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Depolarization for quantum channels with higher symmetries  [PDF]
A. B. Klimov,L. L. Sanchez-Soto
Physics , 2009, DOI: 10.1088/0031-8949/2010/T140/014009
Abstract: The depolarization channel is usually modelled as a quantum operation that destroys all input information, replacing it by a completely chaotic state. For qubits this has a quite intuitive interpretation as a shrinking of the Bloch sphere. We propose a way to deal with depolarizing dynamics (in the Markov approximation) for systems with arbitrary symmetries.
Study of Gaussian Relay Channels with Correlated Noises  [PDF]
Lili Zhang,Jinhua Jiang,Andrea J. Goldsmith,Shuguang Cui
Mathematics , 2010,
Abstract: In this paper, we consider full-duplex and half-duplex Gaussian relay channels where the noises at the relay and destination are arbitrarily correlated. We first derive the capacity upper bound and the achievable rates with three existing schemes: Decode-and-Forward (DF), Compress-and-Forward (CF), and Amplify-and-Forward (AF). We present two capacity results under specific noise correlation coefficients, one being achieved by DF and the other being achieved by direct link transmission (or a special case of CF). The channel for the former capacity result is equivalent to the traditional Gaussian degraded relay channel and the latter corresponds to the Gaussian reversely-degraded relay channel. For CF and AF schemes, we show that their achievable rates are strictly decreasing functions over the negative correlation coefficient. Through numerical comparisons under different channel settings, we observe that although DF completely disregards the noise correlation while the other two can potentially exploit such extra information, none of the three relay schemes always outperforms the others over different correlation coefficients. Moreover, the exploitation of noise correlation by CF and AF accrues more benefit when the source-relay link is weak. This paper also considers the optimal power allocation problem under the correlated-noise channel setting. With individual power constraints at the relay and the source, it is shown that the relay should use all its available power to maximize the achievable rates under any correlation coefficient. With a total power constraint across the source and the relay, the achievable rates are proved to be concave functions over the power allocation factor for AF and CF under full-duplex mode, where the closed-form power allocation strategy is derived.
The Wideband Slope of Interference Channels: The Small Bandwidth Case  [PDF]
Minqi Shen,Anders H?st-Madsen
Mathematics , 2012,
Abstract: This paper studies the low-SNR regime performance of a scalar complex K -user interference channel with Gaussian noise. The finite bandwidth case is considered, where the low-SNR regime is approached by letting the input power go to zero while bandwidth is small and fixed. We show that for all \delta>0 there exists a set with non-zero measure (probability) in which the wideband slope per user satisfies Slope<2/K+\delta . This is quite contrary to the large bandwidth case [ShenAHM11IT], where a slope of 1 per user is achievable with probability 1. We also develop an interference alignment scheme for the finite bandwidth case that shows some gain.
Spin Depolarization in Quantum Wires Polarized Spontaneously in a Zero Magnetic Field  [PDF]
N. T. Bagraev,V. K. Ivanov,L. E. Klyachkin,I. A. Shelykh
Physics , 2004, DOI: 10.1103/PhysRevB.70.155315
Abstract: The conditions for a spontaneous spin polarization in a quantum wire positioned in a zero magnetic field are analyzed under weak population of one-dimensional subbands that gives rise to the efficient quenching of the kinetic energy by the exchange energy of carriers. The critical linear concentration of carriers above which the quasi one-dimensional gas undergoes a complete spin depolarization is determined by the Hartree-Fock approximation. The dependence of the critical linear concentration on the concentration of carriers is defined to reveal the interplay of the spin depolarization with the evolution of the 0.7 (2e2/h) feature in the quantum conductance staircase from the e2/h to 3/2 (e2/h) values. This dependence is used to study the effect of the hole concentration on the 0.7 (2e2/h) feature in the quantum conductance staircase of the quantum wire prepared inside the p-type silicon quantum well using the split-gate technique. The 1D channel is demonstrated to be spin-polarized at the linear concentration of holes lower than the critical linear concentration, because the 0.7 (2e2/h) feature is close to the value of 0.5 (2e2/h) that indicates the spin degeneracy lifting for the first step of the quantum conductance staircase. The 0.7 (2e2/h) feature is found to take however its normal magnitude when the linear concentration of holes attains the critical value corresponding to the spin depolarization. The variations in the height of the 0.7 (2e2/h) feature observed in the hole quantum conductance staircase that is revealed by the p-type silicon quantum wire seem to be related to the evidences of the quantum conductance staircase obtained by varying the concentration of electrons in the 1D channel prepared inside the GaAs-AlGaAs heterojunction.
