Coherent Signal Processing for Loosely Coupled Bistatic Radar

Instrumentation grade, coherent, bistatic radar cross section (RCS) measurement systems require a reliable low-noise method to link the reference, local oscillator (LO) and intermediate frequency (IF) coherent signals between the transmit and receive subsystems. One approach to this is the use of a fiber optic link (FOL). Phase noise tests were performed on a distributed feedback (DFB) type laser transmitter-photodiode receiver link with a delay distance of 2.26 kilometers. The measurements were conducted using a standard Hewlett Packard HP 3048 A phase noise test set, and a variety of link configurations at a variety of frequencies from 100 MHz to 5 GHz. Further, system tests have been performed to incorporate a FOL into a coherent bistatic instrumentation radar system local oscillator path, and perform image processing on an emulated target. A Lintek elan system was configured to emulate bistatic operation by separating the transmit and receive local oscillator lines, allowing one of them to be excited through a fiber optic link. A target with one or two scatterers was emulated using a delay line between a sample of the transmit signal and a receiver injection port. This paper describes the theoretical effects of phase noise (close-in and higher frequency) on target scatterer imaging performance, describes the FOL tests and the architecture of the bistatic radar tests performed, and presents the results of the tests, including baseline system performance and performance using the FOL. Images representing closely spaced scatterers are presented, and the point spread function of a long dwell (40 seconds) data file, representing an ISAR process is presented. One of the limiting factors in the use of a FOL for long dwell times is that of the variable delay, caused by variation of the propagation velocity with laser frequency drift (dispersion), and the change in index of refraction with temperature. A method for mitigating the effects of this drift is presented.

With one exception the topics covered in this chapter are unique to bistatic radars. The exception is noise-limited errors associated with measuring the target range sum, doppler, and angle; these errors are the same as for monostatic measurements. For a bistatic radar, these measurements, along with a measure or estimate of transmitter position, are combined in various ways to solve the bistatic triangle and thus estimate the target position, or location, and the error associated with this location estimate. These topics are covered in Sections 5.1 and 5.2. Often, the measurements are taken in a local coordinate system centered at the receiving (or transmitting) site. These measurements usually must be converted to a transmitter-receiver- referenced coordinate system (the North-referenced coordinate system is used in this book) in order to establish target location, as discussed in Section 5.3. When bistatic range sum and an azimuth angle are displayed directly on a plan position indicator, which normally displays monostatic range versus azimuth angle, the display will be distorted, but can be corrected for most geometries, Section 5.4. Both sensitivity and LOS-constrained coverage of a bistatic radar can deviate significantly from that of a monostatic radar, in which coverage is defined as the region or area on the bistatic plane where the target is 'visible' i.e., detectable by the receiver and within LOS of both the transmitter and receiver. These altered coverage patterns often control bistatic radar operation, Section 5.5. Bistatic radar performance in clutter depends in part on the clutter cell area, which again can deviate significantly from that of a monostatic radar, depending on the bistatic geometry, Section 5.6. Geometry also controls the maximum unambiguous range and PRF. Further, in some bistatic geometries, the area, or volume, common to the bistatic transmitting and receiving beams will be small, which can cause beam scan-on-scan problems (Section 13.1), but also can allow the bistatic radar to operate with a higher pulse repetition frequency before encountering range ambiguities (Section 5.7).

Bistatic radar has transmitter and receiver separated on different platforms. It has many operational advantages for freedom and used in many applications. Especially, bistatic FMCW radar is being developed because of low-weight and cost-effectiveness. However, it is important to take into account the synchronization in time and frequency since transmitter and receiver use different oscillators. Before signal processing at the receive end, time offset and frequency offset must be considered in process. We assumed that the beacon is between Tx and Rx. Using MLE, we find out the time offset and frequency offset in bistatic triangular FMCW system considering Gaussian noise in channel and phase noise in oscillators.

Carrier frequency offset (CFO) and phase noise are challenging problems in single carrier frequency division multiple access (SC-FDMA) system. In this study, the authors have studied single-carrier space frequency block coding (SC-SFBC) to reduce the peak to average power ratio (PAPR) of the multiple-input multiple-output (MIMO) SC-FDMA signal, in which Alamouti SFBC will change the signal spectrum structure, break the single-carrier property and increase PAPR. Also, the authors propose a joint algorithm to suppress the inter-carrier interference (ICI) caused by phase noise and CFO. Conventional methods are to estimate phase noise and CFO in other algorithms, which are pretty difficult and complicated to obtain accurate estimation results. Unlike the conventional works, the novelty of the authors proposed algorithm is that it directly calculates the interference components and then reconstructs the ICI matrix. Thus, it avoids the degrading interactions between phase noise and CFO estimations. The proposed algorithm exploits block-type pilot, which is a common pilot pattern in SC-FDMA communications and is used in other wireless communication standards. Simulation results show that the suppression performance keeps smooth while phase noise and CFO varies, and BER performance degradation can be significantly reduced in 3 dB.

