Multicast Multigroup Precoding and User Scheduling for Frame-Based Satellite Communications

The present work focuses on the forward link of a broadband multibeam satellite system that aggressively reuses the user link frequency resources. Two fundamental practical challenges, namely the need to frame multiple users per transmission and the per-antenna transmit power limitations, are addressed. To this end, the so-called frame-based precoding problem is optimally solved using the principles of physical layer multicasting to multiple co-channel groups under per-antenna constraints. In this context, a novel optimization problem that aims at maximizing the system sum rate under individual power constraints is proposed. Added to that, the formulation is further extended to include availability constraints. As a result, the high gains of the sum rate optimal design are traded off to satisfy the stringent availability requirements of satellite systems. Moreover, the throughput maximization with a granular spectral efficiency versus SINR function, is formulated and solved. Finally, a multicast-aware user scheduling policy, based on the channel state information, is developed. Thus, substantial multiuser diversity gains are gleaned. Numerical results over a realistic simulation environment exhibit as much as 30% gains over conventional systems, even for 7 users per frame, without modifying the framing structure of legacy communication standards.Comment: Accepted for publication to the IEEE Transactions on Wireless Communications, 201

Multibeam Joint Processing in Satellite Communications

Cooperative Satellite Communications (SatComs) involve multi-antenna satellites enabled for the joint transmission and reception of signals. This joint processing of baseband signals is realized amongst the distinct but interconnected antennas. Advanced signal processing techniques –namely precoding and Multiuser Detection (MUD)– are herein examined in the multibeam satellite context. The aim of this thesis is to establish the prominence of such methods in the next generation of broadband satellite networks. To this end, two approaches are followed. On one hand, the performance of the well established and theoretically concrete MUD is analysed over the satellite environments. On the other, optimal signal processing designs are developed and evaluated for the forward link. In more detail, the present dissertation begins by introducing the topic of multibeam joint processing. Thus, the most significant practical constraints that hinder the application of advanced interference mitigation techniques in satellite networks are identified and discussed. Prior to presenting the contributions of this work, the multi-antenna joint processing problem is formulated using the generic Multiuser (MU) Multiple InputMultiple Output (MIMO) baseband signal model. This model is also extended to apply in the SatComs context. A detailed presentation of the related work, starting from a generic signal processing perspective and then focusing on the SatComs field, is then given. With this review, the main open research topics are identified. Following the comprehensive literature review, the first contribution of this work, is presented. This involves the performance evaluation of MUD in the Return Link (RL) of multiuser multibeam SatComs systems. Novel, analytical expressions are derived to describe the information theoretic channel capacity as well as the performance of practical receivers over realistic satellite channels. Based on the derived formulas, significant insights for the design of the RL of next generation cooperative satellite systems are provided. In the remaining of this thesis, the focus is set on the Forward Link (FL) of multibeam SatComs, where precoding, combined with aggressive frequency reuse configurations, are proposed to enhance the offered throughput. In this context, the alleviation of practical constraints imposed by the satellite channel is the main research challenge. Focusing on the rigid framing structure of the legacy SatCom standards, the fundamental frame-based precoding problem is examined. Based on the necessity to serve multiple users by a single transmission, the connection of the frame-based precoding and the fundamental signal processing problem of physical layer multigroup multicasting is established. In this framework and to account for the power limitations imposed by a dedicated High Power Amplifier (HPA) per transmit element, a novel solution for multigroup multicasting under Per Anntenna Constraints (PACs) is derived. Therefore, the gains offered by multigroup multicasting in frame-based systems are quantified over an accurate simulation setting. Finally, advanced multicast and interference aware scheduling algorithms are proposed to glean significant gains in the rich multiuser satellite environment. The thesis concludes with the main research findings and the identification of new research challenges, which will pave the way for the deployment of cooperative multibeam satellite systems

Generalized multicast multibeam precoding for satellite communications

This paper deals with the problem of precoding in multibeam satellite systems. In contrast to general multiuser multiple-input-multiple-output cellular schemes, multibeam satellite architectures suffer from different challenges. First, satellite communications standards embed more than one user in each frame in order to increase the channel coding gain. This leads to the different so-called multigroup multicast models, whose optimization requires computationally complex operations. Second, when the data traffic is generated by several Earth stations (gateways), the precoding matrix must be distributively computed and meet additional payload restrictions. Third, since the feedback channel is adverse (large delay and quantization errors), the precoding must be able to deal with such uncertainties. In order to solve the aforementioned problems, we propose a two-stage precoding design in order to both limit the multibeam interference and to enhance the intra-beam minimum user signal power (i.e., the one that dictates the rate allocation per beam). A robust version of the proposed precoder based on a first perturbation model is presented. This mechanism behaves well when the channel state information is corrupted. Furthermore, we propose a per beam user grouping mechanism together with its robust version in order to increase the precoding gain. Finally, a method for dealing with the multiple gateway architecture is presented, which offers high throughputs with a low inter-gateway communication. The conceived designs are evaluated with a close-to-real beam pattern and the latest broadband communication standard for satellite communications.Peer ReviewedPostprint (updated version

