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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 onboard 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 − 43 dB 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 LowNoise 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 39.0 dB/K of Gain system-noise Temperature ratio (G/T) and an XPD higher than 37 dB. 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 8.41 bit/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 Rand 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 rightand left-hand circular polarizations. key words: satellite communications, small satellite, DVB-S2X, Cross Polarization Discrimination (XPD), polarization multiplexing

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