Power/Performance Bits: Nov. 30

Universal decoding algorithm
Researchers at MIT, Boston University, and Maynooth University built a silicon chip that is able to decode any error-correcting code, regardless of its structure, with maximum accuracy, using a universal decoding algorithm called Guessing Random Additive Noise Decoding (GRAND).

Encoded data traveling over a network is susceptible to noise, which disrupts the signal. Error-correcting code utilizes hashes that allow a decoding algorithm to determine the likely original data based on the structure of the hash.

Instead, GRAND works by guessing the noise that affected the message and uses the noise pattern to deduce the original information. GRAND generates a series of noise sequences in the order they are likely to occur, subtracts them from the received data, and checks to see if the resulting codeword is in a codebook. While the noise appears random in nature, it has a probabilistic structure that allows the algorithm to guess what it might be.

“In a way, it is similar to troubleshooting. If someone brings their car into the shop, the mechanic doesn’t start by mapping the entire car to blueprints. Instead, they start by asking, ‘What is the most likely thing to go wrong?’ Maybe it just needs gas. If that doesn’t work, what’s next? Maybe the battery is dead?” said Muriel Médard, a Professor in the Department of Electrical Engineering and Computer Science at MIT.

The GRAND chip uses a three-tiered structure, starting with the simplest possible solutions in the first stage and working up to longer and more complex noise patterns in the two subsequent stages. Each stage operates independently, which increases the throughput of the system and saves power.

The device is also designed to switch seamlessly between two codebooks. It contains two static random-access memory chips, one that can crack codewords, while the other loads a new codebook and then switches to decoding without any downtime.

Because the codebooks are only used for verification, it works with both legacy codes and those that have yet to be introduced.

“For reasons I’m not quite sure of, people approach coding with awe, like it is black magic. The process is mathematically nasty, so people just use codes that already exist. I’m hoping this will recast the discussion so it is not so standards-oriented, enabling people to use codes that already exist and create new codes,” said Médard.

In tests, the GRAND chip could effectively decode any moderate redundancy code up to 128 bits in length, with about a microsecond of latency.

Developing the chip meant rethinking preconceived notions about hardware design, said Médard. “We couldn’t go out and reuse things that had already been done. This was like a complete whiteboard. We had to really think about every single component from scratch. It was a journey of reconsideration. And I think when we do our next chip, there will be things with this first chip that we’ll realize we did out of habit or assumption that we can do better.”

Next, the team plans to retool the GRAND chip for soft detection, try it on longer and more complex codes, and optimize for energy efficiency.

Data transfer without heat
Researchers from the Australian National University, Swinburne University of Technology, and Carl von Ossietzky Universität Oldenburg propose using atomically thin semiconductors as an energy-efficient means of data transport.

The material is based on monolayer transition metal dichalcogenide crystal tungsten disulfide (WS₂) embedded in an optical microcavity. Early investigations show signs that it does not give off heat, and thus wastes less energy.

“Computers already use around 10 percent of all globally available electricity, a number which comes with a massive financial and environmental cost, and is predicted to double every 10 years due to the increasing demand for computing,” said Matthias Wurdack, a PhD scholar from the ANU Research School of Physics. “A huge amount of the energy used by computers is wasted because the electricity used to power it heats up the device as it performs its tasks.”

The researchers explained that in TMDCs, “bound electron-hole pairs (excitons) are stable at room temperature and interact strongly with light. When TMDCs are embedded in an optical microcavity, excitons can hybridize with cavity photons to form exciton polaritons, which inherit useful properties from their constituents.” They were able to demonstrate polariton trapping and ballistic propagation across tens of micrometers at room temperature.

The team next plans to incorporate the technology into a transistor.

“There are many other options for future research, including the development of energy-efficient sensors and lasers based on this semiconductor technology,” said Professor Elena Ostrovskaya of ANU.