  In this paper, we have studied the energy efficiency of cooperative networks
operating in either the fixed Amplifyand- Forward (AF) or the selective
Decode-and-Forward (DF) mode. We consider the optimization of the M-ary
quadrature amplitude modulation (MQAM) constellation size to minimize the bit
energy consumption under given bit error rate (BER) constraints. In the
computation of the energy expenditure, the circuit, transmission, and
retransmission energies are taken into account. The link reliabilities and
retransmission probabilities are determined through the outage probabilities
under the Rayleigh fading assumption. Several interesting observations with
practical implications are made. It is seen that while large constellations are
preferred at small transmission distances, constellation size should be
decreased as the distance increases; the cooperative gain is computed to
compare direct transmission and cooperative transmission.
