Blind Linear MMSE Receivers for MC-CDMA Systems

This paper studies blind constrained minimum output energy (CMOE)-based and subspace-based linear minimum mean-squared-error (LMMSE) detectors for multi-carrier code division multiple access (MC-CDMA) systems. By imposing quadratic weight constraint, the CMOE detector is made more robust against signature waveform mismatch, and a better performance over the standard CMOE detector is obtained. Because of separation of signal and noise subspaces, the more complicated subspace-based LMMSE detector has better performance than the CMOE detector. The recursive subspace tracking algorithms are also investigated for the subspace-based MMSE receiver. Numerical results show that the steady-state performance of the robust CMOE detector is close to the subspace-based MMSE method. The blind mode decision-directed LMMSE detection is studied where the blind detectors are used for initial adaptation. Numerical simulations illustrate that the blind mode decision-directed MMSE detection substantially improves the system performance when the frequency-selective channel is slowly-varying     

基于恒模算法的天线阵多载波码分多址系统空频盲多用户检测

多载波码分多址(MC-CDMA)可以对抗频率选择性,天线阵可以提高系统的性能.本文首先提出一种基于恒模算法的天线阵MC-CDMA系统空频盲自适应多用户检测,该算法具有MN维权向量,结构相对复杂;进而提出一种空频联合约束恒模算法的盲自适应多用户检测,可以保证算法具有鲁棒性,收敛于期望用户,该算法具有M N权向量,结构简单.仿真表明,本文提出的算法比传统的基于波束成形的两步算法具有更好的误码率性能.     

Joint Optimum Linear Precoding and Power Control Strategies for Downlink MC-CDMA Systems

In this paper we derive two iterative joint precoding and power allocation methods for downlink MC-CDMA systems with multiple transmit antennas at the base station. The first method optimally solves the so called assigned target SINRs problem, i.e. determines the precoding coefficients and the minimum sum-power allocation under a constraint on the SINR of each user. The second method instead solves the maximum sum-rate problem, i.e. maximizes the system throughput under a constraint on the overall transmit power. The main result of this paper is that the proposed joint precoding and power control solutions provide an advantage with respect to suboptimal solutions for a fully loaded system, i.e. when the number of users is equal to the spreading factor times the number of transmit antennas. For lower loads, performance of the various schemes becomes closer.     

MC-CDMA系统中的帧同步

简要介绍了多载波码分多址(MC-CDMA)系统,重点讨论MC-CDMA的帧同步问题.在介绍循环前缀(CP)同步算法的基础上,对基于CP的最大似然估计(ML)的似然函数进行了改进,改进方案大大降低了系统的复杂度,仿真表明性能优异,具有较高的实用价值.     

一种改进的线性受限共轭梯度常模盲多用户检测算法

提出了一种改进的线性受限共轭梯度常模算法(M-LCCGCMA),其核心是采用在最优自适应步长的方法对算法进行优化,并推导出步长的解析形式.将本文算法在加性白高斯和多径衰落信道的环境中进行了仿真.仿真结果表明,该算法的信干比性能和误码率性能均比现有的自适应步长常模算法要好.     

MC-CDMA系统中基于子空间的半盲信道估计

该文提出了MC-CDMA系统中一种基于子空间的半盲信道估计方法。该方法结合MC-CDMA系统信号模型的特点,通过初始信道估计来解决收敛速度慢的问题;通过部分用户的导频符号以及预均衡技术来估计模糊因子;通过迭代算法来降低计算复杂度。这些措施使得半盲信道估计方法的实用化成为可能。计算机仿真验证了该方法的有效性。     

Widely Linear Versus Linear Blind Multiuser Detection With Subspace-Based Channel Estimation: Finite Sample-Size Effects

In a recent paper [A. S. Cacciapuoti et al., “Finite-Sample Performance Analysis of Widely Linear Multiuser Receivers in DS-CDMA Systems, IEEE Transactions on Signal Processing, vol. 56, no. 4, pp. 1572–1588, Apr. 2008], we presented the finite-sample theoretical performance comparison between linear (L) and widely linear (WL) minimum output-energy (MOE) receivers for direct-sequence code-division multiple-access (DS-CDMA) systems, worked out under the assumption that the channel impulse response of the desired user is exactly known. The main scope of this paper is to extend such an analysis, taking into account not only autocorrelation matrix (ACM) estimation effects, but also the accuracy of subspace-based blind channel estimation (CE). We aim to answer the two following questions: Which of the two estimation processes (ACM or CE) is the main source of degradation when implementing the receivers on the basis of a finite sample-size? Compared with the L-MOE one, is the finite-sample WL-MOE receiver with blind CE capable of achieving the performance gains predicted by the theory? To this goal, simple and easily interpretable formulas are developed for the signal-to-interference-plus-noise ratio (SINR) at the output of the L- and WL-MOE receivers with blind CE, when they are implemented using either the sample ACM or its eigendecomposition. In addition, the derived formulas, which are validated by simulations, allow one to recognize and discuss interesting tradeoffs between the main parameters of the DS-CDMA system.      

MC-CDMA系统中自适应调制算法的性能分析

自适应调制能提高系统的频谱利用率和改善系统的误码率性能。该文提出了一种将自适应调制算法与MC-CDMA技术相结合的方案。采用这种方案的自适应MC-CDMA系统在获得频率分集增益和有效频谱利用率的同时,还能获得很好的误码率性能。文章分析了提出的自适应MC-CDMA系统的性能,并通过仿真结果说明了合并方式和信道环境都将影响自适应调制算法在MC-CDMA中应用的有效性。     

Transmission Performance of MC-CDMA Hybrid Fiber-Radio Systems

This paper reports an analysis of the transmission performance of a hybrid broadband access system consisting of a fiber optic feeder and a millimeter-wave radio channel in the last mile. It is also assumed that the system adopts the multi-carrier code division multiple access (MC-CDMA) technique, that allows effective radio resource utilization and does not require a strong equalization. The analysis reveals that the impact of non-linear effects, mostly due to the non-linear optical source and secondarily due to non-ideal RF power amplifier characteristics, could prevent the good performance of the system as a whole, and indicates which are the good operating conditions for the systems.     

一种新的MC-CDMA系统同步算法

多载波CDMA(MC-CDMA)技术结合了正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)和CDMA技术的特点,在无线衰落信道中传输高速数据时具有突出的优势.整个MC-CDMA系统中同步性能起着非常重要的作用.本文在分析现有各种同步算法性能的基础上提出了一种新的MC-CDMA同步算法,理论分析和仿真表明这种新算法具有更好的同步性能且具有更低的系统复杂度.     

Efficient Complex Sphere Decoding for MC-CDMA Systems

Maximum likelihood (ML) joint detection of multi-carrier code division multiple access (MC-CDMA) systems can be efficiently implemented with a sphere decoding (SD) algorithm. In this paper, we examine the application of complex instead of real SD to detect MC-CDMA, which solves many problems in a more elegant manner and extends SD adaptability to any constellation. We first propose a new complex SD algorithm whose efficiency is based on not requiring an estimate of the initial search radius but selecting the Babai point as the initial sphere radius instead; also, efficient strategies regarding sorting the list of possible lattice points are applied. Indeed, complex SD allows complex matrix operations which are faster than real counterparts in double dimension. Next, a novel lattice representation for the MC-CDMA system is introduced, which allows optimum multiuser detection directly from the received signal. This avoids noise whitening operation, and also despreading and equalization procedures are not required further at the receiver side     

Multiuser Detection in MC-CDMA Communication Systems Using MPEM Algorithm

In this paper, we propose a sub-optimum multiuser detection technique in multicarrier code division multiple access (MC-CDMA) communication systems based on missing parameter expectation maximization (MPEM) algorithm. In the proposed detection procedure, first initial values for the bits of all users are estimated from received signal. Then, the proposed MPEM based algorithm uses outputs of carries’ demodulators to improve the accuracy of the initial estimates of the bits. In this paper, the expectation and maximization steps’ functions of the MPEM algorithm for MC-CDMA multiuser detection are derived and the performance of the proposed algorithm is analyzed. Our presented numerical results demonstrate the efficiency of the proposed detection algorithm both in bit error rate performance and computational complexity points of view. Paeiz Azmi was born in Tehran, Iran, on April 17, 1974. He received the B.Sc., M.Sc., and Ph.D. degrees in electrical engineering from Sharif University of Technology (SUT), Tehran, Iran, in 1996, 1998, and 2002, respectively. Since September 2002, he has been with the Electrical and Computers Engineering Department of Tarbiat Modarres University, Tehran, Iran. From 1999 to 2001, he was with Advanced Communication Science Research Laboratory at Iran Telecommunication Research Center (ITRC), Tehran, Iran. From 2002 to 2005, he was with Signal Processing Research Group at ITRC. His current research interests include modulation and coding techniques, digital signal processing, and optical CDMA communications.     

