896 research outputs found
Orthogonal Transform Multiplexing with Memoryless Nonlinearity: a Possible Alternative to Traditional Coded-Modulation Schemes
In this paper, we propose a novel joint coding-modulation technique based on
serial concatenation of orthogonal linear transform, such as discrete Fourier
transform (DFT) or Walsh-Hadamard transform (WHT), with memoryless
nonlinearity. We demonstrate that such a simple signal construction may exhibit
properties of a random code ensemble, as a result approaching channel capacity.
Our computer simulations confirm that if the decoder relies on a modified
approximate message passing algorithm, the proposed modulation technique
exhibits performance on par with state-of-the-art coded modulation schemes that
use capacity-approaching component codes. The proposed modulation scheme could
be used directly or as a pre-coder for a conventional orthogonal frequency
division multiplexing (OFDM) transmitter, resulting in a system possessing all
benefits of OFDM along with reduced peak-to-average power ratio (PAPR)
Coded Index Modulation for Non-DC-Biased OFDM in Multiple LED Visible Light Communication
Use of multiple light emitting diodes (LED) is an attractive way to increase
spectral efficiency in visible light communications (VLC). A non-DC-biased OFDM
(NDC OFDM) scheme that uses two LEDs has been proposed in the literature
recently. NDC OFDM has been shown to perform better than other OFDM schemes for
VLC like DC-biased OFDM (DCO OFDM) and asymmetrically clipped OFDM (ACO OFDM)
in multiple LEDs settings. In this paper, we propose an efficient multiple LED
OFDM scheme for VLC which uses {\em coded index modulation}. The proposed
scheme uses two transmitter blocks, each having a pair of LEDs. Within each
block, NDC OFDM signaling is done. The selection of which block is activated in
a signaling interval is decided by information bits (i.e., index bits). In
order to improve the reliability of the index bits at the receiver (which is
critical because of high channel correlation in multiple LEDs settings), we
propose to use coding on the index bits alone. We call the proposed scheme as
CI-NDC OFDM (coded index NDC OFDM) scheme. Simulation results show that, for
the same spectral efficiency, CI-NDC OFDM that uses LDPC coding on the index
bits performs better than NDC OFDM
Advanced Coding And Modulation For Ultra-wideband And Impulsive Noises
The ever-growing demand for higher quality and faster multimedia content delivery over short distances in home environments drives the quest for higher data rates in wireless personal area networks (WPANs). One of the candidate IEEE 802.15.3a WPAN proposals support data rates up to 480 Mbps by using punctured convolutional codes with quadrature phase shift keying (QPSK) modulation for a multi-band orthogonal frequency-division multiplexing (MB-OFDM) system over ultra wideband (UWB) channels. In the first part of this dissertation, we combine more powerful near-Shannon-limit turbo codes with bandwidth efficient trellis coded modulation, i.e., turbo trellis coded modulation (TTCM), to further improve the data rates up to 1.2 Gbps. A modified iterative decoder for this TTCM coded MB-OFDM system is proposed and its bit error rate performance under various impulsive noises over both Gaussian and UWB channel is extensively investigated, especially in mismatched scenarios. A robust decoder which is immune to noise mismatch is provided based on comparison of impulsive noises in time domain and frequency domain. The accurate estimation of the dynamic noise model could be very difficult or impossible at the receiver, thus a significant performance degradation may occur due to noise mismatch. In the second part of this dissertation, we prove that the minimax decoder in \cite, which instead of minimizing the average bit error probability aims at minimizing the worst bit error probability, is optimal and robust to certain noise model with unknown prior probabilities in two and higher dimensions. Besides turbo codes, another kind of error correcting codes which approach the Shannon capacity is low-density parity-check (LDPC) codes. In the last part of this dissertation, we extend the density evolution method for sum-product decoding using mismatched noises. We will prove that as long as the true noise type and the estimated noise type used in the decoder are both binary-input memoryless output symmetric channels, the output from mismatched log-likelihood ratio (LLR) computation is also symmetric. We will show the Shannon capacity can be evaluated for mismatched LLR computation and it can be reduced if the mismatched LLR computation is not an one-to-one mapping function. We will derive the Shannon capacity, threshold and stable condition of LDPC codes for mismatched BIAWGN and BIL noise types. The results show that the noise variance estimation errors will not affect the Shannon capacity and stable condition, but the errors do reduce the threshold. The mismatch in noise type will only reduce Shannon capacity when LLR computation is based on BIL
LDPC code-based bandwidth efficient coding schemes for wireless communications
This dissertation deals with the design of bandwidth-efficient coding schemes
with Low-Density Parity-Check (LDPC) for reliable wireless communications. Code
design for wireless channels roughly falls into three categories: (1) when channel state
information (CSI) is known only to the receiver (2) more practical case of partial CSI
at the receiver when the channel has to be estimated (3) when CSI is known to the
receiver as well as the transmitter. We consider coding schemes for all the above
categories.
