51 research outputs found
LDPC-coded modulation for transmission over AWGN and flat rayleigh fading channels
La modulation codée est une technique de transmission efficace en largeur de bande qui intègre le codage de canal et la modulation en une seule entité et ce, afin d'améliorer les performances tout en conservant la même efficacité spectrale comparé à la modulation non codée. Les codes de parité à faible densité (low-density parity-check codes, LDPC) sont les codes correcteurs d'erreurs les plus puissants et approchent la limite de Shannon, tout en ayant une complexité de décodage relativement faible. L'idée de combiner les codes LDPC et la modulation efficace en largeur de bande a donc été considérée par de nombreux chercheurs. Dans ce mémoire, nous étudions une méthode de modulation codée à la fois puissante et efficace en largeur de bande, ayant d'excellentes performances de taux d'erreur binaire et une complexité d'implantation faible. Ceci est réalisé en utilisant un encodeur rapide, un décoder de faible complexité et aucun entrelaceur. Les performances du système proposé pour des transmissions sur un canal additif gaussien blanc et un canal à évanouissements plats de Rayleigh sont évaluées au moyen de simulations. Les résultats numériques montrent que la méthode de modulation codée utilisant la modulation d'amplitude en quadrature à M niveaux (M-QAM) peut atteindre d'excellentes performances pour toute une gamme d'efficacité spectrale. Une autre contribution de ce mémoire est une méthode simple pour réaliser une modulation codée adaptative avec les codes LDPC pour la transmission sur des canaux à évanouissements plats et lents de Rayleigh. Dans cette méthode, six combinaisons de paires encodeur modulateur sont employées pour une adaptation trame par trame. L'efficacité spectrale moyenne varie entre 0.5 et 5 bits/s/Hz lors de la transmission. Les résultats de simulation montrent que la modulation codée adaptative avec les codes LDPC offre une meilleure efficacité spectrale tout en maintenant une performance d'erreur acceptable
Advanced constellation and demapper schemes for next generation digital terrestrial television broadcasting systems
206 p.Esta tesis presenta un nuevo tipo de constelaciones llamadas no uniformes. Estos esquemas presentan una eficacia de hasta 1,8 dB superior a las utilizadas en los últimos sistemas de comunicaciones de televisión digital terrestre y son extrapolables a cualquier otro sistema de comunicaciones (satélite, móvil, cable¿). Además, este trabajo contribuye al diseño de constelaciones con una nueva metodologÃa que reduce el tiempo de optimización de dÃas/horas (metodologÃas actuales) a horas/minutos con la misma eficiencia. Todas las constelaciones diseñadas se testean bajo una plataforma creada en esta tesis que simula el estándar de radiodifusión terrestre más avanzado hasta la fecha (ATSC 3.0) bajo condiciones reales de funcionamiento.Por otro lado, para disminuir la latencia de decodificación de estas constelaciones esta tesis propone dos técnicas de detección/demapeo. Una es para constelaciones no uniformes de dos dimensiones la cual disminuye hasta en un 99,7% la complejidad del demapeo sin empeorar el funcionamiento del sistema. La segunda técnica de detección se centra en las constelaciones no uniformes de una dimensión y presenta hasta un 87,5% de reducción de la complejidad del receptor sin pérdidas en el rendimiento.Por último, este trabajo expone un completo estado del arte sobre tipos de constelaciones, modelos de sistema, y diseño/demapeo de constelaciones. Este estudio es el primero realizado en este campo
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
Turbo-Detected Unequal Protection MPEG-4 Wireless Video Telephony using Multi-Level Coding, Trellis Coded Modulation and Space-Time Trellis Coding
Most multimedia source signals are capable of tolerating lossy, rather than lossless delivery to the human eye, ear and other human sensors. The corresponding lossy and preferably low-delay multimedia source codecs however exhibit unequal error sensitivity, which is not the case for Shannon’s ideal entropy codec. This paper proposes a jointly optimised turbo transceiver design capable of providing unequal error protection for MPEG-4 coding aided wireless video telephony. The transceiver investigated consists of space-time trellis coding (STTC) invoked for the sake of mitigating the effects of fading, in addition to bandwidth efficient trellis coded modulation or bit-interleaved coded modulation, combined with a multi-level coding scheme employing either two different-rate non-systematic convolutional codes (NSCs) or two recursive systematic convolutional codes for yielding a twin-class unequal-protection. A single-class protection based benchmark scheme combining STTC and NSC is used for comparison with the unequal-protection scheme advocated. The video performance of the various schemes is evaluated when communicating over uncorrelated Rayleigh fading channels. It was found that the proposed scheme requires about 2.8 dBs lower transmit power than the benchmark scheme in the context of the MPEG-4 videophone transceiver at a similar decoding complexity
On the Asymptotic Performance of Bit-Wise Decoders for Coded Modulation
Two decoder structures for coded modulation over the Gaussian and flat fading
channels are studied: the maximum likelihood symbol-wise decoder, and the
(suboptimal) bit-wise decoder based on the bit-interleaved coded modulation
paradigm. We consider a 16-ary quadrature amplitude constellation labeled by a
Gray labeling. It is shown that the asymptotic loss in terms of pairwise error
probability, for any two codewords caused by the bit-wise decoder, is bounded
by 1.25 dB. The analysis also shows that for the Gaussian channel the
asymptotic loss is zero for a wide range of linear codes, including all
rate-1/2 convolutional codes
On the BICM Capacity
Optimal binary labelings, input distributions, and input alphabets are
analyzed for the so-called bit-interleaved coded modulation (BICM) capacity,
paying special attention to the low signal-to-noise ratio (SNR) regime. For
8-ary pulse amplitude modulation (PAM) and for 0.75 bit/symbol, the folded
binary code results in a higher capacity than the binary reflected gray code
(BRGC) and the natural binary code (NBC). The 1 dB gap between the additive
white Gaussian noise (AWGN) capacity and the BICM capacity with the BRGC can be
almost completely removed if the input symbol distribution is properly
