by Benedict Chua, Associate Editor
Researchers at the Tokyo Institute of Technology and NTT have developed a novel CMOS-based transceiver for wireless communications at the 300 GHz band -- the first wideband CMOS phased-array system to operate at such elevated frequencies.
NTT said the breakthrough opens possibilities "beyond 5G".
One of the key features of the proposed design is that it is bidirectional; a great portion of the circuit, including the mixer, antennas, and local oscillator, is shared between the receiver and the transmitter (Figure 2). This means the overall circuit complexity and the total circuit area required are much lower than in unidirectional implementations.
Another important aspect is the use of four antennas in a phased array configuration. Existing solutions for 300 GHz CMOS transmitters use a single radiating element, which limits the antenna gain and the system’s output power. An additional advantage is the beamforming capability of phased arrays, which allows the device to adjust the relative phases of the antenna signals to create a combined radiation pattern with custom directionality. The antennas used are stacked “Vivaldi antennas,” which can be etched directly onto PCBs, making them easy to fabricate.
The proposed transceiver uses a subharmonic mixer, which is compatible with a bidirectional operation and requires a local oscillator with a comparatively lower frequency. However, this type of mixing results in low output power, which led the team to resort to an old yet functional technique to boost it. Professor Kenichi Okada from Tokyo Tech, who led the study, explains: “Outphasing is a method generally used to improve the efficiency of power amplifiers by enabling their operation at output powers close to the point where they no longer behave linearly–that is, without distortion.
The design achieved maximum rates of 26 Gbaud for transmission and 18 Gbaud for reception.
