Pilot embedding for channel estimation and tracking in OFDM systems

Abstract-We consider the problem of channel estimation and tracking in OFDM systems and explore the idea of adding pilot symbols to the data symbols as a means to conserve bandwidth. The term pilot embedding (PE) is used to refer to this scheme. Compared to the pilot insertion (PI) scheme, i.e., the conventional pilot symbol assisted modulation (PSAM), PE is more bandwidth efficient since no separate subcarriers/timeslots are allocated to pilots. We formalize this by evaluating the capacity of the two schemes and showing that PE indeed has the potential to transmit at a higher rate. The problem of channel tracking using a decision directed approach is reviewed and found to be unreliable, in the sense that the channel estimator fails to track the channel variations after some iterations because of unavoidable decision errors. We propose an ad hoc channel estimation algorithm that uses the embedded pilots along with the past decisions of data for reliable tracking of the channel.