###### Abstract  
This paper presents a new scheme of a cost-effective tunable millimeter-wave (MMW) frequency synthesizer based on an ultra-wideband electro-optic frequency comb. The architecture for the quasi-tunable millimeter-wave frequency synthesizer mainly consists of a compact ultra-wide flat electro-optic frequency comb and a multi-tone frequency generator, which only includes a quantum dot mode-locked laser, a LiNbO\({}_{3}\) dual-driving Mach-Zehnder modulator (DD-MZM) and Uni-traveling-carrier photodiode (UTC-PD). MMW signals generated with a quasi-tunable frequency are experimentally demonstrated. The difference in power is obtained for the different frequencies. The linewidth of the quasi-tunable frequency signals is less than 273 Hz. In addition, the single side band (SSB) phase noise of the 25, 37.5, 50 and 75 GHz is measured as \(-115\), \(-106\), \(-102\) and \(-95\) dBc/Hz at an offset of 1 kHz, respectively. The proposed frequency synthesizer has ultra-low phase noise, quasi-tunable frequency and simple structure. The research results of the frequency synthesizer are applied for 5G+ transmission with radio wave working at K-band and V-band. The flexible, compact and robust MMW frequency synthesizer is suitable for the future of ultra-high capacity 5G+ communication. millimeter-wave generator; ultra-wide electro-optic comb; frequency synthesizer; dual-driving Mach-Zehnder modulator; quantum dot mode-locked laser; high capacity 5G communication  
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and size of the 5G/5G+ transmitter. The generation of various frequencies within the comb lines also allows for high-fidelity frequency transfer to different frequency domains, such as the microwave, MMW and terahertz domains [5]. The latest advances in the MMW generation based on OFC mainly include micro resonator-based Kerr frequency combs [6], high nonlinearity fiber combs [7], electro-optic combs (EO-combs) [8] and mode-locked laser combs [9]. Due to their simple optical architecture and offering wide band and flexible tuning of the spectrum interval [10], electro-optic frequency combs based on external modulation have been revisited in recent years primarily in the context of arbitrary optical waveform generation, high bit-rate optical communication and MMW photonics synthesizers [11]. These generators based on electro-optic modulators (EOMs) have some advantages, including their frequency interval and tunable central-frequency, which are important for applications in telecommunication and spectroscopy [12]. In [13], a novel method to generate a MMW signal was proposed, which employed a uniform fiber Bragg grating based acousto-optic tunable filter (FBG-AOTF) and a DD-MZM with frequency multiplication factors of 2 and 4, paying for a large power penalty. To achieve higher frequency multiplication factors, external modulation techniques employ cascaded electro-optic modulator (EOM) to produce more harmonically related optical modes. In [14], a frequency 16-tupling technique was demonstrated using four cascaded MZMs. With the proper adjustment of the DC bias, the amplitude and phases of the RF local oscillators, an MMW signal with a frequency of 16 times the input RF signals is demonstrated. Unfortunately this method is limited to big RF power and a big power penalty. According to the intrinsic photoelectric characteristics of the EOM, over-driving of conventional electro-optic modulators is provided for EO-comb generation [15]. However, optical comb lines suffer from inherent phase instabilities and big power deviations, leading to excess multiplication of MMW phase noise, which limits their usage in 5G+ transmission [16]. A range of EO-combs can be explored to generate directly or through upconversion of MMW signals [17] but suffer from high phase noise and are not as spectrally pure as photonic based sources [18; 19].