移动通信的介绍

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 移动通信的介绍
一、移动无线电系统的功能
    测试一个可操作的移动无线电系统必须满足的要求是有指导性的, 所以必须定义影响潜在的设计策略的约束。
    多数的移动无线电系统集中在人口中心的周围, 鉴于用户，系统被放置的地方是可以预料到的。急救服务，像警察局、消防部门和急救部门全部使用移动的无线电设备, 正如提供各种不同类型的维护功能的出租车操作员 、负责能源分配，水和与它们相关的服务的公共组织、大众运输和拖运组织和公众成员。 这些部门的两个特征立刻就明显了: 第一，很多的使用者必须能够在没有干扰彼此的情况下同时操作,第二,系统所能提供交流的距离大体上不需要太长, 因为许多系统的操作是在大都市的边界里面或沿着明确限定的主要车辆主干路径或铁路线。另外地，如果移动装置是自由地和它们的基站通信或者彼此在一个给定的区域 (可能或者不可能是系统的所有服务区域) ，被牵涉的所有发射器必须能够为遍及被考虑到的整个区域提供适当的信号能量，这是显然的。在水平面上，有效且密集的天线可以无固定形式地发射，在这些可能的地方，操作频率必须在无线电波的一个频率中被选择。 因为信号随着离发射器的距离增加而快速地衰减, 这也暗示选择的频率必须是容易而且有效率地产生可观的能量。

 Introduction to Mobile Communications
一  Mobile Radio System Fundamentals
    It is instructive to examine the requirements which an operational mobile radio system must fulfill, so that the constraints which influence subsequent design philosophy can be clarified.
  The majority of mobile radio systems exists in and around centers of population, as is to be expected in view of the uses to which systems are put. Emergency services such as police, fire and ambulance all use mobile radio, as do taxi operators, those who provide maintenance functions of various kinds, public authorities with responsibilities for power distribution, water and associated services, public transport and haulage organizations, and members of the public. Two features of these operations are apparent immediately: firstly, a large number of users must be able to operate at the same time without interfering with each other, and secondly, the distances over which the system must provide communication need not in general be great, since much of the system’s operation will be either within the confines of a conurbation or along well-defined major vehicle trunk routes or railway lines. Additionally, it is evident that if mobiles are to communicate freely with their base or each other throughout a given area (which may or may not be the total service area of the system), all the transmitters involved must be able to provide an adequate signal strength over the entire area concerned. Operating frequencies must be chosen in a region of the radio spectrum where it is possible to use efficient, compact antennas that radiate uniformly in the horizontal plane. Since the signal is rapidly attenuated with increasing distance from the transmitter, this also implies that the frequencies chosen must be that substantial power can be easily and efficiently generated.
  The part of the electromagnetic spectrum which includes radio waves extends from 30 kHz (wavelength 10 km) to 300 GHz (0.1 cm ). Electromagnetic energy in the form of radio waves propagates outwards from a transmitting antenna, and there are three main ways in which these waves travel, much depending on the frequency of transmission.

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