蜂窝式移动电话系统

Cellular Mobile Telephone System

One of many reasons for developing a cellular mobile telephone system3and deploying it in many cities is the operational limitations of conventional mobile telephone systems: limited service capability, poor service performance, and inefficient frequency spectrum utilization.

A major problem facing the radio communication industry is the limitation of the available radio frequency spectrum. In setting allocation policy, the5Federal Communications Commission (FCC) seeks systems which need minimal bandwidth but provide high usage and consumer satisfaction.The ideal mobile telephone system would operate within a limited assigned frequency band and would serve an almost unlimited number of users in unlimited areas. Three major approaches to achieve the ideal are:

1. Single-sideband (SSB), which divides the allocated frequency bandinto maximum numbers of channels;

2. Cellular, which reuses the allocated frequency band in different geo-graphic locations;

3. Spread spectrum, frequency-hopped, which generates many codes over a wide frequency band.

In41971,4the4computer5industry5entered5a6new5era.5Microprocessors6and5minicomputers are now used for controlling many complicated features and functions with less power and size than was previously possible. Large-Scale Integrated (LSI) circuit technology reduced the size of mobile transceivers so that they easily fit the standard automobile. These achievements were a few of the requirements for developing advanced mobile phone systems and encouraging engineers to pursue this direction.

Another4factor5was the6price6reduction5of the mobile5telephone unit. LSI technology and mass production contribute to reduced cost so that in the near future an average-income family should be able to afford a mobile telephone unit.

On Jan.4, 1979, the FCC authorized Illinois Bell Telephone Co. to conduct a developmental cellular system in the Chicago area and make a limited commercial offering of its cellular service to the public. In addition, American Radio Telephone Service Inc.3 (ARTS) was authorized to operate a cellular system in the Washington, D.C.4--Baltimore, Md., area. These first systems showed the technological feasibility of cellular service.

Why 800 MHzS? The FCC's decision to choose 800 MHz was made because of severe spectrum limitations at lower frequency bands. FM broadcast- lng services operate in the vicinity of 100 MHz. The television broadcasting service starts at 41 MHz and extends up to 960 MHz. Air-to-ground systems use 118 to 136 MHz~ military aircraft use 225 to 400 MHz. The maritime mobile service is located in the vicinity of 160 MHz. Aisc fixedstation services are al-located portions of the 30 to 100 MHz band. Therefore, it was hard for the FCC to allocate a spectrum in the lower portions of the 30 to 400 MHz band since the services of this band had become so crowded. On the other hand, mobile radio transmission cannot be applied at 10 GHz or above because severe propagation path loss, multipath fading, and rain activity make the medium improper for mobile communications.

Fortunately, 800 MHz originally assigned to educational TV channels. Cable TV service became a big factor in the mid-70s and shared the Icad of providing TV channels. This situation opened up the 800 MHz band to some extent, and the FCC allocated a 40 MHz system at 800 MHz3to4mobile4radio4cellular5systems.                                                             

43A basic cellular system consists of three parts, a mobile unit, a cell site, and a mobile telephone switching office (MTSO), with connections to link the three subsystems.

1. Mobile units. A mobile telephone unit consists a control unit, a trans-ceiver, and an antenna system.                                             

2. Cell site. The cell site provides interface between the MTSO and the

mobile units. It has a control unit, radio cabinets, antennas, a power plant, and data terminals.

3. MTSO. The switching office, the central coordinating element for all cell sites, consists of the cellular processor and cellular switch. It interfaces with telephone company zone offices, controls call processing and handles billing activities.

4. Connections. The radio and high-speed data links connect the three subsystems.

Each mobile unit can only use one channel at a time for its communication link. But the channel is not fixed; it can be any one in the entire band assigned by the serving area, with each site having multichannel capabilities that can connect simultaneously to many mobile units.The MTSO is the heart of the cellular mobile system. Its processor provides central coordination and cellular administration.The cellular switch, which can be either analog or digital, switches calls to connect mobile subscribers to other mobile subscribers and to the nationwide telephone network6. It also contains data links providing supervision links between the processor and the switch and between the cell sites and the processor. The radio links carries the voice and signaling between the mobile unit and the cell site. Microwave radio links or wire lines carry both voice and data between the cell site and the MTSO.

