2.65Gbps Downlink Communications with Polarization Multiplexing in X-Band for Small Earth Observation Satellite

Tomoki KANEKO  Noriyuki KAWANO  Yuhei NAGAO  Keishi MURAKAMI  Hiromi WATANABE  Makoto MITA  Takahisa TOMODA  Keiichi HIRAKO  Seiko SHIRASAKA  Shinichi NAKASUKA  Hirobumi SAITO  Akira HIROSE  

IEICE TRANSACTIONS on Communications   Vol.E104-B   No.1   pp.1-12
Publication Date: 2021/01/01
Publicized: 2020/07/01
Online ISSN: 1745-1345
DOI: 10.1587/transcom.2019EBN0009
Type of Manuscript: POSITION PAPER
Category: Satellite Communications
satellite communications,  small satellite,  DVB-S2X,  Cross Polarization Discrimination (XPD),  polarization multiplexing,  

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This paper reports our new communication components and downlink tests for realizing 2.65Gbps by utilizing two circular polarizations. We have developed an on-board X-band transmitter, an on-board dual circularly polarized-wave antenna, and a ground station. In the on-board transmitter, we optimized the bias conditions of GaN High Power Amplifier (HPA) to linearize AM-AM performance. We have also designed and fabricated a dual circularly polarized-wave antenna for low-crosstalk polarization multiplexing. The antenna is composed of a corrugated horn antenna and a septum-type polarizer. The antenna achieves Cross Polarization Discrimination (XPD) of 37-43dB in the target X-band. We also modify an existing 10m ground station antenna by replacing its primary radiator and adding a polarizer. We put the polarizer and Low Noise Amplifiers (LNAs) in a cryogenic chamber to reduce thermal noise. Total system noise temperature of the antenna is 58K (maximum) for 18K physical temperature when the angle of elevation is 90° on a fine winter day. The dual circularly polarized-wave ground station antenna has 39.0dB/K of Gain - system-noise Temperature ratio (G/T) and an XPD higher than 37dB. The downlinked signals are stored in a data recorder at the antenna site. Afterwards, we decoded the signals by using our non-real-time software demodulator. Our system has high frequency efficiency with a roll-off factor α=0.05 and polarization multiplexing of 64APSK. The communication bits per hertz corresponds to 8.41bit/Hz (2.65Gbit/315MHz). The system is demonstrated in orbit on board the RAPid Innovative payload demonstration Satellite (RAPIS-1). RAPIS-1 was launched from Uchinoura Space Center on January 19th, 2019. We decoded 1010 bits of downlinked R- and L-channel signals and found that the downlinked binary data was error free. Consequently, we have achieved 2.65Gbps communication speed in the X-band for earth observation satellites at 300 Mega symbols per second (Msps) and polarization multiplexing of 64APSK (coding rate: 4/5) for right- and left-hand circular polarizations.