Sharing spectrum the smarter way

[Dave Farber <dave () farber net>]

Delivered-To: dfarber+ () ux13 sp cs cmu edu
Date: Mon, 12 Apr 2004 16:11:16 -0700
From: Dewayne Hendricks <dewayne () warpspeed com>

 Sharing spectrum the smarter way
 By  Patrick Mannion  , EE Times
 April 02, 2004 (12:01 PM EST)
 URL:  <http://eetimes.com/article/showArticle.jhtml?articleId=18700443>

A seismic shift in radio design could break the spectrum-availability 
bottleneck, proponents say, and open up a new frontier of opportunities for 
radio designers and wireless application developers. Cognitive radio — one 
that can sense its environment and location and then alter its power, 
frequency, modulation and other parameters so as to dynamically reuse 
available spectrum — is emerging, but some are already calling it the holy 
grail of wireless design.

 Derived from a doctoral thesis submitted in May 2000 by Joseph Mitola, 
who is now at work on the military's cognitive-radio initiative, CR could 
in theory allow multidimensional reuse of spectrum in space, frequency and 
time, obliterating the spectrum and bandwidth limitations that have slowed 
broadband wireless development in the United States and elsewhere.

This new technology is a kissing cousin of software-defined radio. With 
SDR, the software embedded in a radio cell phone, for example, would define 
the parameters under which the phone should operate in real-time as its 
user moves from place to place. Today's cell phone parameters, by contrast, 
are relatively fixed in terms of frequency band and protocol.

 But cognitive radio is even smarter than SDR. CR "is a radio that's aware 
of and can sense its environment; learn from its environment; and perform 
functions for its user that best serve its user," said Bruce Fette, chief 
scientist at General Dynamics Decision Systems and technical committee 
chairman of the SDR Forum.

 Although the communications and wireless industries are just beginning to 
hear about cognitive radio, the U.S. government is already showing a keen 
interest in its possibilities. The Federal Communications Commission is 
weighing the commercial applications and the Defense Advanced Research 
Projects Agency is proposing a military version.

The FCC in December officially began the effort to extricate the concept 
from academia by issuing a notice of proposed rule making (NPRM) calling 
for input on how CR could be realized. Now in the response stage, that NPRM 
underscores the unprecedented willingness of the commission in recent years 
to explore innovative ways to open new spectrum to commercial unlicensed 
use. Examples include the release of new spectrum in the 5-GHz U-NII band 
last year, as well as the opening up of 7.5 GHz of bandwidth for 
ultrawideband (UWB) signaling in the region between 3.1 and 10.6 GHz. 
Though the power levels allowed for UWB were extremely low — a roof of - 41 
dBm — the move marked the first time the FCC had allowed unlicensed use 
across otherwise licensed bands.

"UWB was a first shot at this [reuse of spectrum]," said Bob Brodersen, 
professor at the University of California, Berkeley's Wireless Research 
Center. "But CR is a more complete solution as it [actively] looks for 
unused spectrum and begins to transmit inside those bands." Cognitive 
radios coexist with primary users, he said, but get out of those bands if 
need be when the primary users show up. "UWB just hid below the noise 
floor," Brodersen said. "This one does it by jumping out really fast. Yet 
it can transmit at reasonable levels."

In his opening keynote at the CTIA Wireless show in Atlanta last month, FCC 
chairman Michael Powell hammered home the need to open up new spectrum and 
to enable spectrum reuse. The commission, he said, formed the Spectrum

 Policy Task Force in 2000 "as we saw the potential for wireless broadband."

 But in the current system under which the FCC operates, formulated in the 
1920s, different bands are assigned to different services and licenses are 
then required to operate inside those bands. As a result, Powell said, the 
FCC has not "been able to faster optimize spectrum use. We're moving 
cautiously but purposefully and will look at policies that will bring it 
all together for consumers."

 UC Berkeley's Brodersen said that President Bush "also came out recently 
with an edict on the same topic of better spectrum use. So politically, 
there's great support for this."

The FCC's Spectrum Policy Task Force (SPTF) came out with a report in 2002 
that recommended the agency migrate toward a "policy-based" solution where 
spectrum could be used on an opportunistic basis. "This is a paradigm 
shift," said Jeffrey Schiffer, director of Intel Corp.'s Wireless Research 
Radio Communications Lab (Santa Clara, Calif.). "You can characterize it as 
the most exciting thing in radio that's come along in a long time, because 
of the opportunities that exist now and the creativity you can have around 
the ability to agilely move across the band and operate legally in multiple 
different places, using things like policy engines as a means to check 
whether you're legal."

