Code Waveform Research Articles

A Frequency Diverse Array SAR Processing Framework Based on the Segmented Phase Code Waveform for HRWS Imaging

A Frequency Diverse Array SAR Processing Framework Based on the Segmented Phase Code Waveform for HRWS Imaging

Photonic generation of frequency-tunable biphase and quadriphase coded pulse signals without background interference enabled by vector modulation and balanced detection.

An approach to generating frequency-tunable biphase and quadriphase coded pulse signals without background interference based on a polarization division multiplexing dual-parallel Mach-Zehnder modulator (PDM-DPMZM) is presented and demonstrated. Two ternary baseband code sequences are separately encoded into a pair of orthogonal optical carriers by exploiting a polyphase encoder on the basis of the principle of vector modulation, which in turn can be mapped to the phase shifts of the generated phase coded waveforms after the balanced detection. The frequency tunability can also be achieved by controlling the bias voltage of the associated modulator, so that the carrier frequency can be tuned to either fundamental or doubled frequency. Additionally, by designing different phase codes, the generated pulse signals can be conveniently switched between the quadriphase and biphase coding waveforms. The major advantage of the proposed approach is that four phase shifts can be obtained by simply adjusting the polarity of the ternary code sequences, overcoming the power-dependent limitation of the previous work. A proof-of-principle experiment is conducted to assess the feasibility of the proposed approach built on the Barker code and Frank code phase coded pulse signals generation. Experimental results show the phase coded pulse signals at 12 and 24 GHz carrier frequency are well behaved in terms of peak-to-sidelobe ratio (PSR), range-Doppler coupling and Doppler tolerance.

A Two-Stage Echo Separation Scheme for Spaceborne MIMO-HWRS SAR System

Multiple-input and multiple-output synthetic aperture radar (MIMO SAR) is a very potential technique for high-resolution and wide-width (HRWS) imaging due to the fact that it can provide more degrees of freedom. In this paper, an innovative MIMO-HRWS SAR imaging system is introduced with segmented phase coding (SPC) waveforms. However, it is well known that the echo separation issue is the most technical challenge for MIMO SAR. It has shown that the separation of orthogonal echoes from cross-correlation interference can be achieved by scan-on-receive (SCORE). However, this method often requires large computing resources on board, especially for null-steering process. For this, we propose a two-stage spaceborne-ground echo separation scheme. This scheme is mainly divided into two-steps: the first step is a real-time SCORE-beamforming process on board to reduce the downlink data volume, and the second step is a bandpass-null steering process to suppress the interfering echoes on the ground. Thus, this scheme allows for the low computing resources on board and sufficient suppression of the interference simultaneously. Following this scheme, the MIMO-HRWS SAR system enables its number of equivalent phase centers nearly double that of the azimuth multichannel SAR system, thus higher-resolution and wider-swath imaging. Finally, detailed simulation experiments for the MIMO-HRWS SAR imaging system are performed to verify the practicability and feasibility of the proposed scheme.

First Demonstration of Echo Separation for Orthogonal Waveform Encoding MIMO-SAR Based on Airborne Experiments

Multiple-input–multiple-output synthetic aperture radar (MIMO-SAR) has extensive application prospects, mainly including the acquisition of multidimensional scattering information, high-resolution and wide-width (HRWS) imaging, and moving target indication (MTI). Its echo separation is the most technical challenge, and so far, the confirmation for orthogonal waveform encoding MIMO-SAR by airborne experiments has not been reported in any literature. Here, an echo separation experiment based on the segmented phase code (SPC) waveforms and an airborne digital beamforming SAR (DBF-SAR) system is demonstrated for the first time. In the experiment, the SPC waveforms are cyclically transmitted within the adjacent pulse repetition intervals (PRIs) to simulate multiple transmitters, and the scattered echoes are received by the 16-channel antennas in elevation at the same time. In the postprocessing, the echo signals of continuous PRIs are added to obtain the mixed echoes, and a detailed echo separation method is adopted. In the method, the mixed echo signals from close arrival angles and far arrival angles are separated by the time shift and weighting, and by the bandpass filtering and DBF technique, respectively. Through the presented method, the mixed echoes of dual-transmit and 16-receive (2T16R) SAR imaging mode are separated and imaged successfully. The experimental results not only validate the echo separation scheme but also indicate that it is very promising in future MIMO-SAR missions.

