Smart radios stretch spectrum
The sky isn't falling. Contrary to apocalyptic hypothesis, there is no shortage of available spectrum upon which to deploy next-generation wireless services. In fact, there's an abundance.
Unfortunately, however, there is an equal abundance of regulations that bar access to that spectrum. In short supply are the policies, technologies and methodologies that would allow a suitably equipped radio to access spectrum intelligently worldwide without incurring the wrath of current users—from TV broadcasters and cellular operators to radio astronomers and the various militaries. Enter cognitive radio.
A cr is "a really smart radio that would be self-, RF- and user-aware, and that would include language technology and machine vision along with a lot of high-fidelity knowledge of the radio environment," said Mitre Corp. researcher Joe Mitola, who coined the term in the late 1990s. For an FCC desperate to appease a spectrum-starved wireless industry, the concept was irresistible. In late 2003, the FCC's newly-formed Spectrum Policy Task Force issued a notice of proposed rule making, calling for input on how CR could be realized commercially. In the interim, the task force also issued regulations for the use of UWB and opened up spectrum for unlicensed use in the 5GHz and 60GHz bands.
The wireless industry has unabashedly embraced it in the belief that cognitive is the next big thing after software-defined radio (SDR). "It's an extremely challenging and interesting application," said John Chapin, CTO at Vanu Inc.
One of the more immediate commercial applications of CR now being studied is the reuse of TV's UHF bands in the 80-850MHz range, where the propagation characteristics allow for long-range, non-line-of-sight, last-mile Internet access.
While the concept of a smart radio that can dynamically adapt to the environment and reuse licensed spectrum on an opportunistic basis sounds attractive to many, incumbent spectrum licensees aren't impressed. They perceive reuse as a threat to their business model. "If there hadn't been such strong government drivers, CR would have been dead on arrival," said Chapin, who credits the FCC for having pushed CR so far over the protests of existing spectrum users. However, pushback from incumbents is only the start of CR's problems, although the wireless-engineering community has recast those problems as innovation opportunities.
The opportunities start at the hardware level, where complex, frequency-agile RF front-ends and SDRs, while not mandatory, are viewed as a foundation for CR in its most extreme form of a radio that can jump in and out of any band. The XG program of the Defense Advanced Research Projects Agency (Darpa) is the most ambitious dynamic spectrum-adaptation effort to date, calling for a radio that can cover the entire swath from 30MHz to 30GHz, although that's considered an endgame approach. In the interim, "hopscotching"—where a radio can jump between specific bands of high interest such as those between 800MHz and 2.45GHz—is the more feasible approach.
Advances in FPGA, DSP and structured-ASIC development tools and programming environments from the likes of Pentek, Spectrum Signal Processing and Mercury Computer can accelerate waveform development and allow rapid baseband reconfigurability at ever-lower power and cost. In addition, the Software Communications Architecture (SCA) 3.0 promises to provide the security, application programming interfaces and code density required to make cross-platform waveform portability a reality.
"SCA is the operating system that defines the interfaces, connections and plug-and-play," said Al Margulies, executive director of the SDR Forum. "It's a functional specification that can be implemented on Corba or Java."
SCA 3.0 is currently in a state of "strategic pause," according to Mark Turner, director of Harris Corp.'s RF communications software and security products engineering organization. But an SDR Forum spokesperson said the organization continues to plan what forms of support for SCA-related standards and extensions, and associated compliance testing would be of greatest value.
As the hardware underpinnings of the most extreme and flexible version of CR get ironed out, academia and industry are tackling a sense of dissonance left in the wake of the FCC's actions on CR. Where radio development once dealt with linear optimization of individual and well-defined bands, researchers must now think in terms of horizontal development across a wide, and possibly contiguous, swath of spectrum.
Universities worldwide have started researching the theory, realization and practical application of CR. The SDR Forum in January last year set up the Cognitive Radio Working Group (CRWG) and the Cognitive Applications Special Interest Group. "The CRWG is a natural extension of our work in SDR and is the next stage in developing intelligent radios," said Margulies. The working group will focus on the radio development and the SIG on applications, he said.
Within the IEEE, the 802.18 Radio Regulatory Technical Advisory Group has spun out the 802.22 working group on wireless regional-area networks. The specific aim is to craft a standard for a CR-based PHY and MAC layer air interfaces that license-exempt devices could use to access spectrum allocated to the TV broadcast service—on a non-interfering basis.
"The singular advantage of this technology is the propagation characteristic in terms of range and power," said John Notor, wireless architect at Cadence Design Systems Inc. and an early participant in CR's development. According to Carl Stevenson, the IEEE group's chair, "the goal is to do something equivalent to wireless DSL or cable, but not 802.11a at 54Mbps."
Covering the spectrum from 54MHz to 860MHz, the PHY/MAC would allow for minimum download and upload rates of 1.5Mbps and 384kbps, respectively, at the fringe of what Stevenson predicts will be a coverage radius of 40km. "The MAC will handle roundtrip delays out to 100km, and CPE down close could get higher rates if they use higher-order modulation schemes such as 64QAM or 256QAM."
According to Stevenson, the group's work takes a divide-and-conquer approach in a relatively narrow range of frequencies. More important, the incumbents are easy to detect and avoid, thanks to their well-documented bands of operation and, in the case of DTV, their use of easily identifiable pilot tones. "I call it cognitive radio 'lite,' " he said.
Nevertheless, Stevenson is well-aware of broadcasters' ongoing concerns about potential interference issues, but he said that representatives from the National Association of Broadcasters, Fox and CBS were attending the group's meetings. "They believe something will happen with spectrum reuse and are helping to make sure we do it right."
While the FCC has stalled in its push of TV band reuse due to fears of interference, Cadence's Notor considers those concerns overblown and is adamant that reuse can be implemented with current technologies.
The IEEE working group's efforts may get a shot in the arm from work under way at Ireland's Centre for Telecommunications Value-Chain Research. Led by researchers from Trinity College, the CTVR is using OFDM as the foundation for a spectrally sculpted CR scheme that it believes will aid signal detection and interference avoidance.
"You need good measurements to make a good estimate of what's going on and a means of controlling the spectrum of the transmitted waveform," said Keith Nolan, a professor at Trinity. "That's why we chose OFDM." OFDM, he said, can be modified at will or "sculpted" to avoid interfering with incumbents.
The CTVR is setting up a test bed in the Dublin area in the bands between 1.6GHz and 2.5GHz that it will open to other researchers worldwide for CR experimentation via a Web interface.
Nolan described how the Trinity group managed to perform frame synchronization, carrier offset estimation and subcarrier allocation in a single symbol to reduce overhead.