They were out to debunk a popular myth: With the explosion of wireless devices, the air is nearly saturated with zinging TV, radio, cell-phone, and BlackBerry signals, right? Not to mention satellite and air-traffic-control signals, police dispatches, and mushrooming Wi-Fi networks. And yet, the duo found that even in a heavily trafficked part of the airwaves above the District of Columbia, only 19% to 40% of the spectrum was occupied at any moment during an eight-hour period.
The experiment highlights a paradox that vexes the wireless industry. Although nearly all of America's best ready-to-use spectrum is rented out to tenants ranging from broadcasters to the U.S. military, most of the time, it's unused -- just vacant space. ``It's as though every operator had his own personal road to go to work instead of sharing the road,'' says McHenry, president of tech startup Shared Spectrum Co. in McLean, Va. The upshot: Wireless inventors with a new idea may not get access to the airwaves.
As any of these inventors might tell you, however, momentous changes are in the air. Digital technologies have already allowed cellular operators to pack more signals into each band than they could under the old analog regime. Now, a wave of intelligent network technologies is sweeping from university and military labs into the marketplace. The innovations are known by various names, including smart antennas, mesh networks, and agile radios -- all of them sharing the same basic breakthroughs in digital signal processing. Together, they appear poised to knock down the lane dividers on the spectrum highway, which were devised about 75 years ago when federal regulators concluded that the airwaves were a scarce resource.
Regulators are applauding the liberation of the spectrum -- especially after witnessing the meteoric rise of Wi-Fi. This wireless networking standard, and the ubiquitous Internet ``hot spots'' it has spawned, took off in the U.S. only because there was a swath of airwaves that regulators left open for unlicensed gadgets such as microwave ovens and garage-door openers. That's why the FCC is dismantling more fences. In mid-November, it offered up a new slice of lightly regulated frequency in the 5-gigahertz range. ``The more people who can play in the sandbox, the higher the probability of technological innovation,'' says Federal Communications Commission Chief Engineer Edmond Thomas.DUG-IN RESISTANCE
Broadcasters, cell-phone carriers, and other longtime licensees of spectrum rights won't give up their exclusive hold without a fight. ``If we have to pay for spectrum and others can gain access to those very bands for free, it becomes a parity issue,'' says Brian F. Fontes, vice-president for federal relations at Cingular Wireless. Still, engineers, inventors, and their financial backers are sure to keep up pressure on the FCC to use the airwaves with greater efficiency and imagination. Together, new ideas about intelligent devices and novel network architectures will open up a wireless frontier. Here's what the engineers have in mind:Smart Antennas: When the first AM radio tower went up in 1920 at KDKA in Pittsburgh, the proud antenna on top beamed out signals 360 degrees around it. Like a pebble thrown into a pond, it sent energy indiscriminately in concentric ripples through the air.
What a waste, say many engineers today. If you could throw all that energy in just the direction of the users you want to reach, the signal could travel much farther and avoid unnecessarily jamming airwaves in other directions. With a ``smart antenna,'' a narrow beam shoots a greater distance in the same way that a water hose sprays farther when the gardener puts a thumb over the nozzle. But that doesn't capture the intelligence of these systems. Wireless consultant Nitin Shah prefers the analogy of a spotlight following individual actors on a stage, as opposed to a room light that illuminates everyone.
There are many approaches to such antennas under study at universities and corporate labs -- including a commercial product from San Francisco startup Vivato Inc. By clustering 128 pencil-size antennas, Vivato can project signals as far as 2.5 miles. They achieve this by squishing a tiny bit of energy -- roughly 100 milliwatts, or half the power of a cell phone -- into narrow seven-to-eight-degree beams. Each antenna, starting at a different moment, sends out its own signals on regular radio waves. When the waves of one start waning, those of another might be cresting. Taken together, these waves can form rays specially shaped to reach a particular target. With the help of software, the antennas can change the shape and direction of these rays at a moment's notice when targets move. In this way, Vivato expects to extend the range of Wi-Fi, currently limited to about 300 feet. ``Smart antennas are the future of spectrum sharing for wireless,'' says Greg Raleigh, CEO of Airgo Networks Inc. in Palo Alto, Calif., which makes a more complex extension of this technology. ``Ten years from now, they'll be in every wireless device.''Mesh Networks: Just as smart antennas free up more airwaves than traditional towers, a new routing technology makes even today's most efficient digital networks look like spectrum hogs. With cell-phone systems today, for example, users must be within the range of a cell tower, also known as a base station, to get a link. The cell tower is the central hub connecting the phones around it.
