HotSpot node for DMR / D-STAR / C4FM / NXDN / P25 / POCSAG

Before talking about hotspots, nodes, access points and repeaters, let's briefly compare the digital modes.

Digital voice modes compared

GMSK - Gaussian Minimum Shift Keying
4FSK - 4-level Frequency Shift Keying
C4FM - Continuous 4-level Frequency Modulation
FDMA - Frequency Division Multiple Access
TDMA - Time Division Multiple Access

User identification

* The callsign is shown when the subscriber ID is in the contact list of the receiving radio; otherwise the subscriber ID is shown. 

Repeater linking

Using the radio

Any hotspot, node or repeater consists of hardware and software and is connected to an amateur radio network.

Hardware

The hardware of a hotspot, node or repeater consists of:

• a computer;
• a modem;
• a level-matching board (optional);
• a radio or a low-power transceiver;
• an information display (optional).

Computer

The usual computer is a Raspberry Pi single-board computer. There are plenty of models to choose from: anything from the Raspberry Pi Zero W to the Raspberry Pi 4 will do. The table below lists the models and their specs.

Modem

While the choice of computer hardly matters, the modem deserves a careful choice. The modem decides the computer and the radio part (the radio modem), and whether the device ends up a hotspot, a node or a repeater. There are countless modems on the market, some with a low-power transmitter on the same board. Here are popular and affordable ones:

• Arduino Due
• MMDVM POG
• MMDVM LJT
• MMDVM HS HAT
• MMDVM HS HAT Dual
• MMDVM HS HAT Dual (LoneStar) N5BOC

As the table shows, the ready-made modem / radio boards are built on Chinese RF7021SE (ADF7021) radio modules and STM32Fxxx modems.

The RF7021SE (ADF7021) radio module:

• Frequency range 80–650 MHz;
• Output power up to +13 dBm (20 mW);
• Sensitivity −115 dBm (0.4 µV);
• Supply voltage 2.2–3.6 V;
• Current draw 26–30 mA.

The RF7021SE (ADF7021) is not outstanding in sensitivity, selectivity, blocking, out-of-band suppression or dynamic range. In a noisy environment expect up to 1 km; with line of sight in the countryside, up to 10 km. A stable modem needs a high-stability TCXO of 2 ppm or better — I recommend the FOX924B-14.7456. Ready-made boards already have one; the Arduino Due needs it added, since its own oscillator is not stable enough.

Level-matching boards

The name says it all: the board matches the levels of the modem and the radio. In most cases the matching circuits are already on ready-made modems and transceivers. One of the few cases that needs a separate board is an Arduino Due used as the modem. You can buy one or build it yourself. Of the ready-made boards I recommend the SP8NTH v 6.2. To build one yourself, I recommend the NTH pinout with bias stabilisation. Below is the version I used: 

Radio or low-power transceiver

This part is simple. A hotspot uses a ready-made radio modem board with a low-power transceiver. A repeater needs a pair of radios, a node just one. The classic choice is the Motorola GM340, but in practice any radio with 9600-baud support will do. When choosing a radio, look at the usage statistics on the BrandMeister site. Keep in mind that the best receiver bandwidth depends on the mode: set the IF width to 25 kHz for DMR/C4FM and 12.5 kHz for D-STAR.

Information display

A display is optional on a hotspot, node or repeater, but makes life much easier, especially without a dashboard. The usual choices are:

• OLED 0.96" or 1.3" (depending on the radio modem board and the case);
• Nextion of any version.

An OLED display is easy. A 0.96" or 1.3" OLED will do; inside they are identical, 128 × 64 px. It is soldered to the radio modem board on the 3V3, GND, SCL and SDA pins and enabled in MMDVM Host. It shows the basics: the mode, the subscriber ID/callsign and name, the current talk group.

A Nextion display shows more and does more than an OLED. The most popular models are NX3224K024_011R and NX4832K035_011R. The Nextion connects either to the radio modem board (5V0, RXD, TXD, GND) or through a USB-UART adapter to a USB port of the Raspberry Pi (when the board has no such pins). It has to be flashed first, either over the same USB-UART with Nextion Editor, or from a microSD card. 

As for firmware: if the Nextion only shows a little (2–3 lines) and does not control the MMDVM device, the simple firmware by G4KLX or ON7LDS plus the MMDVM Host settings is enough. For a rich multi-line display that also controls the MMDVM device, install the Nextion Driver for MMDVM from here or from here. I recommend taking Nextion display firmware from here. When picking a firmware, mind the display model and its screen size.

Software

The software of a hotspot, node or repeater is built on the open MMDVM platform (Open Source Multi-Mode Digital Voice Modem). There are many images for the Raspberry Pi; the most popular come from the PI-STAR project. They come with the most complete set of preinstalled components and a web panel to manage them. Here are the components:

As said at the start, D-STAR uses the AMBE+ vocoder while DMR, C4FM, NXDN and P25 use AMBE+2. So transcoding between DMR, C4FM, NXDN and P25 is done in software, while D-STAR to DMR, C4FM, NXDN or P25 needs AMBE3000 / 3003 / 3006 / 3012 hardware. You can build the transcoder yourself or buy one. Suppliers: DVSI's Hardware Solutions, NW Digital Radio and DVMEGA.

Cross-mode settings are described in detail in this article. As said above, cross modes with D-STAR are impossible without a hardware AMBE transcoder. Ready-made modem/transceiver boards have none, so they have no D-STAR cross modes. The only hotspot I know with an AMBE chip is the OpenSpot3. This page  lists its cross modes.

So Pi-Star is a multi-purpose build and is recommended for hotspots. Download Pi-Star from this link. Installation and setup are described in this manual. Problems with this build are discussed on the forum.

For access points and repeaters, though, lighter and more stable custom builds are better. A good example is the KAVKAZ build, available at this link. You can also make your own build. You will need:

• A tool to write the OS image. I recommend: balenaEtcher or Win32 Disk Imager.
• The operating system image: take the latest or one from the archive of older versions (another archive).
• The software core is available here or here.
• Cross-mode software transcoders are available here.
• And some Linux know-how.