Keyhole Channels Research Articles

On the Secrecy Performance of MIMOME Keyhole Channels Aided with Artificial Noise

On the Secrecy Performance of MIMOME Keyhole Channels Aided with Artificial Noise

The evaporation behavior of zinc and its effect on spattering in laser overlap welding of galvanized steels

In this paper, a computational fluid dynamics model considering zinc evaporation was innovatively established to investigate the interaction between zinc vapor and laser keyhole in both zero-gap and small-gap conditions. Under the zero-gap condition, it was observed by a high-speed camera that the entrance of zinc vapor gas into the keyhole had the cyclic characteristics, and the occurrence of spattering was closely related to the entrance of zinc vapor. Following the cyclic characteristics obtained from experiment, the zinc vapor with properties of mass and momentum was set to periodically entering the keyhole in the numerical model. The numerically calculated results under the zero-gap condition showed that the zinc vapor entered the keyhole at an initial velocity of 81 m/s, resulting in large deformation of the keyhole channel. The vapor flowed upward along the rear wall with a velocity of 15–20 m/s, pushing large amounts of melt in the molten pool upward and generating spatter. After the spattering event, the velocity of the remaining zinc vapor in the keyhole dropped to lower than 5 m/s, leading to shrinkage of the keyhole channel, and both the keyhole and the molten pool restored to a normal and stable state. Under the small-gap condition, the zinc vapor entered the keyhole continuously with an initial velocity of 33 m/s, which had little impact on keyhole geometry. The velocity of the zinc vapor inside the keyhole was significantly reduced comparing with the zero-gap condition, and the velocity of the upward flowing melt was lower than 0.75 m/s, thus no spatter occurred. The numerical analysis sufficiently explained why spatter was depressed under the gap condition.

Influence of the metal flow in the keyhole molten pool on the molten pool stability in continuous variable polarity plasma arc keyhole vertical-up welding

The metal flow of the keyhole molten pool affects weld pool stability and weld formation vitally during continuous variable polarity plasma arc welding (VPPAW). A novel “keyhole inside alternating oscillation” (KIAO) thermal-mechanical coupling model was developed to study the influence of keyhole molten pool metal flow on molten pool stability. This model can achieve the oscillation effect of heat-force on the molten pool during the electrode negative (EN) and electrode positive (EP) phase. A heat source, with keyhole channel backward bending, was used to create a keyhole backward deviation along the workpiece thickness. An asymmetric arc pressure source was adopted to ensure asymmetric distribution of the arc pressure. The metal flow of the stable and cut molten pool are calculated in continuous VPPAW process, and the conditions needed for the stable existence of the molten pool are obtained. The molten pool must have sufficient liquid metal which needs to flow as follows: in the front of the upper molten pool, a part of the metal flows in the form of “two-way rotating confluence” along the keyhole side wall to the rear of the lower molten pool, and part of the metal flows backwards to the rear of the upper molten pool. The metal at the rear of the lower molten pool flows upwards along the keyhole side wall. Under the combined action of these flow behaviors, the expansion speed of the leading edge of the keyhole is approximately equal to the shrinkage speed of the keyhole trailing edge, and the keyhole molten pool is stable. The stability of the bottom molten pool has a significant impact on the overall molten pool stability. The calculated fusion line and the offset distance between the central axis of the keyhole exit and the welding torch corresponded with the measured results.

