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2024年4月1日发(作者：)

mobile and cellular radio移动和细胞广播

in comparison to the relative stability and modest technical developments

which are occurring in long haul wideband microwave communication systems

there is rapid development and expanding deployment of new mobile personal

communication system. These rang from wide coverage area pagers,for simple

data message transmission,which employ common standards and hence achieve

contiguous coverage over large geographical areas,such as all the major urban

centres and transport routes in Europe,Asia or the continental chapter

discusses the special channel characteristics of mobile systems and examines the

typical cellular clusters adopted to achieve continuous communication with the

mobile then highlights the important properties of current,and

emerging,TDMA and code division multiple access(CDMA), mobile digital cellular

communication systems.

Private mobile radio

Terrestrial mobile radio works best at around 250 MHz as lower frequencies

than this suffer from noise and interference while higher frequencies experience

multipath propagation from buildings,etc,section practice modest

frequency bands are allocated between 60MHz and 2GHz. Private mobile

radio(PMR) is the system which is used by taxi companies,county councils,health

authorities,ambulance services,fire services,the utility industries,etc,for mobile

communications.

PMR has three spectral at VHF,one just below the 88 to 108 MHz FM broadcast

band and one just above this band with another allocation at approximately

are also two allocations at UHF around 450MHz. all these spectral

allocations provide a total of just over 1000 radio channels with the channels

placed at 12KHz channel spacings or centre frequency offsets. Within the

12khz wide channal the analogue modulation in PMR typically allows 7khz of

bandwidth for the signal further allowance is made for the

frequency drift in the oscillators of these systems a peak deviation of only 2 to 3

khz is available for the speech traffic. Traffic is normally impressed on these

systems by amplitude modulation or frequency modulation and again the receiver

is of the ubiquitous superheterodyne design,Figure 1.4. A double conversion

receiver with two separate local oscillator stages is usually required to achieve the

required gain and rejection of adjacent channel signals.

One of the problems with PMR receiver is that they are requiredto detect very

small signals,typically—120dBm at the antenna output,corresponding to 0.2

uV,and,after demodulating this signal,produce ann output with perhaps 1W of

audio equipment, the first IF is normally at10.7MHz and the second IF is very orten

at 455KHz . unfortunately,with just over 1000 available channels for the whole of

the UK and between 20000and30000issued licences for these systems,it is

inevitable that the average busuness user will have to share the allocated channel

with other companies in their same geographical area.

There are various modes of operation for mobile radio communications

networks, the simplest of which is singal frequency simplex. In simplex

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