基于SFBC的MIMO-OFDM在频率选择性信道中的应用

针对于多径时延引起频率选择性衰落信道，在MIMO-OFDM的基础上采用了一种空频分组编码（space-frequency block coder——SFBC）技术。空频分组编码是在OFDM符号基础上，对一个OFDM符号中的子载波进行分组编码，使得同一帧上子载波在不同的天线上发送。现给出了SFBC编码及其解码算法，并在不同多径时延衰落条件下进行了仿真，验证了SFBC对于频率选择性衰落系统性能的改善。     

空时分组编码技术在OFDM系统中的应用

介绍了空时分组编码，正交频分复用原理以及STBC—OFDM的一般系统模型。阐述了STBC—OFDM系统的性能和当前对STBC—OFDM技术研究取得的进展，最后时STBC—OFDM技术在未来移动通信中的研究方向做了前景展望。     

基于STBC的OFDM系统

正交频分复用 (OFDM )系统因其良好的抗选择性衰落信道特性 ,在无线通信领域越来越受到关注。为了提高其性能和容量 ,本文提出将空时分组编码(STBC)这一分集新技术引入OFDM系统的方案。文章首先阐述STBC的原理及优势 ,再结合OFDM系统的特点 ,详细分析了STBC -OFDM系统模型。     

OFDM-MIMO系统中发射分集和信道估计技术研究

文章研究了OFDM-MIMO系统中的循环延迟分集（CDD）,以及基于Alamouti的空时分组编码（STBC）与空频分组编码（SFBC）,针对发射分集方案所必须知道的信道信息,对两种信道估计方法,即最小二乘（LS）＋线性插值与最小均方误差（MMSE）插值进行了研究。通过链路仿真还给出了以上分集方案在不同信道条件下的性能曲线,此外,对两种信道估计方法的误帧率做了统计。结果表明,CDD结合信道编译码可获得分集增益,有效对抗无线信道的衰落性。STBC,SFBC在不需要外码的情况下便可获得较大分集增益,且分别适用于频率选择性衰落与时间选择性衰落信道。而MMSE估计可以获得较为理想的误帧率,为分集方案提供较准确的信道参数以提高系统的可靠性。     

频率选择性衰落信道下STBC—DFDM系统的性能

本文将基于平衰落信道提出的空时块编码（STBC）方案应用到OFDM系统中,理论分析和仿真证明在理想信道估计下这种STBC-OFDM系统在频率选择性信道下能够获得对应的STBC在平衰落信道下的性能。     

频率选择性衰落信道下STBC-OFDM系统的性能

本文将基于平衰落信道提出的空时块编码(STBC)方案应用到OFDM系统中.理论分析和仿真证明在理想信道估计下这种STBC-OFDM系统在频率选择性信道下能够获得对应的STBC在平衰落信道下的性能.     

空时/频编码在OFDM系统中的应用

介绍了空时编码(STBC)和空频编码(SFBC)应用于OFDM系统,实现发射分集,并对基于梳状导频的LS(最小二乘)算法采用正交导频设计方案,把多天线信道估计转化为单天线信道估计,大大简化了运算。仿真结果表明,实现的空时和空频编码OFDM方案不仅误码性能良好,且具有系统实现复杂度低、接收机结构简单等优点。     

基于LabVIEW的STBC-OFDM通信系统设计

结合LabVIEW所具有的图形化编程和交互式可控制界面的特点,采用层次化,模块化的设计方案,针对MIMO—OFDM系统进行模块化设计。在802．16d信道模型基础上,构建了一个典型的STBC—OFDM基带系统仿真模型与框架,并进行系统仿真和性能分析,给出了系统性能的基本仿真结论。该系统设计界面直观,数据流程清晰,并具有可扩展性,可用于对STBC—OFDM技术进行深入研究,同时为进一步优化系统算法打下了坚实基础。     

