Performance improvement in detection and estimation of MC-CDMA systems over MIMO channels

Multiple-input Multiple-output (MIMO) multi-carrier code division multiple access (MC-CDMA) is a strong candidate for the downlink of future mobile communications to obtain high data rates. Nevertheless, during any transmission over fading channel, performance of MC-CDMA systems are highly degraded due to the presence of multiple access interference (MAI). Multi-user detection (MUD) and channel estimation play a major role in overcoming MAI and characterising the channel, respectively. In this paper, space time serial interference cancellation (STSIC) detection using random and Gold codes and turbo aided iterative channel estimation (ICE) techniques are extended for MC-CDMA system MIMO channels to overcome MAI. Simulation results show STSIC outperforms optimal MUD and linear MUD techniques in mitigating MAI and turbo aided ICE surpasses ICE in characterising the channel with reduced error rates.     

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。     

基于改进信息最大化法的盲多用户检测

针对多用户检测中的多址干扰（MAI）,给出了改进信息最大化（I infomax）的盲多用户检测算法．该 算法利用扩频码初始化学习向量,实现了多用户的盲检测．仿真结果及分析表明,算法在误码率、收敛速度和用户 容量等方面优于Infomax 算法及传统盲多用户检测算法．     

Blind information-theoretic multiuser detection algorithms for DS-CDMA and WCDMA downlink systems

Code division multiple access (CDMA) is based on the spread-spectrum technology and is a dominant air interface for 2.5G, 3G, and future wireless networks. For the CDMA downlink, the transmitted CDMA signals from the base station (BS) propagate through a noisy multipath fading communication channel before arriving at the receiver of the user equipment/mobile station (UE/MS). Classical CDMA single-user detection (SUD) algorithms implemented in the UE/MS receiver do not provide the required performance for modern high data-rate applications. In contrast, multi-user detection (MUD) approaches require a lot of a priori information not available to the UE/MS. In this paper, three promising adaptive Riemannian contra-variant (or natural) gradient based user detection approaches, capable of handling the highly dynamic wireless environments, are proposed. The first approach, blind multiuser detection (BMUD), is the process of simultaneously estimating multiple symbol sequences associated with all the users in the downlink of a CDMA communication system using only the received wireless data and without any knowledge of the user spreading codes. This approach is applicable to CDMA systems with relatively short spreading codes but becomes impractical for systems using long spreading codes. We also propose two other adaptive approaches, namely, RAKE-blind source recovery (RAKE-BSR) and RAKE-principal component analysis (RAKE-PCA) that fuse an adaptive stage into a standard RAKE receiver. This adaptation results in robust user detection algorithms with performance exceeding the linear minimum mean squared error (LMMSE) detectors for both Direct Sequence CDMA (DS-CDMA) and wide-band CDMA (WCDMA) systems under conditions of congestion, imprecise channel estimation and unmodeled multiple access interference (MAI).     

基于强跟踪滤波器的盲自适应多用户检测

盲自适应多用户检测是码分多址(CDMA)系统中抑制多址干扰的一项重要技术.为克服传统的基于最小化输出能量准则的盲自适应多用户检测技术收敛速度慢的这一缺点,利用一种新近提出的多用户接收器的典范表示方式,构造了多用户检测系统的状态空间模型,再利用强跟踪滤波器(STF)理论,设计了一种新的盲自适应多用户检测方法.仿真结果表明,在异步CDMA系统中,与基于LMS算法、RLS算法及Kalman算法的盲自适应多用户检测相比, 方法具有收敛速度快速稳定, 跟踪性能好等优点.因此,基于强跟踪滤波器的盲自适应多用户检测是有效的.     

TD-SCDMA集群系统中的联合功率控制

TD-SCDMA集群系统中存在各种干扰,传统的功率控制算法无法消除来自集群群组间的用户干扰。采用了先用波束赋形进行主波瓣外干扰抑制,然后进行多群组联合检测来消除波瓣内群组间干扰,再联合功率控制的两步迭代算法来进一步消除集群中的干扰,从而降低了系统对功控的要求,提高了性能。仿真结果证明,该算法能有效降低系统发射功率和抑制干扰。     

Robust adaptive spread-spectrum receiver with neural netpreprocessing in non-Gaussian noise

Multiuser communications channels based on code division multiple access (CDMA) technique exhibit non-Gaussian statistics due to the presence of highly structured multiple access interference (MAI) and impulsive ambient noise. Linear adaptive interference suppression techniques are attractive for mitigating MAI under Gaussian noise. However, the Gaussian noise hypothesis has been found inadequate in many wireless channels characterized by impulsive disturbance. Linear finite impulse response (FIR) filters adapted with linear algorithms are limited by their structural formulation as a simple linear combiner with a hyperplanar decision boundary, which are extremely vulnerable to impulsive interference. This raises the issues of devising robust reception algorithms accounting at the design stage the non-Gaussian behavior of the interference. We propose a multiuser receiver that involves an adaptive nonlinear preprocessing front-end based on a multilayer perceptron neural network, which acts as a mechanism to reduce the influence of impulsive noise followed by a postprocessing stage using linear adaptive filters for MAI suppression. Theoretical arguments supported by promising simulation results suggest that the proposed receiver, which combines the relative merits of both nonlinear and linear signal processing, presents an effective approach for joint suppression of MAI and non-Gaussian ambient noise.     

