Radio-Optical Imaging of ATLBS Survey

We present the radio-optical imaging of ATLBS, a sensitive radio survey (Subrahmanyan et al. 2010). The primary aim of the ATLBS survey is to image low-power radio sources which form the bulk of the radio source population to moderately high red-shifts (z similar to 1.0). The accompanying multiband optical and near infra-red observations provide information about the hosts and environments of the radio sources. We give here details of the imaging of the radio data and optical data for the ATLBS survey.

Object Tracking Using Motion Direction Detection

We present an algorithm for tracking objects in a video sequence, based on a novel approach for motion detection. We do not estimate the velocity �eld. In-stead we detect only the direction of motion at edge points and thus isolate sets of points which are moving coherently. We use a Hausdor� distance based matching algorithm to match point sets in local neighborhood and thus track objects in a video sequence. We show through some examples the e�ectiveness of the algo- rithm.

Mechanical properties of ZnS nanowires and thin films: Microscopic origin of the dependence on size and growth direction

Mechanical properties of ZnS nanowires and thin films are studied as a function of size and growth direction using all-atom molecular dynamics simulations. Using the stress-strain relationship we extract Young's moduli of nanowires and thin films at room temperature. Our results show that Young's modulus of 0001] nanowires has strong size dependence. On the other hand, 01 (1) over bar0] nanowires do not exhibit a strong size dependence of Young's modulus in the size range we have investigated. We provide a microscopic understanding of this behavior on the basis of bond stretching and contraction due to the rearrangement of atoms in the surface layers. The ultimate tensile strengths of the nanowires do not show much size dependence. To investigate the mechanical behavior of ZnS in two dimensions, we calculate Young's modulus of thin films under tensile strain along the 0001] direction. Young's modulus of thin films converges to the bulk value more rapidly than that of the 0001] nanowire.

Pulsed radio emission from the Fermi-LAT pulsar J1732-3131: Search and a possible detection at 34.5 MHz

We report our search for and a possible detection of periodic radio pulses at 34.5 MHz from the Fermi Large Area Telescope pulsar J1732-3131. The candidate detection has been possible in only one of the many sessions of observations made with the low-frequency array at Gauribidanur, India, when the otherwise radio weak pulsar may have apparently brightened many folds. The candidate dispersion measure along the sight line, based on the broad periodic profiles from �20min of data, is estimated to be 15.44 ± 0.32 pccc -1. We present the details of our periodic and single-pulse search, and discuss the results and their implications relevant to both, the pulsar and the intervening medium. © 2012 RAS.

HIGH-RESOLUTION IMAGING OF THE ATLBS REGIONS: THE RADIO SOURCE COUNTS

The Australia Telescope Low-brightness Survey (ATLBS) regions have been mosaic imaged at a radio frequency of 1.4 GHz with 6 `' angular resolution and 72 mu Jy beam(-1) rms noise. The images (centered at R. A. 00(h)35(m)00(s), decl. -67 degrees 00'00 `' and R. A. 00(h)59(m)17(s), decl. -67.00'00 `', J2000 epoch) cover 8.42 deg(2) sky area and have no artifacts or imaging errors above the image thermal noise. Multi-resolution radio and optical r-band images (made using the 4 m CTIO Blanco telescope) were used to recognize multi-component sources and prepare a source list; the detection threshold was 0.38 mJy in a low-resolution radio image made with beam FWHM of 50 `'. Radio source counts in the flux density range 0.4-8.7 mJy are estimated, with corrections applied for noise bias, effective area correction, and resolution bias. The resolution bias is mitigated using low-resolution radio images, while effects of source confusion are removed by using high-resolution images for identifying blended sources. Below 1 mJy the ATLBS counts are systematically lower than the previous estimates. Showing no evidence for an upturn down to 0.4 mJy, they do not require any changes in the radio source population down to the limit of the survey. The work suggests that automated image analysis for counts may be dependent on the ability of the imaging to reproduce connecting emission with low surface brightness and on the ability of the algorithm to recognize sources, which may require that source finding algorithms effectively work with multi-resolution and multi-wavelength data. The work underscores the importance of using source lists-as opposed to component lists-and correcting for the noise bias in order to precisely estimate counts close to the image noise and determine the upturn at sub-mJy flux density.

Precoder optimization in cognitive radio with interference constraints

We consider precoding strategies at the secondary base station (SBS) in a cognitive radio network with interference constraints at the primary users (PUs). Precoding strategies at the SBS which satisfy interference constraints at the PUs in cognitive radio networks have not been adequately addressed in the literature so far. In this paper, we consider two scenarios: i) when the primary base station (PBS) data is not available at SBS, and ii) when the PBS data is made available at the SBS. We derive the optimum MMSE and Tomlinson-Harashima precoding (THP) matrix Alters at the SBS which satisfy the interference constraints at the PUs for the former case. For the latter case, we propose a precoding scheme at the SBS which performs pre-cancellation of the PBS data, followed by THP on the pre-cancelled data. The optimum precoding matrix filters are computed through an iterative search. To illustrate the robustness of the proposed approach against imperfect CSI at the SBS, we then derive robust precoding filters under imperfect CSI for the latter case. Simulation results show that the proposed optimum precoders achieve good bit error performance at the secondary users while meeting the interference constraints at the PUs.

