Overview of the Global System for Mobile Communications 1 History of GSM During the early 1980s, analog cellular telephone systems were experiencing rapid growth in Europe, particularly in Scandinavia and the United Kingdom, but also in France and Germany.Each country developed its own system, which was incompatible with everyone else’s in equipment and operation.This was an
undesirable situation, because not only was the mobile equipment limited to operation within national boundaries, which in a unified Europe were increasingly unimportant, but there was also a very limited market for each type of equipment, so economies of scale and the subsequent savings could not be realized.The Europeans realized this early on, and in 1982 the Conference of European s Posts and Telegraphs (CEPT) formed a study group called the Group Special Mobile (GSM) to study and develop a pan-European public land mobile system.The proposed system has to meet certain criteria: ·Good subjective speech quality ·Low terminal and service cost ·Support for international roaming ·Ability to support handheld terminals ·Support for range of new services and facilities ·Spectral efficiency ·ISDN compatibility In 1989, GSM responsibility was transferred to the European Telecommunication Standards Institute (ETSI), and phase I of the GSM specifications were published in 1990.Commercial service was started in mid-1991, and by 1993 there were 36 GSM networks in 22 countries.Although standardized in Europe, GSM is not only a European standard.Over 200 GSM networks are operational in 110 countries around the world.In the beginning of 1994, there were 1.3 million subscribers worldwide, which had grown to more than 55 million by October 1997.With North America making a delayed entry into the GSM field with a derivative of GSM called PCS1900, GSM systems exist on every continent, and the acronym GSM now aptly stands for Global System for Mobile communications.The developers of GSM chose an unproven (at the time) digital system, as opposed to the then-standard analog cellular systems like AMPS in the United States and TACS in the United Kingdom.They had faith that advancements in compreion algorithms and digital signal proceors would allow the fulfillment of the original criteria and the continual improvement of the system in terms of quality and cost.The over 8,000 pages of GSM recommendations try to allow flexibility and competitive innovation among suppliers, but provide enough standardization to guarantee proper interworking between the components of the system.This done by providing functional and interface descriptions for each of the functional entities defined in the system.2 General Concepts 2.1 Analog Versus Digital Telephony Systems First-generation systems were analog.During the early 1980s these underwent rapid development in Europe.Although the NMT system was used by all the Nordic countries, and the TACS system in the United Kingdom and Italy, there was a variety of systems and no compatibility among them.Compared with these systems, the main advantages offered by GSM, which is the most important of the second-generation digital systems, are: ·Standardization; ·Capacity; ·Quality; ·Security.Standardization guarantees compatibility among systems of different countries, allowing subscribers to use their own terminals in those countries that have adopted the digital standard.The lack of standardization in the first-generation system limited service to within the borders of a country.Mobility is improved, since roaming is no longer limited to areas covered by a certain system.Calls can be charged and handled using the same personal number even when the subscriber moves from one country to another.Standardization also allows the operator to buy entities of the network from different vendors, since the functional elements of the network and the interfaces between these elements are standardized.This means that a mobile phone from any manufacturer is able to communicate with any network, even if this network is built with entities from different vendors.This leads to a large economy of scale and results in cost reduction for both the operator and the subscriber.Furthermore, the phone cost is also reduced, because GSM is an international standard, produced quantities are greater and the level of competition is high.With respect to capacity, the use of the radio resource is much more efficient in a digital system such as GSM than in an analog system.This means that more users can be allocated in the same frequency bandwidth.This is poible with the use of advanced digital techniques, such as voice compreion algorithms, channel coding, and multiple acce techniques.Note that capacity gains are also achieved with radio frequency reuse, which had also used in analog systems.Frequency reuse means that a given carrier can be employed in different areas.The quality in digital transmiion systems is better, thanks to the channel coding schemes that increase the robustne in the face of noise and disturbances such as interference caused by other users or other systems.The quality improvement is also due to the improved control of the radio link, and adaptations to propagation conditions, with advanced techniques such as power control or frequency hopping.This will be explained in greater detail in the next section.In terms of security, powerful authentication and encryption techniques for voice and data communications are enabled with GSM, which guarantees protected acce to the network, and confidentiality.2.2 Cellular Telephony In mobile radio systems, one of the most important factors is the frequency spectrum.In order to make the best use of the bandwidth, the system is designed by means of the division of the service area into neighboring zones, or cells, which in theory have a hexagonal shape.Each cell has a Base Transceiver Station (BTS), which to avoid interference operates on a set of radio channels different from those of the adjacent cells.This division allows for the use of the same frequencies in nonadjacent cells.A group of cells that as a whole use the entire radio spectrum available to the operator is referred to as a cluster.The shape of a cell is irregular, depending on the availability of a spot for the BTS, the geography of the terrain, the propagation of the radio signal in the presence of obstacles, and so on.In dense urban areas, for instance, where the mobile telephony traffic is important, the diameter of the cells is often reduced in order to increase capacity.This is allowed since the same frequency channels are used in a smaller area.On the other hand, reducing the cell diameter leads to a decrease in the distance neceary to reuse the frequencies (that is, the distance between two co-channel cells), increasing co-channel interference.In order to minimize the level of interference, several techniques are used on the radio interface.A basic example of cluster organization is shown in Figure 1.In this example, we see a reuse attern for seven different frequencies, f1 to f7.These frequencies correspond to the beacon carrier of each cell, on which signaling information about the cell is broadcast (see Section 2.7).It can be seen from this figure that a given carrier can be reused in two separate geographical areas, as long as these areas are far enough from each other to reduce the effect of interference.With this technique of dividing the area in cells and clusters, the operator can increase the area it is able to cover with a limited frequency bandwidth.
