Chapter 3 Overview; General; Digital Modulation Technique - Motorola MTM800 Basic Service Manual

OVERVIEW

General

The MTM800 with Enhanced Control Head is Motorola's latest and most advanced digital mobile
TETRA terminal. This terminal generation is based on a new digital platform technology which takes
care of the linear modulation type of terminals to support the TETRA needs. It covers Trunk Mode
Operation (TMO) as well as Direct Mode Operation (DMO) and among other new features it is
supplied with extended code and operating memory capacity to support all new market
requirements. The MTM800 with Enhanced Control Head TETRA terminal ensures a high audio
quality.
To achieve high spectrum efficiency, the MTM800 with Enhanced Control Head uses digital
modulation technology and sophisticated voice-compression algorithm. The voice of the person
speaking into the microphone is converted into a digital bit stream consisting of zeros (0) and
ones (1). This stream is then modulated into a radio-frequency (RF) signal, which is transmitted over
the air to another MTM800 with Enhanced Control Head. The process is called digital modulation.

Digital Modulation Technique

The MTM800 with Enhanced Control Head is a 380–430 or 410–470 MHz mobile that can operate
in dispatch mode. It uses two digital technologies: π/4 DQPSK and Time Division Multiple Access
(TDMA).
π/4 DQPSK is a modulation technique that transmits information by altering the phase of the radio
frequency (RF) signal. Data is converted into complex symbols, which alter the RF signal and
transmit the information. When the signal is received, the change in phase is converted back into
symbols and then into the original data.
The TETRA system can accommodate 4-voice channels in the standard 25 kHz channel as used in
the two-way terminal.
Time Division Multiple Access (TDMA) is used to allocate portions of the RF signal by dividing time
into four slots, one for each unit.
Time allocation enables each unit to transmit its voice information without interference from other
transmitting units. Transmission from a unit or base station is accommodated in time-slot lengths of
15 milliseconds and frame lengths of 60 milliseconds. The TDMA technique requires sophisticated
algorithms and a digital signal processor (DSP) to perform voice compression/decompression and
RF modulation/demodulation.
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CHAPTER 3
OVERVIEW