DMR or TETRA – a technical comparison of coverage

Every year, many communications systems reach the end of their economic life due to obsolescence, 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 do 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.
  For the purposes of this article, the aforementioned factors will be held as constant.
  The following factors which effect 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.

Delay Spread

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

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 over TETRA.

The author acknowledges the information provided by Principal Engineer Ian Graham in the preparation of this article.