External publication | MIMO for Satellite Communication Systems
The multiple antenna technique, MIMO (Multiple Input Multiple Output) is a success story in wireless communication systems. One of the main features of MIMO is the utilisation of the spatial dimension. The spatial dimension in MIMO brings significant performance improvement through array gain, spatial diversity, spatial multiplexing and interference avoidance. In this thesis, the application of MIMO to satellite communications (SATCOMs) is analysed and addressed, especially to Military SATCOM (MILSATCOM) systems operating at UHF, X and Ka band frequencies in geo-stationary orbit.
It is common for a SATCOM channel between the ground and a satellite to have a strong line of sight (LOS) path. The LOS path is essential in achieving a healthy link budget. However, in a MIMO scenario the LOS nature of the channel and the large range distance in the channel path can increase the spatial correlation between the channel paths. Geometrical optimisation is required to achieve extra spatial degrees of freedom. To achieve spatial orthogonality in the LOS SATCOM channel, antenna separation on the order of several kilometres (depending on the wavelength) is required either in space or on the ground.
In this thesis, the investigation begins with analysing the applicability of MIMO to UHF SATCOM. The benefits of spatial multiplexing using multiple satellites are addressed in the analysis. UHF SATCOM has some unique advantages compared to other higher frequency bands, but at the same time, some inherent disadvantages including limited usable bandwidth and significant restrictions on applying frequency reuse in the geostationary arc, resulting in low capacity. Generally, from a commercial perspective, a MIMO scenario using multiple satellites is not considered as a cost effective solution. However, we show that narrowband MILSATCOM in UHF is a good example where using MIMO with multiple satellites can be most useful to increase the overall spectral efficiency through frequency reuse. Utilising orthogonal circular polarizations is another well-known frequency reuse technique in MILSATCOM. However, due to channel depolarization and polarization excitation errors in the antenna, the resulting polarization wave will often be elliptical in practice. Thus, any mismatch in antenna orientation can result in poor cross polar isolation (XPI) and this can severely degrade the system performance in respect of polarization frequency reuse. In this thesis, this problem is addressed within a MIMO framework. Polarization multiplexing is jointly analysed with spatial multiplexing using two X-band satellites in adjacent orbital slots and is shown to achieve a fourfold increase in channel capacity. The analysis also shows that the MIMO processing mitigates the effect of polarization imperfections.
Spatial multiplexing in single satellite systems using Multi-User MIMO (MU-MIMO) is also investigated. Next generation Ka-band SATCOM systems are ambitious interms of throughput and capacity using multiple spot beams. There are two categories of SATCOM systems that have emerged: The first is High Throughput Satellite (HTS) systems aiming to increase the overall throughput of a satellite; the second is High Capacity Satellite (HiCapS) systems, where the aim is to increase the satellite's capacity in a given region. The application of MIMO techniques to improve the system performance is a topic for research in both these scenarios. In this thesis, consideration is given to increase the frequency reuse for HiCapS systems in a high demand geographic region. Practical trade-off, user location sensitivity and MIMO communications signal processing architectures are analysed in this thesis. The results show that a linear increase in channel capacity can be achieved through MU-MIMO for a satellite with multiple spot beam antennas with overlapping frequencies serving the same geographical region.
Finally, channel measurement results are provided using a novel passive MIMO SATCOM channel measurement technique. The channel measurement results are paramount to validate the theory and knowing the channel parameters is necessary to achieve MIMO gain in real world scenarios. Results from two different measurement campaigns are presented: the first was in collaboration with Prof. Knopp and his team at the Munich University of the Bundeswehr, using signals received from two EUTELSAT satellites in Ku-band. The second was a measurement campaign conducted at the Defence Science Technology (DST) Group, Edinburgh, South Australia, in the MILSATCOM X-band.