First Demonstration of Airborne MIMO SAR System for Multimodal Operation

Synthetic aperture radar (SAR) systems have extensively contributed to diverse scientific applications in the recent past decades. For the next generation of top-level SAR systems, the multimodal operation of wide swath imaging, sliding spotlight, and highly sensitive ground moving target indication (GMTI) is becoming increasingly crucial. Following this concept, in this article, a novel airborne multiple-input-multiple-output (MIMO) SAR system, along with the space-time coding (STC) waveform scheme and the phased array, is developed to realize the multimodal operation. In particular, the airborne MIMO SAR system consists of two transmitters and four receivers. The signals of the multiple modes, which overlap in the frequency domain, are interpulse modulated and transmitted by different subarrays concomitantly. The superposed echoes are separated by range-Doppler filtering. Based on the separated echoes, we can obtain eight equivalent transceiver channels and, as a result, enhance the signal-to-noise ratio (SNR) for airborne multimodal operation. In the future, the STC waveform scheme and flight experiments of this novel airborne MIMO SAR system will be further investigated to reduce the pulse repeat frequency (PRF) for spaceborne multimodal operation.

Efficient Shadow Encoding Scheme Towards Power Spectrum Density in Digital Network Communication

Digital communication based on the conversion of data which only possible through digital conversion techniques. From the last couple of years, many digital conversion schemes have been introduced in the field of communication. All designed approaches convert data bits into a signal using various line code waveforms. This type of conversion has various kinds of issues such as synchronization between sender and receivers, bandwidth utilization, direct current components and power spectrum density (PSD) that need the attention of researchers. Although all these issues need a proper solution, PSD is a major concern in digital communication. This paper presents a newly designed scheme with the name of Shadow Encoding Scheme (SES) to transmit data bits efficiently by using physical waveform. SES provides a reliable transmission over the physical medium without using extra bandwidth and ideal power spectrum density (PSD) with the help of a shadow copy of the same bitstream which is being transmitted previously over the network. The SES is validated by mathematical equations which are used to calculate PSD and MATLAB simulator is used to simulate SES. The proposed SES is compared with other state-of-the-art line code techniques. The results show that SES performed well in PSD and bandwidth utilization as compared to other benchmark techniques. The coordinates of PSD are also presented in a tabular form which shows a vivid picture of the working condition of various line codes.

Novel Methodology to Assess RF Performance of Co-located MIMO Radar Systems Transmitting Binary Phased Coded Waveforms

In this work an experimental radar testbed and dual directional couplers (DDC) are used to collect measurements of the forward and reverse waves at each element, its mutual coupling, and beam-patterns. The collected measured data is used to validate a methodology for assessing radio frequency (RF) performance of a co-located multiple-input multipleoutput (MIMO) radar system transmitting binary phased coded (BP) waveforms. The estimated and measured active reflection coefficient (ARC) and beam-patterns are presented. The effect of different excitations and mutual coupling on the radiated fields is also presented.

Moving towards IoT Based Digital Communication: An Efficient Utilization of Power Spectrum Density for Smart Cities

The future of the Internet of Things (IoT) is interlinked with digital communication in smart cities. The digital signal power spectrum of smart IoT devices is greatly needed to provide communication support. The line codes play a significant role in data bit transmission in digital communication. The existing line-coding techniques are designed for traditional computing network technology and power spectrum density to translate data bits into a signal using various line code waveforms. The existing line-code techniques have multiple kinds of issues, such as the utilization of bandwidth, connection synchronization (CS), the direct current (DC) component, and power spectrum density (PSD). These highlighted issues are not adequate in IoT devices in smart cities due to their small size. However, there is a need to design an effective line-code method to deal with these issues in digital IoT-based communication for smart technologies, which enables smart services for smart cities. In this paper, the Shadow Encoding Scheme (SES) is proposed to transmit data bits efficiently by using a physical waveform in the smart cities’ ecosystem. SES provides a reliable transmission over the physical medium without using extra bandwidth and with ideal PSD. In it, the shadow copy of the repeating bitstream is forwarded, rather than repeating the actual stream again and again. The PSD is calculated with the help of mathematical equations to validate SES. MATLAB simulator is used to simulate SES and compared with other well-known digital line-code techniques. The bit error rate is also compared between SES and the chirp spread spectrum (CSS) for the specific data frames. The coordinates of the PSD graph are also shown in tabular form, which shows a vivid picture of the working conditions of various line codes.

