多用户检测技术在DS-CDMA系统中的应用

码间干扰(ISI)、多址干扰(MAI)、以及系统中强信号对弱信号的抑制(远近效应)是CDMA系统存在的几类主要干扰,该文研究了多用户检测技术,它的提出为抑制多址干扰和克服远近效应开辟了一条新的途径。     

一种用于DS/CDMA系统的联合信道估计与多用户检测方案

在DS/CDMA系统中,多用户检测技术可以减小多址干扰和远近效应,但一般情况下,多用户检测总是假设接收机对信道特性已知。本文提出一种慢衰落信道中的有效的联合信道估计和干扰抵消方法,这种方法需要很少的导频符号来进行信道估计,而且实现简单,可以用在快衰落信道中。     

一种用于CDMA系统的信道估计与检测方案

在DS/CDMA系统中,多用户检测技术可以减小多址干扰和远近效应,但一般情况下,多用户检测总是假设接收机对信道特性已知。本文提出一种慢衰落信道中的有效的联合信道估计和干扰抵消方法,这种方法需要很少的导频符号来进行信道估计,而且实现简单,可以用在较快衰落信道中。     

基于UKF的CDMA系统信道参数估计

CDMA系统信道时间延迟估计是一个非线性的迭代过程。UKF算法能够避免EKF由于线性化非线性系统而带来的误差过大等问题,比EKF估计的更加精确。利用UKF算法对CDMA系统信道的幅度衰减参数与延时参数进行了估计。在研究中考虑到了多址干扰和远近效应对信道参数的影响,仿真结果表明UKF算法能有效地抑制远近效应及多址干扰,估计出无线信道参数。     

CDMA系统中多用户检测技术的分析

多用户检测技术是第三代移动通信系统的关键技术,具备良好的抗多址干扰和远近效应性能。从移动通信系统抗干扰的角度出发,分析了CDMA系统中多址干扰(MAI)的产生原因,对多用户检测(MUD)技术的概念系统模型以及常用的MUD算法进行了阐述,并指出其局限性和研究方向。     

最优化波形在MC—DS—CDMA系统中的应用研究

通过对MC—DS—CDMA系统中多址干扰（MAI）的研究,提出了运用最优化波形结合最优化载波间距来减小系统多址干扰的方法。理论分析结果表明,可以用多项式替代法来同时最优化波形和载波间距。仿真结果表明,该方法降低了系统中的多址干扰和误比特率,改善了MC—DS—CDMA系统的性能。     

一种新的混沌扩频序列优选算法

混沌序列因其相关性好、对初值敏感、保密性强等特性成为直序码分多址(DS/CDMA)系统中性能较好的一种扩频码。为了提高DS/CDMA系统保密性、降低多址干扰,改进混沌序列的传统优选算法,研究混沌互相关优选,提出一种新的混沌扩频序列互相关优选算法,对所得的优选序列进行性能仿真比较。结果表明,该算法能提供较大的系统容量且抗多址干扰能力强。     

CDMA系统中自动功率控制对系统容量的影响

CDMA系统的容量主要受限于系统内移动台的相互干扰,所以如果每个移动台的信号到达基站时都达到最小所需的信干比,系统容量将会达到最大值。在CDMA蜂窝系统中,为了解决远近效应的问题,同时避免对其他用户过大的干扰,必须采用严格的自动功率控制。功率控制的目的就是既维持高质量通信。又不对同一频率的其它用户产生不应有的多址干扰。功率控制的目的是控制每个用户的发射功率,而使得基站接收收到的每个用户的功率接近,从而克服远近效应。     

CDMA系统中的联合功率控制技术

功率控制和多用户检测是CDMA系统中克服远近效应、抑制多址干扰(MAI)的关键技术.将两者结合起来的联合功率控制技术具有很大的优越性,并有极大的研究潜力.重点综述了联合功率控制与线性多用户检测技术以及联合功率控制与非线性多用户检测技术的研究进展,并对今后的研究方向进行了概括性的描述.     

环形OCDMA系统中远近效应的研究

为了分析环形OCDMA系统性能.研究了OCDMA系统中的远近效应.首先分析了环形OCDMA系统中远近效应产生的原因,在考虑多址干扰和自环干扰的前提下,推导出在效应下系统的误码率(BER)表达式,并对远近效应对系统性能的影响进行了研究和仿真.仿真结果表明信噪比的提高可以减小远近效应,且系统中存在一个最佳接入用户数,当实际接入用户数大于最佳接入用户数时,系统误码率与耦合系数成反比,反之系统误码率与耦合系数则成正比.最后,给出两种有效减小远近效应的方法.     

