Pre-distortion compensation for optical-based broadband LFM signal generation system

In this paper, we propose an integrated method for windowing and matched filtering in the analog domain based on microwave photonic technology, which utilizes dispersion regulation of optical waveguide to achieve the windowing processing of broadband signals in the optical domain and the surface acoustic wave filter (SAWF) to achieve the following matched filtering processing in the radio frequency (RF) domain, thus realizing their integration processing in the analog domain. The proposed method is validated by simulation and experiment, in which the integrated processing of matched filtering and windowing in the analog domain for a linear frequency modulation (LFM) signal with a bandwidth of 1 GHz is carried out and the peak to sidelobe ratio (PSLR) of the output signal is -19.55 dB and the mainlobe width (MLW) broadens to 0.16 ns with the maximum achievable dispersion value. The proposed method can significantly reduce the pressure of signal processing at the back-end of the radar system and improve the response speed of the broadband radar system in detecting the targets.

A lidar system for simultaneous detection of distance, velocity and direction of a moving target is proposed and experimentally demonstrated based on an optical multi-chirped LFM signal generator. The optical multi-chirped linearly frequency-modulated (LFM) signal generator is realized by an optical frequency shifting loop (OFSL) that is driven by an electrical LFM signal. After the OFSL, a sequence of optical LFM signals with incremental chirps are generated. The signal is then emitted into the space and captured again after being reflected by a target. A lidar system is thus realized. An experiment is carried out. A sequence of LFM signals with chirps incremented from 0.389 to 2.713 GHz/s are generated. A distance of 19.1 m and velocities of -2.203 m/s and 4.170 m/s are measured based on a lidar system built based on the LFM signal.

For pulse compression characteristics and not easily affected by noise, linear frequency modulation signal are widely used in underwater acoustic communication. This paper analyzes the characteristics of hyperbolic frequency modulation signal over underwater acoustic channels. Compared with linear frequency modulation signal, hyperbolic frequency modulation has the same performance of strong anti-noise and anti-multipath, what’s more, hyperbolic frequency modulation signal is better resist the influence of doppler. And discussed the influence of doppler on signal, simulation results show that the hyperbolic frequency modulation signal detection rate is better than linear frequency modulation signal in the doppler environment. DOI: http://dx.doi.org/10.11591/telkomnika.v11i3.2183 Full Text: PDF

In this letter, we experimentally demonstrate multi-band linear frequency modulation (LFM) signal generation subjected to multi-input optical injection in an external cavity-based Fabry-Pérot laser diode (ECFP-LD). The ECFP-LD is a specially designed FP-LD with a self-injected single longitudinal mode tuned up to 10 nm. The non-linear dynamics in period-one oscillation in ECFP-LD subjected to multi-input optical injection shows unique characteristics for multi-band linear frequency modulation. In the proposed scheme, four external input beams are injected to ECFP-LD, among which one is power modulated with the saw-tooth like waveform, and others are with the constant power. The varying power beam red-shifts the self-injected mode of ECFP-LD and mode-hopping on attaining a particular power level. Hence, multi-band LFM signals are generated by beating self-injected and multi-input injected beams with a large time-bandwidth product (TBWP). The total bandwidths of 12GHz (LFM1: 33-36 GHz, LFM2: 29-32 GHz, LFM3: 25-28 GHz, LFM4: 22-25 GHz) for four-band linear frequency modulation signals with a TBWP of 12000 is measured. The generated multi-band LFM signal provides flexibility on central frequency tunability and dynamic chirp rate for a multifunctional radar system.

Linear frequency-modulated continuous wave (LFMCW) is applied in continuous active sonar (CAS). Methods for echo detection and direct path interference (DPI) suppression are studied and evaluated by simulation and sea trial data. To detect the LFMCW echo, the received signal is first demodulated by the transmitting signal, resulting in a de-chirped signal, which is similar as in LFMCW radar. However, in active sonar case the target velocity is no longer negligible comparing with the sound velocity. Thus the de-chirped signal cannot be simplified as a CW signal. Instead, it is a LFM signal, with its sweep rate and center frequency related with the velocity and distance of the target. Time-frequency analysis methods which are generally used to detect and estimate LFM signals can then be used in LFMCW active sonar. In this work, the Fractional Fourier Transform (FRFT) is chosen. Simulations and experiments show that LFMCW active sonar with the de-chirp-FRFT receiver can get much higher processing gain than conventional LFM pulsed active sonar with a matched filter (MF) receiver. To reduce the direct path interference (DPI) from the transmitter, bi-static mode is preferred. DPI is eliminated by filtering the de-chirped signal in the frequency domain. The major drawback of this method is that targets close to the transmitter will be lost. The disadvantage of the de-chirp-FRFT algorithm is that the processing results can be obtained only once every long work cycle, which is not propitious to target tracking. The future work will be focused on the improvement of refresh rate.

