VS600ServiceManual
VOXTELTechnicalDocumentation 8
channels used by cell A). In this system, subscribers in cell A and subscribers in cell D
could simultaneously operate on channels 1 through 8.
The implementation of frequency re-use increases the call handling capability of the
system without increasing the number of available channels. When re-using identical
frequencies in a small area, co-channel interference can be a problem. The GSM
system can tolerate higher levels of co-channel interference than analogue systems, by
incorporating digital modulation, forward error correction and equalization. This means
that cells using identical frequencies can be physically closer, than similar cells in
analogue systems. Therefore the advantage of frequency re-use can be further
enhanced in a GSM system, allowing greater traffic handling in high use areas.
By incorporating Time Division Multiple Access (TDMA) several 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 TDMAframe.
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
