Atmospheric scattering model (ASM) is one of the most widely used model to describe the imaging processing of hazy images. However, we found that ASM has an intrinsic limitation which leads to a dim ...effect in the recovered results. In this paper, by introducing a new parameter, i.e., light absorption coefficient, into ASM, an enhanced ASM (EASM) is attained, which can address the dim effect and better model outdoor hazy scenes. Relying on this EASM, a simple yet effective gray-world-assumption-based technique called IDE is then developed to enhance the visibility of hazy images. Experimental results show that IDE eliminates the dim effect and exhibits excellent dehazing performance. It is worth mentioning that IDE does not require any training process or extra information related to scene depth, which makes it very fast and robust. Moreover, the global stretch strategy used in IDE can effectively avoid some undesirable effects in recovery results, e.g., over-enhancement, over-saturation, and mist residue, etc. Comparison between the proposed IDE and other state-of-the-art techniques reveals the superiority of IDE in terms of both dehazing quality and efficiency over all the comparable techniques.
Mobile network is evolving from a communication-only network towards one with joint communication and radar/radio sensing (JCAS) capabilities, that we call perceptive mobile network (PMN). Radio ...sensing here refers to information retrieval from received mobile signals for objects of interest in the environment surrounding the radio transceivers, and it may go beyond the functions of localization, tracking, and object recognition of traditional radar. In PMNs, JCAS integrates sensing into communications, sharing a majority of system modules and the same transmitted signals. The PMN is expected to provide a ubiquitous radio sensing platform and enable a vast number of novel smart applications, whilst providing non-compromised communications. In this paper, we present a broad picture of the motivation, methodologies, challenges, and research opportunities of realizing PMN, by providing a comprehensive survey for systems and technologies developed mainly in the last ten years. Beginning by reviewing the work on coexisting communication and radar systems, we highlight their limits on addressing the interference problem, and then introduce the JCAS technology. We then set up JCAS in the mobile network context and envisage its potential applications. We continue to provide a brief review of three types of JCAS systems, with particular attention to their differences in design philosophy. We then introduce a framework of PMN, including the system platform and infrastructure, three types of sensing operations, and signals usable for sensing. Subsequently, we discuss required system modifications to enable sensing on current communication-only infrastructure. Within the context of PMN, we review stimulating research problems and potential solutions, organized under nine topics: performance bounds, waveform optimization, antenna array design, clutter suppression, sensing parameter estimation, resolution of sensing ambiguity, pattern analysis, networked sensing under cellular topology, and sensing-assisted communications. We conclude the paper by listing key open research problems for the aforementioned topics and sharing some lessons that we have learned.
A new class of in-phase and out-of-phase power dividers with constant equal-ripple frequency response and wide operating bandwidth is presented in this paper. The proposed design is based on ...microstrip-to-slotline transitions and slotline resonators. A slotted T-junction is adopted to split the power into two parts and obtain wideband isolation between the two output signals at the same time. The characteristic impedance of the transitions and resonators determines the operating bandwidth and in-band magnitude response. By reversing the placement direction of the slotline-to-microstrip transition, the electrical field is reversed, thus resulting in out-of-phase responses between output ports. A thorough analysis of the relations between the structure and the characteristic functions is provided to guide the selection of parameters of the structure in order to meet the design objectives. In the structure, simulation and measurement are conducted to verify the design method. For both in-phase and out-of-phase cases, more than 110% bandwidth has been achieved with excellent matching at all ports and isolation of output signals. Constant in-band ripple is obtained within the operating band of the power dividers, indicating that the proposed design can realise minimal power deviations, which is extremely desired in wireless systems.
This paper introduces a novel and effective image prior, i.e., gamma correction prior (GCP), which leads to an efficient image dehazing method, i.e., IDGCP. A step-by-step procedure of the proposed ...IDGCP is as follows. First, an input hazy image is preprocessed by the proposed GCP, resulting in a homogeneous virtual transformation of the hazy image. Then, from the original input hazy image and its virtual transformation, the depth ratio is extracted based on atmospheric scattering theory. Finally, a "global-wise" strategy and a vision indicator are employed to recover the scene albedo, thus restoring the hazy image. Unlike other image dehazing methods, IDGCP is based on the "global-wise" strategy, and it only needs to determine one unknown constant without any refining process to attain a high-quality restoration, thereby leading to significantly reduced processing time and computation cost. Each step of IDGCP is tested experimentally to validate its robustness. Moreover, a series of experiments are conducted on a number of challenging images with IDGCP and other state-of-the-art technologies, demonstrating the superiority of IDGCP over the others in terms of restoration quality and implementation efficiency.
