Codan Radio P25 Training Guide - Manual (Page 102)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 7: P25 Phase 2 Page 94
Bi-directional Signaling
Phase 2 systems use bi-directional signaling with a dedicated signaling slot in each superframe called
the Slow Associated Control Channel (SACCH). This SACCH occurs in an inverted position, meaning
the outbound signaling slot will appear in the inbound signaling slot and vice versa. This ensures a
transmitting subscriber has time to momentarily switch from transmit to receive, and to look at its
signaling slot from the fi xed site trunking system without disrupting the subscribers transmitted voice
information.
Bi-directional signaling allows for a transmitting subscriber to be notifi ed about other active calls, be
alerted if the subscriber passes out of system coverage while transmitting, be alerted that the subscriber
has been interrupted by another unit, and possibly shut down for emergency situations. Bi-directional
signaling can also include transmitter power control information. A transmitting subscriber can receive
power control updates from the fi xed site trunking system to mitigate interference or low signal quality.
Timing Synchronization
All outbound TDMA channels at each site are synchronized in time. Each of the superframes on all the
outbound paths of TDMA channels are time aligned so that the synchronization of one TDMA channel
will synchronize to all other TDMA channels at that site.
It is not mandatory to synchronize TDMA channels with the FDMA control channel; however, a subscriber
can transmit or receive sooner on a TDMA channel when it is already synchronized to the FDMA control
channel. This allows the subscriber to skip the process of having to acquire this synchronization after
moving to the assigned TDMA channel. The TDMA channels can be synchronized to the FDMA control
channel by regularly transmitting a Synchronization Broadcast message on the control channel.
Physical Layer
Modulation
The P25 Phase 2 TDMA CAI uses two different modulation schemes for over-the-air transmission of
the 12 kbps data stream, one inbound to the fi xed site trunking system (from the subscribers), and a
different one outbound from the fi xed site trunking system (to the subscribers).
The inbound modulation used is Harmonized Continuous Phase Modulation (H-CPM), a common
constant-envelope non-linear modulation. This allows the subscribers to use the same non-linear
amplifi ers currently employed in P25 Phase 1 FDMA. Non-linear amplifi ers help preserve the battery
longevity of the subscribers. The level of sidebands of the transmitted signal is reduced by having a
continuous phase modulation scheme.
The outbound modulation used is Harmonized Differential Quadrature Phase Shift Keyed modulation
(H-DQPSK), a non-coherent modulation scheme that splits the information stream into two channels,
delays one channel by 90° in phase (quadrature) and then recombines the two phase shift keyed
channels using differential coding (encoding the difference of the current data word applied to the
transmitter with its delayed output). Combining two channels in quadrature lowers the transmitted baud
rate, improving the transmitted spectral characteristics. H-DQPSK modulation requires linear amplifi ers
in the fi xed site trunking system.