Finite-Sample Performance Analysis of Widely Linear Multiuser Receivers for DS-CDMA Systems

This paper tackles the theoretical performance analysis of widely linear (WL) multiuser receivers for direct-sequence code-division multiple-access (DS-CDMA) systems, as well as their comparison with conventional linear (L) ones. In particular, receivers based on the minimum output-energy (MOE) criterion are considered, since they offer a good tradeoff between performance and complexity and, moreover, lend to some simplifications in the analysis. After comparing the ideal signal-to-interference-plus-noise-ratio (SINR) performances of the WL-MOE and L-MOE receivers, the paper establishes finite-sample performance results for two typical data-estimated implementations. Specifically, by adopting a first-order perturbative approach, the SINR degradation of the data-estimated WL-MOE receivers is accurately evaluated and compared with that of its linear counterpart. Simulation results are provided to validate and complement the theoretical analysis.     

Fundamental Limits of Linear Equalizers: Diversity, Capacity, and Complexity

Linear equalizers (LEs) have been widely adopted for practical systems due to their low computational complexity. However, it is also well known that LEs provide inferior performance relative to a maximum-likelihood equalizer (MLE) or other near-MLEs, because LEs usually cannot collect the diversity order enabled by the transmitter and at the same time they lose mutual information. More importantly, unlike MLE or near-MLEs, the performance of LEs has not been well quantified. This hinders more general applications of LEs in wireless systems. In this paper, we reveal a fundamental link between a channel parameter-orthogonality deficiency (od) of the channel matrix-and the diversity and capacity of LEs. We identify that when the od of channel matrix has an upper bound strictly less than 1 , the same diversity order as that of MLEs is collected by LEs and the outage capacity loss relative to MLEs is also a constant over signal-to-noise ratio (SNR). These results can be applied to designing a framework for hybrid equalizers. Furthermore, by studying the statistical properties of the od and comparing the complexity of different equalizers, we show that hybrid equalizers can trade off between performance and complexity by tuning the channel matrix od. The theoretical analysis is corroborated by computer simulations.     

On the Rapid Prototyping of Equalizers for OFDM Systems

This article discusses hardware-oriented issues related to the compensation of channel distortions in packed-based mobile Orthogonal Frequency Division Multiplexing (OFDM) systems. The adopted evaluation approach relies upon Field-Programmable Gate Array (FPGA) prototyping. Depending on radio channel parameters and the modulation scheme, the required computation effort for the compensation of channel distortions can depict significant variations. In order to perform a trade-off analysis between complexity and performance, different compensation methods for channel distortions have been simulated within an OFDM simulation chain. Based on these results, hardware models have been created and prototyped onto a FPGA. The performance of the models with regard to the hardware efficiency is evaluated by integrating the prototyped components into the OFDM simulation chain. The hardware designs and simulations have been done according to the high-speed wireless LAN standard HiperLAN/2.     

Adaptive Multiplication-Free Transversal Equalizers with Application to Digital Radio Systems

Recently, multiplication-free digital filters have been proposed for different applications to digital radio systems and signal processing. The absence of multiplication is obtained by constraining each coefficient to be sum of powers of two. In this way, multipliers are substituted by shift registers and, if appropriate, some adders. The optimal power-of-two coefficients must be computed through a nonlinear optimization procedure. Until now the problem of adapting the power-of-two coefficients of base band transversal equalizers in the case of time-varying channels had not been considered. In this paper two algorithms are proposed to cope with this problem and are compared from the viewpoint of complexity, accuracy, and convergence speed. As an application example, transmission of a biphase PSK digital radio system over a multipath fading channel is considered.     

An Analysis of Automatic Equalizers for Orthogonally Multiplexed QAM Systems

A simple structure of a per-channel automatic equalizer for an orthogonally multiplexed QAM system is proposed and the performance is analyzed. It is shown that a dual automatic equalizer withT/2-long tapped delay lines, whereTis a period of individual baseband baud, can equalize not only the transmission channel distortions, but the timing deviations and demodulating carrier phase deviations. Despite its high efficiency, the orthogonally multiplexed QAM system has been regarded as overly costly. However, by using the dual automatic equalizer with fairly simple structure, the system is competitive with conventional single channel data transmission systems.     

