Articles

There exist two approaches for designing communication equipment. The classical approach means that the entire radio link, including filtering, demodulation and generation of signal, is engineered using analogue components or specialized microchips. Only the control and interface parts are based on programmed microcontrollers. However, advancement of technologies (in particular, rise of ADC and DAC rates) made it possible to transfer the major part of functions to software. This approach is called Software Defined Radio (SDR) and is optimum in terms of functionality and cost effectiveness nowadays.

The main advantages of this approach are flexibility in composing modulation/demodulation algorithms, signal characteristic dynamic variation capability, operation in broad spectral band with several types of modulation simultaneously, and possibility of modifying algorithms on-the-fly. Besides, this technology uses program processing of radio signal that enables creating encrypted communication channels with various security levels and application of digital data packet dynamic routing (MESH technology) to considerably increase range and reliability of radio communication. In addition to engineering advantages, this approach allows to reduce the governmental budget expenditures for production of new equipment thanks to commonality of terminal devices and possibility to improve functionality through firmware updating and reflashing.

For example, to solve the problem of commonality in a specialized communication system where two generations of different devices are used, analogue ones with frequency modulation (FM) and digital ones with continuous-phase frequency-shift keying (CPFSK), the use of SDR technology may be employed to enable both types of modulation and automatically switching between the modes depending on external conditions, all within a single software-controlled device. Moreover, it will enable employing such a solution in related areas (other frequencies, modulation types, etc.). Adapting it to other communication systems will require modifying antennas, power amplifier and output filters to fit the performance goals of the used radio frequency path, however, without changes in digital hardware; this will speed up and cheapen future developments.

Presently, the AstroSoft Development Company is developing a software system to design terminal SDR devices (Figure 1); these efforts are aimed at considerable simplifying and standardizing the software-defined radio application development processes. The software system includes digital signal processing algorithm libraries designed for field programmable gate arrays (FPGA) in the form of complex functional blocks, control libraries for central processor, real-time operating system, design tool, and a set of demonstration solutions. The use of existing solutions enables solving these problems within a short period.

A test setup was arranged to debug the software for such problems; the setup includes:

  1. Platform Nuand SDR BladeRF [1]: as a main component of SDR transceiver comprising:
    • FPGA Altera Cyclone 4 (115K logical elements),
    • ARM9 200 MHz with USB3.0 support,
    • LimeMicro LMS6002D transceiver – 300 MHz – 3.8 GHz (28 MHz DAC/ADC band),
  2. Transverter Nuand XB-200 [2]: to shift operating frequency to BladeRF operation range and filtering,
  3. Broadband power amplifier: to have required output power,
  4. Single-board computer RaspberryPi3 [3]: for setup, control and handling digital interfaces.


Software System for Designing Terminal SDR Devices
Fig. 1. Software System for Designing Terminal SDR Devices

 

Based on this test setup and with the use of current solutions from the terminal SDR device designing system, it is possible to develop the prototype software for solving the above described problem; the structure of such prototype software shown in Figure 2 includes:

  1. Complex functional blocks (FB) in FPGA:
    • FB for analogue frequency modulation/demodulation [4]
    • FB for digital continuous-phase frequency-shift keying [5]
    • FB for digital filtering and frequency shifting algorithms
    • FB for FFT [6], base station signal detection and signal characterization algorithms
  2. Interface algorithms for ARM926
  3. Control software for Embedded Linux.

 

Test Setup Block Diagram
Fig. 2. Test Setup Block Diagram


Once this problem is solved, the next logical step should be the transfer of existing network base stations to SDR technology that will help, together with terminal SDR equipment, to achieve a number of additional goals, such as:

  1. Higher reliability and noise immunity thanks to use of additional redundant coding methods,
  2. Using several communication channels simultaneously,
  3. Higher transmission rates,
  4. Implementation of cognitive communication algorithms,
  5. Use of the mesh technologies to increase communication range and reliability,
  6. Integration with other communication networks,
  7. etc.

 

References

  1. https://www.nuand.com/blog/product/bladerf-x40/
  2. https://www.nuand.com/blog/product/hf-vhf-transverter/
  3. https://www.raspberrypi.org/products/raspberry-pi-3-model-b/
  4. T.G. Thomas, S. C. Sekhar. Communication Theory. — Tata-McGraw Hill, 2005. — pp. 131–152.
  5. Leon W. Couch II, "Digital and Analog Communication Systems, 8th Edition". — Prentice-Hall, Inc., 2013. — pp. 359–362.
  6. A. Oppenheim, R. Schafer Digital Signal Processing, 3rd Edition. — Moscow: Tekhnosfera, 2012. — pp. 601–605.


A.A. Spirkov
AstroSoft

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