Cooperative and cognitive satellite systems

1 Introduction

The history of satellite communications to date has been quite short but extraordinary. It has been just about 50 years since the first Geostationary (GEO) earth satellite was launched to relay communication data (Telstar 1962). A few years later (1969), the first global commercial satellites providing broadcasting and telephone trunking services were successfully launched by Intelsat, starting a new era for satellite communications (satcom). In the same year this global satellite network transmitted in real time television images of the moon landing around the world. A record 500 million television viewers worldwide saw Neil Armstrong’s first steps on the moon.

The evolution of satcom systems was rapid and in a couple of decades the creation of Inmarsat (1979) allowed the development of a system with worldwide mobile satellite coverage initially serving ships and airplanes as well nomadic users.

During this time, technological advances in satcom systems have always been motivated by service demand. On one hand, the demand for some service types, e.g., telephone trunking across continents and mobile telephony gradually attenuated with the advent of underwater telephone cables and terrestrial cellular systems. On the other hand, some service types persisted since the dawn of satcoms and have been increasingly supporting the technological progress of satellite systems. The most prominent example in this context is television broadcasting, which still represents the most economically relevant application for the satellite industry. In the meantime, new services such as broadband internet have surfaced creating new challenges in designing and implementing satcom systems. Nowadays, the demand for fixed and mobile broadband access is ever increasing with various applications in business, education, and entertainment. The projections for the increase of broadband demand are exponential and all telecom sectors have been struggling to accommodate the needs through viable technical solutions and sustainable business modes. The main cause behind this trend has been the advent of new demanding traffic applications originating in social networking, on-demand multimedia consumption, and cloud computing. The explosion of machine-to-machine applications in the Internet of Things era is also opening new perspectives to satellite in extending the terrestrial networks coverage for these kinds of applications.

However, the available frequency resources are becoming scarce due to the spectrum segmentation and the dedicated frequency allocation of the standardized wireless systems. This has become evident during the last decades through the fierce battles in the World Radio Conference for securing rights to prime frequency allocations. In addition, the power consumed by current communications systems has become a limiting factor in the face of global warming, leading to the concept of green and sustainable radio. Therefore, it becomes crucial to define and investigate new network architectures which have the ability to support higher system throughput and energy efficiency, while providing large-scale coverage and availability.

In this direction, cooperative and cognitive satellite systems constitute innovative and promising network architectures, which can improve the desired performance metrics. Cooperative satellite networks operate by jointly processing multiple data streams which might belong to multiple users or originating from multiple radiating elements. In this context, advanced encoding and decoding techniques can be utilized to maximize the spectral and energy efficiency. In essence, this is achieved by employing more aggressive frequency reuse while minimizing the intrasystem interference among the data streams of users. Ideally, the additional signal processing required for those techniques should be implemented on the ground station in order to maximize the satellite lifetime and reliability, while minimizing the mass. On the other hand, cognitive satellite systems are based on the coexistence of satellite system with another incumbent system over a common frequency band. This concept is mainly motivated by spectrum scarcity and the underutilization of assigned frequencies. Focusing on a terrestrial incumbent service, hybrid cognitive networks can combine a ground and a satellite component operating over the same frequency bandwidth. Based on cognitive techniques, both ground and satellite components can communicate simultaneously with users without the need of (Frequency Division) orthogonalization, minimizing additional bandwidth requirements. Furthermore, cognitive satellite networks can be even envisaged on top of an incumbent satellite service in order to address the orbital slot scarcity.

This book attempts to provide an overview of recent R$SPI0026SPI$D results and open issues related to cooperative and cognitive satellite systems. We believe that the development and exploitation of these techniques may greatly help in making satellite networks more attractive and competitive in the near future.

1.1 Cooperative Satellite Systems

During the last decade, research in the area of cooperative communications has been very fruitful, producing innovative paradigms based on optimal communication strategies as dictated by Information Theory. The term cooperation has been used in a wide range of contexts, including receiver cooperation, transmitter cooperation, or cooperation through relays. In this direction, cooperation has also been exploited in satellite communications in order to push the boundaries of single-user link optimization. More specifically, novel transceiver design approaches have employed multiuser, MIMO, and network coding models in order to reap the benefits of cooperation. In this context, a number of new techniques have been investigated in order to maximize frequency reuse while simultaneously mitigating intra- and intersystem interference. The following paragraphs summarize the contributions included in this book and their interconnection.

Chapter 1 focuses on multibeam joint decoding, a technique which is utilized to mitigate interbeam interference on the return link of full frequency reuse multibeam satellites. This work covers both fixed and mobile systems using a generic system model and studies the theoretical communication limits in terms of sum rate and outage capacity. Both linear and nonlinear detectors are considered, as well as the impact of practical aspects such as imperfect channel estimation, synchronization issues, and feeder link limitations.

Chapter 2 continues on the path of Chapter 1 by studying the return link, but this time random access (RA) schemes are motivated by the large terminal population, the bursty traffic, and the need for low signaling overhead. These characteristics may apply to a large range of fixed and mobile services, such as consumer broadband access, machine-to-machine communications, supervisory control and data acquisition (SCADA), transaction, and safety of life applications. In this direction, this work reviews existing state-of-the-art RA schemes in the communication literature and examines their application to satellite systems. Furthermore, the capacity bounds for spread-spectrum and non spread-spectrum RA schemes are investigated, while satellite systems and standards which employ RA are reviewed.

Chapter 3 complements Chapter 1 by examining the forward link of full frequency reuse multibeam systems. This topic has received attention recently, when the DVB (digital video broadcasting) issued the DVB-S2 extension (DVB-S2X) with an optional specification that provides the necessary framing and signaling support to interference management techniques. In this context, state-of-the-art precoding and user scheduling techniques are reviewed and compared, while considering the impact of nonideal system aspects, such as channel phase offsets and imperfect/outdated channel estimates. In addition, practical constraints are discussed, such as frame-based algorithms, multiple gateway systems, and feeder link limitations.

Chapter 4 considers the joint processing of multiple carriers instead of multiple beams. To enable the efficient utilization of satellite transponders, multiple carriers have to be relayed through a single High-Power Amplifier (HPA). However, the nonlinear nature of the HPA result results in adjacent channel interference and peak to average power ratio, which limit the expected performance gains. In this context, this work studies signal processing techniques, i.e., predistortion at the gateway and equalization at the user terminal, which can mitigate the intercarrier nonlinear interference and improve the system performance.

Chapter 5 investigates on-ground beamforming techniques for the forward and the return link of multibeam mobile satellite systems and is closely related to the concepts of Chapters 1 and 3. The use of such techniques is motivated by the reduction of the payload complexity, as well as the exploitation of advanced interference mitigation techniques. On the other hand, additional feeder link bandwidth and complex calibration processes are required to implement such systems. This chapter investigates the performance of on-ground beamforming techniques for both forward and return link and discusses the related trade-offs, including on-board payload complexity and calibration issues.

Chapter 6 addresses cooperative relaying in heterogeneous land mobile satellite (LMS) systems by focusing on an urban scenario with intermittent satellite reception due to the shadowing effect of surrounding buildings....