Lagrangian乘子法在多用户检测中的应用

将二进制二次规划问题连续化为一个带等式约束的二次规划问题,借助于求解等式约束的Lagrangian乘子法,推导出Lagrangian乘子与用户信息码及信道噪声之间的关系.同时,根据多用户检测问题的特殊性,还给出了估计Lagrangian乘子的方法,进而得到了一种新的多用户检测器.仿真结果表明,新算法的误码率与最佳多用户检测器大致相同,低于一般的次最优多用户检测器.     

一种新的线性多用户检测器

量子信号处理是一种基于量子检测原理的信号处理框架.本文通过借鉴量子信号处理的思想,给出了一种新的线性多用户检测器:加权正交匹配滤波检测器.理论分析和仿真结果表明:新检测器只需要多用户系统中的扩频码已知,就可以在不同的工作环境下,通过灵活调整影响因子,分别实现与解相关检测器、匹配滤波检测器相同的性能;且新检测器的最佳性能与线性最小均方误差检测器的性能接近.     

采用Rao-Blackwellised粒子滤波的时变多用户检测

传统多用户检测方法通常假定接收方已知活跃用户数,其一般为这个系统所能容纳的最大用户个数.在此前提下,传统多用户检测方法能够获得较好的性能.然而在实际多址移动通信系统中活跃用户个数及其参数往往都是时变的,因此传统多用户检测方法性能恶化.针对这个问题,本文首先采用随机集理论( Random Set Theory,RST)建立多用户动态模型,基于此模型将信道分解为离散部分和连续部分,并通过分析两者的关系得到它们的状态转移概率;然后提出了采用Rao-Blackwellised粒子滤波(RBPF)算法的时变多用户检测器,实现了活跃用户数目变化和信道幅度变化的跟踪及用户发送数据估计;最后给出了算法在抗噪声能力、抗远近效应和系统容量等方面的仿真结果.仿真结果表明本文算法性能明显优于传统多用户检测方法.     

CDMA通信系统中的多用户检测

ＣＤＭＡ由于其用户容量大,抗多径衰落能力强以及便于实现软切换等特点而优于其他多址方式,近年来在移动通信系统中越来越得到重视。但是ＣＤＭＡ通信系统由于存在着严重的多址干扰而限制了其容量的进一步提高。多用户检测利用每个用户的信息对接收信号进行相关处理作统计判决或是进行干扰对消从而恢复出各个用户的发送序列,多用户检测方法可以使用最优多用户检测器、线性多用户检测器及非纡性多用户检测器等几类。     

基于改进FastICA的多用户检测

研究了直接序列扩频码分多址(DS-CDMA)通信系统的多用户检测问题,利用DS-CDMA信号中各用户扩频码和用户信息数据统计独立性建立了基于独立分量分析方法的多用户检测模型,通过对噪声作白化预处理,对FastICA算法的分离性能进行分析并给出精度修正矩阵,提出了一种改进FastICA的多用户检测算法.仿真结果表明,该方法较传统检测算法具有更好的检测性能、更强的抗多址干扰的能力和抗远近效应的性能.     

基于盲源分离的CDMA系统信息码与扩频码盲估计

针对码分多(CDMA)系统盲多用户检测问题,提出了一种基于盲源分离(BSS)的信息码盲检测与扩频码盲估计算法.该算法在对CDMA系统进行多用户检测的过程中,不仅不需要知道用户的扩频码,同时利用最小均方误差(MMSE) 则还可以盲估计出不同用户的扩频码序列.仿真结果证明该算法有较大的实用价值.     

基于子空间约束RLS的半盲多用户检测算法

考虑实际的MC-CDMA上行链路,深入研究了基于RLS算法实现的MOE(RLS-MOE)盲自适应多用户检测,提出了MC-CDMA系统下一种基于子空间约束RLS的半盲多用户检测算法.在MOE盲多用户检测的基础上,利用小区内用户的已知扩频码设计了一种MOE半盲多用户检测器.将子空间方法和RLS算法相结合提出一种基于子空间约束的RLS 算法,使用该算法自适应得到MOE的权向量.本文算法利用所有已知用户的扩频码抑制了小区内用户的干扰,子空间约束的RLS 算法降低了噪声的影响,从而改善了系统的性能.修正的PASTd算法实现了信号的自适应跟踪,大大降低了计算量.仿真实验表明,本文算法的输出信干噪比和误码率性能优于RLS-MOE盲多用户检测,更接近于最优值.     

基于免疫算法的智能多用户检测技术在CDMA中的应用

多用户检测技术是第3代移动通信系统CDMA中的一项关键技术.在多用户检测器中求解最佳矢量的问题可以转化为在免疫算法中求解具有最高适应度抗体的问题.文中用免疫算法来解决CDMA中的多用户检测问题.通过性能分析和计算机仿真,并与最佳多用户检测器和传统检测器相比较,发现基于免疫算法的多用户检测技术能较好地实现多用户检测的功能,这是一种切实可行的方案.     

