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Researchers developing Full duplex technology to double the cellular capacity and alleviate the spectrum crunch for civil and military

Mobile subscribers are growing rapidly, by 2020, around three-fifths of the global population or 4.6 billion users will have a mobile subscription.  In the future billions of machines will use mobile networks to connect with each other. All of these causes is leading to tremendous increase in data traffic. Mobile operators are continuously making their networks more efficient by investing in new generations of mobile technology (e.g. 5G) and rolling out ever increasing numbers of cellular base stations as well as public Wi-Fi.


Even with the use of new wireless technologies and Wi-Fi, the GSMA has calculated, based on traffic growth estimates, 600-800MHz of additional spectrum will need to be made available for mobile broadband use by 2020 in order to meet growing consumer demand. The availability of this additional harmonised spectrum will be critical for the future vitality of mobile services and the broader digital economy.


Current frequency spectrum crunch is one of the biggest challenges researchers are grappling with and it is clear that today’s wireless networks will not be able to support tomorrow’s data deluge. One of the most well-known constraints in wireless RF has been that it is generally impossible to transmit and receive at the same time on the same frequencies because the act of transmission creates a massive amount of interference for the receiver, preventing the receiver from “hearing” the desired signal coming from the environment.


Therefore, today’s wireless networks are only half duplex. Transmitters and receivers either transmit and receive in different time slots (which is called time division duplexing, or TDD) or at the same time but at different frequencies (frequency division duplexing, or FDD). Because time or frequency resources are being used only part of the time, such networks achieve only half the basic network capacity that is possible in an ideal full duplex network.


A full-duplex radio, which transmits and receives simultaneously over the same frequency band, ideally cuts the spectrum requirement to half, i.e., it can either double the spectral efficiency of a half-duplex system, or it has the capacity to accommodate twice the number of users in the same cell zone. To enable full-duplex communication, a radio is required to suppress the self-interference signal to the receiver’s noise floor. Any residual self-interference raises the noise floor for the desired signal, which results in reduced signal-to-noise ratio (SNR), and lower system throughput.


Recent works  have presented different techniques and system architectures to suppress this self-interference for reliable full-duplex transmission. Military is also interested to develop full duplex technology as military spectrum requirements are increasing exponentially. Military operations increasingly rely on access to the wireless spectrum in order to assess the tactical environment and coordinate and execute their critical missions. TrellisWare, a member of the National Spectrum Consortium (NSC), has been selected to develop the Military Full Duplex Radio (MFDR) to simplify frequency planning for civil and military communication systems.

Full Duplex Radios

LG Electronics said  it demonstrated Full Duplex Radio, or FDR, for the first time in the world in partnership with Yonsei University. FDR is a strong candidate communication technology of 5G standard for significantly raising data transmission speed. It is capable of raising the frequency efficiency up to double the current Frequency Division Duplex, LG Electronics said. “We will continue to strengthen 5G competitiveness through leadership in FDR technology, which is expected to become a major standard for 5G,” said Kwak Kook-yeon, a chief of LG Electronics’ next-generation standard research division.


TrellisWare Technologies, Inc. (www.trellisware.com), the leading innovator of tactical networking solutions, today announced a contract award for $15.7M to develop the MFDR program for the NSC. The program objective sets out to simplify frequency planning and reduce the amount of spectrum required to deploy civilian and military communications by enabling systems to transmit and receive on the same frequency at the same time.


Tactical radio users including the United States Army, Joint Services, National Guard and first responders use an array of communication systems that need to work together in congested spectral environments. These systems must co-exist with existing communications and with other wireless systems which can result in significant frequency planning challenges such as channel interference, inefficient spectrum reuse, and addressing contingency plans under dynamic operational scenarios.


The industry leaders involved with the Military Full Duplex Radio all bring communications expertise and technologies needed in order to address same-frequency simultaneous transmit and receive (SF-STAR). TrellisWare is leading a multi-vendor team that includes Kumu Networks, Inc., and Rockwell Collins, to support the effort for a viable SF-STAR system employing sufficient self-interference suppression (SIS), practical for both commercial and military baselines.



