Digital Mobile Radio – an overviewPresented over the Henderson Amateur Radio Club Tek NetSunday 11-29-20By Tony Dinkel, WB6MIE

DMR stands for “Digital mobile radio”. It is a standard for mobile radio communications that is defined in the European Telecommunications Standards Institute (ETSI) Standard TS 102 361 parts 1–4. You can Google that if you want, but it goes far deeper than the scope of tonight’s presentation. In fact, most publications on this topic delve far deeper than I intend to go tonight, please consider this an overview of the system and some simple rules of entry.

Figure 1. A simplified diagram of a 2 timeslot DMR conversation in progress.

DMR is a two time slot, Time Division Multiple Access (TDMA) communication system. It was designed to service the commercial mobile radio market, hotels, taxi cabs, trucking companies, private security and other types of “dispatch” applications. Because it is a commercial system, we have to adapt our operations to it by using its existing feature set. Talk Groups, Group Calls, Private Calls, Color

Codes and Time Slots are most likely all foreign terms to most hams that are analog users.About “time slots”… with regular ham radio analog communication we get just a single time slot. A single conversation occupies all of the time on a given channel. With DMR and TDM we get to have two independent conversations on the same RF channel. Two separate QSOs? Yes, two separate QSOs, on the same channel, at the same time, and all within the FCC’s 2013 12.5khz emissions mask. This makes DMR one of the most spectrally efficient voice communication modes on the ham bands today. Operating under Part 97, we don’t have to meet this emissions mask right now. We can continue occupying our full 25khz bandwidth channels. But this spectral efficiency may become more important to us in the future. I’ll talk a little bit more about this later.The concept of having two independent conversations going on the same channel at the same time may make some peoples brains hurt. It’s called multiplexing and it goes on all the time in the real world. You are a user of multiplexing systems and you don’t even have to know about it. You use voice multiplex systems every day if you make a phone or cellular call. Whether you know it or not, you conversations are taking place simultaneously with millions of other calls, all being carried on massive multiplex systems. Or consider an FM stereo radio station. Two audio channels, left and right are broadcast to your stereo multiplex receiver and we don’t even think about it. Well, I think about it a lot because my job is to maintain systems like this, but you as the end user don’t have to think about it at all, it just happens. Listen to a Beatles record on an oldies FM radio station. If you hear “Flying” or “Revolution #9”, you will hear an example of just how independent these two channels can be.Multiplex systems seem unlimited in scope. In my younger years I worked with Frequency Division Multiplex or FDM. This was a technique where a “baseband” of separately modulated RF carriers was modulated onto a higher frequency RF carrier, around 1 ghz. These systems carried up to 20 independent conversations at the same time.In later years I was working with a system called T-1, which carried 24 simultaneous conversations using a technique called Time Division Multiplex or TDM. These systems grew into T-3 systems

which carried 28 T-1s or 672 conversations. Then 3 T-3 systems were combined onto a system called Optical Carrier 3 or OC-3 for 2016 conversations, or more specifically “voice circuits”. We really have to refer to these as circuits because each time slot also carries channel associated signaling messages such as on hook, off hook and caller information. Or in the case of ham radio, channel keyed, channel unkeyed, caller identification, and which conversation you wish to participate in.

Figure 2. Shows a comparison between conventional analog and two time-slot TDMA.

So, to recap, in analog, what you hear on the repeater is our only current talk group. We don’t really refer to it as a talk group because it’s just where we talk, but it can be considered a talk group for the purpose of this discussion. The only equivalent in DMR terms is “ALL CALL” and that is not really meant for two way communication in this context. jIn DMR, using different talk groups and time slots, it is possible for a conversation to be going on over a repeater channel and not be able to hear it. That’s not really ham radio you might say. I’ll come back to that and other unique aspects of DMR later. First, lets look at the steps involved in generating a DMR signal.

There are 4 steps involved in generating a Digital Mobile Radio signal. Thankfully, they are all taken care of inside the radio, no additional boxes or computers are needed. This is the last time you should have to think of these steps, I present them here to simply explain the process.

Figure 3. A basic block diagram of the DMR signal generation process.

1. Analog to Digital Conversion

When a radio user presses the Push-To-Talk (PTT) button and begins speaking, the voice is received by the radio microphone and converted from an acoustic pressure waveform to an analog electrical waveform. The voice waveform is then sampled by an analog to digital converter. In non two-way radio applications, a 16-bit sample is taken every 8 kHz, this produces a 128,000 bits per second digital bitstream, which contains far too much information to send over a 12.5 kHz or 25 kHz radio channel. Therefore, some form of compression is required.

2. Vocoder and Forward Error Correction (FEC)

Vocoding, for Voice encoding, compresses speech by breaking it into its most important parts and encoding them with a smaller number of bits. Vocoding compresses the voice bitstream to fit the narrow 6.25 kHz equivalent radio channel. The vocoder used in DMR is an AMBE+2TM which was developed by Digital Voice System, Inc. (DVSI). You can Google that if you are interested.

