<![CDATA[Selecting modulation schemes for High-Altitude Platform Station (HAPS) backhaul links at 28 GHz is critical, as higher-order modulations improve spectral efficiency but degrade reliability under tropical conditions. This study evaluates the trade-off for QPSK and 16-QAM through link budget analysis, log-normal fading modeling with 2.5 dB tropospheric scintillation variance, and outage probability estimation via Q-function under three scenarios: clear-sky, rain attenuation, and worst-case (rain + turbulence). Results show that 16-QAM achieves fourfold spectral efficiency improvement (0.8 to 3.2 bps/Hz) but requires 8.2 dB additional Eb/No, degrading receive margin from 12.1 dB to 2.1 dB under rain, with availability dropping from >99.9999% to 80–90%. QPSK maintains a rain margin of 16.3 dB and availability exceeding 99.9999%. These findings confirm that static 16-QAM deployment is unsuitable for tropical environments. Adaptive modulation with dynamic QPSK/16-QAM switching based on real-time channel state information is recommended for reliable HAPS deployment in Indonesia.]]>
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