Tao Jin

Compressive Sensing Based Coprime Array Direction-of-Arrival Estimation

A coprime array has a larger array aperture as well as increased degrees-of-freedom (DOFs), compared with a uniform linear array with the same number of physical sensors. Therefore, in a practical wireless communication system, it is capable to provide desirable performance with a low-computational complexity. In this study, the authors focus on the problem of efficient direction-of-arrival (DOA) estimation, where a coprime array is incorporated with the idea of compressive sensing. Specifically, the authors first generate a random compressive sensing kernel to compress the received signals of coprime array to lower-dimensional measurements, which can be viewed as a sketch of the original received signals. The compressed measurements are subsequently utilised to perform high-resolution DOA estimation, where the large array aperture of the coprime array is maintained. Moreover, the authors also utilise the derived equivalent virtual array signal of the compressed measurements for DOA estimation, where the superiority of coprime array in achieving a higher number of DOFs can be retained. Theoretical analyses and simulation results verify the effectiveness of the proposed methods in terms of computational complexity, resolution, and the number of DOFs.

Sub-Nyquist Sampled Analog-to-Digital Conversion Based on Photonic Time Stretch and Compressive Sensing With Optical Random Mixing

An approach to realizing wideband analog-to-digital conversion based on the techniques of photonic time stretch (PTS) and compressive sensing (CS) is proposed. In the system, a multitone signal within a wide bandwidth (spectrally sparse) signal is slowed down in the time domain by a photonic time stretcher. The stretched signal is then down-sampled and reconstructed by a random-demodulator-based CS scheme, in which random mixing is realized in an optical domain. Thanks to the techniques of PTS and CS, wideband spectrally sparse signals can be acquired with a sampling rate far below the Nyquist rate of the original signal. The optical random mixing applied in the system has the advantages of lower distortions and larger bandwidth compared to its electrical counterpart. In order to construct a Gaussian measurement matrix with zero mean, balanced detection is applied after the optical mixer. In addition, in order to eliminate the dc component and the even-order harmonics of the stretched signal, we propose to use balanced PTS technique in the system. We demonstrate that a system with a time stretch factor 20 and a compression factor 4 can effectively acquire a spectrally sparse wideband signal, which means a sampling rate as low as 1/80 of the Nyquist rate.

Harmonics analysis of the photonic time stretch system

Photonic time stretch (PTS) has been intensively investigated in recent decades due to its potential application to ultra-wideband analog-to-digital conversion. A high-speed analog signal can be captured by an electronic analog-to-digital converter (ADC) with the help of the PTS technique, which slows down the speed of signal in the photonic domain. Unfortunately, the process of the time stretch is not linear due to the nonlinear modulation of the electro-optic intensity modulator in the PTS system, which means the undesired harmonics distortion. In this paper, we present an exact analytical model to fully characterize the harmonics generation in the PTS systems for the first time, to the best of our knowledge. We obtain concise and closed-form expressions for all harmonics of the PTS system with either a single-arm Mach-Zehnder modulator (MZM) or a push-pull MZM. The presented model can largely simplify the PTS system design and the system parameters estimation, such as system bandwidth, harmonics power, time-bandwidth product, and dynamic range. The correctness of the mathematic model is verified by the numerical and experimental results.

Time-Frequency Uncertainty in the Photonic A/D Converters Based on Spectral Encoding

In recent years, some approaches to realize photonic analog-to-digital conversion based on spectral encoding have been proposed. In these approaches, the amplitude information of input analog signals is first converted into the frequency information of the optical carrier via a nonlinear process, which is equivalent to optical frequency modulation; then, the spectral information of the optical carrier is quantized and encoded with the help of an optical filter group. In this letter, we propose, for the first time to our knowledge, that the time-frequency uncertainty principle puts a fundamental limit on this type of photonic analog-to-digital converters (ADCs). The condition for achieving acceptable performance in terms of effective number of bits is discussed. The presented theoretical results are verified by numerical simulations and a proof-of-concept experiment in electrical domain. We believe that the presented result is important to the design of spectral-encoding-based photonic ADCs.

