王海龙——基于MSC-51单片机计算器(7)

哈尔滨远东理工学院学士学位论文 CDMA cellular system can provide full connectivity between the microcells and the overlaying macrocells without capacity degradation. Reference analyzes several factors that determine the cell size, the soft handoff (SHO) zone, and the capacity of the cell clusters. Several techniques for overlay-underlay cell clustering are also outlined. Application of CDMA to microcell/macrocell overlay have the following major advantages:

? A heterogeneous environment can be illuminated uniformly by using a distributed antenna (with a series of radiators with different propagation delays) while still maintaining a high-quality signal.

? SHO obviates the need for complex frequency planning.Reference studies the feasibility of a CDMA overlay that can share the 1850–1990 MHz personal communications services (PCS) band with existing microwave signals (transmitted by utility companies and state agencies). The results of several field tests demonstrate the application of such an overlay for the PCS band. The issue of use of a CDMA microcell underlay for an existing analog macrocell is the focus of. It is shown that high capacity can be achieved in a microcell at the expense of a slight degradation in macrocell performance.Reference finds that transmit and receive notch filters should be used at the microcell BSs. It shows that key parameters for such an overlay are the powers of the CDMA BS and MS transmitters relative to the macrocell BSs and the MSs served by the macrocells. Reference [25] studies spectrum management in an overlay system. A new cell selection method is proposed, which uses the history of microcell sojourn times. A procedure to determine an optimum velocity threshold for the proposed method is also outlined. A systematic approach to optimal frequency spectrum management is described.

Special Architectures There are several special cellular architectures that try to improve spectral efficiency without a large increase in infrastructure costs. Some of

these structures, discussed here, include an underlay/overlay system (which is different from the overlay/underlay system described earlier) and a multichannel bandwidth system. Many cellular systems are expected to evolve from a macrocellular system to an overlay/underlay system. A study that focuses on such evolution is described in [26].

A Multiple-Channel-Bandwidth System — Multiple channel bandwidths can be used within a cell to improve spectral efficiency.In a multiple-channel-bandwidth system (MCBS), a cell has two or three ring-shaped regions with different bandwidth channels [28]. Figure 7 shows an MCBS. Assume that 30 kHz is the normal bandwidth for a signal.Now, for a three-ring MCBS, 30 kHz channels can be used in the outermost ring, 15 kHz channels in the middle ring, and 7.5 kHz channels in the innermost ring. The areas of these rings can be determined based on the expected traffic conditions.

Thus, instead of using 30 kHz channels throughout the cell, different bandwidth channels (e.g., 15 kHz and 7.5 kHz) can be used to increase the number of channels in a cell. The MCBS uses the fact that a wide-bandwidth channel requires a lower carrier-to-interference ratio (C/I) than a narrow-bandwidth channel for the same voice quality. For example, C/I requirements for 30 kHz, 15 kHz, and 7.5 kHz channel bandwidths are 18 dB, 24 dB, and 30 dB, respectively, based on subjective voice quality tests [28]. If the transmit power at a cell cite is the same for all the bandwidths, a wide channel can serve a large cell while a narrow channel can serve a relatively small cell. Moreover, since a wide channel can tolerate a higher level of co-channel interference (CCI), it can afford a smaller D/R ratio (the ratio of co-channel distance to cell radius). Thus, in the MCBS more channels become available due to multiple-bandwidth signals, and frequency can be reused more closely in a given service region due to different C/I requirements.

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哈尔滨远东理工学院学士学位论文 Integrated Wireless Systems Integrated wireless systems are exemplified by integrated cordless and cellular systems, integrated cellular systems, and integrated terrestrial and satellite systems. Such integrated systems combine the features of individual wireless systems to achieve the goals of improved mobility and low cost.

Integrated Terrestrial Systems — Terrestrial intersystem handoff may be between two cellular systems or between a cellular system and a cordless telephone system. Examples of systems that need intersystem handoffs include GSM–Digital European Cordless Telephone (DECT), CDMA in macrocells, and TDMA in microcells. When a call initiated in a cellular system controlled by an MSC enters a system controlled by another MSC, intersystem handoff is required to continue the call [29]. In this case one MSC makes a handoff request to another MSC to save the call. The MSCs need to have software for intersystem handoff if intersystem handoff is to be implemented. Compatibility between the concerned MSCs needs to be considered, too.There are several possible outcomes of an intersystem handoff [29]:

? A long-distance call becomes a local call when an MS becomes a roamer.

