Squeezing More Out of Wireless

Wireless data needs are increasing every day. As users add devices, radio spectrum is becoming increasingly scarce. According to the Ericsson Mobility Report 2020, wireless data traffic needs are expected to increase by almost 50% every year. Currently, most wireless devices operate on the MHz frequency. If devices could move to a higher unoccupied frequency, like GHz or terahertz, it has the potential to offer low latency and high-speed services. This trend is evident from the growth of millimeter-wave technology as part of 5G wireless and initial research in the 6G technologies for wireless. However, it comes at the cost of creating much more specialized hardware that can reliably operate at such high frequencies.

Researchers in Carnegie Mellon University’s WiTech Lab have demonstrated an alternate way to fulfill the traffic demands, by squeezing out more from existing frequency bands using a technique called full duplex.

Traditionally, wireless radios either transmit and receive in the same frequency band but in different time slots or transmit and receive in the same time slot but using two different frequency bands, known as half duplex. Full duplex has the potential to double the data rate by transmitting and receiving on the same frequency band at the same time.

“When faced with the question of using higher frequency bands and dealing with the associated hardware challenges, we tried to see if we can squeeze out more from the existing frequency bands,” said Swarun Kumar, assistant professor of electrical and computer engineering and WiTech Lab PT. “Full Duplex communication, which has been widely studied in theory and with some promising system implementations over the past decade, emerged as an exciting option.”

While full duplex systems have been implemented at lower sub-6 GHz frequencies, the team of researchers have demonstrated the first bidirectional full duplex link at millimeter wave frequencies using novel techniques at the hardware and software domains.

“The main challenge in enabling full duplex communication is that since the radio transmits and receives at the same time and frequency, the receiver is overwhelmed with interference from transmission from its own transmitter, known as self-interference. Hence, to receive and decode any useful signal from the other radio, it needs to remove the self-interference signal which is almost 10-100 million times stronger. At millimeter wave frequencies, the problem is exacerbated by the large signal distortions that are inherent to high bandwidth and high frequency operation,” said Kumar.

Due to the high self-interference signal power that needed to be cancelled, the team broke down the cancellation into three stages, each contributing partially to achieve the required cancellation. At the antenna domain, they used a self-reflector, a tiny strip of metal placed in front of the radio antenna, to reflect a negative copy of the transmit signal into the receiver.

“The best thing about the self-reflector is that it is unique to the millimeter-wave context. Because of its high frequency and as a result, a small wavelength, the self-reflector can be implemented in a small form factor which was not possible earlier at lower frequencies”, says Vaibhav Singh, a Ph.D. student in electrical and computer engineering and one of the project team members.

The team also designed a custom RF self-interference cancellation IC at the analog domain and devised new wideband self-interference cancellation techniques that are robust to high-frequency signal distortions at the digital domain. These millimeter-wave full-duplex circuit techniques in the context of multi-antenna beamforming systems have also been presented by their collaborator in a premiere circuit conference, IEEE ISSCC, in 2019 and 2020.

“The most exciting part of the project was getting to learn about all the three parts of the wireless radio chain: the antenna, analog and digital, from experts in their respective fields,” said Singh.

The project and paper, Millimeter-wave Full Duplex Radios, authored by Vaibhav Singh, Susnata Mondal, Akshay Gadre, Milind Srivastava, Jeyanandh Paramesh, and Swarun Kumar will be presented at the virtual ACM MobiCom Conference.