All-positive-coefficient microwave photonic filter with rectangular response

In conventional approaches to realizing microwave photonic filters (MPFs) with rectangular response, both positive and negative coefficients should be included in the systems, which is a difficulty in incoherent MPFs. Electrical or optical ways to realize the MPFs with bipolar taps, such as those using balanced detectors or based on two opposite modulation slopes of dual-input Mach-Zehnder modulators, usually involve more components compared with the MPFs with unipolar taps. In this Letter, a design of MPFs with rectangular response using all positive coefficients is proposed. There is only one optical link as well as one photodetector utilized in the approach, which largely simplifies the filter structure. Experimental demonstrations of 55-positive-tap MPFs with different passbands are presented to verify the feasibility and potential of the approach.

Photonics-assisted compressive sensing for sparse signal acquisition

Compressive sensing (CS) is a novel technology for sparse signal acquisition with sub-Nyquist sampling rate but with relative high resolution. Photonics-assisted CS has attracted much attention recently due the benefit of wide bandwidth provided by photonics. This paper discusses the approaches to realizing photonics-assisted CS.

Characterization of the photonic generation of phase-coded RF signals based on pulse shaping and frequency-to-time mapping

Photonic generation of arbitrary waveforms based on pulse shaping and frequency-to-time mapping is of great interest since it offers an effective way to generate complex signals with large bandwidth. In this paper, we mathematically study the photonic generation of phase-coded radio frequency (RF) signals based on pulse shaping and frequency-to-time mapping in detail. By mathematically analyzing the process of signal generation and utilizing the less restrictive dispersion requirement, a design criterion for the generation of well-shaped phase-coded RF waveforms is presented, by which we can properly set the spatial light modulator in the pulse shaper and dispersion in the system according to the carrier frequency and modulating frequency of the desired waveform. In addition, the maximum achievable time-bandwidth product of phase-coded RF signals that can be generated is estimated. The theory is verified by numerical and experimental results. The presented results can be used to guide the design of a photonic system for the generation of high-quality phase-coded RF signals.

Analysis of compressive sensing with optical mixing using a spatial light modulator.

Compressive sensing (CS) in a photonic link has a high potential for acquisition of wideband sparse signals. In CS it is necessary to mix the input sparse signal with a pseudorandom sequence prior to subsampling. A pulse shaper with a spatial light modulator (SLM) can be used in photonic CS as an optical mixer to improve the speed of mixing. In this approach, the sparse signal is modulated on a chirped optical pulse and the pseudorandom sequence is recorded on the SLM within the pulse shaper. The optical mixing in the frequency domain is realized based on the principle of frequency-to-time mapping. In this paper, we investigate the performance and limitations of photonic CS with an SLM in detail. A theoretical model to describe optical mixing based on frequency-to-time mapping is presented. We point out that there is an upper limit on the length of the pseudorandom sequence recorded on the SLM that can be mixed with the sparse signal due to the condition of the far-field approximation of the frequency-to-time mapping. Since the length of the pseudorandom sequence is one of the major factors that affect the signal recovery performance in CS, this limitation should be fully considered in the system design of the CS with optical mixing in the frequency domain. We present numerical and experimental results to verify the theoretical findings. Discussion on the performance improvement is also presented.

Photonics-enabled compressive sensing with spectral encoding using an incoherent broadband source

In this Letter, we propose an approach to achieving photonics-enabled compressive sensing of sparse wideband radio frequency signals in which an incoherent broadband source is applied, and the mixing and integration functions are realized in the optical domain. A spectrum shaper is employed to slice and encode the spectrum of the broadband light according to a predetermined random sequence. Because of the dispersion-induced group delay, the mixing between the incoming signal and the random bit sequence is achieved. At the output of the spectrum shaper, an array of photodetectors is employed to realize down-sampling, and the input sparse signal can be captured in a single-shot mode. Since no pulsed laser is employed, our scheme obviates the need for time-domain synchronization between the repetitive ultra-fast pulses and the random sequence. Experimental demonstrations and numerical results are presented to verify the feasibility and potential of the approach.

Improving the performance of the injection-locked optoelectronic oscillator by using an extra feedback loop

Abstract. We describe and demonstrate a practical method for improving the performance of an injection-locked optoelectronic oscillator (IL OEO). An improved IL theory model, which helps to set the optimal locking range and reduce the phase noise of the OEO, is proposed. The IL OEO with an extra feedback loop exhibits good long-term operating stability and can produce a 10 GHz output signal with 80 dB side-mode suppression ratio and better than ±0.0005u2009u2009ppm frequency drift within 12 h. The presented results can be used to guide the design of high-quality OEOs.