With so-called mesh networks, one transmitter can get a connection from the antenna next to it, even if neither is in the range of the hub. All that's required is a connection between users, circuitous or not, that leads to a hub -- which may be an Internet access point or a cell tower. To explain this idea, engineers often invoke the analogy of a crowded cocktail party. Instead of shouting across the room to tell your spouse it's time to go home, thus drowning everyone else out, the guests transmit the message person by person in whispers across the room.
In the case of mesh networks, each device -- whether it's a laptop computer, as in most prototypes today, or a futuristic cell phone -- becomes the equivalent of a base station or network hub. As soon as one of the devices is switched on, it puts out a signal announcing its presence and searches for signals from others. Once the newcomer finds a group of networked devices, it knocks on the door, and the others welcome it to the group. True, wired Internet switches achieve the same effect. But the challenges multiply in the wireless world, where changing topography is ever a threat. If a building goes up across your street, a mesh network would spot the obstruction and route itself around it.
Various companies have experimented with ad hoc networks like these -- so far, without much commercial success. For one thing, the demands of a mesh network can drain everyone's batteries. Even if your laptop doesn't want to communicate across the network at the moment, it ends up being used as a hopping station for other people's messages. Despite that, Nokia (NOK
), Microsoft (MSFT
), Intel (INTC
), and others hope to use the technology to extend the range of Wi-Fi. And it's not just commercial ventures that are interested. In San Mateo, Calif., the Police Dept. is testing a system from a local startup, Tropos Networks, to connect laptops in police cars across the downtown area.Agile Radios: On the blue waters of San Diego Bay, engineers have implemented the first phase of an experiment that could lead to the biggest radio breakthrough of all. Twelve sailors on the USS Coronado, a Navy flagship, are testing a radio from General Dynamics Corp. (GD
) that can communicate in 10 different frequency bands. That matters because the military has access to many swaths of spectrum, but different branches of the Armed Forces use different radios, which often can't talk to one another. The gizmo on the Coronado uses software to transmit and receive in multiple frequences, thus breaking down the barriers. ``Think of these radios as a computer with an antenna,'' says John D. Bard, CEO of Space Coast Communication Systems Inc. in Melbourne, Fla., which is writing new radio software.AN ULTRASMART MACHINE
The military's work in software-defined radio is the prelude to what many consider the ultimate solution for the wireless future: the agile radio. This device can hop in and out of empty spaces in the spectrum, operating in a variety of different bands, in spaces nobody else is using. The Defense Advanced Research Projects Agency (DARPA) is funding the development of this ultrasmart machine, which would be able to scan the airwaves and determine the vacant spaces on its own. As other users pile into these frequencies, the agile radio would see the traffic and instantly seek out ``white space'' in other bands.
It will take at least 10 years to create a workable agile radio, according to Vanu Bose, CEO of Vanu Inc, which is developing software-defined radios. For one thing, it must be smart enough to ensure that the vacant bands are indeed available, and not cause interference with existing users. ``We have to prove we can coexist,'' says Preston Marshall, DARPA's program manager for the agile radio. But if such a sensitive radio can be made, researchers say users can harvest up to 10 times more out of the airwaves.
The lure of such bountiful yields has turned federal regulators into evangelists for these potentially disruptive technologies. At first, the FCC is likely to move slowly, releasing small blocks of spectrum that are under lease to the military and others. But once the dam is compromised, there is no stopping the deluge. On the flood plain, wireless innovators may realize their wildest dreams -- and citizens will gain access to a new frontier in the sky. By Catherine Yang in Washington