Based On RoF System With High Frequency Millimeter-wave Generation Technology Research

RoF is an attractive solution for future broadband wireless communication systems due to its large capacity and high flexibility for both fixed and mobile users. Millimeter-wave generation is a key technique to realize low cost and high transmission performance in RoF systems. In electrical domain, MMW signals beyond 60 GHz are difficult to generate and process due to restrictions on frequency responses of electronic devices. Hence, all-optical generation and distribution of MMW signals have attracted interest in recent years.So far, three optical techniques have been proposed to implement the generation of mm-wave, namely, optical heterodyne, frequency up-conversion and nonlinear modulation of external optical modulator. Among these techniques, the last one has shown great potential for high purity and low phase noise mm-wave signal generation with low cost and simple configuration. For this technique, mm-wave generation without optical filter is an ideal scheme to further reduce the complexity and cost of system, especially for WDM-ROF system. In addition, fiber dispersion is an important influence factor for the transmission performance in ROF system. On one hand, fiber dispersion will lead to code interference because of walkoff effects. On the other hand, it also can lead to the periodical fading variation of the power of the generated mm-wave signal with transmission distance.The fundamental technique to generate a MMW signal in the optical domain is to heterodyne two optical waves of different wavelengths which beat at a photodetector. The frequency of the MMW signal corresponds to the wavelength spacing of the two-tone-based optical sidebands. In the past few years, numerous techniques have been proposed to generate frequency-multiplied MMW signals. For example frequency-quadrupled, sextupled, or octupled MMW signals could be generated by heterodyning two-tone-based optical sidebands. However, only a handful of approaches have been reported using higher frequency multiplication factor of higher than eight times. To achieve a high FMF, external modulation employing configurational Mach-Zehnder modulator was proposed to produce two sidebands. The sidebands have a frequency spacing corresponding to FMF times the frequency of the microwave drive signal. External modulation employing configurational MZM has good performance of the optical sideband suppression ratio and high spectral purity of the generated MMW signal. However, the major limitation of these techniques are high complexity in frequency tuning since more than two parameters are needed to be adjusted simultaneously as well as bias drifting problem, which would lead to poor stability of the microwave generation system. In general, the proposed approaches with high frequency multiplication factor (FMF>8) are based on the idea of beating two optical sidebands at a PD. The power of the generated high frequency MMW signals is very low, even if an optical power amplifier is used.In this paper, we propose a novel optical multi-sidebands modulation (OMSB) to generate a high-power and high-frequency MMW signals. Several periodical optical sidebands are generated by employing a configuration of parallel phase modulators. Unlike heterodyning two waves, more than two optical sidebands will beat at a PD and contribute to generate a MMW signal with designed frequency. Since no optical filters or DC bias are used, the MMW signals have the evident character of high-power output. In additional, the MMW frequency with octupling,12-tupling,16-tupling or even higher FMF of the local oscillator can be implemented. A generalized analytic expression for the generated frequency multi-tupling MMW signal is developed. Simulation verifications and comparison results are also presented. The influences caused by non-ideal factors are discussed in detail, and undesired power ratios versus non-ideal factors are plotted and analyzed in the following sections. The proposed RoF system have better performance on transmission and anti-dispersion, and it’s more suitable for long-distance transmission.