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Drone Small Cells in the Clouds: Design, Deployment and Performance Analysis

Mohammad Mozaffari, Walid Saad, Mehdi Bennis, Merouane Debbah

arXiv:1509.01655v1cs.ITcs.NI

TL;DR

Drone small cells can extend wireless service in overloaded or disrupted environments, but their coverage-efficient deployment is insufficiently characterized. The paper analyzes air-to-ground coverage for low-altitude DSCs, deriving optimal altitude and two-DSC separation under interference-free and full-interference conditions. Analytical and numerical results establish deployment optima for maximizing coverage and minimizing required transmit power in the studied settings.

  • Problem

    The paper addresses limited coverage-performance and deployment analysis for DSCs, where rapid deployment is needed to maximize service coverage in unexpected events.

  • Method

    The paper models low-altitude air-to-ground coverage, derives single-DSC altitude and two-DSC separation optima, and evaluates interference-free and full-interference cases numerically.

  • Results

    The results show an optimal DSC altitude and an optimal separation distance between two DSCs that maximize coverage for a given target area.

  • Takeaways & Limitations

    DSC deployment can be configured by jointly selecting altitude and inter-DSC distance to improve coverage within a specified area.

Abstract

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The use of drone small cells (DSCs) which are aerial wireless base stations that can be mounted on flying devices such as unmanned aerial vehicles (UAVs), is emerging as an effective technique for providing wireless services to ground users in a variety of scenarios. The efficient deployment of such DSCs while optimizing the covered area is one of the key design challenges. In this paper, considering the low altitude platform (LAP), the downlink coverage performance of DSCs is investigated. The optimal DSC altitude which leads to a maximum ground coverage and minimum required transmit power for a single DSC is derived. Furthermore, the problem of providing a maximum coverage for a certain geographical area using two DSCs is investigated in two scenarios; interference free and full interference between DSCs. The impact of the distance between DSCs on the coverage area is studied and the optimal distance between DSCs resulting in maximum coverage is derived. Numerical results verify our analytical results on the existence of optimal DSCs altitude/separation distance and provide insights on the optimal deployment of DSCs to supplement wireless network coverage.

I. INTRODUCTION

Drone small cells are proposed as mobile aerial base stations for overloaded, damaged, or dangerous network environments, but their coverage and deployment performance require systematic study. This paper develops coverage and deployment results for single and paired DSCs in low-altitude platforms.

  • Motivation: DSCs can support cellular networks during high demand, overload, public-safety operations, and disaster management.Their autonomous deployment and mobility are described as advantages in dangerous or changing environments.
  • Research gap: Prior work addressed integration, overload compensation, movement, and trajectories, but did not extensively discuss DSC coverage performance and deployment methods.The paper identifies rapid, efficient deployment as necessary for maximizing coverage during unexpected events.
  • Deployment setting: DSCs may operate from high-altitude platforms above 10 km or low-altitude platforms below 10 km.The paper focuses on low-altitude-platform coverage.
  • Contributions: The paper derives optimal altitude for a single DSC and studies optimal two-DSC deployment under interference-free and interfering conditions.The objectives are maximum coverage and minimum transmit power for specified target areas.
  • Validation: Numerical evaluations validate the analytical coverage and deployment results.The paper examines altitude and inter-DSC distance as deployment variables.

II. SYSTEM MODEL AND THE SINGLE DSC CASE

The single-DSC analysis models a static aerial base station at altitude h serving static ground users and begins by selecting an air-to-ground channel model. It then derives the altitude optimization problem.

  • System model: A static DSC at altitude h transmits to static ground users whose coverage is analyzed using an air-to-ground path-loss model.The channel model supports deriving the optimal altitude for the single-DSC case.
  • Optimization objective: The section’s single-DSC objective is to derive the altitude that optimizes coverage under the adopted air-to-ground channel model.

A. Air to Ground Channel Model

The air-to-ground model separates LOS, strong NLOS, and multipath components, assigning occurrence probabilities and additional losses according to environment and elevation angle. Geometry links DSC altitude, user radius, distance, and elevation angle.

  • Channel components: The receiver’s signal comprises LOS, strong reflected NLOS, and multiple reflected components associated with multipath fading.The model treats these groups separately according to their probabilities of occurrence.
  • Channel components: Small-scale fading is neglected because LOS and strong NLOS components have significantly higher occurrence probabilities than fading components.
  • Path loss: NLOS links incur higher path loss than LOS links because obstacles cause shadowing and signal reflection.The model adds different excessive path-loss values to LOS and NLOS links beyond free-space propagation loss.
  • Geometry: For a DSC at altitude h, a ground user at radius R has link distance d = (R^2 + h^2)^0.5 and elevation angle θ = tan^-1(h/R).
  • Path loss: LOS and NLOS path loss include free-space propagation loss plus environment-dependent additional losses ξ_LoS and ξ_NLoS.The carrier frequency, speed of light, and DSC–receiver distance also enter the path-loss expressions.
  • Occurrence probabilities: LOS probability increases with elevation angle, while NLOS probability equals 1−P(LOS).The environment-dependent parameters α and β determine the LOS-probability function.

