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In this paper, we propose a new adaptive relaying protocol called Threshold-based Adaptive Decode-Amplify- Forward relaying protocol (T-ADAF). In our protocol, the relay compares the signal to noise ratio (SNR) of the received signal to the average SNR of the source-relay link. If the SNR of the received signal is greater than the average SNR of the source-relay link, then the relay performs the Amplify-Forward relaying protocol (AF). On the other hand, if the SNR of the received signal does not exceed the average SNR of the source-relay link, then the relay performs the Adaptive Decode-Forward relaying protocol (ADF). The performance of the proposed protocol is investigated and a closed form of its symbol error probability is derived in the presence of Rayleigh fading channels. Furthermore, a comparison with other relaying protocols such as AF, ADF and SNR-HDAF (SNR-based Hybrid Decode-Amplify-Forward) is made in order to evaluate the performance of our protocol and to show its benefits. We also investigate the T-ADAF protocol with multiple relays and we derived a closed form of its symbol error probability.