中国移动传输建设的切入点——无线接入

本文首先介绍了无线接入的特点,然后从覆盖范围、系统容量、业务能力、适应范围等方面比较了3．5GHz固定无线接入与LMDS无线接入的区别。文章指出,3．5GHz固定无线接入适合于业务发展初期进行宽范围的一般覆盖,从而上提供业务;而LMDS适合于业务量集中的小范围热点地区覆盖,从点上提供业务。     

本地多点分配业务(LMDS)系统技术

介绍本地多点分配业务（LMDS），主要包括LMDS系统的业务应用、LMDS系统结构、LMDS系统技术特点、LMDS系统设备等。     

宽带无线接入方案—LMDS

本地多点分配业务系统（Local Multipoint Distribution Service-LMDS)是宽带无线接入系统的代表。运用LMDS的目的是实现用户远端到骨干网的宽带无线接入，包括语音、数据、图像的传输，也可作为Internet的接入网。本简单介绍了什么是LMDS、LMDS的技术特点、LMDS系统结构，LMDS技术支持的业务以及LMDS的优势和局限性，并讨论了LMDS的应用前景。     

LMDS宽带无线接入技术

本地多点分配业务（LMDS）是解决通信网接入问题的有效技术,国际电联规划的LMDS可用频带比DBS、MMDS的要宽得多。LMDS系统包括基站系统、用户端设备和网络管理系统,可以提供多种业务。目前LMDS的商用系统已经正式投入运营,我国也已在广州等城市开展了LMDS的测试工作。     

LMDS宽带无线接入技术及其应用

LMDS宽带无线接入技术代表了宽带接入技术的一种新的不可忽视的发展趋势.本文介绍了LMDS技术的主要技术特点;讨论了LMDS系统的组成、LMDS系统的主要技术要点;分析了LMDS系统提供的业务及LMDS技术的广泛应用.     

浅谈宽带固定无线数据接入技术

本地多点分配系统(LMDS)利用点对多点的覆盖,可实现包括高速因特网接入、电路(TDM)/电路仿真业务(CES)、虚拟专用网和VoIP等多种宽带固定无线接入应用。本文介绍了该业务的技术特点、系统组网结构、服务范围及业务运营前景。     

盈利"无线"--LMDS商用前景分析

LMDS国内发展前景分析在国内,LMDS受到运营商的越来越广泛而深入的关注,由于中国大城市数量多,人口密集,因此建设LMDS单位成本特别低,对运营商来说效益比别的接入方式更加显著,因而LMDS在中国有特别广阔的发展前景。LMDS具有以下技术特点:*可提供极高的通信带宽:LMDS工作在26GHz微波波段附近,可用频带1GHz以上,支持所有的话音和数据传输标准,理论上可以提供所有业务。*蜂窝式的结构配置可覆盖整个城域范围:每个蜂窝站的覆盖区为2～10公里,覆盖区可相互重叠。每个蜂窝的覆盖区又可以划分为多个扇区,可根据需要在该扇区提供特定业…     

宽带固定无线接入技术的研究

固定无线接入技术主要有多路多点分配业务(MMDS)和本地多点分配业务(LMDS)。一个完善的LMDS网络是由主干网络、网络运行中心(NOC)、基站系统和用户端设备组成。LMDS可同时向用户提供话音、数据及图像等综合业务,还可以提供承载业务,如蜂窝系统或PCS/PCN基站之间的传输等。     

接入技术的革命——LMDS

本地多点分配业务(LMDS)是一种宽带固定无线接入技术,它可以承载包括话音、数据和图像在内的各种业务。其特点为:大容量,多业务,易建设,见效快;可根据用户需要进行建设及网络运行、维护费用低等优点。LMDS网络由骨干网络、网络运行中心、服务区中的基站系统和服务区中的用户端设备组成。详细介绍了LMDS系统的网络结构和系统传输方式。     

LMDS技术——未来的“无线光纤”

LMDS是一种微波宽带系统,它工作在微波频率的高端（20～40GHz）,组网灵活方便,使用成本低,是一种非常有前途的宽带固定无线接入解决方案。它可以在较近的距离点对多点双向传输语音、视频和图像信号等多种宽带交互式数据及多媒体业务,被誉为“无线光纤”。LMDS技术具有以下一些特点：可提供极高的通信带宽—LMDS工作在28GHz微波频段附近,可用频带为1GHz以上;蜂窝式的结构配置可覆盖整个城域范围;可提供多种业务—它在理论上可以提供所有业务,并可支持所有的语音和数据传输标准;能提供模拟和数字视频业务,如远程医疗、高速…     

Study of LMDS Videoconferencing System

LMDS is one kind of fixed broadband wireless access network. It uses radio spectrum between 10GHz and 40GHz to transmit voice, video, and data signals within small cell 3-10 miles in diameter. This paper proposes some key technical scenarios to support videoconferencing services transported on LMDS. Firstly, two kinds of typical LMDS lower layer standards made by DAVIC and 802.16 separately are discussed from the angle of supporting video conferencing services. Then, H.323 is studied and recommended as the standard of LMDS video conferencing system. And some strategies are proposed to support H.323 services on LMDS. After that, IP multicast technology, the key supporting technology of H.323, is studied from the angle of running on LMDS. Scenarios for IGMP, RMTP protocol running on LMDS are suggested and analyzed with Markov network queuing theory. The technical scenarios proposed by this paper make use of the transporting property of LMDS to support LMDS videoconferencing system more effectively.     

