Service manual

The radiotelephones themselves are capable of operation on any channel in the
system, allowing them to operate in any cell. Due to the low power requirements for
communications between radiotelephones in a particular cell and the cell site,
operating channels may be repeated in cells which are outside the coverage area of
each other.
For example, presume that cell A operates on channels arbitrarily numbered 1 through
8, cell B opeates on channels 9 through 16, cell C operates on channels 17 through 24
and cell D operates on channels 1 through 8 (repeating the usage of those channels
used by cell A). In this system,subscribers in cell A and subscribers in cell D could
simultanuously operate on channles 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 TDMA frame, see Figure 2:
“TDMA Transmission” following: