Link Optimization in Future Generation Satellite Systems

In recent years, communication networks have seen a huge growth in the amount of
requested throughput, pushed from the combination of two main drivers: the
introduction of new services and the improvement of existing ones, requiring
increased amount of traffic (e.g. higher quality of video content). These
effects mandate the constant evolution of current systems in order to cope with
the growing user demand and should be tackled from multiple angles. On the one
hand, better utilization of available resources might help in the short term to
keep up with the market and has always been an important priority for operators
of terrestrial and satellite networks alike. On the other hand, acquisition and
exploitation of currently unused resources might fuel the growth for a
significantly longer period of time, ensuring longevity and thus enabling
future-proofing of current systems. Both these topics are addressed in this
thesis with specific applications relevant to satellite communication networks.

In the first part, this thesis focuses on maximization of the user capacity by
better exploiting the available radio resources. Motivated by the substantial
capacity gains enabled by a higher bandwidth allocation, we
investigate the optimization of satellite systems employing full-frequency
reuse on the user downlink. Unlike most of the literature on the subject, usually
resorting to precoding techniques to mitigate the interference, we propose a
combination of predistortion and precoding to jointly counteract on-board non-linear
distortions and multi-user interference. First, a flexible framework for
the optimization of transmit processing schemes in communication chains is
presented. This framework expands on the application of the well known gradient
descent technique by applying it to the maximization of the received Signal
to Noise plus Interference ratio in complex communication systems. To do so, it
identifies a suitable mathematical representation of various key blocks of the
system and exploits the chain rule of the derivative to compute the overall
gradient as a cascade of the single components. Afterwards, this framework is
validated by optimizating the coefficients of the proposed predistortion
architecture for the satellite system in analysis. The obtained results
highlight the flexibility of the developed optimization framework and the
benefits of the suggested predistortion strategy compared to existing state of
the art solutions.

In the second part of the thesis, the focus is shifted towards investigating the
exploitation of novel resources by looking at the use of optical frequencies for
ground-to-space feeder links. The topic is introduced by a survey of existing benefits
and limitations of free space optical communications. Subsequently, the
implications of employing optical frequencies in long distance ground-to-space feeder
links with transparent satellites are addressed. Furthermore, a powerful and flexible
simulation tool was developed and exploited during the course of this thesis to
model and assess the Physical (PHY) layer performance of hybrid optical/Radio
Frequencies (RF) satellite networks. This tool is presented together with the scenarios and results obtained as part of the project ONSET
(Optical Feeder Links Study for Satellite Networks - ESA Contract No. 40000113462/15/NL/NDe).

Finally, the thesis investigates a scenario that combines the transmit processing
techniques analyzed in the first part and the context of optical feeder links
evaluated in the second part. A hybrid optical/RF system is considered with an
electrical predistorter in place to counteract the impairments induced by the
combined effects of electrical and optical non-linearities encountered along the
end-to-end chain. The developed mathematical framework is exploited to jointly
optimize the predistortion coefficients and the working point for the
electro-optical modulator. The performance results obtained after the
optimization procedure demonstrate the efficacy of the proposed approach for
hybrid optical/RF systems with analog modulations.