瑞利衰落下的空时频(STF)分组编码OFDM系统

基于正交频分复用(OFDM)系统，提出了一种发射分集方案——比特交织空时频(BI—STF)分组编码。其基本思路是：应用子载波分群方法并选择合适的系统参数，将OFDM系统转化成分群OFDM(G-OFDM)，对每个群分别进行空时频分组编码(GSTFBC)；在编码比特被重组和映射成GSTF分组编码前进行合理的比特交织，并按一定的规则分配给各个单群子载波进行酉星座旋转(CR)预编码。随后讨论了该方案的频谱利用率和成对错误概率(PEP)。仿真结果表明，同其它编码方案相比，提出的方案能在频率选择性瑞利衰落信道下获得最大的空间分集和频率分集增益，且只有较低的解码复杂性。     

具有子块编码结构和优化编码增益的非相干全分集空频码设计

非相干空频编码是在无法得到精确信道信息的情况下获取MIMO-OFDM系统中空间分集和频率分集的重要手段,而目前非相干空频码的研究还是将整个OFDM符号看作是一个码字,这样大大增加了系统的编译码复杂度.本文提出了具有子块编码的非相干空频码结构,每个空频码字只占用部分子载波.通过一种码设计的新思路,将频率选择性衰落下的传输等效为平衰落下的传输,并在分析成对错误概率的基础上提出了具有全分集子块结构非相干空频码的设计准则和编码增益的优化方法.     

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。     

基于空时Turbo网格码的MIMO-OFDM系统

提出了使用空时Turbo网格编码的MIMO-OFDM系统,分析了系统的性能,给出了衰落信道中的性能上界以及编码和分集增益表达式。通过仿真评估了空时Turbo网格码在慢衰落信道中的性能,与传统的STTC方法相比,该系统可以获得更好的分集增益和编码增益。     

新的GLSFBC-CDMA-OFDMA发射方案

提出了频率选择性衰落信道下GLSFBC(grouplayeredSFBC)-CDMA-OFDMA多天线发射方案.该方案分为三级级联设计:第一级(基于OFDMA)用来抑制多用户间的干扰和对抗频率选择性衰落;第二级(基于CDMA)用来消除组间干扰和获得频率分集增益;第三级(基于GLSFBC)用来获得空间分集增益和复用增益.该方案在接收端只需要一根天线就可以抑制空频码组间的干扰和多用户间的干扰,从而使接收机结构简单.理论分析和计算机仿真结果都证明了该方案的有效性     

基于OFDM 系统的差分空频时编码

基于OFDM(正交频分复用)系统,提出了一种新的空频时三维编码,将网格编码调制和差分空时分组码进行组合链接设计。在信道状态信息很难获取的情况下,它可以很好地工作在频率选择性信道中。进一步分析并推导了非相干检测下该编码的性能。仿真结果表明,这种新的编码方案可以获得编码增益以及满的空间域和频域分集增益,很好地支持了理论分析。     

基于循环延迟分集的分组预编码和空时频编码技术

该文提出一种把循环延迟分集应用在分组线性预编码的新方案。该方案首先应用循环延迟分集来虚拟一个多径时延扩展的信道,在此基础上应用分组线性预编码以获得频率分集增益。该方案的译码复杂度相对较低,可以针对不同的信道模型有相应的编码方案,至少能获得与空时编码同样的分集增益M(M为发射天线数),且信道估计相对简单,不降低传输码率。该方案结合空时分组码可构造出新的空时频编码方案。仿真结果表明,该方案具有良好的译码性能。     

在频率选择性相关信道中的空频分组编码技术

本文提出了频率选择性瑞利衰落信道中的对角空频分组码(DSF)，研究了码的性能。分析表明，在各天线对间的信道相互独立，系统有NT个发射天线，NR个接收天线和信道冲击响应长度是L时，DSF码可实现分集增益NRNTL。此外，相关的发射天线阵列会使DSF码分集增益下降，但是对码的性能影响不大，特别是在较长的多路径信道上。因此，DSF码对于相关的衰落信道显示出优越的鲁棒性能。最终仿真结果证实了本文的分析。     

无需信道估计的一种简单的差分分集OFDM传输方式

空时码在近几年得到了普遍关注,但空时码的应用通常都假定信道具有非频率选择性衰落特性,同时还假定信道状态也是已知的,这些都极大地限制了空时码的应用。该文给出了一种未知频率选择性衰落信道下的差分分集调制方法。应用正交频分复用技术的基本原理,把一个频率选择性衰落信道转换成了多个并行平坦衰落信道。在此基础上,给出了一种无需信道估计的简单的时空频分集结构.仿真结果表明,这种结构在频率选择性衰落信道中无需信道估计就能得到很好的结果.     