Kumu Networks Full-Duplex technology

Kumu addresses the capacity constraints faced by carriers with technology that allows radios to transmit and receive data at the same time and on the same frequency potentially halving the spectrum needs of cellular networks. The company claims that this can work out to “trillions of dollars” in cost savings because it improves the investments already made in spectrum and equipment, while also putting off the need to get more. So it’s no surprise that carriers are describing it as a “breakthrough.”


Deutsche Telekom (DT) has teamed up with startup firm, Kumu Networks to demonstrate a full duplex 5G in a field trial which took place on its local network in Prague, Czech Republic. According to DT, the field trial focused on measuring the stability and robustness of the technology in a variety of challenging, real-world deployment scenarios. The trial successfully demonstrated the potential of the technology to increase spectral efficiency and its relevance as an enabler for 5G networks.


The startup had earlier announced that it has picked up a strategic round of $25 million from several of the mobile operators that are likely to use it when it’s commercially available. The Series C funding is led by Cisco, with participation also from Verizon Ventures, Deutsche Telekom, along with previous investors NEA, Third Point Ventures and Khosla.


The full duplex technology utilizes a hybrid analog-digital design that successfully models all linear and nonlinear distortions as well as transmitter noise, and uses algorithms that can dynamically cancel the self-interference in real time thereby ensuring no degradation to received signals and allows the receiver to “hear” the signal even while the transmitter is active. Kumu Networks is a two-year-old startup spun out of Stanford to commercialize its research on self-interference cancellation technology.


Sachin Katti, Chief Scientist and Co-Founder of Kumu Networks, and also a Professor at Stanford University, said Full Duplex has a number of applications, from device, to cellular and WiFi access to backhaul.


The cancellation technology can work for any frequency and air interface, including any potential new waveforms, Katti said, providing a tunable radio head that could be used to:

– Create universal roaming phones that work on all bands by allowing phones to be tuned to the available frequency band in the market instead of relying on static filters to handle cross-band interference.

– Double the capacity on LTE access networks by sending and receiving on the same channel.

– Double the capacity on wireless backhaul links

– Enable small cells to “self-backhaul” by using the same frequencies at the same time for backhaul and access.

– Enable tight HetNet coordination by creating enhanced interference coordination, enabling what in effect is a very low latency control plane by receiving control channel data at the same time as transmitting, thereby mititgating the need for high capacity fronthaul to carry co-ordination.

– Allow increased use of under-utilised spectrum, for example by allowing LTE-U (LTE in unlicensed bands such as 2.4GHz and 5GHz) and WiFi to co-exist by avoiding LTE signals “leaking” to WiFi channels.


“All of the carriers that are investing today are already using the tech in advanced trials, the company tells me, along with others that are not in this round. For example, Deutsche Telekom is using it in its 5G Haus. SK Telecom in Korea, as well as Telecom Italia and Telefonica have all also been working with the company.”


“Kumu Networks’ Full Duplex technology is a rare technological breakthrough that offers substantial benefit to a multitude of wireless modalities,” said Rachid El Hattachi, SVP Architecture and Blueprints at Deutsche Telekom, in a statement. “Full Duplex offers significant advantages and removes roadblocks from wireless endeavors as diverse as LTE Advanced, general Radio Frequency planning and future standards and protocols. We jumped at a chance to invest in and help foster this important new technology.”

Full duplex technology of Columbia team, led by Electrical Engineering Associate Professor Harish Krishnaswamy

To go from half to full duplex requires canceling that interference at the receiver by subtracting the known transmitter signal. Because that echo can be anywhere from a billion to a trillion times as strong as the signal that needs to be detected, the system can be made to work only by canceling the echo very, very accurately. “What you really need to do is to cancel-out that echo to the point where it’s eliminated almost perfectly and the residual echo is extremely small —smaller than the received signal, the desired signal—that you’re trying to receive from the distant cell tower,” Krishnaswamy says.


