In this paper a simple algorithm for the estimation of the Direct Path (DP) Time of Arrival (TOA) in an OFDM-based telecommunications system is proposed. It is shown that, under certain conditions, it is possible to infer the TOA of the direct path by estimating the phase slope across the subcarriers. The proposed algorithm exploits the intrinsic properties of a multi-carrier OFDM system to perform the estimation, and it can be employed in a fully opportunistic way if known reference signals (intended for purposes other than TOA estimation) are available. The obtained TOA estimations can be used to calculate the pseudo-ranges that can be employed for trilateration-based positioning. The performance of the proposed algorithm and its variants are assessed with simulations.