Interference Exploitation via Symbol-Level Precoding: Overview, State-of-the-Art and Future Directions

Interference is traditionally viewed as a performance limiting factor in wireless communication systems, which is to be minimized or mitigated. Nevertheless, a recent line of work has shown that by manipulating the interfering signals such that they add up constructively at the receiver side, known interference can be made beneficial and further improve the system performance in a variety of wireless scenarios, achieved by symbol-level precoding (SLP). This paper aims to provide a tutorial on interference exploitation techniques from the perspective of precoding design in a multi-antenna wireless communication system, by beginning with the classification of constructive interference (CI) and destructive interference (DI). The definition for CI is presented and the corresponding mathematical characterization is formulated for popular modulation types, based on which optimization-based precoding techniques are discussed. In addition, the extension of CI precoding to other application scenarios as well as for hardware efficiency is also described. Proof-of-concept testbeds are demonstrated for the potential practical implementation of CI precoding, and finally a list of open problems and practical challenges are presented to inspire and motivate further research directions in this area

The Role of Physical Layer Security in Satellite-Based Networks

In the coming years, 6G will revolutionize the world with a large amount of bandwidth, high data rates, and extensive coverage in remote and rural areas. These goals can only be achieved by integrating terrestrial networks with non-terrestrial networks. On the other hand, these advancements are raising more concerns than other wireless links about malicious attacks on satellite-terrestrial links due to their openness. Over the years, physical layer security (PLS) has emerged as a good candidate to deal with security threats by exploring the randomness of wireless channels. In this direction, this paper reviews how PLS methods are implemented in satellite communications. Firstly, we discuss the ongoing research on satellite-based networks by highlighting the key points in the literature. Then, we revisit the research activities on PLS in satellite-based networks by categorizing the different system architectures. Finally, we highlight research directions and opportunities to leverage the PLS in future satellite-based networks

A Tutorial on Interference Exploitation via Symbol-Level Precoding: Overview, State-of-the-Art and Future Directions

IEEE Interference is traditionally viewed as a performance limiting factor in wireless communication systems, which is to be minimized or mitigated. Nevertheless, a recent line of work has shown that by manipulating the interfering signals such that they add up constructively at the receiver side, known interference can be made beneficial and further improve the system performance in a variety of wireless scenarios, achieved by symbol-level precoding (SLP). This paper aims to provide a tutorial on interference exploitation techniques from the perspective of precoding design in a multi-antenna wireless communication system, by beginning with the classification of constructive interference (CI) and destructive interference (DI). The definition for CI is presented and the corresponding mathematical characterization is formulated for popular modulation types, based on which optimization-based precoding techniques are discussed. In addition, the extension of CI precoding to other application scenarios as well as for hardware efficiency is also described. Proof-of-concept testbeds are demonstrated for the potential practical implementation of CI precoding, and finally a list of open problems and practical challenges are presented to inspire and motivate further research directions in this area

