Evolving radio technology: SDR to cognitive radio
Software-defined radio is no longer a military-only technology, as it has evolved beyond the now-defunct Joint Tactical Radio System (JTRS) program, can be found in commercial and defense applications worldwide, and is essentially a solved problem. Meanwhile, a big remaining challenge is the finite amount of available spectrum. Dynamic spectrum management is needed to navigate what is left and cognitive radio is seen as the technology that will make that possible.
Software-Defined Radio (SDR) technology is flourishing in defense and commercial applications despite the demise of its patron – Department of Defense’s (DoD’s) JTRS program. SDR devices are in the hands of today’s warfighters not only via certain JTRS successes but also through various Non-Developmental Item (NDI) programs and independent development by companies such as Harris Corp. in Melbourne, FL. New SDR products continue to be developed and an upgrade is on the way for the Software Communications Architecture (SCA). Yet, challenges still remain for users and designers of SDR solutions – especially the coming spectrum shortage that will likely force a shift toward cognitive radio.
Software-defined radio is a solved problem
Most of the radios in the world today, including handsets, are based on SDR technology, because some level of the physical baseband processing is done on a Digital Signal Processor (DSP), which is a programmable device.
“The key to software-defined radio is really the ability to reprogram a radio to change its functionality,” explains Lee Pucker, chief executive officer of the Wireless Innovation Forum, a nonprofit international industry association that is focused on radio communications and systems worldwide and based in Reston, VA (www.wirelessinnovation.org).
The definition of SDR, developed by the Wireless Innovation Forum in conjunction with IEEE, is a “radio in which some or all of the physical layer functions are software programmable.”
A key distinction is that there’s no relationship between SDR and Radio Frequency (RF), meaning that it can be an SDR and only ever be able to transmit in one band, according to Manuel Uhm, vice president of marketing for Coherent Logix, a provider of high-performance processors to enable next-generation software-defined systems in Austin, TX. Uhm also serves as the Wireless Innovation Forum’s chief marketing officer and treasurer. “Many people confuse SDR with a radio that can receive or transmit anywhere across a wideband spectrum, which isn’t accurate,” Uhm points out. “What it really means is that the actual baseband processing, or some portion of it, is done in software.”
JTRS and its goals for SDR
Like so many wireless technologies, SDR was originally developed by the military. It’s not a new technology; the military has worked with it for more than 20 years and was the first to deploy it in the field through its JTRS program. Most military systems around the world operate systems similar to JTRS, which mandated SDR as the path forward for all radios. A big impetus behind JTRS was enabling warfighters to reduce the number of radios they needed to carry around. The AN/PRC-154 Rifleman Radio from General Dynamics C4Systems in Phoenix, AZ is an example of a successful dismounted radio that came out of the JTRS program. It transmits voice and data simultaneously using the Soldier Radio Waveform (SRW) (see Figure 1).
“A typical communications soldier out in the battlefield today might be carrying between 20 to 30 radios, because you need different radios to talk to different services and forces,” Uhm says. “Each of these radios is like a brick, so imagine running around Afghanistan with 20 to 30 of these bricks on your back ... it’s ludicrous.” Another driver was the pursuit of cost savings. “If a radio waveform is defined by software, in theory, you can readily port the waveform to next-generation radios using next-generation processors that are higher performance and lower power,” Uhm explains.
While the DoD agencies and U.S. government funded the original development of JTRS, they have for all intents and purposes now cancelled the program – after achieving their goals. “Moving forward, they’re not funding development, but are funding procurement of radios. They’re still spending money to buy existing SDRs that were developed under JTRS, but only off-the-shelf,” Uhm notes.
One recent example of spending on off-the-shelf SDRs is a $38 million USD order placed with Harris Corp. to provide tactical radios to expand deployment of wideband tactical communications across the U.S. Air Force. The Air Force is using Harris’ Falcon III AN/PRC-117G multiband manpack and AN/PRC-152A multiband handheld tactical radio systems to accelerate the rollout of JTRS wideband networking capabilities to para-rescue, combat communications, and other Air Force personnel. While this type of investment will continue, in the future if companies want to make new features or waveforms available they’ll likely need to fund it on their own.
Evolution of SDR: Software Communications Architecture 4.0
While SDR devices are becoming more widely used on the battlefield and in law enforcement like the Falcon radios, the enabling technology behind SDR – the Software Communications Architecture (SCA) – is currently undergoing a revision called “SCA 4.0.” The SCA is the key component of any SDR as it is what enables the SDR’s re-configurability. It is the middleware that enables radio waveforms to be designed in software.
SCA 4.0 takes this concept a step further by enabling users to tailor SCA components to specific applications. It will be “more modular and allow you to select portions of the SCA to use for a specific program,” Pucker explains. “One of its capabilities is to dynamically allow you to set up and tear down waveforms.” By adopting a more modular approach, you can remove portions of SCA you’re not using and “shrink the footprint to create something that’s easier to instantiate on a specific kind of radio platform,” Pucker says. “It enables more ‘á la carte’ SCA options vs. the all-encompassing previous versions.”
Cognitive radio: The next frontier
Beyond SDR remains the challenge of a finite amount of available spectrum. Industry experts see cognitive radio as the solution to this problem.
“Cognitive radio is the next frontier,” Uhm says. “It adds dynamic spectrum access ability to an SDR, which means you can still change the type of waveform processing the radio does, but now add a wideband RF front-end and have a radio with the intelligence to scan across a wide bandwidth of spectrum to identify ‘holes’ in the spectrum and transmit through them.”
Why would you want this capability? It’s a well-established fact that the traditional licensed model of spectrum isn’t sustainable moving forward. “At some point we’re going to need to move to spectrum sharing to maximize its usage or else we’ll run out of spectrum – it’s inevitable,” says Uhm.
Cognitive radio will have implications for everyone, including the military, which may end up losing or sharing spectrum in the future. The military uses dedicated spectrum, and so do public safety and mobile operators. Whatever decisions are made in the regulatory environment will have a huge impact on all of these markets.
The scope of this issue also extends much further because one of the battlegrounds is Ultra-High Frequency (UHF) spectrum (~500 to 700 MHz), which is used by TV broadcasters. This is a highly desirable spectrum because this frequency tends to propagate further and go through walls – it provides much better radio characteristics than shorter wavelengths. Today, TV broadcasters own this highly desirable spectrum, surrounded by guardbands or open channels between TV channels, also known as TV whitespace. Since we’ve moved from an analog world to a digital one, these open channels are no longer necessary.
For spectrum sharing of these open channels, cognitive radio technology is one obvious solution. If the military needs to move within the spectrum, for example, cognitive radio will allow them to adapt to changes without having to replace radios, according to Uhm. “It makes sense to use cognitive radios that can change and move within various spectrum bands rather than replacing them with fixed-function radios that could be obsolete again within a few years,” he elaborates. “This is a problem the defense industry is cognizant of because it may have significant ramifications for the them – not just within the military communications market, but part of the spectrum in question is also used by military radar.”
Military radar spectrum is ~3 GHz, which also falls within the range of commercial services. If this range of spectrum is opened up in the future, an entire swath of military radar may need to move to a different frequency.
The bottom line is, no matter what happens in the regulatory environment with spectrum sharing, “moving forward, we’ll begin seeing more cognitive radio and dynamic spectrum access capabilities added to SDR capabilities,” says Pucker.