Full duplex 2×2 MIMO radios

This paper presents the novel design and implementation of our 2 ×2 full duplex MIMO radios for LTE system. Our design first uses multi-tap analog cancellation to reduce self-interference in RF domain. 4 multi-tap cancellers are used respectively for the 4 self-interference chains existed in a radio. RF power detectors and ADCs are introduced to indicate the residual self-interference power arriving at receivers. Multidimensional gradient descent search algorithm aiming at minimizing residual self-interference power is used to find out the optimal setting for the adjustable attenuation, time delay and phase shift of each tap in multi-tap cancellers. In the further digital cancellation, due to the fact that the non-linear component and noise arisen from transmitters are significant in residual self-interference and difficult to be cancelled, we use additional receivers to get the feedback of local transmitted RF signals, and therefore the non-linear component and noise hided in them are known and can easily be cancelled. The feedback makes it not necessary to estimate non-linear component and transmitter noise in digital cancellation algorithm and thus makes the algorithm simple. Actually, a simple channel estimation based method utilizing the feedback can accurately reconstruct self-interference. Then the reconstructed self-interference is subtracted from received signals to complete the digital cancellation. Our implementation can cancel the self-interference caused by 23dBm LTE transmitted signals to the receiver noise floor of -90dBm and achieve 220Mbps full duplex communication.