## 4 Hardware Implementation of the RNCS
### D&F Relay Node Implementation  
In this subsection, a MIMO Decode-&-Forward Relay Node using the developed SDR Platform is implemented. A simplified flowchart of the MIMO Decode-&-Forward Relay Node can be seen in Figure 9.  
Figure 9: MIMO Decode-&-Forward Relay Node Processing Blocks. First, the IQ signals are captured by the RF receive chains of the MIMO NI-USRP 2944R platform. After acquisition of the data symbols, any significant frequency deviation must be estimated and eliminated, without which many errors would propagate. Furthermore, the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) are produced. The PSS and SSS are detected by employing the time- and frequency-domain correlation. Therefore, the time offset synchronization and cell identity in the Time Synchronization & Cell ID Search block are obtained. Then, in order to establish the quality of the correlation, the correlation for each of the three possible primary cell identities is computed. The Basic Structure block is a configuration structure, taking into account the sample rate of the SDR. Duplex mode (FDD), Cyclic Prefix, and a number of resource blocks (depending on the sample rate of the SDR) are assumed. After the steps described above, the MIMO-OFDM signal is demodulated. The first task after MIMO-OFDM demodulation is to perform channel estimation of the first subframe, which is carried out by making use of the Channel Estimation Configurator block. The Channel Estimation block is performed in two steps: First, the frequency responses for the subcarriers of the pilot symbols are determined, based on the Least Squares (LS) estimator. On the other hand, from the first step, the frequency responses of the subcarriers of the data symbols can be derived by interpolation methods employing the adjacent pilot symbols. For this, we use the 2D Bi-Cubic (BC) interpolation method [33]. The outputs of the OFDM Demodulation Signal and Channel Estimation blocks are used as the input of the Decode Structure Signal, which determines the parameters to obtain the full bandwidth. Nevertheless, now that the signal parameters are known, the received signal is resampled to the nominal sampling rate. Afterwards, the Relay Node can carry out the full received signal demodulation and, so, perform the channel estimation. Finally, the demodulated MIMO received signal (\(\mathbf{Y}_{\text{DF}}^{1}\)) and the estimated channel matrix (\(\mathbf{\hat{H}}^{1}\)) are processed through the Demodulation-&-Modulation Algorithm Processing Blocks in Figure 10, which is performed to each subframe. This Processing Block in Figure 10 executes the demodulation and modulation of physical, control, and data channels, as well as determining other reference signals. In the 4G communication networks, the PDCCH used to provide physical layer signaling to support MAC layer operation. Each PDCCH carries the message known as the Downlink Control Information (DCI) for the UE. DCI contains the information about resource scheduling for downlink and uplink, transmit power commands (TPC), etc. On the other hand, PBCH is a control channel (can be found in subframe 0 of each radio frame), which has the aim of transporting basic information about the net, named by Master Information Block (MIB). This information contains: four bits to identify the canalization used in the cell, three bits to define the channel PHICH(Physical Hybrid ARQ Indicator Channel) structure that is used to transport recognition information about the HARQ (Hybrid Automatic Repeat Request) mechanism and seven bits in order to identify the frame number (System Frame Number, SFN). On the other hand, the data symbols are carried out by the PDSCH, particularly, SIBs (System Information Blocks) are transported by this physical channel. Several modulation options can be applied to this channel, including Q-PSK, 16-QAM, and 64-QAM, which the 64-QAM scheme has been employed in our platform, as well as is flexible and can be considered Q-PSK and 16-QAM.