Hosam El-Ocla

Backscattering enhancement for partially convex targets of large sizes in continuous random media for E-wave incidence

Abstract The shape of the target constitutes an important factor in the radar detection problem. In a previous study, the enhancement in the radar cross section (ERCS) has proved to be affected largely by the target parameters as well as the effects of the double passage and the spatial coherence length of incident waves around the target. However, the target size was limited to about less than one wavelength. Here, we estimate numerically the RCS of targets taking large sizes of more than three wavelengths, and analyse the characteristics of RCS. Moreover, we investigate the ERCS phenomenon of such targets under different circumstances of random medium and target configuration. In this regard, we assume partially convex targets in continuous random media and also horizontal incident wave polarization (E-wave incidence).

TCP CERL: congestion control enhancement over wireless networks

In this paper, we propose and verify a modified version of TCP Reno that we call TCP Congestion Control Enhancement for Random Loss (CERL). We compare the performance of TCP CERL, using simulations conducted in ns-2, to the following other TCP variants: TCP Reno, TCP NewReno, TCP Vegas, TCP WestwoodNR and TCP Veno. TCP CERL is a sender-side modification of TCP Reno. It improves the performance of TCP in wireless networks subject to random losses. It utilizes the RTT measurements made throughout the duration of the connection to estimate the queue length of the link, and then estimates the congestion status. By distinguishing random losses from congestion losses based on a dynamically set threshold value, TCP CERL successfully attacks the well-known performance degradation issue of TCP over channels subject to random losses. Unlike other TCP variants, TCP CERL doesn’t reduce the congestion window and slow start threshold when random loss is detected. It is very simple to implement, yet provides a significant throughput gain over the other TCP variants mentioned above. In single connection tests, TCP CERL achieved an 175, 153, 85, 64 and 88% throughput gain over TCP Reno, TCP NewReno, TCP Vegas, TCP WestwoodNR and TCP Veno, respectively. In tests with multiple coexisting connections, TCP CERL achieved an 211, 226, 123, 70 and 199% throughput improvement over TCP Reno, TCP NewReno, TCP Vegas, TCP WestwoodNR and TCP Veno, respectively.

The effect of H-polarization on backscattering enhancement for partially convex targets of large sizes in continuous random media

Abstract In our previous work, we have proved that the spatial coherence length (SCL) of the incident waves around the target together with the target configuration play a leading role in the determination of the enhancement in the radar cross-section (ERCS) of a target in a random medium. Owing to the double-passage effect, the ERCS is almost two when the SCL is much smaller or larger than the target size. However, for a SCL comparable with the target size, the ERCS deviates from two, depending on the target parameters and the SCL. The last conclusion was proved only for E-polarization. The polarization of incident waves is one of the key parameters in scattering problems. In this work, we extend our study and investigate the effect of H-polarization on the radar cross-section and the ERCS for large-size targets.

ON LASER RADAR CROSS SECTION OF TARGETS WITH LARGE SIZES FOR E-POLARIZATION

In this paper a study is presented to handle the behavior of radar cross section (RCS) of partially convex targets of large sizes up to five wavelengths in free space. The nature of incident wave is an important factor in the remote sensing and radar detection applications. To investigate the effects of incident wave nature on the RCS, scattering problems of plane and beam wave incidences are considered. Targets are taking large sizes to be bigger enough than the beam width with putting into consideration a horizontal incident wave polarization (E-wave incidence). The effects of the target configuration together with the beam width on the laser RCS compared to the case with the plane wave incidence are numerically analyzed. Therefore, we will be able to have some sort of control on radar detection using beam wave incidence.

