# Adaptive Relay Selection Scheme by Using Compound Channel
## 1 Introduction  
With the increase in the use of multimedia services such as mobile video and audio, traffic for communication is generated. In particular, micro cells with wide coverage could not guarantee stable long-term evolution (LTE) service in traffic-intensive urban areas or buildings [1, 2]. As spatial density and mobile devices increase, the nodes of cell are overloaded [3]. In 5G, the heterogeneous UDN is proposed for both high data rate and a lot of data transmission with limited resources. The heterogeneous UDN is HetNet with an UDN structure that concentrates more small cells such as pico and femtocells than the HetNet used in the fourth generation [4]. The heterogeneous UDN improves spectral efficiency by increasing the network capacity per unit area of the cell [5]. It also distributes mobile traffic and satisfies 5G performance requirements such as user quality of service (QoS), quality of experience (QoE), lower latency and power consumption [6, 7]. Data rate and reliability are significant elements of communication, but it is difficult to achieve high performance in a wireless system. Especially, if the network condition is poor, communication performance is degraded. One-way or two-way relays are used to overcome poor network condition in cooperative communication systems. For same data rate, two-way relaying protocol improves transmission power consumption and spectral than one-way relaying protocol [8]. For this reason, two way relaying protocols have been studied actively to improve performance [9, 10]. The relay can be used to decentralize the overload of cell nodes in a heterogeneous UDN system [11]. In additionto improvement of reliability for communication, relays can also offer a variety of benefits such as diversity gains, robustness for noise and expanded coverage ranges [12]. When the outage in the system occurs, these benefits cannot be acquired. In other words, if the channel capacity is less than the amount of data to transmit, a network system fails. So, accurate channel state information (CSI) is important to select a relay for the stability of the network system. In the communication systems, channel state information (CSI) is required to achieve the expected benefits. The CSI is mostly classified as full CSI and partial CSI. The researches of partial CSI [13; 14] are progressed to compensate for the weakness of full CSI, which requires a lot of feedback. However, researches using full CSI [15; 16] have been studied. In particular, as the relay node forwards the received signal, the stability between the source and the relay node is very important. Therefore, the relay with a largest channel capacity between the source node and the relay node should be selected using CSI. Researches have been studied steadily to reduce the outage probability in a cellular network using relays [17; 18; 19; 20]. In the studies of the multi-hop system among these researches, it is essential to assume that the base station (BS) node and relay station (RS) nodes are connected with backhaul [21; 22; 23]. Although researches about wireless backhaul are in progress, but issues such as cost, flexibility, and wired and wireless convergence still remain unsolved. Furthermore, due to the difficulty of backhaul connections and the interaction between relays, researches on single hop have been studied actively to improve the performance of single link as an alternative of complex multi-hop systems [24; 25]. These researches only consider system using a single relay. In relaying system, one of the primary issues is how to maximize the diversity gain [26; 27; 28]. The use of multiple relays is one way to maximize diversity gain, and various selection schemes for multiple relays have been studied on a single link [27; 29; 30]. Researches to reduce the outage probability of cellular network systems that generally use relays select a single relay with the lowest outage probability. Furthermore, the technique for relay selection according to the case of outage does not exist.