Simulation Model of Electric Code-Modulated Signal in Russian Systems of Interval Control of Train Movement Based on Track Circuit

Systems of interval control of train movement Signaling systems, which are currently in service in Russian railways, use the electric track circuit as the main data channel between signals and locomotives. Code-modulated electric signals transferred through that channel are frequently get corrupted which leads to railway traffic delays.
 Decoding of the electric signal received from a track circuit can be represented as an image classification problem, and thus the stability of the data channel could be significantly improved.
 However, to build such a classifier based on some machine learning algorithm, one needs a large dataset. In this article, a simulation model to synthesize this dataset is proposed.
 The structure of the computer model matches the main stages of the electric code-modulated signal generation in a track circuit: code signal generator, rails, locomotive receiver.
 Based on code signal generator schematic and waveform diagrams, a generator algorithm is developed. At this stage, we modeled timings of electric code signals according to the specification as well as their random deviations caused by various factors.
 The analysis of substitution circuits of the rail line revealed that it has the properties of a low-pass filter. So, the rail line using the Butterworth digital filter with corresponding parameters is modeled. Additionally, at this stage, random noise during transmission was taken into account.
 A similar technique is applied for modeling of a locomotive receiver which has a band-pass filter as the first signal processing block.
 Thus, the proposed simulation model consists of a set of algorithms which run in series. By varying the parameters of the model, one can synthesize waveform diagrams of the electric code-modulated signal received by the locomotive equipment from a track circuit working in various modes and conditions.

First Demonstration of Joint Wireless Communication and High-Resolution SAR Imaging Using Airborne MIMO Radar System

Special attention has been devoted to joint wireless communication and radar sensing systems in recent years. However, since communication and radar have conflict requirements in terms of waveforms, transceiver developments, and signal processing algorithms, realization of this system concept is still a great challenge. In this paper, we introduce an airborne multi-input multi-output (MIMO) radar, along with the modified orthogonal frequency-division multiplexing (OFDM) and space–time coding (STC) waveform schemes, for the implementation of the joint wireless communication and synthetic aperture radar (SAR) imaging. The proposed system, which simultaneously transmits multidimensional waveforms with reconfigurable channels, can acquire adequate degrees of freedom. Thereby, it becomes a feasible method to perform both data transmission and high-resolution SAR imaging at the same time without intramodal interference. Theoretical analysis is validated by laboratory and flight experiments. Through our analysis, we aim to open up a new perspective of using MIMO radar to realize joint wireless communication and SAR imaging.

A modified stepped frequency phase coding radar waveform designed for the frequency domain algorithm

A great deal of interest has been paid to enhance the radar range resolution for the last decades. One of the techniques is to construct a waveform combining inter-pulse and intra-pulse modulations. In this paper, stepped frequency (SF) waveform is used to represent the former, whereas phase coding (PC) is considered for the latter. To obtain the high-resolution range profile (HRRP) of a target induced by an SF waveform at the receiver, one of the approaches is the frequency domain (FD) algorithm. It has been successfully applied with SF linear frequency modulation waveforms. However, based on our investigations, processing the SFPC waveform with the FD algorithm does not lead to the performance, in terms of peak sidelobe ratio (PSLR) and integrated sidelobe ratio (ISLR), of the single-carrier phase coding (SCPC) waveform processed with a matched filter (MF). To overcome these drawbacks, we propose to split the spectrum of a phase coded pulse into a predetermined number of portions, and then to successively transmit the time-domain transformed versions of these various portions. The received echoes are then processed with a modified version of the FD algorithm. Our analysis and simulations show that the proposed waveform can be characterized by a PSLR and an ISLR close to those of the SCPC processed with MF in some scenarios.

A Novel MIMO⁻SAR Solution Based on Azimuth Phase Coding Waveforms and Digital Beamforming.

In multiple-input multiple-output synthetic aperture radar (MIMO–SAR) signal processing, a reliable separation of multiple transmitted waveforms is one of the most important and challenging issues, for the unseparated signal will degrade the performance of most MIMO–SAR applications. As a solution to this problem, a novel APC–MIMO–SAR system is proposed based on the azimuth phase coding (APC) technique to transmit multiple waveforms simultaneously. Although the echo aliasing occurs in the time domain and Doppler domain, the echoes can be separated well without performance degradation by implementing the azimuth digital beamforming (DBF) technique, comparing to the performance of the orthogonal waveforms. The proposed MIMO–SAR solution based on the APC waveforms indicates the feasibility and the spatial diversity of the MIMO–SAR system. It forms a longer baseline in elevation, which gives the potential to expand the application of MIMO–SAR in elevation, such as improving the performance of multibaseline InSAR and three-dimensional SAR imaging. Simulated results on both a point target and distributed targets validate the effectiveness of the echo separation and reconstruction method with the azimuth DBF. The feasibility and advantage of the proposed MIMO–SAR solution based on the APC waveforms are demonstrated by comparing with the imaging result of the up- and down-chirp waveforms.

