###### Abstract  
Unmanned aerial vehicles (UAVs) are extensively employed in pursuit, rescue missions, and agricultural applications. These operations require substantial data and video transmission, demanding significant spectral resources. The ultra-broad bandwidth of 0.1-10 THz in the Terahertz (THz) frequency range is essential for future UAV-based wireless communications. However, the available bandwidth in the THz frequency spectrum varies with transmission distance. To enhance spectral efficiency over this variable bandwidth, we propose using hierarchical modulation (HM) in the overlapped spectrum and traditional quadrature amplitude modulation (QAM) in the non-overlapped spectrum for closer users. Furthermore, we analyze the single-user case and utilize the block-coordinated descent (BCD) method to jointly optimize the modulation order, subcarrier bandwidth, and sub-band power to improve the system sum rate. Finally, considering the mobility and randomness of UAV users, we design a modulation switching rule to dynamically adjust to changes in distance as users move, thereby enhancing data rates. Simulation results demonstrate superior performance in data rate and design complexity compared to existing methods such as hierarchical bandwidth modulation (HBM) and HM schemes. BCD method; hybrid HM-QAM; modulation scheme; THz communication; UAVs +
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videos, ultrafast massive data transfers among nearby devices, ultrabroadband small cell systems, the novel nano-networks for biomedical and military industries and so on [7; 8; 9; 10; 11; 12; 13]. THz communication could also be used as an alternative to wired backbone connectivity in data centers [14] or as part of large intelligent surface deployments [15] and facilitate mobile wireless mid-range communications such as device-to-device, drone-to-drone, vehicular, or personal communications [16]. Although THz communications present exciting opportunities, they have their own set of challenges that must be addressed before they can reach their full potential. The key technologies in 6G THz wireless communication systems mainly include THz channel modeling, multibeam antenna design, front-end chip design, baseband signal processing algorithms, waveform and modulation design, and resource management schemes. These technologies can provide a strong guarantee for the implementation of THz communication systems. Future research directions on these key components in 6G THz wireless communication systems have also been highlighted [17; 18]. UAVs configured with the THz frequency band can provide users with Tbps-level data transmission rates. However, configuring THz frequency band networks poses many new challenges for UAVs due to the inherent characteristics of THz, such as sensitivity to distance variations in terms of path loss [19]. Currently, there are several studies related to THz communication and drone communication. For example, [PERSON] et al. [20] proposed optimizing the total delay of the system in a single drone network by considering drone positioning, bandwidth allocation, and mobile device transmission power. [PERSON] et al. [21] designed a single drone network that optimizes system capacity by adjusting drone location, resource allocation, and mobile device computing offload. [PERSON] et al. [22] aimed to maximize the transmission rate of users in a single UAV network equipped with an IRS through THz sub-band allocation and power control, trajectory optimization of the UAV, and phase shift of the IRS. [PERSON] et al. [23] investigated the use of low orbit satellites to assist UAVs in providing data transmission services for the terrestrial Internet of Things. Reference [24] analyzed the SNR metrics and channel performance under varying conditions of geometric loss and fading channels for UAV-aided THz communications.