# Proof-of-Concept of a Millimeter-Wave Integrated Heterogeneous Network for 5G Cellular
## 1 Introduction
MmWave communications with high gain antenna technologies offer flexible wireless backhaul with suitably high data rates [18; 19; 20; 21]. This contribution presents our pioneering work on a proof-of-concept (PoC) of mmWave-integrated HetNet. The research and PoC were conducted within a joint European-Japanese research project named Millimetre-wave Evolution for Backhaul and Access (MiWEBA) [22]. The MiWEBA project focuses on the effective use of mmWave technologies in HetNet and especially on 60-GHz technologies. The envisioned HetNet comprises a conventional macro-cellular network and novel small cells that use the mmWave both for backhaul and access. In addition, we introduced into the HetNet the concept of control plane (CP) and user plane (UP) splitting [14; 23]. In a CP/UP splitting HetNet, the CP is provided by a conventional macrocell to deliver broad coverage, while the UP data are provided by mmWave small cells. This architecture enables continuous connectivity with capacity boosting by using the advantages of both the macrocell and the small cells. Some PoCs have already been conducted on mmWave technologies for 5G cellular networks. Early 2000 saw the first development of 60-GHz access by Communications Research Laboratory (CRL, now National institute of Information and Communications Technology, Tokyo, Japan) [24], achieving the world's fastest data rate of 128 Mbps at that time. The main limit on the data rate was the processing speed at the baseband. The CRL study [24] encouraged the creation of the standard IEEE 802.15.3c in 2009, the first 1-Gbps over wireless communication standard [25]. This standard defined the current frequency regulation of the 60-GHz band. However, the actual use of the 60-GHzband only became a reality a few years later with the development of a complementary metal oxide semiconductor (CMOS) radio frequency integrated circuit (RFIC) with low-phase noise [26], built-in self-calibration [27] and an RF phase shifter with low power consumption [28]. The latest wireless standard for the 60-GHz band is IEEE 802.11 ad (also referred to as WiGig), which was standardized in 2012 [29; 30]. Intel (Santa Clara, CA, USA) then released the first WiGig chipset supporting 7 Gbps for docking stations, and in 2014, Qualcomm (San Diego, CA, USA) released a processor combining the LTE and WiGig standards [31; 32]. The first WLAN AP product with the WiGig standard is scheduled for release in 2016 [33]. Samsung (Suwon, South Korea) and Deutsche Telecom (Bonn, Germany) have recently demonstrated 5G on a smartphone integrating 60-GHz technologies [34]. Although the regulations for mobile use have not yet been defined, research and development on mmWave frequencies other than a 60-GHz band is currently being undertaken. Samsung and Qualcomm have investigated the 28-GHz band [34; 35; 36], and Huawei (Shenzhen, China) and Nokia (Uusimaa, Finland) have been working on the 73-GHz band [37; 38], mainly for 5G outdoor access. Two United States cellular operators, Verizon (Basking Ridge, NJ, USA) and T-Mobile (Bellevue, WA, USA), have applied for test radio licenses for the 28- and 38-GHz bands [39], and Ericsson (Stockholm, Sweden), Intel, Samsung, Qualcomm and Nokia are planning to join these tests. As well as mmWave access, much work is being conducted on mmWave backhaul technologies for 5G. InterDigital (Wilmington, DE, USA) has investigated 60-GHz meshed backhaul for small cell BSs in which a self-organizing function is realized by a phased array [40]. NEC (Tokyo, Japan) has released an E-band (71-86 GHz) transceiver supporting 10-Gbps backhaul to enable centralized radio access networks (C-RANs) [41]. This range of activity demonstrates the key role of mmWave technology in boosting the data rates of 5G systems. However, to date, no integrated PoC has been developed for 5G, which combines mmWave access, mmWave backhaul and mmWave antennas. To the best of the authors' knowledge, this contribution is the first to demonstrate a PoC for an mmWave-integrated cellular network.