A Broadband Multistage Self-Interference Canceller for Full-Duplex MIMO Radios

Full-duplex (FD) antenna arrays are desirable for expanding the channel capacity of wireless systems up to 2, which is particularly important in the crowded sub-6-GHz spectral bands. Conventional FD solutions based on circulators do not scale to antenna arrays due to the presence of significant mutual coupling between antenna elements. This article describes a two-stage self-interference cancellation (SIC) method that is suitable for FD arrays. The first-stage SIC uses a replica-antenna (RA)-based network that is passive and linear and can thus be placed before the low-noise amplifier (LNA) without significantly degrading receiver noise figure (NF) and linearity. The RA-based SIC also simplifies the hardware requirements for the second-stage SIC, which uses an analog tapped delay line (ATDL) network. The proposed two-stage SIC was realized using off-the-shelf components and tested in the 1-3-GHz range using strongly coupled pairs of ultrawideband antennas. Network analyzer measurements show 20-30 dB of SIC using the RA network over the entire frequency range and >20 dB of SIC using a single-stage ATDL SIC over instantaneous bandwidths of 0.53-0.65 GHz. The combined transmit (Tx)-receive isolation is 66.5, 73.5, and 85.8 dB around the center frequencies of 1, 2, and 3 GHz, respectively. The amount of isolation is shown to be sufficient for realizing practical FD wireless links at 2 GHz with reasonable Tx power levels (0 dBm).