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Feature Articles: Ultrahigh-speed Transmission Technology to Support High-capacity Communications Infrastructure of the Future

Toward Terabit-class Wireless Transmission: OAM Multiplexing Technology

Doohwan Lee, Hirofumi Sasaki, Yasunori Yagi, Takayuki Yamada, Takana Kaho, and Hiroshi Hamada


NTT is researching and developing technology with the aim of achieving terabit-class wireless transmission that achieves the next generation of 5G (fifth generation). In particular, we are investigating orbital angular momentum (OAM) multiplexing transmission technology utilizing the OAM of electromagnetic waves as a new spatial multiplexing technology capable of transmitting multiple radio waves at the same time using the same frequency band. In an experiment on OAM multiplexing transmission using the 28-GHz band, we successfully demonstrated the world°«s first 120-Gbit/s wireless transmission. This article introduces the details of this technology.

Keywords: high-capacity wireless transmission technology, OAM multiplexing, OAM-MIMO multiplexing


1. Introduction

The coming 5G (fifth generation) will accelerate the use of wireless communications in all fields including connected cars, virtual reality/augmented reality, and high-definition video transmission. Mobile traffic is expected to increase by 1.5 times annually, and from such a trend, it is expected that wireless transmission at the hundreds-of-gigabits to the terabit class will be necessary in the 2030s.

To provide for the ever-increasing demand for wireless communications, NTT is researching and developing technology with the aim of achieving terabit-class wireless transmission. There are three directions that can be taken to increase capacity in wireless communications: increasing the spatial multiplexing*1 order, broadening the transmission bandwidth, and increasing the modulation level (Fig. 1).

Fig. 1. Research directions toward large-capacity wireless transmission.

Of these, the direction involving increasing the modulation level is already reaching the limit. For example, to obtain twice the capacity of 1024QAM (quadrature amplitude modulation), which transmits 10 bits of information at a time, it is necessary to carry out one million QAM modulation to transmit 20 bits of information at a time. Consequently, it is difficult to achieve large capacity with this approach.

NTT is taking the approach of broadening the transmission bandwidth using the quasi-millimeter wave*2 band and increasing the spatial multiplexing order through the use of a new principle based on electromagnetic waves having orbital angular momentum (OAM)*3.

*1 Spatial multiplexing: A signal multiplexing method that sends and receives multiple data streams in parallel by using multiple spatially independent electromagnetic waves.
*2 Millimeter waves, quasi-millimeter waves: Millimeter waves refer to the radio waves with a wavelength of 1°›10 mm, in the frequency band of 30°›300 GHz and that feature strong rectilinear propagation similar to that of microwaves. Quasi-millimeter waves refer to radio waves with a wavelength that, at 1°›3 cm, are close to those of millimeter waves in the frequency band of 10°›30 GHz.
*3 OAM: The nature of radio waves is such that the angular momentum of radio waves is represented by the product of position coordinates and the conjugate momentum; radio waves with different OAM are not correlated, so they can be separated independently even if they overlap.

2. Principles of OAM multiplexing transmission and NTT’s new transmission technology

In this section, we explain OAM multiplexing transmission and introduce a new wireless transmission technology developed by NTT.

2.1 OAM multiplexing transmission

OAM multiplexing transmission is a technique to increase the number (multiplexing number) of data signals to be transmitted at the same time by transmitting different signals using radio waves having different OAM modes [1, 2]. OAM is a property of electromagnetic waves expressing phase rotation on the vertical plane in the propagation direction. The number of phase rotations is called an OAM mode. When an electromagnetic wave has this OAM property, the trace of the same phase takes on a helical shape in the direction of propagation (Fig. 2). For example, the trace of the same phase in OAM mode 1, OAM mode 2, and OAM mode 3 is shown in Fig. 2(a).

Fig. 2. Principle of OAM multiplexing transmission technology.

Radio waves having this OAM property cannot be received without a receiver having the same number of phase rotations at the time of transmission. For this reason, if multiple radio waves having different OAM modes are superposed, they can eventually be separated without mutual interference as long as there is a receiver that can receive radio beams with the same phase rotations corresponding to each OAM mode. For instance, an example is shown in Fig. 2(b) where different signals are put on OAM modes 1, 2, and 3 and transmitted simultaneously. Technology for transmitting multiple data signals using this feature is called OAM multiplexing transmission technology.

Research on radio wave OAM dates back to the beginning of the 20th century. More recently, with the maturing of large-capacity transmission technologies using high-frequency bands in the 2010s, OAM multiplexing transmission in the millimeter-wave band has been attracting attention. An example of a recent achievement is a demonstration at the University of Southern California in the United States of 32-Gbit/s transmission using the 28-GHz band in 2014 and the 60-GHz band in 2016 [3, 4].

