Chapter 3 Overview; General; Digital Modulation Technique; Voice Compression Technology - Motorola MTM800 User Manual

OVERVIEW

General

The MTM800 ENH is Motorola's latest and most advanced digital mobile TETRA radio. This radio
generation is based on a new digital platform technology which takes care of the linear modulation
type of radios 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 operat-
ing memory capacity to support all new market requirements. The MTM800 ENH TETRA radio
ensures a high audio quality.
To achieve high spectrum efficiency, the MTM800 ENH 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 modu-
lated into a radio-frequency (RF) signal, which is transmitted over the air to another MTM800 ENH.
The process is called digital modulation.

Digital Modulation Technique

The MTM800 ENH Mobile Terminal can be operated in dispatch mode. It uses two digital technolo-
gies: 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 trans-
mit the information. When the signal is received, the change in phase is converted back into sym-
bols 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 radio.
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.

Voice Compression Technology

Voice is converted into a digital bit stream by sampling the voice at high rate and converting the
samples into numbers, which are represented by bits.
Voice compression reduces the number of bits per second while maintaining the voice at an accept-
able quality level. The Tetra system uses a coding technique called ACELP (Algebraic Code Excited
Linear Prediction). The compressed voice-data bits modulate the RF signal.
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CHAPTER 3
OVERVIEW