The transmitter and receiver positions of a bistatic radar are highly influential on its performance in radar target identification since the radar cross-section of a target varies with these positions. In this study, radar target identification performance using calculated bistatic scattering data for three full-scale models and measured data for four-scale-model targets is analyzed and compared. FFT-based CLEAN is used for shift-invariant feature extraction from the bistatic scattering data of each target, and a multilayered perceptron neural network is used as a classifier. The optimum receiver position is found by comparing the calculated identification probabilities while changing the position of the bistatic radar receiver. The identification results using calculated data and measured data show that an optimally positioned bistatic radar yields better identification results, demonstrating the importance of the positions of the transmitter and receiver for bistatic radar.

The residual phase noise at close-in to carrier offset frequency is studied for optical links using a laser diode with an external optical feedback. Since the measured FM noise degradation of the modulating signal was found to be insignificantly higher than the expected 20log (n) in dB, the residual phase noise of the laser diode was measured to quantify the expected carrier signal FM noise floor level. The measured residual phase noise of a InGaAsP laser diode at 1 KHz offset carrier signal of 5.08 GHz is measured to be -100 and -90 dBc/Hz with and without a 3-cm-long free-space external cavity, respectively. The close-in to carrier phase noise results of this laser at the external cavity resonance frequency of /spl ap/5 GHz is explained for the first time in terms of laser diode nonlinearity and FM noise theory of injection locked microwave oscillators. A good match between the predicted and measured results was observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

This work describes the use of the finite-difference time-domain (FDTD) method to investigate the capabilities and limitations in the use of a UWB radar system, to detect a human model target inside a realistic room. A 0.8 ns pulse at a center frequency of f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 1.1 GHz is used. The room is modelled with all its components (water & heating pipes, windows, electrical outlets, brick walls, barred windows, etc.). The radar setup is also modelled to simulate a realistic system. The performance of the UWB radar is examined by generating 2-dimensional (2D) images of the room interior with the human body model included. This is done by considering monostatic and multistatic radar scenarios where responses are recorded and processed using the time-domain back-projection method. Images from the FDTD modelling detected the human body model at the correct location inside the room. As expected, the multistatic radar setup provided a cleaner image than that of the monostatic radar setup due to greater diversity in aspect angle.

The Ionospheric Continuous‐wave E region Bistatic Experimental Auroral Radar (ICEBEAR) is a coherent scatter ionospheric radar. It operates at a frequency of 49.5 MHz, which is ideal for observing E region coherent echoes. The radar is located in Saskatchewan, Canada, and is operated by the University of Saskatchewan. The ICEBEAR system uses a continuous‐wave (CW) signal and requires isolation between the receiving and transmitting arrays. This was accomplished through a bistatic setup, where the receiver and transmitter are ≈240 km apart. Currently, the ICEBEAR system implements a pseudo random noise phase modulation on this CW signal to obtain 3‐km range resolution and 5‐s integration time images of E region ionospheric irregularities over a 600 km × 600 km field of view. The center of the field of view is located at ≈58°N, 106°W. The radar design allows for future improvements to temporal and/or spatial resolutions. Each site consists of a linear phased array with 10 equally spaced antennas. This, combined with modern digital radio hardware, provides azimuthal angle of arrival measurements at the receiving array and azimuthal transmission control at the transmitting array. This publication describes the radio hardware and signal processing used by the ICEBEAR radar and emphasizes the unique capabilities of the radar. First ICEBEAR observations from a Kp ≥ 4 event on 10 March 2018, are presented and shown to produce simultaneously the four types of previously characterized E region coherent scatter echoes.

This paper presents an ultra compact bistatic FMCW radar transceiver MMIC at 160 GHz with a mixer-based synthesizer concept. The integrated mixer converts a ramp signal with a stabilized local oscillator (SLO) signal to the RF output signal. The usage of a mixer reduces the frequency multiplication factor of the ramp signal and hence improves the phase noise at 160 GHz. The fixed frequency local oscillator for the up-conversion has a comparably small phase noise level compared to the ramp input signal and does not contribute significantly to the total phase noise level at 160 GHz. Apart from the synthesizer, the MMIC also includes a power amplifier with a maximum output power of 2 dBm, two efficient integrated antennas with a wide radiation pattern, and an IQ-receiver. Two FMCW radar responses recorded with a bandwidth of 20 GHz show the dynamic range of this sensor and its near range behavior. The compact SiGe MMIC requires only a chip area of 1.4mm × 1.0mm and consumes 285mW from a 3V power supply.