Deep Learning-Based Automatic Modulation Classification Over MIMO Keyhole Channels

Automatic modulation classification (AMC) is a significant part of cognitive communication systems. In early researches, likelihood-based (LB) and feature-based (FB) solutions were proposed for the AMC problem. With the developments in the data-driven approaches, a third method based on deep learning (DL) has recently gained prominence among AMC researchers. It is shown that convolutional neural network based classifiers are very efficient in the AMC for both single input single output (SISO) and multiple-input multiple-output (MIMO) systems. However, for most of the works in MIMO-AMC, the channel considered is full rank. This work addresses the problem of AMC over rank deficient channels such as a keyhole channel using a DL-based classifier. The proposed DL-classifier utilizes a CNN, which does not employ pooling layers or dropouts in the convolutional layers. To further improve the classification accuracy, decision cooperation as well as feature fusion is employed. In addition to the keyhole effect, this work investigates the effect of antenna correlation on DL-based AMC. A comparative study of the proposed method and the existing FB AMC method for the MIMO keyhole channel is also presented.

Automatic modulation classification over MIMO amplify and forward (AF)-relay fading channels

Automatic Modulation Classification (AMC) plays an important role in many civilian applications such as spectrum monitoring (SM) and cognitive radio (CR). The majority of the AMC algorithms for MIMO systems are developed for rich scattering environments, which offer a full rank channel matrix. A rank deficient channel such as a keyhole channel severely affects the AMC performance. It is reported that cooperative relaying is an attractive solution to overcome the rank deficiency of multiple-input multiple-output (MIMO) channels. This work investigates the effect of cooperative relaying on the AMC performance. A higher-order cumulant (HOC)-based AMC method, namely the relay assisted RA-AMC method, is proposed to classify a pool of modulation types, namely BPSK, OQPSK, QPSK, π4-QPSK, 8-PSK and 16-QAM over an amplify and forward (AF) relay network. Simulation results show that the proposed method performs reasonably well over AF-relay fading channels.

Massive MIMO under multi-keyhole channels: Does the use-and-then-forget bounding technique work?

This paper considers the uplink of a massive MIMO system under the multi-keyhole channels. Two capacity bounding techniques, namely use-and-then-forget (UF) and full-side-information (FSI), are used to obtain the achievable rates. With UF scheme, the base station (BS) utilizes the mean value of the effective channel gain to detect the signal, while with the FSI scheme, the BS exploits the full channel knowledge for signal detection. By using the law of large numbers and the Jensen’s inequality, we derive the tight approximations of the rates which can be represented in simple and insightful closed-form expressions. These closed-form expressions enable us to get important insights for future design of massive MIMO. Under keyhole effects, the FSI scheme yields a much higher rate than the UF scheme does, especially when the number of keyholes is small. Therefore, though the UF bounding technique is widely used in massive MIMO literature due to its simple closed-form expressions, the FSI bounding technique should be used if the channels are keyhole channels.

Hardening the Channels by Precoder Design in Massive MIMO With Multiple-Antenna Users

In this paper, we investigate the potential of a novel precoder for a massive multiple-input multiple-output (MIMO) system with multiple-antenna users under the influence of both Rayleigh and keyhole channel models. The proposed precoder is based on maximum-ratio processing and aims to enhance the channel hardening effect at the users. This is achieved through dividing each channel vector by its norm square. Closed-form expressions for the achievable spectral efficiency of the conventional and proposed precoders are derived. If modeled with Rayleigh fading, due to the physical hardening property of this channel, a moderate gain in performance is produced by the proposed precoding method, where the highest spectral efficiency gain occurs for a small number of users and user antennas. Under Rayleigh fading, the results also demonstrate that similar performance is returned by increasing the number of users or user antennas. By comparison, the proposed precoder is shown to produce significant performance gains in the keyhole channels under the condition that the system is limited to single-antenna users. Corresponding to this, the results showcase how the performance of massive MIMO under keyhole channels is heavily dependent on increasing the number of served users over user antennas.

