114 research outputs found
Maximum likelihood detection for OFDM signals with strong nonlinear distortion effects
Dissertação para obtenção do Grau de Mestre em
Engenharia Electrotécnica e de Computadore
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)
Receiver design for nonlinearly distorted OFDM : signals applications in radio-over-fiber systems
Tese de doutoramento. Engenharia Electrotécnica e de Computadores. Universidade do Porto. Faculdade de Engenharia. 201
Analytical Characterization and Optimum Detection of Nonlinear Multicarrier Schemes
It is widely recognized that multicarrier systems such as orthogonal frequency division
multiplexing (OFDM) are suitable for severely time-dispersive channels. However, it is
also recognized that multicarrier signals have high envelope fluctuations which make them especially sensitive to nonlinear distortion effects. In fact, it is almost unavoidable to have nonlinear distortion effects in the transmission chain. For this reason, it is essential to have a theoretical, accurate characterization of nonlinearly distorted signals not only to evaluate the corresponding impact of these distortion effects on the system’s performance, but also to develop mechanisms to combat them. One of the goals of this thesis is to address these challenges and involves a theoretical characterization of nonlinearly distorted multicarrier signals in a simple, accurate way.
The other goal of this thesis is to study the optimum detection of nonlinearly distorted,
multicarrier signals. Conventionally, nonlinear distortion is seen as a noise term
that degrades the system’s performance, leading even to irreducible error floors. Even
receivers that try to estimate and cancel it have a poor performance, comparatively to the performance associated to a linear transmission, even with perfect cancellation of nonlinear distortion effects. It is shown that the nonlinear distortion should not be considered as a noise term, but instead as something that contains useful information for detection purposes. The adequate receiver to take advantage of this information is the optimum receiver, since it makes a block-by-block detection, allowing us to exploit the nonlinear distortion which is spread along the signal’s band. Although the optimum receiver for nonlinear multicarrier schemes is too complex, due to its necessity to compare the received signal with all possible transmitted sequences, it is important to study its potential performance gains. In this thesis, it is shown that the optimum receiver outperforms the conventional detection, presenting gains not only relatively to conventional receivers that deal with nonlinear multicarrier signals, but also relatively to conventional receivers that deal with linear, multicarrier signals. We also present sub-optimum receivers which are able to approach the performance gains associated to the optimum detection and that can even outperform the conventional linear, multicarrier schemes
HybridDeepRx: Deep Learning Receiver for High-EVM Signals
In this paper, we propose a machine learning (ML) based physical layer
receiver solution for demodulating OFDM signals that are subject to a high
level of nonlinear distortion. Specifically, a novel deep learning based
convolutional neural network receiver is devised, containing layers in both
time- and frequency domains, allowing to demodulate and decode the transmitted
bits reliably despite the high error vector magnitude (EVM) in the transmit
signal. Extensive set of numerical results is provided, in the context of 5G NR
uplink incorporating also measured terminal power amplifier characteristics.
The obtained results show that the proposed receiver system is able to clearly
outperform classical linear receivers as well as existing ML receiver
approaches, especially when the EVM is high in comparison with modulation
order. The proposed ML receiver can thus facilitate pushing the terminal power
amplifier (PA) systems deeper into saturation, and thereon improve the terminal
power-efficiency, radiated power and network coverage.Comment: To be presented in the 2021 IEEE International Symposium on Personal,
Indoor and Mobile Radio Communication
Simultaneous Wireless Information and Power Transfer in 5G communication
Green communication technology is expected to be widely adopted in future generation
networks to improve energy efficiency and reliability of wireless communication network.
Among the green communication technologies,simultaneous wireless information and
power transfer (SWIPT) is adopted for its flexible energy harvesting technology through
the radio frequency (RF) signa lthati sused for information transmission. Even though
existing SWIPT techniques are flexible and adoptable for the wireless communication
networks, the power and time resources of the signal need to be shared between infor-
mation transmission and RF energy harvesting, and this compromises the quality of the
signal. Therefore,SWIP Ttechniques need to be designed to allow an efficient resource
allocation for communication and energy harvesting.
