# Fibre Wireless Distributed Antenna Systems for 5G and 6G Services
## 4 Experimental Results
#### Effect of Chromatic Dispersion and Power Fading  
One of the main effects of chromatic dispersion (CD) in RoF systems is that it can cause the modulated RF signal to spread out over a larger portion of the fiber, leading to a reduction in the signal strength and an increase in the bit error rate. This is because the different wavelengths of light that make up the modulated RF signal will travel at slightly different speeds through the fiber due to the changes in the refractive index of the fiber with wavelength. As a result, the different wavelengths will arrive at the destination at slightly different times, leading to a spreading out of the signal. It is known that \(CD\) is the result of the combination of waveguide dispersion \(D_{w}\) and material dispersion \(D_{M}\) and is generally represented as [39; 40]:  
\[CD=-\frac{\beta\omega^{2}}{2\pi c} \tag{5}\]  
where \(\beta\) represents the part of the phase constant that is affected by the optical carrier frequency \(\omega\). In general, the value of chromatic dispersion for SMF at 1550 nm is 17 ps/(nm.km). The optical signal created by the laser's modulation of the RF signal travels via an optical fibre and, as a result, each band experiences a distinct phase rotation as a result of the optical fibre's chromatic dispersion. The injected RF signal's strength is amplified by a constant that relies on the phase mismatch of the two optical sidebands before the optical signal at the SSMF's output beats in a PD and retrieves it by heterodyne detection. This condition that occurs due to chromatic dispersion is known as power fading. The power constant \(K_{PF}\) for power fading is represented as follows [41; 42]:  
\[K_{PF}=\cos^{2}(\theta_{LSB}-\theta_{RSB}) \tag{6}\]  
where \(\theta_{LSB}\) represents the rotation in the phase induced by the CD of the fibre in the left sideband, while \(\theta_{RSB}\) represents the one induced in the right sideband. Both chromatic dispersion and power fading can have a significant impact on the performance of 5G and 6G fiber systems, including those using THz and distributed antenna systems (DAS). In order to ensure reliable and high-quality communication, it is important to carefully design and engineer the RoF-based DAS system. This may involve using specialized fibers with lower dispersion characteristics, or using techniques such as wavelength division multiplexing (WDM) to reduce the impact of dispersion on the signal. Other strategies to mitigate the effects of chromatic dispersion in RoF systems may include using repeaters or amplifiers to boost the signal strength, or using error-correction techniques to mitigate the impact of errors on the transmission. The utilization of Digital Predistortion (DPD) for counteracting these nonlinearities can be a good approach [21]. In the proposed system, the length of the SMF in the system being discussed is 10 km, which is a relatively smaller length, and the frequency of the intermediate frequency carrier was relatively low, i.e., 1.14 GHz. As a result, the effects of power fading were not noticeable. Additionally, the use of zero-chirp Mach-Zehnder modulators (MZM) eliminated inter-mixing and the generation of second-order intermodulation distortion (IMD2). This was also true for the sub-terahertz 6G system, where the use of only a single sideband in fibre-wireless transmission eliminated the effects of RF fading. Figure 9 shows below the power fading for the range of 1 GHz to 20 GHz. Similarly, we characterized the 6G THz system performance by increasing the Gbaud rate test for the 6G signal by increasing its baud rate with respect to optical power and saw the behaviour in terms of BER, as shown in Figure 10. To prevent 5G and 6G bands from interfering with each other, the \(f_{1F2}\) of the 6G signal was increased by 250 MHz for every 2 GB increment in the baud rate.