# Proof-of-Concept of a Millimeter-Wave Integrated Heterogeneous Network for 5G Cellular
## 4 Proof of Concept Implementation and Integration
### LTE/WiGig-Integrated HetNet Prototype  
Our PoC hardware for the mmWave-integrated HetNet used an LTE eNB as the macrocell and WiGig APs as the small cells. Figure 12 is a photograph of our PoC hardware. The LTE macrocell comprised an eNB baseband unit (BBU) and a remote radio head (RRH) connected via an optical fronthaul based on the European Telecommunications Standards Institute (ETSI) Open Radio Interface (ORI) standard [66; 67]. This is capable of transferring LTE baseband I/Q signals with a 50% compressed data rate. The WiGig AP was connected to the LTE eNB via Ethernet. The UE had LTE modem and WiGig devices were implemented. The WiGig device is the shown in Figure 2 and supported MCSs up to Index #9, as shown in Table 1.  
Figure 13 shows the protocol stack of the PoC. This was similar to LWIP, but with the security protections omitted for simplicity. Security is an important feature of mobile networks, but was not the focus of the current PoC. We will return to this feature in future work. An LTE-WiGig-integrated CP (multi-RAT CP) application was implemented on top of the CP protocol stack. The multi-RAT CP monitored the real-time traffic load and radio link conditions, such as the received signal-strength indicator (RSSI) of both the LTE and WiGig. The measured link condition on UE was reported to the eNB by CP signaling via LTE. On the basis of the monitored information, the multi-RAT CP on the eNB managed the handling of the UP data. The UP data path was switched between LTE  
Figure 11: Hardware set-up of HetNet PoC.  
Figure 10: Overall illustration of mmWave-integrated HetNet PoC.  
and WiGig by Open vSwitch (OVS) [68]. When the WiGig was used, the multi-RAT CP controlled activation and authentication of the WiGig. PDCP SDUs were sent via the WiGig or LTE link. Since the interface between the LTE-eNB and WiGig-AP was IP-based, various Layer-2 media can be used for the backhaul, including the 1, 2.5, 5 or 10 Gigabit Ethernet or mmWave backhauls presented in Sections 3.1 and 3.2.  
Figure 12: PoC for LTE-WiGig-integrated HetNet: (**a**) LTE eNB, RRH and WiGig AP and (**b**) dual-connectivity UE with LTE and WiGig modules.  
Figure 13: Protocol stack of LTE-WiGig-integrated HetNet PoC.  
To demonstrate the CP/UP splitting concept, we evaluated the performance of LTE-WiGig handover. The experimental setup is shown in Figure 14. The WiGig station (STA) device was attached to a linear actuator and moved horizontally at a constant velocity of 83 mm/s. The vertical distance between the AP and STA antennas was set to 200 mm, so that the STA moved into and out of the coverage area of the WiGig. Switching of the UP data path was based on the availability of the WiGig link and the reported RSSI. If the WiGig link were available and the RSSI exceeded the threshold \(RSSI_{th}\), the UP data was offloaded to the WiGig link. Otherwise, it was carried on the LTE link.  
Figure 15 shows the transition of the instantaneous UP throughput during an LTE-WiGig handover. The downlink UP data were transmitted from the eNB in TCP using Iperf [69]. The handover decision threshold \(RSSI_{th}\) was set to \(-60\) dBm, and the throughput was measured in the MAC layer. From the figure, it can be seen that the data path was switched smoothly, depending on the UE location, and that a throughput of more than 1 Gbps was achieved when the UP path was switched to WiGig.  
Figure 14: Experimental setup.  
Figure 15: Transition of instantaneous UP throughput during LTE-WiGig handover.