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density (Eb/No) was calculated by the signal power and the noise power density measured with a spectrum analyzer aer distributing the signal at the input port of the re-ceiver by the directional coupler. As the satellite loopback system cannot freely control the noise amount, this ex-periment obtained the BER characteristics while varying Eb/No by changing the output power of HPA. e main parameters for this experiment are shown in Table 1. e MBA link of WINDS has 1.1 GHz bandwidth and has a suciently wide bandwidth for this experiment.4Experimental results4.1BER characterization by the DPDTo conrm the BER improvement eect by the DPD, we obtained BER in implementing the DPD on the 350 Msps symbol rate (data rate: 1.36 Gbps) that can most clearly conrm the non-linearity at a transmission rate exceeding 1 Gbps. In the satellite loopback communication path, as the non-linearities of the amplier in the ground station and the amplier mounted on the satellite are re-ected in the whole non-linearity, we set a proper satellite repeater gain and conducted the test so that the BER characteristics can be conrmed in the widest dynamic range. Figure 4 shows the experimental results. e mea-sured result of the translator loopback path shows that BER is improved as Eb/No becomes large (Fig. 4: green solid line). In addition, the required Eb/No seems to increase as BER becomes small. e error oor seems to appear around the point where the output power of the HPA in the ground station exceeds 47.2 dBm. is result tells us that for the translator path, the non-linear eect of the HPA of the ground station happens when the output power is around 47 dBm.Next, the result measured in the satellite loopback path (Fig. 4: blue dashed line) tells us that, like the translator path, the BER seems to degrade as Eb/No becomes large. e error oor appears around the point where Eb/No exceeds 18 dB, and then BER rises as Eb/No rises. is is consistent with the trend that the signal distorts due to the saturation characteristic of the amplier and the bit error determinately starts to arise. As compared to the result of the translator path, the degradation of BER is more notice-able. erefore, it can be presumed that the eects by the non-linearities of not only the HPA in the ground station but also the satellite’s on-board amplier are added. We enabled the DPD to control the eect of these non-linear-ities (Fig. 4: blue solid line). As the I/O characteristics of the satellite’s on-board amplier are unknown, the LUT coecient of DPD adjusted to that BER becomes the minimum. However, Subsection 4.2 will conduct evalua-tion of the eect on BER by the inconsistency of the non-linearity of the communication path with the distor-tion compensation characteristic. As a result of measure-ment, without the DPD, the best value of BER was 3.2 × 10-5 (Eb/No=19 dB), but it became 7.5 × 10-6 (Eb/No=19 dB) with the DPD, and even if increasing the output of HPA, BER did not degrade. In the range that Eb/No was 16 dB or less, BER did not show signicant change regard-less of the DPD. erefore, it was estimated that the bit error by the Gaussian noise was dominant.4.2The Effect of inconsistency of amplifier characteristics and distortion compensation quantity on BERFor the satellite’s on-board DPD which we suggest, it is required that the signal including inverse distortion made by the LUT is consistent with the non-linearity of the amplier, because the LUT compensates the non-linearity of the amplier. If the variation in the input level to the amplier or the gain variation of the amplier arises due to aging degradation, etc., the lowering eect of the DPD is expected since the distortion characteristic is not consis-tent with the compensation characteristic. us, we con-rmed the BER degradation when increasing or decreasing the LUT 0.5 dB from the optimal operating point at a time. Figure 5 shows the DPD sensitivity measured result (Fig. 5: red solid line). e variation of the received power, or the variation of Eb/No, when decreasing or increasing the input power only by 0.5 dB, was -0.18 dB or +0.51 dB, respec-tively. e output power did not change linearly in response to the change in input power because the output power FiF4　BER characteristic change by the DPD1.00E‐071.00E‐061.00E‐051.00E‐041.00E‐031.00E‐0281318BEREb/No[dB]TheoryTranslator RouteSatellite Routewith DPDSatellite Routew/o DPD48.9*50.0*47.7*48.2*48.1*46.8*47.2*48.5*45.6**Earth station HPA output power in dBm3 Ultra-High-Speed Satellite Communication Technology148　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 2 (2017)