In this paper, we develop and analyze an energy efficient ARQ (automatic repeat request) initialized transmit diversity protocol for cooperative communications. Medium access control (MAC) layer packet retransmission limit (similar to aShortRetryLimit or aLongRetryLimit [802.11-1997]) has been used as an actuator for transmit cooperative diversity initialization. We take the channel state information (CSI) as a function of retransmission tries and the number of retransmission tries is modeled as a random variable. Relays close to the source node are chosen for the transmit cooperation. Closed form expressions are obtained for symbol error rate (SER), outage capacity and outage probability for the proposed scheme in shadowed fading channels. This cooperative scheme achieves lower signal-to-noise ratio (SNR), stumpy outage probability, higher bandwidth, and transmit energy efficiencies for desired average symbol error rate (ASER) than the preceding ARQ based cooperative protocols. Finally, the results of computer simulations are included to demonstrate the efficacy of the proposed scheme and to verify the accuracy of the analytical expressions.
