Independent Subbands Research Articles

Improved Wideband Adaptive Transmit Nulling Using Subband Phase-Only Pattern Synthesis

The phase-only transmit nulling method is widely used in narrowband arrays, but there are few reports of its application in wideband arrays. This paper describes the implementation of adaptive transmit nulls in wideband arrays based on subband phase-only pattern synthesis behind each array element. The filter bank behind each array element partitions the transmit signal into independent subbands and utilizes a phase shifter group in each subband to form frequency invariant spatial nulls in the array’s transmit pattern. The scheme developed in this paper includes an algorithm for computing a wideband phase-only weight vector and a subband phase-only adaptive signal processing method based on band partitioning. The validity of the scheme is proven by theoretical analysis and simulation experiments.

Spin drift and diffusion in one- and two-subband helical systems

The theory of spin drift and diffusion in two-dimensional electron gases is developed in terms of a random walk model incorporating Rashba, linear and cubic Dresselhaus, and intersubband spin-orbit couplings. The additional subband degree of freedom introduces new characteristics to the persistent spin helix (PSH) dynamics. As has been described before, for negligible intersubband scattering rates, the sum of the magnetization of independent subbands leads to a checkerboard pattern of crossed PSHs with long spin lifetime. For strong intersubband scattering we model the fast subband dynamics as a new random variable, yielding a dynamics set by averaged spin-orbit couplings of both subbands. In this case the crossed PSH becomes isotropic, rendering circular (Bessel) patterns with short spin lifetime. Additionally, a finite drift velocity breaks the symmetry between parallel and transverse directions, distorting and dragging the patterns. We find that the maximum spin lifetime shifts away from the PSH regime with increasing drift velocity. We present approximate analytical solutions for these cases and define their domain of validity. Effects of magnetic fields and initial package broadening are also discussed.

Robust Transmit Nulling in Wideband Arrays

The ability to create nulls in the transmit pattern of a phased array antenna has many applications in communication and radar systems, including interference and clutter mitigation. This paper describes the implementation of transmit nulls in wideband arrays through the use of a filter bank inserted behind each array element. The filter bank decimates and partitions the transmit signal into independent subbands and utilizes a tapped delay line (TDL) in each subband to form frequency invariant spatial nulls in the array's transmit pattern. New contributions developed in this paper include an algorithm for determining the TDL coefficients and a novel band partitioning scheme based on principal component filter banks (PCFBs), which is shown to be optimal for minimizing spectral errors if the TDL coefficients are quantized. Numerical techniques are presented for approximating ideal PCFBs using practical paraunitary filter banks (PUFBs) and perfect reconstruction filter banks (PRFBs).

METHOD TO GENERATE MULTI-CHANNEL AUDIO SIGNAL FROM STEREO SIGNALS

An exemplary embodiment of the invention can generate multiple output audio signals from multiple input audio signals, in which the number of output signals is equal to or higher than the number of input signals. The embodiment includes computing one or more independent sound subbands representing signal components which are independent between the input subbands; computing one or more localized direct sound subbands representing signal components which are contained in more than one of the input subbands and direction factors representing the ratios with which these signal components are contained in two or more input subbands; generating the output subband signals, where each output subband signal is a linear combination of the independent sound subbands and the localized direct sound subbands; and converting the output subband signals to time domain audio signals.

Distributive Stochastic Learning for Delay-Optimal OFDMA Power and Subband Allocation

In this paper, we consider the distributive queue-aware power and subband allocation design for a delay-optimal OFDMA uplink system with one base station, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> users and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> independent subbands. Each mobile has an uplink queue with heterogeneous packet arrivals and delay requirements. We model the problem as an infinite horizon average reward Markov decision problem (MDP) where the control actions are functions of the instantaneous channel state information (CSI) as well as the joint queue state information (QSI). To address the distributive requirement and the issue of exponential memory requirement and computational complexity, we approximate the subband allocation <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factor by the sum of the per-user subband allocation <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factor and derive a distributive online stochastic learning algorithm to estimate the per-user <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factor and the Lagrange multipliers (LM) simultaneously and determine the control actions using an auction mechanism. We show that under the proposed auction mechanism, the distributive online learning converges almost surely (with probability 1). For illustration, we apply the proposed distributive stochastic learning framework to an application example with exponential packet size distribution. We show that the delay-optimal power control has the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">multilevel water-filling</i> structure where the CSI determines the instantaneous power allocation and the QSI determines the water-level. The proposed algorithm has linear signaling overhead and computational complexity <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">KN</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> ), which is desirable from an implementation perspective.

Novel split-band erbium-doped fiber amplifier

In this paper we design a 120 nm bandwidth erbium-doped fiber amplifier using a seed light injection method for the first time. It is shown that when the wavelength of the seed light varies from 1520 to 1570 nm , the gain at 1610 nm reaches maximum near 1553 nm and increases as the seed light power. Based on the method, a novel split-band EDFA configuration is designed. The split-band architecture employs a 1550/1610 nm waveband multiplexer/demultiplexer and two independent sub-bands which pass in parallel through separate branches of the optical amplifier. The signals at independent sub-bands are combined and flattened before output, resulting in a broadband gain-flattened optical amplifier with 120 nm bandwidth.

Subband architecture for automatic speaker recognition

We present an original approach for automatic speaker identification especially applicable to environments which cause partial corruption of the frequency spectrum of the signal. The general principle is to split the whole frequency domain into several subbands on which statistical recognizers are independently applied and then recombined to yield a global score and a global recognition decision. The choice of the subband architecture and the recombination strategies are particularly discussed. This techniques had been shown to be robust for speech recognition when a narrow band noise degradation occur. We first objectively verify this robustness for the speaker identification task. We also study which information is really used to recognize speakers. For this, speaker identification experiments on independent subbands are conducted for 630 speakers of TIMIT and NTIMIT databases. The results show that the speaker specific information is not equally distributed among subbands. In particular, the low-frequency subbands (under 600 Hz) and the high-frequency subbands (over 3000 Hz) are more speaker-specific than middle-frequency ones. In addition, experiments on different subband system arechitectures show that the correlations between frequency channels are of prime importance for speaker recognition. Some of these correlations are lost when the frequency domain is divided into subbands. Consequently, we propose a particularly redundant parallel architecture for which most of the correlations are kept. The performances obtained with this new system, using linear recombination strategies, are equivalent to those of a conventional fullband recognizer on clean and telephone speech. Experiments on speech corrupted by unpredictable noise show a better adaptability of this approach in noisy environments, compared to a conventional device, especially when pruning of some recognizers is performed.

Exciton dynamics in GaAs/AlGaAs multiple quantum wells investigated by picosecond reflectivity and luminescence measurements

Picosecond time-resolved induced reflectivity measurements were performed in narrow well (∼4.2 nm) GaAs/Al0.4Ga0.6As multiple quantum well systems in order to investigate fast excitonic relaxation. In a wide-barrier sample, - corresponding to independent subbands in each quantum well - a single exponential decay of the induced reflectivity is observed at very low power levels, corresponding to the population decay of the lowest exciton (hh1-e1). At higher excitation levels, a second, faster component appears, which is ascribed to exciton-exciton interactions. The amplitude of both components shows resonances at the energy of the lowest heavy and light hole exciton transitions. Comparison is made with induced transparency measurements on a back-etched specimen. In a thin barrier sample (LB∼4.2 nm) - in which the exciton states couple through the barriers - a short, energy dependent decay time is measured, for which correlation with the photoluminescence decay is not straightforward.