Xiaojun Jin

Performance Improvement of Phase Modulation with Interferometric Detection Through Low-biasing

The key advantage of using phase modulation (PM) instead of intensity modulation (IM) in radio-over-fiber (RoF) link is that no bias circuitry is needed. However, proper demodulation is important for the PM links performance. In this paper, PM with interferometric detection is low biased to get better performance. The theory of this scheme is fully investigated, including the expressions for link gain (G), noise figure (NF) and spur-free dynamic range (SFDR). Both the theory calculation and simulation results verify that the performance improvement of PM RoF link can be realized by tuning the laser wavelength without complicating the system configuration.

Simultaneously Realizing PM-IM Conversion and Efficiency Improvement of Fiber-Optic Links Using FBG

A simple and low-cost method for simultaneously realizing phase-to-intensity modulation conversion and efficiency improvement of fiber-optic links is proposed and demonstrated by utilizing a fiber Bragg grating (FBG). Optical phase modulation (PM) is converted to intensity modulation (IM) by reflecting the phase-modulated light at the slope of the FBG reflection spectrum. As the optical carrier is suppressed by the FBG, the modulation depth can be enhanced at the same time. Experimental results show that the effective optical PM-IM conversion is realized and an additional 5.5 dB electrical signal gain can be obtained with appropriate suppression of the optical carrier.

ULTRA-WIDEBAND BANDPASS FILTER WITH NOTCHED BAND BASED ON ELECTROOPTIC PHASE MODULATOR AND PHASE-SHIFT FIBER BRAGG GRATING

A novel tunable ultra-wideband (UWB) bandpass filter (BPF) with narrow notched band based on photonic method is proposed. The filter is mainly composed of an electrooptical phase modulator (EOPM) and a phase shift fiber Bragg grating (FBG). As the FBG serves as a tilt filter with limited bandwidth and a π-shift, the optical phase-modulation to intensity-modulation conversion induced by the reflection of the FBG generates a bandpass radio-frequency response with narrow notched band. Both theoretical analysis and experimental results confirm the feasibility of the proposed approach.

Microwave Frequency Measurement Based on Phase Modulation to Intensity Modulation Conversion using Fiber Bragg Grating

A simple photonic technique for instantaneous microwave frequency measurement using an optical phase modulator and a fiber Bragg grating (FBG) is proposed and investigated. The FBG acts as tilt filters to offer different slopes of suppression for the optical carrier and the two sidebands when wavelengths are located at different positions of the FBG. A fixed relationship between the microwave frequencies and microwave powers is established. The microwave frequency can be estimated by measuring the microwave power differences. A measurement resolution of ±0.22 GHz over a 15 GHz measurement bandwidth is achieved. Experiments are performed to verify the effectiveness of the proposed approach.

Adaptive median threshold algorithm used in FDIS of DSSS receivers

For direct sequence spread spectrum (DSSS) receivers, the capability of rejecting narrow-band interference can be significantly improved by a process of frequency-domain interference suppression (FDIS). The key issue of this process is how to determine a threshold to eliminate interference in the frequency domain, which has been extensively studied. However, these previous methods are tedious or very complex. A simple and efficient algorithm based on medians is proposed. The elimination threshold is only related to the median by a scale factor, which can be obtained by the numerical analysis. Simulation results show that the algorithm provides excellent narrow-band interference suppression while only slightly degrading the signal-to-noise ratio (SNR). A one-pass algorithm using logarithmic segmentation is further derived to estimate medians with low computational complexity. Finally, the FDIS is implemented in a field programmable gate array (FPGA) of Xilinx. Experiments are carried out by connecting the FDIS FPGA to a DSSS receiver, and the results show that the receiver has an effective countermeasure for a 60 dB interference-to-signal ratio (ISR).

Method of adaptive PLL bandwidth adjustment without phase slipping

In a system based on the phase lock loop(PLL), a trade-off must be made between the tracking precision and the dynamic performance if constant parameters are adopted. To overcome this drawback, a new method called no phase slipping adaptive bandwidth(NPS-AB) is proposed, which can adjust the loop bandwidth adaptively for different working conditions. As a result, both the tracking precision and the dynamic performance can be achieved concurrently. NPS-AB has two features to keep the loop stable: one is the capability of quick response to dynamics; the other is a series of additional constraints when the bandwidth is switched. Compared with other methods, there is no phase slipping during the adjustment process for NPS-AB. The phase integer ambiguity can be avoided and the phase value is kept valid. It is meaningful for carrier ranging systems. Simulation results show that NPS-AB can deal with sudden dynamics and keep the pseudo-range value stable in the entire dynamic process.

Effect of chip rate on the ranging accuracy in a regenerative pseudo-noise ranging system

The ranging accuracy of a pseudo-noise ranging system is mainly decided by range jitter and time delay discrimination. Many factors can affect the ranging accuracy, one of which is the chip rate. In digital signal processing, the time delay discrimination and autocorrelation function of sampled ranging sequences of different chip rates are very different. An approximation simulation model is established according to an in-phase quadrature (I/Q) correlator which is used to evaluate the time delay. Simulation results of the range jitter and time delay discrimination show that the chip rate which provides a non-integer sample-to-chip rate ratio can achieve a higher ranging accuracy, and some test results validate the simulation model. In some design missions, the simulation results may help to select an optimum sample-to-chip rate ratio to satisfy the design requirement on the ranging accuracy.

PN Ranging Based on Noncommensurate Sampling: Zero-Bias Mitigation Methods

This paper proposes two methods to mitigate the zero bias of noncommensurate sampling based code-tracking loops to significantly improve the pseudonoise ranging accuracy. For the compensation-based method, a set of algorithms is developed to directly calculate the zero bias which is then removed from the range measurement. For the compensation-free method, a special category of sampling ratios are selected such that the zero bias is self-cancelled. Simulations validate the effectiveness of the proposed methods.