21 research outputs found
Comparative Analysis between OFDMA and SC-FDMA: Model, Features and Applications
This paper represents Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SCFDMA) techniques along with the Orthogonal Frequency Division Multiplexing (OFDM). The concept, model, features, scopes, applications and limitation for both types of multiple access have been discussed in this paper. In present 4G and 5G cellular communication system, both OFDMA and SC-FDMA have a notable applications. Dividing the available spectrum into overlapping orthogonal narrowband sub bands, OFDMA ensures high spectral efficiency. Besides by allocating multiple sub carriers to each user, OFDMA provides high data rate, reduces inter blockage interference, minimizes frequency selective fading and so on. But it suffers from high peak to average power ration (PAPR) which results in high power consumption at the transmitter end. SC-FDMA is one of the most promising techniques to solve the PAPR problems. Besides it also removes the capacity problem of wireless cellular systems and provides higher spectral efficiency, depending on multiplexing signals based on their spatial signature. On the other hand, in OFDM due to fixed subcarrier allocations for each user and its performance can suffer from narrowband fading and interference
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-
xíveis 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
Capacity of a Massive MIMO-OFDM system using Millimetric waves
In modern day society, wirelesses communications are fundamental to the communications worldwide. But with the saturation of the usable spectrum for communications, the need for capacity is one of the most important challenges faced nowadays.
With the development of technology, there is a much great need for quality of the service and bigger data streams. Increasing the capacity of a telecommuni-cation system will allow to cope with that need.
The solution for the need for capacity, can be solved with the use of massive multiple input multiple output (MIMO) techniques, since MIMO system ap-proach have already shown to increase the capacity of a wireless system.
The implications of this approach will make systems more complex and more energy consuming, in order to sustain the massive MIMO system. But such implementation combined with millimeter waves will allow the increase of the capacity of a system
Performance Evaluation of Low Complexity Massive MIMO Techniques for SC-FDE Schemes
Massive-MIMO technology has emerged as a means to achieve 5G's ambitious goals;
mainly to obtain higher capacities and excellent performances without requiring the use of more
spectrum. In this thesis, focused on the uplink direction, we make a study of performance of low
complexity equalization techniques as well as we also approach the impact of the non-linear elements
located on the receivers of a system of this type. For that purpose, we consider a multi-user
uplink scenario through the Single Carrier with Frequency Domain Equalization (SC-FDE)
scheme. This seems to be the most appropriate due to the low energy consumption that it implies,
as well as being less favorable to the detrimental effects of high envelope fluctuations, that is, by
have a low Peak to Average Power Ratio (PAPR) comparing to other similar modulations, such
as the Orthogonal Frequency Division Multiplexing (OFDM). Due to the greater number of antennas
and consequent implementation complexity, the equalization processes for Massive-
MIMO schemes are aspects that should be simplified, that is, they should avoid the inversion of
matrices, contrary to common 4G, with the Zero Forcing (ZF) and Minimum Mean Square Error
(MMSE) techniques. To this end, we use low-complexity techniques, such as the Equal Gain
Combining (EGC) and the Maximum Ratio Combining (MRC). Since these algorithms are not
sufficiently capable of removing the entire Inter-Symbol Interference (ISI) and Inter-User Interference
(IUI), we combine them with iterative techniques, namely with the Iterative Block with
Decision Feedback Equalizer (IB-DFE) to completely remove the residual ISI and IUI. We also
take into account the hardware used in the receivers, since the effects of non-linear distortion can
impact negatively the performance of the system. It is expected a strong performance degradation
associated to the high quantization noise levels when implementing low-resolution Analog to
Digital Converters (ADCs). However, despite these elements with these configurations become
harmful to the performance of the majority of the systems, they are considered a desirable solution
for Massive-MIMO scenarios, because they make their implementation cheaper and more energy
efficient. In this way, we made a study of the impact in the performance by the low-resolution
ADCs. In this thesis we suggest that it is possible to bypass these negative effects by implementing
a number of receiving antennas far superior to the number of transmitting antennas
Single- versus Multi-Carrier Terahertz-Band Communications: A Comparative Study
The prospects of utilizing single-carrier (SC) and multi-carrier (MC)
waveforms in future terahertz (THz)-band communication systems remain
unresolved. On the one hand, the limited multi-path components at high
frequencies result in frequency-flat channels that favor low-complexity
wideband SC systems. On the other hand, frequency-dependent molecular
absorption and transceiver characteristics and the existence of multi-path
components in indoor sub-THz systems can still result in frequency-selective
channels, favoring off-the-shelf MC schemes such as orthogonal
frequency-division multiplexing (OFDM). Variations of SC/MC designs result in
different THz spectrum utilization, but spectral efficiency is not the primary
concern with substantial available bandwidths; baseband complexity, power
efficiency, and hardware impairment constraints are predominant. This paper
presents a comprehensive study of SC/MC modulations for THz communications,
utilizing an accurate wideband THz channel model and highlighting the various
performance and complexity trade-offs of the candidate schemes. Simulations
demonstrate that discrete-Fourier-transform spread orthogonal time-frequency
space (DFT-s-OTFS) achieves a lower peak-to-average power ratio (PAPR) than
OFDM and OTFS and enhances immunity to THz impairments and Doppler spreads, but
at an increased complexity cost. Moreover, DFT-s-OFDM is a promising candidate
that increases robustness to THz impairments and phase noise (PHN) at a low
PAPR and overall complexity.Comment: 18 pages, 12 figures, journa
Técnicas de pré-codificação para sistemas multicelulares coordenados
Doutoramento em TelecomunicaçõesCoordenação Multicélula é um tópico de investigação em rápido
crescimento e uma solução promissora para controlar a interferência entre
células em sistemas celulares, melhorando a equidade do sistema e
aumentando a sua capacidade. Esta tecnologia já está em estudo no LTEAdvanced
sob o conceito de coordenação multiponto (COMP). Existem
várias abordagens sobre coordenação multicélula, dependendo da
quantidade e do tipo de informação partilhada pelas estações base, através
da rede de suporte (backhaul network), e do local onde essa informação é
processada, i.e., numa unidade de processamento central ou de uma forma
distribuída em cada estação base.