The goal oft his thesisis to design SWIP Ttechniques that allow efficient,reliable and
secure joint communications and power transference. A problem associated to SWIPT
techniques combined with multi carrier signals is that the increased power requirements
inherent to energy harvesting purposes can exacerbate nonlinear distortion effects at the
transmitter. Therefore, we evaluate nonlinear distortion and present feasible solutions to
mitigate the impact of nonlinear distortion effects on the performance.Another goal of
the thesisis to take advantage of the energy harvesting signals in SWIP Ttechniques for
channel estimation and security purposes.Theperformance of these SWIPT techniques is
evaluated analytically, and those results are validated by simulations. It is shownthatthe
proposed SWIPT schemes can have excellent performance, out performing conventional
SWIPT schemes.Espera-se que aschamadas tecnologiasde green communications sejam amplamente ado-
tadas em futuras redes de comunicação sem fios para melhorar a sua eficiência energética
a fiabilidade.Entre estas,encontram-se as tecnologias SWIPT (Simultaneous Wireless
Information and Power Transference), nas quais um sinal radio é usado para transferir
simultaneamente potência e informações.Embora as técnicas SWIPT existentes sejam fle-
xÃveis e adequadas para as redes de comunicações sem fios, os recursos de energia e tempo
do sinal precisam ser compartilhados entre a transmissão de informações e de energia, o
que pode comprometer a qualidade do sinal. Deste modo,as técnicas SWIPT precisam ser
projetadas para permitir uma alocação eficiente de recursos para comunicação e recolha
de energia.
O objetivo desta tese é desenvolver técnicas SWIPT que permitam transferência de
energia e comunicações eficientes,fiáveis e seguras.Um problema associado às técnicas
SWIPT combinadas com sinais multi-portadora são as dificuldades de amplificação ine-
rentes à combinação de sinais de transmissão de energia com sinais de transferência de
dados, que podem exacerbar os efeitos de distorção não-linear nos sinais transmitidos.
Deste modo, um dos objectivos desta tese é avaliar o impacto da distorção não-linear em
sinais SWIPT, e apresentar soluções viáveis para mitigar os efeitos da distorção não-linear
no desempenho da transmissão de dados.Outro objetivo da tese é aproveitar as vantagens
dos sinais de transferência de energia em técnicas SWIPT para efeitos de estimação de
canal e segurança na comunicação.Os desempenhos dessas técnicas SWIPT são avaliados
analiticamente,sendo os respectivos resultados validados por simulações.É mostrado que
os esquemas SWIPT propostos podem ter excelente desempenho, superando esquemas
SWIPT convencionais
Near-Optimal Detection of CE-OFDM Signals with High Power Efficiency via GAMP-based Receivers
Proceeding of: 2022 IEEE Globecom Workshops (GC Wkshps), Rio de Janeiro, Brazil, 4-8 December 2022A quasi-optimum receiver based on the generalized approximate message passing (GAMP) concept is proposed for constant envelope orthogonal frequency division multiplexing (CE-OFDM) signals. Large modulation index results in large power efficiency for CE-OFDM, but the phase modulator introduces nonlinear distortion effects, precluding good performance for a simple phase detector. Our simulation results show that the GAMP receiver can achieve quasi-optimum performance and it can outperform the linear OFDM and CE-OFDM with phase detectors, for both additive white Gaussian noise (AWGN) and frequency selective channels.This work received funding from the European Union (EU) Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie ETN TeamUp5G, grant agreement No. 813391, the Spanish National Project IRENE-EARTH (PID2020-115323RB-C33) (MINECO/AEI/FEDER, UE) and Portuguese FCT Instituto de Telecomunicaçoes project UIDB/50008/2020
Approaching the Optimal Performance of Nonlinear OFDM With FWA Techniques
Telecommunications are part of people’s daily lives. Also, nowadays, a complete
"technological illiteracy" is hard to be found in the so-called developed countries. To the point
of being almost a necessity in everyday life, telecommunications must evolve to meet this need.
