TETRA Transmission planning

Digital levels

Infrastructure

The infrastructure converts TETRA coded speech to linear and to A Law coded data which it exchanges with the PSTN. The charactistics of the A-law CODEC are defined in [G.711] and the 13 bit linear interface to the CODEC is considered here.

Infrastructure conversions

[G.712] defines the CODEC peak signal point of +3.14 dBm0 (Section 3.3). This is mapped onto the 16 bit linear TETRA CODEC at the same point by aligning the most significant bits and inserting or removing the 3 least significant bits.

Going from TETRA the conversion is as defined below

Value_PCM13 = Value_TETRA / 8
Converting from TETRA 16 bit to PCM 13 bit

Going from PCM to TETRA the conversion is

Value_TETRA = Value_PCM13 * 8
Converting from PCM 13 bit to TETRA 16 bit

Terminal

Defining the digital infrastructure level mapping allows the reference level to be set in the terminal. Assuming that the TETRA CODEC has an effective gain of 0dB we can define the peak signal point at the linear interface to the CODEC as being +3.14dB0.

Note: This assumption needs to be validated. If it is incorrect then the input to the CODEC must be scaled to give an effective 0dB gain through the encoding and decoding process.