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.     

时空编码WCDMA下行链路MMSE接收机

ＷＣＤＭＡ下行链路的性能受到信道衰落和多址干扰的影响。本文提出一种带时空编码的ＷＣＤＭＡ下行链路ＭＭＳＥ接收机,可以在减小信道衰落的同时达到抑制多址干扰的目的。应用高斯近似法对接收机性能进行了分析。仿真结果表明,该接收机能使系统性能得以较大提高。     

TDD OFDM系统中下行链路多址接入方式性能分析

对应用于OFDM系统下行链路的几种多址方式,TDMA、FDMA和CDMA进行了比较分析.研究表明,在系统用户数和速率相同时,采用CDMA多址方式的OFDM系统误比特率性能优于TDMA和FDMA.在时分双功(TDD)方式下,发射机已知信道增益时,最优子载波分配的FDMA OFDM系统性能优于其他多址技术.     

两种盲多用户检测常模算法的比较

CDMA作为移动通信一项基本技术,存在多用户干扰问题,多用户干扰也称为多址干扰,严重制约系统容量的提高。多用户检测目的在于抑制多用户干扰。本文研究的是CDMA下行链路的盲多用户接收技术,采用常模算法,针对不同的算法性能进行仿真。通过计算输出信干比和误码率来评价两种常模算法的干扰抑制能力。     

IDMA水声多用户下行通信链路迭代接收算法

水声通信网络的快速发展对多用户水声物理层通信技术提出了迫切的需求。本文主要针对水声多用户下行通信链路中存在严重码间干扰和多址干扰的问题,提出了一种交织多址（IDMA）水声多用户下行通信链路迭代接收算法,能够根据译码器输出的软信息对估计的信道进行迭代更新,同时根据信道估计结果重建各用户对当前用户的多址干扰（MAI）,并使用干扰抵消器予以抵消,通过在均衡器、译码器和干扰抵消器之间的迭代逐次提高当前用户信息的输出信噪比,最终实现正确解调。其中,信道估计算法采用LMMSE初始化的分簇OMP稀疏信道估计算法,多普勒估计采用时频二维搜索结合自相关相位估计的方法,计算机仿真结果表明该接收机具有与已知信道时接近或相当的性能。进行了多用户水声通信海上试验,在4.5km、6km和7km三个距离实现了对2个用户、3个用户和4个用户的同时通信,每个用户的通信净速率约为500bps,证明了本文所提出的方法能够有效应用在多用户水声通信中,为水声通信网络的发展提供了一种重要的物理层通信技术手段。      

基于差分空时分组码的DS-CDMA系统下行MOE自适应接收机

DS—CDMA下行链路的性能受到信道衰落和多址干扰的影响。本文提出了一种MOE多用户检测技术与差分空时分组码相结合的新方法。该方法不需要信道估计,并可在克服信道衰落的同时达到抑制多址干扰的目的。为适应无线信道的时变特性,基于阵列处理中广泛应用的分支干扰相消器结构,我们给出了该方法的RLS自适应实现形式。计算机仿真结果证实了本文提出方法的优良性能。     

平坦瑞利衰落环境下CDMA系统的多用户检测性能

在采用传统信号检测方式的CDMA无线通信网中,多址干扰限制了系统的容量和性能。多用户信号检测利用扩频码互相关信息在高斯信道中可以减小或消除多址干扰,缓解远近效应,增加系统容量。本文将多用户检测器直接用于平坦瑞利衰落的CDMA环境中,用分析与仿真相结合的方法讨论了典型多用户算法的误码性能。结果表明,平坦瑞利衰落信道中多用户检测算法的性能仍然优于传统检测算法。     

DS-CDMA系统下行盲多用户接收机

本文研究多径频率选择性衰落信道下，DS－CDMA系统下行盲多用户检测技术，提出了一种新的基于恒模算法的检测方法。利用下行信号的特征构造约束条件并采用投影梯度法求解优化问题，本文方法克服了常规恒模算法无法保证收敛到期望用户的缺点。仿真结果验证了本文方法具有良好的检测性能，可以在克服信道衰落的同时抑制多址干扰。     

MC-CDMA系统中并行干扰抵消的改进

在CDMA系统中,多址干扰是影响系统性能的主要因素。为此,人们提出了并行干扰抵消的检测方法。针对并行干扰抵消中存在的误判问题,本文应用模糊数学中的基本原理,在MC—CDMA系统模型中提出了一种新的部分并行干扰抵消算法——基于估计干信比的模糊并行干扰抵消,并对其原理作了简要阐述。     

自适应部分干扰抵消多用户接收机

在DS/CDMA系统中,采用多用户检测技术可以有效地克服多址干扰(MAI,MultipleaccessInterference)及远近效应。本文提出了一种自适应部分干扰抵消(APPIC,AdaptivePartialParallelInterferencecancellation)算法,根据匹配滤波器输出的判决统计对多址干扰进行选通抵消,以提高系统性能。同时本文给出了自适应部分干扰抵消接收机的非相干实现方案,并给出了仿真结果。     

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。     

多速率CDMA系统中多址干扰对系统容量的影响

第三代蜂窝移动通信系统能以不同的数据率和服务质量（Quality of Service,QoS）支持不同类型的通信业务。随着同时接入系统的用户数目的增加,多址干扰（MAI）也逐渐增加,不仅严重影响了CDMA通信系统的性能,也限制了系统容量的提高,且在异步传输方式下,多址干扰将更为严重。为此,首先在理论上分析了多址干扰对单速率CDMA系统容量的影响;然后,通过计算机仿真,讨论了异步多速率CDMA通信系统中多址干扰对系统容量的影响。     

