as four different calls on one 25 kHz carrier frequency (One channel within the
system must carry the Main Control Channel, referenced here as the Base
channel).
Speech is digitized and compressed into less than 4.6 kbit/s. Data channels
support up to 7.2 kbit/s; the standard allows to allocate up to four timeslots for
one transmission so that data rates up to 28.8 kbit/s can pass the TETRA air
interface. This is comparable to today’s modems on the fixed-line telephone
system.
The modulation format is π/4 DQPSK (differential quaternary phase-shift keying).
This modulation method shifts phase in steps of ±π/4 or ±3π/4 depending on the
symbol to transmit. With each phase change you can transmit one symbol
containing two bits. In a vector diagram of the modulation signal, the ideal
positions of the eight possible signal vector states are on a circle (see Fig. 10.25).
The error vector is the line drawn between the ideal signal vector and the real
(measured) signal vector. Modulation accuracy is determined by measuring the
RMS (root mean square) difference between the real signal and the ideal signal.
For this the magnitude of the individual vector error on each symbol in a slot is
measured; the RMS vector error gives the square root of the sum of squares
divided by the number of symbols in the slot.
TETRA uses error-tolerant modulation and coding formats. The information to be
transmitted (eg speech) is usually coded adding some redundant information like
CRC. This information can be used to identify reception errors and to reconstruct
the correct bit stream with a highly sophisticated bit error correction algorithm.
But if the correction function is operating at its limits (eg because transmitter and
receiver are far from each other so that the signal quality is very low), speech
quality could suddenly turn from good to bad. Therefore, well-adusted modulators
are still necessary.
error vector
Radius of circle
reflects RF power
Fig. 10.25:
Representation of the modulation in the vec-
tor plane.
TETRA MS Test Appendix
10-52
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