一种MC-CDMA系统LS信道估计的改进算法

LS信道估计算法计算量小,实现简单,但是受噪声的影响信道估计精度低。为降低噪声对信道估计精度的影响,提出一种基于DFT的LS信道估计改进算法。该算法利用MC-CDMA中闲置的虚载波对信道噪声功率进行估计,通过将LS信道估计的时域响应限制在循环前缀长度内并进一步通过设置门限值忽略LS信道时域响应在循环前缀长度内的噪声分量和无效径响应,显著提高了信道估计精度。本文的算法运算量小,实现简单,信道估计精度高,仿真结果验证了算法的有效性。     

Transmission Strategies for MISO Downlink MC-CDMA Mobile Systems

This paper proposes two space-frequency schemes with a multi-user pre-filtering technique for downlink (DL) multicarrier code division multiple access (MC-CDMA) systems. We consider the use of antenna arrays at the base station (BS) and a single antenna at the mobile terminal (MT) and derive the proposed multi-user pre-filtering technique that modulates the transmitted signal to eliminate the effects of multiple access interference (MAI) and channel distortions at the mobile terminals, while maintaining low MT complexity. Two types of detectors are considered at the MT: simple despreading and single user equalizers. The performances of the proposed schemes are compared to those of other transmit signal design approaches that have been recently proposed for DL MC-CDMA, considering both typical indoor and pedestrian scenarios, and channel coding based on UMTS specifications.     

空时分组编码多载波码分多址系统的直接解码

针对宏蜂窝小区中基站使用均匀线阵空时分组编码多载波码分多址系统的上行物理链路,该文提出了一种无需信道估计,直接对多个用户的发射符号和波达方向进行盲估计的算法。该算法通过构筑蕴涵用户编码符号序列和波达方向的辅助矩阵,对其进行特征分解,得到了每个用户波达方向的闭式解。进而,利用空时分组码的正交结构以及发射符号的有限码集特性给出了所有活动用户原始符号序列的闭式估计。计算机仿真实验验证了该算法的有效性。     

线性约束差分恒模自适应阵列性能分析

线性约束差分恒模算法(LCDCM)能有效克服恒模算法(CMA)中存在的干扰捕获问题,但其性能受迭代步长的影响。本文对所需信号功率、干扰信号功率及所需信号和干扰信号空域相关系数对迭代步长取值和算法收敛速度的影响进行了分析,给出了保证算法收敛的迭代步长的取值范围。最后利用计算机仿真对上述理论结果进行了验证。     

CMA盲均衡在MIMO MC DS-CDMA系统中的应用研究

本文首先建立了采用TR-STBC发送分集的MIMO MC DS-CDMA系统模型,然后在此基础上,研究了SDCMA盲均衡在该系统中的应用,最后进行了仿真和分析.结论是对采用TR-STBC方案的MIMO MC DS-CDMA系统进行常模盲均衡,算法简单,较易实现,但当多址干扰较大时,仅采用此均衡方法不能较好地改善系统性能,还需要进一步采用其他方法来减小或消除多址干扰.     

多载波码分多址两跳放大转发中继系统的能量效率与频谱效率关系

本文研究了多载波码分多址两跳放大转发中继系统的能量效率与频谱效率关系,利用随机矩阵理论的渐近分析推导了系统分别采用协作式和分布式放大转发中继时,系统能量效率与频谱效率关系的显式表达式。分析了不同系统参数对能量效率的影响。此外,还分析了系统要使频谱效率最优和能量效率最优时使用系统资源的区别。最后,通过仿真验证了理论分析的有效性。结果表明,能量效率与负载因子和中继个数的关系不是单调的,存在一个使能量效率最优的负载因子和中继个数,此外,使能量效率最优的负载因子和中继个数总小于或等于使频谱效率最优的负载因子和中继个数。      

Neural Network Based Receiver for Multiuser Detection in MC-CDMA Systems

In this paper, we present a multiuser detection technique based on artificial neural network (NN) for synchronous multicarrier code division multiple access systems over Rayleigh fading channels. To test the robustness of the proposed receiver, also the effect of power control problem is studied with a comparative manner. Bit error rate (BER) performance of the NN based receiver is compared with the single user bound and conventional receivers. Although the BER performance of the conventional receiver degrades as the number of the users and power level differences among the users increase, as a decision structure, neural network based receiver gives closer BER performance to the single user bound.