selected. First-order asymptotics of the BICM capacity for arbitrary input
alphabets and distributions, dimensions, mean, variance, and binary labeling
are developed. These asymptotics are used to define first-order optimal (FOO)
constellations for BICM, i.e. constellations that make BICM achieve the Shannon
limit -1.59 \tr{dB}. It is shown that the \Eb/N_0 required for reliable
transmission at asymptotically low rates in BICM can be as high as infinity,
that for uniform input distributions and 8-PAM there are only 72 classes of
binary labelings with a different first-order asymptotic behavior, and that
this number is reduced to only 26 for 8-ary phase shift keying (PSK). A general
answer to the question of FOO constellations for BICM is also given: using the
Hadamard transform, it is found that for uniform input distributions, a
constellation for BICM is FOO if and only if it is a linear projection of a
hypercube. A constellation based on PAM or quadrature amplitude modulation
input alphabets is FOO if and only if they are labeled by the NBC; if the
constellation is based on PSK input alphabets instead, it can never be FOO if
the input alphabet has more than four points, regardless of the labeling.Comment: Submitted to the IEEE Transactions on Information Theor
Bandwidth-efficient communication systems based on finite-length low density parity check codes
Low density parity check (LDPC) codes are linear block codes constructed by pseudo-random parity check matrices. These codes are powerful in terms of error performance and, especially, have low
decoding complexity. While infinite-length LDPC codes approach the capacity of communication channels, finite-length LDPC codes also
perform well, and simultaneously meet the delay requirement of many communication applications such as voice and backbone transmissions. Therefore, finite-length LDPC codes are attractive to employ in low-latency communication systems. This thesis mainly focuses on the bandwidth-efficient communication systems using finite-length LDPC codes. Such bandwidth-efficient systems are realized by mapping a group of LDPC coded bits to a symbol of a high-order signal constellation. Depending on the systems' infrastructure and knowledge of the channel state information (CSI), the signal constellations in different coded modulation systems can be two-dimensional multilevel/multiphase constellations or multi-dimensional space-time constellations.
In the first part of the thesis, two basic bandwidth-efficient coded modulation systems, namely LDPC coded modulation and multilevel LDPC coded modulation, are investigated for both additive white Gaussian noise (AWGN) and frequency-flat Rayleigh fading channels. The bounds on the bit error rate (BER) performance are derived for these systems based on the maximum likelihood (ML) criterion. The derivation of these bounds relies on the union bounding and combinatoric techniques. In particular, for the LDPC coded modulation, the ML bound is computed from the Hamming distance spectrum of the LDPC code and the Euclidian distance profile of the two-dimensional constellation. For the multilevel LDPC coded modulation, the bound of each decoding stage is obtained for a generalized multilevel coded modulation, where more than one coded bit is considered for level. For both systems, the bounds are confirmed by the simulation results of ML decoding and/or the performance of the ordered-statistic decoding (OSD) and the sum-product decoding. It is demonstrated that these bounds can be efficiently used to evaluate the error performance and select appropriate parameters (such as the code rate, constellation and mapping) for the two communication systems.The second part of the thesis studies bandwidth-efficient LDPC coded systems that employ multiple transmit and multiple receive antennas, i.e., multiple-input multiple-output (MIMO) systems. Two scenarios of CSI availability considered are: (i) the CSI is unknown at both the transmitter and the receiver; (ii) the CSI is known at both the transmitter and the receiver. For the first scenario, LDPC coded unitary space-time modulation systems are most suitable and the ML performance bound is derived for these non-coherent systems. To derive the bound, the summation of chordal distances is obtained and used instead of the Euclidean distances. For the second case of CSI, adaptive LDPC coded MIMO modulation systems are studied, where three adaptive schemes with antenna beamforming and/or antenna selection are investigated and compared in terms of the bandwidth efficiency. For uncoded discrete-rate adaptive modulation, the computation of the bandwidth efficiency shows that the scheme with antenna selection at the transmitter and antenna combining at the receiver performs the best when the number of antennas is small. For adaptive LDPC coded MIMO modulation systems, an achievable threshold of the bandwidth efficiency is also computed from the ML bound of LDPC coded modulation derived in the first part
Constellation design for future communication systems: a comprehensive survey
[EN] The choice of modulation schemes is a fundamental building block of wireless communication
systems. As a key component of physical layer design, they critically impact the expected communication
capacity and wireless signal robustness. Their design is also critical for the successful roll-out of wireless
standards that require a compromise between performance, efficiency, latency, and hardware requirements.
This paper presents a survey of constellation design strategies and associated outcomes for wireless
communication systems. The survey discusses their performance and complexity to address the need for
some desirable properties, including consistency, channel capacity, system performance, required demapping
architecture, flexibility, and independence. Existing approaches for constellation designs are investigated
using appropriate metrics and categorized based on their theoretical algorithm design. Next, their application
to different communication standards is analyzed in context, aiming at distilling general guidelines applicable
to the wireless building block design. Finally, the survey provides a discussion on design directions for future
communication system standardization processes.This work was supported in part by the Basque Government under Grant IT1234-19, in part by the PREDOC under
Program PRE_2020_2_0105, and in part by the Spanish Government through the Project PHANTOM (MCIU/AEI/FEDER, UE) under Gran
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