Adaptive mobile line strip multibeam MC-CDMA optical wireless system employing imaging detection in a real indoor environment

Three methods (transmit power adaptation, imaging reception, and Multicarrier Code Division Multiple Access (MCCDMA)) are introduced to the optical wireless (OW) system and a significant improvement is achieved in the presence of very directive noise, multipath propagation, mobility, and shadowing typical in a real indoor environment. In the absence of shadowing, replacing a single non-imaging receiver by an imaging receiver with maximal ratio combining (MRC) improves the signal-tonoise ratio (SNR) by 20 dB in a conventional diffuse system (CDS) operating at 30 Mbit/s at a transmitter-receiver separation of 6m in agreement with previous results in the field. Further SNR improvement of 24 dB is achieved when a line strip multi-beam system (LSMS) replaces the CDS when both systems employ an imaging MRC receiver. Furthermore, our new adaptive LSMS (ALSMS) system coupled with the imaging MRC receiver offers an SNR improvement of 23 dB over the imaging MRC LSMS illustrating the gain achieved through adaptation. The results also show that combining transmit power adaptation with spotdiffusing (i.e. ALSMS) coupled with an imaging receiver based on select best (SB) increases the bandwidth from 46.5 MHz (nonimaging CDS) to 7.53 GHz thus enabling the OW system to achieve higher data rates and provide multi-user capabilities in our case by employing a MC-CDMA scheme. In a 10 user MC-CDMA OW system, a bit error rate (BER) improvement from 4.9 × 10^?1 to 2.1 × 10^?5 is achieved when the imaging MRC ALSMS system replaces the imaging CDS in a shadowed environment.     

Multiple spot diffusing geometries for indoor optical wireless communication systems

In order to improve the performance of indoor optical wireless communication links, two multispot diffusing geometries based on diamond and line strip spot distribution geometries are proposed, analysed and compared to the known uniform spot distribution. Such geometries combine the advantages of the diffuse and the line‐of‐sight systems, giving great robustness and ease of use. The novel line strip multibeam transmitter geometry has resulted in a receiver signal‐to‐noise ratio (SNR) improvement of about 4 dB compared to the conventional diffuse system as well as a significant reduction in the pulse spread. Simulation and comparison results for both the conventional diffuse system and the three multispot diffusing geometries are presented. Further, pulse responses, SNR, and delay spread results at various locations are presented. Copyright © 2003 John Wiley & Sons, Ltd.     

Optimum order of angle diversity with equal-gain combining receivers for broad-band indoor optical wireless communications

Angle diversity is an effective technique to compensate for multipath temporal dispersion in a wireless infrared environment. Diversity is accomplished by using a multibranch receiver capable of resolving multipath. The goal of this paper is to illustrate the effect that the increasing diversity order of a receiver has on the performance of a link in a multispot-diffusing configuration with equal-gain combining outage probability based on probability of bit error is adopted as a performance measure. It is shown that there is an optimal number of branches, which minimizes outage probability. Increasing diversity order beyond this optimal number degrades the performance and increases the receiver complexity, cost, and susceptibility to any shadowing effects.     

Angle diversity and rate-adaptive transmission for indoor wireless optical communications

The main degrading factor in indoor wireless optical communication systems for bit rates up to several megabits per second is the shot noise induced by ambient light (sunlight and artificial light produced by incandescent and fluorescent lamps). Due to the directional nature of both signal and ambient light noise, the spatial distribution of the signal-to-noise ratio in indoor environments can show large variations. This article compares techniques that are able to mitigate the effect of such SNR variations: rate-adaptive transmission and angle diversity. In the first technique, the effective data rate is adjusted to the local SNR conditions by introducing different levels of redundancy. The second technique explicitly explores the directionality of the SNR by combining signals collected from different observation angles. We address the performance of rate-adaptive transmission and angle diversity techniques, and compare them based on experimental results obtained in a typical indoor environment.     

室内MIMO系统采用极化分集时的信道容量

典型的室内传播信道为莱斯衰落信道,可将其分解为LOS传播信道和散射信道.本文假设散射信道为瑞利衰落信道,并根据天线的极化特性,给出了存在极化分集时的LOS传播信道.基于此,本文提出了室内MIMO系统采用极化分集时的信道模型.研究表明:极化分集可以有效提高MIMO信道容量.     

Recovery in the mobile wireless environment using mobile agents

Application recovery in mobile database systems (MDS) is more complex because of an unlimited geographical mobility of mobile units. The mobility of these units makes it tricky to an store application log and access it for recovery. This paper presents an application log management scheme, which uses a mobile-agent-based framework to facilitate seamless logging of application activities for recovery from transaction or system failure. We compare the performance of our scheme with lazy, pessimistic, and frequency-based schemes through simulation and show that compared to these schemes, our scheme reduces overall recovery time by efficiently managing resources and handoffs.     

Joint multipath-Doppler diversity in mobile wireless communications

We introduce a new approach for achieving diversity in spread-spectrum communications over fast-fading multipath channels. The RAKE receiver used in existing systems suffers from significant performance degradation due to the rapid channel variations encountered under fast fading. We show that the Doppler spread induced by temporal channel variations in fact provides another means for diversity that can be further exploited to combat fading. We develop the concept of Doppler diversity and propose a framework that exploits joint multipath-Doppler diversity in an optimal fashion. Performance analysis shows that even the relatively small Doppler spreads encountered in practice can be leveraged into significant diversity gains via our approach. The framework is applicable in several mobile wireless multiple access systems and can provide substantial performance improvement over existing systems     

Two-antenna receive diversity performance in indoor environment

Receive diversity gain performance of a two-antenna setup in an office environment is reported. The terminal is handheld in front of the user simulating a data mode scenario while walking around. The dual polarised base station sector antenna is placed at the end of a corridor. The results indicate that experimentally achieved diversity performance is similar to previously predicted theoretical data.     

240Mbit/s直线运动型无线光通信系统研究

详细分析了直线轨道上运动通信的特点,设计和研制了一种具有双向收发功能的直线运动型无线光通信系统。该系统主要包括1 550 nm半导体激光器、高灵敏度PIN光探测器、球面单透镜式光发射天线和实心光锥耦合式光接收天线等关键部件。130 m运动工作距离的系统性能测试结果表明,在3.5 m/s最高运动速度、240Mbit/s通信传输速率下,运动中的数据丢帧率为零,以太网和视频指标均满足实际应用要求,系统适用于存在直线运动轨道的无线光通信场合。     

一种室内环境下机器人同时定位与地图构建方法

介绍了一种移动机器人室内环境下同时定位与地图构建方法，给出了整个系统的结构框图。将从激光传感信息中提取出的直线特征作为主要的环境描述特征，用EKF算法更新机器人位姿和直线特征的估计值，用一种新的数据关联方法以实现地图的最小描述。直线特征提取采改进的IE2F算法，能够快速地从数据点中得到较为精确的直线参数。实验证明整个系统具有良好的性能。     

Tradeoff between diversity gain and interference suppression in a MIMO MC-CDMA system

In this paper, the uplink of an asynchronous multi-carrier direct-sequence code-division multiple-access (MC-DS-CDMA) system with multiple antennas at both the transmitter and the receiver is considered. We analyze the system performance over a spatially correlated Rayleigh fading channel with multiple-access interference (MAI), and evaluate the antenna array performance with joint fading reduction and MAI suppression. Assuming perfect channel knowledge available at the transmitter, maximal ratio transmission is employed to weight the transmitted signal optimally in terms of combating signal fading. At the receiver, adaptive beamforming reception is adopted to both suppress MAI and combat the fading. Note that while correlations among the fades of the antennas in the receive array reduce the diversity gain against fading, the array still has the capability for interference suppression. We examine the effect of varying the number of transmit and receive antennas on both the diversity gain and the interference suppression.     

海洋湍流下波长分集无线光通信系统性能分析

由于海水的吸收、散射衰减以及海洋湍流效应会引起水下无线光通信（Underwater wireless optical communication,UWOC）系统接收端光信号的闪烁,导致UWOC系统传输性能下降。基于Gamma-gamma分布的海洋湍流信道模型,根据海洋湍流参数和各向异性因子表示的等效结构参数,推导出波长分集UWOC系统中断概率（Outage probability,OP）与平均误码率（Bit error rate,BER）封闭表达式。研究分析随着各向异性因子的增加,具有不同波长分集阶的水下无线光通信系统中断概率与平均误码率的变化,比较接收端使用最佳组合（Optimal combining,OC）与等增益组合（Equal gain combining,EGC）技术的水下无线光通信系统平均误码率,并仿真不同海洋湍流参数、传输距离对波长分集UWOC系统性能的影响。数值结果表明,随着各向异性因子的增加,海洋湍流对水下无线光通信系统产生的影响逐渐减弱,使用波长分集技术的UWOC系统比无波长分集技术的UWOC系统中断概率与平均误码率明显改善。     

MIMO characterization of indoor wireless optical link using a diffuse-transmission configuration

Performance of a diffuse optical link may potentially be degraded by temporal dispersions resulting from surface reflections. In order to devise techniques to alleviate the adverse effect of dispersion, an accurate channel model is needed. Obtaining the impulse response (IR) for a given receiver location requires consideration not only of the direct path, but also of reflections up to nth order. The IR is only valid for a specific location and specific receiver parameters. If a receiver moves, the IR has to be recalculated. We propose a new approach for characterizing diffuse links that results in a tremendous saving in calculation and also gives more insight into the channel characteristics. The new approach is based on consolidating the dependence of receiver parameters, transmitter parameters, and the indoor environment into independent components. Thus, changing one of the parameters of the link requires recalculation of one of these components. The new model is utilized to obtain an accurate profile of delay spread and received power throughout a room.     

