3 - 2 Overview
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
acceptable 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.
High Speed Data
There are two ways to transmit and receive packet data using the MTM5400/MTM800 FuG. The first
is Multi-Slot Packet Data (MSPD), where the data throughput is increased by transmitting in more
than one slot per frame, but using the older π/4 DQPSK modulation. This mode supports continuous
transmission.
The other possibility is the TETRA Enhanced Data System (TEDS), which uses multi-carrier
techniques and more complex modulation to achieve higher data throughput. The modulation
supports channel bandwidths of 25 and 50 kHz with 8 and 16 subcarriers per channel, respectively.
Each subcarrier is modulated using Quadrature Amplitude Modulation (QAM). The standard
supports 4-QAM, 16-QAM or 64-QAM (2, 4 or 6 bits per symbol). This mode also supports
continuous transmission.
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