# Opto-Electronic Oscillators for Micro- and Millimeter Wave Signal Generation
## 4 Conclusions  
The OEO is a very well-known, high-frequency oscillator for generating low-phase microwave and millimeter wave signals and is used daily in various fields of science, technology and engineering. It has already been shown that an OEO can generate a millimeter wave signal up to the W band. The OEO has the advantage that the oscillator's phase noise is independent of the operating frequency. However, the OEO might require stabilization for short-term, long-term and multimode operation. Moreover, the OEO can be developed as a tool/application for various scientific and engineering fields. The wideband OEO was introduced to generate high-frequency signals in the microwave and millimeter wave signal ranges. This enabled the OEO to be tunable up to GHz and above. In 2017, the manufacture of an OEO on a photonic-integrated chip was added to the literature. The next generation of 5G RAN requires a low-phase-noise oscillator in the millimeter wave range. Thus, we propose to implement the OEO in the central station of the 5G RAN, defined as a LO for frequency up-conversion and down-conversion. This eliminates the requirement for a LO in each base station, and the clock distribution is also conducted centrally. Moreover, due to its structure, the OEO does not require opto-electronic/electro-optical signal conversion, which can be classified as another advantage. On the other hand, the simple, single-loop OEO cannot meet the requirements of the centralized OEO for 5G RAN. Thus, an injection-locked, millimeter wave OEO with temperature-stabilized components might be employed to exhibit the best performance as a centralized LO for 5G RAN. Furthermore, by combining the optical fiber path selector with the single-loop OEO, we developed an application to measure the FSR, SMSR and phase noise of the OEO for different optical fiber lengths. This application is also practical to test different electrical bandpass filters and find the optimum performance. In the future, we would like to improve the application so that it can be used for testing/verifying/measuring different configurations of OEOs, such as injection-locked, multi-loop, etc. For injection-locked or multi-loop configurations, an additional optical fiber path selector (one or more) is required to find the optimum configuration of the OEO by comparing the SMSR and the phase noise. In the current state of the art, one of the limitations is designing a stable OEO on the photonic integrated chip. In order to enable mass production of the OEO, which would make it possible to use the OEO in different scientific areas, photonic integration might be useful. However, because of its short delay line length, the photonic integrated OEO has a poor short-term stability. A future study should focus on improving the short-term stability of the integrated OEO. A high Q-factor resonator can be implemented for the photonic integrated chip. If this is achieved, the integrated OEO can easily be combined into the 5G RAN as a centralized local oscillator. On the other hand, the single-loop OEO has its own limitations for long-term stability and multi-mode operation. A variety of different configurations are proposed to reduce the challenges. Multi-loop, injection-locked or coupled OEOs have been used for many years to improve the multimode operating characteristics of the OEO. Moreover, OEOs with a feedback control loop, PLL and temperature stabilization of the optical fiber as well as an electrical bandpass filter are used to reduce the frequency drift of the oscillator signal. Another topic for future research should be the sophisticated design of wideband, tunable, integrated and ordinary OEO operating in the millimeter wave range with acceptable phase noise performance (\(<-\)120 dBc@10 kHz offset from the carrier). Conceptualization, methodology, investigation and resources M.A.I., and B.B. ; formal analysis and validation B.B. ; writing--original draft preparation M.A.I. ; writing--reviewand editing M.A.I. and B.B.B.