###### Abstract  
Millimeter-Wave (mmWave) bands are receiving enormous attention in 5G mobile communications, due to the capability to provide a multi-gigabit transmission rate. In this paper, a two-hop architecture for 5G communications with the capacity to support high end-to-end performance due to the use of Relay Nodes (RNs) in mmWave-bands is presented. One of the novelties of the paper is the implementation of Amplify-and-Forward (A&F) and Decode-and-Forward (D&F) RNs along with a mmWave-band transceiver chain (Tx/Rx). In addition, two approaches for channel estimation were implemented at the D&F RN for decoding the backhaul link. One of them assumes complete knowledge of the channel (PCE), and the other one performs the channel estimation through Least Square (LS) estimator. A large number of simulations, using MATLAB(tm) and Simulink(tm) software, were performed to verify the potential benefits of the proposal two-hop 5G architecture in an outdoor-to-indoor scenario. The main novelty in performing these simulations is the use of signals with 5G features, as DL-SCH transport channel coding, PDSCH generation, and SS Burst generation, which is another of the main contributions of the paper. On the other hand, mmWave transmitter and receiver chains were designed and implemented with off-the shelf components. The simulations show that the two-hop network substantially improves the Key Performance Indicators (KPIs), Bit Error Rate (BER), and Throughput, in the communications between the logical 5G Radio Node (gNodeB), and the New Radio User Equipment (NR-UE). For example, a throughput improvement of 22 Mbps is obtained when a 4 \(\times\) 4 \(\times\) 2 MIMO D&F with LS architecture is used versus a SISO D&F with PCE architecture for Signal-to-Noise Ratio (SNR) = 20 dB and 64-QAM signal. This improvement reaches 96 Mbps if a 256-QAM signal is considered. The improvement in BER is 11 dB and 10.5 dB, respectively, for both cases. This work also shows that the obtained results with D&F RNs are better than with A&F RNs. For example, an improvement of 17 Mbps in the use of SISO D&F with LS vs. SISO A&F, for the 64-QAM signal is obtained. Besides, this paper constitutes a first step to the implementation of a mmWave MIMO 5G cooperative network platform. m 2021 21 1372  
Millimeter-Wave (mmWave) spectrum [6] is emerging as another enabling technology in 5G networks. It has been a topic of interest in recent years by academia, standards organizations, regulatory commissions, and industry, and it is a key candidate for addressing the challenges of bandwidth shortage for 5G systems. However, signals at mmWave bands undergo severe path loss and are highly sensitive to blockage as compared to FR1 frequency bands of 5G. On the other hand, thanks to smaller wavelengths of mmWave allowing more antennas to pack within the same physical area, this drives to use of a large number of antennas at the transceivers, thereby forming massive MIMO systems. In conclusion, the combination of the massive MIMO and mmWaves emerge as a concrete tool to address the requirements of 5G network deployments [7]. However, the hardware implementation of mmWave Massive MIMO systems present significant technological challenges [8]. The common solution for their implementation is the use of Planar Phased Array antennas. Nevertheless, these systems present the problem of mutual coupling between antennas. Some solutions and various isolation improvement mechanisms have been proposed [9; 10]. In our case, horn antennas were used in the mmWave transceiver chain, which, at the cost of reducing the couplings between antennas, limit the massive MIMO capabilities of the system. In the last few years, Relay Node (RN) protocols have been a research topic that has attracted much interest from researchers. In fact, relaying technology has proven to be an optimal solution to guarantee the quality of service in certain scenarios.