509 research outputs found
On Continuous-Time Gaussian Channels
A continuous-time white Gaussian channel can be formulated using a white
Gaussian noise, and a conventional way for examining such a channel is the
sampling approach based on the Shannon-Nyquist sampling theorem, where the
original continuous-time channel is converted to an equivalent discrete-time
channel, to which a great variety of established tools and methodology can be
applied. However, one of the key issues of this scheme is that continuous-time
feedback and memory cannot be incorporated into the channel model. It turns out
that this issue can be circumvented by considering the Brownian motion
formulation of a continuous-time white Gaussian channel. Nevertheless, as
opposed to the white Gaussian noise formulation, a link that establishes the
information-theoretic connection between a continuous-time channel under the
Brownian motion formulation and its discrete-time counterparts has long been
missing. This paper is to fill this gap by establishing causality-preserving
connections between continuous-time Gaussian feedback/memory channels and their
associated discrete-time versions in the forms of sampling and approximation
theorems, which we believe will play important roles in the long run for
further developing continuous-time information theory.
As an immediate application of the approximation theorem, we propose the
so-called approximation approach to examine continuous-time white Gaussian
channels in the point-to-point or multi-user setting. It turns out that the
approximation approach, complemented by relevant tools from stochastic
calculus, can enhance our understanding of continuous-time Gaussian channels in
terms of giving alternative and strengthened interpretation to some long-held
folklore, recovering "long known" results from new perspectives, and rigorously
establishing new results predicted by the intuition that the approximation
approach carries
Performance Analysis and Enhancement of Multiband OFDM for UWB Communications
In this paper, we analyze the frequency-hopping orthogonal frequency-division
multiplexing (OFDM) system known as Multiband OFDM for high-rate wireless
personal area networks (WPANs) based on ultra-wideband (UWB) transmission.
Besides considering the standard, we also propose and study system performance
enhancements through the application of Turbo and Repeat-Accumulate (RA) codes,
as well as OFDM bit-loading. Our methodology consists of (a) a study of the
channel model developed under IEEE 802.15 for UWB from a frequency-domain
perspective suited for OFDM transmission, (b) development and quantification of
appropriate information-theoretic performance measures, (c) comparison of these
measures with simulation results for the Multiband OFDM standard proposal as
well as our proposed extensions, and (d) the consideration of the influence of
practical, imperfect channel estimation on the performance. We find that the
current Multiband OFDM standard sufficiently exploits the frequency selectivity
of the UWB channel, and that the system performs in the vicinity of the channel
cutoff rate. Turbo codes and a reduced-complexity clustered bit-loading
algorithm improve the system power efficiency by over 6 dB at a data rate of
480 Mbps.Comment: 32 pages, 10 figures, 1 table. Submitted to the IEEE Transactions on
Wireless Communications (Sep. 28, 2005). Minor revisions based on reviewers'
comments (June 23, 2006
Precoded FIR and Redundant V-BLAST Systems for Frequency-Selective MIMO Channels
The vertical Bell labs layered space-time (V-BLAST) system is a multi-input multioutput (MIMO) system designed to achieve good multiplexing gain. In recent literature, a precoder, which exploits channel information, has been added in the V-BLAST transmitter. This precoder forces each symbol stream to have an identical mean square error (MSE). It can be viewed as an alternative to the bit-loading method. In this paper, this precoded V-BLAST system is extended to the case of frequency-selective MIMO channels. Both the FIR and redundant types of transceivers, which use cyclic-prefixing and zero-padding, are considered. A fast algorithm for computing a cyclic-prefixing-based precoded V-BLAST transceiver is developed. Experiments show that the proposed methods with redundancy have better performance than the SVD-based system with optimal powerloading and bit loading for frequency-selective MIMO channels. The gain comes from the fact that the MSE-equalizing precoder has better bit-error rate performance than the optimal bitloading method
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