This particular vocoder works by dividing speech into short segments, 30 milliseconds in length. Each segment of speech is analyzed, and the important parameters such as pitch, level, and frequency content are extracted. These parameters are then encoded using a smaller number of digital bits. Forward Error Correction (FEC) is also applied. FEC is a mathematical technique that enables the receiver to both validate the integrity of a received message and determine which, if any, bits have been corrupted. FEC enables the receiver to correct bit errors that may have occurred due to radio frequency (RF) channel impairments. Think noise, phase distortion, multipath, etc. This effectively rejects noise that can distort an analog signal and by comparison enables more consistent audio performance throughout the coverage area.

This error correction has it limits, it can’t correct for gross errors or total loss of signal. It’s only able to correct for the occasional dropout, so don’t expect a miraculous increase in coverage. It can and does operate quite well at eliminating the noise associated with linear signal to noise degradation experienced when traveling away from a repeater or other station. It can and does work out to just about where you would typically find an analog FM signal to be totally unusable, perhaps just slightly farther. There are also timing related distance limitations imposed by DMR but I don’t know if I have ever reached the actual timing limit.

Figure 4. A comparison of voice quality in analog vs digital transmission.

3. Framing

At this stage, the vocoder has compressed the 128,000 bps input signal to 3,600 bps or 3.6 khz. In framing, the vocoded speech is formatted for transmission. This includes organizing the voice and any embedded signaling information, such as color code, group ID, PTT ID, call type, etc. into packets. These packets form a header and payload type of structure with the header containing the call control and ID information, and the payload containing the encoded speech.

This same structure can also relay Internet Protocol (IP) data packets, the IP packets are simply an alternative form of payload to the radio. The two-slot TDMA implemented in DMR uplinks use a 30ms window for each time slot, this is further divided into a 27.5ms frame and a 2.5ms gap. This means when transmitting, your mobile transmitter is only turned on for 27.5ms every 60ms.

In contrast, the DMR repeater transmits a continuous data stream even if only one timeslot is being used, the 2.5ms uplink gap is replaced with a Common Announcement Channel or CACH burst, that is used for channel management and low speed signaling. The 27.5ms frame consists of a total of 264 bits, a 108 bit payload, 48 bits for sync or embedded signaling, and a second 108 bit payload for a total of 216 bits of payload per frame. The vocoder must compress 60ms of audio with FEC into 216 bits of data for transmission. The 2.5ms gap is used for guard time to allow PA ramping and propagation delay. Because of propagation delay and synchronization there is a limit to the usable range of DMR. I’ve never worked out what that range limitation is, but I don’t believe I’ve ever reached it. I do believe that the limit is far enough out that it will never affect us in the Las Vegas area.The header information is repeated periodically throughout the transmission, thereby improving the reliability of the signaling information as well as enabling a receiving radio to join a call that may already be in progress when, for example, it is just turned on or tunes to the channel. This feature is called “late entry”.

4. TDMA Transmission

Finally, the signal is encoded for 4FSK RF transmission. The bits contained in the digital packets are encoded as symbols representing the amplitude and phase of the modulated carrier frequency, amplified, and then transmitted. TDMA organizes a channel into 2 time slots: a given radio’s transmitter is active only for short bursts. By transmitting only on their alternating time slots, two calls can share the same channel at the same time without interfering with one another, thereby doubling spectrum efficiency. Using TDMA, a radio transmits only during its time slot, that is, it transmits a burst of information, waits, then transmits the next burst of information.

Break for questions and radio cool down.

It is important to note that all of this “spectral efficiency”, two time slots, error correction and the other stuff comes with some engineering tradeoffs. When people think of “digital” they think about

extremely high quality audio. Nothing about digital mobile radio is high quality. Digital two-way radio was never about high quality voice transmission. It’s about cramming more users in an existing finite amount of bandwidth and selling more radios. Anything that involves compression loses something in the process, by design. Parts of speech that are familiar to us are simply thrown away by the compression algorithms. Speaker recognition, tone, inflections and other nuances are simply lost. All that is transmitted is the important stuff, the speech, with as few accouterments as possible. The reproduced voice can sound almost mechanical at times. And because of the way the digital signal is recovered, there is no noise. So there are no cues for when you are about to lose a signal. It simply drops out. There may be some unintelligible fragments of something that sounds remotely like a human voice, but that’s it, then nothing. Also, historical problems cannot be solved by digital radio. System issues with coverage and interference are not necessarily eliminated. Adjacent or co-channel interference may sound different to a digital user, but digital technology does not solve interference issues. For example, analog interference will not be heard as voice to a digital radio and vice versa, but disruption of system performance can still occur.

