The objective of this paper is to review state-of-the-art techniques of beamforming in mobile satellite systems and evaluate the potential benefits/drawbacks of on-ground beamforming compared with on-board beamforming approach. The paper also provides a short analysis of beamforming error sources in on-ground beamforming such as propagation effects at feeder link level, on-board degradations at payload level, differential atmospheric perturbations, and Doppler shift effect. An investigation of signal processing techniques is also performed to provide a preliminary assessment of the interest for employing adaptive beamforming and precoding techniques in multi-spots mobile satellite systems

The recent emergence of satellite and terrestrial integrated mobile systems raises new challenges for interference coordination between satellite and terrestrial users. The terrestrial component aims at improving the satellite coverage in highly populated areas where satellite communications often suffer from a high blocking factor caused by the buildings. The term "integrated system" refers to a system composed of a multibeam Satellite Based Network (SBN) and a Complementary Ground Component (CGC) network that re-uses the same frequency band than the satellite's one. This principle is of major interest to improve the spectral efficiency of the overall system and spatially optimize the usage of scarce spectrum resource available at low frequency bands such as L or S-band. Satellite and CGCs are assumed to be controlled by the same resource management entity which allows the coordination of frequency allocation between both components. The understanding of mutual interferences between both components is a key element for the design of such a system taking into account the forecasted balance between the number of terrestrial and satellite users in the system. This paper presents a model of an integrated mobile network composed of a multi-beam satellite, terrestrial base stations and hybrid satellite/terrestrial user terminals. This model is then used for statistical simulations of a continental scale network over Europe in order to analyze interferences between satellite and terrestrial users. The influence of several parameters such as the satellite frequency reuse pattern and the exclusion zone size between terrestrial and satellite cells is then presented.

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