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.     

Interference coordination based on random fractional spectrum reuse in femtocells toward Internet of Things

Femtocell is featured with a low-power and low-cost cellular access in indoor environments, and thus offers an effective yet flexible way to implement information exchange over Internet of Things. In femtocell networks, the dense deployment of home eNodeBs causes severe inter-cell interferences and imposes heavier load on the scarce frequency spectrum. In this paper, we propose an inter-femtocell interference coordination scheme to enable random and fractional reuse of frequency resources in a 3D in-building scenario. Specifically, we consider the regular femtocell deployment, where all the femtocells are divided into two groups and two neighboring femtocells will be classified into different group. Each group is initially allocated with a half of frequency resources. To more sufficiently utilize the spectrum, either group of femtocell is allowed to transmit over the frequency assigned to the other group of femtocell in a random way at the cost of introducing some interferences, i.e., reuse based on a specified probability. This probability is determined by maximizing the geometric mean of users’ average throughput, such that the fairness among users is guaranteed simultaneously. Moreover, an equivalent scheme generated from full frequency reuse between two femtocells groups is also given. Here, either group of femtocell will avoid transmitting over fractional frequency randomly with a certain probability and the interference to the adjacent femtocell can be reduced. The simulation results show that the proposed schemes could obtain the larger system average rate and edge user performance compared with the baseline schemes. Moreover, the geometrical average throughput per user achieved by our method is highest, and a more fair resources allocation can be realized .     

Neural network-based adaptive multiuser detection schemes in SDMA–OFDM system for wireless application

Neural network applications in adaptive multiuser detection (MUD) schemes are suggested here in the context of space division multiple access–orthogonal frequency division multiplexing system. In this paper, various neural network (NN) models like feed forward network (FFN), recurrent neural network (RNN) and radial basis function network (RBFN) are adopted for MUD. MUD using NN models outperforms other existing schemes like genetic algorithm--assisted minimum bit error rate (MBER) and minimum mean square error MUDs in terms of BER performance and convergence speed. Among these NN models, the FNN MUD performs efficiently as RNN in full load scenario, where the number of users is equal to number of receiving antennas. In overload scenario, where the number of users is more than the number of receiving antennas, the FNN MUD performs better than RNN MUD. Further, the RBFN MUD provides a significant enhancement in performance over FNN and RNN MUDs under both overload and full load scenarios because of its better classification ability due to Gaussian nonlinearity. Extensive simulation analysis considering Stanford University Interim channel models applied for fixed wireless applications shows improvement in convergence speed and BER performance of the proposed NN-based MUD algorithms.     

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.     

基于协作中继的多点广播码分多址信息系统设计

为了提高多群多点广播中继网络系统的性能,提出一种改进的协作中继多点广播码分多址(CDMA)系统设计。该方案利用协作中继辅助的分布式波束形成 实现单天线基站多播,获得了较好的空间分集增益。在该系统中,多个基站利用多个中继节点向多个目的地的各个小组传播消息;利用CDMA技术来减少中继节点以及目的地节点的多址干扰(MAI)障碍,同时每个中继节点作为线性预编码波束合成器,可以在合适的代码空间重塑基站信号;对线性波束形成矩阵进行优化,使得中继节点的功率最小化,从而满足QoS在信号干扰噪声比方面的要求。系统性能仿真对比实验的结果表明,提出的改进方案明显优于传统的正交复用方案(FDMA / TDMA)。     

Bursty traffic over CDMA: predictive MAI temporal structure, rate control and admission control

We study data communications for the downlink in code-division-multiple-access (CDMA) networks. The focus of this paper is to exploit the predictive temporal structure of the multi-access interference (MAI) for adaptive resource allocation, particularly for rate control and admission control. We also investigate the impact of fading and traffic burstiness on the system performance. We first present our result that when the input data traffic corresponds to multiple ON–OFF heavy tailed sources, the MAI process is asymptotically self-similar (with Hurst parameter 1/2<H<1). Thus the MAI has a non-trivial predictive temporal structure that enables accurate interference prediction. We exploit the predictive MAI structure to construct a multiple time-scale interference predictor. Rate adaptation is carried out based on the predicted interference level. Our numerical results show that this rate control scheme achieves significantly better performance than that using the packet-level MAI prediction only. We also devise a joint rate control and admission control scheme. To this end, we propose a sliding observation window scheme that has a two-tier flavor: Each observation window is divided into many time slots, rate control based on the interference prediction is conducted in each slot, and the corresponding throughput in the observation window is used for admission control accordingly.     

