Capacity Characterization of UAV-Enabled Two-User Broadcast Channel (1801.00443v2)

Abstract: Although prior works have exploited the UAV's mobility to enhance the wireless communication performance under different setups, the fundamental capacity limits of UAV-enabled/aided multiuser communication systems have not yet been characterized. To fill this gap, we consider in this paper a UAV-enabled two-user broadcast channel (BC), where a UAV flying at a constant altitude is deployed to send independent information to two users at different fixed locations on the ground. We aim to characterize the capacity region of this new type of BC over a given UAV flight duration, by jointly optimizing the UAV's trajectory and transmit power/rate allocations over time, subject to the UAV's maximum speed and maximum transmit power constraints. First, to draw essential insights, we consider two special cases with asymptotically large/low UAV flight duration/speed, respectively. For the former case, it is shown that a simple hover-fly-hover (HFH) UAV trajectory with time division multiple access (TDMA) based orthogonal multiuser transmission is capacity-achieving, while in the latter case, the UAV should hover at a fixed location that is nearer to the user with larger achievable rate and in general superposition coding (SC) based non-orthogonal transmission with interference cancellation at the receiver of the nearer user is required. Next, we consider the general case with finite UAV speed and flight duration. We show that the optimal UAV trajectory should follow a general HFH structure, i.e., the UAV successively hovers at a pair of initial and final locations above the line segment of the two users each with a certain amount of time and flies unidirectionally between them at the maximum speed, and SC is generally needed.

• The paper characterizes capacity regions by formulating an optimization problem under UAV mobility and power constraints.
• The study demonstrates that a hover-fly-hover trajectory with superposition coding or TDMA optimizes performance in varying SNR conditions.
• The findings provide practical insights into UAV trajectory design, enhancing network capacity for diverse flight and communication scenarios.

Capacity Characterization of UAV-Enabled Two-User Broadcast Channel

The paper presents a comprehensive paper on the capacity limits of a UAV-enabled two-user broadcast channel. Given the growing interest in utilizing unmanned aerial vehicles (UAVs) for enhancing wireless communication networks, this research fills a notable gap by theoretically defining the capacity regions of such systems. Specifically, the authors investigate a setup where a UAV provides broadcast services to two ground users while flying at a constant altitude, with the objective of optimizing the UAV's trajectory in conjunction with transmission power and rate allocations.

Key Contributions and Findings:

1. Capacity Region Characterization: The paper formulates the problem of characterizing the capacity region of the UAV-enabled broadcast channel as an optimization problem. It considers constraints such as the UAV's maximum speed and power, aiming to maximize the sum communication rate to the users over a given flight duration.
2. Special Cases Considered:
   • Asymptotically Large Flight Duration: The authors find that a hover-fly-hover (HFH) trajectory combined with time division multiple access (TDMA) can achieve the capacity. In this trajectory, the UAV hovers at the users' locations for a significant time, allowing each user exclusive transmission periods.
   • Asymptotically Low UAV Speed: In scenarios with very low mobility, the UAV should hover closer to the user requiring a higher data rate, employing superposition coding (SC) and interference cancellation.
3. General Case Analysis: For finite UAV speed and flight duration, the HFH trajectory remains optimal, necessitating superposition coding for optimal performance. The UAV flies at maximum speed between the hovering points, emphasizing the significance of UAV mobility in enlarging the capacity region, especially in low signal-to-noise ratio (SNR) scenarios.
4. High SNR Insight: The paper reveals that in high SNR environments, dynamic UAV movements contribute minimally to capacity improvements compared to static positioning directly above a user.
5. Comparison with TDMA: While SC generally offers superior capacity, TDMA can achieve nearly equivalent performance when UAV speed and duration are ample, suggesting its practicality in simpler implementation scenarios.

Implications and Future Directions:

The findings carry significant theoretical implications by advancing existing knowledge on UAV-enabled multiuser communication systems’ capacity limits. Practically, the results provide a foundation for designing UAV communication protocols that can achieve close to optimal capacity in realistic implementations.

For future research, extending this analysis to multiuser and multiantenna UAV systems could elucidate broader capacity regions and provide more generalized design guidelines. Furthermore, considering real-world path loss models and environmental variables such as cityscapes would increase applicability, especially in urban wireless systems.

In conclusion, this paper offers valuable insights into the communication capacity potentials of UAV deployment, underscoring UAV trajectory and transmission strategy as pivotal factors in enhancing communication performance. The methodologies and results provided in this paper lay a solid groundwork for further exploration in UAV communications, pointing towards a future where such systems might play a central role in improving network coverage and efficiency.