For the PDF version of this article, click here.

Software-defined radio or SDR may or may not be the most revolutionary arrival since Edwin Armstrong introduced the superheterodyne in 1918. It is too early to tell, but SDR has all the earmarks of being a powerful engine for innovation because the military's Joint Tactical Radio System (JTRS) is playing a pioneering role by funding what will be landmark advances in radio communications. These advances will, in good time, undoubtedly trickle down into all sectors of consumer and commercial communications. JTRS is drawing its technical sustenance from a variety of sources — in particular, software, digital signal processors (DSPs), general-purpose processors (GPPs), and field-programmable gate arrays (FPGAs). What's more, the military has committed billions of dollars to replace hundred of thousands of traditional radios now deployed, with SDRs.

The JTRS efforts are subdivided into clusters. (See, “JTRS — Cluster by Cluster,” p. 14.) Cluster 1 has encountered some bumps in the road. At the time of this writing there is uncertainty as to whether Boeing will continue as prime contractor for Cluster 1. But whatever the outcome, it is not likely that it will put much of a dent in the overall program. It is true, however, that Cluster 5, as we discuss later in the article, is presenting the greatest challenge with regard to DSP processing since it will employ a highly novel form of networking.

But before we go any further, it makes sense to summarize just what JTRS is.

JTRS is a reconfigurable, software-driven communications architecture (SCA) that aims to introduce a family of software-programmable radios, which will enhance communications capabilities and at a reduced cost of ownership. JTRS architecture will enable wireless devices to break free of predefined functions and capabilities, so that new features can be implemented in real time, including the ability to update and change modulation schemes, protocol standards, and frequency bands. Unlike previous radio systems, SDRs developed under the aegis of JTRS, will be able to interoperate with each other and be upgraded via software to incorporate the latest communications technologies.

Any JTRS SDR will be able to download what are termed ‘waveforms’ but are actually digital representations of standard radio protocols. Each SDR will store these downloads in memory and at any time dynamically transfer the right one into the receiver and transmitter channels when it wants to reconfigure itself as an entirely different radio.

Shown in Figure 1 are the basic ingredients of an SDR. The ‘enabling technologies’ are built with the chips that comprise the processors, the FPGAs, and the RF components, together with the software. They store and deliver the enabling technologies that will ultimately govern each ‘SDR hardware platform.’ At the right are the ‘user markets,’ which now are confined to government entities, but one day may also embrace the consumer and business markets.

The transition from an analog to a digital radio can be viewed as a graduated evolution, as depicted in Figure 2. It maps the SDR hardware along the vertical axis and increasing functional capability along the horizontal axis so that the ultimate in a SDR culminates at the upper right corner of the square.

Although SDR was first conceived more than 15 years ago, there simply did not exist, at that time, the technology to bring it to reality. But that has changed. The current JTRS had its beginnings in 1997. Although low-rate initial production was expected to begin in 2006 it will probably turn out that Cluster 2, managed by the U.S. Special Operations Command, will actually begin production this year of an SDR version of the multiband inter/intra (MBITR). This is a 30 MHz to 512 MHz, 5 W hand-held radio manufactured by Thales Communications Inc. They will be delivered to the U.S. Army, Navy, Marine Corps and Air Force. All told, there are expected to be some 177,000 units involved.

Echotek is a potential SDR hardware supplier whose specialty is in mixed-signal products that marry the analog and the digital together and assure the integrity of the signals transported, as in their platform shown in Figure 4. They are able to maintain an analog-to-digital converter (ADC) or a digital-to-analog converter (DAC) to within a 0.5 dB of its manufacturer's specification — which is no simple feat in what is so often a noisy environment. SDR is engendering some strange bedfollows, according to Ray Blakeman, manager of strategic and international accounts for Echotek, because, in some cases, they are selling product to a customer who was formerly a competitor, and vice-versa. So it is no surprise that the front-ends are moving toward the ADCs and the DACs are moving toward the transmitting portions as larger FPGAs and more effective DSP capability becomes available.

