## 4 Experimental Results  
### CFR Measurement Results with LTE Signals  
Comparing Table 2, it can be seen that CFR reduce the PAPR of the developed system. The PA measurements as DUT include the analysis of the PAPR and ACPR based on a variation of the clipping in the CFR stage. Figures 8 and 9 depict the power spectral density (PSD) performance for the CFR factors as summarized in Table 2 for the 5 and 10-MHz bandwidth signals, respectively. Figure 8 shows the ACPR for three CFR factors, where a PAPR at the Tx shows an improvement of 2.94 dB for a LTE 5-MHz. While an improvement of 3.30 dB is obtained for the LTE 10-MHz as shown in Figure 9, which represents a trade-off between the PAPR and ACPR controlled by the CFR threshold and \(n\) clip factor; this is represented by Equations (1)-(9). The actual signal exhibits a PAPR of around 10.87 dB with an output power \(-39.43\) dBm for an LTE 5-MHz. In this sense, it is considered that the CFR can be targeted from PAPR level outputs from 6.88 to 9.82 for a LTE 5-MHz and from 9.87 to 6.57 for a LTE 10-MHz. This conditions represents a variation that derives directly from the clipping threshold defined by Equation (9), where \(L\) is the desired level of clipping, additionally a CFR of Algorithm 1 is shown with a PAPR target established as a means of optimizing the process.  
LTE Proposal Study Case  
In this work, the performance of LTE signals of 5 and 10-MHz bandwidths are evaluated for commercial LTE applications. For the evaluation a single band, a LTE signal is used as the input stimulus to drive a Mini-Circuits ZX60-5916 MA+ amplifier. In this case, the signal is measured in the Tx under a 64-QAM modulation scheme for both bandwidth cases. In order to highlight the three CFR factor cases related with its residual spectral regrowth, its ACPR associated is calculated at lower/upper \(\pm\)2.5-MHz frequency offsets for the LTE 5-MHz and \(\pm\)5-MHz for the LTE 10-MHz. Figure 10 shows the gain distortion for a given input power using an LTE 10-MHz stimulus in the DUT for a normalized output power using the entire 76,800 samples. To obtain a memoryless model for given input power, it is swept by iteratively increasing the scale factor to obtain the best AM-AM characteristic, which aims to preserve the useful data points in the saturation level. The clipping factor is then applied for three cases in the LTE actual signal and it is shown in Figure 7, where a reduction can be targeted, noting that after applying the CFR, the PAPR value is reduced from around 4 dB of reduction as depicted in the probability function shown in Figure 11.  
\[\text{ACPR (dB)}\triangleq 10\log_{10}\left(\frac{\sum\limits_{n=1}^{N}\cdot|y |^{2}\cdot e^{(i\omega n)}}{\sum\limits_{n=1}^{N}\cdot|y_{\text{low}}|^{2}\cdot \gamma e^{(i\omega_{1}n)}+\sum\limits_{n=1}^{N}\cdot|y_{\text{upp}}|^{2}\cdot \gamma e^{(i\omega_{2}n)}}\right) \tag{13}\]  
Figure 9: PSD of LTE 10–MHz OFDM waveform signal with residual spectral regrowth after CFR using three scale factors.  
Figure 11 shows the CCDF of the input signal after three clipped factors represented by the transmitted signal, respectively. As can be seen, the CFR stage reduces the PAPR of the input signal from 11 dB to 5 dB. This process impacts on an improvement in the EVM, the PAPR distribution of the LTE PA input signal with a desired PAPR of 3 dB, it can be observed that after 10 CCDF iterations reach the probability between \(10^{-3}\) and \(10^{-4}\), in this condition, the optimal PAPR level is reached.