# Wireless Fractal Ultra-Dense Cellular Networks

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## Abstract

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## 1. Introduction

## 2. Wireless Fractal Ultra-Dense Small Cell Networks

#### 2.1. Evolution of Wireless Cellular Coverage Boundary

#### 2.2. Main Features of Fractal Ultra-Dense Small Cell Networks

## 3. Performance of the Fractal Small Cell Networks

## 4. Application Scenarios and Challenges

#### 4.1. Application Scenarios

#### 4.2. Challenges

- Controllability of small cell BSs: With the development of software-defined network (SDN) technology, we can adopt SDN to separate the functions of a small cell BS and to make it only with forwarding function. When the traffic of a cellular network increases sharply (e.g., at large gatherings), the fractal coverage of a community may be realized through adjusting the transmit power of the small cell BS, thus alleviating the burden of traffic.
- Cooperative communication of small cell BSs: Through the discussion above, it can be concluded that the coverage boundary of wireless fractal ultra-dense cellular networks takes on fractal feature. Thus, cooperative communication between small cell BSs is not determined through traditional mutual distance between cells, but through the fractal feature of the border between neighboring cells; therefore, the relationship between user and cooperative cell is still a challenging problem.
- Caching scheme deployment for small cell BSs: In addition to communication, content caching may also be conducted by small cell BSs to decrease the traffic of a cellular network in rush hours. The existing caching schemes are all based on the fact that the coverage boundary of a cellular network is an irregular polygon. When the coverage boundary of a cellular network is fractal, how to conduct deployment for caching content and to increase cache hit ratio of files as per coverage of small cell BS is a challenging problem.

## 5. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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Cellular Network | Third Generation | Fourth Generation | Fifth Generation |
---|---|---|---|

Coverage Feature Deployment | Regular hexagon Macrocells BS | Irregular polygon Macrocells and microcell | Statistical fractal shape Macrocells and Ultra-dense small cells |

BS density | Low (4–5 BSs/km${}^{2}$) | Middle (8–10 BSs/km${}^{2}$) | High (40–50 BSs/km${}^{2}$) |

Transmit Power of Macrocell | High | High | High |

Transmit Power of Small cell | N/A | N/A | Low |

Interference | Low | Middle | High |

Coverage Redundancy | low | Middle | High |

Wireless Fractal Phenomenon | No | No | Yes |

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**MDPI and ACS Style**

Hao, Y.; Chen, M.; Hu, L.; Song, J.; Volk, M.; Humar, I.
Wireless Fractal Ultra-Dense Cellular Networks. *Sensors* **2017**, *17*, 841.
https://doi.org/10.3390/s17040841

**AMA Style**

Hao Y, Chen M, Hu L, Song J, Volk M, Humar I.
Wireless Fractal Ultra-Dense Cellular Networks. *Sensors*. 2017; 17(4):841.
https://doi.org/10.3390/s17040841

**Chicago/Turabian Style**

Hao, Yixue, Min Chen, Long Hu, Jeungeun Song, Mojca Volk, and Iztok Humar.
2017. "Wireless Fractal Ultra-Dense Cellular Networks" *Sensors* 17, no. 4: 841.
https://doi.org/10.3390/s17040841