Space alignment for cognitive transmission with multiple primary users

A space-alignment (SA) scheme with K primary users in different cells working in the same frequency band based on K-symbol extension for cognitive transmission in a multiple-input multiple-output system is proposed. The main idea is that the secondary transmitter sends a supersymbol over the K timeslot; meanwhile, the secondary transmitter aligns its transmit directions by precoding to the kth primary channel eigenmodes unused at the kth timeslot, and at the same time adapts its transmit power to satisfy the interference-power constraints at the other K -1 primary receivers. The simulation results show that the proposed scheme achieves a higher performance compared with both the SA scheme with the single primary user and the scheme of the direct singular value decomposition of the secondary channel with interference-power constraint.