On Downlink Beamforming Methods: Capacity and Error Probability Performance

In space-division multiple access (SDMA), different beamforming or space-domain precoding techniques can be applied. We investigate two different space-domain precoding methods, the maximum capacity (MC) and the minimum mean square error (MMSE) precoders, for the downlink channel. It is shown that the MMSE precoding, which is practically implementable, can provide a reasonable performance in terms of the capacity and error probability, while the MC precoding is not practical (although it is optimum in terms of the capacity). Space-domain precoding methods are also applied to code-division multiple access (CDMA) systems.This work was supported by the HY-SDR Research Center at Hanyang University, Seoul, Korea, under the ITRC Program of MIC, Korea.Jinho Choi was born in Seoul, Korea. He recieved the B.E. degree (magna cum laude) in electronics engineering from Sogang University in 1989 and the M.S.E. and Ph.D. degree in electrical engineering from the Korea Advanced Institute of Science and Technology in 1991 and 1994, respectively. Currently he is a Senior Lecturer in the School of Electrical Engineering and Telecommunications,University of New South Wales, Australia. Dr. Choi received the 1999 Best Paper Award of Signal Processing from EURASIP.Seungwon Choi received the B.S. degree from Hanyang University, Seoul, Korea, in 1980 and the M.S. degree from Seoul National University, Seoul, in 1982, both the electronic engineering. He received the M.S. degree in computer engineering in 1985 and the Ph.D degree in electrical engineering in 1988 from Syracuse University, Syracuse, NY.From 1982 to 1984, he was with LG Electronics Co. Ltd., Seoul, where he helped developed the 8-mm camcorder system. From 1988 to 1989, he was with the Department of Electrical and Computer Engineering, Syracuse University, as an Assistant Professor. In 1989, he joined the Electronics and Telecommunications Research Institute, daejeon, Korea, where he developed the adaptive algorithm for real-time application in secure telephone systems. From 1990 to 1992, he was with yhe Communication Research Laboratory, Tokyo, Japan, as a science and Technology Agency Fellow, developing adaptive antenna array system and adaptive equalizing filters for applications in land-mobile communications. He joined Hanyang University, Seoul, in 1992 as an Assistant Professor. He is a Professor in the School of Electrical and Computer Engineering, Hanyang University. His research interests include digital communications and adaptive signal processing with a recent focus on the real-time implementation of smart antenna system for 3G mobile communication system.     

OFDM系统中基于线性预编码技术的均衡方法

在快速移动环境中,子载波间干扰(ICI)是恶化OFDM系统性能的重要因素。为抑制OFDM系统中的ICI,采用了一种基于线性预编码技术的迫零均衡方法,并对其在无冗余情形下的误码率(BER)性能进行了理论分析,推导出了BER最优的线性预编码矩阵的选取准则。仿真结果表明,在较高信噪比条件下,所用的方法能有效抑制ICI和显著改善OFDM系统的误码率性能。     

MIMO-OFDM系统基于线性预编码的自适应加载方案

在MIMO-OFDM无线通信系统中,采用基于子载波线性预编码设计和变步长加载算法得到一种优化的自适应加载算法,在完全已知信道状态前提下,能在满足固定速率和给定误比特率约束下使系统总发射功率达到最小。仿真结果表明,该算法复杂度比Capello算法低,并且性能与之近似,都远远优于非自适应MIMIO系统的性能。     

移动通信下行链路中的非线性预处理技术

首先介绍了经典的汤姆林森预编码技术,然后分别介绍了在其基础上发展起来的两种预处理技术——网格预编码和传输非线性迫零,并分析了其特点,指出非线性的预处理技术是性能和复杂度的折中,是很具潜力的一种下行链路技术。最后文中给出了一种新的预处理技术,同样具有较好的性能。     

