In this paper, we have studied the energy efficiency of cooperative networksoperating in either the fixed Amplifyand- Forward (AF) or the selectiveDecode-and-Forward (DF) mode. We consider the optimization of the M-aryquadrature amplitude modulation (MQAM) constellation size to minimize the bitenergy consumption under given bit error rate (BER) constraints. In thecomputation of the energy expenditure, the circuit, transmission, andretransmission energies are taken into account. The link reliabilities andretransmission probabilities are determined through the outage probabilitiesunder the Rayleigh fading assumption. Several interesting observations withpractical implications are made. It is seen that while large constellations arepreferred at small transmission distances, constellation size should bedecreased as the distance increases; the cooperative gain is computed tocompare direct transmission and cooperative transmission.
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