Delay spread is caused by the same signal arriving at the
receiver via two (or more) different paths. One signal
may be direct, the other reflected/bent around objects in
its path, so each arrives at a different time but at similar
amplitude. The speed or complexity of the modulation
scheme (how data is applied to the radio signal),
determines how susceptible it is to delay spread.
Delay spread is measured in microseconds (µS) - a
higher value is more tolerant to spreading. DMR has a
delay spread tolerance of 28µS compared with 15µS for
TETRA. Again, this is unlikely to be a factor in urban
areas, or in open rural situations, as typical delay spread
is around 5µS. While it can affect rural environments
with rugged landscapes, escarpments and canyons with
no direct radio path, it is only likely to occur near the
edges of the coverage area.
Link budgets determine the maximum possible signal
loss that can be tolerated between a transmitter and a
receiver before communications fail. If we calculate the
link budget for a base station transmitter (high site) to
a radio terminal receiver (downlink) as well as the radio
signal path between the radio terminal (portable) and
the base station receiver (uplink) we get differences
in performance. This table shows the parameters that
determine radio capability, comparing DMR and TETRA.
The larger the negative value in the radio terminal
sensitivity, the better. A 4.3dB advantage for DMR
(conservative value) equates to around a 1. 5 times
increase in the radius, or over twice the coverage area for
DMR compared with TETRA in the downlink. Whereas
the 6.8dB uplink advantage of DMR over TETRA
equates to over twice the range and a coverage area
increase of nearly three times.
So which provides better coverage – DMR or TETRA?
From this information we can clearly see in the case
of both TDMA gap time and link budget, DMR offers
a measurable improvement in coverage or range.
While the advantage may be limited in a busy urban
environment, when coverage includes rural areas,
DMR demonstrates significant coverage advantages
The author acknowledges the information provided by Principal
Engineer Ian Graham in the preparation of this article.
DMR OR TETRA
Technology DMR TETRA DMR TETRA
Path Downlink (base station) Uplink (portable)
Power output 44.0dBm ERP 44.0dBm ERP 36.0dBm ERP 31.8dBm ERP
Equivalent noise bandwidth 8kHz 18kHz 8KHz 18KHz
Dynamic Co-channel SINR for DAQ
3. 4 15dB 16dB 15dB 16dB
Radio terminal receiver noise figure 8dB 8dB 8dB 8dB
Radio terminal receiver sensitivity
(including diversity) -112dBm -107.7dBm -112.dBm -109.4dBm
Difference 4.3dB 6.8dB