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Why full duplex technology is suddenly hot in 5G

Full duplex technology is more than a decade old, but it's getting renewed interest from some mobile operators and infrastructure vendors because of its ability to more efficiently make use of spectrum.

At a time when operators are trying to secure more spectrum for 5G, any technology that offers a way to more efficiently use existing spectrum is worth closer inspection.

TDD, FDD and full duplex

To understand why full duplex technology is so compelling, it's important to first understand how 3G and 4G networks handle uplink vs. downlink signals. There are two basic configurations:

  • Frequency division duplexing (FDD). With FDD, the transmit and receive signals are in different spectrum bands. In other words, the uplink signal is sent in one block of spectrum and the downlink is transmitted in another spectrum block.
  • Time division duplex (TDD). With TDD, the uplink is separated from the downlink by using different time slots in the same frequency band. Meaning, uplink signals will connect for a few seconds, and then downlink signals will connect for a few seconds.

Full duplex is different. The technology promises to give wireless networks the ability to simultaneously transmit and receive wireless signals over a single spectrum channel at the same time. Indeed, the cable industry is already racing to embrace the concept of full duplex communications in a wired scenario.

However, in wireless, full duplex technology does come with some challenges, mainly the potential for interference between the uplink and downlink signals.

Finally, it's worth noting that full duplex should not be confused with dynamic spectrum sharing (DSS), which uses the same spectrum band to transmit both 4G and 5G signals. DSS doesn't occur simultaneously; instead, operators transmit 4G and 5G signals over the same spectrum band in 1 ms increments.

Full duplex and IAB

Kumu Networks, a startup that makes full duplex technology, says it can filter out interference using special cancellation technology. The company, which is backed by Verizon Ventures, Cisco Investments and others, is currently advocating that full-duplex technology be used for integrated access backhaul (IAB), which is part of the 3GPP Release 16. IAB makes it possible for operators to use their existing 5G spectrum for backhaul as well as for customer access.

Joel Brand, vice president of product management at Kumu Networks, said his company believes full duplex will be essential to IAB, particularly for 5G networks that use millimeter wave (mmWave) spectrum. "What makes full duplex important now is that the mmWave signal doesn't propagate very far and operators do not have access to backhaul for every mmWave cell site," he said, explaining that operators will want to use IAB in conjunction with full duplex to make their backhaul more spectrally efficient. "No one wants to spend billions of dollars on spectrum and then use it for backhaul."

Why now?

Companies that make full duplex technology say that the reason it hasn't caught on before now is because operators didn't have the same urgency when it comes to conserving their spectrum assets as they do with 5G. Plus, full duplex has finally matured to the point where there is less concern about interference.

Lextrum, which is part of the COMSovereign group of companies, has a full-duplex technology that it says can suppress interference and dramatically improve throughput. The company is currently integrating its technology with 3GPP Release 15's software-defined radio and will begin testing its full duplex technology using LTE-Advanced. Lextrum has secured an experimental FCC license to do its testing.

According to Bob Hopkins, CEO of Lextrum, the company is focusing on LTE first because it is talking to fixed wireless equipment makers that produce both basestation equipment and the end user device. This is important because for full duplex technology to work it needs to be deployed both in the network and the device. However, Hopkins said the company plans to also work on 5G and will have a 5G demo of its technology sometime next March. "Our experimental license covers both 4G and 5G," Hopkins said.

Another startup that is generating interest is GenXComm, an Austin, Texas-based company that is a member of the 5G Open Innovation Lab, a new program led by T-Mobile, Intel and NASA that aims to help early stage companies develop 5G products and services.

Sriram Vishwanath, co-founder and CEO of GenXComm, said the company has developed a light-enabled, photonic-based technology that will allow wireless signals to transmit and receive on the same spectrum frequency without interference. "Our photonic piece is unique," Vishwanath said. "That is how we cancel interference. We convert the signal to light and process it in the light domain, filter it and then convert it to RF."

Vishwanath said that while other companies do offer full-duplex capabilities, GenXComm's ability to convert the signal to light is valuable because without it, it is much more difficult to reduce interference.

Vishwanath believes that full duplex will become part of 3GPP Release 17, and he said it could be useful for spectrum bands beyond mmWave. "The sub-6GHz spectrum is gold but it's also crowded. This will double that spectrum," he said of the company's technology.

And while he believes full duplex IAB may be the first iteration of the technology, he also thinks it will make its way into the access network as well. "I believe the access network will go full duplex and it will become standard over time," he said.

— Sue Marek, special to Light Reading. Follow her @suemarek.

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