  Spectrum shortage is a fundamental problem in wireless networks and this
problem becomes increasingly acute with the rapid proliferation of wireless
devices. To address this problem, spectrum sharing in the context of cognitive
radio networks (CRNs) has been considered a promising solution. In this paper,
we propose a practical spectrum sharing scheme for a small CRN that comprises a
pair of primary users and a pair of secondary users by leveraging the
multiple-input and multiple-output (MIMO) technology. In our scheme, we assume
that the secondary users take full responsibility for cross-network
interference cancellation (IC). We also assume that the secondary users have no
knowledge about the primary network, including its signal waveform, frame
structure, and network protocol. The key components of our proposed scheme are
two MIMO-based interference management techniques: blind beamforming (BBF) and
blind interference cancellation (BIC). We have built a prototype of our scheme
on a wireless testbed and demonstrated that the prototyped secondary network
can coexist with commercial Wi-Fi devices (primary users). Experimental results
further show that, for a secondary device with two or three antennas, BBF and
BIC achieve an average of 25dB and 33dB IC capability in an office environment,
respectively.