For the first scenario, we describe a bandwidth efficient scheme which uses highorder
constellations such as QAM over both AWGN as well as fading channels. We
propose a simple design with LDPC codes which combines the good properties of
Multi-level Coding (MLC) and bit-interleaved coded-modulation (BICM) schemes.
Through simulations, we show that the proposed scheme performs better than MLC
for short-medium lengths on AWGN and block-fading channels. For the first case,
we also characterize the rate-diversity tradeoff of MIMO-OFDM and SISO-OFDM
systems. We design optimal coding schemes which achieve this tradeoff when transmission
is from a constrained constellation. Through simulations, we show that with
a sub-optimal iterative decoder, the performance of this coding scheme is very close
to the optimal limit for MIMO (flat quasi-static fading), MIMO-OFDM and SISO OFDM systems.
For the second case, we design non-systematic Irregular Repeat Accumulate
(IRA) codes, which are a special class of LDPC codes, for Inter-Symbol Interference
(ISI) fading channels when CSI is estimated at the receiver. We use Orthogonal Frequency
Division Multiplexing (OFDM) to convert the ISI fading channel into parallel
flat fading subchannels. We use a simple receiver structure that performs iterative
channel estimation and decoding and use non-systematic IRA codes that are optimized
for this receiver. This combination is shown to perform very close to a receiver
with perfect CSI and is also shown to be robust to change in the number of channel
taps and Doppler.
For the third case, we look at bandwidth efficient schemes for fading channels
that perform close to capacity when the channel state information is known at the
transmitter as well as the receiver. Schemes that achieve capacity with a Gaussian
codebook for the above system are already known but not for constrained constellations.
We derive the near-optimum scheme to achieve capacity with constrained constellations
and then propose coding schemes which perform close to capacity. Through
linear transformations, a MIMO system can be converted into non-interfering parallel
subchannels and we further extend the proposed coding schemes to the MIMO case
too
Fast Convergence and Reduced Complexity Receiver Design for LDS-OFDM System
Low density signature for OFDM (LDS-OFDM) is able to achieve satisfactory performance in overloaded conditions, but the existing LDS-OFDM has the drawback of slow convergence rate for multiuser detection (MUD) and high receiver complexity. To tackle these problems, we propose a serial schedule for the iterative MUD. By doing so, the convergence rate of MUD is accelerated and the detection iterations can be decreased. Furthermore, in order to exploit the similar sparse structure of LDS-OFDM and LDPC code, we utilize LDPC codes for LDS-OFDM system. Simulations show that compared with existing LDS-OFDM, the LDPC code improves the system performance
Low-power Secret-key Agreement over OFDM
Information-theoretic secret-key agreement is perhaps the most practically
feasible mechanism that provides unconditional security at the physical layer
to date. In this paper, we consider the problem of secret-key agreement by
sharing randomness at low power over an orthogonal frequency division
multiplexing (OFDM) link, in the presence of an eavesdropper. The low power
assumption greatly simplifies the design of the randomness sharing scheme, even
in a fading channel scenario. We assess the performance of the proposed system
in terms of secrecy key rate and show that a practical approach to key sharing
is obtained by using low-density parity check (LDPC) codes for information
reconciliation. Numerical results confirm the merits of the proposed approach
as a feasible and practical solution. Moreover, the outage formulation allows
to implement secret-key agreement even when only statistical knowledge of the
eavesdropper channel is available.Comment: 9 pages, 4 figures; this is the authors prepared version of the paper
with the same name accepted for HotWiSec 2013, the Second ACM Workshop on Hot
Topics on Wireless Network Security and Privacy, Budapest, Hungary 17-19
April 201
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