IMAGE: Bryan Christie Design

ADAPTIVE ANTENNAS: can form a beam and direct it to a target, greatly reducing the attenuation of power, a serious problem in the 60-GHz band, which is strongly absorbed by air. In a conventional antenna [left], the received power declines with the square of the distance. Adaptive antennas [right] use an array of emitters with delayed phases that make the waves’ peaks and troughs add constructively [insert]. This trick focuses the power into a beam, which can then be steered up (a) or down (b) electronically. Advances in 60-GHz transceiver design now make it possible to fit an adaptive array on a single chip.

Although such designs avoid the need for a 60-GHz oscillator, they still require low-noise amplifiers and down-converters. These circuits typically use passive devices like inductors or transmission lines on the chip to overcome the speed limitations of the transistors. Alas, such passive components have large footprints, which normally forces them to be placed awkwardly far apart, their long interconnections creating lots of parasitic resistance, capacitance, and inductance. To alleviate this problem, designers can nest the inductor loops used to build these passive components so that the connections between them can be kept short [see illustration, “Design Tricks”].

A bigger concern is how to fabricate transmitter circuits that can deliver a lot of oomph to the antenna. For communication across a range of 10 meters at data rates of several gigabits per second, some tens of milliwatts are necessary. Performed by a power amplifier, the task requires large transistors, which are typically slow. The good news is that the upcoming generation of CMOS chips, which boast 45-nanometer gate lengths, may be up to the job of producing this much power at 60 GHz.

But that's not the whole story, because not all the power that goes into an on-chip antenna gets broadcast. The silicon substrate—just 10 micrometers below—absorbs (and hence wastes) some energy, so such antennas radiate only a fourth to a half of the power supplied to them.

Perhaps more research will lead to more energy-efficient antennas. Meanwhile, engineers can resort to off-chip antennas that operate at these tiny wavelengths. Another, cheaper, solution is to incorporate enough transmitters and on-chip antennas to compensate for the power lost to the silicon substrate. The future will tell whether we need to resort to this rather inefficient solution.

It will take time for designers to master all this new technology, because the models that we use to simulate circuits can't easily handle 60 GHz. Today's transistor models are constructed as though all their capacitance and resistance came from small capacitors and resistors connected here and there. In reality, of course, the capacitance and resistance in these transistors are distributed over appreciable dimensions. So lumping things in this way fails to capture some important effects that manifest themselves most obviously at these high frequencies. Also, the electric and magnetic interactions between the passive devices and the silicon substrate are difficult to calculate from basic physical principles. For these reasons, modeling must rely on both the theoretical understanding of the behavior of the devices and on a large number of experimental measurements (which in turn can help refine the models).

The industry's vision is that we can solve these problems and that all the electronic devices in our homes and offices will be chattering furiously and wirelessly in another five to 10 years. Cables will go the way of the buggy whip. Now if the engineers at MIT can finally perfect their idea of using magnetically coupled resonators to charge batteries through the air, we'll eliminate those pesky power cords, too. Only then will we enter the real wireless age.