Codan Radio P25 Training Guide - Manual (Page 103)

P25 RADIO SYSTEMS | TRAINING GUIDE
Chapter 7: P25 Phase 2 Page 95
TDMA Transmission Format
The TDMA transmission format for inbound H-CPM and outbound H-DQPSK emission is the same
except for the fi rst and last 20 bits (12 bits of rampguard and 8 bits of pilot) of a burst:
- Outbound: an ISCH logical channel is emitted at the beginning and end of a burst. The ISCH occurs
at the equivalent location in the outbound burst as the rampguard and pilot portions for the inbound
direction as shown in Figure 7-8.
- Inbound: rampguard and pilot periods as shown in Figure 7-8.
Figure 7-8: TDMA Transmission Format
The outbound emission is continuous (never bursty) and a replacement burst is transmitted in the other
TDMA slot when only one TDMA slot is used.
For inbound bursts, a total of 2 ms of time is reserved as “Guard” time to allow for power ramping and
propagation delay protection. The modulation burst is centred in the 30 ms time slot, with 1 ms of the
“Guard” time at the beginning of the burst and 1 ms at the end of the burst as shown in Figure 7-8.
Of the 2 ms of “Guard” time, a minimum of 0.8 ms shall be allocated for propagation delay protection
while the remaining time up to 1.2 ms shall be allocated for power ramp up and ramp down. Note that
overlapping of the ramps is allowed as long as the portion of the waveform containing useful information
located in the center portion of Figure 7-8 (including data, synchronization and pilot sequences) remains
unaffected.
30 ms
1 ms 28 ms 1 ms
ramp
up
12
bits
pilot
8
bits
ISCH
20 bits
1.67 ms
Outbound
Inbound
ISCH
ramp
down
pilot