That means performing cancellation across several domains: radio frequency, analog, digital, and even within the antenna interface. The cancellation in each domain must be coordinated with the cancellation in all the others, Krishnaswamy says. Since the echo is over a billion times more powerful than the received signal, echo cancellation circuits must operate highly precisely. “We need echo cancellation circuits that are something like one-part-per-billion-level accurate,” Krishnaswamy explains.


Such precision is difficult to achieve in software alone without killing overall device performance. “This really is something that needs to be done in hardware,” Krishnaswamy says. “That level of precision in the echo cancellation, and the need to handle such a loud echo, cannot be done purely in signal processing.”


To achieve optimal quality, the researchers applied multiple layers of echo cancellation to their software. “The echo is at least a billion to 10 billion times more powerful than the signal that we’re trying Columbia to receive, so basically you want to cancel that factor, and that’s Harish Krishnaswamy hard to do with one signal echo canceler,” Krishnaswamy says. “So the way these full duplex systems are likely to be successful is to have multiple layers of echo cancellation, just hitting that echo canceling again, and again, and again.


Krishnaswamy and Zhou plan next to test a number of full-duplex nodes to understand what the gains are at the network level. This work was funded by the DARPA RF-FPGA program. “We are looking forward to being able to deliver the promised performance improvements,” Krishnaswamy says. 


Columbia Researchers Develop Full-Duplex Radio Integrated Circuits that Enable Simultaneous Transmission and Reception

Harish Krishnaswamy, an electrical engineer at Columbia University demonstrated the ability to transmit and receive signals on the same frequency using two antennas in a full duplex radio that he built. Now, Negar Reiskarimian, a PhD student under Krishnaswamy, has embedded this technology on a chip that could eventually be used in smartphones and tablets.


“We designed a prototype of the receiver cum canceler and fabricated it using 65nanometer CMOS technology. Our full duplex receiver can operate at any frequency between 0.8 and 1.4 gigahertz, and the RF self-interference canceler suppresses the transmitter interference, for a variety of antenna types, over a bandwidth that is about 10 times as great as what you can get with existing, conventional cancellation techniques. We achieved this 10x performance advantage with just two Npath filters in the bank. That’s good enough to make it compatible with many advanced wireless standards, including LTE and WiFi. More filters would enable even wider cancellation bandwidths,” says Krishnaswamy.


Harish Krishnaswamy develops echo cancelling receiver

The Columbia team, led by Electrical Engineering Associate Professor Harish Krishnaswamy, has developed full-duplex radio integrated circuits (ICs)—that can be implemented in nanoscale CMOS to enable simultaneous transmission and reception at the same frequency in a wireless radio.


“Having a transmitter and receiver use the same frequency offers the potential to immediately double network data capacity,” Krishnaswamy says. “Our work is the first to demonstrate an IC that can receive and transmit simultaneously,” he says. CMOS is the dominant technology used for radio ICs inside phones and other radio-equipped mobile devices.


“Doing this in an IC is critical if we are to have widespread impact and bring this functionality to handheld devices such as cellular handsets, mobile devices such as tablets for WiFi, and in cellular and WiFi base stations to support full duplex communications.”


Krishnaswamy, along with his Ph.D. student Jin Zhou proposed another approach—frequency domain equalization. It works something like the graphic equalizer in a stereo system, which corrects sound by adjusting the power of audio signals in particular bands of frequencies. “To divide the signal into multiple frequency bands that can be individually manipulated, we use filters that each have a very sharp frequency response (or high quality factor) ; they take the incoming signals and let only a very narrow range of frequencies through. We use a number of filters centered at different frequencies spread out across the signal’s full spectrum.” They implemented sharp radiofrequency filters on nanoscale CMOS chips by using Npath filter that uses switches, instead of conventional filter that uses inductors and capacitors.

A Low-Complexity Full-Duplex Radio Implementation With a Single Antenna

Researchers from Turkey  have developed a novel low-complexity full-duplex radio design, which only uses a single patch antenna without any duplexer or circulator for passive suppression of self-interference, and a computationally efficient technique for linear digital cancellation.