Linear unbiased data estimation in mobile radio systems applyingCDMA

Data estimation in the uplink of a synchronous mobile radio system applying code-division multiple access (CDMA) is considered. In mobile radio systems applying CDMA, multipath propagation leads to intersymbol interference (ISI) and together with time variance, to cross interference between the signals of different users regardless of whether the user codes are chosen orthogonal or not. A linear unbiased data estimation algorithm is presented that eliminates both ISI and cross interference perfectly by jointly detecting the different user signals, leading to unbiased estimates of the transmitted data symbols. By theoretical analysis and simulation, the performance of the linear unbiased data estimation algorithm is examined under the assumption that the radio channel impulse responses are known at the receiver. The price to be paid for the interference elimination are SNR degradations, which are calculated for typical mobile radio situations in urban areas. The resulting average uncoded bit error probabilities lead to the conclusion that systems applying the linear unbiased data estimation algorithm are well suited for mobile radio applications     

Synchronous fiber-optic CDMA using hard-limiter and BCH codes

In this paper, synchronons code division multiple access (S/CDMA) for fiber-optic local area networks is considered. The performance of the fiber-optic S/CDMA network with negligible thermal and shot noises is interference limited. Here we derive the bit error rate of the S/CDMA system as a function of code length and number of active users, and the performance characteristics are also discussed. Furthermore, we analyze the performance of the fiber-optic S/CDMA system with an ideal optical hard-limiter, and the error probability with error control coding is also derived. In addition, the optimization between the S/CDMA and BCH codes of a constant bandwidth system is presented. The results show that by using an ideal hard-limiter in conjunction with BCH codes in this system, the influence of interference arising from other users can be greatly reduced, and the number of active users can also be increased significantly     

On Next Generation CDMA Technologies: The REAL Approach for Perfect Orthogonal Code Generation

All currently available code-division multiple-access (CDMA) technologies used in second-generation and third-generation mobile cellular systems are interference limited and can be appropriately called first-generation CDMA, whereas next-generation CDMA should provide a nearly interference-free performance. This paper addresses the issues on spreading code generation that is suitable for next-generation CDMA systems. The real environment adaptation linearization (REAL) approach is proposed to generate perfectly orthogonal complementary (POC) codes characterized by multiple access interference (MAI)-free and multipath interference (MI)-free operation. The REAL approach takes into account almost all major impairing factors in real applications, such as multipath propagation, asynchronous transmission, random data signs, and burst traffic, such that a CDMA system using them can offer an interference-resist operation. Two important conclusions are drawn in this paper: First, implementation of an interference-free CDMA will not be possible unless using complementary codes, such as the POC codes. Second, to enable interference-free CDMA, the flock size of the signature codes should preferably be equal to the set size of the codes. A fast algorithm to generate supercomplementary codes (a subset of POC codes) is also presented, and their ideal orthogonality is explicitly proven.      

Multicarrier CDMA for future generation mobile communication

Future generations of wireless communication systems will be designed with the aim of making the best possible use of the limited radio spectrum in order to further increase throughput as well as user-capacity. In this paper, the application of multicarrier CDMA (code division multiple access) within mobile communication systems is discussed. An overview of the main types of multicarrier spread-spectrum systems is given. The multicarrier CDMA system is described in detail, including its relationship with OFDM (orthogonal frequency division multiplexing), its general performance in comparison with direct sequence CDMA and some specific implementation problems. The paper also presents some results comparing the performance of a selection of algorithms that can be used to isolate the individual signal of each user. Potential capacity increases achievable with adaptive loading in time and frequency slots is briefly demonstrated. By allocating users multiple codes, very high data rates can be achieved, and the paper gives a comparison of multicode, multicarrier CDMA with OFDM-based wireless LAN systems     

CDMA／SDMA系统性能分析

空分多址接入技术（SDMA）可以通过信号不同的空间传播路径来区分用户，从而提高了蜂窝移动通信系统容量．而此技术又可以和其他多址方式相互兼容，比如频分多址，时分多址，码分多址．CDMA／SDMA系统就是码分多址和空分多址两种接入技术相结合的系统．该系统可通过时空编码实现的．文中分析此种系统的性能，并与多用户环境下的CDMA系统进行了比较．     

Spectrum Efficiency Enhancement in Dynamic Space Coded Multiple Access (DSCMA) System

In cellular mobile communication systems using coded modulations, the spectrum efficiency of the system is related to the number of available codes. Recently, large area synchronous (LAS) CDMA codes, which exhibit a region called interference free window (IFW) within some delay-spread, have been accepted as one of fourth-generation (4G) wireless communication systems. However, the number of synthesized LAS codes is very low due to the low duty ratio of the sequence. In this paper, a dynamic space coded multiple access (DSCMA) scheme which utilizes the spatial diversity from smart antenna system is proposed to overcome the low spectrum efficiency in LAS CDMA system. In the DSCMA a modified version of LAS codes called LAS even ternary (LAS-ET) codes is proposed. These codes are used together with a novel algorithm called dynamic space code (DSC), which will decrease the code length so that the spectrum efficiency can be increased. By taking advantage of dynamic code reuse assignment in spatial diversity, the spectrum efficiency of the DSCMA can be further increased significantly.     

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.