Switching a multistable solid-state laser from one stable steady state to another-dynamic stability analysis

The switching of a multistable laser from one stable steady state to the next has been studied using a longitudinally pumped laser with a nonlinear mirror. To achieve switching, use has been made of a second mode that is oscillating at the pump frequency. This pump mode is kept just below its threshold for oscillation. We find through an analysis of the dynamic gain functions of both the laser and pump modes that the steady state of the laser mode is very sensitive to changes in the steady state of the pump mode that are brought about by fluctuations in the pump intensity. It is found that for switching to be possible, the difference in frequency between the pump and laser mode should not be much larger than the linewidth of the laser atomic medium. We conclude that a conventionally pumped multistable and multimode solid-state laser can have one of its modes switched from one of its stable steady states to another such state, of a lower or higher intensity, by longitudinally injecting a pulse of suitable magnitude, frequency, and duration as a perturbation. This can happen provided that one of the other N-1 modes, which are kept below oscillation threshold, has been pushed by such a perturbation to just above threshold for oscillation in order that it couples with the oscillating laser mode. We have estimated the switching time to be less than the laser cavity photon decay time but of the order of the round-trip time in the Fabry-Perot cavity. The analysis also shows that the laser threshold intensity and its stability are not dependent on the difference between the frequencies of the pump and laser modes. However, the threshold is found to be sensitive to changes in the overlap parameter which is defined as a ratio of the beam waists.     

Burst-Error Correction Strategies in a Slotted DS-CDMA Mobile Satellite Channel

Various burst-error handling strategies exist to reduce the effects of bursts of errors such as those that characterise a heavily shadowed, slowly fading land mobile satellite channel using low earth orbit satellites (LEOS). It is proposed that a combination of the techniques with code rate adjustment and channel-state sensing leads to high performance gains for the direct sequence CDMA channel.This paper presents studies of the performance of code interleaving in co-ordination with code rate reduction in a channel with known burst statistics where provision is made for channel-state sensing. The results are compared with those from concatenated coding and hybrid automatic repeat request (hybrid ARQ) with selective repeat (SR).Results are presented for throughput and delay. The combination of the various techniques for specific channel conditions results in significant performance improvements in terms of throughput and average time delay. The proposed error-control technique offers significant performance gains over the use of hybrid ARQ, concatenated coding or interleaving in isolation.     

A Channel Hopping Algorithm for Guaranteed Rendezvous in Cognitive Radio Ad Hoc Networks Using Swarm Intelligence

Wireless Personal Communications - Achieving fast Time to Rendezvous (TTR) on a common coordination channel in an Ad hoc network remains a contemporary issue in Cognitive Radio research. In this...     

Cross Layer Relay Selection in Device-to-Device Communication

Cross layer optimization for relay assisted wireless communication is important as it caters for parameters in more than one layer thereby a more holistic insight to the performance of proposed algorithms is realised. Due to the nature of the wireless channel, the availability of perfect channel state information may not be guaranteed. Moreover for relay selection algorithms, where mobile user equipment are used as relays, optimizing a relay defined utility can motivate relaying. This work proposes a cross layer stable matching based relay selection algorithm for relay assisted device-to-device communication. The cross layer parameter defined by this work is a function of data rate at the physical layer and queueing state information at the media access control sub-layer. The selection problem is modelled as an optimization problem for which stable matching is used to suitably allocate relay-enabled user equipment to source-destination pairs. To evaluate the performance of the proposed algorithm, spectral efficiency, energy efficiency and fairness were used as performance metrics. The proposed cross layer algorithm was compared with the best source-relay selection algorithm, best relay-destination selection algorithm and a random selection algorithm. Unlike other works, our proposed algorithm is channel uncertainty aware. The proposed cross layer selection algorithm was shown through simulations to offer improved spectral efficiency  performance of 16% over the random selection algorithm. Furthermore, there was a fairness improvement of 0.01 and 0.02 over the best source—relay and the best relay—destination selection algorithms respectively.

