Cooperative diversity is a technique where the diversity associated with spatially distributed users is exploited to overcome violent fading in wireless channels. In vehicle-to-vehicle (V2V) communication systems, where fading is severe due to mobility of both transmitters and receivers, cooperative diversity is an acceptable technique to enhance the performance of high-rate communications. In this paper, we investigate a single-relay half-duplex system and deal with the performance of three time-division-multiple-access (TDMA) amplify-and-forward protocols over double-Rayleigh fading channels. In a mobile-to-mobile (M2M) communication system, the communication channel is often modeled as a cascaded Rayleigh fading channel. We determine pairwise error probability (PEP) for the studied protocols, then we investigate optimal power allocation for these cooperative protocols, where the optimality is determined in terms of minimizing the bit error rate (BER) of the system. Based on the relay position and modulation type, optimization of power allocated to source and relay results performance improvement up to 3dB for Protocols I and II, and up to 7dB for Protocol III.