GSM (Global System for Mobile Oommunication)

The success of mobile systems across the world is a sign that communication is moving towards a more personalized, convenient system:. People who have to use a mobile phone on business soon begin to realize that the ability to phone any time, any place in one's personal life rapidly becomes a necessity, not a convenience. 

The speed and rapidity with which the personal communications revolution takes place is, unlike fixed transmission systems, highly dependent on technology and communication standards.
For mobile the three key elements to achieving service take-up are the cost,the size and the weight of the phone,and the cost and quality of the link3. If any of these are wrong,especially the first two, then market growth is liable to be severely restricted.

The fixed telephone service is global and the interconnection varies from coaxial cable to optical fibre and satellite. The national standards are different, but with common interfaces and interface conversion, interconnection can take place. For  mobile the problem is far more complex, with the need to roam creating a need for complex networks and systems. Thus in mobile the question of standards is far more crucial to success than fixed systems. In addition, there is also the vexed question of spectrum allocation in the mobile area.

Mobile systems originally operated in analog mode in the 450 MHz band, moving later to 900 MHz with digital GSM and then to 1 800 MHz with personal communication systems. The history of mobility can split into generations. The first generation systems were the advanced mobile phone systems (AMPS) in the US, total access communication system (TACS) in most of Europe and Nordic mobile telephone system (NMT), which were all analogue systems. The second generation is much dominated by the standard first set out in Europe by the group special mobile (GSM) committee, which was designed as a global mobile communication system.

The GSM system is based on a cellular communications principle which was first proposed as a concept in the 1940s by Bell System engineers in the US. The idea came out of the need to increase network capacity and got round the fact that broadcast mobile networks, operating in densely populated are as, could be jammed by a very small number of simultaneous calls5, The power of the cellular system was that it allowed frequency reuse.

The cellular concept is defined by two features, frequency reuse and cell splitting. Frequency reuse comes into play by using radio channels on the same frequency in coverage areas that are far enough apart not to cause co-channel interference. This allows handling of simultaneous calls that exceed the theoretical spectral capacity. 

Cell splitting is necessary when the traffic demand on a cell has reached the maximum and the cell is then divided into a microcellular system. The shape of cell in a cellular system is always depicted as ahexagon and the cluster size can be seven, nine or twelve.

The GSM system requires a number of functions to be created for a fully operational mobile system. The cell coverage area is controlled by a base station which is itself made up of two elements. The first element is the transmission system which communicates out to the mobile and also receives information from it to set up and maintain calls when actually in operation. Tine base station transceiver (BST) is controlled by the base station controller (BSG), which communicates with the mobile switching center (MSC)  the essential link to the local public switched telephone network (PSTN), and to the subscriber data which is stored in registers within the system. The subscriber registers allow the GSM system to check a subscriber who requests the use of the network, allow access and then set up the charging function, etc.

The GSM system was allocated part of the 900 MHz band at the 1978 World Administration Conference (WAC), the actual bands being 890 to 915 MHz for the uplink transmission and 935 to 960 MHz for the downlink. The access method is time division multiple access (TDMA).

The GSM system operates in a burst transmission mode with 124 radio channels in the 900 MHz band, and these bursts can carry different types of information. The first type of information is speech, which is coded at 6.5 kbit/s ,or 13 kbit/s. The second type is data, which can be sent at 3. 6 kbit/s, 6 kbit/s or 12.6 kbit/s. These two forms of transmission are the useful parts of the transmission, but have to be supported by overhead information which is sent in control channels (CCH).

The use of digital radio transmission and the advanced handover algorithms between radio cells in GSM network allows for significantly better frequency usage than in analogue cellular systems, thus increasing the number of subscribers that can be served. Since~ GSM provides common standard, cellular subscribers will also be able to use their telephones over the entire GSM service area. Roaming is fully automatic between and within all countries covered by GSM system. In addition to international roaming, GSM provides new services, such as high-speed data communication, facsimile and short message service, The GSM technical specifications are designed to work in concert with other standards, e.g. ISDN. Interworking between the standards is in this way assured. In the long term perspective cellular systems, using a digital technology, will become the universal method of telecommunication.