PERFORMANCE OF THE ZERO FORCING PRECODING MIMO BROADCAST SYSTEMS WITH CHANNEL ESTIMATION ERRORS

In this paper, the effect of channel estimation errors upon the Zero Forcing (ZF) precoding Multiple Input Multiple Output Broadcast (MIMO BC) systems was studied. Based on the two kinds of Gaussian estimation error models, the performance analysis is conducted under different power allocation strategies. Analysis and simulation show that if the covariance of channel estimation errors is independent of the received Signal to Noise Ratio (SNR), imperfect channel knowledge deteriorates the sum capacity and the Bit Error Rate (BER) performance severely. However, under the situation of orthogonal training and the Minimum Mean Square Error (MMSE) channel estimation, the sum ca- pacity and BER performance are consistent with those of the perfect Channel State Information (CSI) with only a performance degradation.     

Pre-Compensation for Rayleigh Fading Channels in Time Division Duplex OFDM Systems

Coherent demodulation results in good detection performance but requires channel estimation. Fading pre-compensation (precoding) at the transmitter can lead to low-complex receiver structures with good performance capabilities, without the need for channel estimation. Time division duplex systems based on orthogonal frequency division multiplex (OFDM) are particularly suited to this because intersymbol interference effects can be neglected, simplifying transmitter adjustments. Methods that involve amplitude and/or phase pre-compensation are compared in terms of resulting bit error rate and increase in peak-to-average power ratio. Dynamic channels degrade the performance as the block lengths get longer. For a certain block length the performance degrades below that of traditional differential decoding. A block length of up to 40 times that used in the Digital European Cordless Telephone system, DECT, is possible using channel estimation and ideal Wiener prediction.     

基于迫零检测的分布式发射天线V-BLAST OFDM系统中的载波频偏补偿

 在采用分布式发射天线的垂直贝尔实验室分层空时结构(V-BLAST)正交频分复用(OFDM)系统中,由于各发射天线与接收机之间的载波频偏(CFO)均可能不同,传统系统中针对单个频偏的简单补偿方法不再适用。不同于已有的使用高复杂度矩阵求逆运算的线性频域均衡方法,本文利用迫零(ZF)检测的固有特性,提出一种适用于低时延扩展多径信道的无需矩阵求逆运算的载波频偏补偿方法。仿真结果表明,在ITU-R M.1225信道模型的三种低时延扩展测试环境下,使用本文所提频偏补偿方法进行频偏补偿能有效改善系统误符号率(SER)性能。     

大阻力液压支架柱窝结构设计与分析

对ZF21000/25/45D大阻力支架柱窝结构进行专业性设计,并根据Hertz接触模型算出载荷接触半径与分布规律,最后利用ANSYS进行静力学仿真,验证其结构设计的合理性。     

VBLAST系统的译码算法研究

对已有的传统译码算法如迫零算法（ZF）、最小均方（MMSE）算法、连续抵消（SIC）算法等的性能进行了研究,并在此基础上提出将迫零算法与连续抵消算法、最小均方算法与连续抵消算法相结合,构成迫零-连续抵消算法（ZF-SIC）与最小均方-连续抵消算法（MMSE-SIC）,从而明显改善系统的误码性能。此外,对收发两端采用不同天线数时的系统误码性能进行了仿真与分析,同时仿真分析了系统采用QPSK与16QAM调制方式的误码性能,最后给出仿真分析结果。     

异步协作通信中三协作方信道与延迟估计算法

无线通信是采用多节点协作分集方式发送信号的,故具有很大的优势,但缺点是所传输的信号是异步的,因此采用协作分集方式发送信号亟需消除这种异步的影响。为此,给出了块平坦衰落信道延迟估计算法,采用训练序列附加在信息码上作为循环前缀,通过循环前缀来获得已知符号,再利用已知符号来消除异步。结果表明,该方案完全能消除异步,信道估计达到了克拉默拉奥下限,误码率和MRC（最大比合并）方式相同,且带有完全的分集阶数,初步达到了消除信号异步影响的目的。