Transmission Techniques and Channel Calibration for Spatial Interweave TDD Cognitive Radio Systems

We study the problem of beamforming design for a Cognitive Radio (CR) system in which a multiple-input multipleoutput (MIMO) link, identified as secondary link, wants to opportunistically communicate without harming a licensed MIMO system, called primary. In the proposed solution 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 whitespace of the primary system, causing no interference to the latter. This is spatial interweave. To solve this beamforming design problem the knowledge of channel state information at the transmitter (CSIT) is crucial. In our model we do not require any a priori knowledge of the channel information but we rely on channel reciprocity in Time-Division Duplex (TDD) transmission. We provide beamformer design algorithms and channel estimation procedures which allow the secondary communication, without need of cooperation with the primary system. However, in practice uplink and downlink channels are not reciprocal due to non reciprocal Radio Frequency front-ends. To compensate the channel mismatch we introduce a new calibration algorithm that outperforms the performance of previously proposed solutions for MIMO channels. We prove that channel calibration is a crucial operation and, more importantly, that it can be done without cooperation between primary and secondary systems. Finally, after a model extension to multiple primary pairs, we show how it is possible to implement in practice the proposed CR settings using the frame structure of the recent cellular communication standard Long Term Evolution (LTE).