World's First Successful 1.4-Tbit/s Wireless Transmission in the Sub-THz Band
-Contributing to the Creation of New Wireless Services Enabled by IOWN and 6G-

Figure 1: Image of OAM-multiplexing transmission technology and trends in high-capacity wireless transmission

1. Background of research

Aiming to support the high-capacity network and information processing infrastructure of the IOWN and 6G and prepare for the increasing future demand for wireless communications, NTT is engaged in research and development with the goal of achieving terabit-class wireless transmission. Capacity of wireless communications can be increased by taking three approaches: (i) increasing the spatial multiplexing order^*3, (ii) increasing transmission bandwidth, and (iii) increasing number of modulation levels. Taking approaches (i) and (ii), NTT is increasing the spatial multiplexing order by applying on a new principle using radio waves with "orbital angular momentum" (OAM) to increase wireless-transmission capacity and using the sub-terahertz (sub-THz) band to increase transmission bandwidth.
　OAM is a physical quantity that describes the properties of radio waves, and radio waves with OAM (so-called "OAM waves") have identical phase trajectories with a helical structure that spirals in the direction of travel (Figure 1). OAM waves can only be received by receivers with the same helical structure (like the relationship between a nut and bolt, for example). Accordingly, multiple OAM waves with different spiral structures can be superimposed and separated without causing them to interfere with each other if we use a receiver that can receive each OAM wave with its corresponding spiral structure. OAM multiplexing transmission technology uses this feature to transmit multiple data streams simultaneously. Since the 2010s, results of research on OAM multiplexing transmission using millimeter wave bands, have been reported globally. NTT has also conducted demonstration experiments^*4 of OAM multiplexing transmission over 100 m at 100 Gbit/s by using millimeter wave bands (Fig. 1).

2. Results of current research

The approach taken by NTT is to use an analog circuit called a "Butler matrix" to increase the spatial multiplexing order by multiplexing multiple OAM waves. This approach reduces the enormous amount of digital signal processing required to eliminate interference between multiplexed data streams in high-capacity communications exceeding one terabit.
　In this research, NTT has promoted research and development of sub-THz-band waveguide technology and succeeded in developing an antenna-integrated Butler matrix that operates with a wide bandwidth and low loss (Figure 2). The antenna-integrated Butler matrix is designed to simultaneously generate and separate eight different OAM waves over a very wide bandwidth, i.e., 135 to 170 GHz, which can be used to multiplex and transmit eight data signals. Moreover, by performing OAM multiplexing transmission with two different polarizations, it is possible to multiplex and transmit twice as many data signals simultaneously without them interfering. We conducted transmission tests using the antenna-integrated Butler matrix, and we achieved the world's first successful large-capacity wireless transmission totaling 1.44 Tbit/s in the sub-THz bands of 135.5 to 151.5 GHz and 152.5 to 168.5 GHz (Figure 2). This transmission capacity is equivalent to the speed of simultaneous transmission of approximately 35,000 ultra-high-definition 4K videos (at about 40 Mbit/s) that can be available on current video websites. This speed enables the simultaneous transmission of 140 or more uncompressed 4K videos (at about 10 Gbit/s) for applications that require ultra-low latency.

Figure 2: Antenna-integrated Butler matrix developed for the sub-THz band and transmission experiments using the Butler matrix

3. Technological points of note

Figure 3: Schematic illustration of multilayer three-dimensional path of the Butler matrix

4. Future developments

This technology is expected to enable (i) broadband and high-speed wireless transmission comparable to optical transmission systems and (ii) seamless connection between wireless and optical transmission systems without the need for a complex digital signal-processing system for spatial multiplexing, since the analog circuit (i.e., the Butler matrix) handles the multiplexing of OAM waves. The next step in this development is to conduct demonstrations over longer distances (exceeding 100 m) while envisioning various applications of this technology in the real world, including wireless backhaul/fronthaul between base stations and relay transmission (Figure 4). We expect the eventual development of this technology to support the creation and spread of various future services, such as VR/AR (virtual reality/augmented reality), high-definition video transmission, connected cars, and telemedicine, as innovative wireless-communication technology in the IOWN and 6G era.

Figure 4: Example applications of this technology (top: wireless backhaul/fronthaul; bottom: temporary wireless link to transmit uncompressed 8K/16K video)

【Glossary】

^*1Orbital angular momentum. One of the properties of radio waves, expressed as the product of the positional coordinate and its conjugate momentum, is the angular momentum that radio waves with different orbital angular momenta are uncorrelated and can be superimposed and separated independently.

^*2Sub-terahertz band
The sub-terahertz band refers to the frequency range between 100 GHz and 1 THz. It is characterized by extremely short wavelengths (ranging from several hundred micrometers to several millimeters) and strong linearity.

^*3Spatial multiplexing
A transmission method by which multiple data series are transmitted in parallel by using multiple spatially independent radio waves simultaneously and in the same frequency band.

^*4NTT news release: "NTT successfully demonstrates 100 Gbps wireless transmission using a new principle (OAM multiplexing) as a world's first --New groundbreaking technology for achieving next generation of 5G systems--"
https://group.ntt/en/newsrelease/2018/05/15/180515a.html