People not only talk on the airwaves, they increasingly expect their gadgets to do the same. The trend began in the late 1990s with Bluetooth, which provided 1 megabit of data per second. Then Wi-Fi and the IEEE 802.11 standard pushed the rate to 100 Mb/s. Now ultrawideband systems are going five times as fast as that.
In principle, such radio links, operating over short ranges, could replace the cables that now clutter our homes and offices, eliminate the speed penalty of going wireless, and even allow portable devices to off-load computing work to a nearby base station. The devices could thus shed hardware to become smaller, lighter, and cheaper.
But it won’t happen until engineers lay their hands on more bandwidth. The various 2.4- and 5.8-gigahertz systems now in common use are rapidly running out of spectrum, and the inflexible 100-microwatt constraint on ultrawideband power will likely limit it to about 1 gigabit per second. Where do we go next?
Clearly, we must look upward, but just how far up isn’t so obvious. One tempting thought is to use really high frequencies—infrared light. Although that tactic works fine if all you want to do is switch TV channels with your remote or operate a wireless mouse, it turns out that it’s hard to modulate the output of infrared light-emitting diodes fast enough for more demanding applications. So for the moment anyway, RF makes more sense, and the best prospects to be found there reside in 7 GHz of unlicensed spectrum near 60 GHz. Those frequencies are 10 times as high as anything in common use today, and with the bandwidth they provide they can carry a lot more data. Until now, engineers designing products for the consumer market have shied away from 60 GHz because of various technical difficulties, but bandwidth hunger is finally awakening their interest.
Developments in chip design have also played a part. Today’s 60-GHz technology depends on relatively expensive and power-hungry gallium-arsenide semiconductors, but various researchers—including engineers at IBM, the University of California, Berkeley, and those in my group at University of California, Los Angeles—have shown that silicon chips can do the job with much less power and at a fraction of the cost. The silicon option is what makes 60-GHz communications attractive.
The allure is so strong that a special task force is now working on an extension of the IEEE 802.15.3 standard for wireless personal area networks in the 57- to 64-GHz band, and in 2006 a number of companies—including Matsushita, NEC, and Sony—came together to define a specification for transmitting high-definition video in this slice of the radio spectrum. Their group, called Wireless HD, of Sunnyvale, Calif., wants to link TV sets to disc players, video cameras, game consoles, laptops, and other devices at rates as high as 5 Gb/s—fast enough to transmit an HD feature movie in about a minute [see “May the Power Be With You”].
There are other advantages besides extra bandwidth and faster data rates. Because the wavelengths are so short, the antenna needn’t be much bigger than the head of a pin, small enough to go on the transceiver chip. Indeed, it is feasible to integrate many antennas and transceivers into a single chip so that together they can, with proper phasing, form a beam to steer transmissions in a particular direction [see illustration, “Adaptive Antennas”]. Such phased-array antennas can also be used to boost reception. These operations can be conducted automatically so that the sender and the receiver can find each other without human intervention, constituting an adaptive-array (or “smart”) antenna system.
The integration of the antenna avoids the need for wires to carry signals to and from the chip, reducing the cost of packaging by one to two orders of magnitude. Further, the absence of exposed inputs and outputs makes the transceiver less vulnerable to electrostatic discharge during fabrication and assembly. Manufacturers could thus dispense with antistatic devices, which add capacitance and degrade performance.
These benefits do not come for free, however. Communication at 60 GHz involves significant challenges at the system, circuit, and device levels—challenges that account for why this bandwidth has lain fallow for so long. Designers can, however, get around these obstacles by taking advantage of the capabilities available at one level to relax the requirements imposed at another.
The difficulties begin with the propagation of the 60-GHz wave itself. As with any electromagnetic signal, the number of watts passing through each square meter diminishes in proportion to the square of the distance from the transmitter. On top of that, the size of the antenna scales with the wavelength, so its effective area—and so the power it can capture—varies in direct proportion to the square of the wavelength (and therefore in inverse proportion to the square of the frequency). Hence a signal broadcast at 60 GHz will convey to the typical receiving antenna just 1 percent as much power as it would have done had it been broadcast at 6 GHz. Making matters worse, 60-GHz rays are blocked by solid objects.