# Opto-Electronic Oscillators for Micro- and Millimeter Wave Signal Generation
## 3 Recent Development of the OEO's Application
### Optical Signal Distribution for the 5G Radio Access Network  
The next-generation 5G radio access network (RAN) has certain requirements to improve bit rate and spectral efficiency [87]. It is expected that millimeter wave bands will be used to meet these requirements [88]. The current 4G-RAN technology uses an electrical local oscillator (LO) in each base station to perform the frequency up-conversion and down-conversion of the data signal. Unfortunately, the phase-noise performance is degraded when they are used for the millimeter-wave signal range. The centralized oscillator's signal distribution was reported in the literature from several sources [89; 90; 91; 92]. For example, in [93], we proposed the use of an OEO as a LO in a 5G RAN fronthaul. The centralized oscillator signal was distributed to the base stations without the need for electro-optical and opto-electronic conversion. Radio over fiber [94; 95; 96] is proposed as an efficient and sophisticated solution to distribute the centralized OEO signal. The idea is shown in Figure 10. The OEO signal is distributed from the central office to the base station through an optical distribution network (ODN). The data signals and the OEO signal can be transmitted over the same fiber using a dense wavelength-division-multiplexing (DWDM) approach. This approach has the advantage of keeping the ODN infrastructure simple, as well as the base station, because the LO is no longer required. Since the LOs would need to be temperature stabilized to provide a stable clock signal, it is more cost-effective to only develop one well-stabilized oscillator in the central office and distribute its signal to the base stations. In addition, the proposal has the advantage that the total number of oscillators in the whole fronthaul system is reduced. This would have a positive impact on the energy savings of the complex system if the number of oscillators was reduced. Figure 10: Use of an OEO as a local oscillator (LO) for a 5G radio access network (RAN) fronthaul. Reprinted from Reference [93]. On the other hand, the optical signal distribution of the OEO via an optical fiber presents some challenges. One of the main challenges is the power penalty due to chromatic dispersion [97]. The C band is widely used in optical communications because of its advantages such as a low optical loss (0.2 dB/km), but chromatic dispersion is very dominant in this band, ranging between approximately 16 and 17 ps/nm/km. Intensity-modulated links are negatively affected by the power penalty due to chromatic dispersion. Therefore, we proposed to use the tunable dispersion-compensation module (TDCM) in each base-station [93], which is shown in Figure 10. The TDCM is a device based on Bragg gratings that provides a way to compensate for the chromatic dispersion in the C band with respect to the actual fiber length. To test the performance of the optical signal distribution, we employed the experimental setup shown in Figure 11.  
For the experiment, we set up an optical delay line in combination with a vector network analyzer (VNA). This configuration provided the ability to apply a wideband RF signal to the optical delay line to track the full spectrum of the dispersion penalty. When the OEO was used instead of the described experimental setup, we only had the opportunity to investigate a single frequency. Therefore, we could obtain limited experimental data relevant to the dispersion penalty. The VNA provided the RF signal with a range of about 45 GHz. The results for the power penalty with and without the TDCM are shown in Figure 12.  
Figure 11: Experimental setup for dispersion penalty measurements with and without tunable dispersion-compensation module (TDCM). Figure 12: Power-penalty measurements from 10 MHz to 45 GHz over a 20-km optical fiber. Reprinted from Reference [93]. The results show that the TDCM can be used in any base station to compensate for chromatic dispersion in the C band.