NTT, NTT DOCOMO, and NEC recently announced that they demonstrated a real-time bidirectional wireless transmission in the millimetre wave band between 71GHz and 86GHz that achieved a bit rate of 140Gbps --- unprecedented for sub-100GHz 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. The achievement is expected to help with realising high-capacity wireless transmissions to meet future demand expected to happen in the 2030s.
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 100Gbps enable both optical-fibre 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 results of the research 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.
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, all of which increase 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.
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 synthesised 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 100GHz, where wide bandwidth is not readily available. While there is a report of a successful real-time transmission test that achieved 14.7Gbps over 40 metres in a single direction using OAM-mode multiplexing technology with digital signal processing circuits in the 71GHz to 86GHz 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.
