多址干扰对CDMA系统的影响及干扰抑制措施

对CDMA通信系统的容量进行了理论分析,并使用SPW(Signal Processing Worksystem)软件对CDMA系统的多址性能进行了仿真,表明了多址干扰对CDMA通信系统的影响。然后,分析了几种现有抑制多址干扰的方法的性能。     

干扰消除技术在CDMA网络中的应用

CDMA网络中存在前反向的多址干扰,降低了前反向容量。为了减少多址干扰对前反向容量的影响,对多址干扰产生的原因和机理进行深入分析,详细介绍了多用户检测技术和干扰消除技术。通过对造成干扰的所有用户信号信息进行检测,根据多址干扰信号的结构特征进行干扰消除和信号恢复,从而改善各个信号的信噪比,提高CDMA系统的前反向容量。重点介绍了准线性干扰消除技术QLIC（Quasi-Linear Interference Cancellation）和反向干扰消除技术RLIC（Reverse Link Interference Cancellation）的基本原理,分别对QLIC技术和RLIC技术的容量增益进行了理论推导,并计算了QLIC和RLIC对CDMA网络容量提升的程度。QLIC和RLIC是现有CDMA网络容量提升的重要技术手段,能够为运营商进行技术升级和网络部署提供重要参考。     

CDMA移动通信中的功率控制技术

移动通信的多址通信方式码分多址(CDMA)是一个自干扰系统,它对发射信号的功率控制在解决远近效应问题和增加系统容量方面是非常重要的。本文介绍了CDMA的系统模型和功率控制技术。     

码分多址（CDMA）系统MMSE-MUD算法研究

CDMA系统由于用户容量大、频谱利用率高等特点而优于其他多址方式,近年来在通信系统中得到越来越多的重视,如何克服多址干扰问题已成为提高CDMA系统容量的关键。而作为3G的关键技术之一的多用户检测技术,近年来取得了多个方面的发展。本文就TD—SCDMA系统的多用户检测技术（MUD）之一的最小均方误差（MMSE）技术进行了详细分析,并给出仿真结果。     

码分多址系统中的多用户检测

CDMA通信系统是一个自干扰受限系统，如何克服CDMA系统中的多址干扰问题已成为提高CDMA系统容量的关键。多用户检测是克服CDMA多址干扰最有潜力的关键技术之一。本文研究了CDMA移动通信系统中几种常见的多用户检测技术，分析了他们的原理，并对各种多用户检测的优缺点进行了比较。     

S──CDMA

Q-CDMA是一种窄带码分多址技术，W-CDMA则是一种宽带的码分多址技术，而且都是异步的码分多址技术；S-CDMA则是同步的码分多址技术。Q-CDMA、W-CDMA是异步码分多址技术，即每个用户发射的码道信号到基站的时间不是同时的，使码道方向非正交，有很大的码道间干扰，因此严重影响系统容量的进一步增加，同时系统的功率控制也很复杂。S—CDMANtJ应用扩频码的正交特性，大大降低同一射频信道中来自其它码道的多址干扰，使系统容量达到更大值（可比Q—CDMA系统容量高出4～5倍）。N此，S—CDMA又名正交码分多址，而且它的功率控…     

扩频码速率异步时直扩系统码分多址能力

直接序列扩频码分多址系统是目前应用最广泛的扩频系统,影响系统容量的主要因素是系统内多址信号的干扰。分析了采用Gold码的码速率异步直扩系统多址干扰,对扩展积分周期和单一积分周期均进行了数值仿真。仿真结果表明,码速率异步的Gold序列互相关函数不再具有三值特性;随着积分长度的扩展,互相关函数的统计值逐渐递减,即多址干扰减小;多址干扰与码偏移量大小有关,在某些特定码偏移量,积分长度为最小公倍数周期时,互相关函数接近为单值,而这些码偏移量正负对称出现;积分长度为单一周期时,多址干扰随码偏移量的变化趋势与扩频码长度无关;在相同码偏移量的情况下,随着扩频码的长度增加而减小。     

