移动通信中基于Turbo码的混合ARQ方案

在移动环境中,由于信道的时变特性,单纯的FEC或ARQ机制往往无法提供所希望的系统性能。为了提高数据及图像的传输可靠性,需采用ARQ与FEC相结合的混合ARQ差错控制方案。首先介绍了几种常见的以Turbo码作为纠错码的混合ARQ方案,然后研究了基于Turbo码组合译码的有限次重传混合ARQ方案,给出了传输方案及组合译码算法。仿真结果表明,低Eb/No时,该方案对通过率有明显改善。     

判断多原子分子中有无大π键的方法

大π键是分子结构和分子轨道理论中的重要概念。如何判断分子中是否含有大π键是无机化学、有机化学和结构化学中的一个值得研究的问题。本文提出的通过中心原子杂化态的推测来判断多原子分子中有无大π键的方法,容易掌握,可供读者研究参考。     

Nested Transaction Concurrency Control in Parallel Real-Time Databases

An integrated method for concurrency control in parallel real-time databases has been proposed in this paper. The nested transaction model has been investigated to offer more atomic execution units and finer grained control within in a transaction. Based on the classical nested locking protocol and the speculative concurrency control approach, a twoshadow adaptive concurrency control protocol, which combines the Sacrifice based Optimistic Concurrency Control （OPT-Sacrifice） and High Priority two-phase locking （HP- 2PL） algorithms together to support both optimistic and pessimistic shadow of each sub-transaction, has been proposed to increase the likelihood of successful timely commitment and to avoid unnecessary replication overload.     

改进粒子群算法在作业车间调度问题中的应用

调度问题是一类典型的NP-hard问题,传统粒子群优化算法在解决该类问题上具有一定的局限性.通过分析其优化机理,提出了改进粒子群算法,结合了粒子群优化算法的全局搜索能力和交换粒子位置的局部搜索能力,提出了新的粒子编码方法--基于粒子坐标值排列编码(PPP),发展了一种快速、易实现的新的混合启发式算法.大量实验仿真结果表明本算法可以有效求解作业车间调度问题,通过与遗传算法比较,验证了改进粒子群算法是求解Job-shop调度问题可行而高效的方法.     

求解作业车间调度问题的粒子群优化算法

讨论了作业车间调度问题的数学表达模型,考虑将粒子群优化算法与差异演化算法的优点结合起来,提出求解作业车间调度问题的新的混合粒子群优化算法,对7个标准算例的仿真结果表明了算法的有效性和优越性.     

基于面积坐标的四边形杂交应力有限元方法

本文针对二维线弹性问题提出了一种基于面积坐标的新型杂交应力四边形有限元AGQ-LQ6. 该方法基于广义 Hellinger-Reissner 变分原理，位移逼近采用含内部位移的四节点广义协调元，应力逼近则采用九参数线性应力模式. 数值算例显示，本文构造的有限元既能保持面积坐标广义协调元对网格畸变不敏感及粗网格精度较高的优点，又能有效克服泊松闭锁现象.     

Effect of spark plasma sintering and reinforcements on the formation of ultra-fine and nanograins in AA2024-TiB2-Y hybrid composites

This study reports the influence of sintering mechanism and reinforcement materials on the formation of ultra-fine and nanograins in Al-TiB₂-Y nanohybrid composites. The mechanical properties of the composites and their corresponding micro and nanostructures are correlated. The experimental and characterization results revealed that despite the addition of TiB₂, the hard and brittle Al₃Ti phase formation in the composites was suppressed and hence, their ductility was retained. It was found that yttrium content of 0.3 ?wt% was the optimum amount which created advantageous spark plasma sintering conditions, and the addition of 0.3 ?wt% promoted the formation of bi-modal size grains (ultra-fine and nano) along with micro grains, Ω and other nano precipitates, resulting in a significant enhancement in the composite properties. The formation of ultra-fine and nanograins may be attributed to the combined effect of melting and rapid solidification at necking zones due to Joule's heating and thermo-mechanical fatigue. Among all the sintered composites, the highest hardness (137 HV), ultimate tensile strength (UTS) (496 ?MPa), yield strength (YS) (438 ?MPa) with 15.7% elongation were obtained in the sintered sample reinforced with 1.0 ?wt% TiB₂ and 0.3 ?wt% yttrium.     

二十一世纪四川林业发展的布局和任务

本文探讨了四川林业发展布局及任务,分析现有森林资源,提出林业可持续发展技术措施。