Evaluating tactical software-defined radios (SDRs) poses no small challenges for test engineers. Even when considering only the analog hardware attributes of these radios, a test engineer is faced with verifying radios that have enough bandwidth for multiple communications standards, the ability to transmit and receive signals at high data rates, and the capability to simultaneously operate multiple radio channels. Even if spread-spectrum operation, such as frequency-hopped signals, is ignored, the analog portion of these radios resembles multiple radios and requires special testing.
An SDR is not unlike a personal computer (PC), wherein the function of the computer is defined by its software. In the same way, the functionality of the radio is defined by software loaded into the radio. The hardware must be relatively generic but extremely broadband, with the software controlling frequency, modulation, channel bandwidth, security functions, and waveform requirements.
Although the concept of an SDR was developed by the military to meet the requirements for reliable and secure communications across different branches of the armed forces, there is increasing interest in the technology for commercial applications. FlexRadio Systems, for example, currently manufactures the SDR-1000, a commercial SDR for use by amateur radio operators. It incorporates a direct-digital synthesizer (DDS) for frequency agility and waveform flexibility from 12 kHz to 60 MHz. This analysis, however, will focus on tactical SDRs for military use.
The United States Department of Defense (DoD) is driving the development of SDR technology through its $1 billion Joint Tactical Radio System (JTRS) program. The goal of the program is to replace traditional hardware radios with units covering 2 MHz to 2 GHz and beyond that can emulate any radio by changing software. Software upgrades sent via wireless networks will keep fielded JTRS devices current and compatible. In addition to the US DoD, most European defense agencies have SDR development programs similar to the JTRS program.
Most development programs specify SDRs in a wide range of footprints, from compact, manportable units to vehicle-mounted and shipboard platforms. According to the SDR Forum , an industry group organized to foster and standardize SDR technology, SDRs are "radios that provide software control of a variety of modulation techniques, wide-band or narrow-band operation, communications security functions (such as hopping), and waveform requirements of current and evolving standards over a broad frequency range."1
In the military, SDR applications are emerging rapidly as technology advances enable their effective use. SDR solves the existing incompatibilities between the command and control radio systems of the various branches of the armed services as well as with the communications systems of allied and coalition forces, enabling all units to work together as a single team. Future SDRs will seamlessly operate with the latest single-channel ground and airborne radio system (SINCGARS) units, for example.
The SDR Forum requires an SDR to provide software control of a variety of modulation techniques, wideband or narrowband operation, communications security functions (such as frequency hopping and encryption), and waveform requirements for current and evolving standards. The SDR should be able to store a large number of waveforms and add new ones via software download.
Current multiband tactical radios operate from 2 to 512 MHz. SDRs will cover that range and more, with current SDR designs extending from 2 MHz to above 2 GHz to handle high data rate waveforms. The tactical radios that SDRs emulate, of course, differ widely in design and operating characteristics. High-frequency (HF) radios (operating from 1.6 to 30 MHz) were developed for tactical (short-range) and strategic (long-range) communications in World War II, and this is still one of the most widely used communications bands. But large power amplifiers, antennas, and robust antenna matching circuitry are needed for long-range communications, and such components are not well suited to extended battery life. As a compromise, HF tactical radios often provide medium-range communications when operating from a battery pack and longer-range communications when connected to a vehicular power supply (Figure 1).
1. HF tactical radios provide long-range communications capabilities using frequencies from 1.6 to 30MHz. (Photo courtesy of Harris Corp.)
Due to signal congestion in the over-crowded HF band, HF radios must often compete with interference from other transmitters. To combat interfering signals, HF radio front ends are usually designed with high intercept points and employ sharp cutoff filters to eliminate unwanted signals. Documentation such as MIL-STD-188-141B4 clearly defines the performance requirements of a tactical HF radio. Information bandwidths are limited, often to only 3 kHz wide, requiring stable frequency synthesizers in the radio's front-end circuitry along with narrow intermediate-frequency (IF) filters so that transmitted signals do not spill into adjacent bands.