Excessively Long Channel Estimation for CDD OFDM Systems Using Superimposed Pilots

To fully exploit frequency diversity in cyclic delay diversity orthogonal frequency division multiplexing (CDD-OFDM) system, accurate channel estimation is crucial. Due to the excessive channel delay spread in CDD, traditional in-band pilot assisted channel estimation with limited frequency resolution fails to track the considerable variation in the frequency domain. Alternatively, increasing pilot overhead will degrade the system throughput significantly. In this paper, we propose to use superimposed pilots to estimate highly frequency selective channels in CDD-OFDM systems. Compared to in-band pilot based channel estimation, the proposed scheme with pilot symbols superimposed over each subcarrier has full frequency resolution, hence it is more robust to severe frequency selectivity from the excessively long CDD channel. Expectation-Maximization (EM) algorithm is employed to estimate the channel iteratively based on superimposed pilots and tentative soft decisions. At the end of each iteration, to exploit the inherent channel sparsity and refine the estimate, channel taps are sorted and selected according to power. Simulation results show that the performance of the proposed scheme is promising in time varying fading channels without an increase in pilot overhead.