There is rising interest in full duplex technology – which allows radios to transmit and receive simultaneously in the same frequency – to boost spectral efficiency for 5G backhaul and access networks. It effectively doubles spectral efficiency, by using self-interference cancellation (SIC) to reduce the noise of simultaneous transmission.
Kumu has attained the highest profile among the start-ups, but others are attracting funds, including GenXComm, which has just closed a Series A round led by FAM Capital Partners, with investments from the UT Horizon Fund and other investors.
GenXComm has developed simultaneous self-interference cancellation (S-SIX) technology, which combines full duplex technology, with the ability to allow channels to be stacked next to each other, with no empty spectrum between them.
Currently, multiple transmitters must be spaced apart to avoid interfering with one another. GenXComm says its technology enables multiple transmitters to coexist side-by-side, without interference.
“We will be like MIMO or OFDM, a dominating technology in wireless, doubling bandwidth, reducing latency, and enabling better wireless security,” said Sriram Vishwanath, co-founder and president of GenXComm and a professor at University of Texas at Austin, where the technology originated.
The Series A round, worth $.15m, will pay for 130nm and 90nm test chips and a round of lab tests, says the start-up, but it hopes that will provide the confidence to enable it to raise a further $10m to $15m and so afford commercial silicon. GenXComm has filed for four patents, concerned with adaptive techniques for modeling and canceling signal reflections while consuming less than one watt in a 90-nm device. This would achieve far lower power consumption levels than Kumu does, claims the firm.
“Our solution can get into mobile phones. Other full duplex solutions are more bulky and power-hungry,” Vishwanath told EETimes. He claimed his company already has one OEM design win and two memorandums of understanding with chipset makers. “If we meet certain milestones, they will integrate our technology into their chip,” he added. The company has so far created proofs-of-concept for an indoor WiFi access point, a point-to-point millimeter wave relay, and a base station offering access and backhaul over a single frequency.
Kumu has been trialling its technology with Deutsche Telekom and has investment from that operator as well as Swisscom, Verizon and Cisco.
In October 2016, DT conducted the world’s first public operator trial of SIC, a technology which has been defined as a Technology Building Block for 5G by the NGMN Alliance. DT said SIC technology could be used in existing networks and potentially solve small cell issues by ususing the same spectrum for access and backhaul.
The operator’s 5G:haus research arm conducted the tests in Prague, Czech Republic, with Kumu Networks. The aim was to determine whether SIC could enable full duplex communication, and whether that would remain robust and stable in real world conditions.
Bruno Jacobfeuerborn, CTO at Deutsche Telekom, said in a statement in January: “I’m delighted to see the first experimental results of a potential 5G technology in DT’s real network environment. We use field trials to get a better understanding of a technology’s potential and that helps us to identify use cases and applications in the context of 5G.”
Kumu came to the world’s attention in 2014 when it first demonstrated incoming and outgoing signals using the same frequency without interference, effectively doing away with the FDD/TDD difference. Then it found a way to incorporate its technology relatively easily into existing base stations, potentially paving the way for near term market adoption, rather than having to wait for the next wave of wireless systems.
Kumu’s Steven Hong said then: “There is no DD anymore – not FDD or TDD”.
As well as the DT project, Kumu has been working on proof of concept tests with other carriers, notably Telefonica and SK Telecom – which demonstrated the system on their stands at Mobile World Congress in March 2015.
The company addresses the interference issue by connecting a smart echo-canceller in place of the usual RF rx/tx combiner/splitter in a standard base station. The Kumu radio generates an additional signal that cancels out the interference – a different approach to established interference cancellation techniques, and one which could be applied to cellular networks or to WiFi.
According to Kumu, the system is constantly scanning the radio environment, gauging the state of the airwaves at any given time, as well as the way the transmission is changing the signal environment at the receiver. Its algorithms then compensate for those changes as the signals arrive from the opposite direction, in effect applying echo cancellation to radio waves.
As well as improving speeds, Kumu says spare macrocell capacity created via this approach could be harnessed for backhauling nearby small cells, dividing the full 100Mbps of capacity in the macro sector between several metrocells, which would not need their own backhaul, and could also act as relays. In this scenario, the small cell would need to implement the special receiver but no special handsets or macrocells would be required. Another potential, and topical, application is to combine WiFi and LTE-U or LAA (LTE using 5GHz carriers for supplemental capacity) in one box while avoiding the usual interference issues.
Another full-duplex specialist, MIMOtech of South Africa, is focusing on backhaul first. The company says it can double backhaul throughput in microwave bands using its Air Division Duplexing (ADD) technique, which combines full-duplex with MIMO. At MWC 2015, the company launched its Janus AirDuplex range of microwave backhaul radios based on this technology, claiming data rates of up to 1Gbps in a single 28MHz channel, and 2Gbps in 56MHz.
However, there are downsides to full duplex – limited coverage, and interference risks despite SIC. Researchers at New York University Tandon School of Engineering and Trinity College, Dublin last year proposed a hybrid technology which would address these issues. Shivendra Panwar, professor of electrical and computer engineering at NYU Tandon, says full duplex networks create higher levels of interference, despite SIC and this leads to “many more outages and dropped calls”.
The researchers believe the advantages of both systems could be realized with a hybrid network that mixes full and half duplex cells. Carriers could then optimize their networks to meet their particular levels of demand and spectral efficiency without excess interference. They believe theirs is the first study to investigate the impact of mixed-cell base stations on spectral efficiency and outages and to create models to help wireless engineers improve their networks.
“The beauty of this system is that it’s tunable and would allow providers to adjust the mix of cells based on the needs of a region,” said Sanjay Goyal, a doctoral student. “If you’re designing an urban network, the demand for bandwidth is much greater than the need for wide area coverage. More full duplex cells would provide that bandwidth, even at the cost of a few more dropped calls.” The reverse is true in less dense areas, where spectral efficiency is less important but a carrier might prioritize excellent QoS.