Efficient satellite downlink with Ka d band

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Credit: Tokyo Tech

Satellites in low Earth orbit (LEO) are increasingly used for environmental monitoring of the Earth, as well as for logistical purposes, such as the navigation of maritime vessels. The data transmitted from the satellite terminals to the terrestrial receivers, called “downlink”, is constantly increasing and therefore requires new frequency domains. In this regard, the Ka band for satellite communications in the microwave frequency range between 17 and 31 GHz offers several advantages. Ka-band allows for smaller antennas than conventional downlink frequencies and in-phase grating-based transmission, allowing control of the transmission link independent of satellite position. However, transmission devices in the Ka range have so far been largely inefficient due to degradation of signal beam polarization with changing direction of the transmitter. The degraded polarization appears as noise in the receiver, reducing the signal-to-noise ratio.

In this context, researchers from the Tokyo Institute of Technology (Tokyo Tech), Japan, have proposed, in a new study, what is the first reported dual circular polarization transmitter for Ka-band satellites based on a standard silicon CMOS process. Their work has been featured in the IEEE Radio Frequency Integrated Circuits Symposium and published in the conference proceedings. “In our study, we were able to reduce the losses and increase the transmission efficiency by using an adaptive impedance matching circuit, realized with a load tuner based on a low-loss pi-type transformer. Thanks to this innovation, a satellite will be able to transmit to a terrestrial receiver at a wide range of angles,” says Associate Professor Atsushi Shirane, one of the authors of the study.

In addition to the adaptive impedance circuit, a circular polarization detection circuit has also been integrated to calibrate circular polarization during beam steering. “As the load is adjusted and the transmitter forms a greater angle with the receiver, the circular polarization deteriorates. This has been a fundamental limitation of using dual circular polarization. But, with our internal calibration of deterioration , this traditional constraint is lifted. This could enable a practical device based on our technology,” says Dr Shirane.

Indeed, as demonstrated by the researchers, their innovative transmitter design showed high power efficiency as well as high equivalent isotropically radiated power over a wide range of angles (from -60° to +60°).

These discoveries could pave the way for a significant advance in downlink technology. Additionally, with growing environmental concerns, monitoring the Earth in low orbit is likely to become more important. In this light, these results highlight the Ka frequency band as a strong candidate for satellite communication in the coming years.

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