Research On Microwave Photonic Measurement And Processing Of Radio-Frequency Signal

Microwave photonics is an interdisciplinary research between microwave engineering and photonics,which focus on photonic generation,transmission,and processing of radio frequency(RF) signals.With the increase of the information and communication technology(ICT) data traffic,modern wireless systems have been developing towards broadband and high-frequency operation.However,conventional electronics-based microwave and RF technique,which suffers from limited bandwidth,electromagnetic interference(EMI),and large volume,weight,and power consumption,cannot meet the requirement of the nextgeneration wireless systems.Thanks to the inherent merits of wideband operation,low transmission loss,and immunity to EMI,microwave photonic signal measurement and processing are capable of functions that are difficult or even impossible with conventional RF technique.It is promising to be applied to applications such as wireless communication,radar,electronic support measures,and radio astronomy.According to the requirements of the wireless applications and the unique advantages provided by photonics,this dissertation investigates the microwave photonic measurement and processing of RF signals.What follows are the detailed works:Using the polarization-dependent characteristic of Lithium Niobate modulators,we propose an instantaneous frequency measurement technique based on optical phase modulation.When polarization is properly set,simultaneously intensity and phase modulation can be implemented using a single phase modulator.After transmission through a length of dispersive fiber,two optical signals with different modulation formats are sent to two photodiodes,respectively.The resulted RF power ratio and frequency have a unique relationship.Thus,the unknown RF frequency can be fast estimated by calculating the RF power ratio.Experimental results show that measurement errors are less than ±0.3 GHz for a measurement range of 1.6–24.6 GHz.For angle-of-arrival(AOA) measurement of wideband RF signals,we propose an AOA estimation scheme based on microwave photonic filtering.The RF signals from two separate antenna elements are applied to two sub-modulators of a dual-polarization binary phase-keying(DPBPSK)modulator,respectively.Since orthogonally-polarized lights are incoherently detected in photodiodes,a two-tap microwave photonic notch filter can be implemented using a single laser source and one photodiode.Given the antenna element spacing,the unknown AOA can be estimated by measuring the notch position in RF spectrum.Experimental results show that the delay measurement errors are less than ±0.35 ps for a measurement range of-14–16 ps.We propose an RF self-interference cancellation technique based on optical-domain predistortion for in-band full-duplex communication application.Given the self-interference is a prior knowledge,intensity-modulation direct-detection system is modified,where a predistortion light that carries the self-interference is utilized instead of the pure optical carrier.The pre-distortion light is generated by an intensity modulator and a variable optical delay line.The amplitude and time delay matching can be implemented by adjusting the bias voltage of the modulator and the optical delay line,respectively.When the matching conditions are satisfied,the weak signal of interest can be recovered from the strong self-interference after photodetection.Experimental results show that,at the 2.4-GHz frequency band,the system achieves 32.6 dB of cancellation across 100-MHz bandwidth and 57 dB of cancellation for 10-k Hz narrowband interference.Due to the signal-to-signal beating inference(SSBI) resulted from the direct detection locating at low frequency,heterodyne receiver has to choose a proper intermediate frequency(IF).We propose a microwave photonic RF receiver,which can operate at a lower IF,based on Kramers-Kronig(KK)algorithm.The RF signal and the local oscillator(LO) signal are respectively applied to the two null-biased sub-modulators of a dual-parallel Mach-Zehnder modulator(DPMZM),where the main modulator is also biased at null.After direct detection,the resulting signal includes both desired signal and SSBI.KK algorithm is capable of recovering the phase of a minimum-phase signal from its amplitude,thus mitigating the SSBI.Experimental results show that,under the condition of low-IF reception,error vector magnitude(EVM) of the desired signal is 4.31%,while the EVM is 13.1% without KK processing.We propose an RF image-reject frequency down-converter with a cascade-modulator structure for antenna remoting scenario.The RF signal and the LO signal are respectively applied to a phase modulator and a DPMZM.The upper and lower sidebands of the twice-modulated light are separated by an optical de-multiplexer and then sent to two independent channels.When the main bias of the DPMZM is properly set,two down-conversion channels become a pair of in-phase quadrature(IQ)channels.According to Hartley image-reject structure,the image signal can then be canceled by an IF hybrid.This system can be applied to antenna remoting scenario by inserting a length of fiber between the two modulators.Experimental results show that image rejection ratios of 32.5 dB in a 10-MHz instantaneous bandwidth and 51.6 dB for a single frequency can be obtained.