The duplex is the concept of accommodating two directional communication between two nodes: forward and reviser links. In the current wireless communication systems, the half duplex technique is used, in which the forward and reverse links are separated via different time slots (time-division duplexing) or frequency resource (frequency-division duplexing).
To increase the spectral efficiency, the concept of full duplexing is studied, in which the forward and reverse communication take place concurrently. In the full duplex communication, however, very strong signal transmitted by itself is received at the receive antenna at each node, as shown in the figure above. This strong self-interference significantly degrades the quality of communication, since the desired signal transmitted by the other node is in general dominated by the strong self-interference.
Therefore, sophisticated RF and baseband techniques should be developed to reduce this self-interference to a noise level, thereby achieving the doubled spectral efficiency compared to the half duplex communication.