固定式宽带无线接入技术--LMDS

本地多点分配业务(LMDS)是近年来在国际上兴起的固定式宽带无线接入技术,具有传输容量大、起始费用低的优点,受到了国内外电信业务经营者的普遍关注.文章介绍了LMDS提供的业务类型、工作频率、系统结构和标准化情况,并将LMDS与有线电视网络、ADSL、MMDS等现有的接入技术进行了比较.文章认为LMDS的商业推广尚需解决信号质量、服务区尺寸、费用等制约因素.     

Parallel concatenated convolutional coding for an LMDS system withmultimedia services

The performance of LMDS (local multipoint distribution service) system with parallel concatenated convolutional coding (PCCC) is analyzed and simulated. The channel of a LMDS system is modeled as a Rician fading channel with strong Rician factor because it is generally accepted that there always exists a line-of-sight (LOS) path between base station and subscriber units. The performance is evaluated in terms of codeword error probability. From the simulation results, it is confirmed that the performance is significantly improved by employing the PCCC. The results in the paper can be applied to radio modem design of a LMDS system     

Turbo码在LMDS系统中的应用

介绍了LMDS(LocalMultipointDistributionService)系统结构及系统的信道模型和前向纠错(FEC—ForwardErrorCorrection)机制。易受环境影响的LMDS系统信道特性决定了需要采用纠错能力强的纠错编码方法,在IEEE802.16工作组草案建议的MSUI(ModifiedStanfordUniversityIn-terim)信道模型下,分析了LMDS系统前向纠错采用Turbo码时的译码性能并仿真比较了RS(Reed-Solomon)码和Turbo码的误码率,同时也比较了不同Turbo码译码方法的性能。从仿真结果来看,Turbo码能够明显提高LMDS系统的前向纠错性能,可以应用到实际LMDS系统中去。     

Two‐layer LMDS system architecture: DAVIC‐based approach and analysis

Despite the growing interest for LMDS systems there have been only a few commercial implementations until now especially outside of the U.S.A. The use of hierarchical structure through two‐layer networking has been even rarer. In many cases LMDS systems have strong advantages against their competitors to cover the last mile. In this article, we review and analyze the standards currently available and describe the European two‐layer trial system developed in 1996–2000. We show why further development towards IP based LMDS is useful in the future. Most of our recommendations are based on results derived from the European Union supported research project CABSINET. It had the aim of demonstrating the viability of a 40 GHz cellular digital television system with a return channel to offer interactive services. Two systems were tested: a line of sight link using QPSK, and a non‐line of sight with COFDM modulation scheme. In the RF‐subsystems, the greatest difficulty of any viable LMDS system is to obtain a moderately low price for the user receiver, while fulfilling the hard OFDM requirements in terms of phase noise, stability and spectrum restrictions. Several options have been studied in order to design the subsystems with the smallest cost. This paper will present the architectures of the transmitters, nomadic terminals, and the design of the IF/RF subsystems for both types of modulations. The discussion is focused on system engineering and selections required in order to build a full two‐layer LMDS system. Copyright © 2002 John Wiley & Sons, Ltd.     

Site diversity against rain fading in LMDS systems

Local Multipoint Distribution Services (LMDS) system is a fixed cellular radio based point-to-multipoint technology in the microwave band providing broadband services. These systems need very accurate network planning method because of the variable carrier-to-interference ratio (C/I) in the whole service area and high sensitivity for meteorological phenomena especially for precipitation. The effects of a moving rain cell over an LMDS system was analyzed and site diversity as one possible countermeasure technique is demonstrated. The location dependent C/I in the LMDS service area under rainy conditions with and without site diversity technique was calculated applying a rain shower profile.     

激光金属沉积成形基板预热的研究

为了降低激光金属沉积成形过程中试样和基板间的温度梯度,减小和抑制成形过程的热应力,提高试样的成形质量,提出并设计了一种用于激光金属沉积成形的基板预热系统。该系统由基板预热器、智能比例微分积分控制器以及计算机串口温度采集反馈控制等3部分组成。利用自行研制的激光金属沉积成形设备和基板预热系统进行了成形实验。实验结果表明,基板预热可以改善试样的成形质量,降低成形过程的热应力。     

无线固定宽带技术LMDS的研究

本地多点分配技术(LMDS)是一种崭新的宽带无线接入技术,支持话音、数据、因特网及视频图像等业务,工作频段高于20GHz.本文主要介绍了LMDS的技术特点、系统组成、制约因素、对策和发展现状.另外,还给出LMDS系统中有待于研究的问题.