Line-dependent veiling in very active T Tauri Stars

The T Tauri stars with active accretion disks show veiled photospheric spectra. This is supposedly due to non-photospheric continuum radiated by hot spots beneath the accretion shocks at stellar surface and/or chromospheric emission lines radiated by the post-shocked gas. The amount of veiling is often considered as a measure of the mass-accretion rate. We analysed high-resolution photospheric spectra of accreting T Tauri stars LkHa 321, V1331 Cyg, and AS 353 A with the aim of clarifying the nature of the line-dependent veiling. Each of these objects shows a highly veiled, strong emission line spectrum and powerful wind features indicating high rates of accretion and mass loss. Equivalent widths of hundreds of weak photospheric lines were measured in the observed spectra and compared with those in synthetic spectra with the same spectral type. We found that the veiling is strongly line-dependent: larger in stronger photospheric lines and weak or absent in the weakest ones. No dependence of veiling on excitation potential within 0 to 5 eV was found. Different physical processes responsible for these unusual veiling effects are discussed in the framework of the magnetospheric accretion model. The observed veiling has two origins: 1) an abnormal structure of stellar atmosphere heated up by the accreting matter, and 2) a non-photospheric continuum radiated by a hot spot with temperature lower than 10000 K. The true level of the veiling continuum can be derived by measuring the weakest photospheric lines with equivalent widths down to $\approx$10 m\AA. A limited spectral resolution and/or low signal-to-noise ratio results in overestimation of the veiling continuum. In the three very active stars, the veiling continuum is a minor contributor to the observed veiling, while the major contribution comes from the line-dependent veiling.Comment: 10 pages, 10 figures. Accepted for publication in Astronomy and Astrophysic

TCM coding of PPM based modulations for Infrared WLAN's impaired by ISI

In this communication we consider the use of PPM based modulation methods, such as the hybrid modulation method called Amplitude and Pulse-Position Modulation (APPM) and Overlapping Pulse-Position Modulation (OPPM) to improve the performance of infrared WLAN’s, by the use of Trellis-Coded Modulation (TCM) codes. We describe the best code search and results, which demonstrate that, even with trellis codes of moderate complexity, non-negligible coding gains can be obtained without bandwidth expansion. Monte Carlo simulations have been done to compare uncoded PPM against APPM and OPPM TCM coded systems performances, on ceiling-bounce channel models for various values of delay spread. Although our codes were derived for the AWGN channel without Inter-Symbol Interference (ISI) awareness, their behavior in multipath dispersion channels is quite effective

Performance of extended space-time coding techniques for MIMO MC-CDMA systems

In this paper we consider a transmission system based on MC-CDMA, where signal spreading is performed entirely in the frequency domain. An extended spacetime coding technique, the Double Alamouti, is evaluated considering a MIMO channel. This assessment is made against standard Alamouti coding, for two different Tx/Rx antenna schemes (2x1 and 2x2). Numerical results, attained through system model simulations, are presented for performance evaluation under realistic scenarios considering some typical system impairments. These results show that in practical systems significant improvements can be achieved by using the Double Alamouti coding scheme

Simulation-based evaluation of spectrum opportunities in UMTS cellular networks

A scenario based on an UMTS TDD opportunistic cellular system that operates over UMTS FDD licensed cellular networks is considered. Therefore we develop a simulation tool that addresses the goal of analysis and assessment of UMTS TDD opportunistic radio system in a coexistence environment with UMTS FDD primary cellular networks. The communication presents the scenario considered, the main features of the tool, discusses and proposes metrics to evaluate the communication opportunities in UMTS FDD primary cellular networks, and presents numerical simulation results. These show that a moderate number of UMTS TDD available frequencies and a reasonable UMTS TDD opportunistic radio transmission power will allow the deployment of UMTS TDD opportunistic radio networks in a coexistence environment with existing licensed systems

MIMO-OFDM channel estimation: a pilot sequence design for time-domain processing

A pilot sequence design for MIMO-OFDM systems is investigated where all transmit antennas share the same sub-carriers to convey pilot symbols. The pilot sequences are endowed with phase-shifting properties, granting the receiver the possibility of extracting the impulse responses for all channels directly from the antennas’ time-domain received signals, without co-channel interference. A linear processing of the timedomain samples is all that is needed to get the channel impulse responses, resulting in an algorithm with very low computational load. The feasibility of the investigated method is substantiated by system simulation using indoor and outdoor broadband wireless channel models

Lossy source coding using belief propagation and soft-decimation over LDGM codes

This paper focus on the lossy compression of a binary symmetric source. We propose a new algorithm for binary quantization over low density generator matrix (LDGM) codes. The proposed algorithm is a modified version of the belief propagation (BP) algorithm used in the channel coding framework and has linear complexity in the code block length. We also provide a common framework under which the proposed algorithm and some previously proposed algorithms fit. Simulation results show that our scheme achieves close to state-of-the-art performance with reduced complexity

Frequency domain equalizer for multirate UMTS-TDD systems

Direct-sequence spread spectrum (DS-SS) signals exhibit cyclostationary properties which imply a redundancy between frequency components separated by multiples of the symbol rate. In this paper we present a multiple access interference canceller that explores this property and applies to UMTS-TDD. This linear frequency domain canceller operates in the spreaded signal in such a way that the interference and noise at its output is minimized (minimum mean squared error criterium). The performance is evaluated in two configurations: one including the frequency shift canceller (FSC) and the other concatenated with parallel interference canceller (PIC). The results are benchmarked against the performance of the conventional RAKE detector and the conventional PIC detector

Sensing opportunities in UMTS spectrum

The UMTS radio frequency spectrum is a highly expensive commodity. While the UMTS standards make very efficient use of the allocated bands there is however opportunity for further advancements. This paper focuses on opportunistic use of the UMTS spectrum as a means of ensuring that the maximum possible use of this valuable resource is made. In particular we focus on the local detection of UMTS TDD signals through the use of a cyclostationary feature detector. Simulation results for the use of this detector in the presence of multipath propagation and shadowing effects are presented

Binary dirty paper coding

This paper proposes a practical scheme for implementing binary dirty paper coding (DPC) using a low density generator matrix code (LDGM) concatenated with a high rate low density parity check (LDPC) code. We also propose a new algorithm, a modified version of the belief propagation algorithm (BP), for doing lossy source coding at the encoder, with linear complexity in the block length. In contrast to the superposition coding framework, where high order alphabet codes are used, we propose to implement binary DPC using only binary codes. Through application of approximate density evolution and linear programming we optimize the degree distribution of the proposed code. Simulation results show that our scheme achieves close to state-of-the-art performance with reduced complexity

Mitigating CFO in OFDM systems by exploring the symbol structure

In this paper we revisit the carrier frequency offset (CFO) estimation and removal for broadband orthogonal frequency division multiplexing (OFDM) systems. We propose an algorithm that explores the time-domain (TD) properties of transmitted symbols carrying pilots and data. A careful examination of the components that constitute the TD received symbols leads to the definition of a simple, yet effective, cost function. In a real scenario, the cost function presents a bowl shape with a single global minimum easily identifiable (no local minima). The proposed method is independent of the channel estimation and decision, presenting a very low computational load. The feasibility of the investigated method is substantiated by system simulation using BRAN-A broadband wireless channel model. Simulation results show that the impact of the residual CFO in the system¿s performance is minimal