Characterization and calibration techniques for multi-channel phase-coherent systems

Electronic Warfare (EW) and Radio Detection and Ranging (Radar) are two of the many applications that rely on multi-channel and phase-coherent configurations for signal processing. We provide herein an overview of the complexities and requirements of a multi-channel phase-coherent measurement system. Multiple Input Multiple Output [MIMO] systems have to overcome key technical challenges related to phase, time and frequency synchronization in order to coherently receive and process the data acquired/generated from each input/output. In practical MIMO systems, the radio hardware should be capable of acquiring and/or generating such phase coherent signals across the multiple channels. Further, the systems need to be able to sustain the phase coherence over considerable duration of time, depending on the sensitivity of the system. However, drifts will occur owing to the effects like temperature, thermal expansion, mismatched cable lengths, uncorrelated phase noise, ADC sample clock phase noise and quantization noise. Thus, a calibration process is required to compensate for the drift, whenever it crosses a particular threshold value that defines the accuracy of the phase-coherent system. In this paper, an FPGA based software-defined calibration method is presented for synchronizing the phase and magnitude across multiple channels of a system. This method allows the phase/magnitude drift over time to be periodically monitored and calibrated, when there is a need. With the FPGA built into the system, the calibration can take place remotely without the need of connecting the system to an external calibration kit. Also, measurement results are provided for a state of the art super heterodyne receiver system to show that the phase drift is lesser than ±1 degree across a 500MHz – 26.5GHz frequency range at 23°C ± 5°C for 2 and 4 channels configurations. Such systems can find use in a variety of real-world MIMO implementations such as Direction Finding, Beam Steering, Passive Radars, MIMO and Phased-Array Radar Systems, where phase coherence, alignment and /or synchronization has added advantages to multi-channel systems.

Passive radar systems operate by exploiting signals of opportunity that are designed for other applications, such as broadcasting, communications, and satellite navigation. Passive radars have recently attracted attention in military applications, mainly because of their covertness, low-cost implementation, and no requirement of signal emissions [1], [2]. Whereas the primary use of passive radars is in the defense sector, these same features make them attractive in commercial applications. Unlike conventional active radar systems, which typically operate in a monostatic mode, passive radar can operate in a multistatic mode (thus called passive multistatic radar, or PMR) by using multiple transmitters. Multiple networked receivers can also be exploited for an expanded PMR network size.

A new Phased Array Radar (PAR) system for meteorological application has been developed by Toshiba Corporation and Osaka University under a grant of NICT, and installed in Osaka University, Japan in 2012. The PAR system has the unique capability of scanning the whole sky with 100 m and 10 to 30-second resolution up to 60 km. The system adopts the digital beam forming technique for elevation scanning and mechanically rotates the array antenna in azimuth direction within 10 to 30 seconds. The radar transmits a broad beam of several degrees with 24 antenna elements and receives the back scattered signal with 128 elements digitizing at each element. Then, by digitally forming the beam in the signal processor, the fast scanning is realized. After the installation of the PAR, SIP project has started to develop a new PAR with dual polarimetric capabilities, which was installed in Tokyo metropolitan area in 2017. Within the project, studies are being conducted to perform very short predictions of torrential rainfall events with deep neural networks.

Passive coherent location (PCL), passive covert radar (PCR), passive bistatic radar (PBR), passive radar, parasitic radar, and hitchhiking radar are all common names for passive radar systems exploiting cooperative, noncooperative transmitters, or transmitters of opportunity, [1, pp. 3]. Recent review of the various names resulted in the compromise PBR, [2, pp. 248]. However, it should be noted that the PCL name is still being used for full systems like Lockheed Martin's - Silent Sentry [3], [4], Thales' - Homeland alerter 100 [5], [6] seen in Figure 1, and Airbus “Passive Radar” [7]–[15]. The PCL systems may be considered as systems of PBR systems. This work focuses on the system perspective of the technology, i.e. PCL, while relying on the technical depths of the PBR technology.

Phase coherence is an important measure in nonlinear science. Whereas there is no generally accepted definition for phase and therefore for phase coherence, many works associate this feature with topological aspects of the systems, such as having a well-defined rotating center. Given the relevance of this concept for synchronization problems, one aim of this paper is to argue by means of a couple of counterexamples that phase coherence is not related to the topology of the attractor. A second aim is to introduce a phase-coherence measure based on recurrence plots, for which probabilities of recurrences for two different trajectories are similar for a phase-coherent system and dissimilar for non-phase-coherent systems. The measure does not require a phase variable defined a priori.

Distributed passive radars confront the high dynamic range challenge by splitting the receiver physically to provide high SNR samples of the transmitter as well as reduced cochannel interference. The receivers are mutually coherent and must therefore exchange voltage level data for computation of the cross-ambiguity function. The data bandwidth required for full- precision (16 bit) data is about 2 Mbps to support for a single channel. This data bandwidth demand can stress the cost and latency of distributed passive radars. Therefore we investigate the effect the sensitivity of a passive radar system using several methods to compress the voltage level samples prior. (4 pages)

The present work illustrates the Fraunhofer FHR contribution to the MAPIS project field trials held in Livorno in June 2017. The MAPIS project, funded by EDA, aims at demonstrating radar imaging capability of passive radar systems in a (military) maritime scenario. In particular, two different Passive Bistatic Radar (PBR) systems, one exploiting Digital Video Broadcasting - Terrestrial (DVB-T) signals, the other one exploiting Digital Video Broadcasting - Satellite (DVB-S) signals have been used to experimentally assess the imaging performance. This paper describes the systems developed by Fraunhofer FHR and used during the trials. In addition, a small overview of the trial itself is provided together with some preliminary results concerning the passive radar imaging.

