System Introduction - LG LGC-500W Service Manual

LGC-340/345W
CHAPTER 1. System Introduction
1. Introduction to the Cellular System
1.1 Basic Concept for the Cellular System
The design objective of early mobile radio systems was to achieve a large coverage area by using a single, very
high powered transmitter with antenna mounted on the tall tower. While this approach achieved very good
coverage, it also meant that it was impossible to reuse those same frequency throughout the system. But the
increasing demand for mobile service required the radio telephone system to achieve high capacity with limited
radio spectrum, while at the same time covering very large areas.
The cellular concept was a major breakthrough in sloving the problem of spectral congestion and user capacity.
The cellular concept ,which starts from the frequency reuse technique, is a system level idea which calls for
replacing a single, high power transmitter (large cell) with many low power transmitter(small cells), each
providing coverage to only a small portion of the service area. Each base station is allocated a portion of the total
number of channels available to the entire system, and nearby base stations are assigned different groups of
channels so that all the available channels are assigned to a relative small number of neighboring base stations.
Neighboring base stations are assigned different groups of channels so that the interference between base stations
(and the mobile users under their control) is minimized. This technique is called channel assignment stategy.
In the cellular system, the channel handoff operation is inevitably needed to keep a call while a user moves one
cell to another. This function not only involves identifying a new base station, but also eruires that the voice and
control signals be allocated to channel associated with new base station. The handoff between different swithcing
systems is called roaming. It is call state can be maintained continuously by the information exchange between
switching systems when the busy subscriber moves from one cellular system area to the other cellular system
area.
1.2 Multiple Access Techniques for the Cellular System
In cellualr systems, it is often desirable to allow the subscriber to send simultaneously infoermation to the base
station while receiving information from the base station. This is called deplexing. Duplexing is done using
frequency or time domain rechniques. Frequency Division Duplexing(FDD) provides two distinct bands of
freqeuncies for every user. In FDD, any duplex channel actually consist of two simplex channels, and a device
called a duplexer is used inside handsets and base stations. Time Division Duplexing(TDD) uses time instead of
frequency to provide both a forward and reverse link. If the time split between the forward and reverse time slot
is small, then the transmission and reception of data appears simultaneous to the user.
Multiple access techniques are used to share the available channel resources(frequency bandwidth). Frequency
Division Multiple Access(FDMA), Time Division Multiple Access(TDMA), Code Division Multiple
Access(CDMA) are the three major techniques in cellular systems.
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