Transmission techniques and channel estimation for Spatial Interweave TDD Cognitive Radio systems

In this paper we propose a new method of designing the beamformer subspace in MIMO interference channel with a Time-Division Duplex (TDD) transmission scheme. In particular, this method is applied to a Spatial Interweave Cognitive Radio scenario. In our model we do not require a priori knowledge of the Channel State Information (CSI) at the transmitters. The primary and the opportunistic (cognitive) users are able to obtain information required for Tx beamforming through smart exploitation of received signal during a TDD time slot, exploiting channel reciprocity thus reducing overhead for channel estimation. The opportunistic user designs its beamformer in order to span the noise subspace at the primary receiver, thus intertwining its signal with the primary´s so that its signal lies within the spatial whitespaces of the primary, possibly licensed system, causing no interference to the latter.