Link adaptation for wireless systems

To improve the robustness and reliability of wireless transmissions, two complementary link adaptation techniques are employed: adaptive modulation and coding (AMC) at the physical layer and hybrid automatic retransmission request (HARQ) at the medium access control layer. Because of their effectiveness in combating errors induced by the wireless channel, AMC and HARQ are now integral components of most emerging broadband wireless system standards, for example, LTE and WiMAX. Spectral efficiency (SE) as measured in bit per second per Hertz is one important parameter used to characterize a wireless system for comparison between different systems or between different configurations of the same system. This work provides a holistic approach of cross‐layer optimizations with the intent of maximizing SE by combining AMC and HARQ. It formulates closed‐form equations for calculating the average SE for wireless systems with the Rayleigh fading channel model. A new online algorithm is developed to optimize SE for both Rayleigh and non‐Rayleigh fading channel. Simulations using proven LTE model are performed to compare SE obtained from closed‐form equations and the developed algorithm for different system configurations. With the developed algorithm to determine how many retransmissions required in addition to the initial transmission in advance depending on the current wireless channel condition, the latency can be reduced up to 24 ms when sending the initial transmission and all of its retransmissions sooner than waiting for retransmission requests as is done previously. Copyright © 2012 John Wiley & Sons, Ltd.     

基于模糊逻辑的3G无线链路层控制策略

在分析WCDMA的无线链路控制（RLC）层的自动请求重传(ARQ)的实现机制的基础上,综合考虑RLC队列长度、TCP分组到达强度、无线网络纠错强度以及TCP发送端的应用时延要求等因素,提出了基于模糊逻辑控制的自适应的重传纠错次数主动队列管理策略。仿真结果表明,该策略能够有效提高TCP通过无线网络的吞吐量,高效地利用无线资源。     

自动粒度选择的半结构化页面信息抽取

半结构化页面的数据记录间存在结构相似性,在先序遍历DOM树生成的标记序列中表现为重复出现的模式,可利用后缀树进行挖掘。由于标记序列可以在块粒度和文本粒度两个级别上展现,而不同粒度下产生的最佳抽取模式在抽取效果方面又表现出不确定性,因此提出一种自动粒度选择的半结构化页面信息抽取方法。算法从后缀树获取的重复模式中选取最大重复和串联重复构成候选模式集,通过特征参数确定两个粒度各自的最佳模式集,最后引入抽取结果规则度参数并进行综合评价,以确定抽取模式完成半结构化页面数据记录的自动抽取。     

Self-efficacy as a marker of cardiac function and predictor of heart failure hospitalization and mortality in patients with stable coronary heart disease: Findings from the Heart and Soul Study.

Objective: The authors sought to evaluate the association of self-efficacy with objective measures of cardiac function, subsequent hospitalization for heart failure (HF), and all-cause mortality. Design: Observational cohort of ambulatory patients with stable CHD. The authors measured self-efficacy using a published, validated, 5-item summative scale, the Sullivan Self-Efficacy to Maintain Function Scale. The authors also performed a cardiac assessment, including an exercise treadmill test with stress echocardiography. Main Outcome Measures: Hospitalizations for HF, as determined by blinded review of medical records, and all-cause mortality, with adjustment for demographics, medical history, medication use, depressive symptoms, and social support. Results: Of the 1,024 predominately male, older CHD patients, 1013 (99%) were available for follow-up, 124 (12%) were hospitalized for HF, and 235 (23%) died during 4.3 years of follow-up. Mean cardiac self-efficacy score was 9.7 (SD 4.5, range 0–20), corresponding to responses between “not at all confident” and “somewhat confident” for ability to maintain function. Lower self-efficacy predicted subsequent HF hospitalization (OR per SD decrease = 1.4, p = .0006), and all-cause mortality (OR per SD decrease = 1.4, p Conclusion: Among patients with CHD, self-efficacy was a reasonable proxy for predicting HF hospitalizations. The increased risk of HF associated with lower baseline self-efficacy was explained by worse cardiac function. These findings indicate that measuring cardiac self-efficacy provides a rapid and potentially useful assessment of cardiac function among outpatients with CHD. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Optimal selection of ARQ parameters in QAM channels