同步DS-CDMA系统中的最优时空线性多用户检测器

本文给出了一种通用的时空二维线性多用户检测方法,它将所有线性多用户检测器归结到一个统一的数学模型之下.基于此模型,本文将时空线性多用户检测问题转化为一个权矢量的优化问题,并在线性约束最小方差(LCMV)准则下给出了权矢量的优化解.对现有的几种典型的线性多用户检测器的分析表明,这些方法都可以看作是本文提出的通用时空二维线性多用户检测方法的特例,且只能在某种限定条件下得到局部最优解,其抑制多址干扰和噪声的能力都低于文中的最优时空线性多用户检测器.最后,本文给出了数值仿真结果.     

BPSK通信系统的部分最优MIMO检测算法

在BPSK调制下,基于最大似然（Maximum Likelihood,ML）准则的MIMO检测器是一个二进制二次规划问题,其计算复杂度随着天线数的增多呈指数增加,当天线数较多时,其计算量太大,无法满足实时通信的要求。本文提出了一种新的MIMO检测算法。使用新算法,可以在很小的计算开销下,求解出ML检测器的部分全局最优解,然后,将优先检测出的部分最优解从原二进制二次规划问题中剔除得到一个相对小规模问题,最后使用传统的次最优检测算法对该小规模问题进行求解。这样,新算法不仅可以得到比传统的次最优检测器更低的误码率,计算量又远小于ML最优检测器。本文的仿真结果验证了新算法的有效性。      

Population Declining Ant Colony Optimization Multiuser Detection in Asynchronous CDMA Communications

In this paper we present a population declining ant colony optimization (PDACO) multiuser detector for asynchronous CDMA communications. Ant colony optimization (ACO) algorithms have already been used in multiuser detection in CDMA systems; however, as the pheromone accumulates, we may not get a global optimum because it stops searching early. PDACO can enlarge searching range through increasing the initial population of the ant colony, and the population declines in successive iterations. So, the performance of PDACO is superior with the same computational complexity. PDACO is applied to multiuser detection in asynchronous CDMA systems over slowly multipath Rayleigh-fading channels in this paper. Via computer simulations it is shown that the performance of PDACO detector is much better in bit-error rate and near-far effect resistance than conventional detector, ACO detector and genetic algorithm detector, and is close the optimal multiuser detector.     

Design of Large Scale MU-MIMO System with Joint Precoding and Detection Schemes for Beyond 5G Wireless Networks

The large scale multiuser multiple input multiple output (MU-MIMO) is one of the promising communication technology for 5G wireless networks as it offers reliability, high spectral efficiency and high throughput. The lattice reduction (LR) precoding based user level local likelihood ascent search (ULAS) detection scheme is proposed in this paper for efficient signal detection in large scale MU-MIMO system. The initial solution of ULAS algorithm is obtained from the LR precoding assisted zero forcing detector. The LR precoding transforms the non-orthogonal channel matrix into nearly orthogonal channel, which helps to mitigate inter antenna interference (IAI) exists at each user. The remaining multiuser interference (MUI) imposed to each user from undesired users is cancelled by the proposed ULAS multiuser detection scheme. Thus, the proposed LR precoding assisted ULAS mitigates both IAI and MUI unlike the classical detector, those try to moderate either IAI or MUI. By contrast, the proposed ULAS detector provides performance close to optimal maximum likelihood detector with just a fraction of its complexity.

     

异步CDMA系统中的CM多用户检测器

本文将恒模（CM）算法应用于异步CDMA系统的多用户信号检测。该CM检测器只要求了解目标用户的信息，运算较简单，且由于采用了自适应方法，检测器无需估计信号的幅度。分析及仿真结果均表明，CM检测器的误码性能和抗-近效应能力均优于传统检测器，且对于用户数目的变化有很好的适应性。     

Turbo greedy multiuser detection

Previously, a novel scheme for iterative multiuser detection and turbo decoding was proposed by Damnjanovic and Vojcic (2000, 2001). In this scheme, multiuser detection and single-user turbo decoding are tightly coupled to maximize the overall gain. The extrinsic probabilities for the coded bits of the interfering users, obtained after each turbo decoding iteration, are used as a priori probabilities in the following multiuser iteration and the extrinsic information for the systematic bits of the desired user is used as a priori information in the next single-user turbo decoding iteration. Turbo decoding of parallel concatenated convolutional codes is carried out in parallel fashion. It has been shown that the proposed detector approaches the multiuser capacity limit within 1 dB in the low signal-to-noise ratio region. However, the main drawback of the scheme is its exponential complexity in the number of users, which is due to the complexity of the maximum a posteriori probability (MAP) multiuser detector. In this paper, we show that the complexity of the scheme can be significantly reduced by replacing the (MAP) multiuser detector with an iterative detector derived from the greedy multiuser detector proposed by AlRustamani and Vojcic (2000). In this paper, we demonstrate that, for both the additive white Gaussian noise and the frequency-nonselective Rayleigh fading, the substantial reduction in complexity of the iterative scheme proposed by Damnjanovic and Vojcic when the greedy detector is utilized introduces a slight degradation in performance     

