How far can you get on DMR?
The amount of coverage one can expect from a MOTOTRBO (or any DMR) radio system is dependant on a number of factors. In this post I will briefly discuss the five which play a bigger role.
The TLDR answer is up to 150km but that depends on the other factors below.
Link Budget
The link budget is basically the sum of all gains and losses between the radio and repeater. This includes:
- Transmit power on the repeater
- Loss in the cables and filtering equipment between the repeater and antenna.
- Gain of the antenna used at the repeater site.
- Path loss between repeater antenna and radio antenna.
- Gain/loss of the radio's antenna.
- Loss in the cable between the radio and antenna - if any.
- Receiver sensitivity for a certain DAQ (Digital Audio Quality) or bit error rate.
The path loss will increase in proportion to the distance between radio and repeater but this will also be affected by any obstacles in the way. Since all of the other in the above list are known/fixed, there is a finite distance one can achieve with a specific configuration.
The only optimisations one can make are using better antennas; using lower loss cable and choosing a better location for the repeater but there are limits to what is feasible.
Propagation
In practice, the path loss is not only affected by the inverse square law but also by terrain. It is well known that a signal will travel further if the terrain is flat. It's also well known that moving an antenna way from obstacles will improve signals.
In order to get a better understanding of the coverage area a particular configuration will provide, a coverage prediction tool can be used to generate a coverage map. These tools will generally only provide reliable results for outdoor coverage. Some tools can provide indoor coverage but I have no experience with those.
These tools use either the Longley Rice; Okumura or Hata models. The tool basically takes the input data and runs a series of calculations to arrive at a plot on a map. The tools also include the parameters from the link budget in their calculations.
Remember that all of these tools are prediction tools and that there are some circumstances where the tool will get things wrong. For example, many tools rely on data from the NASA SRTM. Some of this data dates from 2003. Some cities (e.g. Dubai) look a lot different now when compared to 20 years ago. This means that those tools will miss the obstruction a newly constructed skyscraper places on radio waves.
Above is a coverage simulation I done for a local ham repeater I see from my Idstein office window. This simulation shows a heat map / rainbow (composite SSI) for the inbound signal (radio to repeater) for a 4W portale held at head height. The areas shaded blue ought to produce -120dBm whereas the area shaded red will produce up to -30dBm.
While this format is okay for technical staff, users may not understand what the map shows. I prefer using a go/no-go plot which shades the go area (with a >0,97 probability of service) a single colour - this is very easy to understand (and is used by mobile phone operators to show clients where they will get 3G or 4G service and where not).
I also prefer showing the inbound coverage as this gives a conservative estimation of what the coverage will be - anything extra can be a pleasant surprise later.
In Capacity Max systems, it is important to only consider inbound coverage as the radio needs to register on the site - being able to receive a (good) signal is not enough. In some cases, it's desirable to have very good outbound coverage so that the user can at least hear the dispatcher but this won't work in all cases.
A list of coverage prediction tools can be found here. The above plot was created using Motorola Solutions' in-house tool available to employees only (sorry).
Near-Far Effect
Radio waves travel at 2,997x10⁸ ms⁻¹ in air. This means that a signal will take 500µs to travel around 150km.
This means that, radio A and C (using timeslot 1) cannot be more than 150km apart. The same is true for the distance between A and B (on timeslot 2). The difference between the two (i.e. the difference between x and y) does not come into play since the interslot guard period is 2,5ms.
This is commonly known as the near-far effect and would be relevant in the following system configurations where both slots are used:
- On simplex with Dual Capacity Direct Mode.
- On simplex with Extended Range Direct Mode.
- On a repeater operating in digital mode. This could be a single site; IP Site Connect; Capacity Plus or Capacity Max.
If the radios are being used on simplex without DCDM or ERDM then the near-far effect does not come into play.
Multipath
Multipath occurs when a signal is is received directly from the transmitter together with signals that have been reflected off objects along the path (e.g. buildings; mountains; water). In some cases, the reflections can be as strong as the direct signal.
The signal from the portable radio (right) will arrive at the repeater (left) directly (red line) and the reflections from mountain and buildings will arrive a few nanoseconds later.
If you are old enough to remember analogue over the air television, you might have seen ghosting. This is (was) multipath. Since analogue TV transmitted the picture line by line, a reflected signal would arrive at the receiver a few nanoseconds later and would be seen as a ghost image to the right of the original image (note the shoulder of this newsreader).
The effects of ghosting on analogue TV could, in most cases, be reduced or eliminated by using a directional antenna (Yagi) at the receiver.
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| Image credit: dxarchive.com |
The relatively low bitrate and use of 4FSK mean that the effects of delay spread are comparatively small. DMR also employs interleaving and parity which aid in countering the effects of Rayleigh scattering (ETSI TS 102-361-1 [v2.6.1] §B.2 and §B.4). This means that DMR is more tolerant to the effects of multipath when compared to technologies like GSM.
Despite being tolerant to the effects of multipath, there may be circumstances where this will affect the received signal which has an impact on coverage to some extent.
Other things to consider...
- The TX power on ATEX portables is set to 1W maximum. Special attention needs to be paid to ensuring good inbound coverage where these radios are used
- Most portable antennas have less than unity gain under ideal circumstances - see this post.
- The DM4000e series is available in a 25W and 45W variant. The latter is not allowed/sold in some countries due to regulations.
- Changing a 50W SLR5500 out for a 100W SLR8000 will only improve the outbound coverage by a factor of 3dB - see this post.
Frequency
Generally speaking:
UHF (and 800MHz if you have it) tends to work better in cities; underground and indoors.
VHF tends to work better in suburban and rural settings.
Although you can use ither in both settings, one will outperform the other depending on the terrain.
Interference
A system which receives interference will have poorer performance when compared to an interference-free system with the same properties. Interference on DMR is different from interference on an analogue system in that the interference can be heard on analogue whereas on digital it results in garbled audio; missed calls amongst others - some of these symptoms can be incorrectly attributed to coverage problems.
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