In this paper we have developed a novel Local Oscillator Module for down conversion application in Signal Source Analyzer with low phase noise, which covers 3-10 GHz frequency range. The local oscillator is a multi-loop YTO based frequency synthesizer, which is implemented with ultra low phase noise OCXO, low phase noise YIG-tuned oscillator, low conversion loss, and low noise sampling phase detector. In this project in order to have a good phase noise of local oscillator, key circuits have been analyzed, designed and implemented. The output signal of local oscillator has a pretty good phase noise, and the measured phase noise at 10 kHz offset is about -128dBc/Hz at 3.05GHz and -120dBc/Hz at 9.95GHz respectively. The frequency switching time is less than 2 millisecond in the entire operating range.

Summary form only given. Microwave signals of extreme spectral purity are of great interest in a large variety of applications ranging from deep-space navigation systems to synchronization of large scale facilities as well as pulse-Doppler radars and precision spectroscopy. Photonic microwave generation schemes, in which a mode-locked laser is synchronized to an ultra-stable laser reference, allow unparalleled levels of phase noise through photodetection of the low-jitter optical train of pulses [1]. However, the photodetection process is known to introduce excess phase noise by transferring the baseband amplitude noise of the mode-locked laser to the phase noise of the output radio-frequency carrier [2, 3], hereby spoiling the quality of the phase transfer. Although this conversion is easily observed, complete understanding of its process and its relation with other parameters remains unattended.We will report on the impact of the dispersion of the ultra-short optical pulses from fiber-based optical frequency combs on the amplitude to phase noise conversion coefficient. In our scheme, we use femtosecond pulse shaping techniques to introduce known amounts of phase delay in the path of the laser while monitoring the amplitude to phase noise conversion coefficient. In order to simulate the effect of dispersion we introduce linear frequency-dependent phase delay by performing quadratic spectral phase filtering of the output spectrum of the comb.

A performance analysis of an optical clock extraction circuit based on a Fabry-Perot filter (FPF) is presented. Two analytical methods, time-domain and frequency-domain analysis, are developed in this paper. Time-domain analysis shows that there is no phase jitter in the extracted optical clock if the free spectral range (FSR) of the FPF is exactly equal to the signal clock frequency. Based on this, we obtain an analytical expression for root mean square (rms) amplitude jitter of the extracted optical clock in time domain, in which we have taken the impacts of carrier frequency drift and carrier phase noise into account. When the FSR of the FPF deviates from the signal clock frequency, both phase jitter and amplitude jitter will occur in the extracted optical clock. In this situation, a more general frequency-domain method is developed to deal with the timing performance under the assumption that carrier phase noise is negligible. This method allows us to calculate both rms phase jitter and rms amplitude jitter of the extracted optical clock. Using the developed two methods, we present a detailed numerical investigation on the impacts of finesse of the FPF, carrier frequency drift, resonator detuning, carrier phase noise, and optical pulse chirp on the timing performance. Finally, the application of this circuit in multiwavelength clock recovery is discussed.

