Self Interference Cancellation on a Full Duplex DFTs-OFDM System using GNU Radio and USRP

The physical layer of an orthogonal frequency division multiplexing (OFDM) wireless communication system, operating in full-duplex (FD) mode is implemented in this paper, using GNU Radio software and universal software radio peripheral (USRP) hardware technology. The self-interference cancellation (SIC) is achieved on the FD node by applying four different techniques, two in the radio frequency (RF) domain and two in the digital domain. In the RF domain, a passive suppression is obtained by placing the transmit (Tx) and the receive (Rx) antennas such that they generate orthogonal electromagnetic fields. Also, an active signal suppression is achieved by changing the phase and attenuating the self-interference (SI) Tx signal, to generate a local SIC signal. The SIC signal and the Rx signal are then combined before going to the low noise amplifier at the FD node. In the digital domain, two techniques are applied to improve the interference cancellation and the quality of the link: firstly, virtual differentiation of the Tx and Rx signals is achieved by using OFDM symbols with different sizes; secondly, a spreading and precoding technique utilizing the discrete Fourier transform (DFT) is considered. In order to test the practical operation of the FD node, two simultaneous links between the three nodes are established. The results of the implemented system show that the SI can be reduced up to 58 dB which, along with the two digital techniques, allows to have two simultaneous links with a bit error rate similar to the one achieved with just one link without interference, increasing the spectral efficiency.