蜂窝式移动电话系统 

由于移动电话系统存在容量有限、性能差、频谱利用率低的缺点，发展蜂窝式移动电话系统并在一些城市推广成了当务之急。

无线通信领域面临的一个主要问题是可使用的无线频谱有限。在确定分配政策时，美国联邦通信委员会寻求的是只需最小的带宽却能提供高使用率并使用户满意的系统。理想的移动电话系统将在有限的给定频段上工作，但却可以向任意多的地区中几乎是任意多的用户提供服务。实现这种理想系统的方法主要有三种：

单边带(SSB)，可将给定的频段分为最多的信道。

蜂窝式，可使给定的频段在不同的地理位置上重复使用。

扩展频谱与跳频，能在宽频带上产生许多代码。

1971年，计算机工业进入了一个新纪元。现在许多复杂的特性和功能都使用微处理机和小型计算机进行控制，这比以前的控制方法减少了功率，缩小了体积。

规模集成电路技术减小了移动收发两用机的体积，而容易安装在标准汽车中。这些成果满足了发展先进的移动电话系统的一些要求，促使工程人员进一步向这个方向努力。 

另一个因素是移动电话机的价格下降。大规模集成电路技术和批量生产带来的价格下降，使在不远的将来普通收入水平的家庭也用得起移动电话。

1979年的1月4日，美国联邦通信委员会授权伊利诺斯贝尔电话公司在芝加哥地区开发蜂窝式系统，并向公众提供一定的商业性服务。另外，美国无线电话服务公司(ARTS)被授权在首都华盛顿到马里兰州的巴尔的摩地区建立蜂窝式系统。这些早期系统显示了蜂窝式移动通信的技术可行性。

为什么选择800 MHz作为蜂窝移动电话通信的频率?美国联邦通信委员会选择800 MHz是因为低频段的频谱非常有限。调频广播工作在100 MHz附近，电视广播工作在41～960 MHz。空对地系统使用118—136 MHz，军用飞机使用225～400 MHz，海事移动服务在160 MHz附近，地面固定无线电台使用30～100MHz。30～400 MHz低频段这样拥挤使得联邦通信委员会很难将移动电话频率安排在此波段。另一方面，移动无线传输不能使用10 GHz以上波段，因为严重的传输路径损耗、多径衰落和降雨，使该波段的空间媒质不适宜移动通信。所幸的是，800 MHz原来安排的是教育电视。70年代中期，有线电视的发展分担了电视频道的业务，在某种程度上缓解了800 MHz波段的压力，使联邦通信委员会在800 MHz处为移动无线蜂窝系统分配了40 MHz的频带。

一个蜂窝式系统基本上由三部分组成：移动机、无线基地站和移动电话交换局(MTSO)，并且这三部分互相联通。

移动机。移动电话机包括控制单元、收发信号机和天线系统。

     无线基地站。无线基地站在移动电话交换局和移动机之间提供接口，它有控制单元、无线机柜、天线、电源装置和数据终端。

移动电话交换局。它是所有无线基地站的协调中心，色括蜂窝处理机和蜂窝交换机。它与电话公司的地区交换局接口，控制呼叫处理并进行计费业务。

     连线。无线线路和高速数据线连接以上三个部分。

每个移动机每次只占用通信线路的一个频道。频道不是固定的，可以是服务区分配的全部频道中的任何一个。每一个无线基地站具有多频道能力，可同时联通许多移动机。移动电话交换局是蜂窝移动系统的中心，它的处理机起到了中心协调和蜂窝管理的作用。蜂窝交换机既可以是模拟的也可以是数字的，它对移动用户之间和移动用户与全国电话网之间的呼叫进行交换连接。它还有在．处理机与交换机间和在无线基地站与处理机间起监控作用的数据线。无线线路在移动机和无线基地站间传输话音信号和信令。微波无线线路或有线线路在无线基地站和移动电话交换局之间传送音频信号和数据信号。

