全速率满分集准正交空时分组码设计研究

自Alamouti提出空时分组码以来,空时分组码的构造得到广泛的研究。研究表明,复正交空时分组码当发射天线数大于3时,它的传输码率小于1。准正交空时分组码虽然能达到全速率,却牺牲了一定的分集增益。根据空时编码秩准则,通过对传统准正交空时分组码进行矩阵旋转处理,得到一种全速率满分集的空时分组码。数值仿真显示,这种全速率满分集空时分组码的性能较传统准正交空时分组码至少有4 dB的提高。     

一种新的准正交空时分组码设计

根据正交设计理论,当发送天线数大于2时,不存在可以获得完全分集增益和全速率的复正交空时分组码。对空时分组码采用准正交设计,能够保证数据以全速率传输,但是会使其误码性能降低。本文根据矩阵正交理论提出了一种新的全速率准正交空时分组码并给出了一种基于QR分解的最大似然译码方法。仿真结果表明,本文方案与已有典型的准正交空时分组码相比,具有更好的误码性能。     

基于坐标间交织正交设计的空时分组码方案

依据空时分组码的复正交设计理论的推广,推导了基于坐标间交织正交设计（CIOD）的空时分组码设计方法,并根据Alamouti正交接收原理,对接收信号依次经过矢量正交变换、矢量虚部交换和矢量加权旋转处理,进行解调运算。最后在准静态平坦Rayleigh衰落信道中,对802.16d/e中使用的4发射天线基于CIOD的发射分集配置方案A进行了性能仿真,结果表明这些码具有较低的译码复杂性,与基于复数正交设计的空时分组码的译码复杂度相当,但性能要好。     

一种优化的准正交空时分组码预编码方法

基于准正交空时分组码的时域均衡技术提出一种新的块预编码方法。该方法将平坦衰落信道符号编码扩展到时域上数据块编码,构造了一种嵌入式准正交结构,从而简化了接收端信号处理,降低了复杂度。同时,对准正交空时分组码部分解耦和全解耦两种方式进行了比较,蒙特卡罗仿真结果证明了该方法的优势。     

旋转星座准正交空时分组码的改进性研究

当发射天线数大于2时,复信号空时分组码不能实现满速率编码,可以通过星座旋转来设计发送矩阵,使系统同时获得满分集增益和最大的编码速率。通过对传统的旋转星座准正交空时分组码加以改进,使每根天线在不同时隙发射的信号属于不同星座,在保证满分集和最大的编码速率的同时,减小了码间干扰,更有利于译码器译码,提高了系统性能。     

一种快衰落下正交空时分组码的译码

准静态衰落信道下正交空时分组码MIMO系统的一种简单最大似然检测算法性能接近于最大似然检测算法．但在实际时变信道干扰下，性能表现较差。本文分析提出了在多个发送和接收天线下能有效消除时变信道对简单最大似然检测算法产生干扰的算法改进。计算机仿真表明，针对时变信道下的正交空时分组码MIMO系统，该译码算法性能实现简单、计算复杂度低，性能接近最大似然检测算法。     

基于四元数正交设计的三极化正交空时极分组码*

构建了一种四元数正交设计的三极化正交空时极分组码,该码满足正交设计关系,能够通过三极化天线进行发射和接收。在相同的系统条件下,对相同天线数的Alamouti码和双极化正交空时极分组码进行仿真比较,结果表明正交极化空时分组码在不增加空间体积和发射天线及接收天线数的情况下,可以有效降低误码率,提高系统性能。     

全分集高速率复正交空时分组码设计

基于典型的多输入-多输出无线通信系统,研究了一种适用于任意发射天线数的高速率复正交空时分组码设计方法。证明了采用该算法所设计的复正交空时分组码能达到满发送分集度和未对信号进行线性处理时的最大码率（m+1）/2m（m为自然数）。在此基础上,对不同发送天线数（n=2,3,4,5,6,7,8）的复正交空时分组码进行了蒙特卡洛仿真,从而验证了算法的有效性,并得出发射天线数与系统误比特率性能呈正比,且随着发射天线数目的增加,误比特率性能改善的程度越来越小。     

发射分集系统中正交空时分组码的研究

分析了空时分组码在实际应用中较空时格码的优势.以Alamouti发射分集方案为例简要介绍了空时分组码的编译码方法.针对带来性能优势的信号正交性设计,进行了详细的讨论.对瑞利衰落信道下,针对不同天线配置和不同调制方式条件下的空时分组码进行了性能仿真.     

空时码在Rician衰落信道下性能分析

基于Alamouti提出的BPSK调制下空时分组码在Rayleigh衰落信道中的简单分集方案。推导出多发射和多接收天线系统中正交空时分组码在Rician衰落信道的BPSK调制下的比特差错率的最小距离球界,并推广到在高阶调制下衰落信道中系统符号差错率的性能。仿真分析和比较了空时分组码的多天线系统中发射和接收天线分集增益,以及信道相关参数的变化对系统误比特性能的影响。     

快速的CCSDS压缩方法的编码选项选择算法

为提升“空间数据系统咨询委员会”( CCSDS)提出的通用无损压缩编码方法的时间效率和压缩性能,提出一种新的编码选项选择算法.该算法利先用块内J个数据的和,粗略搜索最优编码选项参数的范围,再从该范围内精确查找.证明:最佳的搜索范围为3J/2,算法的时问复杂度为O(K+J).相对于CCSDS图像压缩标准中的快速编码选择方法,新算法可以使压缩性能最优;相比启发式的编码选择算法,该算法执行时间缩短了15％ ～25％.     

