14 research outputs found
Analysis of random orientation and user mobility in LiFi networks
Mobile data traffic is anticipated to surpass 49 exabyte per month by 2021. Smartphones, as
the main factor of generating this huge data traffic (86%), are expected to require average speed
connection of 20 Mbps by 2021. Light-fidelity (LiFi) is a novel bidirectional, high-speed and
fully networked optical wireless communication and it is a promising solution to undertake
this huge data traffic. However, to support seamless connectivity in LiFi networks, real-time
knowledge of channel state information (CSI) from each user is required at the LiFi access point
(AP). The CSI availability enables us to achieve optimal resource allocation and throughput
maximization but it requires feedback transmitted through the uplink channel. Furthermore,
the important aspects of the indoor LiFi channel such as the random orientation of user device,
user mobility and link blockage need to be carefully analysed and effective solutions should be
developed.
In contrast to radio frequency (RF) channels, the LiFi channel is relatively less random. This feature
of LiFi channel enables a potential reduction in the amount of feedback required to achieve
high throughputs in a dynamic LiFi network. Based on this feature, two techniques for reducing
the amount of feedback in LiFi cellular networks are proposed: 1) limited-content feedback
scheme based on reducing the content of feedback information and 2) limited-frequency feedback
scheme based on the update interval. It is shown that these limited-feedback schemes can
provide almost the same downlink performance as full feedback scheme. Furthermore, an optimum
update interval which provides maximum bidirectional user equipment (UE) throughput,
has been derived.
Device orientation and its statistics is an important determinant factor that can affect the users
throughput remarkably in LiFi networks. However, device orientation has been ignored in many
previous performance studies of LiFi networks due to the lack of a proper statistical model. In
this thesis, a novel model for the orientation of user device are proposed based on experimental
measurements. The statistics of the device orientation for both sitting and walking activities are
presented. Moreover, the statistics of the line-of-sight (LOS) channel gain are calculated. The
influence of random device orientation on the received signal-to-noise-ratio (SNR) and bit-error
ratio (BER) performance of LiFi systems has been also evaluated.
To support the seamless connectivity of future LiFi-enabled devices in the presence of random
device orientation, mobility and blockage, efficient handover between APs are required. In this
thesis, an orientation-based random waypoint (ORWP) mobility model is proposed to analyze
the performance of mobile users considering the effect of random device orientation. Based on
this model, an analysis of handover due to random orientation and user mobility is presented.
Finally, in order to improve seamless connectivity, a multi-directional receiver (MDR) configuration
is proposed. The MDR configuration shows a robust performance in the presence of user
mobility, random device orientation and blockage
Advanced Technique and Future Perspective for Next Generation Optical Fiber Communications
Optical fiber communication industry has gained unprecedented opportunities and achieved rapid progress in recent years. However, with the increase of data transmission volume and the enhancement of transmission demand, the optical communication field still needs to be upgraded to better meet the challenges in the future development. Artificial intelligence technology in optical communication and optical network is still in its infancy, but the existing achievements show great application potential. In the future, with the further development of artificial intelligence technology, AI algorithms combining channel characteristics and physical properties will shine in optical communication. This reprint introduces some recent advances in optical fiber communication and optical network, and provides alternative directions for the development of the next generation optical fiber communication technology
Enhanced carrierless amplitude and phase modulation for optical communication systems
This thesis develops and investigates enhanced techniques for carrierless amplitude and phase
modulation (CAP) in optical communication systems. The CAP scheme is studied as the
physical layer modulation technique due to its implementation simplicity and versatility, that
enables its implementation as a single carrier (CAP) or multi-carrier technique (m-CAP).
The effect of timing jitter on the error performance of CAP is first investigated. The
investigation indicates that synchronization is a critical requirement for CAP receiver and as
a result, a novel low-complexity synchronization algorithm is developed with experimental
demonstration for CAP-based visible light communication (VLC) systems. To further reduce
the overall link complexity, a fractionally-spaced equalizer (FSE) is considered to mitigate the
effects of inter-symbol interference (ISI) and timing jitter. The FSE implementation, which
eliminates the need for a separate synchronization block, is shown through simulation and
VLC experimental demonstration to outperform symbol-spaced equalizers (SSE) that are
reported in literature for CAP-based VLC systems.