水声CDMA多用户通信系统仿真研究

针对水声环境通信系统码间干扰和相位偏移问题是水下多用户通信面临的主要问题,水下有限的发射功率使接收机对远距离用户信号的接收性能较低。提出了一种水声码分多址多用户通信系统,系统采用多输入多输出(M IMO)判决反馈均衡器补偿码间干扰、抑制多址干扰,同时利用扩频增益提高接收性能并进行仿真验证。计算机仿真结果表明,当扩频码长度为15、输入信噪比为16dB、远距离用户信号功率比近距离用户低14dB时,接收机能够对各用户信号无误码地接收,证明了系统的有效性。     

QR分解卡尔曼盲自适应多用户检测算法

码分多址（Code-Division Multiple-Access,CDMA）系统中,采用盲多用户检测算法抑制多址干扰（Multiple Access Interference,MAI）和“远-近”效应（Near-Far effect）的影响,是CDMA技术研究热点之一。为避免基于卡尔曼（Kalman）滤波的多用户检测算法中,协方差矩阵失去非负定性而面临的数值稳定性问题,提出利用QR分解方法重新构造系统模型协方差阵,建立一种收敛速度更快、数值鲁棒性更好的卡尔曼盲自适应多用户检测算法。仿真结果表明,该算法具有较好的抗多址干扰和抗“远-近”效应能力。     

Adaptive MMSE decision feedback equalizer for asynchronous CDMA with erasure algorithm

Adaptive decision feedback equalizer with erasure algorithm (E-DFE) is proposed for asynchronous code division multiple access (CDMA) transmission, which not only combats intersymbol interference (ISI) and multiple access interference (MAI) but also reduces the effects of error propagation in the presence of Gaussian background noise. To reduce the possibility of feeding back the wrong decisions, the output of the feedforward filter of the E-DFE is processed before it is fed back to the feedback filter. Specifically, the focus is on the performance of E-DFE using soft-slicer based on a novel erasure algorithm. In addition, the fully connected feedback filter of E-DFE has been used to eliminate ISI due to other active users. Comparison of the performance of conventional decision feedback equalizer (C-DFE) and E-DFE over slowly varying frequency selective fading channel is presented to show the advantages of E-DFE in terms of reduced average bit error rate performance. Simulation results are also presented to show the substantial improvement in its performance under near-far situations and the sudden change in the signal power of the desired user.     

Adaptive multiuser receiver with joint channel and time delay estimation of CDMA signals based on the square-root unscented filter

The paper discusses an adaptive multiuser receiver for CDMA systems in which the scaled unscented filter (SUF) and the square root unscented filter (SURF) are used for joint estimation and tracking of the code delays and multipath coefficients of the received CDMA signals. The proposed channel estimators are more near-far resistant than the conventional extended Kalman filter (EKF) and present lower complexity than the conventional particle filter (PF) based methods. To present meaningful performance measures, the modified Cramer–Rao lower bound (CRLB) and computational complexity metrics are derived for the proposed and existing channel estimators. Computer simulation results demonstrate the superior performance of the proposed channel estimators. The proposed estimators are also shown to exhibit lower complexity relative to the PF.     

An adaptive least squares channel estimation based on conjugate gradient approach for multipath fading CDMA systems

Multiple access interference (MAI) is the main factor affecting the performance of channel estimation techniques for code division multiple access (CDMA) systems. Although, several multi-user channel estimation algorithms have been proposed to mitigate MAI, these algorithms require high computational complexities. In this paper, we address the problem of iterative least squares (LS) mobile channel estimation at high channel efficiency that requires a short training sequence along with the spreading sequences. We employ an efficient iterative method based on conjugate gradient (CG) algorithm to reduce the computational complexity of the estimation method. Computer simulations illustrate that the proposed method performs almost identical to the exact LS estimate for reasonable training lengths.     

Performance evaluation of cyclic prefix CDMA systems with frequency domain interference cancellation

Direct-sequence code division multiple access (DS/CDMA) systems transmitting over multipath channels suffer from intersymbol interference (ISI) and multiple access interference (MAI). Recently, DS/CDMA with frequency domain equalization (FDE) has attracted much attention for its ability to obtain an excellent performance even in strong frequency selective fading channels. However, the presence of residual interference after the FDE degrades the orthogonality among the spreading codes and hence the BER performance deteriorates. Parallel interference cancellation (PIC) is an effective method to suppress the MAI and improve the CDMA system capacity. In this paper, we propose three interference cancellation architectures for downlink cyclic prefix CDMA systems. The common thread between these three architectures is their implementation in frequency domain. A comparison between such architectures is presented. Simulation results show that the combination of PIC and FDE provides an efficient solution to suppress the MAI in downlink CDMA systems over frequency selective fading channels.     

伪卫星的远近效应研究

介绍了全球卫星导航系统及其伪卫星在近些年的部署情况和应用,对伪卫星的远近效应进行了深入的研究。用频分多址、码分多址、时分多址结合伪卫星的特点来解决其远近效应问题。得出实际应用中,在多颗伪卫星的复杂应用场合,最好的解决方案应该是时域、频域和码域的技术组合。     

CDMA系统中一种基于代价函数的联合功率控制算法

功率控制技术是CDMA系统中克服远-近效应、降低多址干扰、增大系统容量的一项关键技术。首先比较了传统功率控制和联合功率控制算法,然后提出了一种基于联合系数的功率控制算法,该算法结合了功率控制技术和MM SE多用户检测技术。仿真结果表明,采用提出的功率控制算法,在系统内发射总功率一定的条件下,可以使各用户获得更高的信干比服务质量,系统容量也具有进一步提高的潜力。