     

Trellis code‐aided high‐rate M‐QAM space‐time labeling diversity using a unitary expansion

In this paper, we present a high‐rate M‐ary quadrature amplitude modulation (M‐QAM) space‐time labeling diversity (STLD) system that retains the robust error performance of the conventional STLD system. The high‐rate STLD is realised by expanding the conventional STLD via a unitary matrix transformation. Robust error performance of the high‐rate STLD is achieved by incorporating trellis coding into the mapping of additional bits to high‐rate codes. The comparison of spectral efficiency between the proposed trellis code‐aided high‐rate STLD (TC‐STLD) and the conventional STLD shows that TC‐STLD with 16‐QAM and 64‐QAM respectively achieves a 12.5% and 8.3% increase in spectral efficiency for each additional bit sent with the transmitted high‐rate codeword. Moreover, we derive an analytical bound to predict the average bit error probability performance of TC‐STLD over Rayleigh frequency‐flat fading channels. The analytical results are verified by Monte Carlo simulation results, which show that the derived analytical bounds closely predict the average bit error probability performance at high signal‐to‐noise ratios (SNR). Simulation results also show that TC‐STLD with 1 additional bit achieves an insignificant SNR gain of approximately 0.05 dB over the conventional STLD, while TC‐STLD with 2 additional bits achieves an SNR gain of approximately 0.12 dB.     

Compensator-based position control of an electrorheological actuator

This paper addresses the question of whether compensator-based controllers can be used to successfully control the position of a model scale robot arm attached to the output shaft of an electrorheological (ER) actuator, with a view of recommending the actuator for robotic applications and providing an alternative control strategy to proportional plus integral plus derivative, (PID), control. An ER actuator is a device that uses a ‘smart’ material, called an ER fluid or suspension, as a coupling medium between the prime mover running at a constant speed and a load. Three compensator-based controllers—the phase-lag, the phase-lead and lead-lag compensators—were designed and implemented on the ER actuator-arm system. The phase-lead and phase-lag compensators gave reasonable performance. The lead-lag compensator, on the other hand, was impossible to implement.     

Mesh IP Video Surveillance Systems Model Design and Performance Evaluation

This paper proposes a model for Internet Protocol (IP) mesh video surveillance systems and presents the performance evaluation of Ah-hoc On-demand Distance Vector (AODV) and Open Shortest Path First (OSPF) routing protocols in wireless mesh video surveillance system. A wireless mesh video surveillance network consists of IP cameras linked to mesh routers which are then linked to the mesh gateways. Local monitoring can be done by inserting a switch or router between the gateways and the Internet while remote monitoring can be done through the Internet. Routing provides selection, constructing and management of routes in order to maximize throughput and minimize video packet losses, end-to-end delays and Jitter. Results show that the OSPF routing protocol outperformed the AODV in throughput, packet loss, end-to- end delay and Jitter terms with a throughput advantage of 35%.     

Factors associated with human immunodeficiency virus testing among tuberculosis patients receiving treatment at health facilities in Uganda

地点:乌干达的1个近郊区和4个郊区.目的:了解结核病患者中人类免疫缺陷病毒(HIV)检测水平及相关影响因素.设计:2007年8-11月间对选取的5个地区进行了横断面调查.本研究纳入了在结核病和艾滋病医疗卫生机构接受抗结核治疗,并返回取药的、年龄≥18岁的患者,排除重症、不会讲英语或当地学习用语的患者.诊断为结核病的患者自我报告是否接受过HIV检测,用门诊记录进行确认.结果:本研究纳入261例患者.其中,169例(65％)患者在诊断为结核病之后接受了HIV检测.经多变量分析发现,患者年龄＞45岁(比值比0.27,95％可信区间0.08～0.87)、患者未获得TB/HIV相关性信息(比值比0.35,95％可信区间0.15～0.77)、医疗卫生机构/医务人员未提供HIV检测(比值比0.02,95％可信区间0.006～0.042)、患者对就诊的私密空间不满意(比值比2.49,95％可信区间1.11～5.55),以及患者在门诊就诊时花费了30～60 min(比值比4.48,95％可信区间1.66～12.10),这些因素明显影响了HIV检测水平.结论:按照政策,所有结核病患者均应接受HIV检测.但本研究发现结核病患者的HIV检测水平不甚令人满意.乌干达卫生部应在本研究强调的重点地区的医疗卫生机构继续扩展HIV检测工作及其他TB/HIV合作服务.     

Performance of type-II hybrid ARQ in a shadowed Rician fading LEOsatellite-based spread slotted ALOHA communication channel

Type-II hybrid ARQ is applied to a shadowed Rician fading low earth-orbiting (LEO) satellite-based spread slotted ALOHA communication channel. In particular it is found to be effective in combating heavy shadowing