1 Cooperative Communication in LTE systems using Amplify-and-Forward & Decode-and-Forward 1 Kien Trung Ngo, 1 Anh Hoang Bui, 2 Oanh Tu Thi Truong, 2 Hoa Tung Le, 2 Sy Quy Nguyen, 2 Hung Viet Nguyen Posts and Telecommunications Institute of Technology, Hanoi, Vietnam. 1 {keeningo, hoanganhbui.ptit}@outlook.com; 2 {oanhttt, hoalt, synq, hungnv - vt1}@ptit.edu.vn Abstract— Cooperative Communications has been proved to be capable of providing high data rates for wireless access services as well as of broadening coverage area of the wireless networks. Long term evolution (LTE), LTE-Advanced (LTE-A) have been recently developed and deployed in a large scale. In this paper, different models of LTE-based cooperative communications are proposed and investigated, where replaying schemes may be applied in LTE-based system for improving the spatial diversity gain. More specifically, cooperative transmission in our proposed system is formed by employing the Mobile station (MS), Relay Station (RS) and Base Station(BS) in LTE environment. The system can be operated in both Amplify-and-forward (AF) regime and Decode-and-forward (DF) regime, in order to take advantages of cooperative communications in various scenarios of relay positions. The advantages of cooperative communication are demonstrated by evaluating frame error ratio (FER) performance in numerous AF and DF scenarios in comparison to the point- to-point conventional transmission of the LTE systems. I. I NTRODUCTION In recent years, cooperative communication considered as an exciting research topic has attracted numerous research efforts [1]–[7]. In wireless systems employing cooperative transmission, Relay Stations (RS) have a vital role in collabo- rating with MS, in order to transmit signals to the Base Station (BS) [4], [8]. In the typical scenarios, users not only transmit signals but also process signals separately, thereby signifi- cantly enhancing the performance of wireless communication system [2]. The main focuses of research in the cooperative communications are to increase the data throughput for user devices moving around cell-edge and to extend coverage area of the BS by exploiting RSs [6], [7]. The ideal is that user devices in the same coverage area collaborates with each other and transmits its messages to the same or different destination. Furthermore, all user devices uses the same resources in short distance. As suggested in [3], the pros of collaboration can be seen in ability transmission, reliability, coverage and quality-of-service (QoS). Additionally, the cost of transmission may be partly reduced since transmission time is decreased. Meanwhile, users collaboration may improve throughput of the system, save the power consumed at each user device as well as simultaneously reduce the noise generated in the entire system [2]. It is obvious that the cost of the RS is lower than that of the BS, hence it may be more economically appropriate when employing RS-based solution in the wireless infrastructure. Particularly, RSs located at appropriate positions are capable of increasing throughput conveyed in a cell as well as of enhancing the system reliability [9]. In another scenario, cooperative communication can make use of multiple antennas for duplicating messages forwarded to the destination. As a result, standards proposed for LTE, LTE-A and beyond infrastructure are supposed to support the above-mentioned cooperative transmission scenarios [9], [10]. In the context of cooperative communications, a variety of solutions can be used with respect to different infrastructure constraints and specific network requirements [1]. Among the existing cooperative strategies, Amply-and-Forward (AF) and Decode-and-Forward (DF) are the most basic and widely adopted [3]. In AF regime, the signals transmitted to the RS from MS are amplified at the RS and then forwarded to the BS without further processing, namely decoding and demodulation. As a result, the AF solution is intuitively simple and straightforward to deploy in the existing wireless network infrastructure [5], [3]. However, noise and interference added during the transmission from the MS to the RS over the wireless channels are also amplified and present at the BS. This characteristic of the AF-based solution makes its more appropriate for being employed in the systems capable of providing with the signal-to-noise ratio (SNR) that is higher than normal [1], [11]. In the contrary, the signals transmitted in DF-based systems are decoded and then re-encoded at the RS before forwarding to the BS. Since the signals are recovered before sending to the BS, this solution helps to eliminate noise and interference added during the transmission from the MS to the RS. Upon exploiting the advantage, the DF-based systems tend to be used in the scenarios when a low SNR is frequently seen [12]. Additionally, cooperative communications may also be used in conjunction with transmission protocols in order to improve the system performance in specific collaboration scenarios [2]. The transmission protocols aim to specifically identify the re- sponsibility of individual nodes in different time slots (TS). In other words, the protocols describe when or what information blocks are transmitted from given nodes. Examples of suchlike protocols are Multi-hop, Split-Combine and Diversity [2], [13]–[15]. In Multi-hop scenario, the MS only transmits to the RS in the first TS, then the RS forwards its information to the BS in the second TS [13]. By contrast, in the Split-Combine scenario, the MS transmits to RS in the first TS, while both the MS and RS simultaneously transmit their information to the BS during the second TS [2]. In the Diversity-oriented scenario deemed as one of the best transmission protocols [14], [15], the MS transmits to both the RS and BS during the first TS, and subsequently only the RS is activated for forwarding messages