一种新的满符号传输率满分集度的分布式空时码

结合分布式空时码的系统模型和DBOAST空时分组码的编码方法,提出了一种新的分布式空时码--DBO分布式空时码(DBO-DST),可达到满发送分集度和满符号传输率.对该分布式空时码的编码和解码方法进行了仿真,结果表明,DBO分布式空时码比已有的线性分布式空时码的误码率低,性能更优.     

The NIRVANA code: Parallel computational MHD with adaptive mesh refinement

I report on a new version of the magnetohydrodynamics code NIRVANA¹ which is targeted at the study of astrophysical problems. The new version allows for distributed-memory simulations supporting adaptive mesh refinement. Numerical algorithms include dissipative terms (viscosity, Ohmic diffusion, thermal heat conduction) in a conservative form. Domain decomposition is preferably block-wise in case of unigrid applications but adopts space-filling curve techniques for adaptive mesh applications with a hierarchical block-structured mesh. The code architecture facilitates workload balancing among processors for arbitrary mesh refinement depths maintaining intra-level data locality via space-filling curve mappings and, at the same time, ensuring inter-level data locality by applying a novel technique called block sharing. This way, it is demonstrated that comparable performance can be achieved for problems with locally highly refined grid. The data transfer between processors extensively utilizes the coarse-granularity concept of parallel computing and makes use of the MPI library. Conservation properties of the numerical method carry over to the parallel framework. In particular, the solenoidality condition for the magnetic field is preserved to roundoff precision applying the constrained transport machinery. This paper has its focus of discussion on implementation details related to the parallelization and on a code performance analysis.     

分组相关快衰落信道下Turbo码译码算法研究

研究了分组相关快衰落信道的特性，推导出该信道下Turbo码的译码算法，并从推导结果确定：平均信噪比和信噪比弥散度是影响系统性能的两个重要参数。仿真结果表明，在分组相关快衰落信道下，采用该文提出的Turbo码译码算法，可以提高整个系统性能，在误比特率为10^-3时，与信道编码采用卷积码的系统相比，信噪比改善2．9dB；在大平均信噪比和小信噪比弥散度条件下，系统性能较好，误比特率较低。     

瑞利信道下线性分组码的分集增益研究

比较AWGN信道与瑞利信道的误码性能,研究瑞利信道下分集支数与信息传输可靠性之间的关系。推导线性分组码在瑞利信道下误码率上界的理论表达式,得到分集增益与其最小汉明距离的关系。基于蒙特卡洛法对误码性能进行仿真,并与理论误码率上界进行比较,结果表明线性分组码在瑞利信道下获得的分集增益等于其最小码距。     

NAS Parallel Benchmarks with CUDA and beyond

NAS Parallel Benchmarks (NPB) is a standard benchmark suite used in the evaluation of parallel hardware and software. Several research efforts from academia have made these benchmarks available with different parallel programming models beyond the original versions with OpenMP and MPI. This work joins these research efforts by providing a new CUDA implementation for NPB. Our contribution covers different aspects beyond the implementation. First, we define design principles based on the best programming practices for GPUs and apply them to each benchmark using CUDA. Second, we provide ease of use parametrization support for configuring the number of threads per block in our version. Third, we conduct a broad study on the impact of the number of threads per block in the benchmarks. Fourth, we propose and evaluate five strategies for helping to find a better number of threads per block configuration. The results have revealed relevant performance improvement solely by changing the number of threads per block, showing performance improvements from 8% up to 717% among the benchmarks. Fifth, we conduct a comparative analysis with the literature, evaluating performance, memory consumption, code refactoring required, and parallelism implementations. The performance results have shown up to 267% improvements over the best benchmarks versions available. We also observe the best and worst design choices, concerning code size and the performance trade-off. Lastly, we highlight the challenges of implementing parallel CFD applications for GPUs and how the computations impact the GPU's behavior.     

SigFree: A Signature-Free Buffer Overflow Attack Blocker

We propose SigFree, an online signature-free out-of-the-box application-layer method for blocking code-injection buffer overflow attack messages targeting at various Internet services such as web service. Motivated by the observation that buffer overflow attacks typically contain executables whereas legitimate client requests never contain executables in most Internet services, SigFree blocks attacks by detecting the presence of code. Unlike the previous code detection algorithms, SigFree uses a new data-flow analysis technique called code abstraction that is generic, fast, and hard for exploit code to evade. SigFree is signature free, thus it can block new and unknown buffer overflow attacks; SigFree is also immunized from most attack-side code obfuscation methods. Since SigFree is a transparent deployment to the servers being protected, it is good for economical Internet-wide deployment with very low deployment and maintenance cost. We implemented and tested SigFree; our experimental study shows that the dependency-degree-based SigFree could block all types of code-injection attack packets (above 750) tested in our experiments with very few false positives. Moreover, SigFree causes very small extra latency to normal client requests when some requests contain exploit code.