2.2 OAM-MIMO (multiple-input multiple-output) wireless multiplexing transmission technology

In theory, the OAM mode order (number of phase rotations) can be infinitely increased, so the spatial multiplexing order can be infinitely increased by OAM multiplexing transmission. However, radio waves with the property of OAM have the characteristic such that the electric energy diverges spatially as the propagation distance becomes larger as the OAM mode becomes higher. Thus, there is a practical limitation to the use of the higher order OAM modes to increase the spatial multiplexing order.

NTT has devised OAM-MIMO multiplexing transmission technology that integrates the widely used MIMO*4 technology in order to efficiently increase the spatial multiplexing order while overcoming the practical limitation [5]. The devised OAM-MIMO multiplexing transmission technology allows the usage of the same OAM modes. In particular, multiple sets of low OAM mode signals are simultaneously transmitted to address the divergence issue of higher OAM modes. The nature of OAM multiplexing that generates multiple independent channels among different OAM modes is still applicable.

At the same time, the MIMO concept is exploited to increase the number of concurrently transmitted data streams. By properly exploiting MIMO technology, it is possible to separate signals between multiple sets of the same OAM modes while maintaining noninterfering properties between different OAM modes.

A configuration example of OAM-MIMO multiplexing transmission technology is illustrated in Fig. 3. This figure shows an example in which a total of 11 multiplexing transmissions to be described later are performed. The left figure represents four uniform circular array antennas (UCAs) concentric with one in the center. UCAs #1 to 4 can generate OAM modes –2, –1, 0, 1, and 2, respectively, and five OAM modes can be generated and transmitted. The antenna in the center (described as UCA #0 for notation convenience) is used for the axis alignment and/or transmission of OAM mode 0. The signals multiplexed in the same mode are separated by MIMO technology at the receiver.

Fig. 3. Example of configuring OAM-MIMO multiplexing transmission.

*4 MIMO: Radio signal processing technology for improving the quality of communications by using multiple antennas at both the transmitter and receiver. It has attracted the attention of the wireless communications industry since it can greatly improve throughput and transmission distance without having to increase bandwidth or power.

3. World-first transmission of over 100 Gbit/s using 28-GHz band

To demonstrate the effectiveness of the OAM-MIMO multiplexing transmission technology, NTT has developed a transmitter and receiver that uses a bandwidth of 2 GHz in the 28-GHz band (Fig. 4). The developed transmitter/receiver consists of four concentric UCAs with different radii and one antenna in the center. Each UCA is composed of 16 antenna elements and can transmit and receive radio waves in 5 OAM modes. Simultaneous transmission of a total of 21 data signals is possible using these antenna elements. Although frequency bands under 6 GHz have been used in the past, the development of equipment that can simultaneously transmit up to 21 signals in quasi-millimeter-wave and higher frequency bands is a world first.

Fig. 4. Developed transceiver.

The configuration of this transmitter/receiver and the experimental setup are shown in Fig. 5. On the transmitter side, signals are generated and converted into analog intermediate frequency (IF) band signals using arbitrary waveform generators that work as digital-to-analog converters. IF signals are inserted into the developed transmitter. The input signal is transmitted by each UCA via an OAM mode generation circuit after up-converting IF signals to the 28-GHz band. On the receiver side, the wireless signals received by each UCA are separated into each OAM mode by OAM mode separation circuits, and then separated signals are down-converted into IF band signals. Those IF band signals are converted to digital signals by the digital oscilloscope, which is used as an analog-to-digital converter. Finally, the digital signals are processed to extract each transmitted signal. Here, the OAM mode generation circuit and the OAM mode separation circuit were respectively achieved by designing a 5 × 16 and a 16 × 5 wideband Butler matrix circuit. Five OAM modes can be simultaneously generated and separated by the wideband Butler matrix circuits.

Fig. 5. Configuration of transmitter and receiver.

Using this transmitter/receiver, we conducted a transmission experiment at a distance of 10 m in a shield room. Multiple data signals were wirelessly transmitted using multiple OAM multiplexed radio waves. We confirmed that wireless transmission was possible according to the OAM principle. In addition, we succeeded in the world-first large-capacity wireless transmission of 120 Gbit/s [6] by multiplexing 11 data signals of 9.6 to 13.3 Gbit/s. We also successfully implemented the corresponding signal processing technology that can handle such multiplexed signal transmission and reception.

4. Expectation for broadband technology

Technology to utilize wide frequency bands is currently being studied. For example, the mixer integrated circuit (IC) and module in the 300-GHz band shown in Fig. 6 make it possible to both expand the transmission bandwidth and improve the signal-to-noise ratio, which had been a problem in the conventional 300-GHz band radio front end [7]. We used this device to carry out 100-Gbit/s transmission in one stream using a wide bandwidth of 25 GHz. By combining such wideband IC technology and OAM multiplexing transmission technology, we can expect to achieve even larger capacity.

Fig. 6. 300-GHz band mixer and IC module.

