Digital Modulation Technology MTH800 TETRA Handportable Terminal / Basic Service Manual 3 - 1
CHAPTER 3
OVERVIEW
To achieve a high spectrum efficiency, the MTH800 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 terminal. The
process is called digital modulation.
Digital Modulation Technology
The MTH800 is a 380-430 or 440-470MHz TETRA Handportable Terminal that
can operate in dispatch and phone modes. The terminal can also operate in TMO
(Trunked Mode Operation) and DMO (Direct Mode Operation) modes. 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 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
compressions/decompressions and RF modulation/demodulation.
Voice Compression Technology
Voice is converted into a digital bit stream by sampling the voice at a 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 acceptable quality level. The MTH800 uses a coding technique called
ACELP (Algebraic Code Excited Linear Prediction). The compressed voice-data
bits modulate the RF signal.
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