In particular, the FCC is looking at a means by which the 6-MHz-wide 
licensed spectrum in the UHF band, currently assigned to TV broadcasters, 
can be reused in secondary markets as a path to last-mile data access (see 
story, opposite page). This proposal would set power levels up to 8 dB 
higher than those currently allowed in those bands, thereby greatly 
increasing range and coverage area. In addition, said Schiffer, the 
propagation characteristics at the lower UHF frequencies are particularly 
attractive, offering the possibility of longer distance and lower power.

 "Still, you have to operate in 6-MHz channels, so you won't get the 54 
Mbits/ second you get with .11g or a [Wi-Fi]," he said. Nonetheless, 
Schiffer says, data rates "will be respectable," especially in rural areas 
where channels can be concatenated.

 But the FCC's interest also extends to higher frequencies. "If you look 
at the entire RF frequency up to 100 GHz, and take a snapshot at any given 
time, you'll see that only 5 to 10 percent of it is being used," said Ed 
Thomas, chief engineer at the FCC. "So, there's 90 GHz of available bandwidth."

 Military angle
The military, too, has latched on to the concept of cognitive radio under 
the umbrella of the Darpa-funded XG — or Next Generation Communications — 
program. Its aim is to develop technology that allows multiple users to 
share spectrum in a way that coexists with, and complements, sharing 
protocols included in today's Wi-Fi technologies. "They're exploring the 
idea in the most extreme way," said John Notor, a system architect at 
Cadence Design Systems Inc. (San Jose, Calif.) and a leading researcher in 
the area of CR. "They're working on a 'dc-to-daylight' box covering a broad 
range of frequency bands up to the microwave area and which can use any 
spectrum at any time and adapt accordingly."

 The government's interest in — and definition of — CR goes way beyond 
spectrum reuse, said Fette of General Dynamics. Indeed, CR and Darpa's XG 
technology are not identical, as many in the industry believe, he said. 
"The spectral-efficiency technologies — as exemplified by XG [and the FCC's 
efforts] — are one important facet of CR, but not the only facet."

 From a military perspective, CR's ability to handle functions that best 
serve its user translates to sufficient situational and mission awareness 
to help the soldier reach an objective. "The kind of data required for this 
is still in the labs," said Fette. "Academicians are studying this and we 
want to work with them." Examples of data for a soldier, he said, could 
include local topography, mission objectives and time scales for those 
objectives, as well as knowledge of — and access to — the radio networks in 
the area as well as "the location of friendly — and enemy — positions and 
artillery."

WLAN launch pad
Researchers believe the technical foundations established by wireless LANs 
provide a launching pad for CR. WLANs already incorporate essential CR 
features such as dynamic frequency selection and transmit power control. 
Also, while the RF front ends may require wideband receivers and 
transmitters, Fette said, the hardware exists now, "and the software is a 
matter of sitting down and doing the software engineering" to make 
functions like filtering, band selection and interference mitigation 
"available as plug-in software modules for the radios. That's stuff we know 
how to do."

 Fette sees the technology being implemented in a different style for the 
commercial market, "but the FCC's interest in spectrum efficiency will make 
that happen fairly quickly — in five years. And that's not so bad," he said.

That's not to say there are no technical hurdles to surmount. "Some 
fundamental ideas need to be ironed out, such as interference temperature," 
said Brodersen of UC Berkeley. Interference temperature, a metric the FCC 
included in its December rule-making notice, involves helping a radio 
that's intending to transmit determine how much interference it will cause 
on other radios in the locale to ensure the interference doesn't cross a 
certain yet-to-be-determined threshold, said Brodersen. "But the problem is 
how you, as a transmitter, can detect how much interference you're causing 
this other receiver? That's a tough problem," he acknowledged.

 Researchers are also grappling with how to identify channels and 
so-called "white space" within the TV spectrum, as well as the uncertainty 
surrounding FCC certification rules for radios with no assigned frequency 
bands.

 Protocols and communications channels are also a thorny issue, since 
they're required to help disparate radios negotiate optimum communication 
parameters to minimize their respective interference levels.

 "We're working on a prototype CR right now, but the problem is the 
protocol and how to figure out how to set up these calls," said Brodersen, 
referring to the kind of data that should go back and forth between the 
physical and media-access control layers. "So you have to design a protocol 
stack that's very different from what we've had before as you have this 
sensing thing built into it now."

 "There's some research to be done, especially in the incumbent profile 
detection space," said Cadence's Notor. "Radios don't exist in isolation. 
Turn on a bunch of radios and how do they detect each other? How do they 
get out of the way if a licensed user comes up? None of this has been 
addressed in any standards I'm aware of."

Archives at: <http://Wireless.Com/Dewayne-Net>
Weblog at: <http://weblog.warpspeed.com>

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