For a quadrature amplitude modulation (QAM) communication channel with forward error correction (FEC) and automatic repeat request (ARQ), the bit error rate (BER) is estimated as a function of the channel signal‐to‐noise ratio (SNR). A method is presented for trading off the number of allowed transmissions and the level of margin (or the line ARQ gain) applied to the channel SNR. The line ARQ gain maximizing the channel throughput is shown to be a sum of the channel SNR and some function of the bit load and FEC parameters. Using this result, we formulate a fast method of determining the optimum ARQ parameters at the actual channel conditions, based on their values at some reference conditions, the latter being either stored in the modem memory or pre‐calculated prior to channel initialization. The derivations are presented both for AWGN and slowly fading channel conditions. Copyright © 2004 John Wiley & Sons, Ltd.     

一种改进的短波链路数据传输协议

因特网的迅猛发展,使得IP协议成为事实上的工业标准,各种无线网络也纷纷开始使用IP技术组网,短波IP网络也已经出现。在短波链路上传输IP报文通常采用ARQ模式,以对网络层提供报文的无错传输。目前ARQ传输协议普遍采用选择式重发策略,在数据帧中加入帧序号,以确保收方能对数据帧重新排序。为提高IP报文在短波链路上的传输效率,对现有的短波链路选择式重发ARQ协议进行了分析,针对收发双方的通信流程进行了改进,提出了一种无帧序号的ARQ传输协议。测试结果表明,在同样的信道状况下,改进后的协议吞吐率性能有了一定的提高。最后就如何进一步提高该协议性能进行了思考。     

LCL并网逆变器新型电流双闭环控制策略研究

提出了一种新型的LCL型并网逆变器电流双闭环控制策略。内环采用电容电流反馈增加LCL并网逆变器系统阻尼,以抑制LCL输出滤波器带来的高频谐振问题;外环采用重复PR复合控制策略实现对并网电流的高性能控制,以抑制电网电压波动和非线性负载对并网电流的影响,实现对基频信号的无静差控制和高功率因数并网。在此理论分析的基础上研究了控制系统的稳定性,提出LCL并网逆变器电流双闭环控制器优化设计方案。最后通过仿真验证了理论分析的正确性和控制策略的可行性。     

时延反馈下的自适应STBC-Beamforming跨层设计

为提高无线通信网络的频谱效率,提出了一种自适应STBC-Beamforming跨层设计方案. 该方案利用自适应编码调制技术和混合自动重传技术交互协作,使发射端可以根据反馈的延时信息自适应调节调制模式,选择发射权以及自动重传数据.推导了系统在时延反馈条件下的频谱效率和中断概率的闭合表达式,分析了时延反馈对于系统性能的影响.仿真结果表明,对比单发/单收跨层系统和Alamouti’s跨层系统,自适应STBC-Beamforming跨层系统的性能有明显提高.     

Mechanisms of the FMR1 Repeat Instability: How Does the CGG Sequence Expand?

A dynamic mutation in exon 1 of the FMR1 gene causes Fragile X-related Disorders (FXDs), due to the expansion of an unstable CGG repeat sequence. Based on the CGG sequence size, two types of FMR1 alleles are possible: “premutation” (PM, with 56-200 CGGs) and “full mutation” (FM, with >200 triplets). Premutated females are at risk of transmitting a FM allele that, when methylated, epigenetically silences FMR1 and causes Fragile X syndrome (FXS), a very common form of inherited intellectual disability (ID). Expansions events of the CGG sequence are predominant over contractions and are responsible for meiotic and mitotic instability. The CGG repeat usually includes one or more AGG interspersed triplets that influence allele stability and the risk of transmitting FM to children through maternal meiosis. A unique mechanism responsible for repeat instability has not been identified, but several processes are under investigations using cellular and animal models. The formation of unusual secondary DNA structures at the expanded repeats are likely to occur and contribute to the CGG expansion. This review will focus on the current knowledge about CGG repeat instability addressing the CGG sequence expands.