Highly stable and small size local oscillators is a very important component of any remote sensing, imaging, and telecommunication systems. The key factor influencing the frequency stability is quality factor of the resonance structure. Related close-in to carrier phase noise for offset frequencies above active device corner frequency reduces at least at a rate of 20 dB/decade while offset below corner frequency increases at rate of 30 dB/decade. A new approach in realization of the electrical oscillators is to augment electrical quality factor by optical energy storage components (e.g., fiber optic delay lines and ring resonators) to enhance effective quality factor (Q) to 10^4-10^6, well beyond available electrical Q of 2000 in dielectric resonator oscillators (DRO). Moreover, stability of resonance frequency has been impacted by the environmental induced (e.g., piezo and pyro properties of material) sensitivities. In addition, micro-scale design approaches are reported using multi-mode (both semiconductor and solid state) lasers for low phase noise intermodal oscillations. Finally, forced oscillation techniques of injection locking, phase locking, and mode-locking are attractive to further stabilize electrical and optical oscillation methods. In this thesis, development of broadband stable frequency sources is reported covering both X- and K-band using forced opto-electronic oscillator technique. The broadband frequency tuning is achieved using coarse electrical tuning of YIG bandpass filter combined with fine-tuned optical transversal filter using dispersive delay component and wavelength control of a fiber laser, as optical source. Forced opto-electronic oscillation techniques of self-injection locking (SIL), self-phase locked loop (SPLL), and self-injection locked phase locked loop (SILPLL) are optimized for further frequency stabilization of a frequency synthesizer. In SILPLL application, long delay line is crucial for substantial phase noise reduction; therefore, kilometer long optical fiber delay lines have been used to construct the optical feedback since their loss is extremely low compared to electrical delay lines. Meantime, open loop gain is another key parameter to achieve low phase noise in these feedback systems, as the phase noise reduction is proportional to phase locking loop gain. The phase noise and pull-in time can be greatly reduced in SILPLL as opposed to SIL or SPLL alone. A forced oscillation of fixed frequency DRO at 10 GHz system provide -137 dBc/Hz phase noise at 10kHz offset with under 10 fs aperture jitters, while OEO based X-band and K-band synthesizers provide phase noise of -137 dBc/Hz (&lt;10 fs jitters) and -127 dBc/Hz (&lt;12 fs jitters) at offset 10 kHz respectively in a computer controlled 19" rack-mountable OEO systems. After realization of the desired modular frequency synthesizer, development a compact opto-electronic oscillator system is demonstrated in my PhD study. Monolithic design and testing of multi-section quantum well lasers are reported on 5x2 mm2 InP chip using a commercial foundry service (i.e., SmartPhotonics). The designed lasers consist of 4 major sections of semiconductor optical amplifier (SOA), optical phase modulator (PM) and distributed Bragg reflector (DBR) mirrors. The DBR section provides wavelength selectivity while phase section and gain medium provide output wavelength tuning. An electro-absorption modulator (EAM) is prepared for mode number control using its wavelength sensitive absorption coefficient. The experiments supported predicted performance of this multi-mode laser structure and impact of combined forced oscillation of intermodal oscillations with mode locking method to realize a frequency stabilized and tunable RF signal source. Meantime, forced technique SILPLL and optimum mode locking techniques are applied in order to achieve improved phase noise performance. The free-running inter-modal oscillation frequencies suffers from poor phase noise of -5 dBc/Hz at 1 kHz and -32 dBc/Hz at 10kHz offset around 11.54 GHz, while under forced oscillation a -58 dBc/Hz at 1 kHz offset and -98 dBc/Hz at 10 kHz offset are achieved. A frequency tuning range of 800 MHz is also measured. In mode locked laser operation, the RF beat-notes forced with SILPLL topology provides a better performance. Improved phase noise of -79 dBc/Hz at 1kHz, -110 dBc/Hz at 10 kHz is measured with under 50 fs aperture jitters, while a broadband frequency tuning from 11 GHz to 13 GHz range is measured. The tuning range is potentially to be further improved, if the shared DBR laser pair symmetry could be improved. In summary, this thesis provides the solution for building RF synthesizer using forced opto-electronic oscillator and laser beat-notes oscillation output. The experimental results are corroborated with detailed analytical and numerical modeling to optimize oscillator phase noise performance. The modular design solution shows great stability and low phase noise, while the compact monolithic design solutions are quite attractive for a smaller size. Potential for full integration of multi-mode multi-section MQW laser with optical delay lines are also presented as a logical development path towards a compact frequency synthesizer.

In passive bistatic noise radar, cross-correlation is a popular processing choice for range compression of received signals. However, when using cross-correlation, the noise floor is dependent upon the characteristics of the clutter in the scene. For similar computational requirements, least-mean-square (LMS) processing can yield signal-to-noise ratio (SNR) performance which is independent of the clutter characteristics, making it a superior choice relative to cross-correlation. The superior SNR performance of LMS relative to cross-correlation may be even more striking when the transmitted noise waveform is not spectrally uniform. This paper will study the results achieved in a zero-Doppler range profile containing a single point scatterer and multiple point scatterers when cross-correlation, whitened cross-correlation, and LMS processing are used with a transmitted waveform whose spectrum is flat (white noise) and notched (colored noise).

Radar sensors are one of the key elements in highly automated driving systems. Their performance is robust independently of weather and visibility conditions, being able to detect targets up to hundreds of meters distance. In vehicles, radar sensors are required to estimate the range, doppler shift and direction of arrival of the reflected wave. In this contribution we will focus on the direction of arrival (DoA) estimation, which, while being crucial for environmental perception, still has considerable room for improvement. In particular, we aim to measure the performance of Compressive Sensing (CS) based algorithms applied for reconstruction of the DoA. In contrast to the traditional FFT approach, these algorithms are able to exploit sparse antenna configurations with a reduced number of Tx and Rx channels and a large effective aperture. For this purpose, a series of relevant metrics are defined and applied to measurements in representative open-air driving scenarios acquired with an automotive 4x8 MIMO radar operating at 77GHz. The results show that, when compared with the FFT, these algorithms display an overall enhanced angular estimation accuracy, resolution and false alarm ratio.