A ±45°-Polarized Antenna System with Four Isolated Channels for In-Band Full-Duplex (IBFD)

An in-band full-duplex (IBFD) antenna system having four ports with good self-interference cancellation (SIC) is presented in this paper. Two of the four ports provide +45°-polarized radiation and the other two generate -45°-polarized radiation. Every port of the proposed antenna is well isolated from the other three ports, enabling simultaneous transmitting and receiving with both two polarizations. To achieve this goal, three different types of couplings need to be addressed simultaneously. In this work, a novel isolation feed network is proposed, which is the first of its kind that can achieve three targets at the same time, i.e., feeding the antenna differentially, forming the original horizontal and vertical polarizations into ±45° polarizations, and cancelling out the reflected and coupled signals between the transmitting (TX) and receiving (RX) ports. Then, a double-antenna IBFD system with four isolated channels is attained by combining the proposed feed network, defected ground structure (DGS), cross-shaped walls, and metal baffles, which effectively mitigated all the three types of couplings at the same time. The double-antenna system has high isolations of > 39 dB between any two of the four ports from 3.3 to 3.9 GHz. The four radiation patterns obtained by exciting the four ports are very similar to each other and are stable with frequency, which makes it an excellent candidate for 5G sub-6 GHz IBFD applications.