Metal flow of weld pool and keyhole evolution in gas focusing plasma arc welding

Establishing an open keyhole throughout work-piece is the key to penetrate work-piece fully in gas focusing plasma arc welding. In order to study heat transfer, metal flow and keyhole surface evolution in gas focusing plasma arc welding, the three dimensional mathematical model is developed. Arc heat model and plasma arc pressure model are used to describe heat and force action in gas focusing plasma arc welding. Temperature field, fluid flow in weld pool and keyhole evolution are simulated. It is found that metal in weld pool front flows bypass keyhole wall toward weld pool back. Center line of keyhole channel is crocked instead of going along z-axis. The calculated weld shape agrees with the experimental data basically. This work may enrich theoretical knowledge about the complicated thermal physical process and provide basic data to guide welding application in gas focusing plasma arc welding.

Effect of Heat Input on Weld Formation and Tensile Properties in Keyhole Mode TIG Welding Process

Keyhole mode Tungsten Inert Gas (K-TIG) welding is a novel advanced deep penetration welding technology which provides an alternative to high power density welding in terms of achieving keyhole mode welding. In order to facilitate welding procedure optimisation in this newly developed welding technology, the relationship among welding parameters, weld formation and tensile properties during the K-TIG welding was investigated in detail. Results show that except for travel speed, the heat input level also plays an important role in forming undercut defect by changing the plasma jet trajectory inside keyhole channel, leading to the formation of hump in the weld centre and exacerbation of undercut formation. Both undercut defect and root side fusion boundary can act as a stress concentration point, which affects the fracture mode and tensile properties considerably. The research results provide a practical guidance of process parameter optimisation and quality assurance for the K-TIG welding process.

On the Performance of OAM in Keyhole Channels

The orbital angular momentum (OAM) is considered as a new degree of freedom (DOF) that could be utilized to enhance the capacity of wireless communication systems. However, it is known that uniform circular array (UCA)-based OAM in line-of-sight channels is a special case of the multiple-input multiple-output (MIMO) system. In this letter, we investigate the performance of UCA-based OAM communications in a “spatial keyhole” channel and reveal that in contrast to traditional MIMO, UCA-based OAM may provide additional DOF. In particular, the phenomenon that diffraction at the “spatial keyhole” does not change the helical phase structure of OAM modes provides a promising way to overcome the keyhole effect for traditional multi-antenna wireless communications.

Blind Modulation Recognition of the Lower Order PSK Signals Under the MIMO Keyhole Channel

The presence of a keyhole can severely degrade the signal identification performance of the multiple input multiple output (MIMO) system. In this letter, a novel algorithm, namely, the direct modulation recognition (DMR) algorithm is presented to classify the lower order PSK constellations under the spatially correlated MIMO keyhole and Rayleigh channels. In the proposed algorithm, the ratios of higher order cumulants derived from the received signal are used directly as classifying features. The advantage of the DMR algorithm is that it works without the knowledge of the channel state information. Simulation results are presented to show the efficacy of the proposed DMR algorithm.

Throughput and Delay Analysis of HARQ With Code Combining Over Double Rayleigh Fading Channels

This paper proposes the use of hybrid automatic repeat request (HARQ) with code combining (HARQ-CC) to offer reliable communications over double Rayleigh channels. The double Rayleigh fading channel is of particular interest to vehicle-to-vehicle communication systems as well as amplify-and-forward relaying and keyhole channels. This paper studies the performance of HARQ-CC over double Rayleigh channels from an information theoretic perspective. Analytical approximations are derived for the $\epsilon$ -outage capacity, the average number of transmissions, and the throughput of HARQ-CC. Moreover, we evaluate the delay experienced by Poisson-arriving packets for HARQ-CC. We provide analytical expressions for the average waiting time, the packets sojourn time, the average consumed power, and the energy efficiency. In our investigation, we take into account the impact of imperfect feedback on different performance metrics. Additionally, we explore the tradeoff between energy efficiency and the throughput. The proposed scheme is shown to maintain the outage probability below a specified threshold $\epsilon$ , which ensures the link reliability. Meanwhile, HARQ-CC adapts implicitly the transmission rate to the channel conditions such that the throughput is maximized. Our results demonstrate that HARQ-CC allows improving the achievable communication rate compared to fixed time diversity schemes. To maximize the throughput of HARQ-CC, the rate per HARQ round should be less than the rate required to meet the outage constraint. Our investigation of the performance of HARQ-CC over Rayleigh and double Rayleigh channels shows that double Rayleigh channels have a higher severity of fading and result in a larger degradation of the throughput. Our analysis reveals that HARQ with incremental redundancy achieves a larger throughput compared to HARQ-CC, whereas HARQ-CC is simpler to implement, has a lower decoding complexity, and requires less memory resources.

