Codan Radio P25 Training Guide - Manual (Page 100)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 7: P25 Phase 2 Page 92
Media Access Control Layer
Vocoder
P25 Phase 1 equipment originally used the IMBE™ full-rate vocoder, but was replaced in all P25
equipment (Phase 1 and 2) in 2009 by the AMBE+2™ dual-rate vocoder, which consists of the Phase
1 enhanced full-rate vocoder (7.2 kb/s) for Phase 1 FDMA, and the enhanced half-rate vocoder (3.6
kb/s) for Phase 2 two-slot TDMA operation. The full-rate enhanced vocoder in the AMBE+2™ allows
for backwards compatibility with all other P25 Phase 1 radio systems (including IMBE™ vocoders) and
direct or “talk-around” communications between Phase 2 subscribers. TDMA systems typically require
a high degree of synchronization from fi xed site equipment that makes direct subscriber to subscriber
communications more diffi cult.
An AMBE+2™ vocoder does not reproduce the input speech signal on a sample by sample basis, but
constructs a synthetic speech signal which contains the same perceptual information as the original
speech signal. The AMBE+2™ vocoder divides a digital speech input signal into overlapping speech
segments (or frames) spaced 20 ms apart and uses a robust speech model and sophisticated parameter
estimation algorithms to achieve a low data rate while maintaining most of the quality, intelligibility and
speaker recognizability found in the original speech signal.
The National Institute of Standards and Technology (NIST) have found that there is no signifi cant
statistical difference between the quality of the full and half rate AMBE+2™ enhanced vocoders.
Bit Rate
A Phase 2 two-slot TDMA system uses an overall bit rate of 12.0 Kbps, equaling 6.0 Kbps per time slot.
The half-rate vocoder uses a net bit rate of 2450 bps for voice information plus 1150 bps for Forward
Error Correction (FEC) information, resulting in a total channel bit rate of 3600 bps per time slot. Each
time slot also uses 2400 bits per second for signaling and control functions.
P25 FDMA Common Air Interface – 9.6 kbs
Full Rate Vocoder – 7.2 kbs
Signaling – 2.4 kbs
Digital Voice – 4.4 kbs FEC – 2.8 kbs Imbedded Signaling – 2.4 kbs
P25 TDMA Common Air Interface – 12.0 kbs
Time Slot 1 – 6.0 kbs
Time Slot 2 – 6.0 kbs
1/2 Rate Vocoder – 3.6 kbs Signaling – 2.4 kbs 1/2 Rate Vocoder – 3.6 kbs Signaling – 2.4 kbs
Digital Voice
– 2.45 kbs FEC
– 1.15 kbs Imbedded
Signaling – 2.4 kbs Digital Voice
– 2.45 kbs FEC
– 1.15 kbs Imbedded
Signaling – 2.4 kbs
Table 7-1: Phase 1 and Phase 2 Bit Rates