全球移动通信系统，世界范围移动通信的成功标志着通信正在向着更加个人化、更加方便的通信系统迈进。商务今需使用移动电话的人们很快就认识到在午人生活中随时随地打电话不仅仅带来了方便，而且是一种必需。不像固定通信系统，个人通信的快速发展在很大程度上依赖于技术和通信标准的发展。移动通信取得业务腾飞的三个关键因素是费用、手机的大小和重量以及链路的收费和质量。如果其中任何一个不能令人满意，尤其是前两项，移动通信市场增长就有可能受到严重的限制。

固定电话业务是全球性的，系统的相互连接采取同轴电缆、光纤甚至卫星。虽然各国标准不同，但却采用共同的接口和接口转换装置，使得相互连接可以进氘对移动通信来说，问题要更加复杂，这是由于需要漫游业务就要有更复杂的网络系统。这样比起固定通信系统，移动通信的标准问题是取得成功更为关键的因素. 此外，在移动通信领域，频谱的分配也是令人苦恼的问题。

移动通信系统最初以模拟方式工作(现在仍在运行)，其频带在450 MHz，后来逐步发展到900 MHz的数字GSM系统，下一步是向工作在1 800 MHz的个人通信系统发展。移动通信的发展可分为几代。第一代系统如美国的高级移动电话系统(AMPS)、欧洲多数国家的全接入通信系统(TACS)以及北欧的移动电话系统(NMT)，这些都是模拟系统。第二代移动通信系统在很大程度上是由欧洲特别移动小组(GSM)委员会所制订的标准决定的，设计这一系统作为一种全球移动通信系统。

GSM系统基于蜂窝通信系统原则。这一概念是由美国贝尔系统工程师在40年代首先提出的。这一思想出自于增加网络容量的需要以及解决网络堵塞的问题。在人口稠密地区运行的广播式移动网络系统会由于很少的几个用户同时呼叫而引起堵塞。蜂窝系统的威力在于允许频率的再利用。

蜂窝概念由两个特点确定，即频率重复利用和小区分割。在相邻覆盖区域相隔足够远而不至于引起共用信道干扰时，过使用同一频率的无线信道，频率再利用才起作用。这样可以处理同时出现并超过了理论频谱容量的呼叫。  当小区的业务需求增到最大时，就要进行小区划区，小区再被分成更小的蜂窝系统区域。蜂窝系统的小区形状常被描绘成六边形，一群六边形小区的数量可以是7个、9个或12个。对于一个充分运营的移动系统来说，GSM系统需要建立一些功能。小区的覆盖面由一个基站控制，基站本身由两个单元组成。第一个单元是传输系统，它在实际运行中与移动台进行通信以建立并保持通话。第二个是基站收发信机(BTS)，由基站控制器(BSC)控制，而BSC与移动交换中fu(MSC)进行通信。这条通信链路，对于MSC与本地公共交换电话网(PSTN)的链接以及连接存储在系统寄存器中的用户数据来说，都十分重要。用户寄存器使GSM系统核查需要使用网络的用户，允许接人并建立收费功能等。1978年由世界管理会议(WAC)分配给GSM 900 MHz的部分频带，实际的频带是890～915 MHz用于上行传输，935—960 MHz用于下行传输。接Af式是时分多址(TDMA)方式。

GSM系统以突发脉冲序列传输方式工作。在900 MHz频带有124个无线信道，这些突发脉冲序列可以传送不同的信息。第一类信息是话音，编码速率为6．5 kb/s或13 kb/s。  第二类信息是数据，可以3．6 kb/s，6 kb/s或12．6kb/s的速率传输。这两类信息是传输信息的有用部分，但是传输还必须得到额外开销信息的支持，开销信息通过控制信道传输。

GSM网络使用数字无线传输和先进的无线越区切换算法，可以得到比模拟蜂窝系统好得多的频率利用，因而增加了服务的用户数。  由于GSM可提供共同的标准，蜂窝用户就可以在整个GSM服务区使用其电话。在GSM覆盖的国内或国际区域内，其漫游是完全自动的。除了国际漫游外，GSM还可提供新型用户服务，如高速数据通信、传真和短电丈业务。GSM技术规范可与其他通信标准保持一致，如ISDN。这样可保证标准间的互通。展望未来，利用数字技术的蜂窝系统将成为通信的通用方式。