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.
