Abstract
The chapter tries to provide an introduction to DMR (digital mobile radio) targeted at the unexperienced operator and anyone interested in this topic. I try to hide details until it gets absolutely necessary to explain them. The majority of DMR introductions I've found, are more or less extensive glossaries (if you are interested in that, see Glossary). They are hard to comprehend, unless one has at least some experience with DMR.
The perceived complexity of DMR comes from its origin as a radio standard for commercial applications at large events and companies (i.e., trunked radio). Therefore, I will first describe an example how DMR is used commercially before I describe how it is used in amateur radio. I hope that this approach will make some of the weird terms and concepts of DMR clearer.
This section briefly describes the common amateur radio FM-repeater operation. The majority of all licensed operators will be familiar with this topic.
If you are not yet licensed and interested into amateur radio, consider reading this section. Otherwise, skip right to the section called “DMR Introduction & Origin”
The majority of connections between HAM operators are made in the so-called simplex mode. That is, the two operators transmit and receive alternately on the same frequency[1] and the connection is direct. This works very well on HF where world-wide direct connections can be made.
For this example, DM3MAT transmits directly to DL2XYZ on the frequency 144.500 MHz. The latter then also answers directly on the same frequency.
On higher frequencies, however, radio waves behave more like light and it gets increasingly difficult[2] to bridge significant distances beyond the horizon. This fact limits the operating range of simple hand-held radios. To still cover a large area, repeaters can be used.
Repeaters are autonomous amateur radio stations that are usually located on a mountain, hill or high tower. This allows them to easily cover a large area. They receive signals from HAM operators and retransmit them at the same time. To do that, they cannot send and receive on the same frequency (otherwise they would interfere with themselves). Therefore, repeaters operate in the so-called duplex mode. That is, the repeater receives on one frequency (the so-called input frequency) and transmits the received signal on another frequency (the output frequency) simultaneously.
For this example, DM3MAT sends on the input frequency 431.9625 MHz to the repeater DB0LDS. The repeater transmits the received signal on its output frequency 439.5625 MHz. On that frequency, DL2XYZ receives the original call.
The Example 1.2, “Simple FM repeater operation” shows a common repeater operation on UHF. Here, the operator DM3MAT transmits its call to DL2XYZ not directly but on the input frequency of the repeater DL0LDS (431.9625 MHz). The repeater receives the call and transmits it simultaneously on its output frequency (439.5625 MHz). This signal is then received by DL2XYZ. Consequently, the call has reached its destination, although DM3MAT and DL2XYZ may not be able to communicate directly. The reply of DL2XYZ to DM3MAT follows the same path. Here DL2XYZ transmits on the repeater input frequency, and DM3MAT receives that call on the repeater output frequency. This way the two operators can communicate with each other even if they are not able to reach each other directly.
However, there are situations, where two operators are far away and they cannot reach the same repeater. For these cases, it is possible to connect two repeaters via the EchoLink network.
DM3MAT connects repeaters DB0SP (near Berlin) and DB0LEI (near Leipzig) via EchoLink. Now, they are able to communicate with each other.
This network allows to link FM repeaters via internet™ or to connect directly to a remote repeater via internet. Many FM repeaters are connected to the EchoLink network, allowing for world-wide communication with simple hand-held radios.
Frequently, it is also possible to control a repeater over-the-air and to connect that repeater to some other repeater via EchoLink. Usually, this can be done by sending the EchoLink number of the destination repeater using DTMF to the local repeater. This is shown in Example 1.3, “Repeater operation with Echolink” above. There, DM3MAT sends the EchoLink number 662699 of the repeater DB0LEI near Leipzig to his local repeater DB0SP near Berlin. Then the local repeater (DB0SP) will link with the destination repeater (DB0LEI) via the EchoLink network. For some limited time, both repeaters will act like one logical repeater. That is, everything that is received by one repeater will also be transmitted by the other. This way, the two operators DM3MAT and DL2XYZ can communicate although they cannot reach a common repeater.
Note
Once two repeaters are linked via EchoLink, they behave like a single repeater.
All over the world, there are FM repeater that are part of the EchoLink network. Therefore, it is possible to communicate world-wide at any time using simple hand-held radios that are as cheap as 40€ or even less.
[1] This is actually called semi-duplex, however the term “simplex” stuck. The term simplex actually refers to the situation, where there is only one transmitter and possibly many receivers (e.g., broadcast).
[2] Also on VHF and UHF, larger distances can be bridged using an elevated location and larger antennas.