We deal here with distributed cooperative communication systems' in which a single source transmits its information with the help of multiple relays to a destination. Specifically, we consider decode and forward relay networks based on orthogonal frequency division multiplexing (OFDM). Most previous work needs a priori information for antenna allocation to obtain cooperative diversity advantages. We propose a simple method that does not require additional information in order to coordinate relay transmission. Our main proposal is to retransmit randomly cyclic-shifted sequences of the received symbol at relays. Simulation results verify that full gain in diversity is achieved when the number of subcarriers is sufficiently large.