No Downlink Pilots Are Needed in TDD Massive MIMO

We consider the Massive Multiple-Input Multiple-Output downlink with maximum-ratio and zero-forcing processing and time-division duplex operation. To decode, the users must know their instantaneous effective channel gain. Conventionally, it is assumed that by virtue of channel hardening, this instantaneous gain is close to its average and hence that users can rely on knowledge of that average (also known as statistical channel information). However, in some propagation environments, such as keyhole channels, channel hardening does not hold. We propose a blind algorithm to estimate the effective channel gain at each user, that does not require any downlink pilots. We derive a capacity lower bound of each user for our proposed scheme, applicable to any propagation channel. Compared with the case of no downlink pilots (relying on channel hardening), and compared with training-based estimation using downlink pilots, our blind algorithm performs significantly better. The difference is especially pronounced in environments that do not offer channel hardening.

A unified 3D model for an interaction mechanism of the plasma arc, weld pool and keyhole in plasma arc welding

A 3D model is developed to perform numerical investigation on the coupled interaction mechanism of the plasma arc, weld pool and keyhole in plasma arc welding. By considering the traveling of the plasma arc along the welding direction, unified governing equations are solved in the whole domain including the torch, plasma arc, keyhole, weld pool and workpiece, which involves different physical mechanisms in different zones. The local thermodynamic equilibrium-diffusion approximation is used to treat the interface between the plasma arc and weld pool, and the volume-of-fluid method is used to track the evolution of the keyhole wall. The interaction effects between the plasma arc, keyhole and weld pool as well as the heat, mass and pressure transport phenomena in the whole welding domain are quantitatively simulated. It is found that when the torch is moving along the joint line, the axis of the keyhole channel tilts backward, and the envelope of molten metal surrounding the keyhole wall inside the weld pool is unsymmetrical relative to the keyhole channel. The plasma arc welding tests are conducted, and the predicted keyhole dimensions and the fusion zone shape are in agreement with the experimentally measured results.

How Many Antennas Should Be Activated in Keyhole Channels Under a Holistic Power Model

In this letter, we investigate the problem of how many transmit or receive antennas should be activated in keyhole channels with the consideration of a holistic power model. In conventional works where the circuit power is ignored, the system performance in terms of both ergodic capacity and outage probability improves with the increase of active antennas. However, when a holistic power model which includes the circuit power of RF chains as well as the transmission power is considered, there will exist a fundamental trade-off between the number of active antennas and the residual transmission power. Based on this trade-off, we derive a closed-form formula for the optimal number of active transmit or receive antennas in keyhole channels. In particular, by adopting the Meijer's G-function, we present concise expressions for the optimal numbers of active antennas that maximize the ergodic capacity and minimize the outage probability. Simulations will be shown in agreement with the theoretical results in a large extent.

Numerical simulation of plasma arc welding with keyhole-dependent heat source and arc pressure distribution

A three-dimensional and transient model with keyhole geometry-dependent heat source and arc pressure distribution is developed for plasma arc welding process. The influence of the keyhole evolution on heat flux and arc pressure distribution on the keyhole wall is taken into consideration. The dynamic variation of the temperature field and fluid flow in the weld pool as well as the keyhole shape and size are quantitatively analyzed. The experimentally observed phenomena, i.e., the backward curved keyhole channel and the very thin layer of molten metal at the front edge of weld pool, are first demonstrated with numerical simulation. The predicted establishment time of open keyhole, displacement of keyhole exit, and fusion line generally agree with the experimental ones.

