Electronically Reconfigurable Fluid Antenna System. Image source: arxiv 

Introduction 

5G is touted to operate at frequencies ranging up to tens of GHz, as illustrated below. 

5G Spectrum: Image source: 5G Store 

And while there are relatively few commercially available handsets capable of operating at these lofty frequencies, the telecommunications industry is not stopping there – not even a little bit!  

The next target is 6G, which is proposed to operate at frequencies as high as 3 Terahertz (THz). Now, just to keep this in perspective, 1 THz is equal to 1,000 GHz, so we’re talking about 3,000 GHz – That’s about 100 times the highest frequencies even proposed for 5G!  

Communications at these frequencies, when available to ordinary smartphone customers, will provide astonishing bandwidths and unimaginably low latencies. Achieving these feats for the masses won’t be happening any time soon, as chips that operate at these lofty frequencies presently exist largely in the dreams of semiconductor manufacturers. In a previous Electropages article, we spoke about what exists at the present time.  

But let’s look at it under the optimistic maxim that if you can imagine it, someday it will be true.   

The Limitations of Terahertz-Level Communications  

Electromagnetic waves at these unbelievably high frequencies don’t travel very far, and they are easily interrupted by buildings, hills, trees and a myriad of other physical obstructions. Our intrepid phone must search for another cell tower. But there may not be another terahertz-level 6G tower available close enough to serve, so the system may have to search for a lower frequency savior. But here’s where our antenna trouble starts: 

• Searching for a new tower means the antenna can’t zero in on any one location 
• Every frequency band requires an antenna compliant with its wavelength, and that means changing its apparent physical size. 
• The newly targeted cell tower will be in a different direction, so when it is found, the antenna must then zero in on it. 
• There’s no room in a commercial smartphone for many different physical antennas, so one antenna must serve all purposes.  

Electronically Configurable Antennas (ERA) for 6G  

To answer these many challenges, our potential 6G smartphone’s antenna must be able to respond to a wide array of challenges and tasks. It would be completely impractical for any conventional mechanical system to be small enough, cheap enough and reliable enough to exist within a smartphone that consumers can carry in their pockets or backpacks.   

This brings us to the concept of Electromagnetically Reconfigurable Antenna (ERA). These are antennas whose various characteristics can be dynamically altered through electronic or electromagnetic control mechanisms, without mechanical movement of their structure. As described in an article published by Cornell University/arXiv^[1], these antennas face three primary challenges:   

• Frequency Reconfigurability 
• Polarization Reconfigurability 
• Pattern Reconfigurability  

It should be noted that many ERAs offer one or more of the capabilities listed above, but not all of them.  

Frequency Reconfigurability  

The resonant frequency of an antenna is defined by its electrical size. As such, one way to alter its resonant frequency is to alter its physical size. To achieve this, specialized diodes are used simply to connect or disconnect metallic segments to the antennas, effectively altering its size to accommodate the desired frequency.  

Another method is to add or subtract reactive or capacitive loads to the antenna. This can be achieved through varactor diodes. This powerful methodology allows the ERA’s resonant frequency to be continuously varied.  

Frequency Reconfigurable Antenna. Image source: Cornell University/arXiv  

Polarization Reconfigurability  

The purpose here is to optimize communications between the smartphone and the cell tower by adjusting polarization to match the cell tower’s orientation. This improves signal coupling and minimizes signal loss caused by polarization mismatch between the transmitter and receiver.  

One way of doing this is by again employing varactor diodes. This time, they’re used to adjust the phase between orthogonal antenna elements to rotate the polarization. 

Pattern Reconfigurability 

This is altering the radiation pattern of an antenna; in other words, the angular distribution of the resulting electromagnetic radiation pattern. Examples include:  

• Steering the main transmission lobe effectively controls the beam direction. 
• Enhance or suppress radiation in any direction 
• Change from broad to narrow beam  

The methods used to achieve this in monolithic ERAs involve controlling power distributions and, importantly, phase relations between various on-chip antenna elements.  

A typical method involves phase control of multiple tiny antennas, aimed in different directions. This is achieved via a tunable phase shifter composed of combinations of one or more varactors, MOS-based variable capacitors and FET-based phase shifters.  

About Monolithic Electromagnetically Reconfigurable Antennas  

ERAs have been implemented on monolithic CMOS chips, but the technology isn’t quite ready for prime time. However, several prototypes have been developed by both academic and commercial groups. The most promising results have been for the mmWave (30–300 GHz) and THz bands. These ultra-high frequencies call for antennas in the range of 100s of micrometres, allowing for easier fabrication. 

A very promising note for the future of the as-yet-unborn 6G!   

The technologies employed include our earlier-mentioned varactor and MOS capacitor-based methodologies. MEMS (see Glossary of terms) have also been employed, as well as tunable materials such as graphene and liquid crystals. 

These techniques are all amenable to inclusion on the same CMOS die. The result is an Antenna-on-Chip (AoC). But monolithic ERA are not yet part of the commercial mainstream. 

Wrapping Up  

6G will be like 5G on steroids. If a 6G user connects with a THz-level tower, they’ll reap even greater benefits than a 5G user connected to a 10 G tower will experience. But that THz signal has a very limited range and is easily disrupted. The user device looks for another THz source, but if one isn’t found, well, any port in a storm. The user device must be ready to communicate with the best available source.  

This calls for maximum flexibility, especially for the antenna, which may well have to adjust to communicating with frequencies orders of magnitude smaller.  

This is where reconfigurable antennas come in, because there is no room in consumer devices for multiple physical antennas to communicate on vastly different frequencies.  

The antennas we describe in this article are monolithic, ideal for space-starved mobile devices, but they aren’t the only types.  

Challenges and Opportunities  

The utilization of electronically reconfigurable antennas face many challenges. Some of the most daunting include the high cost of specialized components. These include the radio-frequency microelectromechanical systems (RF MEMS), varactors, or liquid crystals required to change the antenna’s properties. As it stands now, this level of hardware complexity is currently too costly and difficult to implement in mass-produced consumer devices like smartphones. 

Another problem is power requirements. Presently, integrating the necessary switches and bias circuits requires managing power consumption, a significant challenge for battery-powered mobile devices. 

Finally, ERAs are seen as a 6G technology. It might be safe to say that while ERAs would be useful for 5G mobile devices, they aren’t “must haves”. It is 6G that will require the greater adaptability and efficiency at the antenna level that electronically reconfigurable antennas have the potential to provide. 

The sheer complexity of reconfigurable antennas is daunting. As described, there is an array of separate, complex technologies involved – and they all must be fabricated onto a single silicon die. So, while you won’t find monolithic reconfigurable antennas in your dealer’s catalogs as yet, it’s a good bet that they’ll be up and running by the time 6G arrives. 

References  

1. Electromagnetically Reconfigurable Antennas for 6G: Enabling Technologies, Prototype Studies, and Research Outlook. Cornell University/arXiv 

Glossary of Terms 

• Electromagnetically Reconfigurable Antenna (ERA). An antenna whose polarization, operating frequency, and/or radiation pattern can be dynamically altered through electronic or electromagnetic control mechanisms, without mechanical movement of its structure.  
• Varactor Diodes. Diodes whose capacitance is altered through varying its applied reverse voltage. 
• MEMS (Microelectromechanical Systems). Ultra-small mechanical components are fabricated directly onto silicon dies.