Backscattering from conducting targets in continuous random media for circular polarization

The polarization of incident waves is one of the key factors in radar detection and remote sensing problems. In a previous study, attention was drawn to the anomalous increase in the radar cross-section (RCS) of a target in a random medium that occurs with H-polarization. This large increase occurs with a small-size target and is attributed to the coupling between the direct and creeping waves. In this work, we aim to probe the effect of the creeping waves on the scattering waves for circular wave polarization and compare it with the previous results. Therefore, we can control the target detection by choosing the proper polarization, which does not lead to anomalous phenomena. In doing so, we present numerical results for RCS and analyse the characteristics of the enhancement in the RCS (ERCS) behaviour of targets in random media. In this regard, we assume partially convex targets of different configurations. We consider the case in which a directly incident wave is produced by a line source distributed uniformly along the axis parallel to the conducting cylinder (target) axis. Then we can deal with this scattering problem two-dimensionally under the condition of strong continuous random media with large local scale size.

An indirect estimate of RCS of conducting target in random medium

This paper discusses an indirect estimate of the radar cross section (RCS) of a conducting cylinder with a partially convex cross-section under the condition that backscattering enhancement occurs in a random medium. Once we evaluate the spatial coherence length (SCL) of the incident waves around the cylinder in the random medium, we may estimate the RCS approximately from the RCS in free space with beam wave incidence in which the spot size equals the SCL. This method is restricted only to the case where the SCL is larger than the target size.

Laser backscattered from partially convex targets of large sizes in random media for E-wave polarization.

The characteristics of a radar cross section (RCS) of partially convex targets with large sizes up to five wavelengths in free space and random media are studied. The nature of the incident wave is an important factor in remote sensing and radar detection applications. I investigate the effects of beam wave incidence on the performance of RCS, drawing on the method I used in a previous study on plane-wave incidence. A beam wave can be considered a plane wave if the target size is smaller than the beam width. Therefore, to have a beam wave with a limited spot on the target, the target size should be larger than the beam width (assuming E-wave incidence wave polarization. The effects of the target configuration, random medium parameters, and the beam width on the laser RCS and the enhancement in the radar cross section are numerically analyzed, resulting in the possibility of having some sort of control over radar detection using beam wave incidence.

Target configuration effect on wave scattering in random media with horizontal polarization

In a previous study, the radar cross-section (RCS) was shown to be influenced largely by the curvature of the illumination region of the target. Scattering data from smooth concave–convex contours reveals the obvious impact of specular reflections on the behavior of RCS. This effect needs to be investigated more especially with relatively complex surfaces. Here, we work on a numerical calculation of the RCS and analyze its characteristics with different target configurations including complexity and size. We postulate a concave illumination region and consider targets that are taking large sizes of about five wavelengths. In this communication, we assume wave propagation and scattering from targets in free space and random medium with consideration of horizontal incident wave polarization.

Electromagnetic Scattering From 3-D Targets in a Random Medium Using Finite Difference Frequency Domain

In this paper a three-dimensional (3-D) numerical technique based on the finite difference frequency domain (FDFD) is implemented to calculate the scattering from arbitrary shaped objects embedded in a continuous random medium. The total field/scattered field (TF/SF) algorithm is integrated with the FDFD to minimize the memory consumption and speed up the calculations. For validation purposes, the radar cross-section of a 2-D conducting cylinder in random medium is calculated using the FDFD technique and compared to previously published data based on the current generator method. An upgrade to a 3-D solver was then inspired once the idea of using the multi-grid technique was introduced to accelerate the convergence rate of the BICGSTAB iterative solver. Thus, allowing the FDFD technique to become a robust method to solve the scattering problem from large targets embedded in random medium. Therefore, using the introduced simulating scheme, one can easily elucidate any scattering information out of real life targets surrounded by random environmental effects.

TCP congestion avoidance model with congestive loss

This paper presents a model for the TCP congestion avoidance algorithm in networks with only congestive loss. The model predicts the congestion window evolution, throughput and packet loss rate of sustained TCP flows. In other TCP models, packet loss is regarded as exogenous random process. While such assumption is valid in wireless networks, in the wired networks the packet losses are actually the result of network congestion. In this regard, the packet loss rate, the window evolution and throughput of a TCP connection are determined by the bandwidth, delay, buffer size of the underlying network path, the transmitted packet size and number of concurrent TCP flows. We derive formulas to express the TCP congestion window evolution, throughput and packet loss rate as functions of these parameters. The results are verified with simulations and compared with prior works in this area.