Spectral efficient signaling for half-duplex relay channels

We study two-hop communication protocols where one or two relay terminals assist in the communication between two transceiver terminals. All terminals operate in half-duplex mode, i.e., may not receive and transmit simultaneously at the same time and frequency. This leads to a loss in spectral efficiency due to the pre-log factor 1/2 in corresponding expressions for the achievable rate (capacity). We propose and analyze two relaying protocols that avoid the pre-log factor 1/2 but still work with half-duplex relays. Firstly, we consider a relaying protocol where two half-duplex relays, either amplify-and-forward (AF) or decode-and-forward (DF), alternately forward messages from a source terminal to a destination terminal (two-path relaying). It is shown that the protocol can recover a significant portion of the half-duplex loss. Secondly, we propose a relaying protocol where a bidirectional connection between two transceiver terminals is established via one half-duplex AF or DF relay (two-way relaying). It is shown that the sum rate of the two-way half-duplex AF relay channel achieves the rate of the one-way full-duplex AF relay channel, whereas the sum rate of the two-way half-duplex DF relay channel achieves the rate of the one-way full-duplex DF relay channel only in certain cases.