一种时空频分组码编码方案和检测算法

空时编码作为对抗多径衰落的有效方法，近年来已经成为热门研究领域。空时分组码(STBC)的正交设计可以在接收端进行线性最大比合并解码，实现了高编码增益、大分集度和低译码复杂度。这种空时分组码在单个载波多个不同天线和时间上编码，实现了空间和时间的分集，但并没有实现频率的分集，分集度有限。本文将单载波的STBC扩展到多个载波，提出了一种时、空、频分组码(STFBC)的编码方案。这种编码在相同的码速率下，实现了比STBC更大的分集度和更高的编码增益。给出了基于这种编码方案的两种检测算法一线性合并最大似然检测(MLD)算法和线性合并解相关检测(DD)算法。仿真结果表明这种时空频分组码的编码方法和检测算法较STBC具有更好的性能。     

Unit-rate complex orthogonal space-time block code concatenated with turbo coding

Space-Time Block （STB） code has been an effective transmit diversity technique for combating fading due to its orthogonal design, simple decoding and high diversity gains. In this paper, a unit-rate complex orthogonal STB code for multiple antennas in Time Division Duplex （TDD） mode is proposed. Meanwhile, Turbo Coding （TC） is employed to improve the performance of proposed STB code further by utilizing its good ability to combat the burst error of fading channel. Compared with full-diversity multiple antennas STB codes, the proposed code can implement unit rate and partial diversity; and it has much smaller computational complexity under the same system throughput. Moreover, the application of TC can effectively make up for the performance loss due to partial diversity. Simulation results show that on the condition of same system throughput and concatenation of TC, the proposed code has lower Bit Error Rate （BER） than those full-diversity codes.     

Noncoherent space-frequency coded MIMO-OFDM

Recently, the use of coherent space-frequency coding in orthogonal frequency-division multiplexing (OFDM)-based frequency-selective multiple-input multiple-output (MIMO) fading channels has been proposed. Acquiring knowledge of the fading coefficients in a MIMO channel is already very challenging in the frequency-flat (fast) fading case. In the frequency-selective case, this task becomes significantly more difficult due to the presence of multiple paths, which results in an increased number of parameters to be estimated. In this paper, we address code design for noncoherent frequency-selective MIMO-OFDM fading links, where neither the transmitter nor the receiver knows the channel. We derive the code design criteria, quantify the maximum achievable diversity gain, and provide explicit constructions of full-diversity (space and frequency) achieving codes along with an analytical and numerical performance assessment. We also demonstrate that unlike in the coherent case, noncoherent space-frequency codes designed to achieve full spatial diversity in the frequency-flat fading case can fail completely to exploit not only frequency diversity but also spatial diversity when used in frequency-selective fading environments. We term such codes "catastrophic.".     

Performance and design of space-time coding in fading channels

The pairwise-error probability upper bounds of space-time codes (STCs) in independent Rician fading channels are derived. Based on the performance analysis, novel code design criteria for slow and fast Rayleigh fading channels are developed. It is found that, in fading channels, the STC design criteria depend on the value of the possible diversity gain of the system. In slow fading channels, when the diversity gain is smaller than four, the code error performance is dominated by the minimum rank and the minimum determinant of the codeword distance matrix. However, when the diversity gain is larger than, or equal to, four, the performance is dominated by the minimum squared Euclidean distance. Based on the proposed design criteria, new codes are designed and evaluated by simulation.     