A low complexity multiuser detector for synchronous CDMA system

In a code-division multiple-access (CDMA) system, multiuser detection (MUD) can exploit the information of signals from other interfering users to increase the system capacity. However, the optimum MUD for CDMA systems requires the solution of an NP-hard combinatorial optimization problem. It is well known that the computational complexity of the optimum multiuser detector is exponential with the number of active users in the system. In this paper, we apply a hybrid algorithm to develop a suboptimal MUD strategy. The result of symmetric successive overrelaxation (SSOR) preconditioned conjugate-gradient method is first used to initialize the reduced-complexity recursive (RCR) multiuser detector. Then, RCR algorithm is applied to detect the received data bit by optimizing an objective function in relation with the linear system of decorrelating detector. Simulation results for the synchronous case show that the performance of our proposed SSOR-RCR multiuser detector is promising and outperform the decorrelator and linear minimum mean squared error multiuser detector with lower computational complexity.     

Linear Differentially Coherent Multiuser Detection for Multipath Channels

By combining multipath processing, differential signal detection, and multiuser detection techniques, we develop a class of near-far resistant linear detectors for differentially coherent multipath signals. We derive and establish performance relationships among the following detectors: an optimally near-far resistant detector, a suboptimum detector which does not require knowledge of the signal coordinates, and a minimum mean square error (MMSE) detector which achieves near-optimum asymptotic efficiency. We present an adaptive multiuser detector which converges to the MMSE detector without training sequences and which requires less information than the conventional single user rake receiver.     

Blind Multiuser Detection in Frequency Domain for MC-CDMA Systems Using Particle Swarm Optimization

In digital communication systems, the constant modulus algorithm (CMA) for blind multiuser detector has some disadvantage such as slow convergence speed and weight phase rotation. In this paper, we put forward a novel receiver, which combines CMA blind adaptive multiuser detection and particle swarm optimization (PSO) technique in frequency domain for multi-carrier direct sequence code division multiple access (MC DS-CDMA) systems. By this approach we present an adaptive multistage structure based on PSO technique. In this structure, the weights are obtained by PSO technique between the received signal and its estimate through a CMA. The resulting weights contain reliability information for the hard decisions made in the previous stage. The simulation results show that BER performance of the PSO-based CMA blind multiuser detector is superior to that of steepest decreasing constant modulus algorithm (SDCMA), linearly constrained constant modulus algorithm (LCCMA), CMA based multiuser detection and is worthy due to its significant detection performance to achieve desirable improvement in convergence speed and tractability.     

Randomly spread CDMA: asymptotics via statistical physics

This paper studies randomly spread code-division multiple access (CDMA) and multiuser detection in the large-system limit using the replica method developed in statistical physics. Arbitrary input distributions and flat fading are considered. A generic multiuser detector in the form of the posterior mean estimator is applied before single-user decoding. The generic detector can be particularized to the matched filter, decorrelator, linear minimum mean-square error (MMSE) detector, the jointly or the individually optimal detector, and others. It is found that the detection output for each user, although in general asymptotically non-Gaussian conditioned on the transmitted symbol, converges as the number of users go to infinity to a deterministic function of a "hidden" Gaussian statistic independent of the interferers. Thus, the multiuser channel can be decoupled: Each user experiences an equivalent single-user Gaussian channel, whose signal-to-noise ratio (SNR) suffers a degradation due to the multiple-access interference (MAI). The uncoded error performance (e.g., symbol error rate) and the mutual information can then be fully characterized using the degradation factor, also known as the multiuser efficiency, which can be obtained by solving a pair of coupled fixed-point equations identified in this paper. Based on a general linear vector channel model, the results are also applicable to multiple-input multiple-output (MIMO) channels such as in multiantenna systems.     

An effective multiuser receiver for DS/CDMA systems

Multiuser detection (MUD) is a key technique for combating multiple access interference (MAI) in CDMA systems. We propose a simple and effective multiuser receiver, which we refer to as the objective function based multiuser detector (OFMUD). This is a successive multistage joint detector based on the statistical optimization of an objective function. Two objective functions are considered in this paper. The first one is based on a maximum likelihood solution, and the second one is based on the least squared error solution. A key feature of the proposed detector is that among all the K user bits, we update the estimate of one and only one bit (the one that is most likely to be wrong) in every stage. Analytical and numerical results show that significant performance improvement can be achieved compared with the conventional DS/CDMA receiver. At the same time, the near-far problem is alleviated. In addition, it is shown that the proposed receiver can outperform the popular multiuser parallel interference cancellation receiver in most cases with a smaller computational cost     

Robust blind multiuser detection against signature waveform mismatch based on second-order cone programming

Blind signal detection in multiuser code division multiple access (CDMA) system is particularly attractive when only the desired user signature is known to a given receiver. A problem common to several existing blind multiuser CDMA detectors is that the detection performance is very sensitive to the signature waveform mismatch (SWM) which may be caused by channel distortion. In this paper, we consider the design of a blind multiuser CDMA detector that is robust to the SWM. We present a convex formulation for this problem by using the second-order cone (SOC) programming. The resulting SOC problem can be solved efficiently using the recently developed interior point methods. Computer simulations indicate that the performance of our new robust blind multiuser detector is superior to those of many existing methods.