? A long-distance call becomes a local call when a roamer becomes a home mobile unit. ? A local call becomes a long distance call when a home mobile unit becomes a roamer. ? A local call becomes a long-distance call when a roamer becomes a home mobile unit. There is a growing trend toward service portability across dissimilar systems such as GSM and DECT [30]. For example,it is nice to have intersystem handoff between cordless and cellular coverage. Cost-effective handoff algorithms for such scenarios represent a significant research area. This article outlines different approaches to achieving intersystem handoff. Simulation results are presented for handoff between GSM and DECT/Wide Access Communications System (WACS). The paper shows that a minor adjustment to the DECT specification can greatly simplify the implementation of an MS capable of intersystem handoff between GSM and DECT.

Integrated Terrestrial and Satellite Systems — In an integrated cellular/satellite system, the advantages of satellites and cellular systems can be combined. Satellites can provide widearea coverage, completion of coverage, immediate service, and additional capacity (by handling overflow traffic). A cellular system can provide a high-capacity economical system. Some of the issues involved in an integrated system are discussed in [31]. In particular, the procedures of GSM are examined for their application to the integrated systems.The future public land mobile telecommunication system (FPLMTS) will provide a personal telephone system that enables a person with a handheld terminal to reach anywhere in the world [32]. The FPLMTS will include low Earth orbit (LEO) or geostationary Earth orbit (GEO) satellites as well as terrestrial cellular systems. When an MS is inside the coverage area of a terrestrial cellular system, the BS will act as a relay station and provide a link between the MS and the satellite.When an MS is outside the terrestrial system coverage area, it will have a direct communication link with the satellite.Different issues such as system architecture, call handling, performance analysis of the access, and transmission protocols are discussed in [32]. The two handoff scenarios in an integrated system are described below.

Handoff from the Land Mobile Satellite System to the Terrestrial

System — While operating, the MS monitors the satellite link and evaluates the link performance. The received signal strengths (RSSs) are averaged (e.g., over a 30 s time period) to minimize signal strength variations. If the RSS falls below a certain threshold N consecutive times (e.g., N = 3), the MS begins measuring RSS from the terrestrial cellular system.If the

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哈尔滨远东理工学院学士学位论文 terrestrial signals are strong enough, handoff is made to the terrestrial system, provided that the terrestrial system can serve the MS.

Handoff from the Terrestrial System to the Land Mobile Satellite System — When an MS is getting service from the terrestrial system, the BS sends an acknowledge request (called page) at predefined intervals to ensure that the MS is still inside the coverage area. If an acknowledge request signal from the MS (called page response) is not received at the BS for N consecutive times, it is handed off to the land mobile satellite system (LMSS).Reference [33] focuses on personal communication systems with hierarchical overlays that incorporate terrestrial and satellite systems. The lowest level in the hierarchy is formed by microcells. Macrocells overlay microcells and form the middle level in the hierarchy. Satellite beams overlay macrocells and constitute the topmost hierarchy level. Two types of subscribers are considered, satellite-only and dual cellular/satellite. Call attempts from satellite-only subscribers are served by satellite systems, while call attempts from dual subscribers are first directed to the serving terrestrial systems with the satellites taking care of the overflow traffic. An analytical model for teletraffic performance is developed, and performance measures such as traffic distribution, blocking probability, and forced termination probability are evaluated for low-speed and high-speed users.

Handoff Evaluation Mechanisms

Three basic mechanisms used to evaluate the performance of handoff algorithms include the analytical, simulation, and emulation approaches. These mechanisms are described here. The Analytical Approach This approach can quickly give a preliminary idea about the performance of some handoff algorithms for simplified handoff scenarios. This approach is valid only under specified constraints (e.g., assumptions about the RSS profiles). Actual handoff procedures are quite complicated and are not memoryless.This makes the analytical approach less realistic. For real-world situations, this approach is complex and mathematically intractable. Some of the analytical approaches appearing in the literature are briefly touched on below.

