TETRA base station that goes to the edge of physical limits

Using FPGA technology was quite an innovative solution at that time

In 2012, a customer contacted us with an idea of expanding their team of specialists. That is why we were responsible for a design of a suitable system architecture, including the simulation of the radio module. We started with the receiver part at that time. One thing that was already quite innovative about the project was the fact we used the FPGA technology (Field Programmable Gate Array) for digital signal processing and one ARM processor core that enables a real-time TETRA and TEDS data link layer. Another ARM processor core hosted Linux OS and applications for station monitoring and control.

This was one of the quite common solutions in software-defined radios that use digital signal processing, are widely configurable, and can be transparently connected to Wide Area Networks.

“It made our implementation of various other support protocols easier – for example, the SNMP protocol,” recalls Martin Tesař the beginnings of cooperation between the customer and Consilia company, which was responsible for the atypical challenge.

The architecture includes a design of a signal path from an antenna, adjustment of the high-frequency signal, frequency mixing, analog-to-digital converter, signal processing in FPGA, and a HAL (Hardware Abstraction Layer) design for Linux. The digital part is based on the Zynq platform.

After the successful indoor base station, the outdoor project came to life

“The essential part of the product line was quite a large-sized indoor base station. It is basically a set of modules built into a mobile rack in the controlling building, from which a number of cables lead to the antennas on the roof. After developing this base station, we designed our own test system for type and certification tests. Then we created test scripts used on the production line. Later on, the customer launched an outdoor base station project with us. This station is primarily designed for mobile use in larger vehicles – police cars, firefighter engines – and also at public events such as large concerts or similar events, where the safety requirements and medical support are strict. A base station is typically assembled on a mast near the antenna,” adds Martin Tesař.

Unique oscillators and power-efficient amplifier

According to Martin Tesař, the greatest challenge during the development process was to find an ideal solution for a power amplifier in the receiver, which was also supposed to be able to work in energy-saving modes while retaining the requested linearity. The final power level had a peak power of 300 W, in the case of the indoor basement, to provide a sufficient reserve for the crest factor of PSK and QAM modulation. The extraordinary spectral purity of the transmitter ensures compliance with strict regulatory laws.

“The station also contains unique oscillators that provide a pure signal with low noise – this is what we call a spectral mask. The lower noise of the spectral mask an oscillator has, the more immune it is to various kinds of interference. That was quite a unique thing because commonly available parts or modules you can buy from distributors of electronics parts were not suitable. That is why one of our colleagues developed voltage-controlled oscillators using resonators, enabling us to achieve quite interesting parameters.

Our customized oscillators, along with great energy efficiency of our solution for the final transmit level, put this product among the top competitive products.”

Inbuilt software controls radio communication

The inbuilt software is based on Linux OS, for which the team designed special controlling and monitoring applications. These applications exchange data via inter-process communication on strictly defined interfaces. The connection with the outer world is dealt with similarly, for example, with control units for network management.

“To develop inbuilt software, we implemented, debugged, and tested applications in C++ on an ARM Cortex-A9 processor. We designed a hardware abstract layer using object-oriented techniques and modeling in UML. We ported Arch-based Linux distribution on the target processor. We have also developed a hardware emulator and configured software profiling to recognize potential instability during maximum data traffic,” explains Martin Tesař.

From the very beginning to a tested product

The engineers from Consilia also provided support during the production of the first product lines in the Chinese factory. During the development, Consilia engineers liaised with many teams, including systems engineers, software engineers, PCB design engineers, buyers, and production line support.

“The first conceptual phase was launched in 2012. In 2013, we went on with the prototypes for indoor units, and we launched the production in 2014. Between 2015 and 2017, we designed a concept and carried out the realization of an outdoor unit including the production launch.

I am convinced that the product has not become morally obsolete yet; on the contrary, it is still among the top of what is possible to achieve using current technologies,” concludes Martin Tesař.