See my recent paper in IEEE Transactions on Wireless Communications:
A. W. Eckford, J. P. K. Chu, and R. S. Adve, “Low complexity and fractional coded cooperation for wireless networks,” IEEE Trans. Wireless Commun., vol. 7, no. 5, pp. 1917-1929, May 2008. (PDF)
We consider demodulate-and-forward as a low-complexity cooperation technique, and develop a low-complexity "fractional" cooperation scheme, in which every node contributes as much as it can -- possibly not its full resources.