91 research outputs found
On the Convergence Speed of Spatially Coupled LDPC Ensembles
Spatially coupled low-density parity-check codes show an outstanding
performance under the low-complexity belief propagation (BP) decoding
algorithm. They exhibit a peculiar convergence phenomenon above the BP
threshold of the underlying non-coupled ensemble, with a wave-like convergence
propagating through the spatial dimension of the graph, allowing to approach
the MAP threshold. We focus on this particularly interesting regime in between
the BP and MAP thresholds.
On the binary erasure channel, it has been proved that the information
propagates with a constant speed toward the successful decoding solution. We
derive an upper bound on the propagation speed, only depending on the basic
parameters of the spatially coupled code ensemble such as degree distribution
and the coupling factor . We illustrate the convergence speed of different
code ensembles by simulation results, and show how optimizing degree profiles
helps to speed up the convergence.Comment: 11 pages, 6 figure
Massive MIMO: How many antennas do we need?
We consider a multicell MIMO uplink channel where each base station (BS) is
equipped with a large number of antennas N. The BSs are assumed to estimate
their channels based on pilot sequences sent by the user terminals (UTs).
Recent work has shown that, as N grows infinitely large, (i) the simplest form
of user detection, i.e., the matched filter (MF), becomes optimal, (ii) the
transmit power per UT can be made arbitrarily small, (iii) the system
performance is limited by pilot contamination. The aim of this paper is to
assess to which extent the above conclusions hold true for large, but finite N.
In particular, we derive how many antennas per UT are needed to achieve \eta %
of the ultimate performance. We then study how much can be gained through more
sophisticated minimum-mean-square-error (MMSE) detection and how many more
antennas are needed with the MF to achieve the same performance. Our analysis
relies on novel results from random matrix theory which allow us to derive
tight approximations of achievable rates with a class of linear receivers.Comment: 6 pages, 3 figures, to be presented at the Allerton Conference on
Communication, Control and Computing, Urbana-Champaign, Illinois, US, Sep.
201
On Time-Bandwidth Product of Multi-Soliton Pulses
Multi-soliton pulses are potential candidates for fiber optical transmission
where the information is modulated and recovered in the so-called nonlinear
Fourier domain. While this is an elegant technique to account for the channel
nonlinearity, the obtained spectral efficiency, so far, is not competitive with
the classic Nyquist-based schemes. In this paper, we study the evolution of the
time-bandwidth product of multi-solitons as they propagate along the optical
fiber. For second and third order soliton pulses, we numerically optimize the
pulse shapes to achieve the smallest time-bandwidth product when the phase of
the spectral amplitudes is used for modulation. Moreover, we analytically
estimate the pulse-duration and bandwidth of multi-solitons in some practically
important cases. Those estimations enable us to approximate the time-bandwidth
product for higher order solitons.Comment: Accepted for ISIT 201
Scattered EXIT Charts for Finite Length LDPC Code Design
We introduce the Scattered Extrinsic Information Transfer (S-EXIT) chart as a
tool for optimizing degree profiles of short length Low-Density Parity-Check
(LDPC) codes under iterative decoding. As degree profile optimization is
typically done in the asymptotic length regime, there is space for further
improvement when considering the finite length behavior. We propose to consider
the average extrinsic information as a random variable, exploiting its specific
distribution properties for guiding code design. We explain, step-by-step, how
to generate an S-EXIT chart for short-length LDPC codes. We show that this
approach achieves gains in terms of bit error rate (BER) of 0.5 dB and 0.6 dB
over the additive white Gaussian noise (AWGN) channel for codeword lengths of
128 and 180 bits, respectively, at a target BER of when compared to
conventional Extrinsic Information Transfer (EXIT) chart-based optimization.
Also, a performance gain for the Binary Erasure Channel (BEC) for a block
(i.e., codeword) length of 180 bits is shown.Comment: in IEEE International Conference on Communications (ICC), May 201
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