DFT-S-OFDM系统发射端IQ失衡的广义迭代均衡器

该文研究了离散傅里叶变换扩展正交频分复用 (DFT-S-OFDM) 系统的零中频(ZIF)射频(RF)前端非理想特性导致同相与正交分路失衡(IQI)干扰时,改进的频域迭代均衡技术。利用广义正交原理和迭代均衡原理,本文推导了频域广义线性最小均方误差(WLMMSE)迭代均衡器,并通过矩阵变换降低其复杂度。仿真结果表明,在DFT-S-OFDM系统发射端IQ失衡的情况下,所提出的迭代均衡算法性能显著优于常规迭代均衡算法,而复杂度只有少量提高。     

A Unified Approach to Dynamic Length Algorithms for Adaptive Linear Equalizers

The number of taps is an important factor which affects nearly all the performance and complexity measures of adaptive equalizers. Recently, dynamic length algorithms for the adaptive finite impulse response (FIR) filters have attracted considerable attention and several methods have been proposed. Nevertheless, the relationship among these techniques is still unclear which makes the performance comparison remaining difficult. In this paper, a unified explanation of these techniques is presented which summarizes all proposed algorithms and provides a platform for their performance comparison. Moreover, a new type of dynamic length algorithms, the varying threshold dynamic length algorithm (VT-DLA) is proposed. Simulation results show that the performance of the new algorithm outperforms that of existing algorithms     

Performance Comparison of WDM Systems with Different Duty Cycle

The performances of nonlinear WDM systems with different duty cycle are compared by means of numerical simulation.The numerical results show that the optical pulse with duty cycle of 0.5 is superior to the conventional NRZ modulation schene.The conclusion is different from that of some references.The reason is that inter symbol interference is not included in some references.In fact,inter symbol interference plays an important role in monlinear WDM system.Although the larger the duty cycle is,the stronger the effect of the cross-phase modulation and self-phase modulation on nonlinear WDM is however,the larger the duty cycle is ,the stronger the inter symbol interference is.     

Applications of Linear Programming to the Time-Domain Design of Digital-Filter Equalizers

A time-domain technique is presented for the design of frequency-sampling and transversal digital filters for use in equalizing channels with known impulse responses. The technique uses a linearprogramming algorithm to specify filter multiplier coefficients that satisfy constraint equations based upon specified values and derivatives of the desired output signal. It is shown that the transversal equalizer design requires fewer multipliers and performs in a superior fashion in the presence of timing jitter. The effects of changing the number and type of constraints upon such parameters as equalizer complexity, transmitted energy, and residual distortion are presented.     

基于不同信道状态信息的MIMO中继系统收发信机设计

该文研究了在中继站已知完全信道信息或统计信道信息情况下的多输入多输出(MIMO)中继系统收发信机设计的问题。中继站利用获得的信道状态信息对接收信号进行滤波转发,接收节点采用线性最小均方误差接收机(MMSE)恢复出原始信号。仿真结果表明,所提中继转发策略性能优于传统的放大转发。在高信噪比条件下,基于部分信道状态信息反馈的设计方案可以获得与完全信道状态信息反馈接近的性能。     

基于宽线性迭代平均仿射LMS算法的DS-CDMA MAI抵消

基于宽线性处理可以改善线性多用户接收机的性能以及仿射和迭代平均可以加快算法收敛的思想,提出了一种宽线性迭代平均仿射LMS算法,分析了算法在均方意义下的收敛性,给出了算法稳定条件和失调公式,以及算法应用于码分多址系统多址干扰抵消的效果.     

线性时滞系统的时滞相关稳定性

本文讨论了线性时滞系统x(t)=Ax( ) Bx(t-τ)渐近稳定的时滞界限，利用Lyapunov—Krasovskii泛函及H∞范数理论，获得了线性时滞系统时滞相关稳定的充分条件，同时得到了一个新的时滞界限的代数估计式。将本文结果应用于实际系统，计算简单，并且扩大了时滞的稳定区域，降低了对时滞的保守性。分析结果还表明，线性时滞系统的稳定性可以转化为一定常无时滞的线性系统的H∞控制问题。     

Two Proposed Blind Equalizers Using Different Constellation Matched Error Functions for QAM Signals

