Research On Generation And Transmission Performance Optimization Of Ultra-wideband Signal Based On Microwave Photonics

With the rapid development of the communications industry,the related technologies such as high-definition live broadcast,smart home,industrial automation,navigation and positioning,and unmanned aerial vehicles have brought great convenience to people.Meanwhile,communication technology requires higher channel capacity and transmission rate.Besides,the spectrum competition is getting more intense,and there are few available spectrum resources.Consequently,ultra-wideband(UWB)technology that can be used in short-range wireless communication systems has attracted the attention of academia and industry in recent years.UWB is a short-range high-throughput wireless communication technology defined by the Federal Communications Commission(FCC)of the United States.It has the advantages of no carrier,low interference,anti-multipath propagation,high transmission rate and low power spectral density(PSD),among which the low PSD is the crucial solution to the spectrum competition issue.However,there are many challenges in the development of UWB wireless systems,such as UWB transceiver design,interworking and coexistence between UWB systems and narrowband systems,the extension of UWB transmission distance and so on.The wireless propagation distance of UWB is usually several meters or tens of meters because of the low PSD.As a result,UWB-over-fiber that combines UWB technique and fiber distribution technique has emerged.It can expand the transmission range benefiting from the low-loss property of the fiber and provide uninterrupted service across different networks.In consequence,the UWB signal is desired to be generated in the optical domain,which avoids additional electro-optical conversion and makes it easier to tailor the spectrum of the UWB signal to meet with the FCC mask.However,UWB-over-fiber technology still has several problems,such as high cost and complex structure,and its performance is desired to be improved.In this paper,the research substance is the UWB-over-fiber system based on microwave photonics(MWP).And the research focus is the generation methods of UWB signal using MWP techniques.The research target is increasing the spectral power efficiency,reducing the signal distortion,and improving the system's flexibility and stability.In addition,suitable coding,decoding,modulation and demodulation techniques are studied for improving the transmission performance.In the designed system,the transmission and reception of the UWB signals with high spectral power efficiency,large bandwidth and high rate have been realized.And the long-distance transmission with low loss,low distortion and low bit error ratio(BER)has been realized.The proposed schemes can provide the solutions for various applications of UWB communication technology.The contents and innovations are briefly concluded as follows:1.Centimeter-wave(CMW)UWB signal generators based on external modulation technique and delay-line filter are proposed.Firstly,a dual-drive Mach-Zehnder modulators(DDMZM),a balanced photodetector(BPD)and a tunable optical delay line are used to construct a second-order CMW signal generator.Moreover,a tunable notch can be added at the required frequency by adjusting the optical delay line to alleviate the mutual interference between the narrowband signals and the UWB signal.Subsequently,to accomplish the FCC-compliance and attain a higher spectral power efficiency,a BPD and a dual-polarization quadrature phase shift keying(DP-QPSK)modulator are employed to construct a fourth-order CMW-UWB signal generator.The waveform of the generated CMW-UWB signal is similar to the fourth-order Gaussian pulse,and its spectrum has high power efficiency and great FCC-compliance.By simply controlling the input pulse sequence,pulse position modulated(PPM)and onoff keying modulated(OOK)UWB signal can be attained.The designed CMW-UWB signal generator has great performance to be the signal source in the UWB-over-fiber communication system.2.The millimeter-wave(MMW)UWB signal generators based on stimulated Brillouin scattering(SBS)effect are proposed.Since the SBS effect is used to generate a single-sideband(SSB)optical signal,the generated MMW-UWB signal can overcome the power fading caused by dispersion in fiber,which has a good performance in longdistance transmission.Firstly,a DDMZM and a photodetector(PD)are used to generate a first-order CMW-UWB signal.A dual-parallel Mach-Zehnder modulator(DPMZM)is employed to modulate the CMW-UWB signal to the optical carrier.Then the CMWUWB modulated signal is combined with the local oscillation(LO)modulated signal to obtain the UWB-modulated double-sideband(DSB)optical signal.Subsequently,the gain spectrum and loss spectrum generated by the SBS effect are introduced to amplify and attenuate the optical sidebands to obtain the UWB-modulated SSB optical signal.After beating in the PD,the first-order CMW-UWB signal can be up-converted to the MMW band to generate the first-order MMW-UWB signal.In order to simplify the structure and increase the spectral power efficiency,the DPMZM is replaced by a Mach-Zehnder modulator(MZM),and a PD is removed.Besides,a BPD is introduced to realize first-order differentiation.Eventually,the first-order and second-order MMWUWB signals with high power efficiency can be generated.The designed MMW-UWB generation system can realize three common modulation formats: OOK,pulse amplitude modulation(PAM)and PPM.3.A method of MMW-UWB signal generation based on twice SBS effect is proposed.In this method,a DPMZM is used to produce the UWB-modulated signal and optical sidebands.Then it is input into an optical loop composed of two optical circulators and a length of single-mode fiber to generate twice SBS effect.The carriersuppressed UWB-modulated SSB optical signal is obtained by amplifying the negative second-order optical sideband and suppressing the optical carrier.After photoelectric conversion,the baseband UWB signal is up-converted to the MMW band,and the spectrum of the obtained MMW-UWB signal meets well with the FCC mask.The proposed MMW-UWB signal generator has the potential to be used as the signal source in the UWB radar system.4.A CMW-UWB and MMW-UWB signal generator based on SSB up-conversion technology is proposed.The generator can generate CMW-UWB signal and MMWUWB signal flexibly.The DPMZM is used to generate the carrier-suppressed UWBmodulated signal,an MZM and a tunable optical filter are used to generate the SSB optical signal.Then,these two optical signals are combined and photodetected to generate CMW-UWB or MMW-UWB signals.The differential detection and the adjustment of the power ratio between the optical branches are employed to suppress the low-frequency components in the frequency spectrum.After adjusting the modulation index of the DPMZM,the excess LO component is eliminated successfully.The obtained CMW-UWB and MMW-UWB signals fully meet the requirements of FCC and have high power efficiency.5.The methods of optimizing the transmission performance of the UWB-overfiber system are studied.Suitable multi-pulse modulation technology,coherent reception technology,channel coding and decoding technology are studied for the UWB-over-fiber system.The presented CMW-UWB and MMW-UWB signal generator is used as the signal source.Direct sequence binary phase shift keying modulation and convolutional coding are applied to the transmitter of the UWB-over-fiber communication system.The dispersion and the loss of the optical fiber can be compensated by a chirped fiber Bragg grating and an erbium-doped fiber amplifier,respectively.A coherent receiving technology is applied for demodulation,and the Viterbi decoding is used for channel decoding.The BER of the long-haul transmission of the UWB-over-fiber system is tested.In consequence,it is proved that the proposed UWB signal generator has a great performance in long-haul transmission.The appropriate modulation and demodulation,channel coding and decoding and dispersion compensation technology can effectively improve the transmission performance.