NTT Corporation, NTT DOCOMO, INC. and NEC Corporation announced that they demonstrated a real-time bidirectional wireless transmission in the mmWave band between 71 GHz and 86 GHz that achieved a bit rate of 140 gigabits per second — unprecedented for sub-100 GHz frequencies. The test demonstrated that Orbital Angular Momentum (OAM) mode multiplexing transmission technology can increase the capacity of wireless transmission, and that OAM-mode control technology can increase wireless transmission distances by using reflected paths (Figure 1). The achievement is expected to help realize high-capacity wireless transmissions to meet future demand anticipated in the 2030s.
Figure 1 Positioning of these results in real-time wireless transmission technology (below 100 GHz) in actual environments, and the OAM-mode multiplexing transmission device.
OAM-mode multiplexing technology increases the capacity of wireless transmissions between fixed stations by transmitting multiple radio waves, each carrying signals multiplexed with different OAM modes, at the same frequency and time. Transmissions exceeding 100 Gbps enable both optical-fiber and wireless connections for communication lines between fixed stations, facilitating the construction of flexible backhaul networks, wireless connections with mobile base stations during events, and temporary lines during disasters. The research results are expected to contribute to the development of high-capacity wireless backhaul for future services such as virtual reality (VR), augmented reality (AR) and high-definition video transmission in 6G and beyond.
The results were presented at the Wireless Communications and Networking Conference.
1. Research Background
In the 6G era, the demand for wireless communications will accelerate with the emergence of high-definition video transmission, autonomous driving, remote medical surgery, and advanced applications such as VR and AR, increasing the need for high-capacity wireless communication. NTT, DOCOMO and NEC are working to increase capacity using a novel spatial multiplexing method that uses OAM, a property of electromagnetic waves (Figure 2).
Figure 2. Illustration of OAM-mode multiplexing transmission.
OAM, a physical quantity that describes certain properties of electromagnetic waves, is generated by adjusting the phase difference of signals from the transmitting antenna so that the same-phase trajectory spirals in the direction of propagation. On the receiving side, the phase of the received signal can be synthesized at the antenna by rotating in the opposite direction of transmission, so that radio signals corresponding to multiple OAM modes with different helix structures can be superimposed and separated without interfering with each other. Using this feature, OAM-mode multiplexing technology can transmit different data that has been spatially multiplexed, enabling large amounts of data to be sent over limited bandwidths. High-capacity wireless transmission is possible even in frequency bands below 100 GHz, where wide bandwidth is not readily available. While there is a report of a successful real-time transmission test that achieved 14.7 Gbps over 40 meters in a single direction using OAM-mode multiplexing technology with digital signal processing circuits in the 71 GHz to 86 GHz band (E-Band), which is used in existing wireless systems, real-time transmission capacity needs to be further increased for 6G and beyond wireless systems.
2. Research Achievements
Real-time wireless transmission using OAM-mode multiplexing technology in high-frequency bands has now been demonstrated by NTT, DOCOMO and NEC, supported by research on the practical application of OAM-mode multiplexing transmission (Figure 3).
Figure 3. Role of each company.
NEC has developed an OAM-mode multiplexing transmission system capable of real-time transmission at a maximum rate of 70 gigabits per second per direction, using a 1 GHz bandwidth signal in the E-Band. This was achieved by extending conventional digital signal processing circuits3 to increase the modulation rate by about 2.6x to 300 Mbaud while also enabling bidirectional communication in the E-Band.
DOCOMO explored expanding the application scenarios of OAM-mode multiplexing transmission and conducted demonstration experiments on reflection-based transmissions, using OAM-mode inverse-reception technology for transmissions via reflections off surfaces such as walls.
NTT has devised a circuit that doubles the transmission bandwidth and an OAM-mode control technology to support long-distance transmission and reflection scenarios.
Leveraging these developments, the three companies conducted demonstrations based on three scenarios:
- Bidirectional transmission over 22.5 meters
- Bidirectional transmission over 45 meters
- Bidirectional transmission over 22.5 meters using a reflector.
The result was successful transmission of 139.2 Gbps, 104.0 gigabits per second and 139.2 gigabits per second, respectively (Figure 4).
Figure 4. Transmission experiment setup and transmission scenario (Left)Transmission experiment in progress, scenario 1 (Right).
3. Key Research Achievements
Broadband Real-Time OAM-Mode Multiplexing Transmission Technology: Using eight orthogonal OAM modes and high-order modulation up to 256QAM, the team achieved wireless transmissions with a bandwidth of 500 MHz in four frequencies — 74.875 GHz and 75.375 GHz for the uplink and 84.875 GHz and 85.375 GHz for the downlink. The system achieved real-time bidirectional wireless transmission of 139.2 gigabits per second at 22.5 meters in a sub-100 GHz band.
OAM-Mode Control Technology: In OAM-mode multiplexing transmissions, the antenna size was optimized to maximize transmission capacity according to the transmission distance. However, when operating beyond designed distances, transmission capacity typically decreased. In response, an OAM-mode control technology was developed to automatically adjust transmission parameters such as transmission power, OAM-mode and modulation method. As a result, the system successfully maintained 104.0 gigabits per second of real-time wireless transmission over a distance of 45 meters, twice the initially designed range.
Expanded Application Scenarios: To date, OAM-mode multiplexed transmissions have focused on line-of-sight conditions that require precise alignment of transmitters and receivers. To support flexible backhaul solutions for 6G and beyond, non-line-of-sight communication using wall reflections was investigated. Using OAM-mode inverse-reception technology, the test demonstrated that wall reflections can achieve 139.2 Gbps transmissions at 22.5 meters.
4. Future Outlook
The demonstration validated real-time bidirectional wireless transmission exceeding 100 Gbps using OAM-mode multiplexing technology. Such high-capacity wireless transmission technology can expand backhaul infrastructure beyond traditional fiber-optic connections, enabling more flexible backhaul configurations. This includes applications such as wireless connectivity to mobile base stations during events and temporary backhaul lines during disasters, ultimately supporting wireless communication demands expected in the 6G era and beyond.
The three companies will explore applications such as relay transmission using real-time, high-capacity wireless communications and will apply OAM-mode multiplexing technology to wireless backhaul and fronthaul networks. They will also seek to increase wireless transmission capacity and extend transmission distances in mmWave and higher frequency bands. Ultimately, they aim to establish flexible network infrastructure for future services such as VR, AR, ultra-high-definition video transmission, connected vehicles and remote medical applications in the 6G era and beyond (Figure 5).
Figure 5. Application examples for wireless backhaul/fronthaul.