Originally, Echotek began putting FPGAs behind the ADCs, simply because they had to come up with a way to shrink the data, so that it would be manageable and would travel over standard buses. But their customer base took a look at that and said: “That's wonderful, you are now providing an ADC that is enabling me to manage a large amount of data before I send it down to my processor board and throughout the rest of my system. If you put a little larger FPGA on there we could do some real pre-processing on your card!”

As Jim Brown, a senior program manager at Raytheon in Fort Wayne, Ind., points out the bedrock beneath SDR is the software communications architecture (SCA) and that architecture has already proven itself and is continuing to evolve and improve so that it can become more robust in dealing with SDR issues. Raytheon was one of the principal developers of the SCA program and the leader of a consortium, including BAE Systems, ITT and Rockwell-Collins, that delivered version 2.2 of SCA in 2002.

The efforts on the part of the software suppliers are to develop waveforms, which is software that provides mission-specific communication protocols. Right now, in Cluster 1, there are more than 30 waveforms, which all perform digital processing. Here are the objectives of the SCA software:

• Portability

  Maintaining the portability of JTRS is a crucial goal of the SCA program. When software is written you would like to be able to move it, which means it must be ‘SCA compliant.’ Right now, that portability is well under way and has already been demonstrated. Where the challenge will really come is when the SCA is extended into higher-frequency regions such as SHF and EHF — there it will be a challenge to maintain portability, said Brown.

• Networking

  This will be essential, especially in Cluster 5 applications, where a linkage of multiple radios will be implemented for hand-held radios that are on the move, but will have no node such as a cell tower, that is commonly employed in cellular telephone systems.

• Security

  Because the radios will be reprogrammable and handle multiple channels, this will pose challenges when some channels are secret, and others are not. Another aspect is having the radio certified by the National Security Administration. They have stringent rules that require effective isolation and demand that security not be compromised in any way.

• Interoperability

  Interoperability within JTRS will be supported through the use of the software-based waveforms. The waveform software developed for JTRS includes the actual RF signal in space, as well as the entire set of radio functions that are manifested from the user input to the RF output, and vice versa. For example, in the transmit path the waveform software will control the receipt of the data, analog or digital, from the input device and manage the encoding. The encoded data is passed to the encryption engine. The resultant encoded/encrypted datastream is impressed as modulation upon the intermediate frequency (IF) signal. Finally, the IF signal is converted into a RF signal and transmitted to the antenna. These same functions are then reversed in the receive path.

Ideally, you would simply want to hang the ADC directly on the antenna. But you cannot, as Dave Duff, business development manager, communications products at Analog Devices, pointed out because there would be no preselection and the ADC would be overwhelmed. Digitizing at the antenna would require an astronomical dynamic range. So some prefiltering is still essential and is performed after downconverting to the IF frequency.

The ideal SDR would also have the capability to tune the center frequency as well as the bandwidth. But some of that has to remain in the analog domain. So unless you have a perfect ADC, sampling several GHz/sec with 16 bits to 20 bits of resolution, you are always going to be forced to do some amount of analog preselection, said Duff.

“Once you get into the digital domain, you do have a lot of flexibility to tune and filter at some sub-data rate, which is much lower than what you would capture with an analog filter,” he added.

Handling the transmit side digitally is really a lot easier because you are not dealing with a multichannel spectral environment — and you are not dealing with interferers — simply because you are the source of the data. Instead you are concerned with the signal you are generating and with not introducing any spurious content, said Duff.

Until now, the processing capacity demanded by SDR, particularly by Cluster 5, required a lot of power and, therefore, produced a good deal of heat. But the next generation of FPGAs from Altera and Xilinx are likely to halve the power usage for a given level of DSP performance.