Furthermore, in this thesis, spectrally-efficient index modulation techniques are developed for
CAP. The proposed techniques can be divided into two broad groups, namely spatial index CAP
(S-CAP) and subband index CAP (SI-CAP). The proposed spatial index techniques leverage
the fact that in VLC, multiple optical sources are often required. The spatial CAP (S-CAP)
transmits CAP signal through one of Nt available LEDs. It is developed to reduce equalization
requirement and improve the spectral efficiency of the conventional CAP. In addition to the bits
transmitted through the CAP symbol, the S-CAP encodes additional bits on the indexing/spatial
location of the LEDs. The generalised S-CAP (GS-CAP) is further developed to relax the
S-CAP limitation of using a single LED per symbol duration. In addition to the S-CAP scheme,
multiple-input multiple-output (MIMO) techniques of repetitive-coded CAP (RC-CAP) and
spatial multiplexing CAP (SMux-CAP) are investigated for CAP. Low-complexity detectors
are also developed for the MIMO schemes. A key challenge of the MIMO schemes is that they
suffer power penalty when channel gains are similar, which occur when the optical sources are
closely located. The use of multiple receivers and power factor imbalance (PFI) techniques
are proposed to mitigate this power penalty. The techniques result in significant improvement
in the power efficiency of the MIMO schemes and ensure that the spectral efficiency gain is
obtained with little power penalty.
Finally, subband index CAP (SI-CAP) is developed to improve the spectral efficiency of
m-CAP and reduce its peak-to-average power ratio (PAPR). The SI-CAP encodes additional
information bits on the selection of ‘active’ subbands of m-CAP and only modulate data
symbols on these ‘active’ subbands. The error performance of the proposed SI-CAP is
evaluated analytically and verified with computer-based simulations. The SI-CAP technique is
also experimented for both VLC and step-index plastic optical fibre (SI-POF) communication
links. The experimental results show that for a fixed power efficiency, SI-CAP achieves higher
data rate compared tom-CAP. For example, at a representative bit error rate (BER) of 10-5, the
SI-CAP achieves a data rate and power efficiency gain of 26:5 Mb/s and 2:5 dB, respectively
when compared to m-CAP. In addition, an enhanced SI-CAP (eSI-CAP) is developed to
address the complexity that arises in SI-CAP at higher modulation order. The results of the
experimental demonstrations in VLC and 10 m SI-POF link shows that when compared with
m-CAP, eSI-CAP consistently yields a data rate improvement of between 7% and 13% for
varying values of the SNR
Améliorations des transmissions VLC (Visible Light Communication) sous contrainte d'éclairage : études théoriques et expérimentations
Abstract : Indoor visible light communication (VLC) networks based on light-emitting diodes (LEDs)
currently enjoy growing interest thanks in part to their robustness against interference,
wide license-free available bandwidth, low cost, good energy efficiency and compatibility
with existing lighting infrastructure. In this thesis, we investigate spectral-efficient modulation
techniques for the physical layer of VLC to increase throughput while considering
the quality of illumination as well as implementation costs. Numerical and experimental
studies are performed employing pulse amplitude modulation (PAM) and carrierless amplitude
and phase (CAP) modulation under illumination constraints and for high modulation
orders. Furthermore, the impact of LED nonlinearity is investigated and a postdistortion
technique is evaluated to compensate these nonlinear effects. Within this framework,
transmission rates in the order of a few hundred Mb/s are achieved using a test bench made
of low-cost components. In addition, an imaging multiple input multiple-output (MIMO)
system is developed and the impact on performance of imaging lens misalignment is theoretically
and numerically assessed. Finally, a polynomial matrix decomposition technique
based on the classical LU factorization method is studied and applied for the first time to
