Transmission Control And Communication - Ericsson EF738 Service Manual

System Introduction
In order to reduce the risk of interference still further, and to remove noise, the sys-
tem uses the compander technique. Basically, this method improves the signal/noise
ratio of the radio channel by compressing the signal before transmission, and
expanding the signal on reception.
The ETACS system features full duplex operation, with a duplex separation of 45
MHz. Full duplex means that a conversation can progress simultaneously in both
directions, as it does on an ordinary telephone. The duplex separation is the fre-
quency separation between the transmitter and receiver operating frequencies. The
900 MHz-band has been divided into an upper and a lower frequency band. The
lower band (872.0125 - 904.9875 MHz) contains the mobile telephone transmitter
channels, and the upper band (917.0125 - 949.9875 MHz) contains the correspond-
ing mobile telephone receiver channels.

Transmission Control and Communication

The heavy traffic and the many functions applied in the system requires the tele-
phone exchanges to be program-controlled. The exchange has to observe which
base station is now serving the mobile telephone. When the car moves away from
the original base station and comes closer to another, the exchange must request a
switch-over to the more favourable station. This must occur without interrupting or
cancelling the call. Switching a call in progress from one base station to another is
called Handoff or Handover. Before transferring the communication line to a new
base station, the exchange makes sure that this is really the station that will provide
the best conditions for transmission and reception.
To enable the exchange to supervise transmission as indicated above, there is a con-
tinuous flow of data between the mobile telephones and the exchange via the base
stations. This information, which cannot be heard in the telephone, concerns chan-
nel selection, transmission power, identification of sender and receiver, start and ter-
mination of call, and so on. The information enables the exchange that holds the
subscriber registered to follow the movements of the mobile telephone. Should the
subscriber's unit come into the control area of a different exchange, this exchange
will immediately report the new location to the home exchange.
When a mobile telephone approaches the limit of the system's coverage, or when
neighbouring base stations cannot offer a free channel, conversation shall continue
on the active channel until reception quality gets below a specified level.
The channels of the system are automatically scanned and analyzed with regard to
channel number, voice channel, control channel and service area. Normally, the
mobile telephone is locked on a control channel in the service area where it is
located. If the telephone becomes unlocked, it will immediately start scanning for
another control channel .
Each base station sends a search signal over the control channel. When a call is
made to a mobile telephone, the setup information will be transmitted over this con-
trol channel. Once the mobile telephone has responded to the call, the base station
searches for a free voice channel and orders the mobile to switch to the chosen voice
channel, after which the mobile subscriber will hear a ringing signal on the mobile
telephone. The entire conversation between the two subscribers takes place over the
voice channel selected by the base station. If a zone boundary is passed during the
call, the channel will be switched over automatically to the new base station and the
call will continue without any disturbance.
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