In today's world of digital communications, 5G, the fifth-generation mobile network, is fast becoming the global standard for wireless communications. It offers multi-gigabit-per-second data rates, very low latency, high reliability and massive network capacity.

Industry observers, however, are already predicting the development of 6G technology, a successor to 5G that could potentially offer terabit data rates, ultra-low latency and improved network coverage and connectivity. So, what's holding us back?

The Role of the Electronic Oscillator

Inside every digital device in your life — think: cellphone, laptop, tablet, etc. — is a tiny circuit called an electronic oscillator. The oscillator generates a periodic signal, typically a sine wave or a square wave, whose frequency serves as the clock, or pacemaker, for your device. The higher the clock rate, the faster your device can process, send and receive information.

A cellphone, for example, uses the clock to synchronize timing between signals it receives (what you hear) and signals it transmits (what you say). Poor synchronization can result in poor sound quality, interference between signals and dropped calls. Laptops use an electronic oscillator to synchronize the operations of the computer's microprocessor and other components. The clock defines the rate at which instructions are executed and data is transferred within the computer.

Electronic oscillators, which typically operate at frequencies ranging from a few hertz (Hz) to several gigahertz (GHz), are also used in consumer audio equipment, such as music synthesizers, and military applications, such as radar and space communication systems.

Minimizing Phase Noise

Unfortunately, the frequency of the signal generated by an electronic oscillator is degraded by something called phase noise, which is a measure of random, short-term fluctuations in the phase of that waveform.

"In the time domain, phase noise is observed as 'jitter,' or a distortion in the signal," explains Sid Ghosh, a consulting engineer for optical technologies in Northrop Grumman's Strategic Space Systems Division. "Increased phase noise in an electronic system contributes to a lower signal-to-noise ratio, which reduces the accuracy and quality of a signal."

Systems based on 6G technology, which promises operating speeds in the 100s of gigahertz — the upper limit for 5G is 40 GHz — will require components that can minimize jitter to reach their full performance potential.

Taking a New Approach

That's why Northrop Grumman has teamed up with UCLA, Nexus Photonics and the University of New Mexico to develop a new type of oscillator — a compact, portable device based on microwave photonic technology, one that can deliver a very stable, low-phase-noise signal. This research is funded by the Defense Advanced Research Projects Agency (DARPA) Generating RF with Photonic Oscillators for Low Noise (GRYPHON) program.

"Think of this new device as an integrated circuit based on optoelectronic components," says Ghosh. "Nexus Photonics will use a technique called heterogeneous integration to lay out components, such as lasers, photodetectors and passive components, such as waveguides and modulators, on a conventional semiconductor substrate."

Northrop Grumman will provide high-performance photodetectors for the chip-scale device and perform all required integration, packaging and testing. When fully packaged, Ghosh expects the new device to be about one-third the size of an average smartphone.

Operating at Light Speed

A powerful characteristic of the new oscillator is its ability to integrate optical and RF technologies.

"The RF input to our electronic oscillator goes to an electro-optic modulator, which modulates the RF signal onto a laser operating in the optical domain at 193 terahertz [THz]," explains Ghosh. "We do all of our signal processing in the optical domain, then use a photodetector on the chip to convert the signal back to RF. The large amount of fractional bandwidth available using the terahertz signal would dwarf the bandwidth required to process the RF signal."

The RF output (carrier) frequency for a system using this new oscillator would notionally be in the 10s of GHz, he notes, but could be scaled up as high as 500 GHz in the future with novel detector technologies. Certain 6G technology applications would require RF carrier frequencies in the 100s of GHz.

Where Would 6G Technology First Appear?

The overriding characteristic of Northrop Grumman's new electronic oscillator would be its very high stability. Graphically, that means its output power is focused narrowly on a single frequency with very low power (phase noise) residing in adjacent frequencies.

"Having an extremely stable clock is a necessary condition for us to even contemplate a system that would use 6G technology," he says.

Commercial applications most likely to see the arrival of 6G first, Ghosh predicts, will be augmented reality (AR), virtual reality (VR) and gaming systems — all applications that require the high precision, high bandwidth and very low latency offered by 6G technology.

Should We Expect 6G Technology in Cellphones?

Whether 6G handsets become a consumer reality remains to be seen.

"Our new electronic oscillator will certainly be small enough to go into a cellphone," says Ghosh. "The question is whether there will be a need for it. So far, standards for 6G handsets have not been set."

The more likely endpoint for consumer applications of 6G technology, he believes, will be wearable, high-fidelity headsets for AR and VR applications.

What's Needed to Reach a 6G Reality?

Ghosh is confident that 6G will push the frequency limits of certain applications to the 100s of GHz. He also believes Northrop Grumman's low-phase-noise photonic oscillators will play a key role in the success of 6G devices and networks. But other key challenges to the deployment of 6G remain.

To reach 6G nirvana, the wireless industry will also have to:

Figure out how to open the sub-THz spectrum for increased bandwidth and take advantage of that bandwidth.
Push the limits of semiconductor technologies to operate in sub-THz bands.
Achieve microsecond latencies at the network level to achieve 6G key performance indicators.

First Things First

For now, the Northrop Grumman team's work developing a microwave photonic-based electronic oscillator is strictly a technology demonstration. They expect to have their first products packaged in the 2024–2025 timeframe.

As to if and how this technology will speed the development and deployment of 6G, Ghosh advises, "We'll just have to wait and see."

Are you interested in science and innovation? We are, too. Learn more about our people and life at Northrop Grumman, or check out our career opportunities to see how you can be a part of defining possible.

* The views, opinions and/or findings expressed are those of the author and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government.