MIMO VLC systems in large space indoor environments.Les réseaux de communication en lumière visible (VLC) s’appuyant sur l’utilisation de diodes électroluminescentes (LED) bénéficient actuellement d’un intérêt grandissant, en partie grâce à leur robustesse face aux interférences électromagnétiques, leur large bande disponible non-régulée, leur faible coût, leur bonne efficacité énergétique, ainsi que leur compatibilité avec les infrastructures d’éclairage déjà existantes. Dans cette thèse, nous étudions des techniques de modulation à haute efficacité spectrale pour la couche physique des VLC pour augmenter les débits tout en considérant la qualité de l’éclairage ainsi que les coûts d’implémentation. Des études numériques et expérimentales sont réalisées sur la modulation d’impulsion d’amplitude (PAM) et sur la modulation d’amplitude et de phase sans porteuse (CAP) sous des contraintes d’éclairage et pour des grands ordres de modulation. De plus, l’impact des non-linéarités de la LED est étudié et une technique de post-distorsion est évaluée pour corriger ces effets non-linéaires. Dans ce cadre, des débits de plusieurs centaines de Mb/s sont atteints en utilisant un banc de test réalisé à partir de composants à bas coûts. Par ailleurs, un système multi-entrées multi-sorties (MIMO) imageant est également développé et l’impact du désaxage de l’imageur sur les performances est étudié. Finalement, une technique de décomposition polynomiale basée sur la méthode de factorisation classique LU est étudiée et appliquée aux systèmes MIMO VLC dans des grands espaces intérieurs
Spectrum and energy efficient digital modulation techniques for practical visible light communication systems
The growth in mobile data traffic is rapidly increasing in an unsustainable direction
given the radio frequency (RF) spectrum limits. Visible light communication (VLC)
offers a lucrative solution based on an alternative license-free frequency band that is safe
to use and inexpensive to utilize. Improving the spectral and energy efficiency of intensity
modulation and direct detection (IM/DD) systems is still an on-going challenge in
VLC. The energy efficiency of inherently unipolar modulation techniques such as pulse-amplitude
modulation discrete multitone modulation (PAM-DMT) and asymmetrically
clipped optical orthogonal frequency division multiplexing (ACO-OFDM) degrades at
high spectral efficiency. Two novel superposition modulation techniques are proposed
in this thesis based on PAM-DMT and ACO-OFDM. In addition, a practical solution
based on the computationally efficient augmented spectral efficiency discrete multi-tone
(ASE-DMT) is proposed. The system performance of the proposed superposition
modulation techniques offers significant electrical and optical power savings with up
to 8 dB in the electrical signal-to-noise ratio (SNR) when compared with DC-biased
optical orthogonal frequency division multiplexing (DCO-OFDM). The theoretical bit
error ratio (BER) performance bounds for all of the proposed modulation techniques
are in agreement with the Monte-Carlo simulation results. The proposed superposition
modulation techniques are promising candidates for spectrum and energy efficient
IM/DD systems.
Two experimental studies are presented for a VLC system based on DCO-OFDM with
adaptive bit and energy loading. Micrometer-sized Gallium Nitride light emitting
diode (m-LED) and light amplification by stimulated emission of radiation diode (LD)
are used in these studies due to their high modulation bandwidth. Record data rates are
achieved with a BER below the forward error correction (FEC) threshold at 7.91 Gb/s
using the violet m-LED and at 15 Gb/s using the blue LD. These results highlight
the potential of VLC systems in practical high speed communication solutions. An
additional experimental study is demonstrated for the proposed superposition modulation
techniques based on ASE-DMT. The experimentally achieved results confirm the
theoretical and simulation based performance predictions of ASE-DMT. A significant
gain of up to 17.33 dB in SNR is demonstrated at a low direct current (DC) bias.
