## 7 Conclusions  
In this paper, a detailed study and implementation of a Relay Node Co-operative System based on a Software Defined Radio (SDR) Platform and the MatlabTM tool has been performed in order to enhance the average system throughput and BER in indoor-to-indoor environments. The architecture implements a Relay Node Cooperative Network, which uses in-band Relaying Protocols. Two protocols of Relay Nodes have been developed: Amplify-&-Forward (A&F) and Decode-&-Forward (D&F). Moreover, Pre-Equalization stages for the A&F relay were studied, considering the Zero Forcing and Minimum Mean Square Error Pre-Equalization schemes. The cooperative system deploys Relay Node with one antenna at transmission and one antenna at reception, as well as, multiple antennas. Furthermore, the 4G signal employs features that are standardized by the 3 GPP. Our measurement results demonstrated the viability and flexibility of the Co-operative System developed through the SDR and MatlabTM software.  
On the other hand, the results demonstrated that the Co-operative System clearly outperforms the Non-Co-operative architecture. Furthermore, it can be seen that the A&F protocol with the pre-equalization technique greatly enhances the Key Performance Indicators of the system in comparison with the A&F conventional strategy. For example, the maximum throughput of the A&F protocol with Pre-equalization stage is 11.83 Mbps at \(\mathrm{SNR}=16\) dB. However, with A&F conventional strategy, the reached result is less, which has obtained losses of approximately 1.94 Mbps of the maximum throughput achieved by the A&F-ZF protocol at \(\mathrm{SNR}=16\) dB. From the results, it is concluded that the A&F-MMSE strategy presents higher performance than the A&F-ZF scheme, due to the less EVM of the re-transmitted Resource Grid to the UE in the second stage. It can be noticed that the SISO D&F Co-operative Network presents approximately the same performance that 2 \(\times\) 2 MIMO A&F-ZF protocol. Nevertheless, the implemented MIMO D&F Relay Node showed the best performance of the different strategies implemented. In this context, the maximum throughput of the proposed Two-Hop Co-operative System for 64-QAM signal (13.95 Mbps) is reached from \(\mathrm{SNR}=8\) dB, when the MIMO D&F Relay Node is used, being the only one strategy that can achieve it. Furthermore, it can be concluded from the results that employing Relay Node together MIMO technique led to a substantial benefit, in terms of the KPIs of the network in the indoor-to-indoor environment. The MIMO scheme achieved a reduction of the Bit Error Rate (BER) and increasing the throughput in the 64-QAM modulation scheme, which can be concluded increasing the number of antennas can lead to higher performance and spectral efficiency of the system. In addition, the D&F algorithm is the most complex between the developed protocols, due to the decoding and encoding processes. We believe that the implemented platform provides a cost-effective, scalable, and easy to update solution for enabling 5G signals standardized by the 3 GPP.  
[PERSON] conceived the paper, designed the experiments, performed experiments, analyzed the data and wrote the paper; [PERSON] and [PERSON] reviewed and edited the paper. All authors have read and agreed to the published version of the manuscript.  
This work has been supported by the Spanish Ministry of Science, Innovation and Universities within the project TEC2017-87061-C3-1-R (CIENCIA/AEI/FEDER,UE) and by the China Science and Technology Exchange Center-MST of the People's Republic of China, within the project 23016 YFE0200200. The work of [PERSON] is supported by a Pre-doctoral Contract PRE2018-085032 from the Ministry of Science and Innovation.  
The authors declare no conflict of interest.