  The cooperative automatic repeat request (C-ARQ) is a link layer relaying
protocol which exploits the spatial diversity and allows the relay node to
retransmit the source data packet to the destination, when the latter is unable
to decode the source data correctly. This paper presents a cross-layer link
adaptation design for C-ARQ based relay channels in which both source and relay
nodes employ adaptive modulation coding and power adaptation at the physical
layer. For this scenario, we first derive closed-form expressions for the
system spectral efficiency and average power consumption. We then present a low
complexity iterative algorithm to find the optimized adaptation solution by
maximizing the spectral efficiency subject to a packet loss rate (PLR) and an
average power consumption constraint. The results indicate that the proposed
adaptation scheme enhances the spectral efficiency noticeably when compared to
other adaptive schemes, while guaranteeing the required PLR performance.
