Diversity Analysis of Smart Relaying

It has been known that relaying can provide spatial diversity while satisfying the size-limited constraint of the users' devices in wireless communications systems. For a practically attractive decode-and-forward (DF) relaying system with maximal-ratio combining at the destination, spatial diversity is lost, except when the source-relay link is reliable. To deal with the problem and inspired by the work of Wang et al., this paper considers and analyzes an adaptive transmission scheme, which is referred to as smart relaying, when only the average relay-destination (R-D) signal-to-noise ratio (SNR) is available at the relay. In the system under consideration, the source
continuously
transmits the information to the destination. The relay adaptively scales its transmitted power to changes in the channel condition but never exceeds the total power that the conventional relaying used. Performance analysis proves that a diversity order of 2 is always obtained for binary phase-shift keying (BPSK) and quaternary phase-shift keying (QPSK). A diversity order of 2 is also observed for higher order rectangular quadratic-amplitude modulation (QAM) constellations through numerical results. The result on diversity order does not depend on how perfect the R-D feedback channel and how exact the quantization of the power scaling are. All the corresponding results of Wang et al. are subsumed in our analysis.