# Proof-of-Concept of a Millimeter-Wave Integrated Heterogeneous Network for 5G Cellular
## 1 Introduction  
The fifth-generation mobile networks (5G) are expected to be deployed by 2020 and to be a primary source of Internet connection for the year 2020 and beyond. Driven by the rapid growth in the number of mobile-connected devices and the emergence of new services, research and development on 5G have been expanding [1, 2]. The Working Party 5D (WP5D) of the International Telecommunication Union Radiocommunication sector (ITU-R) has established a work plan for International Mobile Telecommunications (IMT) for 2020 and beyond (IMT-2020) [3]. The 3 rd Generation Partnership Project (3 GPP) has initiated studies that aim to make functional specifications for 5G available in 2019 [4]. The leading 5G use case is enhanced mobile broadband, closely followed by critical and massive machine type communications used for wireless sensors, meters or actuators [5]. It is anticipated that 45 trillion networked sensors will be deployed in 20 years [6]. 5G should consider the ability to support a massive number of connected devices and various types of traffic. The requirements for 5G include extremely high data rates, very low latency and the capacity to handle numerous devices, ultrahigh reliability, energy efficiency and reasonable costs [1]. Dealing with ever-growing traffic demand is one of the most important requirements. A 1000-fold increase in traffic is expected between 2010 and 2020 or 2025, with a further 10-100-times growth in the period from 2020 to 2030 [7; 8]. To fulfill all of the requirements at reasonable cost, 5G is expected to require a heterogeneous network (HetNet) architecture, comprising multiple radio access technologies (RATs). The 5G HetNet will involve both improvement of existing RATs, such as 3 GPP Long Term Evolution (LTE)-Advanced Pro and non-3 GPP radios, like Wi-Fi, as well as the development of novel RAT(s) specifically designed to meet the 5G requirements. The HetNet architecture offers a promising approach by using a range of RATs in a flexible manner. To achieve a greater than 1000-fold capacity improvement, the following three primary approaches [9] will need to be combined: (1) improvements in spectrum efficiency by advanced modulation or antenna techniques, such as massive multiple-input multiple-output (MIMO), non-orthogonal multiple access (NOMA) and multiuser superposition transmission (MUST) [9; 10; 11]; (2) expansion of the bandwidth by using high-frequency bands, including both centimeter- and millimeter-wave (mmWave) bands [12]; and (3) an increase in spectrum reuse by network densification [13], for example by deploying many small cells overlaid on a macrocell. In this contribution, we focused on mmWave-integrated HetNet, in which novel mmWave technologies are introduced into traditional cellular networks [14; 15]. Unlike the traditional cellular frequencies below 6 GHz, mmWave signals are inappropriate for long-distance transmission because they suffer from higher propagation losses and blockages. However, the mmWave bands offer more than 1 GHz of contiguous spectrum and are capable of delivering multi-gigabit per second (Gbps) data rates. Solutions combining massive MIMO with beamforming, in which a large number of antennas compensate for the propagation losses, are becoming increasingly feasible as antenna sizes become smaller and chip-scale antenna solutions become available [14; 16]. These make the mmWave attractive for use on small cells in dense deployments. A key challenge in the deployment of a large number of small cells is the provisioning of a backhaul connection [17]. Backhaul is a transport link of aggregate communication signals/data from macrocell or small cell base stations to the core network. Optical fiber links might be too expensive or even impossible, in view of the large number of locations. An alternative is the use of wireless backhaul. The high data rates of the access links incur equally high requirements on the backhaul, in terms of both the data rates and latency.