Finally, the perception that VLC systems cannot work under the presence of sunlight is
addressed in this thesis. A complete framework is presented to evaluate the performance
of VLC systems in the presence of solar irradiance at any given location and time. The
effect of sunlight is investigated in terms of the degradations in SNR, data rate and
BER. A reliable high speed communication system is achieved under the sunlight
effect. An optical bandpass blue filter is shown to compensate for half of the reduced
data rate in the presence of sunlight. This thesis demonstrates data rates above 1 Gb/s
for a practical VLC link under strong solar illuminance measured at 50350 lux in clear
weather conditions
Space time transceiver design over multipath fading channels
Imperial Users onl
Multiuser non coherent massive MIMO schemes based on DPSK for future communication systems
The explosive usage of rich multimedia content in wireless devices has overloaded the
communication networks. Moreover, the fifth generation (5G) of wireless communications
involves new requirements in the radio access network (RAN) which require higher network
capacities and new capabilities such as ultra-reliable and low-latency communication
(URLLC), vehicular communications or augmented reality. All this has encouraged a remarkable
spectrum crisis in the RF bands. A need for searching alternative techniques
with more spectral efficiency to accommodate the needs of future emerging wireless communications
is emerging. In this context, massive MIMO (m-MIMO) systems have been
proposed as a promising solution for providing a substantial increase in the network capacity,
becoming one of the key enabling technologies for 5G and beyond. m-MIMO
provides high spectral- and energy-efficiency thanks to the deployment of a large number
of antennas at the BS. However, we have to take into account that the current communication
technologies are based on coherent transmission techniques so far, which require
the transmission of a huge amount of signaling. This drawback is escalating with the
excessive available number of antennas in m-MIMO. Therefore, the differential encoding
and non coherent (NC) detection are an alternative solution to circumvent the drawbacks
of m-MIMO in coherent systems. This Ph.D. Thesis is focused on signal processing
techniques for NC detection in conjunction with m-MIMO, proposing new constellation
designs and NC detection algorithms, where the information is transmitted in the signal
differential phase.
First, we design new constellation schemes for an uplink multiuser NC m-MIMO system
in Rayleigh fading channels. These designs allow us to separate the users' signals
at the receiver thanks to a one-to-one correspondence between the constellation for each
user and the received joint constellation. Two approaches are considered in terms of BER:
each user achieves a different performance and, on the other hand, the same performance
is provided for all users. We analyze the number of antennas needed for those designs
and compare to the required number by other designs in the literature. It is shown that
our designs based on DPSK require a lower number of antennas than that required by
their counterpart schemes based on energy. In addition, we compare the performance to
their coherent counterpart systems, resulting NC-m-MIMO based on DPSK capable of
outperforming the coherent systems with the suitable designs.
Second, in order to reduce the number of antennas required for a target performance
we propose a multi-user bit interleaved coded modulation - iterative decoding (BICM-ID) scheme as channel coding for a NC-m-MIMO system based on DPSK. We propose a novel
NC approach for calculating EXIT curves based on the number of antennas. Then using
the EXIT chart we find the best channel coding scheme for our NC-m-MIMO proposal.
We show that the number of users served by the BS can be increased with a 70% reduction
in the number of antennas with respect to the case without channel coding. In particular,
we show that with 100 antennas for error protection equal design for all users and a coding
rate of 1/2 we achieve the minimum probability of error.
Third, we consider that current scenarios such as backhaul wireless systems, rural
or suburban environments, and even new device-to-device (D2D) communications or the
communications in higher frequencies (millimeter and the emerging ones in terahertz frequencies)
can have a predominant line-of-sight (LOS) component, modeled by Rician
fading. For all these new possible scenarios in 5G, we analyze the behavior of the NC
m-MIMO systems when we have a Rician fading. We present a new constellation design
to overcome the problem of the LOS channel component, as well as an associated detection
algorithm to separate each user in reception taking into account the characterization
of the constellation. In addition, for contemplating a more realistic scenario, we propose
grouping users which experience a Rayleigh fading with those with Rician fading, analyzing
the SINR and the performance of such combination in a multi-user NC m-MIMO
system based on M-DPSK. The adequate user grouping allows unifying the constellation
for both groups of users and the detection algorithm, reducing the complexity of the
receiver. Also, the number of users that may be multiplexed may be further increased
thanks to the improved performance.
