47 research outputs found
Analysis and Design of Algorithms for the Improvement of Non-coherent Massive MIMO based on DMPSK for beyond 5G systems
Mención Internacional en el tÃtulo de doctorNowadays, it is nearly impossible to think of a service that does not rely on wireless communications.
By the end of 2022, mobile internet represented a 60% of the total global online traffic.
There is an increasing trend both in the number of subscribers and in the traffic handled by each
subscriber. Larger data rates, smaller extreme-to-extreme (E2E) delays and greater number of
devices are current interests for the development of mobile communications. Furthermore, it
is foreseen that these demands should also be fulfilled in scenarios with stringent conditions,
such as very fast varying wireless communications channels (either in time or frequency) or
scenarios with power constraints, mainly found when the equipment is battery powered.
Since most of the wireless communications techniques and standards rely on the fact that the
wireless channel is somehow characterized or estimated to be pre or post-compensated in transmission
(TX) or reception (RX), there is a clear problem when the channels vary rapidly or the
available power is constrained. To estimate the wireless channel and obtain the so-called channel
state information (CSI), some of the available resources (either in time, frequency or any
other dimension), are utilized by including known signals in the TX and RX typically known as
pilots, thus avoiding their use for data transmission. If the channels vary rapidly, they must be
estimated many times, which results in a very low data efficiency of the communications link.
Also, in case the power is limited or the wireless link distance is large, the resulting signal-tointerference-
plus-noise ratio (SINR) will be low, which is a parameter that is directly related to
the quality of the channel estimation and the performance of the data reception. This problem
is aggravated in massive multiple-input multiple-output (massive MIMO), which is a promising
technique for future wireless communications since it can increase the data rates, increase the
reliability and cope with a larger number of simultaneous devices. In massive MIMO, the base
station (BS) is typically equipped with a large number of antennas that are coordinated. In these
scenarios, the channels must be estimated for each antenna (or at least for each user), and thus,
the aforementioned problem of channel estimation aggravates. In this context, algorithms and
techniques for massive MIMO without CSI are of interest.
This thesis main topic is non-coherent massive multiple-input multiple-output (NC-mMIMO)
which relies on the use of differential M-ary phase shift keying (DMPSK) and the spatial
diversity of the antenna arrays to be able to detect the useful transmitted data without CSI knowledge. On the one hand, hybrid schemes that combine the coherent and non-coherent
schemes allowing to get the best of both worlds are proposed. These schemes are based on
distributing the resources between non-coherent (NC) and coherent data, utilizing the NC data
to estimate the channel without using pilots and use the estimated channel for the coherent
data. On the other hand, new constellations and user allocation strategies for the multi-user
scenario of NC-mMIMO are proposed. The new constellations are better than the ones in the
literature and obtained using artificial intelligence techniques, more concretely evolutionary
computation.This work has received funding from the European Union Horizon 2020 research and innovation
programme under the Marie Skłodowska-Curie ETN TeamUp5G, grant agreement No.
