Cognitive Interference Alignment for Spectral Coexistence

Interference Alignment (IA) has been widely recognized as a promising
interference mitigation technique since it can achieve the optimal degrees of freedom
in certain interference limited channels. In the context of Cognitive Radio (CR)
networks, this technique allows the coexistence of two heterogeneous wireless systems
in an underlay cognitive mode. The main concept behind this technique is the
alignment of the interference on a signal subspace in such a way that it can be filtered
out at the non-intended receiver by sacrificing some signal dimensions. This
chapter starts with an overview of IA principle, Degree of Freedom (DoF) concept,
and the classification of existing IA techniques. Furthermore, this chapter includes a
discussion about IA applications in CR networks.Moreover, a generic system model
is presented for allowing the coexistence of two heterogeneous networks using IA
approach while relevant precoding and filtering processes are described. In addition,
two important practical applications of the IA technique are presented along with the
numerical results for underlay spectral coexistence of (i) femtocell-macrocell systems,
and (ii) monobeam-multibeam satellite systems. More specifically, an uplink
IA scheme is investigated in order to mitigate the interference of femtocell User
Terminals (UTs) towards the macrocell Base Station (BS) in the spatial domain and
the interference of multibeam satellite terminals towards the monobeam satellite in
the frequency domain.