In the fourth part of this Thesis, we analyse the performance of multi-user NC m-
MIMO based on DPSK in real environments and practical channels defined for the current
standards such as LTE, the future technologies such as 5G and even for communications
in the terahertz band. For this purpose, we use a metric to model the time-varying characteristics
of the practical channels. We employ again the EXIT charts tool for analyzing
and designing iteratively decoded systems. This analysis allows us to obtain an estimate
of the degradation of the system's performance imposed by realistic channels. Hence, we
show that our proposed system is robust to temporal variations, thus it is more recommendable
the employment of NC-m-MIMO-DPSK in the future communication standards
such as 5G. In order to reduce he number of hardware resources required in terms of RF
chains, facilitating its implementation in a real system, we propose incorporating differential
spatial modulation (DSM). We present and analyze a novel multiuser scheme for
NC-m-MIMO combined with DSM with which we can see that the number of antennas
is not a
affected by the incorporation of DSM, even we have an improvement on the
performance with respect to the coherent case.
Finally, we study the viability of multiplexing users by constellation schemes against
classical multiplexing techniques such as time division multiple access (TDMA). In order
to fully characterize the system performance we analyze the block error rate (BLER)
and the throughput of a NC-m-MIMO system. The results show a significant advantage
regarding the number of antennas for multiplexing in the constellation against TDMA.
However, in some cases, the demodulation of multiple users in constellation could require
an excessively large number of antennas compared to TDMA. Therefore, it is necessary to
properly manage the tradeoff
between throughout and the number of antennas, to reach
an optimal operational point, as shown in this Thesis.El inmenso uso de contenido multimedia en los dispositivos inalámbricos ha sobrecargado
las redes de comunicaciones. Además, la quinta generación (5G) de sistemas de
comunicaciones demanda nuevos requisitos para la red de acceso radio, la cual requiere
ofrecer capacidades de red mayores y nuevas funcionalidades como comunicaciones ultra
fiables y con muy poca letancia (URLLC), comunicaciones vehiculares o aplicaciones
como la realidad aumentada. Todo esto ha propiciado una crisis notable en el espectro
electromagnético, lo que ha llevado a una necesidad por buscar técnicas alternativas con
más eficiencia espectral para acomodar todos los requisitos de las tecnologÃas de comunicaciones
emergentes y futuras. En este contexto, los sistemas multi antena masivos,
conocidos como massive MIMO, m-MIMO, han sido propuestos como una solución prometedora
que proporciona un incremento substancial de la capacidad de red, convirtiéndose
en una de las tecnologÃas claves para el 5G. Los sistemas m-MIMO elevan enormemente el
número de antenas en la estación base, lo que les permite ofrecer alta eficiencia espectral
y energética. No obstante, tenemos que tener en cuenta que las actuales tecnologÃas de comunicaciones
emplean técnicas coherentes, las cuales requieren de información del estado
del canal y por ello la transmisión de una enorme cantidad de información de señalización.
Este inconveniente se ve agravado en el caso del m-MIMO debido al enorme número de
antenas. Por ello, la codificación diferencial y la detección no coherente (NC) son una
solución alternativa para solventar el problema de m-MIMO en los sistemas coherentes.
Esta Tesis se centra en las técnicas de procesado de señal para detección NC junto con
m-MIMO, proponiendo nuevos esquemas de constelación y algoritmos de detección NC,
donde la información sea transmitida en la diferencia de fase de la señal.