Nesta tese, são propostas técnicas de pré-codificação e alocação de
potência considerando várias estratégias: centralizada, todo o
processamento é feito na unidade de processamento central; semidistribuída,
neste caso apenas parte do processamento é executado na
unidade de processamento central, nomeadamente a potência alocada a
cada utilizador servido por cada estação base; e distribuída em que o
processamento é feito localmente em cada estação base. Os esquemas
propostos são projectados em duas fases: primeiro são propostas soluções
de pré-codificação para mitigar ou eliminar a interferência entre células,
de seguida o sistema é melhorado através do desenvolvimento de vários
esquemas de alocação de potência. São propostas três esquemas de
alocação de potência centralizada condicionada a cada estação base e com
diferentes relações entre desempenho e complexidade. São também
derivados esquemas de alocação distribuídos, assumindo que um sistema
multicelular pode ser visto como a sobreposição de vários sistemas com
uma única célula. Com base neste conceito foi definido uma taxa de erro
média virtual para cada um desses sistemas de célula única que compõem
o sistema multicelular, permitindo assim projectar esquemas de alocação
de potência completamente distribuídos.
Todos os esquemas propostos foram avaliados em cenários realistas,
bastante próximos dos considerados no LTE. Os resultados mostram que
os esquemas propostos são eficientes a remover a interferência entre
células e que o desempenho das técnicas de alocação de potência
propostas é claramente superior ao caso de não alocação de potência. O
desempenho dos sistemas completamente distribuídos é inferior aos
baseados num processamento centralizado, mas em contrapartida podem
ser usados em sistemas em que a rede de suporte não permita a troca de
grandes quantidades de informação.Multicell coordination is a promising solution for cellular wireless systems
to mitigate inter-cell interference, improving system fairness and
increasing capacity and thus is already under study in LTE-A under the
coordinated multipoint (CoMP) concept. There are several coordinated
transmission approaches depending on the amount of information shared
by the transmitters through the backhaul network and where the
processing takes place i.e. in a central processing unit or in a distributed
way on each base station.
In this thesis, we propose joint precoding and power allocation techniques
considering different strategies: Full-centralized, where all the processing
takes place at the central unit; Semi-distributed, in this case only some
process related with power allocation is done at the central unit; and Fulldistributed,
where all the processing is done locally at each base station.
The methods are designed in two phases: first the inter-cell interference is
removed by applying a set of centralized or distributed precoding vectors;
then the system is further optimized by centralized or distributed power
allocation schemes. Three centralized power allocation algorithms with
per-BS power constraint and different complexity tradeoffs are proposed.
Also distributed power allocation schemes are proposed by considering
the multicell system as superposition of single cell systems, where we
define the average virtual bit error rate (BER) of interference-free single
cell system, allowing us to compute the power allocation coefficients in a
distributed manner at each BS.
All proposed schemes are evaluated in realistic scenarios considering LTE
specifications. The numerical evaluations show that the proposed schemes
are efficient in removing inter-cell interference and improve system
performance comparing to equal power allocation. Furthermore, fulldistributed
schemes can be used when the amounts of information to be
exchanged over the backhaul is restricted, although system performance is
slightly degraded from semi-distributed and full-centralized schemes, but
the complexity is considerably lower. Besides that for high degrees of
freedom distributed schemes show similar behaviour to centralized ones
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
Orthogonal frequency division multiplexing for optical access networks
Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme with numerous advantages that has for years been employed as the leading physical interface in many wired and wireless communication systems. Recently, with advancements made in digital signal processing, there has been a surge of interest in applying OFDM techniques for optical communications. This thesis presents extensive research on optical OFDM and how it is being applied in access networks. With the aid of theoretical analysis, simulations and experiments, it is shown that the system performance of direct-detection optical OFDM (DD-OOFDM) in the presence of MZM non-linear distortion can be improved by proper biasing and selection of appropriate drive to the MZM. Investigations are conducted to illustrate how a variation in the number of subcarriers and the modulation format influences the sensitivity of the DD-OOFDM system to the MZM non-linear distortion. The possibility of improving the spectral efficiency by reduction of the width of the guard band is also investigated. This thesis also looks into the radio-over-fibre (RoF) transmission of Multiband OFDM UWB as a transparent and low-cost solution for distributing multi-Gbit/s data to end-users in FTTH networks. Due to relaxed regulatory requirements and the wide bandwidth available, UWB operation in the 60-GHz band is also considered for this FTTH application scenario. Four techniques for enabling MB-OFDM UWB RoF operation in the 60-GHz band are experimentally demonstrated. The impacts of various parameters on the performance of the techniques as well as the limitations imposed by fibre distribution are illustrated. Finally, a digital pre-distorter is proposed for compensating for the MZM non-linearity. Experimental demonstration of this digital pre-distortion in an UWB over fibre transmission system shows an increased tolerance to the amplitude of the driving OFDM signal as well as an increase in the optimum modulation index of the OFDM signal