For the purposes of this article, the aforementioned
factors will be held as constant.
The following factors which affect coverage will be
discussed in this article:
• The gap between Time Division Multiple Access
(TDMA) time slots
• Delay spread and its effect on modulation
• The link budget, which influences the radio signal path
TDMA GAP TIME
This is a guard period between two consecutive
time slots. A radio at distance from a base station
transmitting, followed immediately by a second radio
close to the base station transmitting on the following
time slot can cause a collision of the two messages.
The distant signal is likely to fail as it is weaker.
This is caused by the extra time (at the speed of light)
it takes the signal from the distant radio to arrive at the
base station. The message from the distant radio may
still be arriving when the near radio starts to transmit.
This is described as the Near/Far effect.
In practice, if this situation occurs, the DMR range is
around 75kms and TETRA around 30kms. This is due
to the gap time being longer in DMR. This is unlikely to
be a limiting factor in an urban environment but in a flat
rural setting, it could be significant.
Every year, many communications systems reach the end of their economic life due to bsolescence, regulatory requirements or because they cannot deliver the business benefits of digital radio.
Early in your upgrade process, you need to decide which
technology will give you the best coverage without the
expense of additional radio sites. Large organizations
such as utilities normally purchase trunked radio
solutions to provide wide-area dispatch and team-based
communications. Ideally, you will be able to reuse your
existing radio sites and experience similar coverage from
the new investment.
Two digital trunked radio technologies to consider are
DMR and TETRA. To decide which provides better
coverage, we need an objective approach.
KEEPING THE COMPARISON SIMPLE
Many discussions over the last few years have obscured
the answer, by imposing other functionality or feature
comparisons which do not impact coverage. In essence,
factors which impact radio coverage include:
• The frequency band used, where a lower frequency
gives greater coverage. For example, all things being
equal, a 150MHz system provides double the coverage
of a 400MHz system.
• Geographic topology or manmade obstacles that
screen the radio signals path affect coverage.
DMR OR TETRA