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哈尔滨远东理工学院学士学位论文 附录 B

蜂窝系统切换技术

蜂窝系统部署方案

其无线电传播环境和相关切换的难度是在于针对不同的单元结构.在不同的系统环境中,固定参数切换算法不能发挥优势,.而通信系统的具体特点,应考虑而设计的切换算法是:几个基本的单元结构（如宏蜂窝,微单元,并覆盖系统）及（如:特殊结构,多通道带宽系统和演化架构）是描述将来.集成的无线和蜂窝系统,蜂窝系统集成,综合地面卫星系统也作了相关说明.

宏单元

宏的定义是达数公里半径.由于低单元传输率,集中式切换是可能的,尽管大量的各成员国已进行了海安管理.但是在上行和下行信号质量大约是相同的.考虑基地台之间的过渡区,大切换计划应允许一些延迟,以避免信号不稳定.然而,延误应足够短,以保持信号的质量,应对新的穿透移动台单元的干扰增加.这被称为的“单元拖动”.宏单元有相对平缓的路径损耗特性.在平均间隔（即使用一段时间的平均信号强度变化）应得到足够长的波动减少衰退.第一代和第二代蜂窝系统,即在城市大面积的报道.通常,在收发器宏单元传输与高输出功率的天线安装在塔高数米,覆盖大片区域.

微单元

某些技术的提高（例如,更大的带宽,编码,改进方法,语音信道编码和调制）将不足以满足需要的服务需求.无线资源管理目前变得更加困难,因此微单元增加容量的能力被认为是增加单元系统的一个最有效的手段.微单元可分为一,二,或了三维,取决于他们是否沿着道路或公路这些区域进行覆盖,如相邻道路,或在多层建筑物内分别编号.微单元可分为焦点（与通信密度较高或覆盖不足地区服务的地区）,市中心聚集微单元（行人和手机间的相邻地区服务）,并在建设3 – D单元（在职办公大楼和行人）.

通常,在一个微蜂窝系统收发天线传输与低输出功率在路灯水平（大约离地面5米）.安装的移动台转送保持低功耗,从而延长电池寿命.使天线具有比周围的建筑物更低的高度,射频信号的天线大多集中在街区.信号发射有可能包括在每个街道方向100-200米,服务几个街区.此传播环境具有较低的时间分散,这使得可以提高数据速率.

微单元比宏单元通信更敏感.由于短期变化（如干扰,交通和干扰变化）,中/长期变化（如新的建筑物）,以及无线网络（例如,新的增量增长基地台）.每单元交接数目增加了一个数量级,以及可利用的时间作出交替下降.利用中转单元是其中一个不错方法,以减少切换率.由于微蜂窝接口要处理预期的高增长的通信负荷,让切换过程中的权限下放程度较高成为一个必要.

微单元遇到阻塞现象称为拐角效应.拐角效应的特点是围绕一个角落移动时信号处理突然大幅下降（例如,在20-30分贝信号强度（例如）在10-20米的距离）时.由于从服务基站组件到移动台的视线丢失（LOS）.拐角效应需要一个更快的切换和可以快速提高的信号质量.角落里的效果是很难预测.测量平均间隔的不可取是由于拐角效应.运动障碍可以暂时阻碍之间的信号传递,这类似于一个角落效应.参考了在曼哈顿式的环境对称单元计划的研究.单元计划影响的信号来干扰比（SIR）的上行和下行的性能显着.对称单元计划同时距离基地台有四个最近的交流路径.这种单元计划可以分为半平方米（房协）,全方（财经事务）,和矩形（R）的单元计划.这些单元是描述未来的计划.

半广场单元计划

此单元格计划在每个路口基站全向天线的,并且每个微蜂窝占地50份在所有范围的四个方向.此单元格计划避免了街角的效果,并提供最高的容量.此计划是穿透单元交接.图2显

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哈尔滨远东理工学院学士学位论文 示了一半的系统计划,其中在一平方单元微孔为例1. 全区域单元计划

有其他的十字路口位于每个天线与全方位微蜂窝,微蜂窝的每块覆盖在所有四个方向.这是可能的移动台体验这个单元街道拐角效应的计划.计划的全方位单元可以有一个例子,一个全方位的单元计划计划在微孔视角.从楼院或矩形单元的非视距切换.图3显示了每一个微蜂窝学位包括一小部分的水平或垂直的街道正中,及位于微蜂窝该单元格.此单元格图可以很容易地适应市场的需要.较少的基地台发射功率高,可用于初步.随着用户密度的增加,可以添加新的基地台与基地台发射功率降低适当.街道拐角效应是可能的这个单元的计划.计划的R单元可以有非视距或视线交接.图4显示矩形系统单元中的微孔计划的一个例子.