Discrete Frequency and Phase Coding Waveform for MIMO Radar

Discrete Frequency and Phase Coding Waveform for MIMO Radar

MIMO SAR Using Orthogonal Coding: Design, Performance Analysis, and Verifications

Multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) is a promising technology in radar imaging which provides a better balance of azimuth resolution and swath width compared with traditional single-input single-output (SISO) SAR. It has the potential to help scientists and engineers to design ambitious SAR system with higher resolution and wider swath. This paper studies the principle of MIMO SAR using orthogonal coding waveform and then provides the performance analysis in resolution and swath width. By using orthogonal coding waveform, lower channel interference is obtained, which makes MIMO SAR achieve wider unambiguous range swath and lower azimuth ambiguity. Simulations are carried out by means of the system parameters of real spaceborne SAR platform. A ground-based MIMO SAR imaging system with up and down chirp modulation is also designed. The performances of MIMO SAR and SISO SAR are compared, and the validity and advantage of MIMO SAR are verified.

Design Discrete Frequency Coding Waveform Based OFDM for MIMO-SAR

Design Discrete Frequency Coding Waveform Based OFDM for MIMO-SAR

Discrete Frequency Code Waveform Based on MicPSO Cooperated with Grefenstette Coded

Discrete Frequency Code Waveform Based on MicPSO Cooperated with Grefenstette Coded

Recognition of the stacked two-dimensional bar code based on iterative deconvolution

Traditionally, the method of bar code recognition is based on edge detection. Because of the high density of the stacked two-dimensional bar code, the signal is severely blurred by the point spread function of the optical system. And this method is not suitable. In order to deblur the image, a deconvolution technique is necessary. Under the influence of noise, deconvolution is a type of ill-posed problem. Based on the idea of bar codes as bilevel waveforms, a novel bar code recognition algorithm rooted in iterative deconvolution is proposed in this paper. First, the bar code is rotated to be horizontal using the interpolation of sixteen points. After analysing the waveform, the system identification is accomplished. At last, the bar code waveform is reconstructed based on iterative computations. The results show that the performance of the algorithm proposed here is excellent. It can achieve higher recognition rates than previous models.

A self-synchronizing method for asynchronous code acquisition in band-limited spread spectrum communications

This paper addresses the problem of initial synchronization of pseudo-noise code. A new code acquisition technique for spread-spectrum communication systems using band-limited chip waveforms is presented. Unlike conventional power detector based on testing the estimated maximum of the ambiguity function, the devised detector exploits a fast parabolic interpolation, running on three estimated ambiguity samples in the neighborhood of the coarse estimate. Performance analysis is carried out in comparison with conventional detector. Mathematical expressions for the probability of false alarm and detection are derived. They are numerically evaluated, under operating settings, by a reduced Tayloriquests expansion up to the second order. The theoretical results, substantiated by computer simulations, have evidenced that the devised method is well suited for asynchronous spread-spectrum communications. In particular, the acquisition performance depends on the actual offset between the received and the reference code waveforms, which are randomly distributed (in chip-asynchronous systems) within one sampling period. In fact, the parabolic interpolation technique outperforms the conventional detector for a wide range of code offsets because it is able to self-synchronize the testing variable around the true offset.

A Virtual Antenna Beamforming (VAB) Approach for Radar Systems by Using Orthogonal Coding Waveforms

An innovative approach is introduced to form virtual transmitting and receiving radar antenna beams simultaneously by transmitting orthogonal coding waveforms from the antenna elements and digitally processing of their echoes at the receiver. Multiple virtual transmitting-receiving beams can be formed simultaneously by employing an equal number of beamforming filters without increasing transmitting power or antenna gain or resolution loss. The virtually formed antenna beams can provide equivalent antenna gains and spatial resolutions as the conventional phased arrays of the same size. Because the actual antenna radiation pattern can be made almost isotropic, the new system has low probability of intercept (LPI) property. With both transmitting and receiving beams virtually implemented through digital filtering, costly radiation phase shift used in phased arrays is not needed for beam scanning in the proposed system.

Concurrent Extraction of Target Range and Doppler Information by Using Orthogonal Coding Waveforms

In this paper, a novel approach for concurrent extraction of target range and Doppler information in radar systems is introduced. A pair of orthogonal waveforms with different pulse repetition frequencies (PRF) are bundled and transmitted as a single hybrid radar waveform for target measurement. At the receiver, two orthogonal signal processing channels consisting of a low PRF (LPRF) channel and a high PRF (HPRF) one, perform the extraction of target range and Doppler information, respectively. The CLEAN algorithm is innovatively used to suppress the interference from the cross-correlation residues and the autocorrelation sidelobe in the LPRF channel. The processing results, based on the proposed signal processing approach, demonstrate the technical feasibility of implementing such a signal processing scheme in radar systems.