Codan Radio P25 Training Guide - Manual (Page 49)

P25 RADIO SYSTEMS | TRAINING GUIDE
Chapter 3: P25 Technical Information Page 41
ANALOG VS. P25 DIGITAL COVERAGE
There is much discussion about the RF coverage area of an analog radio signal versus a digital radio
signal. In theory, a P25 digital radio signal will allow for a slightly greater coverage area when placed
in the same location as an analog radio. There are some factors, however, that may interfere with the
digital signal to a greater degree than the interference to an analog signal.
P25 Phase 1 uses C4FM modulation. Because C4FM is a constant amplitude modulation, it allows use
of nonlinear power amplifi ers. Use of nonlinear amplifi ers results in digital equipment that produces
RF power levels that are equal to the power levels of current analog equipment. Systems can be
implemented with little or no loss of coverage. An analog transmitter can be replaced with a P25 Phase
1, digital transmitter that produces the same transmitter output level of the analog transmitter. This
is currently not necessarily true for higher power analog systems that are replaced by some TDMA
systems when bandwidth of the resultant signal is a critical issue.
In order to occupy a limited bandwidth, some TDMA systems use modulations that require linear power
amplifi ers and system transmitter power in these systems can be a signifi cant issue. GSM™, for
example, uses a 200 kHz wide channel for 8 voice slots, and uses the fi xed amplitude GMSK modulation.
TETRA on the other hand, uses 4 slots in a 25 kHz wide channel, and TETRA uses DQPSK that
contains amplitude and phase components to the modulation. By using a linear amplifi er, the variable
amplitude modulation implementations produce a relatively lower output power. Their higher data rate
also tends to limit the coverage area because of the bit rate and the resultant bit timing. This can result
in a much larger infrastructure to support TDMA systems as opposed to the Phase 1 FDMA systems.
FDMA also promotes use of a very reliable direct mode of operation because of the power levels of
subscriber equipment and the lack of a requirement for any supporting infrastructure. This direct, or
talk around, mode insures reliable unit-to-unit operation without the need for any infrastructure. Again,
because of the use of non-linear power amplifi ers, portable and mobile radio transmitter power of P25
digital equipment is comparable to the power level available in current FM analog equipment.