NTT Corporation and researchers with the Tokyo Institute of Technology announced the successful demonstration of a phased-array transmitter module to enable instantaneous ultra-high capacity data transmission to mobile receivers. The researchers succeeded in achieving the world’s first wireless data transmission via beamforming in the 300 GHz band, which is expected to be utilized in the realization of 6G communications.

Researchers presented details of the technology at the 2023 IEEE MTT-S International Microwave Symposium (IMS2023), held in San Diego, Calif. in June 2023.

Research Background

In 6G wireless communications systems, ultra-high speed wireless communications are expected to be achieved by utilizing the 300 GHz band. This band has the advantage of being able to use a wide frequency range. On the other hand, it faces the problem of large path loss during signal propagation through space. Beamforming technology is being studied to overcome this problem.

Beamforming concentrates and directs radio energy toward the receiving device. In 5G wireless systems that use radio waves in the 28 GHz and 39 GHz bands, beamforming has been realized with CMOS1 ICs. On the other hand, CMOS-IC alone lacks sufficient output power in the 300 GHz band. Combining CMOS-IC with III-V compound IC3, capable of high output power, is therefore being attempted around the world to achieve beamforming in the 300 GHz band. However, because high output power is prevented by the large energy loss occurring inside the III-V compound IC and in the connection between the III-V compound IC and the CMOS-IC, high speed wireless data transmission by beamforming has not been achieved until now.

Research Results

NTT has developed indium phosphide integrated circuit (InP-IC) chips that integrate NTT’s proprietary high output power amplifier circuit and antenna circuit. This achievement is made possible with NTT’s proprietary indium phosphide-based heterojunction bipolar transistor (InP HBT)4 technology. Tokyo Tech has succeeded in fabricating a highly scaled CMOS-IC containing frequency conversion and control circuits. NTT and Tokyo Tech have now developed a compact four-element phased-array transmitter module5 by mounting the aforementioned CMOS-IC and InP-IC on the same printed circuit board. With a steering range of 36 degrees, maximum data rate of 30 Gbps, and communication distance of 50 cm, this transmitter module has succeeded in achieving the world’s first high speed wireless data transmission in the 300 GHz band using beamforming.

Key Technological Points

In NTT and Tokyo Tech’s achievement, the following two technologies made the beamforming and high speed wireless data transmission possible.

1. Design of 300 GHz band high output power amplifier circuit 
   NTT and Tokyo Tech have designed a power amplifier circuit6 that achieves high output power in the 300 GHz band and realized its fabrication by using NTT’s proprietary InP HBT technology. For the power amplifier circuit, high output power is achieved by combining electrical power output from multiple amplifier elements using a low-loss power combiner. The circuit amplifies the signals output from the CMOS-IC and radiates the radio wave to the receiving device from the antenna packaged on the same chip. NTT and Tokyo Tech’s achievement enables the delivery of high output power to the receiving device, which is needed for high speed data transmission.
2. High frequency band low loss mounting technology
   Conventionally, to connect different types of ICs for the 300 GHz band, each IC is mounted on a waveguide module7, and the modules are connected together. However, this approach has the problem of energy loss when radio waves pass through the waveguides. NTT and Tokyo Tech’s achievement overcomes this problem through flip-chip bonding8 of the CMOS-IC and the InP-IC and connecting them using metal bumps of several 10 micrometers in size. This packaging approach reduces connection loss and achieves high output power.

Future Research

Short-distance mobile communication devices are expected to be deployed in 6G networks in the future. NTT and Tokyo Tech’s developed technology promises to expand these applications, such as interactive kiosks and femtocells. The developed technology demonstrated one-dimensional beamforming. NTT and Tokyo Tech are working on demonstrating two-dimensional beamforming with a 2D array and extending the communication distance by increasing the number of arrays. NTT and Tokyo Tech are also engaged in the development of receiver modules to meet the needs of 6G applications, and in the practical implementation of wireless communication with transmission capacity ten-fold greater than is available today.