Recently small satellites start playing important roles in earth observation missions. It, however, is true that small satellites have drawbacks of sensor resolutions and down link data rate. As a solution to the latter drawback, we have developed novel communications system for 320Mbps down link with 16QAM for small satellites with 50kg class. We developed a new GaN HEMT X-band amplifier with high efficiency and small distortion, digital filter and pre-distortion processing with relatively low clock frequency in FPGAs, and small X-band on-board antennas. As ground segments, we are developing a compact S/X dual band ground antenna station and a high performance demodulator with turbo equalizer/decoder based on CCSDS high rate telemetry standard. These technologies will be demonstrated in 2014 by Japanese Hodoyoshi-4 satellite with 50 kg mass