What is Software Defined Radio?

True Software Defined Radio (SDR) is defined by the use of a general purpose processor (GPP) to accomplish the signal processing required for communications. Traditionally, Field Programmable Gate Arrays (FPGAs) have been the dominant technology in developing and deploying military waveforms. i2SDR is challenging this paradigm with game-changing communications technology that rivals (and often surpasses) standard FPGA waveform implementations.

As GPPs become increasingly more powerful, SDRs likewise grow in capability. Since high-level source code may be compiled across GPPs, upgrading an SDR’s processing power is as simple as replacing the GPP and re-compiling software. The equivalent upgrade on an FPGA implementation requires an in-depth redesign that typically takes months of labor.

The use of high-level coding languages (C, C++) also allows for a significantly accelerated development time when implementing new waveforms. What takes months of FPGA development time has been shown to be completed in weeks of software development using these methods.

How Can the Air Force Utilize Software Defined Radio?

SDRs can equip the Air Force with next-generation communications technology by implementing modular radio systems driven by hardware-agnostic software. By implementing SDRs on operational platforms, the Air Force gains resiliency to the rapidly growing demands of communications on the battlefield. This capability is achieved by combining fast waveform development time with low-cost Commercial Off-the-Shelf (COTS) hardware to create a complete SDR-based communications system.

Why is SDR important to the Air Force?

  • Software Defined Radio enables rapid/agile waveform development in response to emerging threats.
  • Hardware-agnostic source code allows for easy portability across many platforms.
  • Multi-waveform radios may serve as bridges between various platforms on the battlefield.
  • SDR combined with custom RF frontend development enables the creation of versatile communication solutions.
image of radio equipment

An AFRL test engineer configures radio equipment for a flight demonstration at Naval Air Station Patuxent River. Photo Credit: AFRL

SDR Systems at AFRL

AFRL has demonstrated the capabilities gained by SDR technology through the design and manufacturing of various SDR systems equipped with optimized waveform software. The airborne-compliant SPARTAN Radio, manufactured by Collins Aerospace, is capable of running two simeultaneous, high-throughput data links between different network nodes.

The STAWCR, manufactured by Jacobs/KeyW, is a small form-factor single-channel SDR prototype that could be embedded on a small Unmanned Aircraft System (sUAS).

These SDRs are versatile thanks to a growing military waveform library. A variety of waveforms have been implemented and optimized in C++. These include ISR waveforms (BE-CDL, STD-CDL), tactical waveforms (DDL, TDL) and SATCOM waveforms (DVB-S). The foundation for this collection of waveforms is the optimized library of signal processing functions, which enables the rapid development of new waveforms. Using these functions as a toolbox, waveforms may be assembled modularly, which allows for efficient development across different teams.

illustration of radio model

A Model of the SPARTAN radio manufactured by Collins, capable of 300MHz instantaneous bandwidth and two simulatenous RF Links. Photo Credit: Collins, AFRL

Radio Hardware Development

SDRs still require reliable radio hardware, and AFRL has designed and implemented both COTS and custom-built RF systems that enhance connectivity at the physical layer. Three areas of interest for i2SDR are (1) RF translators (2) Antenna design, and (3) aircraft tracking systems.

RF translators are used to expand the functionality of existing radios for high-band frequency operations while providing minimal phase distortion and signal loss. Antenna design has consisted of custom-building and characterizing Ku-Band Active Electronically-Steerable Arrays (AESAs) and Circularly Polarized Ku-Band Dishes, both of which have high directivity. Lastly, low-cost and highly-reliable aircraft tracking systems have been developed to position these high-gain antennas for aircraft tracking demonstrations.

Thanks to the power of SDRs, these matured designs have demonstrated the ability to maintain connectivity in operational environments over long ranges (>120 Miles). SDRs continue to be used in AFRL’s large-scale communications demonstrations.

image of test equipment

A custom-built Ku-band AESA mounted to an aircraft tracking system during beam pattern characterization measurements. Photo Credit: AFRL