## 6 Experimental Results and Discussion
### Performance Evaluation of MIMO RNCS  
Figure 13 shows the BER performance as a function of SNR for a MIMO A&F RN with or without pre-qualification techniques and for a D&F RN for two different transmission schemes (SISO and MIMO). Another plot corresponding to the performance of a 2 \(\times\) 2 MIMO link between the eNB and the UE has been included in the graph. We note that, in the figure, DL represents the received signal of the UE through the direct link, while AL symbolizes the signal received at the UE by means of the access link. From the achievable BER performance shown in Figure 13, it can be seen that the RNCS significantly increased the system performance, in comparison with the Non-Co-operative System with 2 \(\times\) 2  
Figure 12: EVM of the Re-transmitted Resource Grid from A&F Relay Node. MIMO scheme. Furthermore, we note that the A&F with pre-equalization techniques obtained better BER performance than the conventional 2 \(\times\) 2 MIMO A&F strategy. In this context, it can be seen that the A&F-ZF pre-equalization system achieved a BER level of 1.39 \(\times\) 10\({}^{-2}\) at around SNR \(=\) 14 dB while, with A&F-MMSE pre-equalization, the RNCS reached the same BER level at around SNR \(=\) 11 dB, such that a performance gain of 3 dB was achieved. Therefore, the MMSE pre-equalization scheme is capable of improving the system performance, which was achieved as this scheme considered the noise in its process of equalization. On the other hand, when we used the SISO D&F strategy, the system reached approximately the same BER level as the A&F-ZF protocol, at around SNR \(=\) 10 dB, implying that 6 dB performance gain was successfully achieved, compared to the performance of the conventional MIMO A&F protocol without pre-equalization scheme. This is due to the improvements that the decoding and encoding processes carried out by the D&F Relay Nodes introduce into the re-transmitted signal, which in this case are sufficient to compensate for the improvements due to the MIMO capacity and the Zero-Forcing pre-equalization technique of the A&F Relay Node. It can be seen, from Figure 13, that the 2 \(\times\) 2 MIMO D&F protocol generally achieved better BER performance than both the 2 \(\times\) 2 MIMO Non-Co-operative System and the 2 \(\times\) 2 MIMO A&F conventional with or without pre-equalization schemes. More specifically, the 2 \(\times\) 2 MIMO D&F protocol achieved the same performance as the 2 \(\times\) 2 MIMO A&F-MMSE at SNR \(=\) 8 dB, such that an 8 dB performance gain was attained. Finally, the 2 \(\times\) 2 MIMO D&F co-operative system obtained the better BER performance between the considered co-operative techniques, which is achieved by means of the decoding and encoding steps that the received signal is subjected before being re-transmitted to the UE through the \(\mathbf{H}^{2}\) channel. The achievable capacity performance recorded for the 2 \(\times\) 2 MIMO Non-Co-operative System and 2 \(\times\) 2 MIMO RNCS, the latter with various Relay Node schemes, is shown in Figure 14. It can be observed, from the figure, that the MIMO Co-operative System achieved higher average throughput than the MIMO Non-Co-operative System. It may also be seen that, when the pre-equalization techniques were introduced before re-transmitting the signal in the A&F protocol, the achievable throughput of the RNCS increased. Figure 14 indicates that the throughput of the 2 \(\times\) 2 MIMO A&F-ZF Co-operative System at SNR \(=\) 12 dB  
Figure 13: BER performance of the implemented RNCS, in comparison with a conventional MIMO system. was approximately 10.35 Mbps while, with the 2 \(\times\) 2 MIMO A&F-MMSE protocol, a throughput of 11.37 Mbps could be reached with the same SNR value, such that a performance gain of about 1.02 Mbps was achieved. Moreover, with the SISO D&F protocol, a maximum capacity of 11.20 Mbps was obtained, associated with a performance gain of 5.92 Mbps. in comparison with MIMO Non-Co-operative System.