The linear frequency modulated signal is widely used in radar systems, and two methods, which are DDS and DDWS, are mainly used to generate the baseband signal. This system adopts the DDWS method, based on FPGA parallel theory and high-speed DAC framework, to generate the broadband linear frequency signal with bandwidth of 500MHz. The principle of filter-based pre-distortion compensation presented in the paper is used for the amplitude and phase compensation of system, and the ration of the main lobe to the side lobe after pulse compression has a significant improvement. Detailed steps about the implementation are given in this paper. (4 pages)

A novel automatic bias control (ABC) is proposed to generate arbitrary signals with a dualparallel Mach-Zehnder modulator. We demonstrated the practical generation of arbitrary modulated signals using the proposed scheme.

It is shown that synthetic-aperture radar (SAR) motion can be compensated by using an antenna-mounted strapdown inertial measurement unit (IMU) as the motion sensing system, but sensor and system errors affect SAR image quality. A strapdown IMU consists of three accelerator channels and three gyro channels. Strapdown IMU errors include gyro-scale and accelerometer-scale factor and bias errors, velocity error, platform tilt, and errors induced by limited inertial sensor bandwidth. The effects of these errors on the SAR image quality are presented in terms of the SAR impulse response. IMU errors that cause low-frequency phase errors (less than one cycle per array time) are categorized in terms of quadratic and cubic phase errors. IMU errors that cause high-frequency phase errors (greater than one cycle per array time) are categorized in terms of the integrated sidelobe ratio and peak sidelobe ratio. A motion compensation system conceptualization is described wherein a strapdown IMU is attached to an antenna and transfer-aligns to the aircraft's master navigator.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A linear frequency modulation (LFM) signal and its corresponding de-chirp operation are one of the basic methods for wideband radar signal processing, which can reduce the burden of the radar system sampling rate and is more suitable for large-bandwidth signal processing. More importantly, most existing methods against interrupted sampling repeater jamming (ISRJ) are based on time–frequency (TF) or frequency domain analysis of the de-chirped signal. However, the above anti-ISRJ methods cannot be directly applied to multiple-input multiple-output (MIMO) radar with multiple beams, because the angular waveform (AW) in mainlobe directions does not possess the TF properties of the LFM signal. Consequently, this work focuses on the co-optimization of transmit beampattern and AW similarity in wideband MIMO radar systems. Different from the existing works, which only concern the space–frequency pattern of the transmit waveform, we recast the transmit beampattern and AW expressions for wideband MIMO radar in a more compact form. Based on the compact expressions, a co-optimization model of the transmit beampattern and AWs is formulated where the similarity constraint is added to force the AW to share the TF properties of the LFM signal. An algorithm based on the alternating direction method of multipliers (ADMM) framework is proposed to address the aforementioned problem. Numerical simulations show that the optimized waveform can form the desired transmit beampattern and its AWs have similar TF properties and de-chirp results to the LFM signal.