Primero, diseñamos nuevas constelaciones para un sistema multi usuario NC en m-
MIMO en enlace ascendente (uplink) en canales con desvanecimiento tipo Rayleigh. Estos
diseños nos permiten separar las señales de los usuarios en el receptor gracias a la correspondencia
unÃvoca entre la constelación de cada usuario individual y la constelación
conjunta recibida en la estación base. Hemos considerado dos enfoques para el diseño en
términos de probabilidad de error: cada usuario consigue un rendimiento distinto, mientras
que por otro lado, todos los usuarios son capaces de recibir las mismas prestaciones
de probabilidad de error. Analizamos el número de antenas necesario para estos diseños y
comparamos con el número requerido por otros diseños propuestos en la literatura. Nuestro
diseño basado en DPSK requiere un número menor de antenas comparado con los
sistemas basados en detección de energÃa. También comparamos con su homólogo coherente, resultando que NC-m-MIMO basado en DPSK es capaz de superar a los sistemas
coherentes con los diseños adecuados.
En segundo lugar, para reducir el número de antenas requerido para un rendimiento
dado, proponemos incluir un esquema de codificación de canal. Hemos optado por un
esquema de modulación codificado por bit entrelazado y decodificación iterativa (BICMID).
Hemos empleado la herramienta EXIT chart para el diseño de la codificación de canal,
proponiendo un nuevo enfoque para calcular las curvas EXIT de forma NC y basadas en
el número de antenas. Los resultados muestran que el número de usuarios servidos por
la estación base puede ser incrementado reduciendo un 70% el número de antenas con
respecto al caso sin codificación de canal. En particular, para un array de 100 antenas
y un diseño que ofrezca iguales prestaciones a todos los usuarios, con un código de tasa
1=2, podemos conseguir la mÃnima probabilidad de error.
En tercer lugar, consideramos escenarios donde el canal tenga una componente predominante
de visión directa (LOS) con la estación base modelada mediante un desvanecimiento
tipo Rician. Por ejemplo, sistemas inalámbricos de backhaul, entornos rurales
o sub urbanos, comunicaciones entre dispositivos (D2D), también cuando nos movemos
hacia frecuencias superiores como son en la banda de milimétricas o más recientemente,
la banda de terahercios para buscar mayores anchos de banda. Todos estos escenarios
están contemplados en el futuro 5G. Los diseños presentados para canales Rayleigh ya no
son válidos debido a la componente LOS del canal, por ello presentamos un nuevo diseño de constelación que resuelve el problema de la componente LOS, asà como una guÃa para
diseñar nuevas constelaciones. También proponemos un algoritmo asociado al diseñno de
la constelación para poder separar a los usuarios en recepción. Además, para contemplar
un escenario más realista donde podamos encontrar tanto desvanecimiento Rayleigh como
Rice, proponemos agrupar usuarios de ambos grupos, analizando su rendimiento y relación
señal a interferencia en la combinación. El adecuado agrupamiento permite unificar el
diseño de la constelación para ambos desvanecimientos y por tanto reducir la complejidad
en el receptor. También, el número de usuarios multiplicados en la constelación podrÃa
ser incrementado, gracias a la mejora en el rendimiento.
El cuarto módulo de esta tesis es dedicado a analizar el rendimiento de los diseños
propuestos en presencia de canales reales, donde disponemos de variabilidad temporal y en
frecuencia. Proponemos usar una métrica que modela las caracterÃsticas de la variabilidad
temporal y, usando de nuevo la herramienta EXIT, analizamos los sistemas decodificados
iterativamente considerando ahora los parámetros prácticos del canal. Este análisis nos
permite obtener una estimación de la degradación que sufre el rendimiento del sistema
impuesto por canales reales. Los resultados muestran que los sistemas NC-m-MIMO basados
en DPSK son muy robustos a la variabilidad temporal por lo que son recomendables
para los nuevos escenarios propuestos por el 5G, donde el canal cambia rápidamente.
Otra consideración para introducir los sistemas NC con m-MIMO es la problemática
de necesitar muchas cadenas de radio frecuencia que llevarÃan a tamaños de dispositivos
enormes. Para reducir este número se propone la modulación espacial. En esta Tesis,
estudiamos su uso con los sistemas NC, proponiendo una solución de modulación espacial
diferencial para esquemas con múltiples usuarios combinado con NC-m-MIMO.
