Motorola MT712, Basic Service Manual

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OVERVIEW MTM700 Mobile Terminal / Basic Service Manual 3 - 1
CHAPTER 3
OVERVIEW
General
The MTM700 is Motorola’s latest and most advanced digital mobile TETRA radio. This radio gener-
ation is based on a new digital platform technology which takes care of constant envelop type of
radios as well as 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 operating memory capacity to support all new market require-
ments. The MTM700 TETRA radio ensures a high audio quality.
To achieve a high spectrum efficiency, the MTM700 uses digital modulation technology and sophisti-
cated voice-compression algorithm. The voice of the person speaking into the microphone is con-
verted 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 MTM700. The proc-
ess is called digital modulation.
Digital Modulation Technique
The MTM700 is a 380-430 MHz or 806-870 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 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.