The ultimate goal of communication technologies is to improve the link performance between the transmitter and receiver in two aspects: reliability and data rate. In general, the link performance is enhanced as the transmitter and receiver are located closer to each other due to the reduced path loss.
Thus, employing a relay between the transmitter and receiver can shorten the distance as shown in the left figure above, thereby achieving enhanced end-to-end link performance. A variety of functionalities can be considered at the relay according to the complexity and performance trade-off. For instance, the link performance can be further enhanced if the relay performs complicated and sophisticated signal processing, while we can trade-off the performance to reduce the computational complexity at the relay.
One of the promising advanced relay techniques is multi-way relaying. A brief sketch of two-way relaying is depicted in the right figure above. The relay receives the signals from the two end nodes at the same time in the first phase, and broadcasts its signal in the second phase. Doing so, the amount of resource required can be halved, thereby achieving the doubled spectral efficiency.