One of the most widely used transmission techniques in mobile communications today
is Orthogonal Frequency-Division Multiplexing (OFDM). Adopted in the fourth generation of
mobile communications (4G), much for its ability to deal with frequency-selective channels and
good spectral efficiency, it is one of the candidate schemes to be in the fifth generation of
mobile communications (5G). Despite the advantages, OFDM signals have high envelope
fluctuations, making them sensitive to nonlinear effects. Several techniques were proposed to
reduce these fluctuations, however they required nonlinear operations that worsened the
performance of receivers. Nevertheless, it has recently been shown that the distortion effects
caused by nonlinearity is no longer seen as a problem, but as information. In fact, with this
discovery, it becomes possible to employ optimal receivers in order to improve the
performance. Despite that, these receivers have a very high complexity and, to try to solve this
problem, sub-optimal receivers have been proposed.
The sub-optimal receiver presented in this thesis is based on an optimization algorithm
called Fireworks Algorithm (FWA). The thesis includes: a study of the parameters of the
algorithm in order to understand its true impact on Bit Error Rate (BER) performance; a
comparison of the BER for different channels: Additive White Gaussian Noise (AWGN) and
Frequency-Selective; and a proposal for an FWA variant that tries to reduce the receiver’s
complexity even more
Enhanced Receivers for OFDM signals with super-QAM constellations
Nowadays, there is a high demand for wireless communication systems with higher through-
put. One popular technique widely used in current and developing wireless technologies is
Orthogonal Frequency-Division Multiplexing (OFDM) due to its robustness against fre-
quency selective fading and high spectral efficiency. To further extend OFDM capacity
to meet the near future’s expected demanding needs, OFDM systems with very large
Quadrature Amplitude Modulation (QAM) constellations, the so-called super-QAM, are
being proposed. However, OFDM signals are prone to nonlinear distortion effects due to
their high envelope fluctuations which reduces the system’s performance and this issue
is aggravated by the increase in the size of the constellation. For the implementation of
effective super-QAM OFDM systems, it is crucial to develop receivers that expect and
mitigate the nonlinear distortion on the transmitted signal.
In this work, nonlinear distortion on OFDM small QAM and super-QAM constellations
signals is studied, along with distortion models and methods to estimate them solely
from the transmitted signal, and application of Bussgang noise cancellation receivers and
analysis of their performance over a wide range of scenarios.Nos dias de hoje, há uma grande necessidade de criar sistemas de telecomunicação com
maior ritmo de dados. Uma técnica popular em tecnologias de telecomunicação atuais e
em desenvolvimento é Ortogonal Frequency-Devision Multiplexing (OFDM) devido à sua
robustez contra atenuação seletiva na frequência e alta eficiência espectral. Para aumentar
ainda mais a capacidade do OFDM de forma a preparar para ritmos ainda mais altos que
são expectáveis num futuro próximo, estão a ser propostos sistemas OFDM com enormes
constelações de Quadrature Amplitude Modulation (QAM), o chamado super-QAM. O
problema é que sinais OFDM são suscetÃveis a efeitos de distorção não linear devido à s
altas flutuações de envolvente e que traz pior desempenho do sistema, sendo esse problema
agravado pelo aumento do tamanho da constelação. Para a implementação de sistemas
super-QAM OFDM eficazes é crucial desenvolver recetores que mitiguem a distorção não
linear no sinal transmitido.
Neste trabalho, estuda-se a distorção não linear em sinais OFDM de pequenas cons-
telações QAM e super-QAM, modelos de distorção e métodos para estimá-los a partir do
sinal transmitido, aplicação de recetores de cancelamento de ruÃdo Bussgang e análise de
seu desempenho em diversos cenários
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