In this paper, the problems of simultaneously detecting and localizing multiple targets are considered for noncoherent multiple-input multiple-output (MIMO) radar with widely separated antennas. By assuming a prior knowledge of target number, an optimal solution to this problem is presented first. It is essentially a maximum-likelihood (ML) estimator searching parameters of interest in a high-dimensional space. However, the complexity of this method increases exponentially with the number G of targets.Besides, without the prior information of the number of targets, a multi-hypothesis testing strategy to determine the number of targets is required, which further complicates this method. Therefore, we split the joint maximization into G disjoint optimization problems by clearing the interference from previously declared targets. In this way, we derive two fast and robust suboptimal solutions which allow trading performance for a much lower implementation complexity which is almost independent of the number of targets. In addition, the multi-hypothesis testing is no longer required when target number is unknown.Simulation results show the proposed algorithms can correctly detect and accurately localize multiple targets even when targets share common range bins in some paths.
A method to improve the spurious-free dynamic range (SFDR) of analog photonic links has been proposed and experimentally demonstrated, which only consists of a phase modulator (PM), a polarizer and an optical filter. Such structure could compensate for the chromatic dispersion and the nonlinearity of the modulator simultaneously. In addition, by adjusting the states of polarization (SOPs) launching into the PM and the polarizer, the proposed scheme could also be reconfigured to mitigate the second harmonic nonlinearity induced by the photodetector. Experimental results show that the suppressions of the second-order and third-order intermodulation distortions (IMD2 & IMD3) are larger than 14-dB and 25.4-dB, respectively. Furthermore, the SFDR can achieve ~110-dB · Hz(4/5) for 40-km fiber transmission, which is 26-dB higher than that of the link without compensation.
A method to improve the linearity of the analog photonic link is proposed and experimentally demonstrated, which consists of a phase modulator and an optical tunable bandpass filter. By carefully optimizing the bandwidth and center wavelength of the filter, we can significantly suppress the third-order intermodulation distortion by ~32 dB. Subsequently the spurious-free dynamic range of the link is improved by ~14 dB.
A photonics-based multi-band linearly frequency-modulated (LFM) waveform generator with reconfigurable center frequency, bandwidth and time duration is proposed and demonstrated. By introducing two coherent optical frequency combs (OFCs) with a frequency shift and different free spectral ranges (FSRs) as multi-frequency optical LOs, a set of LFM signals with different center frequencies will be generated if one of the combs is modulated by an intermediate-frequency (IF) LFM signal. The center frequencies of the generated RF-LFM signals can be flexibly tuned by adjusting the frequency shift between the two OFCs. In addition, by introducing a series of proper time delays to the LFM signals and combining them, a frequency-stepped LFM signal can be generated. Furthermore, when the bandwidth of the IF-LFM signal equals the difference of the comb FSRs, and the time duration of IF-LFM signal equals the time delay of the consecutive channels, a LFM signal with both bandwidth and time duration multiplied can be obtained. With N comb lines, the maximum achievable timebandwidth product (TBWP) is N × N times of the applied IF LFM signal. A proof-of-concept experiment is carried out. A set of LFM signals with frequencies ranging from L to Ka bands are generated. By introducing proper time delays, a frequency-stepped LFM signal with frequency steps between 10 GHz and 20 GHz is also produced. In addition, LFM signals with the bandwidth and time duration multiplied by 2 and 5 are realized (4-GHz bandwidth, 2-μs time duration and 10-GHz bandwidth, 5-μs time duration), respectively. Correspondingly, the TBWPs are increased by 4 and 25 times.
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