Service manual

The data blocks are modulated onto the carrier using Gaussian Mininum Shift Keying
(GMSK), a very efficient method of phase modulation.
Each time an information burst is transmitted, it may be transmitted on a different
frequency. This process is known as frequency hopping. Frequency hopping reduces
the effects of fading, and enhances the security and confidentiality of the link. A GSM
radio telephone is only required to transmit for one burst in each frame, and not
continually, thus enabling the unit to be more power efficient.
Each radio telephone must be able to more from one cell to another, with minimal
inconvenience to the user. The mobile itself carries out signal strength measurements
on adjacent cells, and the quality of the traffic channel is measured by both the mobile
and the base station. The handover criteria and thus be much more accurately
determined, and the handover made before the channel quality deteriorates to the
point that the subscriber notices.
When a radio telephone is well within a cell, the signal strength measured will be
high. As the radio telephone moves towards the edge of the cell, the signal strength
and quality measurement decreases.
Signal information provides an indication of the subscriber’s distance from the base
station. As the radiotelephone moves from cell to cell, its control is handed from one
base station to another in the next cell.
This change is handled by the radiotelephone and base stations, and is completely
transparent to the user.
Service Area
The area within which calls can be placed and received is defined by the system
operators. (Because this is a radio system, there is no exact boundary that can be
drawn on a map.) If the telephone is outside a coverage area, the ∅ (no service)
indicator will illuminate and calls will be unable to be placed or received. If this
happens during a conversation, the call will be lost. There may also be small areas
within a particular service area where communications may be lost.