# Application Research of Multi-Mode Relay in Future Heterogeneous Networks
## 1 Introduction  
With the fast growth of mobile terminals, traffic of mobile data communication is growing extremely fast, which is becoming one of the challenges of network capacity and transport rate of community networks. Traditional cellular networks will soon become incapable of addressing these problems effectively. Massive multi-input multi-output (massive MIMO) is believed to be a significant approach to meet the massive growth in data traffic. However, the growing number of antennas increases hardware cost dramatically, especially regarding the overheads of radio frequency (RF) links. There are many researchers focusing on the analog beamforming technique to decrease RF resources [1; 2; 3]. These techniques need extra computational cost at both transceiver sides. Heterogeneous hierarchical wireless networks are also considered to be an important architecture candidate to meet the demands of wireless networks in the future, due to the scarcity of RF resources [4]. 5G network architecture is a perfect example of future heterogeneous hierarchical wireless networks that guarantee the user reliability and integrity [5; 6]. Usually a 5G heterogeneous network manages all sorts of networks by using software-defined virtual intelligent network systems. Network function visualization (NFV) represents a significant transformation for telecommunications/service-provider networks, driven by the goals of reducing cost, increasing flexibility, and providing personalizedservices. Software-defined network (SDN) is suggested to control, initialize, manage, and change network behavior automatically by opening interfaces and abstraction of lower-level functionality programmatically. As is shown in Figure 1, heterogeneous hierarchical wireless networks (HHWN) introduces a lot of low-power base stations, such as picocells, femtocells, and distributed antenna systems (remote radio heads, such as relays). These heterogeneous elements can not only increase the flexibility of the network, but also fulfill huge user requirements for random complex data traffic, and raise its quality of service. Studies have been done to improve the ability of the HHWN. In [7] the hybrid grid routing protocol (HGRP) is used to take on the cost advantages of routing and energy consumption. To cope with the handover management and co-channel interference, by combining cloud ratio access network with small cells, a novel network architecture is proposed in [8], applying an effective CoMP clustering scheme to mitigate cell edge user interference. To save system energy, reference [9] showed an energy-efficient approach, and the use of heterogeneous-aware enhanced hierarchical clusters (HEHC) can obtain better energy availability. To handle the problem of cross-tier interference in spectrum-sharing deployment, [PERSON] et al. presented a resource-allocation scheme [10], set up a model for the sub-channel and power allocation, and presented an iterative sub-channel and power-allocation algorithm. To cope with the problem of severe inter-cell interference when we increase the wireless network capacity, reference [11] investigated the coexistence problems of sharing an unlicensed spectrum between Wi-Fi and 4G cellular networks, and proposed a new scheme to avoid interference based on estimating the density of Wi-Fi access points near small cells. To improve system reliability, reference [12] indicated that a resource management unit can be introduced into HHWN to implement the distributed grids, then proposed a semi-centralized hierarchical architecture to balance the load of HHWN. The increase of the low-power base stations may result in inconvenience since a lot of base stations are equipped in a certain area. There may even appear the complex situation that multi-BSs (picocell, femtocell) and multi-HotPoints (wireless router, Wi-Fi relay) coexist, especially at the edge of a macrocell. To solve the aforementioned problems, we unify two or more kinds of BSs to one single piece of equipment.