Finalmente, estudiamos la viabilidad de multiplexar usuarios en la constelación frente
a usar técnicas clásicas de multiplexación como TDMA. Para caracterizar completamente
el rendimiento del sistema, analizamos la tasa de error de bloque (BLER) y el throughput
de un sistema NC-m-MIMO. Los resultados muestran una ventaja significativa en cuanto
al número de antennas para multiplexar usuarios en la constelación frente al requerido
por TDMA. No obstante, en algunos casos, la demodulación de múltiples usuarios en
la constelación podrÃa requerir un número de antennas excesivamente grande comparado
con la multiplexación en el tiempo. Por ello, es necesario gestionar adecuadamente un
balance entre el throughput y el número de antenas para alcanzar un punto operacional
óptimo, como se muestra en esta Tesis.Programa Oficial de Doctorado en Multimedia y Comunicaciones por la Universidad Carlos III de Madrid y la Universidad Rey Juan CarlosPresidente: Ana Isabel Pérez Neira.- Secretario: Máximo Morales Céspedes.- Vocal: MarÃa del Carmen Aguayo Torre
Spatial Modulation for Generalized MIMO:Challenges, Opportunities, and Implementation
A key challenge of future mobile communication research is to strike an attractive compromise between wireless network's area spectral efficiency and energy efficiency. This necessitates a clean-slate approach to wireless system design, embracing the rich body of existing knowledge, especially on multiple-input-multiple-output (MIMO) technologies. This motivates the proposal of an emerging wireless communications concept conceived for single-radio-frequency (RF) large-scale MIMO communications, which is termed as SM. The concept of SM has established itself as a beneficial transmission paradigm, subsuming numerous members of the MIMO system family. The research of SM has reached sufficient maturity to motivate its comparison to state-of-the-art MIMO communications, as well as to inspire its application to other emerging wireless systems such as relay-aided, cooperative, small-cell, optical wireless, and power-efficient communications. Furthermore, it has received sufficient research attention to be implemented in testbeds, and it holds the promise of stimulating further vigorous interdisciplinary research in the years to come. This tutorial paper is intended to offer a comprehensive state-of-the-art survey on SM-MIMO research, to provide a critical appraisal of its potential advantages, and to promote the discussion of its beneficial application areas and their research challenges leading to the analysis of the technological issues associated with the implementation of SM-MIMO. The paper is concluded with the description of the world's first experimental activities in this vibrant research field
Otimização do fronthaul ótico para redes de acesso de rádio (baseadas) em computação em nuvem (CC-RANs)
Doutoramento conjunto (MAP-Tele) em Engenharia Eletrotécnica/TelecomunicaçõesA proliferação de diversos tipos de dispositivos moveis, aplicações e serviços
com grande necessidade de largura de banda têm contribuÃdo para o aumento
de ligações de banda larga e ao aumento do volume de trafego das
redes de telecomunicações moveis. Este aumento exponencial tem posto
uma enorme pressão nos mobile operadores de redes móveis (MNOs). Um
dos aspetos principais deste recente desenvolvimento, é a necessidade que as
redes têm de oferecer baixa complexidade nas ligações, como também baixo
consumo energético, muito baixa latência e ao mesmo tempo uma grande
capacidade por baixo usto. De maneira a resolver estas questões, os MNOs
têm focado a sua atenção na redes de acesso por rádio em nuvem (C-RAN)
principalmente devido aos seus benefÃcios em termos de otimização de performance
e relação qualidade preço. O standard para a distribuição de sinais
sem fios por um fronthaul C-RAN é o common public radio interface (CPRI).
