Signal alignment for two-cell CR networks

In this paper, we study interference-free uplink transmission for two-cell multiple-antenna cognitive radio (CR) networks. We consider a scenario that the primary user (PU) transmission is based on orthogonal frequency-division multiplexing (OFDM) and the nullspace generated by the cyclic-prefix (CP) can be used for CR transmission using Vandermonde-subspace frequency division multiplexing (VFDM). Besides interference to the PU system, we will take intra- and inter-cell interference within CR networks into consideration. A signal alignment scheme will be developed to protect the PU transmission while aligning inter-cell interference into a lower dimensional-subspace. When the interference signals are perfectly aligned, the number of interference-free symbols that can be transmitted by the whole CR network will increase with the number of CR users. Numerical results show significant performance gain can be obtained by using the proposed scheme.