MTMO—OFDM系统中空时／频码的性能分析

MIMO（多输入多输出）和OFDM（正交频分复用）技术的结合可以实现高数据传送速率并具有强的可靠性。文中分别研究基于STBC（空时分组编码）与SFBC（空频分组编码）的MIMO-OFDM系统和在不同的车载速度和时延扩展变化时的误比特率变化情况,结果表明：在不同环境中只有选择合适的系统参数才能发挥系统的最佳性能。     

瑞利快衰落信道下STBC的译码器设计

Alamouti提出的空时分组码(STBC)发射分集技术,其优良性能及低复杂性的译码方案是限制在准静态衰落信道的情况下的,通过研究发现此系统在时变快衰落信道时性能会有严重的损失。为了改善系统在快衰落信道下的性能,提出一种最小均方误差(MMSE)线性译码器,该译码器能够同时抑制噪声和干扰。理论及计算机仿真证明,MMSE译码器用于空时分组码发射分集系统中可以有效地改善无线信道的时变快衰落对系统性能的影响。     

Performance analysis of two-branch transmit diversity block-coded OFDM systems in time-varying multipath Rayleigh-fading channels

Based on Alamouti code, Lee and Williams proposed two-branch transmit diversity block-coded orthogonal frequency-division multiplexing (TDBC-OFDM) systems, namely, space-time block-coded OFDM (STBC-OFDM) and space-frequency block-coded OFDM (SFBC-OFDM). However, they employed the simple maximum-likelihood (SML) detector, which was designed under the assumption that the channel is static over the duration of a space-time/frequency codeword. Therefore, STBC-OFDM/SFBC-OFDM suffers from the high time/frequency selectivity of the wireless mobile fading channel. In this paper, besides the original SML detector, three detectors proposed by Vielmon et al. are applied to improve the two-branch TDBC-OFDM systems. Additionally, assuming sufficient cyclic prefix, the performances of all systems in spatially uncorrelated time-varying multipath Rayleigh-fading channels are evaluated by theoretical derivation and computer simulation, as well. According to the derived bit-error rate (BER), we further derive the bit-error outage (BEO) to provide a more object judgment on the transmission quality within a fading environment. Numerical results have revealed that significant performance improvement can be achieved even when the systems are operated in highly selective channels.     

A novel orthogonal space-time-frequency block code for OFDM systems

In this letter, we propose an orthogonal spacetime- frequency block code for orthogonal frequency division multiplexing (STFBC-OFDM) systems, newly designed to be robust against both time- and frequency-selective fading. The performance of the proposed system is analyzed in spatially uncorrelated time-varying multipath Rayleigh-fading channels. The analytic results, which closely match the numerical results, show that the proposed system outperforms STBC-OFDM in time-varying channels and SFBC-OFDM in frequency-selective fading channels.     

Iterative receivers for space-time block-coded OFDM systems indispersive fading channels

We consider the design of iterative receivers for space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) systems in unknown wireless dispersive fading channels, with or without outer channel coding. First, we propose a maximum-likelihood (ML) receiver for STBC-OFDM systems based on the expectation-maximization (EM) algorithm. By assuming that the fading processes remain constant over the duration of one STBC code word and by exploiting the orthogonality property of the STBC as well as the OFDM modulation, we show that the EM-based receiver has a very low computational complexity and that the initialization of the EM receiver is based on the linear minimum mean square error (MMSE) channel estimate for both the pilot and the data transmission. Since the actual fading processes may vary within one STBC code word, we also analyze the effect of a modeling mismatch on the receiver performance and show both analytically and through simulations that the performance degradation due to such a mismatch is negligible for practical Doppler frequencies. We further propose a turbo receiver based on the maximum a posteriori-EM algorithm for STBC-OFDM systems with outer channel coding. Compared with the previous noniterative receiver employing a decision-directed linear channel estimator, the iterative receivers proposed here significantly improve the receiver performance and can approach the ML performance in typical wireless channels with very fast fading, at a reasonable computational complexity well suited for real-time implementations     

A New Performance Enhancement Algorithm for Space-time Block Coding in OFDM Systems

