## 4 Conclusions  
The OSNR was obtained with an optical resolution bandwidth of 100 MHz. We proposed a simple and flexible design scheme of a high capacity tunable 5G+ MMW synthesizer based on ultra-wideband electro-optical OFCs. The architecture is compact and integrated and mainly consists of a monolithic Fabry-Perot QD laser and a conventional MZM and UTC-PD. The QD laser demonstrated excellent properties of equal frequency spacing, which greatly reduced the system complexity and device noise.  
In addition, the comb lines produced by QD lasers were very flexible and tunable and were modulated by a DD-MZM. The EOM-OFCs involved an 11 nm spectrum with a 10 dB bandwidth. The frequency interval OFCs of 75, 37.5 and 25 GHz had 28, 58 and 84 flat spectral lines, respectively. The OSNR of the comb lines was more than 37 dB. The SSB phase noise of the MMW signals generated was \(-106\) dBc/Hz and \(-115\) dBc/Hz at  
Figure 4: Electrical spectra of the K-band and V-band at different intervals as measured from the MMW frequency synthesizer. (**a**) MMW generated from the 25 GHz OFC; (**b**) MMW generated from the 37.5 GHz OFC.  
Figure 5: Single sideband (SSB) phase noise of the comparisons of the 25, 37.5, 50 and 75 GHz generated from different OFCs.  
1 kHz offset for 37.5 GHz and 25 GHz. The results show that the linewidth of the MMW frequency synthesized based on the combs was less than 273 Hz.  
The MMW signal was generated by the optical comb lines beating each other in the UTC-PD. The MMW signal frequency depended on the frequency interval of the comb lines, and the power was related to the power of the beating lines. Thus, the difference of power was obtained for the different frequencies, related to the beating frequency generation mechanism.  
In this work, a flexible MMW frequency synthesizer was demonstrated. If the DD-MZM is adequately modulated by adequate power and appropriate bias voltage, tunable frequency synthesis from 7.5 to 75 GHz can be achieved by this scheme, involving from the K-band to V-band MMW. Experiments showed that the technology is simple in generating high-quality MMW and suitable for high-capacity 5G/5G+ communication transmission. Small size and ultra-wide broadband OFCs are extensively applied in 5G wireless access, satellite communications and intelligent wireless techniques.  
Conceptualization, L.L., Y.L. and X.Z.; methodology, L.L.; software, L.L.; validation, L.L., Y.L., X.-Z.G.; formal analysis, L.L.; investigation, L.L.; resources, L.L.; data curation, L.L.; writing--original draft preparation, L.L.; writing--review and editing, Y.L. and X.-Z.G.; visualization, L.L.; supervision, L.L.; project administration, L.L.; funding acquisition, L.L. All authors have read and agreed to the published version of the manuscript. This research was funded by in part by the Zhejiang Provincial Natural Science Foundation of China grant number LQ18F010008, the Humanities and Social Science Research Youth Foundation of the Ministry of Education grant number 19 YJC880053 and the Zhejiang Philosophy and Social Science Planning Project grant number 19 NDJC103 YB. The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.