B. Optimal Altitude for Single DSC

The single-DSC optimization seeks an altitude that maximizes coverage or minimizes transmit power for a fixed target area. Because coverage is concave in altitude under typical parameters, an interior optimum exists unless the altitude limit binds.

  • Coverage criterion: A ground point is covered when its SNR exceeds threshold γ_th, and maximum coverage at fixed transmit power satisfies γ(R,h) = γ_th.
  • Coverage optimization: The optimal altitude for a fixed target radius is obtained by solving for µ_opt = h_opt/R_max and then recovering h_opt and R_max.
  • Altitude constraint: With maximum allowable altitude h_max, the feasible optimum is ĥ_opt = min{h_max, h_opt}.
  • Optimal altitude: Coverage radius increases with altitude up to an optimal point and decreases thereafter because R is concave as a function of h.This behavior reflects the trade-off between improved LOS probability and increased path loss at higher altitude.
  • Minimum transmit power: For a fixed target radius R_c, the paper differentiates transmit power with respect to altitude to find the altitude requiring minimum transmit power.
  • Optimality result: Any local minimum of the path-loss function is unique, so the corresponding elevation angle is the optimal one when it exists.The altitude can therefore be increased until path loss begins to rise.

III. CASE OF TWO NON-INTERFERING DSCS

For two DSCs operating together, the paper analyzes their optimal separation distance in both interference-free and interfering conditions.

  • The optimal distance between two jointly operating DSCs is analyzed under both interference-free and interfering conditions.

A. Two DSCs in interference free situations

For two non-interfering DSCs covering a rectangular target, deployment is optimized through their altitudes, separation, and coverage-circle geometry. The identical-DSC case yields non-overlapping coverage regions and a maximum total covered area.

  • Two DSCs cover a rectangular target area whose length is a and width is b.
  • Maximum coverage requires jointly determining the DSC altitudes and distance between them.
  • Fig. 2 depicts two DSCs at optimal altitudes, with D denoting their separation and Rmax representing the maximum coverage radius.
  • The deployment equations place the coverage areas as separate as possible while making them tangent to the target-area borders.
  • For identical DSCs at equal altitude and transmit power, equal-radius coverage regions do not overlap, so total coverage is the sum of their individual areas.

B. Case of Two Interfering DSCs

The interfering-DSC case studies how separation affects coverage when transmissions share a channel. Because interference depends on user location, optimal deployment requires coverage analysis and, in the general case, a three-dimensional search over separation and altitudes.

  • Interference occurs when independently controlled DSCs use the same transmit channel, including because wireless networks have limited channel availability.
  • An optimal DSC separation exists because excessive distance covers area outside the target, whereas insufficient distance produces high interference.
  • A ground point is covered when its signal-to-interference-plus-noise ratio exceeds the threshold γth.
  • With equal DSC altitudes, the SINR depends on the user geometry through the distances and angle between the user direction and DSC separation vector.
  • The total effective coverage is the sum of the portions provided by DSC1 and DSC2 inside the target area.
  • The optimal distance is defined as the separation that maximizes total coverage inside the target area.
  • For fully interfering DSCs, user-location-dependent SINR prevents deriving a closed-form expression for total coverage.
  • When DSCs may differ in height and coverage, optimal deployment requires a three-dimensional search over D, h1, and h2.

IV. NUMERICAL RESULTS

Numerical results show that DSC coverage depends on altitude, interference, and separation distance, with deployment parameters that maximize effective coverage under the evaluated urban-area conditions.

  • Urban-area numerical analysis uses environment-specific α and β parameters to compute path loss effects.The paper notes that these parameters differ across urban, dense-urban, and suburban environments.
  • 310 m is the optimal altitude for providing 500 m coverage radius with minimum transmit power.At low altitude, shadowing reduces LOS probability; at high altitude, increased path loss reduces coverage performance.
  • Interference between two DSCs creates coverage holes, so their separation must balance interference mitigation against coverage outside the target area.The illustrated configuration uses 300 m altitude and 1100 m separation for a 2000 m by 700 m target rectangle.
  • 1100 m with interference and 900 m without interference are the optimal two-DSC separation distances for maximum effective coverage.The effective coverage ratio is evaluated against separation distance; excessive separation shifts coverage outside the target, while close placement increases interference.
  • Without interference, the optimal separation is lower because DSCs can be placed closer without losing coverage performance.The non-interference case also achieves higher overall coverage than the interference case.
  • As target-area length increases from 1800 m to 2400 m, optimal DSC separation increases from 1000 m to 1350 m.The results indicate that separation distance scales approximately linearly with target-area size.

V. CONCLUSIONS

The paper establishes optimal altitude and separation choices for maximizing DSC coverage in LAP deployments, including interference-free and full-interference settings. These results provide a foundation for extending analysis to larger DSC deployments.

  • The study determines DSC altitude values that maximize downlink ground coverage and minimize required transmit power.
  • For two DSCs, the paper presents optimal deployment in interference-free conditions and derives an optimal separation distance that maximizes coverage under full interference.
  • The reported results provide a stepping stone toward analyzing deployments with higher numbers of DSCs.
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