宏/微小区覆盖

微单元会拥塞缺乏某种服务的某些地区,而有些用户的切换率过高等一些原因会令微单元负载[13].为了减轻这些问题,一些单元的混合单元结构（称为覆盖/衬底系统重叠）伞组成的大面积单元或宏单元（称为）和小尺寸微单元（称为衬底单元）都可以使用.图5显示了一覆盖系统.

在宏蜂窝/微蜂窝覆盖架构中提供了一个切换最大化之间的平衡,每个单位面积的用户数量和减少社区间的网络控制与负荷相关的切换.超出微蜂窝覆盖面积宏单元提供广泛的服务领域,并确保做得更好[14].微小区的提供能力利用和覆盖的地区,高密度的通信区（调用）有较大的频率点.例如机场,火车站或停车场.在不太拥挤的通信需求地区（如城市以外地区的主要街道中心或市外）不是很高,宏单元可以提供足够的覆盖面等方面的研究.宏单元也为各成员国和高速微单元所覆盖地区的不足（例如,由于缺乏渠道或范围的移动台正在走出微单元）.此外,在微孔系统用于溢出的通信其最充分的程度是可以路由到宏小区.覆盖/衬垫系统是宏单元和微单元测定的最佳分配的渠道 [15].

参考文献[16] 两层之间的计算方法有四种共享的可用频谱. 方法1用于宏区域的TDMA和CDMA微蜂窝. 方法2用于宏区域的CDMA和TDMA微蜂窝.

爱他们使用在这两个层次的TDMA中,且在这两个办法的第4层采用正交频渠道. 覆盖/衬底系统微蜂窝系统的一些优势 [17]:

?基地台只在需要高流量负荷的地区使用.因为它不是必需的,使该地区与微单元全程服务,同时基础设施的成本节省.

?在系统的覆盖的数目远远低于宏微蜂窝系统,因为在快速移动的车辆可连接到覆盖. ?调用移动台和通信位置可以很容易地做到通过微蜂窝系统.伞单元有几类[17].

第一类在正交渠道分布微单元和巨单元之间.另一个类在微通道使用,微循环利用的巨单元的渠道已经可以处理发射功率稍高的水平分配,抵消了宏单元的干扰.在环境中覆盖/衬底系统中,有4种切换需要管理[19]:微单元,以微蜂窝,微蜂窝向宏,宏向宏,微蜂窝和宏蜂窝.参考文献[20]描述了单元分裂和结合覆盖.单元再利用两个渠道,是通过建立一个覆盖小单元到单元送达相同的大单元基地.小单元可用分裂的单元通道,但因为长的距离限制,大单元不能再利用这些渠道.（覆盖单元约百分之五十多光谱）

效率比细分为（干涉的过程,以避免分配渠道,加强小和大面积的单元.宏蜂窝覆盖系统的实际做法从现行制度的实施中提出了微蜂窝[17].该参考介绍了通道隔离（自组织动态信道分配）和自动传输功率控制,以避免需要设计信道分配和发送微蜂窝系统功率控制.重用现有的渠道之间自动微单元和巨单元,轻微增加传输系统功率的微蜂窝弥补了宏到微单元的干扰.仿真结果表明,本地通信如果是努力安置渠道管理微单元所设下宏单元将没有任何意义.参考方法论的基础）,全球移动通信系统（GSM系统推广到宏蜂窝/微蜂窝覆盖系统[21] .一个覆盖系统和相关的切换对使用随机的频率和自适应跳频频率来规划不同的频率问题的进行了探讨.四个战略的目的都是确定一个合适的覆盖系统为用户的单元[22].有两个战略是基于驻留时间设定（切换时间而没有一个电话可以维持在一个单元格）,和另外两

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