Signal integration, as an effective method of detecting weak target, is widely used in areas of radar, sonar, etc. In previous studies of long-time coherent integration, researchers usually established a multi-pulse echo model with linear frequency modulation (LFM) signal due to its good compression performance and large Doppler tolerance. Then, perfect analytical formula can be deduced to compensate for range migration and Doppler spread, which is helpful in analyzing the mechanism of long-time coherent integration in depth. However, besides LFM, a wide variety of signal waveforms are also used in modern sonar and underwater guidance system to meet the requirements for diverse applications. For instance, continuous wave (CW) pulse is often used in signal detection, high resolution direction of arrival (DOA) estimation, and velocity estimation, while large time-bandwidth product waveforms such as modulated signal, coded signal, and pseudo-random signal are utilized for special tasks like anti-interference detection, channel matching, and active concealed detection. Therefore, the formulas and corresponding instructive conclusions deduced by LFM have no generality when other sonar waveforms are used in pulse integration. In this paper, we focus on long-time coherent integration for arbitrary signal reflected by underwater target moving with a uniform velocity and propose a motion-compensated coherent integration method for arbitrary complex envelop signal. A kind of general ambiguity function (GAF) for transmitted signal is defined to present a unified expression based on GAF for the output of the matched filter. The operation not only helps us to describe and calculate the pulse compression form of the arbitrary complex envelop by using a general mathematical model, but also provides information about the range migration and Doppler frequency shift of the multi-pulse echo, which is needed in pulse range alignment and FFT integration. For the matched filter output expressed by the GAF, Keystone transform is utilized to correct the complex envelop of the multi-pulse echo and eliminate the range walk. Then, Doppler frequency shift is compensated for by performing FFT transform, and the long-time coherent integration for arbitrary complex envelop is realized. To verify the correctness of the proposed method, we carry out the computer simulation on both signal integration and detection performance by using four sonar waveforms, i.e., CW signal, LFM signal, m-sequence phase-coded signal, and Costas frequency hop coded signal. The simulation results show that the proposed motion-compensated coherent integration method is applicable to arbitrary complex envelop signal. We also design an anechoic water tank experiment scheme which can successfully obtain the multi-pulse echoes of constant moving target. The motion-compensated coherent integration of the experimental data of the above-mentioned four waveforms further validates the effectiveness of the proposed method.

A novel in-band OSNR monitoring technique based on fractional Fourier transform of LFM signal

This paper considers the localization of an emitter with an unknown linear frequency modulation (LFM) signal in the passive radar systems based on the direct position determination (DPD) method. To improve the localization accuracy, a novel strategy, wherein the position of the emitter and the unknown transmitted signal are jointly estimated, is proposed. As a result, a six-dimensional estimation problem is formulated, which is, however, computationally intractable for any general application scenario. By combining the DPD algorithm with short time Fourier transform (STFT), a fast and practical suboptimal algorithm, i.e., DPD-STFT, is also designed here to achieve the joint estimation for the emitter position and the unknown parameters of the LFM signal. Simulation results show that the localization performance of the DPD-STFT method is much better than the existing DPD algorithms which do not estimate the transmitted signal.

A tunable high-order single-sideband signal generator with optical carrier suppression (SSB-OCS generator) is proposed, which is based on frequency multiplication operation using dual-parallel Mach–Zehnder modulator (DPMZM) and Mach–Zehnder interferometer (MZI). By changing all the modulators at maximum or minimum transmission point simultaneously, the amplitude of input radio frequency (RF) signals and the phase difference between input RF signals, two high-order even or odd signals with opposite amplitude of one sideband, are generated by upper DPMZM and lower DPMZM, respectively. Then, two optical signals are coupled to reserve one sideband, and the coupled SSB-OCS signal is injected into an MZI to suppress high-order intermodulation components. The simulated results show that the first-, second-, and third-order SSB-OCS signals can be achieved, and the suppression ratio is over 30 dB, which agrees well with the theoretical prediction. This scheme is featured by the capability to generate tunable high-order SSB-OCS signal with high suppression ratio, and the operation is simple and flexible. Finally, an experiment was carried out to demonstrate the feasibility of the proposed scheme, and a first- and second-order SSB-OCS signal were obtained, respectively, and the suppression ratio is over 24 dB.

Generator of Signals with Quadruple Frequency and Triangular Waveform Tunable in Symmetry Based on Dual-Parallel Mach-Zehnder Modulator and Balanced Photodetector

A photonic scheme generating ternary modulated signals, which include amplitude, phase, and frequency modulated signals, has been proposed and demonstrated. This scheme is based on a dual-polarization dual-parallel Mach-Zehnder modulator (DP-DPMZM), where the X-DPMZM is used to generate an optical frequency comb (OFC), and the Y-DPMZM is utilized to accomplish ternary modulation. The key feature of this scheme is that ternary modulated signals with eight wavebands can be obtained by using only the DP-DPMZM in situations of different input signals. In the simulation, a 2.5 Gbit/s amplitude shift keying (ASK), phase shift keying signal (PSK), or an 8 GHz bandwidth linearly frequency modulated (LFM) signal are obtained. The generated signals have eight wavebands with a large frequency range from 10 GHz to 80 GHz. Moreover, the recovery and pulse compression performances of the generated signals are evaluated.