In this paper, a novel pulse-agile waveform design based on random frequency modulation (RFM) waveforms is proposed for range sidelobe suppression and range ambiguity mitigation to improve the system performance of radar detection and imaging. The time-frequency relationships of the proposed RFM waveforms are characterized by random Fourier coefficients, which are different from other RFM waveform models. To determine the number and the range of random Fourier coefficients, an RFM waveform design framework based on the theoretical derivation of spectral distribution is presented. The proposed framework has the ability to generate spectral-compact pulse-agile RFM waveforms with high efficiency even under a large waveform number and a large time-bandwidth product. The designed RFM waveforms have random sidelobe, good quasi-orthogonality, high spectral compactness, and constant amplitude. By exploiting the advantage of pulse-agile RFM waveforms, range sidelobes can be reduced by the plural accumulation of the pulse compression results of multiple RFM waveforms. Meanwhile, range ambiguity can be mitigated using different filter banks for the corresponding range intervals. Compared with other low sidelobe quasi-orthogonal waveforms, the designed pulse-agile RFM waveforms using random Fourier coefficients perform better in peak-to-sidelobe level ratio (PSLR) and cross correlation peak (CCP) levels with a large waveform number. The PSLR and the average CCP level for a thousand RFM waveforms are -49.0 dB and -25.8 dB, respectively. Finally, a series of simulations and experiments of microwave imaging that transmit pulse-agile RFM waveforms are conducted, which demonstrate the ability of range sidelobe suppression and range ambiguity mitigation.

A microwave photonic (MWP) radar system with improved signal-to-noise ratio (SNR) performance is proposed and experimentally demonstrated. By improving the SNR of echoes through properly designed radar waveforms and resonant amplification in the optical domain, the proposed radar system can detect and image weak targets that were previously hidden in noise. Echoes with a common low-level SNR obtain high optical gain and the in-band noise is suppressed during resonant amplification. The designed radar waveforms, based on random Fourier coefficients, reduce the effect of optical nonlinearity while providing reconfigurable waveform performance parameters for different scenarios. A series of experiments are developed to verify the feasibility of the SNR improvement of the proposed system. Experimental results show a maximum SNR improvement of 3.6 dB with an optical gain of 28.6 dB for the proposed waveforms over a wide input SNR range. From a comparison with linear frequency modulated signals in microwave imaging of rotating targets, significant quality enhancement is observed. The results confirm the ability of the proposed system to improve SNR performance of MWP radars and its great application potential in SNR-sensitive scenarios.

The difficulty in designing quasi-orthogonal radar waveforms is to ensure low levels of autocorrelation sidelobe peak (ASP) and cross-correlation peak (CCP) under a large number of waveforms. This letter proposes a waveform model with reciprocating nonlinear frequency modulation (RNLFM) for the quasi-orthogonal waveform design. The RNLFM waveform both enjoys the low sidelobe characteristics of nonlinear frequency modulation (NLFM) and high degrees of freedom from reciprocating frequency modulation. A greedy optimization framework is presented to decompose the waveform design problem into two optimization problems that minimize ASP and CCP levels, respectively. Simulation results show that the ASP and the CCP levels of the optimized RNLFM waveforms are reduced by 5.5~9.5 dB and 0.2~2.4 dB respectively, compared with other quasi-orthogonal waveforms under the same waveform number.

A signal-to-noise ratio (SNR) improvement method for microwave photonic (MWP) links enhanced by optical injection locking (OIL) and channelized spectrum stitching (CSS) is investigated and experimentally demonstrated. By exploiting the resonant amplification characteristics of OIL, both optical gain and in-band noise suppression of the input radio frequency signal can be achieved. The injection bandwidth is channelized to further suppress noise during OIL, and the input signal can be well reconstructed by spectrum stitching in the digital domain. Experimental results show that the optimal improvement in SNR of 3.6 dB is achieved for linear frequency modulated signals and at least an additional improvement of 7.2 dB can be obtained by adopting CSS. Other broadband signals for radar and communication are used to further verify the ability to improve SNR. The potential for application scenarios with large operating bandwidth and high optical gain is also demonstrated.

Fiber transfer delay (FTD) variations influence the coherence of distributed radars with fiber networks, resulting in a performance degradation in target detecting and imaging. To measure and compensate for the variation, a phase-derived ranging based FTD measurement using a composite signal is proposed. The composite signal comprises a sinusoidal component and a linear frequency modulation (LFM) component. As the composite signal passes through a fiber under test (FUT), the sinusoidal component generates a phase shift that corresponds to the FTD. The phase shift can be represented by two parameters: the number of complete periods of 2π that can be estimated by using the LFM component, and a phase shift less than 2π that be measured employing the sinusoidal component. When using the proposed measurement system to measure FTD variations in a distributed radar, only an additional sinusoidal component is needed, which minimizes interference with radar signals. Moreover, the proposed measurement system can share core function modules such as signal generation and process modules with distributed radars, which enhances the compatibility and reduces the overall complexity. Experiments are carried out to measure a variable optical delay line and a long optical fiber. The experiment results verify the feasibility of the measurement system and show that a measurement range of more than 15 km, an accuracy of ±0.1 ps and a measurement time of 105 ms can be achieved.