5. Future work

In order to use OAM multiplexing technology in a real environment, it is necessary to address practical issues that occur in the real environment and to conduct experimental evaluations in various environments. We are now preparing to verify the feasibility of OAM multiplexing in the field by conducting outdoor wireless transmission experiments. In addition, we will utilize frequency bands that can use wider bandwidths and lead research and development toward realizing terabit-class wireless communications.


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Doohwan Lee
Distinguished Researcher, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received a B.S. (Hons.) and completed an M.S. course in electrical engineering from Seoul National University, South Korea, in 2004 and 2006. He received a Ph.D. in electrical engineering and information systems from the University of Tokyo in 2009. He has been with NTT Network Innovation Laboratories since 2009. From 2012 to 2014, he was an assistant professor at the Research Center for Advanced Science and Technology, the University of Tokyo. From 2016 to 2018, he was a part-time lecturer at Kanagawa University. His research interests include compressed sensing, software/cognitive radio, signal processing, and OAM multiplexing. He received the Best Paper Award and Best Technical Exhibition Award from the Institute of Electronics, Information and Communication Engineers (IEICE) Software Radio in 2011, the IEICE Communications Society Excellent Paper Award in 2012, the IEICE Short Range Wireless Communications Young Researcher’s Award in 2016, the Best Technical Exhibition Award from SmartCom 2014, the Best Paper Award from SmartCom 2016, and the Best Paper Award and Special Technical Award in Smart Radio from IEICE Smart Radio in 2017. He served as a General Co-Chair in the Institute of Electrical and Electronics Engineers (IEEE) ICC 2019 Workshop on Electromagnetic Wave Transmission with Orbital Angular Momentum.
Hirofumi Sasaki
Researcher, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received a B.E. and M.E. in engineering from Osaka University in 2011 and 2013. He joined NTT Network Innovation Laboratories in 2013, where he researched a heterogeneous wireless network system in spectrum sharing scenarios. Since 2015, he has been engaged in research on super high-speed wireless communication in millimeter-wave regions. He received the Best Paper Award from IEICE SmartCom 2016 and the Best Paper Award and Special Technical Award in Smart Radio from IEICE Smart Radio in 2017.
Yasunori Yagi
Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received a B.E. and M.E. in electrical engineering from Tokyo University of Science in 2015 and 2017. He joined NTT Network Innovation Laboratories in 2017. Since then, he has been researching high-capacity wireless systems.
Takayuki Yamada
Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
He received a B.E. in electronics from Doshisha University, Kyoto, in 2005 and an M.E. in communications and computer engineering from the Graduate School of Informatics, Kyoto University, in 2007. Since joining NTT Network Innovation Laboratories in 2007, he has been engaged in research and development (R&D) of flexible wireless systems, high-capacity wireless systems, and wireless signal processing. He received the Best Paper Award from the IEICE Communications Society in 2012 and the Young Researcher’s Award from IEICE in 2013. He also received the Best Technical Exhibition Award at SmartCom2014 and the Best Paper Award from IEICE in 2015.
Takana Kaho
Senior Research Engineer, Wireless Systems Innovation Laboratory, NTT Network Innovation Laboratories.
She received a B.S. and M.S. in physics from Tokyo Metropolitan University in 1994 and 1996, and a Dr. Eng. in communication engineering from Tokyo Institute of Technology in 2007. She joined NTT Radio Communication Systems Laboratories in 1996 and has since been engaged in R&D of monolithic microwave integrated circuits (MMICs). Since 2009, she has been an Expert Committee Member on the Information and Communications Council of the Ministry of Internal Affairs and Communications, Japan. From 2010 to 2012, she was a Visiting Associate Professor at the Research Institute of Electrical Communication, Tohoku University, Miyagi. From 2014 to 2019, she was a Visiting Associate Professor at the Graduate School and Faculty of Information Science and Electrical Engineering, Kyushu University, Fukuoka. Since November 2016, she has also been a Visiting Professor at the Graduate School of Information Science and Technology, Osaka University. Dr. Kaho is a senior member of IEICE. She received the Japan Microwave Prize at the 1998 Asia Pacific Microwave Conference and the Young Researchers’ Award in 2004 presented by IEICE. She received a Best Paper Award from IEICE in 2015. She was a treasurer of the Japan Chapter of the IEEE Microwave Theory and Techniques Society from 2011 to 2012, and from 2012 to 2014, she was a Committee Member of Tokyo Section, IEICE.
Hiroshi Hamada
Researcher, Photonics-Electronics Convergence Laboratory, NTT Device Technology Laboratories.
He received a B.E. and M.E. in electrical engineering from Tokyo Institute of Technology in 2009 and 2011. He joined NTT Photonics Laboratories in 2011 and studied millimeter-wave/THz MMICs for wireless communications. He is currently involved in R&D of THz MMICs for very high-speed wireless communications. He received the 2016 IEEE International Microwave Symposium (IMS) Best Industry Paper Award, 2016 URSI Asia Pacific Radio Science Conference (URSI AP-RASC) Young Scientist Award, and 2018 IEICE Asia-Pacific Microwave Conference (APMC) APMC Prize.