requency is divided into four timeslots, allowing four simultaneous communications. The carrier spacing is 25 kHz width, the modulation is /4 DQPSK and (differential quadrature phase shift keying) the gross transmitting rate is 36 kbps. Figure D.4 shows the relevant modulation constillation. The bandwidth efficiency is very high: 36 kbps ==1.44 bps/Hz 25 kHz Figure D.5 compares the available number of channels in the 200 kHz bandwidth for various technolo- gies. The TETRA standard provides 32 channels as against 16 or 8 of the GSM and traditional PMR technologies in the 200 kHz bandwidth. In fact, the TETRA standard considers 25 kHz radio channel spacing and therefore provides 8 radio channels in the 200 kHz bandwidth, since each radio channel is divided into four timeslots, in the same bandwidth the TETRA standard can provide 32 channels. On the other hand, the technologies like GSM and traditional PMR provide a smaller number of channels in the same frequency bandwidth. The GSM standard provides 8 channels, the half-rate GSM 16 channels and the traditional PMR applying only frequency division multiplexing (FDM) 8 or 16 channels depending on the radio channel spacing (12.5 or 25 kHz). The TETRA speech codec is based on the code-excited linear predictive (CELP) coding model. This is a parametric codec using linear predictive coding with a bit rate of about 4.5 kbps. This technique is used to transmit information about the excitation sequence and the filter coefficients. The TETRA codec uses an excitation generator based on a codebook of algebraic codes (ACELP) and contains many excitation sequences of pulses with different amplitudes and positions. Figure D.6 shows the process applied to TETRA Main Features Figure D.4 TETRA modulation constellation 200ml 200 kHz /carrier 8 channels 200 kHz / carrier 16 channels Half-rate GSM 111213141516 25 kHz / carrier 8 channels / 200 kHz Traditional PMR 12.5 kHz carrier 25 kHz 16 channels/ 200 kHz Traditional PMR 12.5 kHz 25 kHz / carrier 4 channels / carrier 32 channels / 20