NTT and researchers with the Tokyo Institute of Technology (President: Kazuya Masu; “Tokyo Tech”) have 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 sixth-generation (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 losses 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 CMOS integrated circuits (ICs). On the other hand, CMOS-IC alone lacks sufficient output power in the 300 GHz band. Combining CMOS-ICs with III-V compound ICs, 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
Figure 1
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) 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 module by mounting the aforementioned CMOS-IC and InP-IC on the same printed circuit board. With a steering range of 36 degrees, a maximum data rate of 30 Gbps, and a 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 (Figure 1).
Key Technological Points
In NTT and Tokyo Tech’s achievements, the following two technologies made beamforming and high-speed wireless data transmission possible:
1. Design of 300 GHz band high-output power amplifier circuit
Figure 2
NTT and Tokyo Tech have designed a power amplifier circuit 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 (Figure 2).
2. High-frequency band, low-loss mounting technology
Figure 3
Conventionally, to connect different types of ICs for the 300 GHz band, each IC is mounted on a waveguide module, 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 bonding of the CMOS-IC and the InP-IC and connecting them using metal bumps of several ten micrometers in size. This packaging approach reduces connection loss and achieves high-output power (Figure 3).
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 a transmission capacity ten-fold greater than that available today.
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