18 research outputs found
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
Non-Coherent Massive MIMO-OFDM Down-Link Based on Differential Modulation
Orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) are wireless radio technologies adopted by the new Fifth Generation (5G) of mobile communications. A very large number of antennas (massive MIMO) is used to perform the beam-forming of the transmitted signal, either to reduce the multi-user interference (MUI), when spatially multiplexing several users, or to compensate the path-loss when higher frequencies than microwave are used, such as the millimeter-waves (mm-Waves). Usually, a coherent demodulation scheme (CDS) is used in order to exploit MIMO-OFDM, where the channel estimation and the pre/post-equalization processes are complex and time consuming operations, which require a considerable pilot overhead and also increase the latency of the system. As an alternative, non-coherent techniques based on a differential modulation scheme have been proposed for the up-link (UL). However, it is not straightforward to extend these proposals to the down-link (DL) due to the (usually) reduced number of antennas at the receiver side. In this paper we overcome this problem, and assuming that each user equipment (UE) is only equipped with one single antenna, we propose the combination of beam-forming with a differential modulation scheme for the DL, enhanced by the frequency diversity. The new transmission and reception schemes are described, and the signal-to-interference-plus-noise ratio (SINR) and the complexity are analysed. The numerical results verify the accuracy of the analysis and show that our proposal outperforms the existing CDS with a significant lower complexity.This work was supported by project TERESA-ADA (TEC2017-90093-C3-2-R) (MINECO/AEI/FEDER, UE)
Advanced wireless communications using large numbers of transmit antennas and receive nodes
The concept of deploying a large number of antennas at the base station, often called massive multiple-input multiple-output (MIMO), has drawn considerable interest because of its potential ability to revolutionize current wireless communication systems. Most literature on massive MIMO systems assumes time division duplexing (TDD), although frequency division duplexing (FDD) dominates current cellular systems. Due to the large number of transmit antennas at the base station, currently standardized approaches would require a large percentage of the precious downlink and uplink resources in FDD massive MIMO be used for training signal transmissions and channel state information (CSI) feedback. First, we propose practical open-loop and closed-loop training frameworks to reduce the overhead of the downlink training phase. We then discuss efficient CSI quantization techniques using a trellis search. The proposed CSI quantization techniques can be implemented with a complexity that only grows linearly with the number of transmit antennas while the performance is close to the optimal case. We also analyze distributed reception using a large number of geographically separated nodes, a scenario that may become popular with the emergence of the Internet of Things. For distributed reception, we first propose coded distributed diversity to minimize the symbol error probability at the fusion center when the transmitter is equipped with a single antenna. Then we develop efficient receivers at the fusion center using minimal processing overhead at the receive nodes when the transmitter with multiple transmit antennas sends multiple symbols simultaneously using spatial multiplexing
Limited Feedback Techniques in Multiple Antenna Wireless Communication Systems
Multiple antenna systems provide spatial multiplexing and diversity benefits.These systems also offer beamforming and interference mitigation capabilities in single-user (SU) and multi-user (MU) scenarios, respectively. Although diversity can be achieved without channel state information (CSI) at the transmitter using space-time codes, the knowledge of instantaneous CSI at the transmitter is essential to the above mentioned gains. In frequency division duplexing (FDD) systems, limited feedback techniques are employed to obtain CSI at the transmitter from the receiver using a low-rate link. As a consequence, CSI acquired by the transmitter in such manner have errors due to channel estimation and codebook quantization at the receiver, resulting in performance degradation of multi-antenna systems. In this thesis, we examine CSI inaccuracies due to codebook quantization errors and investigate several other aspects of limited feedback in SU, MU and multicell wireless communication systems with various channel models.
For SU multiple-input multiple-output (MIMO) systems, we examine the capacity loss using standard codebooks. In particular, we consider single-stream and two-stream MIMO transmissions and derive capacity loss expressions in terms of minimum squared chordal distance for various MIMO receivers. Through simulations, we investigate the impact of codebook quantization errors on the capacity performance in uncorrelated Rayleigh, spatially correlated Rayleigh and standardized MIMO channels. This work motivates the need of effective codebook design to reduce the codebook quantization errors in correlated channels.
Subsequently, we explore the improvements in the design of codebooks in temporally and spatially correlated channels for MU multiple-input single-output (MISO) systems, by employing scaling and rotation techniques. These codebooks quantize instantaneous channel direction information (CDI) and are referred as differential codebooks in the thesis. We also propose various adaptive scaling techniques for differential codebooks where packing density of codewords in the differential codebook are altered according to the channel condition, in order to reduce the quantization errors. The proposed differential codebooks improve the spectral efficiency of the system by minimizing the codebook quantization errors in spatially and temporally correlated channels.
Later, we broaden the scope to massive MISO systems and propose trellis coded quantization (TCQ) schemes to quantize CDI. Unlike conventional codebook approach, the TCQ scheme does not require exhaustive search to select an appropriate codeword, thus reducing computational complexity and memory requirement at the receiver. The proposed TCQ schemes yield significant performance improvements compared to the existing TCQ based limited feedback schemes in both temporally and spatially correlated channels.
Finally, we investigate interference coordination for multicell MU MISO systems using regularized zero-forcing (RZF) precoding. We consider random vector quantization (RVQ) codebooks and uncorrelated Rayleigh channels. We derive expected SINR approximations for perfect CDI and RVQ codebook-based CDI. We also propose an adaptive bit allocation scheme which aims to minimize the network interference and moreover, improves the spectral efficiency compared to equal bit allocation and coordinated zero-forcing (ZF) based adaptive bit allocation schemes
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
Harnessing the Potential of Emerging Technologies to Break Down Barriers in Tactical Communications
peer reviewe