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

Novel Efficient Precoding Techniques for Multiuser MIMO Systems

In Multiuser MIMO (MU-MIMO) systems, precoding is essential to eliminate or minimize the multiuser interference (MUI). However, the design of a suitable precoding algorithm with good overall performance and low computational complexity at the same time is quite challenging, especially with the increase of system dimensions. In this thesis, we explore the art of novel low-complexity high-performance precoding algorithms with both linear and non-linear processing strategies. Block diagonalization (BD)-type based precoding techniques are well-known linear precoding strategies for MU-MIMO systems. By employing BD-type precoding algorithms at the transmit side, the MU-MIMO broadcast channel is decomposed into multiple independent parallel SU-MIMO channels and achieves the maximum diversity order at high data rates. The main computational complexity of BD-type precoding algorithms comes from two singular value decomposition (SVD) operations, which depend on the number of users and the dimensions of each user's channel matrix. In this thesis, two categories of low-complexity precoding algorithms are proposed to reduce the computational complexity and improve the performance of BD-type precoding algorithms. One is based on multiple LQ decompositions and lattice reductions. The other one is based on a channel inversion technique, QR decompositions, and lattice reductions to decouple the MU-MIMO channel into equivalent SU-MIMO channels. Both of the two proposed precoding algorithms can achieve a comparable sum-rate performance as BD-type precoding algorithms, substantial bit error rate (BER) performance gains, and a simplified receiver structure, while requiring a much lower complexity. Tomlinson-Harashima precoding (THP) is a prominent nonlinear processing technique employed at the transmit side and is a dual to the successive interference cancelation (SIC) detection at the receive side. Like SIC detection, the performance of THP strongly depends on the ordering of the precoded symbols. The optimal ordering algorithm, however, is impractical for MU-MIMO systems with multiple receive antennas. We propose a multi-branch THP (MB-THP) scheme and algorithms that employ multiple transmit processing and ordering strategies along with a selection scheme to mitigate interference in MU-MIMO systems. Two types of multi-branch THP (MB-THP) structures are proposed. The first one employs a decentralized strategy with diagonal weighted filters at the receivers of the users and the second uses a diagonal weighted filter at the transmitter. The MB-MMSE-THP algorithms are also derived based on an extended system model with the aid of an LQ decomposition, which is much simpler compared to the conventional MMSE-THP algorithms. Simulation results show that a better BER performance can be achieved by the proposed MB-MMSE-THP precoder with a small computational complexity increase

Performance of Orthogonal Beamforming for SDMA with Limited Feedback

On the multi-antenna broadcast channel, the spatial degrees of freedom support simultaneous transmission to multiple users. The optimal multiuser transmission, known as dirty paper coding, is not directly realizable. Moreover, close-to-optimal solutions such as Tomlinson-Harashima precoding are sensitive to CSI inaccuracy. This paper considers a more practical design called per user unitary and rate control (PU2RC), which has been proposed for emerging cellular standards. PU2RC supports multiuser simultaneous transmission, enables limited feedback, and is capable of exploiting multiuser diversity. Its key feature is an orthogonal beamforming (or precoding) constraint, where each user selects a beamformer (or precoder) from a codebook of multiple orthonormal bases. In this paper, the asymptotic throughput scaling laws for PU2RC with a large user pool are derived for different regimes of the signal-to-noise ratio (SNR). In the multiuser-interference-limited regime, the throughput of PU2RC is shown to scale logarithmically with the number of users. In the normal SNR and noise-limited regimes, the throughput is found to scale double logarithmically with the number of users and also linearly with the number of antennas at the base station. In addition, numerical results show that PU2RC achieves higher throughput and is more robust against CSI quantization errors than the popular alternative of zero-forcing beamforming if the number of users is sufficiently large.Comment: 27 pages; to appear in IEEE Transactions on Vehicular Technolog