Cascade Fading Channel Models for Wireless Communication- A Survey

Statistical theory plays an important role in channel modelling which in turn can be applied in design and performance evaluation of various communication systems. The design and analysis of cascade fading models have been an active area of interest in recent years due to its application in numerous real world scenarios such as keyhole channel and multihop communication system. These cascade fading models are developed by the product of independent but not necessarily identically distributed random variables. Many researchers are currently working in this area and new cascade fading models have been suggested recently in the literature. Due to ever-increasing demand and ubiquitous access of personal communication services, wireless systems are required to operate in increasingly hostile environments so there is a need of better models for wireless communication. So in order to find the gap areas in the literature pertaining to cascaded models, an exhaustive survey has been done here in this paper. This effort will surely help the researchers working in this area, to be able to identify the most appropriate fading channel model for an efficient wireless communication system design.

On the Error Probability of Spatial Modulation over Keyhole MIMO Channels

The keyhole effect causes a dramatic performance degradation to Multiple-Input Multiple-Output (MIMO) systems. The main contribution of this letter is the analytical performance evaluation of Spatial Modulation MIMO (SM-MIMO) systems over the keyhole channel. The closed-form Pairwise Error Probability (PEP) is derived with accuracy verified by simulations. Based on the PEP, the achievable diversity gain is extracted and shown to be no more than 1 under any configuration. In addition, a tight Average Bit Error Probability (ABEP) upper bound is given. Numerical results demonstrate that compared to independent fading scenarios, the keyhole channel causes a significant performance degradation to the SM-MIMO system. It is shown that SM-MIMO schemes outperforms Space-Time Block Code (STBC) and V-BLAST schemes over the keyhole channel. Therefore, among multiple antenna transmission schemes, SM-MIMO schemes are better choices for systems that may experience the keyhole effect. However, owing to the inherent unfitness of the keyhole channel for multiple antenna transmission, SM-MIMO schemes are still inferior to SISO/SIMO schemes.

On the Performance of Dual-Hop Systems with Multiple Antennas: Effects of Spatial Correlation, Keyhole, and Co-Channel Interference

In this paper, taking into account realistic propagation conditions, namely, spatial correlation, keyhole channels, and unequal-power co-channel interference, we investigate the performance of a wireless relay network where all the nodes are equipped with multiple antennas. Considering channel state information assisted amplify-and-forward protocol, we present analytical expressions for the symbol error rate (SER) and outage probability. More specifically, we first derive the SER expressions of a relay system with orthogonal space-time block coding (OSTBC) over correlated/keyhole fading channels. We also analyze the outage probability of interference corrupted relay systems with maximal ratio combing (MRC) at the receiver as well as multiple-input multiple-output MRC (MIMO MRC). Numerical results are given to illustrate and verify the analytical results.

Diversity in cascaded N*Nakagami channels

Short term fading conditions that are worse than those existing in typical Nakagami channels can be modeled using the cascaded approach where the received signal-to-noise ratio is treated as the product of N identical or nonidentical random variables that are either dependent or independent. Cascaded channels also permit the modeling of fading seen in amplify and forward relaying channels as well as keyhole channels. The simplest of these models, referred to as the N*Nakagami model, assumes that the received signal to noise ratio is the product of N independent and identically distributed gamma variables. The fading mitigation achieved in such cascaded channels through diversity is studied in this work. Through analytical approach and simulation, the performance of wireless channels is examined for the selection combining, equal gain combining, generalized selection combining, and maximal ratio combining algorithms. Results demonstrate the existence of severe degradations in cascaded fading channels and the comparative usefulness of the various algorithms in improving the data transmission capabilities. Even though the analysis was undertaken for the case of a coherent binary shift keying modem, results can easily be extended to other modems.