| Microwave photonics(MWP)combines the technologies of microwave and photonics,features the advantages of large bandwidth,multiplexing,low transmission loss and antielectromagnetic interference of photonics technology,and flexibility,reconfigurability of microwave technology.The technique of microwave photonic can address the bandwidth and rate bottlenecks faced by traditional microwave technology.Up to now,the mian research topics of MWP include the generation,transmission,receiving and processing of microwave signal based on photonic technique,which are supposed to promote the technological evolution and performance improvement of the progressing electronic information systems such as radar,electronic warfare,and communications.For the research status and problems in the field of MWP,this thesis focuses on two aspects of the photonic generation of microwave signal and the transmission of radio over fiber(Ro F).On one hand,the generation methods of diverse,tunable microwave signal are explored,on the other hand,the low frequency efficiency in digital Ro F transmission system for mobile fronthaul is researched.In terms of the photonic generation of microwave signal,a generation scheme of multi-level frequency hopping(FH)signal with FH speed up to GHz is proposed firstly;then,a generation scheme of microwave signal with multiple modulation formats is proposed based on one single optical structure;finally,a microwave signal generation method with flexible modulation format is proposed,and the modulation format of the generated signal can be tuned digitally.With respect to the transmission of Ro F,a system of digital Ro F downlink is built,and a compression quantization algorithm using two-dimensional discrete cosine transform and vector quantization(2D-DCT-VQ)is proposed,the spectral efficiency of the transmission system can be greatly improved.Specifically,the main research contents of this thesis are as follows:(1)A generation scheme of multi-level FH microwave signal is designed based on polarization digital modulation and analog and digital mixed intensity modulation.Using the digital modulation characteristics of the polarization modulator(Pol M),an optical switch module can be formed by cascading a Pol M,a polarization controller(PC)and a polarization beam splitter(PBS);Utilizing the modulation characteristic of intensity modulator(IM),a digital signal and a radio frequency signal are applied to the IM in the meantime,an FH module can be formed by cascading two IMs;A multi-level FH microwave signal can be generated by connecting the optical switch modules and the FH modules in series and parallel.By controlling the parameters of the electronic signals applied to the modulators,the carrier frequency and the FH speed of the generated signal can be tuned precisely.The availability of the proposed scheme is verified by the VPI simulation and experimental results,the tunability of the generated signal and the factors affecting the quality of the generated signal are also analyzed.(Chapter 3,IEEE Photon.J.2019)(2)A photonic generation scheme of microwave signal with several kinds of modulation formats is proposed.Based on the essential principles of optical heterodyning to generate binary modulated microwave signal,designing a phase-coded optical frequency comb(OFC)through a Mach-Zehnder interferometer(MZI),the frequency tones of the OFC located at different points of the MZI transmission response curve will present or not according to whether the adjacent two bits are the same.An optical tunable filter is placed after the MZI,microwave signal with different modulation format can be generated by optical to electrical conversion while the specific frequency tones are filted.In the experiments,while the free spectral range(FSR)of the MZI is 10.7 GHz,2ASK/2PSK signals with carrier frequencies of24.075 and 29.425 GHz,and 2FSK signals with carrier frequencies of 10.7/21.4 and 21.4/32.1GHz are generated at the bit-rate of 10.7 Gbps;while the FSR of the MZI is 12.8 GHz,2ASK and 2PSK signals with carrier frequencies of 16 and 35.2 GHz respectively,and 2FSK signal with carrier frequencies of 16/35.2 GHz are generated at the bit-rate is 12.8 Gbps.(Chapter 4,Opt.Express 2017)(3)A microwave signal generation scheme with flexible and fast tuning of modulation format is proposed based on a single polarization-division-multiplexed dual-driving MachZehnder modulator(PDM-DDMZM).The PDM-DDMZM is composed of two DDMZMs in parallel.For a DDMZM,a radio frequency signal and a digital signal are applied for analog and digital mixed modulation,and microwave signal with amplitude or phase modulation can be achieved by controlling the pattern of the digital signal;by utilizing another digital signal,the light wave can be controlled digitally to go through the upper or the lower DDMZM of the PDM-DDMZM,microwave signal with frequency modulation can be realized when the carrier frequencies of the radio frequency signal applied two DDMZM is different.Microwave signals with three different modulation formats of 2ASK,2PSK and 2FSK can be generated by manipulating the combination of the two digital signals.And the modulation formats can be switched to each other flexibly and quickly with the switching speed up to GHz.In the experiments,2ASK,2PSK and 2FSK hybrid modulation signals with different modulation rates and different carrier frequencies are generated.(Chapter 4,IEEE Photon.J.2020)(4)For the application scenario of 5G mobile fronthaul,a compression quantization scheme of 2D-DCT-VQ is proposed to improve the spectral efficiency in digital Ro F transmission system.Appling the 2D-DCT to the orthogonal frequency division multiplexing(OFDM)signal samples,the frequency coefficients after transforming is concentrative.The frequency coefficients center on a range with low-frequency components and the tail high-frequency components are almost zero.Discarding the segment which is almost zero,the amount of data can be greatly reduced.Subsequently,vector quantization using 2D k-means clustering algorithm is implemented,which can properly quantify the entire data according to the distribution of the input signal and optimize the quantization noise.A 20 km 5 GBaud/λ PAM-4 optical transmission link with intensity modulation/direct detection is built to verified the proposed scheme.The experiments show that,the transmission performance of the proposed2D-DCT-VQ scheme is comparable to that of the traditional scheme,and the effective transmission rate of the system is improved by 69.49%.(Chapter 5,Opt.Express 2021)... |
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