Some historical perspective

Now why is all of this a big deal? Because back in 2008, the FCC decided it was a big deal. Back then commercial users were facing a spectrum crunch. More entities than ever before were using the radio spectrum, and every single one of them were asking for more. Voice users, data users, cellular and broadband companies, television and radio broadcasting all have tremendous bandwidth requirements. The FCC decided that decreasing the bandwidth that we all occupy with our voices would be a great idea. By January 1, 2013 all voice radio communication systems would occupy no more than 12.5 khz bandwidth for a single voice circuit. Since the radio spectrum is not an infinite resource, we have to try and fit as many users into that limited resource as possible.

That spectrum crunch may have been relieved for a little while, but it never really ended. It is now worse than ever.As Ham radio operators, we historically have not really ever faced a spectrum crunch. Yes, we have lost spectrum slices here and there. Some of it was for noble causes and some of it was not. But it still seems that we have plenty of bandwidth. On the hf bands there always seems to be plenty of room to camp out on a frequency and call CQ. And not so much on the 2 meter band but on the UHF band there seems to be plenty of dead space to put repeaters, simplex channels, different forms of data communications and other emissions. These days are about to come to a close. Our usable frequencies are under intense scrutiny in the form of commercial interests that need our frequencies to build their 5G, 6G, 7G and whatever comes next communication systems. Our frequencies are a commodity and they will be traded as such, for big money, with the blessing of the FCC and the rest of the US Federal government. “Alternative” spectrum may be offered, but it very well may not be appropriate for our needs or just not enough.Eventually, and possibly sooner than we all think, we will need systems like DMR to utilize the meager slices of spectrum that we will be left with. Applications such as “Trunked Radio” may become popular. Trunking is popular in commercial radio and public safety systems. It involves a set of common radio channels that are shared between a large number of users. A rack of 5 repeaters on only 5 RF channels would give us a total of 10 logical channels. Each user would select the group of users that they want to communicate with and a channel would be assigned to us automatically as everyone else talks around us. Common talkgroups could be assigned so we could have our basic channel that everybody in Las Vegas talks on, kind of like what we have now on the harc network.The trunking application is called Capacity Plustm in the Motorola lexicon. More specifically, Linked Capacity Plustm would be more correct, since we would be linking our repeaters together over Internet Protocol Site Connecttm or IPSCtm.

Now I will get off of my soapbox and get on with the rest of the show.

But first…

Break for questions and radio cool down.

As I said earlier, DMR has some unique requirements. The first thing you are going to need is a Radio ID number. Go to:

https://www.radioid.net/register#!

and follow the instructions. After you receive your Radio ID number, you need to program it into the radio ID field of your radio codeplug. If you will be using more than one radio at the same time, in the case of a family or spousal unit, you will need more than one radio ID number. Don’t even try to key up on a DMR channel without a proper radio ID number. It’s the same as saying your callsign.To find your first repeater, either use repeaterbook or just Google Search for DMR repeater (your area). For example, a Google search for DMR repeaters Las Vegas sent me to:

https://lasvegasdmr.com/repeater-list/

I clicked the Potosi UHF link and ended up with a plethora of listings of talkgroups, timeslots and frequencies with the appropriate color codes.When selecting a talk group, keep in mind they are usually arranged by geographic area. A good place to start would probably be 91 or Worldwide. Other good ones to try would be 31327 for Las Vegas Local. Also try 3132 for Nevada Statewide. Explore, don’t be afraid to try things.Be sure to use the specified time slots for the various talk groups. The Color Code also needs to be correct. Color Code takes the place

of PL or Digital PL in a DMR system. There are 15 “CC”s so make sure you enter them correctly.Networking or linking of repeaters is an integrated feature of DMR systems. There are several worldwide and many regional networks setup by individuals who are so inclined. One of the friendliest and easiest to enter I have found is the Brandmeister Network. Visit their Wiki at:

https://wiki.brandmeister.network/index.php/What_is_BrandMeister

Click around and learn about them. They really are an incredible system.

My professional and amateur experience is with the Motorola® MOTOTRBO® product line and only with their XPR® series of professional radios. Some of the features I discussed may differ in functionality or not be available on other brands of DMR equipment.

This paper was sourced from my brain and used the Motorola® publication MOTOTRBOtm System Planner as subject matter reference. The part number for it is 6880309t12. If you are seriously interested in DMR I strongly suggest that you purchase a copy for yourself. In this paper I have made references to Motorola trademarks, patents or other intellectual property. These references were made for the purpose of technical discussion and no infringement upon the intellectual property rights of Motorola, Inc. was or is intended. All of the graphics used in this paper were copied from the MOTOTRBOtm System Planner publication copyright 2016. These were presented for the purpose of technical discussion and no copyright infringement was or is intended.

That said, DMR is a fun mode so program away and talk to some people. The longer you use it, the better it will sound.

Questions