## 4 Hardware Implementation of the RNCS
### Co-Operative System Implementation  
In this subsection, we describe the developed MIMO SDR platform for testing and characterizing the A&F and D&F Relay Nodes. For their implementation, the NI-USRP  
Figure 4: Two-hop D&F MIMO Relay Node Co-operative System.  
2944R and NI-USRP 2901R were used. Their operating frequencies range from 70 MHz to 6 GHz, which is considered as sufficient to take on the flexibility demands of our platform. Each board can hold two RF chains (2 Tx/2 Rx). Furthermore, they can support a sample rate up to 56 MHz, which is sufficient to record an LTE signal (with maximum bandwidth of 20 MHz). These technical parameters make them suitable for the required functionalities.  
The developed testbed system is shown in Figure 5. It is composed of one evolved Node-B (eNB), which is achieved with one NI-USRP 2944R board and the MatlabTM software. Connection from a PC to the NI-USRP 2944R is performed through the NI-IMAQdx GigE Vision High-Performance Driver, a 10 Gigabit Ethernet Card for Desktop, and a 10 Gigabit ethernet cable. Figure 6 shows the occupied bandwidth (4.57 MHz) of the transmitted signal by emulated eNB, which was measured at the output of the NI-USRP 2944R through the MS2090A Field Master Pro of Anritsu. The transmitted frequency error was approximately 344 Hz, which could be perfectly corrected with the implemented frequency offset correction algorithms. The total channel power transmitted by the emulated eNB was approximately 16 dBm, which fulfils the requirement demanded by the 3 GPP in [44] for the downlink at 5 MHz channel bandwidth.  
Figure 5: Testbed Environment for A&F and D&F Relay Nodes system.  
Figure 6: Bandwidth occupied by the signal transmitted by the emulated eNB.  
The demodulation of the transmitted signal by means of the MS2090A equipment is presented in Figure 7. The obtained information from Figure 7 shows the Error Vector Magnitude (EVM) of the PBCH and PDSCH. Furthermore, the power of the several reference signals and of the PBCH can be observed, which were adjusted to the 3 GPP standard in [44].  
On the other hand, the A&F and D&F Relay Nodes were implemented using a PC, the MatlabTM software, and an NI-USRP 2944R. The NI-IMAQdx GigE Vision High-Performance Driver, 10 Gigabit Ethernet Card for Desktop, and 10 Gigabit ethernet cable were also used for the connection. The User Equipment (UE) was developed by employing a PC, the MatlabTM software, and an NI-USRP 2901R, where the connection from the PC to the USRP was performed through USB 3.0. The antennas used for the experiments were general-purpose LTE and Wi-Fi antennas which work on the 700-960 MHz and 1710-2700 MHz bands, respectively. They had a gain of 5 dBi and an impedance of 50 \(\Omega\). They had SMA connectors, which were compatible with the USRP connectors. Furthermore, the implemented Relay Nodes are an in-band RNs, that is, the reception and transmission frequencies are the same, but the transition from one to the other is done in different time intervals. On the other hand, the problem of coupling and interferences between the antennas has been studied experimentally. In this sense, several measurements have been made at different positions of the receiving and transmitting antennas of the eNB, RNs, and UE to determine their optimal position and minimize interference.