Spectra for the product of Gaussian noises  [PDF]
L. B. Kish,R. Mingesz,Z. Gingl,C. G. Granqvist
Physics , 2012,
Abstract: Products of Gaussian noises often emerge as the result of non-linear detection techniques or as a parasitic effect, and their proper handling is important in many practical applications, including in fluctuation-enhanced sensing, indoor air or environmental quality monitoring, etc. We use Rice's random phase oscillator formalism to calculate the power density spectra variance for the product of two Gaussian band-limited white noises with zero-mean and the same bandwidth W. The ensuing noise spectrum is found to decrease linearly from zero frequency to 2W, and it is zero for frequencies greater than 2W. Analogous calculations performed for the square of a single Gaussian noise confirm earlier results. The spectrum at non-zero frequencies, and the variance of the square of a noise, is amplified by a factor two as a consequence of correlation effects between frequency products. Our analytic results is corroborated by computer simulations.
Image Cipher Technique for Covert and Low Bandwidth Channels  [PDF]
Sangeeta Solanki,A.K.Vats,Shikha Maan
International Journal of Computer Technology and Applications , 2011,
Abstract: Security of images during the transmission over covert low bandwidth channel has importance in today's image communications for confidential, integrated and secure real time communication. The major security problems during communication over covert and low bandwidth channel is to reduce no of bits, efficient and secure cryptographic techniques such that output gain and performance may lead towards more secure and efficient mechanism. Thus, in this paper, we have purposed a secure, reliable and efficient mechanism using arithmetic coding techniques followed by IMAES (Improved Modified Advanced Encryption standard) techniques. The output of encrypted images reveals that proposed technique presents higher performance, quit reliable and robust.
Zero-error capacity of binary channels with memory  [PDF]
Gérard Cohen,Emanuela Fachini,János K?rner
Computer Science , 2014,
Abstract: We begin a systematic study of the problem of the zero--error capacity of noisy binary channels with memory and solve some of the non--trivial cases.
Zero vs. epsilon Error in Interference Channels  [PDF]
Ilia Levi,Dan Vilenchik,Michael Langberg,Michelle Effros
Computer Science , 2013,
Abstract: Traditional studies of multi-source, multi-terminal interference channels typically allow a vanishing probability of error in communication. Motivated by the study of network coding, this work addresses the task of quantifying the loss in rate when insisting on zero error communication in the context of interference channels.
Analysis of Epsilon-Near-Zero Metamaterial Super-Tunneling Using Cascaded Ultra-Narrow Waveguide Channels
Jian Bai;Shouyuan Shi;Dennis W. Prather
PIER M , 2010, DOI: 10.2528/PIERM10080205
Abstract: The Epsilon-Near-Zero (ENZ) super-tunneling structure with weakly coupled cascaded ultra-narrow channels is proposed and demonstrated to have notably wider bandwidth than single stage tunneling structure. An extensive parametric study for such structures is performed to investigate the factors which can affect super-tunneling performance. It is found that the coupling between the ultra-narrow channels is required to be weak enough to ensure a continuous supertunneling band. In addition, electric field in the cascaded channels is enhanced, compared with that in the single channel structure.
The Wideband Slope of Interference Channels: The Large Bandwidth Case  [PDF]
Minqi Shen,Anders H?st-Madsen
Mathematics , 2010,
Abstract: It is well known that minimum received energy per bit in the interference channel is -1.59dB as if there were no interference. Thus, the best way to mitigate interference is to operate the interference channel in the low-SNR regime. However, when the SNR is small but non-zero, minimum energy per bit alone does not characterize performance. Verdu introduced the wideband slope S_0 to characterize the performance in this regime. We show that a wideband slope of S_0/S_{0,no interference}=1/2 is achievable. This result is similar to recent results on degrees of freedom in the high SNR regime, and we use a type of interference alignment using delays to obtain the result. We also show that in many cases the wideband slope is upper bounded by S_0/S_{0,no interference}<=1/2 for large number of users K .
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