“The FPGAs can contribute a lot of work in the IF, which is the upconverter/downconverter side, and we can also do a lot in the baseband-processing domain, as well,” said Joel Seely, marketing manager, military, industrial and automotive products, Altera Corporation.

“The biggest challenge in the SDR program is in upconversion and downconversion. It is here that DSPs can help significantly,” said Ram Sathappan, the DSP new business development manager at Texas Instruments.

However, DSPs are not yet at the stage where they can completely take over. The problems are the MIPs-intensiveness of SDRs in certain applications and also the perception by many that DSPs simply cannot do it.

“The latter is simply not true,” he said.

One of the largest obstacles lies in the power consumption limitation in the JTRS Cluster 5 handset. Using DSPs is entirely feasible from the performance point-of-view, but not the power point of view. It is Sathappan's view that hardware coprocessors within the device will ultimately solve the problem.

“We want people to understand that a DSP can do a lot more than it has been used for traditionally,” said Sathappan. “Today, DSP is used mostly for voice functions and some modulation functions — but not much else. They can also use it for networking, and eventually for the entire upconversion and downconversion processes.”

In conclusion, the ultimate question is: How close can you push the digital realization to the antenna? This applies to both the receive and transmit mode. And, will DSP co-processors eventually supplant the FPGAs that are playing a major role in the initial SDR implementations?

But regardless of the outcome of the above, there is no question that the SDR will free traditional wireless transceivers devices from predetermined functions and capabilities, enabling new features to be implemented in real time including the ability to instantaneously update and change modulation schemes, protocol standards and frequency bands.

1. “Software-Defined Radio” by Chad Hart, Venture Development Corporation, www.vdc-corp.com.

2. “Software-Defined Radio,” a white paper, Wipro Technologies, www.wipro.com.

3. “New Technology Facilitates True Software-Defined Radio,” by Ronald M. Hickling, RF Design, April 2005, p. 18.

4. “Interface Considerations for SDR Using Digital Transmitters,” by Walid K.M. Ahmed and Elias Kpodzo, RF Design, April 2005, p. 10.

Steve Grossman began his career as a design and project engineer with Radio Engineering Laboratories in Long Island City, NY developing troposcatter and satellite microwave communications equipment for the U.S. Air Force, the British Army Royal Corps of Signals, and Comsat. He later served as a technical editor at McGraw-Hill's Electronics and then at Electronic Design. Grossman earned a bachelor of arts from New York University, a bachelor of electrical engineering degree from the City University of New York and pursued graduate studies in electromagnetics and microwaves at the Polytechnic Institute of New York. He can be reached at stevegrossman@zianet.com.

• Cluster 1

  Cluster 1 is developing two distinct radio systems: a ground vehicular joint tactical radio (JTR) set with a tactical air control party (TACP) variant, and an Airborne JTR set. These systems will interact using JTRS waveforms, including legacy waveforms and the wideband network waveform (WNW). The ground vehicular JTR set will be scalable, to provide from one to six channels, and will include an embedded GPS receiver. The TACP JTR set will operate six simultaneous waveforms and will support additional simultaneous capabilities for a six-channel JTR set. It will also include an embedded GPS receiver.

• Cluster 2

  The United States Special Operations Command manages JTRS Cluster 2. The cluster will acquire hand-held radios for the Army, Navy Marine Corps and Air Force. The strategy for Cluster 2 hand-held radios comprises evolutionary development of the current MBITR hand-held radio. Cluster 2 hand-held JTR sets include implementation of waveforms in the 30 MHz to 512 MHz range.

• AMF

  The JTRS airborne and maritime/fixed station (AMF) program resulted from the merger of Cluster 3 (maritime/fixed station) and Cluster 4 (airborne) requirements. The JTRS AMF program will acquire JTR sets for airborne, maritime and fixed station platforms for all services.

• Cluster 5

  The Cluster 5 effort will oversee acquisition development and production of JTRS hand-held and manpack units and forms suitable for embedment into platforms requiring a small form fit (SFF) radio.