1　Introduction　WirelesscommunicationusingMIMOsystems ,some timescalleda“volumetovolume”wirelesslink ,hasre centlyemergedasonofthemostsignificanttechnicalbreakthroughsinmoderncommunications[1] .Inaddi tion ,space timecoding[2～ 4 ] isa good proposalthatcombinessignalprocessingatthereceiverwithchannelcodingtechniquesappropriatetomultipletransmitan tennasandcansignificantlyimproveperformanceofMI MOsystems .Atthesametime ,OFDMiseffectiveinpresenceoffrequencyselectivefadinginthewirelessap …     

空时编码在瑞利衰落信道下的性能分析

以正交空时分组码为例讨论了空时编码技术的性能特征。给出了各种STC-OFDM系统的BER性能。理论分析和仿真结果表明,STBC-OFDM系统在频率选择性的瑞利衰落信道下,性能优于其它STC-OFDM系统。     

Performance of Multiple-Access DCSK Communication over a Multipath Fading Channel with Delay Spread

The bit error rate (BER) performances of variant delay multiple-access differential chaos-shift keying (VDMA-DCSK) communication systems over a multipath fading channel with delay spread are investigated. The BER formula of the VDMA-DCSK over the fading channel is evaluated. A two-ray Rayleigh fading channel model is used to simulate the VDMA-DCSK system. The theoretical and simulation results are presented and they match each other, which supports the theoretical analysis. The multipath performance of the VDMA-DCSK is compared with that of a benchmark coherent MA-CSK system and with that of an invariant delay MA-DCSK system. The results show that in the multipath fading channel with delay spread environment the VDMA-DCSK system performance decreases least.     

A space-time block-coded OFDM scheme for unknownfrequency-selective fading channels

We introduce a space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) selective fading channels which does not require channel knowledge either at the transmitter or at the receiver. The decoding algorithm is based on generalized maximum-likelihood sequence estimation. We investigate the performance of the proposed scheme over two-tap Rayleigh fading channels. Simulation results show the performance to be near optimum     

Interference Modelling in DS/CDMA Cellular Systems Operating in a General Fading Environment

Multiple access interference and the mobile radio environment are the primary limitations on the performance of DS/CDMA cellular systems. This paper presents the analysis of a DS/CDMA cellular system operating in a general fading environment. In particular the issues of signal fading, multiple access interference, and power control are addressed. A computationally efficient statistical method is used in the estimation of system performance. It is assumed that the variability of each received signal can be represented by fast Nakagami_m fading plus slower log-normal shadowing. Average bit-error-rate (BER) and outage probability are estimated as system performance indicators. The analysis shows, that as the variability of the fast fading of the received signal reduces, the performance of the system improves. However, when the signal undergoes both fast fading and shadowing, it is largely the shadowing that determines system performance. Use of forward link power control (that compensates for the variability due to shadowing) results in minimal performance improvement. However, in the reverse link significant improvement in performance can be achieved using a similar power control scheme.     

Super-orthogonal space–time trellis codes for two transmit antennas in fast fading channels

Super-orthogonal space–time trellis codes (SOSTTC) are full rate, full diversity space–time codes with high coding gains for quasi-static fading channels. In this paper, this design approach is extended to fast fading channels and new SOSTTC are proposed for BPSK and QPSK modulations based on the design criteria valid for this type of channels. The new full rate codes have 4- and 16-state trellises for BPSK and QPSK cases, respectively, to avoid parallel transitions which restrict the error performance. The frame error performances are evaluated through computer simulations and it is shown that the proposed codes have superior performance in the fast fading case compared to their counterparts previously given in the literature for fast and quasi-static fading channels. Copyright © 2007 John Wiley & Sons, Ltd.     

衰落信道的快速跳频通信系统抗干扰分析

介绍了跳频通信原理,利用Simulink仿真工具,建立快速跳频通信系统仿真模型,在不同干扰模式下进行仿真,分析了抗干扰性能,并在不同信道下完成仿真,接近真实干扰环境。最终得出结论,快速跳频系统具有较强的抗干扰与多径衰落能力,且随着调频速率的增高,该能力也在增强。