813391. The PhD student was the Early Stage Researcher (ESR) number 2 of the project.
This work has also received funding from the Spanish National Project IRENE-EARTH
(PID2020-115323RB-C33) (MINECO/AEI/FEDER, UE), which funded the work of some coauthors.Programa de Doctorado en Multimedia y Comunicaciones por la Universidad Carlos III de Madrid y la Universidad Rey Juan CarlosPresidente: Luis Castedo Ribas.- Secretario: Matilde Pilar Sánchez Fernández.- Vocal: Eva Lagunas Targaron
Interference mitigation using group decoding in multiantenna systems
fi=vertaisarvioitu|en=peerReviewed
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
Foundations of User-Centric Cell-Free Massive MIMO
Imagine a coverage area where each mobile device is communicating with a
preferred set of wireless access points (among many) that are selected based on
its needs and cooperate to jointly serve it, instead of creating autonomous
cells. This effectively leads to a user-centric post-cellular network
architecture, which can resolve many of the interference issues and
service-quality variations that appear in cellular networks. This concept is
called User-centric Cell-free Massive MIMO (multiple-input multiple-output) and
has its roots in the intersection between three technology components: Massive
MIMO, coordinated multipoint processing, and ultra-dense networks. The main
challenge is to achieve the benefits of cell-free operation in a practically
feasible way, with computational complexity and fronthaul requirements that are
scalable to enable massively large networks with many mobile devices. This
monograph covers the foundations of User-centric Cell-free Massive MIMO,
starting from the motivation and mathematical definition. It continues by
describing the state-of-the-art signal processing algorithms for channel
estimation, uplink data reception, and downlink data transmission with either
centralized or distributed implementation. The achievable spectral efficiency
is mathematically derived and evaluated numerically using a running example
that exposes the impact of various system parameters and algorithmic choices.
The fundamental tradeoffs between communication performance, computational
complexity, and fronthaul signaling requirements are thoroughly analyzed.
Finally, the basic algorithms for pilot assignment, dynamic cooperation cluster
formation, and power optimization are provided, while open problems related to
these and other resource allocation problems are reviewed. All the numerical
examples can be reproduced using the accompanying Matlab code.Comment: This is the authors' version of the manuscript: \"Ozlem Tugfe Demir,
Emil Bj\"ornson and Luca Sanguinetti (2021), "Foundations of User-Centric
Cell-Free Massive MIMO", Foundations and Trends in Signal Processing: Vol.
14, No. 3-4, pp 162-47
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Leveraging UAVs for 6G Networks
Advancing towards 6G networks emphasizes integrating communication with sensing functionalities, promising unparalleled connectivity, efficiency, and intelligence in forthcoming networks. In this context, uncrewed aerial vehicles (UAVs) emerge as pivotal assets, offering versatile solutions for both communication and sensing tasks. Leveraging their mobility and flexibility, optimizing the UAV deployment or trajectory can enhance the network performance to meet the new demands.Anticipating next-generation 6G networks, traditional cellular architectures' constraints have led to the exploration of new network topologies, including cell-free architectures. These architectures abandon the concept of cell, allowing users to connect to multiple base stations and mitigating the effects of cellular boundaries for fairer scenarios. Combining cell-free architectures with UAVs offers substantial performance gains by leveraging UAV adaptability for dynamic coverage and capacity optimization. To fully leverage this potential, we propose a comprehensive framework for cell-free UAV networks. Initially, UAVs operate as flying base stations within a framework of perfect fronthaul connectivity. This paradigm is extended to accommodate wireless fronthaul scenarios, prompting UAVs to function as flying relays instead of flying base stations.Moreover, UAVs hold significant potential beyond their role in communication. Equipped with sensors and video cameras, UAVs can serve a dual purpose, enabling efficient data collection and sensing tasks. One critical application is wildfire tracking, addressing the pressing need for early detection and monitoring of wildfires. With the escalating frequency and intensity of wildfires globally, efficient wildfire tracking has become imperative for mitigating their devastating impact. Integrating the strengths of cell-free UAV networks with artificial intelligence, our aim is to optimize UAV trajectories to achieve two primary objectives: (i) cover the fire perimeter with cameras and (ii) ensure reliable transmission of captured images to the network. This design significantly enhances resilience, allowing UAVs to transmit images even if certain base stations are compromised by fire incidents. However, the complexity of the overall problem presents a challenge, leading to the utilization of reinforcement learning in this scenario.In addition to the aforementioned applications in cell-free networks and wildfire tracking, this dissertation also explores similar scenarios with cellular connectivity. This includes exploring the integration of communication, sensing, and data collection functionalities withintraditional cellular networks. These methodologies, which involve optimizing trajectories via traditional techniques or more sophisticated such as reinforcement learning, contribute to enhancing the efficiency and reliability of cellular networks as well