## 5 Experimental Results
### Synchronization and Performance of the Backhaul Link in the D&F Relay Node
In this campaign, we also assess the control and data channels behavior of the received D&F relay node signal by the backhaul link. For each channel, the EVM in dB for the received symbols is measured by  
\[\text{EVM}_{\text{ch}}[\text{dB}]=20\times\log_{10}[\frac{\sum_{n}^{\text{N}_{ \text{s}}}|\hat{\mathbf{S}}_{\text{s},n}-\mathbf{S}_{\text{s},n}|^{2}}{\sum_{ n}^{\text{N}_{\text{s}}}|\mathbf{S}_{\text{s},n}|^{2}}/100], \tag{7}\]  
where \(\hat{\mathbf{S}}_{\text{s},n}\) is the decoded symbol s matrix at the D&F protocol and \(\mathbf{S}_{\text{s},n}\) describes the originally transmitted data or control symbol s, s indexes the subcarrier position and n is the OFDM data symbols index, the median EVM of the PBCH obtained at D&F relay node by two signals (1 and 2 Table 1), LOS and NLOS channels, and equalization step is shown in Figure 12. In addition, the measured PBCH through the Anritsu spectrum analyzer is employed for comparison. It should be highlighted that the PBCH is transmitted by the Q-PSK modulation scheme. It can be seen from Figure 12 that lower transmit power levels at the base station translate into a lower desired signal quality at the D&F relay node. In this sense, lower signal-to-interference noise ratio (SINR) induces higher EVM for the data symbols of the evaluated channels. On the contrary, the SNR increases for higher power  
Figure 10: PBCH demodulation reference signal estimated SNR versus iSSB. levels \(\mathbf{P_{BS}}\) at the gnodeB, therefore enhancing the EVM results at the PBCH of the D&F protocol, as is shown in Figure 12.  
It should be explained that the considered NLOS backhaul link corresponds to the scenario in Figure 7, where a non-line-of-sight between base station and relay node has been established. The aim of this experimental environment is to verify the functioning of the synchronization and decoding steps in the D&F protocol under a scenario with adverse communication conditions. It can be observed, from Figure 12, that as expected, the NLOS  
Figure 11: Received PBCH and equalized received PBCH constellation diagrams. Figure 12: Median EVM at physical broadcast channel for two modulation schemes, LOS and NLOS environment, and equalization step under real test. experimental scenario presented worse EVM (dB) than the LOS environment. Furthermore, the developed MMSE equalization method in the relay node considerably improves the performance of the decoded received symbols at the PBCH. For example, when considered the signal 1, LOS scenario, and \(\mathrm{P_{\mathbf{BS}}}=7\) dBm, the EVM of the PBCH after the MMSE equalization technique is approximately \(-38.29\) dB; however, before the MMSE technique it is \(-35.98\) dB; therefore, it can be appreciated that it introduces a significant improvement on the order of 2.31 dB. On the other hand, to verify the obtained results through D&F protocol, two new graphics have been inserted (black and blue) for its comparison, as is shown in Figure 8:0 The obtained information from the figure shows that the results with the Anritsu spectrum analyzer have similar behavior to the proposed algorithms of the implemented D&F in both scenarios, LOS and NLOS channels. The authors have also measured the physical downlink control channel in the D&F relaying technology, and the obtained results are drawn in Figure 8:1 The symbols of the downlink control channel are transmitted by the Q-PSK modulation scheme. In addition, from the figure, it can be seen that the performance behavior by the decoded PDCCH in the relay node is similar to that achieved in Figure 12, where the best performance is reached with the LOS scenario and post-MMSE technique. Figure 14 shows the EVM results at physical downlink shared channel for 64-QAM and 256-QAM constellations, which correspond to signal 1 and signal 2, respectively. It can be seen that the EVM (dB) increased when the NLOS channel was considered. Moreover, the MMSE technique significantly improved the system performance, compared to the measured pre-MMSE algorithm.