arXiv:2611.07724v1 Announce Type: new Abstract: The proliferation of low-altitude aerial Internet of Things applications, such as UAV-based sensing/monitoring, demands high-performance uplinks, yet terrestrial-optimized cellular networks with downtilted base station (BS) antennas form coverage holes that severely degrade aerial uplink. To address this challenge without modifying infrastructure, active reconfigurable intelligent surfaces (RISs) are introduced. Unlike existing studies with complicated active RIS optimizations, we conduct a detailed noise analysis to simplify the transmission design. By deploying one active RIS per BS sector to fully exploit multi-sector reception, we develop a joint sector maximal signal combining scheme that improves signal quality through diversity gain. Active RISs introduce amplifier noise, raising questions about their net benefit. We derive an upper bound on the power ratio between RIS-introduced noise and BS-inherent noise, revealing that the impact of RIS noise is negligible under typical operating conditions. Based on this insight, we maximize a derived lower bound of the received signal-to-noise ratio by sequentially designing the amplitude and phase of reflection coefficients with closed-form expressions. Simulation results validate the theoretical analysis on negligible RIS-introduced noise in typical systems and demonstrate that under the same array gain, a larger RIS size rather than a higher reflection gain is preferred for enhancing the low-altitude uplink transmission.