“For passive suppression, a single antenna is employed without a circulator/duplexer element and complex active analog cancellation hardware. Our solution with the dual polarized slot coupled antenna can provide an isolation of 56−60 dB in IEEE 802.11g 2.4 GHz wireless band. For linear digital cancellation, frequency domain estimation is implemented and a frequency domain reconstruction technique is proposed, which not only offers reduced complexity cost that is one third of the complexity of the existing techniques, but it outperforms the existing techniques by providing 5−7 dB higher digital cancellation in frequency selective fading channels. ”


The proposed full-duplex design is tested for IEEE 802.11g Wireless Standard on the WARP (v3) software-defined radio implementation platform. It is shown that this design provides a total suppression of 88 dB, which is sufficient for low-power or short-range full-duplex communication.


The proposed design can easily enable full-duplex for any orthogonal frequency division multiplexing (OFDM) based wireless system at low cost, since it only requires minimal changes in the digital baseband hardware and just a single antenna, with no additional analog circuitry.

Does Full-Duplex Double the Capacity of Wireless Networks?

Full-duplex has emerged as a new communication paradigm and is anticipated to double wireless capacity. Existing studies of full-duplex mainly focused on its PHY layer design, which enables bidirectional transmission between a single pair of nodes. Although FD has the capability of enhancing spectral efficiency, simultaneous downlink and uplink operations on the same band generate additional interference, which is likely to erode the performance gain of FD cells.


Researchers from New York University Tandon School of Engineering and Trinity College in Dublin, Ireland, have conceived a method to improve spectrum efficiency by deploying a mix of half and full duplex radios in base stations. Although FD cells have the potential of enhancing the area spectral efficiency (ASE) of the network, they also increase the interference, with a consequent drop in terms of coverage. Authors have shown that increasing the proportion of FD cells increases ASE but reduces coverage and, therefore, can be used as a design parameter of the network to achieve either a better ASE at the cost of limited coverage or a lower ASE with improved coverage, depending on the desired tradeoff between these two performance metrics


In practice, however, wireless networks are more sophisticated than a single-link. Real-world wireless networks, such as wireless LANs and mesh networks, are distributed in nature, involving multiple contention domains over large areas, thus entangling both self-interference and inter-link interference. Xiufeng Xie and Xinyu Zhang in their paper, establish an analytical framework to quantify the network-level capacity gain of full-duplex over half duplex.


“Our analysis reveals that inter-link interference and spatial reuse substantially reduces full-duplex gain, rendering it well below 2 in common cases. More remarkably, the asymptotic gain approaches 1 when interference range approaches transmission range. Through a comparison between optimal half- and fullduplex MAC algorithms, we find that full-duplex’s gain is further reduced when it is applied to CSMA based wireless networks. ”


Fujitsu is developing technology that doubles the capacity of individual cells in cellular networks through task-sharing and efficient scheduling.

Fujitsu is sidestepping the interference problem by putting transmitters and receivers in different places or allocating sending and receiving to different base stations, a spokesman said. In its envisioned system, large base stations would transmit data to mobile devices, while smaller base stations would receive signals from the devices. The technology would automatically choose two devices in a small cell that have the least mutual interference when using the same frequency. The transmission power would be controlled to reduce interference on the same frequency.


Meanwhile, a scheduling algorithm developed by Fujitsu cuts down on the signal-processing workload by ranking devices in the network according to transmission power and which devices are receiving or transmitting. Another algorithm “accurately picks transmission-power candidates based on the grouping of devices, which reduces the processing workload to about one-fortieth overall,” the company said. The result is that single small cells can double their communication capacity, the company said. The technology could be particularly useful in crowded public areas such as shopping malls and sports stadiums, where cellular capacity can be squeezed. Fujitsu hopes to implement the technology around 2023. It presented its research at the IEEE Vehicular Technology Conference in Boston.


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