Motorola D10 Service Manual page 51

BenQ
calls can share the same carrier. The carrier is divided into a
continuous stream of TDMA frames, each frame is split into eight time
slots. When a connection is required the system allocates the
subscriber a dedicated time slot within each TDMA frame. User data
(speech/data) for transmission is digitized and sectioned into blocks.
The user data blocks are sent as information bursts in the allocated
time slot of each TDMA frame.
The data blocks are modulated onto the carrier using Gaussian
Minimum 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 radiotelephone is only
required to transmit for one burst in each frame, and not continually,
thus enabling the unit to be more power efficient.
Each radiotelephone must be able to move 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 can thus be much more accurately
determined, and the handover made before the channel quality
deteriorates to the point that the subscriber notices.
When a radiotelephone is well within a cell, the signal strength
measured will be high. As the radiotelephone 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 new
cell.
This change is handled by the radiotele-phone and base stations,
and is completely transparent to the user.
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