Next generation multibeam satellite systems

Satellite communication will play a central role towards fulfilling next generation 5G communication requirements. As a matter of fact, anytime-anywhere connectivity cannot be conceived without the presence of the satellite segment. Indeed, satellite communication industry is not only targeting popular markets but also to high dense populated areas where the satellite will become an essential element to decongest the terrestrial wireless network. In order to deliver broadband interactive data traffic, satellite payloads are currently implementing a multibeam radiation pattern. The use of a multibeam architecture brings several advantages in front of a single global beam transmission. First, as an array fed reflector is employed, the antenna gain to noise ratio can be increased leading to high gain in the achievable throughput. Second, different symbols can be simultaneously sent to geographically separated areas, allowing a spatially multiplexed communication. Last but not least, the available bandwidth can be reused in sufficiently separated beams, increasing the spectrum reuse in the overall coverage area. Whenever the system designers target the terabit satellite system the aforementioned multibeam architecture shall be reconsidered. Indeed, the achievable rates can be extremely increased in case a more aggressive frequency reuse is deployed and interference mitigation techniques are implemented either at the user terminal (multiuser detection) or in the transmitter (precoding). Our study deals with the problem of precoding and linear filtering receiving methods for multibeam satellite systems when full frequency reuse is considered. Concretely, we consider the particular restrictions of satellite communications which, in contrast to terrestrial communication systems, suffer from additional drawbacks. First, the feeder link shall aggregate the overall data traffic leading to a very large rate requirement. This required data rate is even increased whenever linear filtering at the return link and precoding in the forward link are deployed. This is because the feed signals, which are larger than the number of beams, shall be computed on ground. In order to solve this problem, we propose a hybrid architecture where the satellite payload is equipped with a fixed processing. This on-board processing linearly transforms the received and transmitted data in order to keep the feeder link rate requirement low. The on-board processing results to be the same for both return and forward links, leading to a large reduction of the payload complexity, mass and cost. Second, as the data traffic can be generated by different gateways, the precoding method shall be designed accordingly. In contrast to previous works, this work studies the case where the collaboration between different gateways is limited. In addition to the aforementioned contribution, in this work some unexplored aspects of multi-gateway multibeam precoding are also investigated. Finally, we consider an important phenomena that currently needs to be treated in multibeam systems: the fact that a single codeword is embedded the information of multiple users in each beam. This leads to the difficult so-called multigroup multicast model, whose optimization requires computationally complex operations. In order to solve this problem: i) we propose a two-stage precoding design in order to both limit the multibeam interference and to enhance the intra-beam minimum user signal power, ii) a robust version of the proposed precoder based on a first perturbation model is presented. This mechanism behaves well when the channel state information is corrupted, iii) we propose a per beam user grouping mechanism so as its robust version in order to increase the precoding gain. Forth, a method for dealing with the multiple gateway architecture is presented that offers high throughputs with a low inter-gateway communication.La comunicación por satélite desempeñará un papel central en el cumplimiento de los requisitos de comunicación 5G de próxima generación. Como cuestión de hecho, la conectividad cualquier momento y lugar no se puede concebir sin la presencia del segmento satelital. De hecho, la industria de la comunicación por satélite no sólo se dirige a los mercados populares, sino también a la alta densas zonas pobladas donde el satélite se convertirá en un elemento esencial para descongestionar la red inalámbrica terrestre. Para entregar el tráfico de datos interactiva de banda ancha, las cargas útiles de satélites están implementando un diagrama de radiación de haces múltiples. El uso de una arquitectura multihaz aporta varias ventajas frente a un único haz de transmisión global. En primer lugar, como se emplea un reflector alimentado matriz, la ganancia de antena a ruido puede aumentar dando lugar a una alta ganancia en el rendimiento alcanzable. En segundo lugar, diferentes símbolos pueden ser enviados simultáneamente a las áreas separadas geográficamente, lo que permite una comunicación multiplexada espacialmente. Por último, pero no menos importante, el ancho de banda disponible puede ser reutilizado en las vigas suficientemente separadas, el aumento de la reutilización del espectro en el área de cobertura global. Cada vez que los diseñadores de sistemas se dirigen el sistema de satélites terabit se reconsideró la arquitectura multihaz mencionado. De hecho, las tasas alcanzables pueden ser extremadamente aumentaron en caso de reutilización de frecuencias más agresiva está desplegado y las técnicas de reducción de interferencias se implementan ya sea en el terminal de usuario (detección multiusuario) o en el transmisor (precodificación). Nuestros estudio aborda el problema de precodificación y filtrado lineal recibir métodos para sistemas de satélites multihaz cuando se considera la reutilización de frecuencias completa. Concretamente, consideramos las restricciones particulares de comunicaciones por satélite que, en contraste con los sistemas de comunicación terrestres, sufren de desventajas adicionales. En primer lugar, el enlace de conexión deberá agregar el tráfico global de datos que conduce a un requisito tasa muy grande. Esta velocidad de datos requerida es incluso aumentó cada vez filtrado lineal en el enlace de retorno y precodificación en el enlace directo se despliegan. Esto se debe a que las señales de alimentación, que son más grandes que el número de haces, se computarán en el suelo. Con el fin de resolver este problema, se propone una arquitectura híbrida, donde la carga útil del satélite está equipado con un procesamiento fijo. Este procesamiento a bordo transforma linealmente los datos recibidos y transmitidos con el fin de mantener el requisito de baja tasa de enlace de conexión. Los resultados del procesamiento de a bordo para ser el mismo para ambos enlaces directo y de retorno, dando lugar a una gran reducción de la complejidad de carga útil, la masa y el coste. En segundo lugar, como el tráfico de datos puede ser generada por diferentes puertas de enlace, el método de precodificación deberá ser diseñado en consecuencia. A diferencia de los trabajos anteriores, este trabajo estudia el caso en que la colaboración entre las diferentes pasarelas es limitado. Además de la contribución anterior, en este trabajo también se investigan algunos aspectos inexplorados de multi-gateway multihaz precodificación. Finalmente, consideramos un fenómeno importante que necesita actualmente para ser tratados en sistemas multihaz: el hecho de que una sola palabra de código se incrusta la información de múltiples usuarios en cada viga. Esto conduce a la denominada modelo de multidifusión multigrupo difícil, cuya optimización requiere operaciones computacionalmente complejos. En tal escenario, el diseño de precodificación en el enlace directo será dirigido

On the use of multiple satellites to improve the spectral efficiency of broadcast transmissions

We consider the use of multiple co-located satellites to improve the spectral efficiency of broadcast transmissions. In particular, we assume that two satellites transmit on overlapping geographical coverage areas, with overlapping frequencies. We first describe the theoretical framework based on network information theory and, in particular, on the theory for multiple access channels. The application to different scenarios will be then considered, including the bandlimited additive white Gaussian noise channel with average power constraint and different models for the nonlinear satellite channel. The comparison with the adoption of frequency division multiplexing (FDM) is also provided. The main conclusion is that a strategy based on overlapped signals is convenient with respect to FDM, although it requires the adoption of a multiuser detection strategy at the receiver