# Fibre Wireless Distributed Antenna Systems for 5G and 6G Services
## 4 Experimental Results  
The results of the experimental bench are discussed. The 3 GPP has a standardized performance metric \(EVM\) and \(CSR\). The \(EVM\) is defined as [35]:  
\[EVM\left(\%\right)=\sqrt{\frac{\frac{1}{M}\sum_{m=1}^{M}\left|S_{m}-S_{0,m} \right|^{2}}{\frac{1}{M}\sum_{m=1}^{M}\left|S_{m}\right|^{2}}} \tag{3}\]  
Here, \(M\) represents the constellation symbols, \(S_{m}\) gives the constellation's real symbol associated with the symbol \"\(m\)\" and \(S_{0,m}\) denotes the real symbol linked with \(S_{m}\). The other performance metric used is Carrier Suppression Ratio (\(CSR\)). It is the measure of the RF carrier leakage relative to the modulated output signal. It is defined as [36]:  
\[CSR_{dB}=10\ \log_{10}\left[\frac{P_{Carrier\ Leakage}}{P_{RF\ output}}\right]_{ Measured} \tag{4}\]  
where \(P_{Carrier\ Leakage}\) implies the measured carrier leakage power and \(P_{RF\ output}\) represents the measured RF output power. Figure 5 shows the spectra of the waveforms that are seen just after the 10 km fibre length. The 5G and 6G THz receiver, consisting of a frequency mixer chain, performs down-conversion, converting the signal to the IF band of \(f_{IF1}\) and \(f_{IF2}\), respectively, for performance processing. Simplifying the system configuration and management of the IF bands is particularly advantageous when compared to coherent systems that require THz mixers and local oscillators to be frequency/phase-locked to the oscillator at the central unit [37]. Similarly, the 5G RAU and 6G RAU outputs are visualized to see the filtering process and, in the case of the 6G RAU, the wireless reception. It can be seen that the 5G RAU received at the VSA1 is received at 1.14 GHz and it is received with a slight degradation in the spectra. However, the 6G RAU signal received over a 3 m wireless distance with a 12 GHz carrier is received with a degradation of 18 dB, peak to peak, compared to 25 dB, peak to peak, at the photodiodes output. \begin{table}
\begin{tabular}{c c} \hline
**Parameters** & **Values** \\ \hline \multirow{4}{*}{Signal} & \(f_{IF1}=1.14\times 10^{9}\) Hz \\  & \(f_{IF2}=12\times 10^{9}\) Hz \\  & F/G/O—FDM signal \\  & Modulation Data Rate = 256 QAM \\ \hline \multirow{2}{*}{Laser Type} & Wavelength \(\lambda_{1}=1310\) nm, \(\lambda_{2}=1315\) nm \\  & Mach Zehnder Modulator \\ \hline \multirow{4}{*}{Fibre} & SMF \\  & Link length = 10 km \\  & Fiber Dispersion = 16 ps\(\left(\text{nm.km}\right)^{-1}\) \\  & Attenuation = 0.32 dB\(\left(\text{km}\right)^{-1}\) \\ \hline Photoreceiver & R402 PIN: \(\mathcal{R}\), \(\textit{Responsivity}\) = 0.84 A/W \\ \hline \multirow{2}{*}{Filter} & Bandpass Filter (\(\sim\)1.5 nm and \(\sim\)12 dB/GHz) \\  & Lowpass Filter (1.14 GHz) \\ \hline \multirow{4}{*}{Antenna} & Directional Gain = 20 dBi \\  & 3 dB Beamwidth = 23.71 deg \\ \cline{1-1}  & Cross Pol. Isolation = 35 dB \\ \hline \end{tabular}
\end{table} 
Table 1: Testbed link component parameters. Figure 6 shows the impact of the carrier suppression ratio (CSR) in dB after 10 km of SMF and a 3 m wireless distance. The BER performance was substantially boosted as the CSR increased from 0 to 20 dB. The BER started to degrade when the CSR was further increased to compensate for the EDFA issues such as noise. One of the reasons for this behavior is that higher input power can lead to nonlinearities in the transmission system, which can result in the generation of additional sidebands. Another reason is that higher input power can lead to increased noise in the system, which can also contribute to an increase in the CSR. Figure 5: RF power vs. frequency showing (**a**) multiband signal of 5G after down conversion back to IF of 1.14 GHz and 6G signal at 12 GHz. (**b**) 5G signal at 5G RAU showing the received signal after 10 km of fibre length. (**c**) 6G signal at 6G RAU showing the received signal after 10 km of fibre length and 3 m wireless distance. Figure 6: Bit error ratio (BER) for 10 km SMF with 3 m wireless distance for CSRs.