多用户检测算法去除用户间干扰,得到期望用户的接收信号,然后采用考虑信道估计误差的软干扰抵消迭代检测算法,对期望用户的接收信号进行检测以去除天线间干扰。仿真结果说明,所提算法对于多用户Turbo-BLAST系统是有效的,可在不增加复杂度的条件下达到良好的迭代效果。     

Interference cancellation scheme with simple structure and betterperformance

The authors propose a new interference cancellation (IC) scheme which has a hybrid structure of the successive IC scheme and the parallel IC scheme. Based on computer simulation results, it is observed that the proposed IC scheme outperforms these two IC schemes for CDMA systems