高速移动环境下无线信道的检测与估计

在高速移动的环境下,无线信道呈现快速变化的特征,导致对信道的估计难度增加。针对高速移动环境下的信道特征,提出一种新的信道估计方法,根据当前信道信息选取最佳负载因子,对时变信道进行更准确的估计。仿真结果证明了该估计算法的有效性。     

Fast-polarization-hopping transmission diversity to mitigate prolonged deep fades in indoor wireless communications

Fast-polarization-hopping (FPH) transmission diversity is herein proposed to mitigate prolonged deep fades at the mobile receiver in, for example, the indoor propagation environment. Even if the individual multipaths are each sufficiently strong for detection, deep fades may occur due to the multipath signals' destructive summation at the receiver. The relative immobility of the transmitter, the propagation environment, and the receiver in the indoor environment means that a deep fade may last for a very long duration, dropping calls or severing links. By rapidly hopping the transmission polarization (say, alternating transmission between a vertically-polarized-dipole antenna and a horizontally-polarized-dipole antenna - or between two "X"-oriented dipoles), the effective propagation channel experiences consecutive polarization modes (each involving a different multipath summation), all within the duration allowed by the channel-coder's interleaving depth. This scheme is usable for either frequency-shift keying (with incoherent demodulation), or for channel-coded phase-shift keying (with differential coding, or with pilot-symbol phase synchronization). This scheme requires no change in the mobile receiver (which does not need to be dual polarized). The base station also needs no spatially separated antenna array, nor any other additional hardware, no mechanical movement of the transmitting antenna(s), and no sophisticated signal processing (such as channel estimation or closed-loop feedback) nor any additional software. The proposed scheme's cost - relative to using antenna arrays at the base station and/or the mobile - is a potentially doubling of the transmission bandwidth. The proposed scheme's potential is illustrated by limited computer simulations using CINDOOR, a polarization-sensitive indoor wireless-propagation ray-tracing simulation software package based on geometrical optics and the uniform theory of diffraction (GO/UTD).     

Study of a frequency diversity system for indoor digital TV

A wide band measurement campaign has been carried out in the DVB-T frequency band. The coherence bandwidth has been measured and its variation with the antennas separation is studied. Also, the relation between the coherence bandwidth and the RMS delay spread is investigated     

复杂环境下基于CFSFDP的自适应室内定位方法

针对复杂环境下的WI-FI定位受限于多径效应等因素影响,提出一种基于CFSFDP（Clustering by Fast Search and Find of Density Peaks）的自适应室内定位算法。该算法分为三个阶段:第一预处理阶段,采用CFSFDP方法训练原始指纹,从中挖掘出稳定且有效的指纹特征;第二离线阶段进一步构建多层覆盖的采样点策略,建立指纹地图;第三在线阶段针对提取到的RSS信号进行参数训练,建立一种自适应信号传播模型,结合离线阶段的指纹地图实现指纹匹配。指纹地图可弥补自适应传播模型测距方案精度不高的缺陷,而测距方案降低在线阶段指纹批匹配开销。仿真结果表明:本文提出ALCCE算法在复杂环境下具有明显的优势,且使用的测距模型性能较高。      

Cochannel interference rejection in a mobile radio space diversity system

Diversity is often used in radio systems to alleviate the problem of fading. It is shown here that the use of diversity in a mobile radio system also will result in better utilization of the frequency spectrum. In a system which reuses channels in physically separated areas, probability distributions of signal-to-cochannel interference ratios in 1-, 2-, and 4-branch diversity systems are developed by Monte Carlo methods. We consider two cases: first, we assume signal strength to be Rayleigh distributed, which is a valid model for reception within a small localized area; second, we take shadow losses into account by assuming a more realistic long-term fading model. The effect of using diversity in a mobile radio system is considerable with either model. Assuming a hexagonal cell pattern of frequency reuse, it is shown that frequencies may be reassigned more often in a diversity system than in a system which does not use diversity. The conclusion, therefore, is that a diversity system utilizes the frequency spectrum more efficiently than an equivalent nondiversity system.     

Optimization methods for optimal transmitter locations in a mobile wireless system

A combination of a simple indoor propagation model with different optimization methods enables optimal single and multiple transmitter locations and antenna sectorization in wireless systems.