Although Constant Modulus Algorithm (CMA) is effective to equalize non-minimum phase channels blindly, it suffers from residual intersymbol interference (ISI) and large Mean Square Error (MSE) when applied to higher order constellations (QAM). Methods based on cost function matched to the signal constellation namely alphabet matched algorithm (AMA) were previously reported and proves its superiority on CMA concerning the MSE. Thus dual mode algorithms between CMA and AMA were introduced. A hybrid technique combining CMA and AMA using a cosine square function as a constellation matched error (CME) was lately reported. In this paper two different CME functions are introduced. The MSE of the proposed algorithms are calculated using Matlab simulation under multipath slow fading channels for different signal to noise ratios (SNR) and different levels of QAM constellations. A comparison is established among them. Depicted results show the effectiveness of the two proposed CME functions. Hebat-Allah M. Mourad received her B.Sc., M.Sc. and Ph.D. degrees in electrical communication engineering from Cairo University, Egypt, in 1983, 1987 and 1994 respectively. Since 1983, she has been with the Department of Electronics and Communications, Faculty of Engineering, Cairo University, and is currently associate professor there. Her research interests include optical fiber communications, mobile and satellite communications.     

Linear Systems, Sparse Solutions, and Sudoku

In this paper, we show that Sudoku puzzles can be formulated and solved as a sparse linear system of equations. We begin by showing that the Sudoku ruleset can be expressed as an underdetermined linear system: ${mmb{Ax}}={mmb b}$, where ${mmb A}$ is of size $mtimes n$ and $n>m$. We then prove that the Sudoku solution is the sparsest solution of ${mmb{Ax}}={mmb b}$, which can be obtained by $l_{0}$ norm minimization, i.e. $minlimits_{mmb x}Vert{mmb x}Vert_{0}$ s.t. ${mmb{Ax}}={mmb b}$. Instead of this minimization problem, inspired by the sparse representation literature, we solve the much simpler linear programming problem of minimizing the $l_{1}$ norm of ${mmb x}$, i.e. $minlimits_{mmb x}Vert{mmb x}Vert_{1}$ s.t. ${mmb{Ax}}={mmb b}$, and show numerically that this approach solves representative Sudoku puzzles.      

State Estimation for Linear Constrained Systems with Unknown Input

This paper deals with the filtering problem for linear discrete constrained dynamic systems with unknown input. The constraint matrix and constraint vector in this system are allowed to vary in the value and in the dimension. The original full state is separated into two parts, and the estimate of the state is reduced to find the optimal estimate of a singular system. The estimable condition is researched and a recursive estimator for the original full state is presented. A rigorous mathematical induction is given to compare the performance of our approach to that of the existing method without constraint. A numerical example is presented to demonstrate the effectiveness of the new method.     

Periodic, Strict, and Quasi-Invariance in Linear Systems

This brief deals with shift invariance in linear systems (LSs), using a group theoretical approach. The group approach is necessary to obtain general definitions, which can be applied to continuous-time, discrete-time, multirate and also multidimensional LSs. Thus, a unique definition is given for periodic invariance and strict invariance (SI) and also for quasi-invariance (QI), a new concept, intermediate between periodic and SI. The importance of QI relies upon the fact that it is comprehensive of practically all multirate LSs     

基于Hammerstein模型的非线性信道广义线性盲均衡算法

为了提高非线性信道盲均衡的性能、降低运算复杂度,本文以Hammerstein模型代替传统的Volterra级数模型来模拟非线性信道,利用非线性信道接收信号呈现非圆性的特点,构造了一种新的基于Wiener非线性模型的广义线性盲均衡器,并在常模准则的基础上提出了NCWL-CMA和NCWL-CMA Newton-like两种非线性信道广义线性盲均衡器抽头系数更新算法.理论分析和仿真实验结果表明,与传统盲均衡算法相比,新算法显著地降低了剩余码间干扰,提高了收敛速度.     

Orthogonal Precoding for Systems with Spatial Multiplexing with a Linear Receiver

Journal of Communications Technology and Electronics - An approach is proposed for the development of algorithms for orthogonal precoding to achieve the effect of full diversity in communication...     

不同DTTB系统抗多径衰落的性能比较

介绍了数字电视地面传输(DTTB)标准中抗多径的方法以及ADTB-T系统在抗多径技术上的改进,通过截止率计算公式,仿真并比较了各系统的抗多径衰落的性能.