基于协方差反馈的多天线系统优化发送研究

为了克服多天线信道相关性的影响，提出一种新的自适应发送方案。应用空时分组码特征波束成型技术和格形编码调制（TCM）来获得分集增益和编码增益。针对采用和不采用交织器两种情况，基于成对差错概率（PEP）准则。分析了系统的统计性能，分别得到了使系统编码增益和分集增益最大化的TCM设计准则。根据注水法则和Lagrange乘子法求得波束间功率分配算法最优解。此外，码距作为优化功率加载算法中的权重因子，有效降低了获取波束成形分集的信噪比门限。分析和实验结果表明此方案复杂度低。能有效克服相关衰落。     

基于信道编码及空时编码级联的两用户协同分集

传统协同分集通过使网络中各单天线用户共享彼此天线,形成虚拟多天线阵列来实现空间分集,使得体积和功耗受限的网络终端也能获得分集增益,然而这并没有将信道编码和空时编码结合起来以使系统得到编码增益。为了能够获得编码增益来进一步改善系统性能,本文提出了一种基于信道编码和分布式空时分组码级联方式下的两用户协同分集方案,并且在准静态的瑞利衰落信道下对系统误码性能进行了理论推导和系统仿真,给出了误比特率的上限解析表达式。在协同用户间信道存在噪声的情况下,我们分别对CRC-DSTBC和CC-DSTBC级联下的发射方案进行了性能分析和系统仿真。仿真结果表明:即使协同用户间的信道存在噪声,本文所提出的协同分集方案与传统协同分集相比,不但获得了分集增益,同时也得到了编码增益,系统误比特率大大降低,从而显著提高了系统性能,并且这也和理论分析相吻合。     

多天线对角空频编码传输

将平坦衰落信道的对角代数空时码(DAST)推广到频率选择性衰落信道,提出了对角空频分组码(DSF).基于多输入多输出天线和正交频分复用(OFDM),DSF码将满秩的旋转信号星座和子载波分组结合起来,以对角发送方式(每时刻只有一个天线发射)发射旋转信息符号向量的每个分量.成对错误概率分析表明:在频率选择性信道中,通过选择最佳的旋转矩阵,这种DSF-OFDM系统能实现满分集增益和最大的编码增益.系统采用了球型解码器对DSF码实施最大似然解码,它的解码复杂性是中等的,并且,解码算法的复杂性与信号星座的维数无关.此外,和先前所提出的一些方法相比,提出的空频码还具有频谱效率高(1symbol/s/Hz)的性能特点.     

An optimal two transmit antenna space-time code and its stacked extensions

A space-time code is proposed that exhibits the highest coding gain among competing full-rate full transmit diversity space-time codes for the two transmit and receive antenna coherent quasi-static fading channel. The proposed code is derived from a layered architecture with real rotation of quadrature amplitude modulation (QAM) information symbols in two dimensions. The existing codes of similar architecture concentrate on application of complex full modulation diversity rotations or asymmetric real rotations. An analytic evaluation illustrates the significant improvement in coding gain achieved with the proposed code. Moreover, the coding gain of the proposed code is independent of its rate. This implies that the proposed code achieves the optimal diversity-multiplexing tradeoff curve for the two transmit antenna system. A stacked extension of the proposed code offers a reduced complexity capacity optimal alternative to the full diversity codes for larger number of transmit antennas. Performance enhancement in several scenarios is verified through simulations.     

Performance of multidimensional multicode DS-CDMA using codediversity and error detection

High rate transmission can be realized using multiple orthogonal codes (MOC), as proposed in the third-generation wide-band code-division multiple-access (W-CDMA) standard. However, the linear sum of MOC channels is no longer constant amplitude, and a highly linear, power-inefficient amplifier may be required for transmission. Recently, a nonlinear block coding technique called precoding is introduced to maintain a constant amplitude signal after superposition of MOC channels. This is achieved by adding redundancy. In this paper, we first describe a multidimensional signaling scheme that recovers some information rate loss by precoding. Second, we propose a self-interference (SI) cancellation scheme resulting from a code diversity between the in-phase and quadrature subchannels among MOC channels. In a typical wireless channel with multipath fading, this type of SI can be detrimental especially when the number of parallel MOC channels is large. Third, we show that the error detection capability of precoding can be combined with code diversity, resulting in a diversity gain. In addition, we show that the diversity gain can be achieved using antenna diversity to assure the degree of freedom in code diversity, and even with the large number of MOC channels, the error performance can be maintained reliably while outperforming the variable spreading factor scheme in W-CDMA     

MIMO-OFDM系统中的一种空时频编码方案

基于MIMO-OFDM系统,本文提出了一种空时频编码方案,其利用扩展码,使数据符号在空域、时域、频域得到扩展,通过设置合适的参数,能够在频率选择性衰落信道下获得满空间分集和满频率分集,同时该方案的编解码复杂度不高。仿真结果表明,该方法在低信噪比下,也具有良好的误码率性能。