CDMA通信系统系多址干扰的仿真研究

码分多址(Code Division Multiple Access,CDMA)作为第三代移动通信系统关键技术,在移动通信领域备受人们的关注。它可以以不同的数据率和服务质量(Qo S)支持不同的通信业务。随着同时接入系统的用户数目的增加,多址干扰成为CDMA通信系统的一个主要干扰。文章利用Matlab提供的可视化仿真工具Simulink建立了CDMA通信系统仿真模型,对系统用户在码分多址机制下多用户信号同时传输的互干扰情况以及信号信噪比恶化情况进行仿真分析,验证抑制多址干扰的必要性。     

CDMA系统与GSM系统兼容性分析及计算

随着中国联通CDMA网络的开通，大量的CDMA基站及直放站被建设，系统覆盖日趋完善，其高效的频谱利用率及优良的语音通话质量，使CDMA迅速发展壮大起来。但是，CDMA系统与GSM系统的电磁环境兼容问题也日益突出，严重时导致系统无法工作。1 系统间干扰问题分析GSM、CDMA蜂窝通信系统不同的通信特点决定CDMA系统与GSM系统的电磁环境兼容问题主要是CDMA系统对GSM系统的干扰。CDMA是一种以扩频通信为基础的调制和多址连接技术。扩频通信技术在信号发端用一高速伪随机码与数字信号相乘，由于伪随机码的速率比数字信号的速率大…     

CDMA系统的导频干扰抵消

CDMA蜂窝移动通信系统是一种采用多址技术的通信系统,即采用不同的地址码来区分用户、基站和信道.然而由于其在多径衰落信道中的自相关和互相关特性的不理想造成了码道之间的多址干扰.在扩频通信系统中,有效地抵消导频信道引入的多址干扰是进一步改善系统性能和提高系统容量的重要途径.     

Performance analysis of MC‐CDMA by employing the MMSE‐PIC scheme in wireless communications

From the past decade, the multicarrier code division multiple access (MC‐CDMA) transmission schemes have placed major role in wireless communications. It is providing a secured wireless communication to the users with guaranteed performance. In many situations, the performance of the MC‐CDMA is restricted due to the interference caused by multiple access interference (MAI), which also influences the frameworks of CDMA. To overcome this issue, we concentrated on developing the efficient technique for data transmission with interference cancellation for downlink MC‐CDMA. In the proposed method, the interference cancellation procedure is done by using the regeneration and subtraction of MAI from the signal. The simulation results are evaluated using the MC‐CDMA system with different decision functions. Results proved that the proposed system is efficient in reducing the MAI along with an improved bit error rate (BER).     

基于子空间盲多用户检测算法的设计和仿真

随着数字移动通信技术的飞速发展,以码分多址(CDMA)为核心的第三代移动通信(3G)技术已经走入人们的日常生活。CDMA系统具有容量大、软容量、抗多径衰落强等优点,但多址干扰(MAI)的存在却严重的影响了系统的性能和容量,因此如何消除和抑制这种干扰就成了CDMA技术研究的热门话题之一,第三代移动通信系统已经将多用户检测技术(MUD)作为克服多址干扰的有效方法之一。文中主要研究了直接序列扩频码分多址(DS-CDMA)系统中的盲多用户检测算法,深入的研究了基于卡尔曼滤波的盲多用户检测算法,通过在卡尔曼滤波算法上引入子空间提高了原有卡尔曼滤波算法的效率。     

Minimum Cross Correlation Spreading Codes

Multiple access interference (MAI) appears in Code Division Multiple Access (CDMA) systems when the communication channel is a multi-path channel and the spreading codes are not orthogonal. Orthogonality between spreading codes cannot be maintained at the CDMA receiver because the codes may be asynchronous due to channel delay and multi-path spread. The receiver cannot perfectly separate the different signals of the multiple access users, and the resultant MAI limits the capacity of CDMA systems. MAI is a function of the cross correlation property between used spreading codes. In this paper we focus on the cross correlation of the spreading codes, we propose a method to find spreading codes with minimum magnitude of cross correlation. Employing these codes will reduce the resultant MAI in the CDMA system; hence it will increase the system capacity. A great enhancement is shown by comparing found minimum cross correlation spreading codes (MCCSC) with Hadamard and Gold codes.