Phased array radar systems are used by the military for tracking targets and providing weapon engagement support. Each radar system consists of multiple antenna modules arranged in a 2-dimensional grid. An n-by-n antenna array radar system can operate with up to a given number of antenna module failures, with acceptable degraded performance, as long as the failed antennas are not adjacent to each other. In this paper, an improved RBD (Reliability Block Diagram) configuration is proposed to analyze the reliability for phased radar systems. Other reliability metrics such as point availability and mean availability also are evaluated.

Passive Bistatic Radar (PBR), or Passive Coherent Location, is gaining interest in the radar community as it provides some advantages with respect to active radar. Although, passive radar does not aim to replace active radar, it provides a good complement to it. The computational effort that is required to implement the required signal processing is one of the drawbacks that affect passive radar. In this paper a computationally affordable algorithm is proposed and its performance tested by using a numerical simulation. (4 pages)

In this paper, we propose an interference alignment (IA) method for interference elimination in co-existing Phased Array (PA) radar and multi-user multiple-input multiple-output (MIMO) communication systems. Although ergodic IA method has been proposed to cancel the mutual interferences, it will definitely reduce the degrees-of-freedom (DoFs) of communication systems. Differently, our proposed IA method allows to improve the DoFs. It is examined by the Neyman-Pearson (NP) test for radar functionality, while the communication performance is evaluated by DoFs and bit error rate (BER) analysis. Both theory analysis and numerical results are provided to validate the effectiveness of the proposed IA method.

Phase linear characteristic is an important parameter of RF transceiver module in phased array radar system. This paper analyzes the causes of phase nonlinear based on the mechanism of which, and puts forward four methods of optimizing the phase linear. Finally, an X-band RF transceiver module is designed and manufactured to verify the effectiveness of the methods. The test results show that by using the methods proposed, the phase distortion of the RF transceiver module can be improved obviously.

Phased array radar systems are used for a wide variety of applications including the precise tracking of airborne craft for air traffic control and providing accurate atmospheric condition information important in weather forecasting. Reducing the cost and size of these radar systems will open new fields to the use of this technology. Using phase control implemented through liquid crystal materials we have created a compact, phased array radar system operating in the microwave range. We report on the construction and testing of a linear, eight element phased array antenna system operating at 32 GHz with element phase controlled by a dual frequency nematic liquid crystal media used as a tunable dielectric. The system was designed using CST Design Studios and Ansys HFSS software. Dual frequency liquid crystals are used to improve beam steering response times. We demonstrate 42 millisecond beam switching times, defined as the time to change the beam focus from one point to another point, controllable beam formation, and beam steering profiles consistent with analytical results and simulation models. The device footprint is a square with sides 9.5 cm long and a thickness less than 2.5 mm. Such a module is easily stackable to create an 8 × 8 phased array system. Our design incorporates a modular construction using PCB for the antennas and input circuitry and a liquid crystal phase control cell with microwave glass substrates. This design simplifies design, construction, and testing as compared to on-glass designs. The device shows an improvement in point-to-point scanning speeds by a factor of 3 as compared to similar liquid crystal based devices and provides continuously variable tuning. Such a device can be used in a system for reduced visibility, directional range finding suitable for automobile collision avoidance systems and rotary wing aircraft landing aids.

In this study, a new approach is proposed for accurate target identification, required by the phased array radar systems that are employed in through‐the‐wall imaging applications. In radar imaging, the effects of the multipath propagation are materialised in fake impressions of the true target, known as ghost images. The developed algorithm removes these ambiguities related to the existence of a target by cancelling, in the final radar image, the ghost images by means of improving the global signal‐to‐spurious‐ratio (SSR) and the target‐to‐ghost‐ratio (TGR) that characterise the real target signature. The development of the approach is based on beamforming and on coherent signal processing upon the returned signal and clutters, for a phased array monostatic radar system. The performances of the algorithm are measured in terms of both the global SSR gain and the TGR and they are verified through simulations and measurements in scenarios that quantify the robustness of the approach. The obtained numerical results concerning the global SSR gain and the TGR certify that the proposed method improves the target localisation and removes the ghost images for radars that operate in rich scattering environments. In the end, the limitations of the approach are also presented.

Use of multiple transmit and receive antennas have become a popular research area in radar community after the success of the same concept in communication. It is shown by Fishler et al. that multi-input multi-output (MIMO) radar has considerable advantages compared to traditional radar and phased array radar systems. In this paper, detection performance of MIMO radar using Neyman-Pearson detector is investigated in K-distributed sea clutter for a practical scenario. Also, spikiness of K-distributed MIMO radar clutter is discussed with respect to the number of nodes.

Accurately measurement of target's range and angle are essential parts for the purpose of real time target tracing. The range between target and radar can be calculated by the time delay of target echo. Meanwhile, there are a variety of mature algorithms on target angle measuring. The monopulse angle measurement is a commonly adopted method. In monopulse radar system, we use the data sampled by sum channel and diff channel to calculate the target angle information. So the channel sampled data is important for angle calculation. The phase coherence between sum channel and diff channel is essential condition. On a PCB board based on high speed AD converter and FPGA, the length of multiple difference pairs of data lines usually are not equal, which will result in the different arrival time to the FPGA logic. It will cause phase offset between different data channels. In this case, it would not be carry out angle measurement precisely. On this background, this paper proposes an auto data alignment method based on first-in-first-out (FIFO) in FPGA. This paper introduces the method in detail and makes a comparison on the data results of alignment. The results validate the effectiveness. This method has been successfully used in radar systems.