Codan Radio P25 Training Guide - Manual (Page 51)

P25 RADIO SYSTEMS | TRAINING GUIDE
Chapter 3: P25 Technical Information Page 43
Although it appears that the digital radio signal performs with greater coverage area than an analog
radio signal, other factors must also be taken into consideration, such as multipath refl ections. Multipath
refl ections of the RF carrier occurs when two or more signals of the same origin arrive at the receive
antenna delayed in time because they traveled different path lengths or because of refl ections and
scattering in the propagation environment. This deterioration of the signal must be considered when
planning coverage areas.
Figure 3-3: The Effects of Multipath
RF multipath is a frequency dependant problem with higher frequencies being more vulnerable. When
a subscriber radio is in motion, multipath interference results in the amplitude modulation effect familiar
to mobile FM listeners as “picket fencing”. In the worst case, when the subscriber radio is stopped in
a signal null, the signal is severely degraded and a single, strong specular refl ection may completely
cancel the transmitted signal. Where analog reception can become noisy, digital signals could be lost
altogether. Increasing power is not a viable remedy because both the direct and refl ected signal will
increase proportionally, preserving the interference nulls.
Coverage studies
Many different agencies and organizations such as the federal government, state departments, fi re
departments and other public safety organizations have conducted studies on analog and digital
propagation of RF communication systems and publish their studies online or in magazines. These
studies are typically specifi c to the type of communications systems the organization uses and the
environment they are deployed in. While these studies are extremely useful, they usually compare
different communication systems (eg. digital trunking vs. simplex analog) in different RF propagation
environments.
Direct Path
Analog Mode
Reflections
Reflections