CDMA系统中一种新的基于MMSE多用户检测的自适应功率控制算法

功率控制技术是CDMA通信系统中克服“远一近效应”，降低多址干扰、增大系统容量的一项关键技术。第三代移动通信系统对功率控制提出了新的研究课题。在本文提出CDMA系统中一种新的白适应功率控制和MMSE多用户检测的联合优化的算法，仿真结果表明，这种新的联合优化算法对提高系统容量，改善系统性能有很大的作用。     

多用户检测技术的应用与发展

第三代移动通信系统采用码分多址(CDMA)方式。由于CDMA系统是一个自干扰系统,存在多址干扰及远近效应,因而限制了系统的容量和性能。多用户检测技术能从根本上消除多址干扰,提高系统的性能。本文主要介绍了多用户检测的思想、分类和性能指标,还提出了多用户检测技术的新发展和在几个具体系统中的应用。     

Output MAI distributions of linear MMSE multiuser receivers inDS-CDMA systems

Multiple-access interference (MAI) in a code-division multiple-access (CDMA) system plays an important role in performance analysis and characterization of fundamental system limits. We study the behavior of the output MAI of the minimum mean-square error (MMSE) receiver employed in the uplink of a direct-sequence (DS)-CDMA system. We focus on imperfect power-controlled systems with random spreading, and establish that in a synchronous system (1) the output MAI of the MMSE receiver is asymptotically Gaussian, and (2) for almost every realization of the signatures and received powers, the conditional distribution of the output MAI converges weakly to the same Gaussian distribution as in the unconditional case. We also extend our study to asynchronous systems and establish the Gaussian nature of the output interference. These results indicate that in a large system the output interference is approximately Gaussian, and the performance of the MMSE receiver is robust to the randomness of the signatures and received powers. The Gaussianity justifies the use of single-user Gaussian codes for CDMA systems with linear MMSE receivers, and implies that from the viewpoints of detection and channel capacity, signal-to-interference ratio (SIR) is the key parameter that governs the performance of the MMSE receiver in a CDMA system     

Welch bound analysis on generic code division multiple access codes with interference free windows

Code-division multiple access (CDMA) technology has been applied to many wireless communication systems. CDMA system suffers from both multiple access interference (MAI) and inter-symbol interference (ISI) over a multipath channel. To suppress MAI and ISI, this paper proposes the spreading codes with interference free windows. In particular, we will develop several upper bounds on the efficiency of generalized spreading codes (for both unitary and complementary codes) in terms of the width of their interference free windows.     

MC–CDMA receiver design using recurrent neural networks for eliminating multiple access interference and nonlinear distortion

Multicarrier code division multiple access (MC–CDMA) is a promising wireless communication technology with high spectral efficiency and system performance. However, all multiple access techniques including MC–CDMA were most likely to have multiple access interference (MAI). So this paper mainly aims at designing a suitable receiver for MC–CDMA system to mitigate such MAI. The classical receivers like maximal ratio combining, minimum mean square error, and iterative block–decision feedback equalization fail to cancel MAI when the MC–CDMA is subjected to severe nonlinear distortions, which may occur due to saturated power amplifiers or arbitrary channel conditions. Being highly nonlinear structures, the neural network receivers such as multilayer perceptron and recurrent neural network could be better alternative for such a case. The feasibility, efficiency, and effectiveness of the proposed neural network receiver are studied thoroughly for MC–CDMA system under different nonlinear conditions.     

Blind multiuser channel estimation in asynchronous CDMA systems

In asynchronous code division multiple access (A-CDMA) systems transmitting over multipath channels, both intersymbol interference (ISI) as a result of interchip interference (ICI) and multiple access interference (MAI) cannot be easily eliminated. Although it is possible to design multiuser detectors that suppress MAI and ISI, these detectors often require explicit knowledge of at least the desired users' signature waveform. Recently, Liu and Xu (see Proc. 29th Asilomar Conf. Signals, Systems and Computers, 1996) introduced a blind estimation algorithm for synchronous CDMA (S-CDMA) systems to estimate the multiuser channels. However, this algorithm cannot be directly applied to an asynchronous CDMA (A-CDMA) system. In this paper, we study a similar blind estimation scheme that provides estimates of the multiuser channels by exploiting the structure information of the data output and the users' delays. In particular, we show that the subspace of the data matrix contains sufficient information for unique determination of channels and, hence, the signature waveforms. By utilizing antenna arrays, we extended our approach to overloaded systems, where the number of users may exceed the spreading factor