No entanto, ligações óticas baseadas em interfaces CPRI necessitam de uma
grande largura de banda. Estes requerimentos podem também ser atingidos
com uma implementação em ligação free space optical (FSO) que é um sistema
ótico que usa comunicação sem fios. O FSO tem sido uma alternativa
muito apelativa aos sistemas de comunicação rádio (RF) pois combinam a
flexibilidade e mobilidade das redes RF ao mesmo tempo que permitem a
elevada largura de banda permitida pelo sistema ótico. No entanto, as ligações
FSO são suscetÃveis a alterações atmosféricas que podem prejudicar
o desempenho do sistema de comunicação. Estas limitações têm evitado o
FSO de ser tornar uma excelente solução para o fronthaul. Uma caracterização
precisa do canal e tecnologias mais avançadas são então necessárias
para uma implementação pratica de ligações FSO. Nesta tese, vamos estudar
uma implementação eficiente para fronthaul baseada em tecnologia
á rádio-sobre-FSO (RoFSO). Propomos expressões em forma fechada para
mitigação das perdas de propagação e para a estimação da capacidade do
canal de maneira a aliviar a complexidade do sistema de comunicação. Simulações
numéricas são também apresentadas para formatos de modulação
adaptativas. São também considerados esquemas como um sistema hibrido
RF/FSO e tecnologias de transmissão apoiadas por retransmissores
que ajudam a alivar os requerimentos impostos por um backhaul/fronthaul
de C-RAN. Os modelos propostos não só reduzem o esforço computacional,
como também têm outros méritos, tais como, uma elevada precisão na estimação
do canal e desempenho, baixo requisitos na capacidade de memória
e uma rápida e estável operação comparativamente com o estado da arte
em sistemas analÃticos (PON)-FSO. Este sistema é implementado num recetor
em tempo real que é emulado através de uma field-programmable gate
array (FPGA) comercial. Permitindo assim um sistema aberto, interoperabilidade,
portabilidade e também obedecer a standards de software aberto.
Os esquemas hÃbridos têm a habilidade de suportar diferentes aplicações,
serviços e múltiplos operadores a partilharem a mesma infraestrutura de
fibra ótica.The proliferation of different mobile devices, bandwidth-intensive applications
and services contribute to the increase in the broadband connections
and the volume of traffic on the mobile networks. This exponential growth
has put considerable pressure on the mobile network operators (MNOs). In
principal, there is a need for networks that not only offer low-complexity,
low-energy consumption, and extremely low-latency but also high-capacity
at relatively low cost. In order to address the demand, MNOs have given significant
attention to the cloud radio access network (C-RAN) due to its beneficial
features in terms of performance optimization and cost-effectiveness.
The de facto standard for distributing wireless signal over the C-RAN fronthaul
is the common public radio interface (CPRI). However, optical links
based on CPRI interfaces requires large bandwidth. Also, the aforementioned
requirements can be realized with the implementation of free space
optical (FSO) link, which is an optical wireless system. The FSO is an appealing
alternative to the radio frequency (RF) communication system that
combines the flexibility and mobility offered by the RF networks with the
high-data rates provided by the optical systems. However, the FSO links are
susceptible to atmospheric impairments which eventually hinder the system
performance. Consequently, these limitations prevent FSO from being an
efficient standalone fronthaul solution. So, precise channel characterizations
and advanced technologies are required for practical FSO link deployment
and operation. In this thesis, we study an efficient fronthaul implementation
that is based on radio-on-FSO (RoFSO) technologies. We propose closedform
expressions for fading-mitigation and for the estimation of channel
capacity so as to alleviate the system complexity. Numerical simulations
are presented for adaptive modulation scheme using advanced modulation
formats. We also consider schemes like hybrid RF/FSO and relay-assisted
transmission technologies that can help in alleviating the stringent requirements
by the C-RAN backhaul/fronthaul. The propose models not only
reduce the computational requirements/efforts, but also have a number of
diverse merits such as high-accuracy, low-memory requirements, fast and
stable operation compared to the current state-of-the-art analytical based
approaches. In addition to the FSO channel characterization, we present
a proof-of-concept experiment in which we study the transmission capabilities
of a hybrid passive optical network (PON)-FSO system. This is
implemented with the real-time receiver that is emulated by a commercial
field-programmable gate array (FPGA). This helps in facilitating an
open system and hence enables interoperability, portability, and open software
standards. The hybrid schemes have the ability to support different
applications, services, and multiple operators over a shared optical fiber
infrastructure