Frequency-hopping (FH) MIMO radar-based dual-function radar communication (FH-MIMO DFRC) enables communication symbol rate to exceed radar pulse repetition frequency, which requires accurate ...estimations of timing offset and channel parameters. The estimations, however, are challenging due to unknown, fast-changing hopping frequencies and the multiplicative coupling between timing offset and channel parameters. In this article, we develop accurate methods for a single-antenna communication receiver to estimate timing offset and channel for FH-MIMO DFRC. First , we design a novel FH-MIMO radar waveform, which enables a communication receiver to estimate the hopping frequency sequence (HFS) used by radar, instead of acquiring it from radar. Importantly, the novel waveform incurs no degradation to radar ranging performance. Then , via capturing distinct HFS features, we develop two estimators for timing offset and derive mean squared error lower bound of each estimator. Using the bounds, we design an HFS that renders both estimators applicable. Furthermore , we develop an accurate channel estimation method, reusing the single hop for timing offset estimation. Validated by simulations, the accurate channel estimates attained by the proposed methods enable the communication performance of DFRC to approach that achieved based on perfect timing and ideal knowledge of channel.
Owing to the characteristics of directional couplers, series-fed multibeam receiving antenna arrays require different treatments from their transmitting counterparts. In this article, the theory and ...strategies for the feed network synthesis of multibeam receiving antennas employing the generalized joined coupler (GJC) matrix are presented. Given <inline-formula> <tex-math notation="LaTeX">M </tex-math></inline-formula> incident waves in any set of directions, we first derive the output signal matrix as a function of the incident signal matrix. This serves as an important tool for both synthesizing and analyzing GJC receiving matrices. Then, we present three different synthesis strategies and show how they are related to the array patterns. We also reveal how the receiving power efficiency of the GJC matrix changes with the antenna beam pattern. We further demonstrate that, despite the employment of matched loads, high receiving power efficiencies can be achieved using the GJC matrix.
In this article, a new design approach for developing hybrid couplers with arbitrary power division and the phase difference is presented. The design is based on a transformer-based coupler with high ...isolation between sum and difference ports. Based on this model, a modified coupler is developed by including two L-shaped networks to produce constant phase differences at two output ports. Slotlines and microstrip-to-slotline transitions are used for expanding the bandwidth and introducing a 180° phase difference at the difference port. Two prototypes with equal and unequal power division are designed and verified. Experimental results reveal that the proposed designs can achieve constant phase difference, low insertion loss, and satisfactory matching across a broad bandwidth of more than 40%. Based on the proposed designs, a simplified multibeam Nolen matrix is also designed utilizing only three hybrid couplers and no phase shifters. The compact beamforming network demonstrates that the proposed component is very useful for size reduction in wireless communication systems.
Despite the rapidly increasing interest in analog multibeam antennas, there has been a lack of systematic theoretical approaches to synthesizing circuit-type multiple beamforming networks, such as ...the Blass matrix and the Nolen matrix. To address the issue, this article presents a new concept, the generalized joined coupler (GJC) matrix, which encapsulates both the Blass matrix and the Nolen matrix, as well as their variants, and presents a novel theoretical framework for generating individually and independently controllable multiple beams using the GJC matrix. A GJC matrix has <inline-formula> <tex-math notation="LaTeX">N </tex-math></inline-formula> columns to feed <inline-formula> <tex-math notation="LaTeX">N </tex-math></inline-formula> antenna elements and <inline-formula> <tex-math notation="LaTeX">M </tex-math></inline-formula> rows to feed <inline-formula> <tex-math notation="LaTeX">M </tex-math></inline-formula> beams, and the direction of each individual beam can be controlled by tuning the phase shifters in the associated row of the GJC matrix. In this article, a matrix theory is developed, and an optimization algorithm is proposed to provide a mathematical tool for synthesizing such matrices and, consequently, the multiple beams. Using a particle swarm optimization algorithm, numerical results demonstrate that multibeams with independent control of individual beam directions and sidelobes can, indeed, be synthesized in a systematic manner. Specifically, two GJC matrix variants, the Blass-like matrix and the Nolen-like matrix, are investigated.
A new approach to reducing the monostatic radar cross section (RCS) and preserving the radiation characteristics of a slot array antenna by employing polarization conversion metasurfaces (PCMs) is ...presented in this communication. The PCM is arranged in a chessboard configuration consisting of fishbone-shaped element. It is placed on the surface of the slot array antenna. The characteristics and mechanism of the RCS reduction are analyzed. Simulated and experimental results show that the monostatic RCS reduction band of the antenna with PCM ranges between 6.0 and 18.0 GHz for normally impinging both x- and y-polarized waves. The radiation characteristics of the antenna are well preserved simultaneously in terms of the impedance bandwidth, radiation patterns, and realized boresight gains.
This article presents a reconfigurable millimeter-wave (mm-wave) dual-polarized antenna array system featuring wide-range continuous beamwidth control (WCBC) using the polarization-mixing method. By ...simultaneously exciting dual polarizations, the system achieves wider beams, with a continuously adjustable beamwidth ranging from 21° to 100°. Additionally, the system retains its original beam scanning capabilities when using the two polarizations separately. Importantly, the ability in flexible beam scanning and wide beamwidth tuning is accomplished with only phase shifters without the need for amplitude control. All antenna elements are fully excited, ensuring high overall efficiency. The robustness and stability of the system are confirmed through error analysis. The WCBC array system is designed, fabricated, and experimentally validated. By employing a control circuit, it realizes reconfigurable dual functionality, i.e., beam scanning and wide beamwidth variation, in a simple and cost-effective manner.
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