Ultra-Dense Networks for 5G and Beyond

5The Role of Massive MIMO and Small Cells in Ultra‐dense Networks

Qi Zhang¹, Howard H. Yang² and Tony Q. S. Quek²

¹The Jiangsu Key Laboratory of Wireless Communications, Nanjing University of Posts and Telecommunications, China

²Information Systems Technology and Design Pillar, Singapore University of Technology and Design, Singapore

5.1 Introduction

To deal with the explosive increase of mobile data traffic, the fifth generation (5G) communications system has come at the forefront of wireless communications theoretical research [1]. Two main approaches in 5G are massive antennas and dense deployments of access points, which lead to the massive multiple‐input multiple‐output (MIMO) and small cell techniques [2].

Massive MIMO employs hundreds of antenna elements at the base station (BS) to serve tens of users simultaneously at the same time‐frequency resource block (RB) [3–5]. The large size of transmit antenna array not only significantly increases the capacity with excessive spatial dimensions [6–9] but also averages out the effect of fast channel fading and provides extremely sharp beamforming concentrated into small areas [ 3,4]. Aside from these, the huge degrees‐of‐freedom offered by massive MIMO can also reduce the transmit power [10].

On the other track, a small cell improves the system capacity by densely deploying low‐power access points into traditional high‐power macrocells [ 2,11]. In this fashion, the distance between the transmitter and receiver can be significantly ...