  Due to ever increasing usage of wireless devices and data hungry
applications, it has become necessary to improve the spectral efficiency of
existing wireless networks. One way of improving spectral efficiency is to
share the spectrum amongst different coexisting networks and serve multiple
devices simultaneously. Spectrum sharing mechanisms for coexistence of a
licensed network, such as LTE, with an unlicensed network, such as Wi-Fi, are
being considered in the recent literature and standardizations. In order to
enable the coexistence between licensed and unlicensed users, it is necessary
to include interference mitigation techniques to protect the licensed primary
users (PUs) from harmful interference. Typical interference mitigation
mechanisms are based on spectrum sensing and cognitive radio (CR), wherein
unlicensed secondary users (SUs) observe the spectrum and utilize it when
licensed PUs are inactive. Thus, the SUs utilize empty time-slots in the shared
spectrum to avoid the interference. The spectral efficiency can be further
improved if the SUs are allowed to transmit concurrently with PUs by exploiting
the spatial dimension provided by multiple antenna techniques. The underlay CR
paradigm allows such coexistence where SUs transmit its signal in the same
time-slots as PUs by exploiting the spatial and frequency resources in the
network. In order to exploit the spatial dimension, SUs can utilize the
location coordinates of PUs to steer its signal away from PUs to mitigate the
interference. The SU transmitter can also employ multiple antenna techniques to
serve a large number of devices. Further, the SUs can utilize frequency bands
occupied by PUs by dynamically selecting the frequency band that provides the
highest rate. In this work, we develop techniques for PU location estimation,
spatial resource allocation and frequency band selection for SUs in underlay CR
networks.