Figure 1 Example of a Cell Planning
在80 年代初期期间，模式手机系统在欧洲体验迅速增长，特别在斯堪的那
维亚和英国，以及法国和德国。 每个国家开发了它自己的系统，在设备和操作 上是互不相容的。这是一个不受欢迎的情况，因为不仅移动通信设备被限制在国 界之内，这在统一的欧洲是越来越不重要的，而且每种类型的设备都只有一个非 常有限的市场，因此经济尺度及随后的储蓄不可能体会。
欧洲人早就意识到了这点，并且在1982 年欧洲邮电管理联合会(CEPT)上形 成了移动通信特别研究组(GSM)来研发一个能在全欧洲大陆通用的移动通信系 统。提出的系统必须符合以下标准： 〃通话质量好
〃终端及服务费用低 〃支持国际漫游
〃能够支持可随身携带的终端 〃支持一系列新的服务和设备 〃效率高
〃与ISDN 兼容
1989 年，GSM 的责任转移了到欧洲电信标准学院(ETSI)，并且GSM 规格的
第一阶段也在1990 年出版了。商业服务在1991 年中期开始了，截至1993 年有 36 个GSM 网络在22 个国家诞生了。GSM 不仅仅是欧洲标准，尽管它是在欧洲 规范化的。现已有200 多个GSM 网络在全球110 个国家和地区运行。1994 年初 期全球有1,300,000 用户，截至1997 年10 月已突破55,000,000 用户。随着美国
后来在GSM 领域制造了一个词条PCS1900 作为GSM 的拓展,GSM 系统已发展至 各大洲,并且缩写词GSM 现在已可以代表移动通信的全局系统。
GSM 开发商选择了一个当时未经证明的数字系统，与后来的标准模式蜂窝系 统相对，就象美国叫AMPS，而英国叫TACS 一样。他们坚信能够推进压缩算法 和数字信号处理器使其能够根据质量和费用将原始标准和系统连续改善。 GSM 推荐8,000 页设法使得在与供应商竞争之中体现灵活性和创新，但提供 足够的标准来保证在系统的组分之间适当配合工作。完成提供功能和接口描述的 功能个体在系统中都有定义。
第一代系统是模拟的。上个世纪80 年代初期在欧洲迅速发展。虽然所有北 欧国家使用TACS 系统，英国和意大利使用NMT 系统，它们之间还有各式各样 的系统不兼容。和这些系统相比，GSM 提供的更重要的第二代数字系统主要优势 体现在： 〃标准化； 〃容量； 〃质量； 〃安全。
标准化保证了不同国家系统之间的兼容性，允许采取了数字化标准的国家的 订户使用他们自己的终端。在第一代系统中缺乏标准化的有限服务为在国家的边 界之内。流动性被改进，漫游不再被限制到某一系统包括的面积之内。既使当用 户从一个国家移动到另一个国家，电话也可以使用同一个个人号码。 因为网络的功能元件和这些元素之间的接口被规范化，操作员可以从不同的 供营商购买。这意味着所有制造商的移动电话能与任何网络沟通，即使这个网络 是由不同的供营商的功能元件建立的。这为操作员和订户带来费用成本的降低。 此外，电话费用也降低了，因为GSM 是国际标准，生产数量大，竞争水平高。 能力方面，数字系统如GSM 比模拟系统使用的无线资源更有效。这意味着 更多的用户可以使用同一频带。这就有可能利用先进的数字技术，如语音压缩算 法，信道编码和多址接入技术。请注意，无线电频率复用也取得了一定增益，这 也已用于模拟系统。频率复用意味着同一载波可以在不同的地区重复使用。 由于这种信道编码方案，提高了可靠性及对噪音和其他用户或系统的干扰能 力，数字传输系统的质量更好了。质量改进的原因还在于改善了对无线连接的控 制，并适应传播条件下，使用先进的技术，如电源控制或跳频。质量改进的原因 还在于使用先进的技术如电源控制或跳频改善了无线连接的控制,，传播条件的适
应性等。这些将在下文中做更详尽的解释。
在安全方面，GSM 具有语音和数据通信的强大的认证和加密技术的功能，保 证了保护接入网络和保密性。
在移动通信系统中，一个最重要的因素就是频谱。为了最有效地利用带宽， 系统设计的目的是通过基站的服务区到周边小区，理论上是一个正六边形。每个 小区有一个收发基站（BTS），其中，以避免相邻小区的不同无线信道在运行时 产生相互干扰。这样可以在不相邻的小区使用同一载频。一组小区，作为一个整 体，给运营商提供整个无线电频谱是指一个区群。小区的形状是不规则的，这取 决于是否有现成的基站，地理地形，无线电信号传播中存在的障碍，等等。 例如，在人口密集的城市地区，移动电话的通行非常重要，小区的直径往往 会减小，以提高其通行能力。
这是允许的，因为同一频道只在一个较小的地区内使用。另一方面，小区直 径的减小导致使用同一频率的小区间的距离（即两个公共通道小区间的距离）缩 小了，增强了各频道之间的相互干扰。为了尽量提高抗干扰的能力，无线接口使 用了一系列技术。
一个基本的区群组织的结构如图1 所示。在这个例子中，我们看到了一个复 用模式的7 个不同的频率，f1 至f7。这些频率对应于每个小区的信标载波，整个
小区的信号信息就靠它们被传播出去（见第2.7 节）。从图中可以看出，某一载 波可在两个不同的地理区域重复使用，只要这两个地区离的足够远，相互之间的 干扰足够小。这一技术把地区分成小区和区群，经营者可以利用有限的带宽增加 覆盖区域的面积。
