Beamforming & mmWave
MmWave communications are receiving a lot of attraction from industry and research communities since they were designated as 5G enablers and because of proliferation of satellite applications requiring steerable antennas in their user terminals. It is envisaged that the wide deployment of this technology will generate a new interference landscape. Therefore, this group is investigating large beamforming arrays jointly with interference mitigation techniques for spectrum sharing scenarios. As a matter of fact, digital beamforming (DBF) antenna arrays offer the best performance in terms of beamforming capabilities. Unfortunately, the high gain requirements of mm-waves translate in high number of antennas that make the DBF solution unaffordable in terms of cost and complexity. At the other end, analog beamforming (ABF) has a limited processing capability (i.e. the one offered by the analog components) and their implementations become bulky and lossy when the number of antennas is large. In this context, current investigations have been focusing on hybrid analog-digital beamforming (HADBF) which allows balancing the number of RF chains, the size of the analog beamforming networks and the processing capabilities. In particular we analyzed different HADBF architectures including full- and partially-connected beamforming networks and alternative solutions such us butler matrices. This analysis considered both the implementation complexity and its related cost as well as the beamforming performance.
Different HADBF beamforming algorithms have been developed for each of the architectures by tackling the optimization problem directly, which guarantees the fulfilment of the interference power limit constraints. These restrictions might be violated whenever the other approaches are elected (i.e. the resulting beamforming approximation might violate the interference constraints).
In addition, other antenna technologies not requiring complex beamforming networks are being studied as low cost alternatives to traditional arrays. This is the case of reflectarrays antennas that are based on a spatial feeding mechanism, in which a feed illuminates a reflector surface composed by a grid of unit cell radiators. In particular, we analyzed the multi-beam and null-steering capabilities of single- and multi-feed reflectarray configurations. Moreover, we developed a new technique for reducing the complexity of controlling steerable reflectarrays based on addressing the unit cell radiators by rows and columns instead of element by element.
Besides reflectarrays, we are also investigating metasurface antennas as a low cost alternative, especially when low–profile solutions are required. In this case, the high gain beams are synthesized from the interaction between an excited surface wave and the subwavelength radiators forming the metasurface, which creates leaky-waves. The focus has been on the development of multi-beam and null-steering techniques.