Power/Performance Bits: Oct. 15

Perfect circuits; cellular overload.


Perfect ICs
Are integrated circuits “too good” for current technological applications? Christian Enz, the new director of the Institute of Microengineering at Ecole Polytechnique Fédérale de Lausanne (EPFL) believes perfection is overrated.

Enz said the reason why we should build our future devices with unreliable circuits, and adopt the “good enough engineering” trend is that non-fully reliable circuits can lead to a substantial reduction of energy consumption. Even better, they will allow scientists to stay in the miniaturization race, which has been compromised of late. The size of transistors that constitute circuits cannot be reduced boundlessly. As they get smaller and smaller, they produce more and more mistakes. Some hardware must therefore be added and additional margins taken, which annuls the benefits of miniaturization, and increases energy consumption. Imperfect circuits require less silicon area, and are therefore less energy consuming and less expensive. But Industry remains to be convinced, where giving up perfection is concerned.

Less than perfect chips don’t adversely affect the performance of the device they’re in because circuits are generally resilient to a certain statistically small proportion of errors, with only a negligible impact on the final output. While not true for all applications, a “good enough” approach can be used for “perceptual” uses like audio and video playback. For instance, the screen on a smartphone: here, any impact on image quality will be too small to be perceived. Human sight is an extremely robust system, one that automatically corrects any small errors, he pointed out.

The crucial step in building inexact circuits is determining where there is room for error, starting by looking for spots on the circuits that are underutilized. For example, if there is a circuit dedicated to adding numbers and there aren’t too many decimals in the numbers being added, the engineering team can try to get rid of the part of the circuit that handles decimal places and see what happens. This sort of “inexact” approach will of course lead to lower numbers on quality metrics like signal-to-noise or image quality, but the result will still be “good enough.” This decimal-place technique is known as “inexact arithmetics”, while more generally, the approach is known as “good-enough engineering”.

This approach is driven by the fact that for last four decades, every two years the semiconductor industry has doubled the number of transistors that fit onto a given silicon chip, i.e., Moore’s Law. Miniaturization has driven the development of computers, tablets and smartphones that are at once powerful, energy efficient and increasingly compact. Today’s transistors measure around 20nm and the circuits have become so dense that 100% error-free functionality is simply no longer possible, given the increase in manufacturing tolerances. This means extra circuits must be added to correct the errors and extend the design margins, which cancels out the space gains from miniaturization – and the energy savings. In fact, with this approach, more energy may end up being used. In a word, we are beginning to hit a wall on miniaturization, Enz concluded.

Preventing cellular overload, dropped calls
When too many people take to their mobile phones at once, cellular networks easily overload but a University of British Columbia graduate student has developed a solution to ensure that calls don’t get dropped and texts make it to their destination.

Mai Hassan, a PhD student in the Department of Electrical and Computer Engineering, found a way to opportunistically use television and radio channels to transmit cellular signals when systems are pushed beyond capacity. She has proposed a more effective way to use any channel in the neighborhood, even if those channels are being used by radio or television stations. The challenge was finding a way to make sure the cellular signals didn’t interfere with the people using those channels in the first place.

The solution involves changing the shape of the wireless signal so they could be transmitted on channels that use radio or television frequencies. Then, the direction of transmission had to be changed away from the original channel. Instead of using traditional antennas, which transmit signals in all directions, smart antennas in mobile phones were used. Smart antennas transmit signals in a single direction and can steer the beam to any direction. By manipulating the direction of the cellular signals, calls and texts can be transmitted to a receiver while avoiding any interference with the original radio and televisions signals.