A novel algorithm for cooperative distributed sequential spectrum sensing in Cognitive Radio

This paper considers cooperative spectrum sensing in Cognitive Radios. In our previous work we have developed DualSPRT, a distributed algorithm for cooperative spectrum sensing using Sequential Probability Ratio Test (SPRT) at the Cognitive Radios as well as at the fusion center. This algorithm works well, but is not optimal. In this paper we propose an improved algorithm- SPRT-CSPRT, which is motivated from Cumulative Sum Procedures (CUSUM). We analyse it theoretically. We also modify this algorithm to handle uncertainties in SNR's and fading.

Performance analysis of data and voice connections in a cognitive radio network

We study the performance of cognitive (secondary) users in a cognitive radio network which uses a channel whenever the primary users are not using the channel. The usage of the channel by the primary users is modelled by an ON-OFF renewal process. The cognitive users may be transmitting data using TCP connections and voice traffic. The voice traffic is given priority over the data traffic. We theoretically compute the mean delay of TCP and voice packets and also the mean throughput of the different TCP connections. We compare the theoretical results with simulations.

Correlation between corona current and radio interference due to high voltage insulator string

The Radio Interference (RI) from electric power transmission line hardware, if not controlled, poses serious electromagnetic interference to system in the vicinity. The present work mainly concerns with the RI from the insulator string along with the associated line hardware. The laboratory testing for the RI levels are carried out through the measurement of the conducted radio interference levels. However such measurements do not really locate the coronating point, as well as, the mode of corona. At the same time experience shows that it is rather difficult to locate the coronating points by mere inspection. After a thorough look into the intricacies of the problem, it is ascertained that the measurement of associated ground end currents could give a better picture of the prevailing corona modes and their intensities. A study on the same is attempted in the present work. Various intricacies of the problem,features of ground end current pulses and its correlation with RI are dealt with. Owing to the complexity of such experimental investigations, the study made is not fully complete nevertheless it seems to be first of its kind.

Isotropic heating of galaxy cluster cores via rapidly reorienting active galactic nucleus jets

Active galactic nucleus (AGN) jets carry more than sufficient energy to stave off catastrophic cooling of the intracluster medium (ICM) in the cores of cool-core clusters. However, in order to prevent catastrophic cooling, the ICM must be heated in a near-isotropic fashion and narrow bipolar jets with P-jet = 10(44-45) erg s(-1), typical of radio AGNs at cluster centers, are inefficient in heating the gas in the transverse direction to the jets. We argue that due to existent conditions in cluster cores, the supermassive black holes (SMBHs) will, in addition to accreting gas via radiatively inefficient flows, experience short stochastic episodes of enhanced accretion via thin disks. In general, the orientation of these accretion disks will be misaligned with the spin axis of the black holes (BHs) and the ensuing torques will cause the BH's spin axis (and therefore the jet axis) to slew and rapidly change direction. This model not only explains recent observations showing successive generations of jet-lobes-bubbles in individual cool-core clusters that are offset from each other in the angular direction with respect to the cluster center, but also shows that AGN jets can heat the cluster core nearly isotropically on the gas cooling timescale. Our model does require that the SMBHs at the centers of cool-core clusters be spinning relatively slowly. Torques from individual misaligned disks are ineffective at tilting rapidly spinning BHs by more than a few degrees. Additionally, since SMBHs that host thin accretion disks will manifest as quasars, we predict that roughly 1-2 rich clusters within z < 0.5 should have quasars at their centers.

Cognitive radio implementation for a frequency hopping primary signal

Frequency hopping communications, used in the military present significant opportunities for spectrum reuse via the cognitive radio technology. We propose a MAC which incorporates hop instant identification, and supports network discovery and formation, QOS Scheduling and secondary communications. The spectrum sensing algorithm is optimized to deal with the problem of spectral leakage. The algorithms are implemented in a SDR platform based test bed and measurement results are presented.

SARAS: a precision system for measurement of the cosmic radio background and signatures from the epoch of reionization

SARAS is a correlation spectrometer purpose designed for precision measurements of the cosmic radio background and faint features in the sky spectrum at long wavelengths that arise from redshifted 21-cm from gas in the reionization epoch. SARAS operates in the octave band 87.5-175 MHz. We present herein the system design arguing for a complex correlation spectrometer concept. The SARAS design concept provides a differential measurement between the antenna temperature and that of an internal reference termination, with measurements in switched system states allowing for cancellation of additive contaminants from a large part of the signal flow path including the digital spectrometer. A switched noise injection scheme provides absolute spectral calibration. Additionally, we argue for an electrically small frequency-independent antenna over an absorber ground. Various critical design features that aid in avoidance of systematics and in providing calibration products for the parametrization of other unavoidable systematics are described and the rationale discussed. The signal flow and processing is analyzed and the response to noise temperatures of the antenna, reference termination and amplifiers is computed. Multi-path propagation arising from internal reflections are considered in the analysis, which includes a harmonic series of internal reflections. We opine that the SARAS design concept is advantageous for precision measurement of the absolute cosmic radio background spectrum; therefore, the design features and analysis methods presented here are expected to serve as a basis for implementations tailored to measurements of a multiplicity of features in the background sky at long wavelengths, which may arise from events in the dark ages and subsequent reionization era.