A medium access control protocol for voice and data integration in receiver-oriented DS-CDMA PCNs

Primarily used in military communications in the past, code division multiple access (CDMA) is recently found to be attractive for personal communications as well. As a large number of mobile hosts are supported within a cell and a wide range of services are provided, one of the most important issues in a CDMA personal communication network is how to control the uplink access to the shared wireless spectrum. In this paper, we address this issue in a realistic situation where the receiver-oriented transmission protocol is employed and the packet loss due to multiple access interference (MAI) cannot be ignored. A medium access control protocol for voice and data integration is proposed. It solves the problems of code assignment and MAI control at the same time. A Markov chain model is used to analyze the protocol and the analytical results are shown to be very close to simulations. Based on the modeling, the effectiveness of the protocol's MAI control is demonstrated and some system design issues are investigated.     

Adaptive GLR-, Rao- and Wald-based CFAR detectors for a subspace signal embedded in structured Gaussian interference

The problem of detecting a subspace signal embedded in subspace Gaussian interference and thermal noise is studied in this paper. In this problem, both the signal-independent and signal-dependent interferences are assumed to be present, therefore the overall interference subspace covers the signal subspace. The approach of this paper extends previous works involving either of those two kinds of interferences. A set of secondary data containing only interference plus noise is employed to estimate the interference covariance matrix and the noise power. Three new detectors are designed via the generalized likelihood ratio (GLR), Rao and Wald tests, respectively. Their probabilities of false alarms (PFAs) and detections are analytically derived. The PFAs show that the new detectors have the constant false alarm rate (CFAR) property against the interference and noise. Numerical results show that the new detectors outperform their counterparts for the studied problem. Furthermore, the new detectors are less sensitive to the secondary data size and to the mismatched subspace signal than some other detectors, such as the GLR detector (GLRD), the adaptive matched filter (AMF), the adaptive subspace detector (ASD), etc.     

Design of MC-CDMA receiver using radial basis function network to mitigate multiple access interference and nonlinear distortion

Multicarrier code division multiple access (MC-CDMA) is a novel 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 and minimum mean square error fail to cancel MAI when the MC-CDMA is subjected to nonlinear distortions, which may occur due to saturated power amplifiers or arbitrary channel conditions. Being highly nonlinear structures, the neural network (NN) receivers such as multilayer perceptron and radial basis function networks could be better alternative for such a case. The possibility NN receiver for a MC-CDMA system under different nonlinear conditions has been studied with respect to both performance and complexity analysis.

     

A memory interference model for regularly patterned multiple streamvector accesses

Most existing analytical models for memory interference generally assume random bank selection for each memory access. In vector computers, however, memory accesses are typically regularly patterned with a number of data items being accessed concurrently from different banks. Very little is known about the queueing behavior of memory interferences in multiple stream vector accesses. This paper presents an analytical model for memory interferences due to vector accesses in multiple vector processor systems. The model captures the effects of both bank conflicts among elements within one vector access stream and conflicts among multiple vector access streams on system performance. The model is based on a closed queueing network assuming an ideal interconnection network. An approximation technique is proposed to solve the memory queueing system that serves customers in a complicated way (non-FIFO). We also carry out extensive simulation experiments to study memory interference and validate our analytical model. Simulation results and analytical results are in a very good agreement, indicating that the model is very accurate. We further validate our analysis by comparing the numerical results obtained from our analytical model with those measurement results that were published by other researchers. Based on our analytical model and simulations, we carry out performance evaluation of the multiple vector processor systems. Our numerical results show that memory access conflicts pose a severe limitation on the number of useful processors in the system, implying that memory system design is essential to high-performance computing     

多址干扰抑制在CDMA系统中的仿真实现

通过运用SystemView工具,建立了具有多址干扰抑制能力的CDMA仿真系统,验证了抑制多址干扰的必要性和多用户检测的优越性,为更加深入的研究CDMA系统奠定了基础。     

基于线性预测的盲多用户检测技术的研究与应用

CDMA系统多用户检测技术(MUD)是第三代移动通信系统的关键技术之一.常规CDMA用户检测方法是使用期望用户的扩频序列对接收信号直接进行相关运算,再经过符号硬判决得到检测符号,这样的处理方式将解扩后的多址干扰成分看作噪声而未加任何处理,导致误码率增加.围绕自适应信号处理及盲多用户检测两方面技术,对CDMA系统的最佳盲多用户检测、次佳盲多用户检测进行研究,提出了一种具有工程实用价值的基于线性预测的盲多用户检测方法.仿真结果表明,在低SNR情况下,其运算复杂度大为降低、BER性能明显优于其他两种盲多用户算法.     

认知无线电中的干扰抑制合并算法

认知无线电中,认知用户允许接入授权频谱会对主用户造成严重干扰。为解决这一问题,结合多天线技术,提出一种干扰抑制算法。该方法不需要干扰信号的先验信息,可有效降低主用户与认知用户之间的干扰。从理论上推导了采用新算法后系统的SINR(信干噪比)表达式,给出了误码率下限。仿真结果表明,SIR(信干比)为0 dB、接收天线数目为2、BER为10-3时,干扰抑制合并算法性能优于传统的最大比合并算法6 dB。