To increase capacity and offload traffic from the current macro cell cellular system, small cells have been deployed extensively. In the current deployments small cells coexist with their respective macro cells in the same spectrum, due to the difficulty and costs involved in acquiring new spectrum licenses. This leads to considerable interference between the two systems. In this context, we propose an underlay cognitive interference alignment technique to remove the interference from the small cell to the macro cell system. In the proposed scheme the small cells sense the unused resources in the space dimension of the macro cell, i.e. its null space, and precode their signals such that they lie in the sensed null space. We demonstrate that only the sign of the null space components is necessary to obtain close to full null space information performance. This results in less overhead in the sensing process making the system more efficient and practically feasible.